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GEODYNAMICS & TECTONOPHYSICS
PUBLISHED BY THE INSTITUTE OF THE EARTH'S CRUST SIBERIAN BRANCH OF RUSSIAN ACADEMY OF SCIENCES
ISSN 2078-502X
2016 VOLUME 7 ISSUE 4 PAGES 555-592
http://dx.doi.org/10.5800/GT-2016-7-4-0223
Sources of Quaternary potassic volcanic rocks from
Wudalianchi, China: Control by transtension at the lithosphere-asthenosphere boundary layer
S. V. Rasskazov1, 2, I. S. Chuvashova1, 2, Yi-min Sun3, Chen Yang3, Zhenhua Xie3, T. A. Yasnygina1, E. V. Saranina1, Zhenxing Fang3
1 Institute of the Earth's Crust, Siberian Branch of RAS, Irkutsk, Russia
2 Irkutsk State University, Irkutsk, Russia
3 Institute of Volcano and Mineral Spring, Heilongjiang Academy of Science, Wudalianchi, Heilongjiang, China
Abstract: Introduction. Transtension is a system of stresses that tends to cause oblique extension, i.e. combined extension and strike slip. Syn-volcanic transtensional deformations of the lithosphere may provide two possible scenarios for control of magmatic processes. One scenario assumes ascending sub-lithospheric melts that mark the permeable lithosphere in a transtension area without melting of the lithospheric material; products of volcanic eruptions in such a zone show only the sub-lithospheric mantle material; components of magmatic liquids do not reveal any connection to the lithospheric structure. Another scenario yields a direct control of melting in lithospheric sources in an evolving transtensional structure. In this case, spatial-temporal changes of lithospheric and sub-lithospheric components are a direct indication of the evolving transtensional zone. In this paper, we present arguments in favor of the transtensional origin of the lithosphere-derived melting anomaly along the Wudalianchi volcanic zone, which are based on the study of components in the rocks sampled from the volcanic field of the same name.
Analytical methods. Trace elements were determined by ICP-MS using a mass-spectrometer Agilent 7500ce and isotopes using a mass-spectrometer Finnigan MAT 262. The methods used were described in the previous papers by Rasskazov et al. [2011] and Yasnygina et al. [2015]. Major oxides were measured by "wet chemistry".
Structural setting of the Wudalianchi zone. This zone extends north-south for 230 km at the northern circuit of the Songliao basin, subsided in the Late Mesozoic - Early Cenozoic (Fig. 1).
Timing of volcanism and variations of K2O contents in rocks from the Wudalianchi zone. Rocks, dated back to the Pliocene and Quaternary, show the stepwise increasing K2O content interval along the Wudalianchi zone from the southernmost Erkeshan volcanic field (5.6-5.8 wt %) to the northernmost Xiaogulihe-Menlu volcanic field (2.0-9.5 wt %) (Fig. 2).
Spatial-temporal clustering of volcanoes in the Wudalianchi field. In terms of the general Quaternary evolution of volcanism in Asia [Rasskazov et al., 2012], spatial-temporal distribution and compositional variations of volcanic products, we distinguish three time intervals of the volcanic evolution: (1) 2.5-2.0 Ma, (2) 1.3-0.8 Ma, and (3) <0.6 Ma. The Central group of volcanoes showed persistent shifting of eruptions from Wohushan (1.33-0.42 Ma) to Bijiashan (0.45-0.28 Ma) to Laoheishan (1720-1721, possibly earlier) to Huoshaoshan (1721) (Figs 3, 4, 5). No spatial-temporal regularity of eruptions in volcanoes of the Erkeshan field and Western and Eastern groups of the Wu-dalianchi field reflected background activity.
Sampling. Representative sampling of rocks from the Wohushan-Huoshaoshan volcanic line was aimed to identify changing geochemical signatures along the whole volcanic line and in the course of eruptions in each volcano (Figs 3, 6, 7). For comparisons, the background volcanoes were also sampled.
Silica and alkalis oxides. On the total alkalis-silica (TAS) diagram (Fig. 8), data points of background rocks are distributed along the dividing lines between highly and moderately alkaline series mainly in tephriphonolite and trachyandesite fields with a few samples in the phonotephrite field. Background rocks from some volcanoes (e.g. Yaoquanshan and Weishan) are highly alkaline (phonotephrites and tephriphonolites). Background rocks from other volcanoes (Longmenshan, Jiaodebushan etc.) are moderately alkaline (mostly trachyandesites). In background rocks, Na2O+K2O range from 8.6 to 9.7 wt %, SiO2 from 51.6 to 55.0 wt %. Phonotephrites from the Erkeshan field are comparable with the Wudalianchi background rocks of this type.
Data points of rocks from the Central group of volcanoes are also distributed along the discriminating line of highly and moderately alkaline series, mainly in the phonotephrite and trachyandesite fields. Almost all samples from the first volcano (Wohushan) fall within the data field of background rocks. Rock compositions of the second and third volcanoes (Bijiashan and Laoheishan) changed on each of them from similar to the background to the ones distinguished by the lower silica and alkalis contents. On the Bijiashan volcano, eruptions were exhibited by trachyande-
sites of a lava shield and by basaltic trachyandesites and phonotephrites of a volcanic cone. The trachyandesites were comparable to the background rocks, the basaltic trachyandesites and phonotephrites differed from them. On the Laoheishan volcano, rocks were subdivided into three groups: (1) basaltic trachyandesites and phonotephrites, (2) trachyandesites, and (3) phonotephrites. The first group was recorded in pyroclastic material from the late volcanic cone and lavas from the northern bocca, the second group in pyroclastic material from the northwestern edge of the late crater, and the third group in bombs from its southwestern edge. On the fourth volcano (Huoshaoshan), rocks are basaltic trachyandesites and phonotephrites.
In terms of Na2O, K2O, and SiO2 contents, peripheral lavas of volcanic fans in the Bijiashan, Laoheishan, and Huoshaoshan volcanoes were close to background rocks. The contents of these oxides, differed from the background signatures, characterize rocks from volcanic cones in a linear progression that demonstrates the transition from compositions of the Wohushan volcano, close to background ones, through the intermediate values in the Bijiashan and Laoheishan volcanoes to the final compositions in the Huoshaoshan volcanic cone.
In the background rocks, K2O concentrations range from 4.8 to 6.0 wt % with its relative decrease in the rocks of the beginning and end of volcanic evolution. Initial lava flows with K2O contents as low as 4.0 wt % erupted along the Laoshantou - Old Gelaqiushan north-south locus from 2.5 to 2.0 Ma and in the final cone of the Huoshaoshan volcano, erupted in 1721, fell to 3.2 wt %. Since 1.3 Ma, irregular spatial-temporal distribution of volcanic activity reflected dominated background processes. Between 1.3 and 0.8 Ma, eruptions took place at the South Gelaqiushan volcano and along the west-east locus of the Lianhuashan, Yaoquanshan, West Jaodebushan, West Longmenshan volcanoes. In the last 0.6 Ma, three groups of volcanoes erupted: Western (North Gelaqiushan, Lianhuashan, Jianshan-Jianshanzi, Central (Wohushan, Bijiashan, Laoheishan, Huoshaoshan), and Eastern (Weishan, East Jaodebushan, Xiaogoshan, West and East Longmenshan, Molabushan). Background eruptions continued in the Western and Eastern groups, whereas the Central group displayed stepwise shift of activity from the southwest to the northeast. Under such a regular volcanic evolution, relative reduction of K2O abundances took place in final eruption products of the Huoshaoshan volcano (Fig. 9).
Other major oxides. Changes of rock compositions along the Wohushan-Huoshaoshan line, from the close to the background signatures at the first volcano (Wohushan) through the contrast major oxide contents at the Bijiashan and Laoheishan edifices to notably different from the background ones at the Huoshaoshan cone, are illustrated further by diagrams of SiO2 vs. MgO, AhO3 vs. MgO, CaO vs. MgO, and P2O5 vs. MgO (Figs 10, 11).
Trace elements. No sufficient difference is found between primitive mantle-normalized patterns plotted for rocks from different volcanoes (Fig. 13). Nevertheless, specific variations of rock compositions in the Central group of volcanoes close to the background and different from them are shown on the diagrams of Ni, Cr, Rb, Zr, Ba, Th, Sr, and La/Yb vs. MgO (Figs 12, 14, 15). A similar behavior was observed, on the one hand, for Rb and Zr, on the other hand, for Ba, Th, Sr, and La/Yb. In rocks from the Central group of volcanoes, which are compositionally close to the background ones, Rb concentrations increase from the first volcano (Wohushan) through the second (Bijiashan) to the third (Laoheishan). In rocks that differ from the background ones, Rb concentrations increase from the second to the fourth volcano and decrease in its final edifice. In rocks, close to the background ones, Zr concentrations decrease from the first to the second volcano and increase to the third volcano. In rocks, distinguished from background ones, relatively low concentrations of Zr at the first volcano change to elevated concentrations at the third and fourth volcanoes with relative decrease at the final Huoshaoshan edifice.
Discussion. Sub-lithospheric continuum of components under East Asia comprises a material from convective mantle domain with subducted slab (paleoslab) fragments of oceanic (paleooceanic) crust as well as delaminated lithospheric blocks of orogens. Volcanic rocks from the Wudalianchi field show a sub-lithospheric end-member, which belongs to this continuum. Lithospheric components of these rocks, however, have no connection with other sub-lithospheric components. We refer the Wudalianchi rocks to a sub-lithospheric-lithospheric cluster of components from the boundary between the lithosphere and sub-lithospheric convective mantle (Fig. 17). From the comparative analysis of K2O, other major oxides, and trace elements in rocks of early and late eruption phases in the Central group of volcanoes, we infer that rocks were compositionally almost similar to the background ones in edifices of the first volcano (Wohushan), partially close to the background rocks and partly differed from them in edifices of the second and third volcanoes (Bijiashan, Laoheishan), and significantly different from the background rocks in the cone of the fourth volcano (Huoshaoshan) (Figs 18, 19). We suggest that magma generation under the Wudalianchi volcanic field was controlled by developing transtension of a layer at the base of the lithosphere that divided and shielded sources of the underlying homogeneous sub-lithospheric convective mantle and the overlying enriched heterogeneous lithosphere. The sub-lithospheric magma source had 87Sr/86Sr=0.7052, sources of the boundary shielding layer the same and lower Sr-isotopic ratios, and sources of the overlying region the same and higher ratios (Fig. 20). Through the extremely low row of data points for rocks from the Huoshaoshan volcanic cone in 87Sr/86Sr vs. 87Rb/86Sr plot, we get an estimate of about 98 Ma for the isotopic system closure at the base of the lithosphere with the initial 87Sr/86Sr apatite-related value 0.70485 and the underlying convective mantle domain with Rb/Sr=0.092 (Fig. 21). We infer that the development of transtension governed time and space of the locally introduced convective mantle component through the boundary shielding layer on background of melting enriched mantle material above the latter (Fig. 22). The 2.5-2.0 Ma local eruptions of sub-lithospheric liquids, derived from the axial part of the north-south zone of transtension, were followed by the 1.3-0.8 Ma background melts from a wider transtensional segment of the enriched lithospheric region. Afterwards, in the past 0.6 Ma, background melting of the enriched lithosphere sharply outlined edge portions of the transtensional segment, whereas simultaneous local sub-lithospheric melting propagated along a crack that originated within the boundary shielding layer due to concentrating tectonic forces at the central portion of the transtensional segment.
Key words: Potassic rocks; trace elements; Sr isotopes; transtension; asthenosphere; lithosphere; Wudalianchi;
Northeast China
Recommended by E.V. Sklyarov
For citation: Rasskazov S.V., Chuvashova I.S., Yi-min Sun, Chen Yang, Zhenhua Xie, Yasnygina T.A., Sarani-na E.V., Zhenxing Fang. 2016. Sources of Quaternary potassic volcanic rocks from Wudalianchi, China: Control by transtension at the lithosphere-asthenosphere boundary layer. Geodynamics & Tectonophysics 7 (4), 555-592. doi:10.5800/GT-2016-7-4-0223.
Источники четвертичных калиевых вулканических пород Удаляньчи, Китай: контроль транстенсией в пограничном слое литосферы-астеносферы
С. В. Рассказов1, 2, И. С. Чувашова1, 2, Йи-минь Сунь3, Чэнь Янг3, Чжэньхуа Сие3, Т. А. Ясныгина1, Е. В. Саранина1, Чжэньсин Фан3
1 Институт земной коры СО РАН, Иркутск, Россия
2 Иркутский государственный университет, Иркутск, Россия
3 Институт вулканов и минеральных источников Хэйлунцзянской академии наук, Удаляньчи, Хэйлунцзян, Китай
Аннотация: Введение. Транстенсия представляет собой систему напряжений, вызывающую косое растяжение литосферы - сочетание ее растяжения и сдвига. Синвулканические транстенсионные деформации литосферы могут обеспечить два возможных сценария контроля магматических процессов. Один из них предполагает восходящие подлитосферные расплавы, обозначающие проницаемые участки литосферы в области транстенсии без плавления литосферного материала. Продукты вулканических извержений в такой зоне представлены только подлитосферным мантийным материалом. Компоненты магматических расплавов не обнаруживают никакой связи с транстенсионной структурой литосферы. Другой сценарий выражается в непосредственном контроле плавления литосферных источников в эволюционирующей транстенсионной структуре. В этом случае пространственно-временная смена литосферных и подлитосферных компонентов служит прямым признаком эволюционирующей транстенсионной зоны. В настоящей статье мы приводим аргументы в пользу транстенсионной природы литосферной расплавной аномалии вулканической зоны Удаляньчи на основе исследований компонентов вулканических пород одноименного поля.
Аналитические методы. Содержания микроэлементов определены методом ICP-MS на масс-спектрометре Agilent 7500се, а отношения изотопов - на масс-спектрометре Finnigan MAT 262. Использованные методики охарактеризованы в предшествующих работах [Rasskazov et al., 2011; Yasnygina et al., 2015]. Петрогенные оксиды определены методом «мокрой химии».
Структурное положение зоны Удаляньчи. Зона простирается субмеридионально более чем на 230 км на северном замыкании бассейна Сунляо, образовавшегося в позднем мезозое - раннем кайнозое (рис. 1).
Возраст и содержания K2O в породах зоны Удаляньчи. Породы зоны Удаляньчи, датированные плиоценом и квартером, характеризуются последовательным увеличением интервала содержаний K2O от наиболее южного поля Еркешан (5.6-5.8 мас. %) к наиболее северному полю Сяогулихе (2.0-9.5 мас. %) (рис. 2).
Пространственно-временное группирование вулканов поля Удаляньчи. По пространственно-временному распределению и составу продуктов извержений мы различаем вулканические интервалы 2.5-2.0 млн лет назад, 1.3-0.8 млн лет назад и последние 0.6 млн лет. В линии четырех вулканов Центральной группы имела место единая возрастная последовательность извержений: Уохушан (1.33-0.42 млн лет назад), Бид-зиашан (0.45-0.28 млн лет назад), Лаохейшан (1720-1721 гг., возможно, более ранние извержения) и Хуо-шаошан (1721 г.) (рис. 3-5). Отсутствие пространственно-временной регулярности извержений вулканов на поле Еркешан и в Западной и Восточной группах вулканов поля Удаляньчи отражало фоновую активность. В фоновых породах определен диапазон концентраций K2O 4.8-6.0 мас. % с относительным снижением содержаний этого оксида в породах начала и конца вулканической эволюции. В начальных лавовых потоках, излившихся в субмеридиональной полосе Лаошантоу - Древний Гелацюшан в интервале 2.5-2.0 млн лет назад, содержания K2O составляли 3.9-5.2 мас. %, на финальном конусе вулкана Хуошаошан, образовавшемся в 1721 г., - снижались до 3.2 мас. %. Фоновая активность проявилась на вулкане Южный Гелацюшан и вулканах субширотной полосы Лианхуашан, Йаоцюаншан, Западный Дзяодебушан, Западный Лонгменшан во временном интервале 1.3-0.8 млн лет назад. В последние 0.6 млн лет извергались вулканы трех групп: Западной (Северный Гелацюшан, Лианхуашан, Дзианшан-Дзиамшанзи), Центральной (Уохушан, Бидзиашан, Лаохейшан, Хуошаошан) и Восточной (Уэйшан, Восточный Дзяодебушан, Сяогошан, Западный и Восточный Лонгменшан, Молабушан). В Западной и Восточной группах фоновая активность продолжалась, в то время как в Центральной группе активность вулканов последовательно смещалась с юго-запада на северо-восток. Такая упорядоченная вулканическая эволюция сопровождалась относительным снижением содержаний K2O в продуктах финальных извержений вулкана Хуошаошан.
Опробование. Представительное опробование пород вулканов в линии Уохушан-Хуошаошан проводилось с целью выявления меняющихся геохимических характеристик в ходе извержений на каждом вулкане и от вулкана к вулкану (рис. 3, 6, 7). Для сопоставлений использованы данные по составу пород вулканов фоновых извержений.
Оксиды кремнезема и щелочей. На классификационной диаграмме щелочи - кремнезем (рис. 8) фигуративные точки пород фоновых извержений сконцентрированы вдоль разделительной линии серий высокой и умеренной щелочности преимущественно в полях тефрифонолитов и трахиандезитов, в меньшей степени - в поле фонотефрита. Фоновые породы одних вулканов (например, Йаоцюаншан и Уэйшан) имеют высокощелочной (фонотефритовый и тефрифонолитовый) состав, фоновые породы других вулканов (Лонгменшан, Дзяодебушан и др.) - умеренно щелочной (трахиандезитовый). Сумма щелочей №20+К20 в фоновых породах находится в интервале 8.6-9.7 мас. %, SiO2 - в интервале 51.6-55.0 мас. %. Фонотефриты поля Еркешан сопоставимы с фоновыми породами поля Удаляньчи.
Фигуративные точки пород Центральной группы вулканов также распределяются вдоль разделительной линии серий высокой и умеренной щелочности диаграммы щелочи - кремнезем, преимущественно в полях фонотефритов и трахиандезибазальтов. Почти все образцы первого вулкана (Уохушан) находятся в фигуративном поле фоновых пород. Составы пород второго и третьего вулканов (Бидзиашан и Лаохейшан) менялись в ходе извержений каждого из них от близких к фоновым до отличавшихся от фоновых. На вулкане Бид-зиашан продукты вулканических извержений были представлены трахиандезитами периферии щитовой постройки и трахиандезибазальтами-фонотефритами вулканического конуса. Трахиандезиты были сопоставимы с фоновыми породами, трахиандезибазальты-фонотефриты заметно отличались от них. На вулкане Лаохейшан выделилось три группы пород: 1) трахиандезибазальты-фонотефриты, 2) трахиандезиты и 3) фонотефриты. Породы первой группы представлены в пирокластическом материале позднего вулканического конуса и в лавовых потоках северной бокки, породы второй группы - в пирокластическом материале западной кромки позднего кратера, породы третьей группы - в бомбах его юго-западной кромки. По соотношениям №20, К2О и SiO2 породы периферических лавовых шлейфов вулканов Бидзиашан, Лаохейшан и Хуо-шаошан сопоставимы с продуктами фоновых извержений. Содержания этих оксидов, отличающиеся от фоновых, свойственны породам линии вулканических конусов, в которых проявился переход от составов пород вулкана Уохушан, близких к фоновым, через промежуточные составы пород вулканов Бидзиашан и Лаохей-шан к финальным составам пород вулканического конуса Хуошаошан (рис. 9).
Другие петрогенные оксиды. Смена составов пород вдоль линии Центральной группы вулканов от близких к фоновым в первой постройке (Уохушан) через контрастные в постройках Бидзиашан и Лаохейшан к заметно отличающимся от фоновых на конусе вулкана Хуошаошан дополнительно иллюстрируется диаграммами Я02-М^0, Al20з-Mg0, Са0-М^0 и P205-Mg0 (рис. 10, 11).
Микроэлементы. Для пород разных вулканов не отмечено заметных различий в элементных спектрах, нормированных к примитивной мантии (рис. 13). Особые вариации пород вулканов Центральной группы, близких к фоновым и отличающихся от них, подчеркнуты, тем не менее, на диаграммах Rb-Mg0, Zr-Mg0, Ва-Mg0, Th-Mg0, Sr-Mg0 и La/Yb-Mg0 (рис. 12, 14-15). Сходное поведение обнаружено, с одной стороны, для Rb и Zr, с другой стороны - для Ва, Т^ Sr и La/Yb. В составах пород вулканов Центральной группы, близких к фоновым, наблюдалось последовательное повышение концентраций Rb от первого вулкана линии (Уохушан) через второй вулкан (Бидзиашан) к третьему вулкану (Лаохейшан). В породах, отличающихся от фоновых, выявлены общие повышенные концентрации Rb от второго до четвертого вулкана линии и относительное снижение концентраций этого элемента в финальной постройке четвертого вулкана. В породах, близких к фоновым, концентрации Zr снижаются от первого вулкана ко второму, а к третьему - возрастают. В породах, отличающихся от фоновых, сравнительно низкие концентрации Zr первого вулкана сменяются повышенными концентрациями третьего и четвертого вулканов с дальнейшим относительным снижением концентраций этого элемента в финальной постройке четвертого вулкана.
Обсуждение. Подлитосферный компонентный континуум в Восточной Азии содержит материал из домена конвектирующей области мантии с субдуцированными слэбовыми (палеослэбовыми) фрагментами океанической (палеоокеанической) коры, а также деламинированных литосферных блоков орогенов. Вулканические породы поля Удаляньчи содержат подлитосферный конечный компонент, который принадлежит к этому континууму. Литосферные компоненты этих пород, однако, не имеют никакого отношения к другим подлитосферным компонентам. Породы Удаляньчи относятся к подлитосферно-литосферному компонентному кластеру, характеризующему границу между литосферой и подлитосферной конвектирующей мантией (рис. 17). Из анализа содержаний К2О, других петрогенных оксидов и микроэлементов в породах ранних и поздних фаз извержений Центральной группы вулканов следует, что составы построек первого вулкана (Уо-хушан) почти не отличались от фоновых, второго и третьего вулканов (Бидзиашан и Лаохейшан) были частично близки к фоновым и частично отличались от них, а четвертого (Хуошаошан) существенно отличались от фоновых (рис. 18, 19). Предполагается, что генерация магм под вулканическим полем Удаляньчи контролировалась развитием транстенсии в граничном слое основания литосферы, разделявшем и экранировавшем источники подстилающей гомогенной подлитосферной конвектирующей мантии и перекрывающей гетерогенной обогащенной литосферы. Подлитосферный источник магм обладал отношением 8^г/8^г=0.7052, источники экранирующего слоя - таким же и более низкими отношениями, источники вышележащей литосферы - такими же и более высокими отношениями (рис. 20). По предельно низкому ряду фигуративных точек пород вулканического конуса Хуошаошан на диаграмме 8^г/8^г-8^Ь/8^г получена оценка закрытия изотопной системы в подошве литосферы около 98 млн лет с начальным 8^г/8^г значением 0.70485 в апатит-содержащем источнике и подстилающем домене конвектирующей мантии с Rb/Sr=0.092 (рис. 21). Развитие транстенсии определяло время и место локального поступления компонента конвектирующей мантии из-под граничного экранирующего слоя на фоне плавления обогащенного материала над ним (рис. 22). Локаль-
ные извержения подлитосферных выплавок из осевой части субмеридиональной магистральной зоны тран-стенсии интервала 2.5-2.0 млн лет назад сменились в интервале 1.3-0.8 млн лет назад извержениями фоновых выплавок из более широкого транстенсионного сегмента обогащенной области литосферы. В последние 0.6 млн лет фоновые выплавки из обогащенной литосферы резче обозначили краевые части транстенсионного сегмента, а локальные подлитосферные выплавки распространились вдоль разрыва, образовавшегося в граничном экранирующем слое при концентрации тектонических усилий в центральной части транстенси-онного сегмента.
Ключевые слова: калиевые породы; микроэлементы; изотопы Sr; транстенсия; астеносфера; литосфера;
Удаляньчи; Северо-Восточный Китай
1. Introduction
Transtension is a system of stresses that tends to cause oblique extension, i.e., combined extension and strike slip [Neuendorf et al., 2011]. Syn-volcanic transtensional deformations of the lithosphere may provide two possible scenarios for control of magmatic processes. One scenario assumes ascending sub-lithosphe-ric melts that penetrate through the lithosphere in a transtension area without melting the lithospheric material. Products of volcanic eruptions in such a zone show only the sub-lithospheric mantle material. Components of magmatic liquids do not reveal any connection to a lithospheric structure. This was the case for 16-13 Ma volcanic eruptions in the Vitim Plateau [Chu-vashova et al., 2016]. Another scenario yields a direct control of melting in lithospheric sources in an evolving transtensional structure. In this case, spatial-temporal changes of lithospheric and sub-lithospheric components are a direct indication of the evolving transten-sional zone.
In East Asia, lateral motions are well recognized along north-south faults (e.g., Tan-lu, Tsushima, Central Sikhote-Alin). Right- and left-lateral displacements in some of these faults, however, are controversial [Jolivet et al., 1994; Gilder et al., 1999; Zhang et al., 2003; Utkin, 2013]. It is not clear whether these faults affected the lithospheric magma sources. In this paper, we present arguments in favor of transtensional origin of the litho-sphere-derived melting anomaly along the Wudalianchi volcanic zone that extends north-south for 230 km at the northern circuit of the Songliao basin, central Hei-longjiang Province, Northeast China. The Wudalianchi volcanic zone is exhibited by rocks of potassic series distributed along a chain of volcanic fields (from south to north): Erkeshan - Wudalianchi - Keluo - Xiaogu-lihe-Menlu [Wang, Chen, 2005; Zhao et al., 2014]. Three volcanic fields (Wudalianchi, Erkeshan, and Keluo) were described also as the WEK zone [Zhang et al., 1991,1995].
The potassic rocks were examined together with Cenozoic potassic-sodic basalts from the whole East
China within a common continuum of mantle sources as the end-member with enriched Sr and Nd isotopic signatures (87Sr/86Sr>0.7055, 143Nd/144Nd<0.5123) and low Pb isotopic ratios (206Pb/204Pb<16.55, 208Pb/204Pb~ ~36.5) [Zou et al., 2000]. From such a taxonomy of sources, the most enriched Wudalianchi-type component was assumed to occur in the mantle of any part of East China or at least within the asthenospheric mantle domain of the North China craton and Xing-Meng block [Choi et al., 2006; Chen et al., 2007]. The common continuum of Cenozoic rocks was assumed also to exhibit the direct generation of the erupted Wudalianchi liquids from the mantle transition zone [Kuritani et al., 2013]. This idea was based on the crystallization temperature estimate for the nucleus of olivine pheno-crysts from rocks of the Laoheishan and Huoshaoshan volcanoes, which exceeded the temperature estimate assuming a conductive heat transfer in the lithosphere.
Firstly detected essential geochemical differences between potassic and potassic-sodic basalts from East China [Zhang et al., 1995] have been confirmed in many studies. Sources of potassic basalts were referred to the enriched post-Archean sub-continental lithospheric mantle, comparable to the global EM1 OIB component, unlike sources of potassic-sodic basalts that were considered as derivatives from the depleted sub-lithosphe-ric mantle, comparable with the depleted global DMM OIB component [Zhang et al., 1995; Zou et al., 2003, 2008; Choi et al., 2006; Chen et al., 2007; Chu et al., 2013]. Significant discrepancies were found between geochemical signatures of mantle xenoliths from potas-sic-sodic basalts of China and their host lavas [Basu et al., 1991]. On the contrary, geochemical characteristics similar (although not identical) to those of host lavas were defined in xenoliths of spinel peridotites from potassic volcanic rocks [Zhang et al., 2000, 2011; Chu et al., 2013]. In such a change of compositional relations between mantle xenoliths and volcanic rocks, clear specifics of lithospheric origin of the potassic rocks from the Wudalianchi zone stood out, unlike sub-litho-spheric nature of potassic-sodic basalts from other areas. Potassic melts were considered to be generated
from a phlogopite-bearing garnet peridotite source [Zhang et al., 1995, 2000; Zou et al., 2003; Chuvashova et al., 2009; Chu et al., 2013].
Rock compositions from the Laoheishan and Huo-shaoshan volcanoes were subdivided to groups with various SiO2, K2O, and other major oxide contents. From mutually convergent trends of data points for rocks from the Wudalianchi and Nuominhe volcanic fields on a plot of initial 87Sr/86Sr versus 103/Sr, the initial 87Sr/86Sr=0.7052 was accepted as a signature for a common component, the origin of which was explained by the involvement in melting of a material from the sub-lithospheric convective mantle. From calculations of mixing models in coordinates of Sr and Pb isotope ratios, sources for potassic liquids were referred to the lithosphere-asthenosphere boundary and mostly to the sub-continental lithosphere [Chuvashova et al., 2009; Rasskazov et al., 2011].
Rocks from the Wudalianchi zone remain geochemi-cally poorly studied. Compositions of most volcanoes have not been described yet or have been characterized by one or two rock compositions. The controversy over the issue of potassic volcanic rock sources requires additional arguments based on detail sampling of eruption products. To elaborate the idea of the spatial-temporal evolution of magmatic processes at the lithosphere-asthenosphere boundary and connect these processes with tectonic features of the study area, we present and discuss the data on the Wohushan-Huoshaoshan volcanic line that has the key significance for understanding the structure of a source region for volcanic rocks of the Wudalianchi field.
2. Analytical methods
Analytical studies of rock samples were performed in the Laboratory for Isotopic and Geochronological Studies of the Institute of the Earth's Crust SB RAS. Trace elements were determined by ICP-MS technique using a mass-spectrometer Agilent 7500ce and isotopes using a mass-spectrometer Finnigan MAT 262. The methods used were described in the previous papers by Rasskazov et al. [2011] and Yasnygina et al. [2015]. Major oxides were measured by "wet chemistry".
3. Structural setting of the Wudalianchi zone
Terranes of the study area were initially assembled during the Early Paleozoic and accreted to the North China craton by the Xilamulun River-Yanji Suture Zone at the end of the Permian [Wang et al., 2016]. In the eastern part of the Central Asian Orogenic Belt, Middle Phanerozoic oceanic crust subducted to the south un-
der the North China craton and to the north under the Erguno-Hinggan block. The Sonidzuoqi-Xilinhot magmatic arc was active at about 320 and 280 Ma, as it was inferred in the study of zircons from sandstones of the southern, central, and northern Greater Hinggan. This arc included the Zhengdashan pluton, located to the south-east of Nenjiang city. The Nenjiang-Balihan left-lateral strike slip fault bounded the western side of the Songliao basin that was subsided in the Late Mesozoic -Early Cenozoic [Han et al., 2012a, 2012b; Wang et al., 2016] (Fig. 1, b). This displacement might result in stretching crust of the Songliao basin limited by the area of the northern end of the Wudalianchi zone.
According to Wang et al. [2007], the Songliao basin includes the lower unit, characterized by a fault-bounded 130-110 Ma sequence of intercalated volcanic, pyroclastic and epiclastic rocks (geochemically active continental margin type) and the upper unit dominated by non-marine sag-style sedimentary sequence of siliciclastics and minor carbonates. In the recent study by Wang et al. [2016], it was suggested that the Aptian and pre-Aptian retroarc evolution was followed by the Albian to Companian strike-slip motions under transtension and afterwards (since the Maastrichtian) by inverse development under compression. The three tectonic cycles are referred to the syn-rift (150-105 Ma), post-rift (105.0-79.1 Ma), and structural inversion (79.1-64.0 Ma) stages. Volcanism was characteristic of the syn-rift and post-rift stages.
Volcanic fields of the Wudalianchi zone are controlled by the Xiaogulihe-Wudalianchi fault - one of the structural elements at the northern circuit of the Song-liao basin [Wang, Chen, 2005] (Fig. 1). Potassic volca-nism is limited to the north by the Greater Hinggan Frontal and Nenjiang faults and to the south by the Central Fault of the Songliao basin. The Keluo and Xiao-gulihe-Menlu volcanic fields are separated from each other by the Erenhot-Hegenshan suture. The Keluo, Wudalianchi, and Erkeshan volcanic fields correspond to structural elements, characterized by local thinning of the crust to 33.5 km, and occupy the southeastern flank of the Nenjiang-Balihan fault with the thinned crust (Fig. 1, a), whereas the Nuominhe field reside in its northwestern flank with the thickened crust. The transition from the Songliao basin to the Greater Hinggan corresponds to the Marginal Gravity Step that extends along the entire eastern margin of Asia with stagnating slabs beneath [van der Hilst et al., 1991; Fu-kao et al., 1992, 2001; Bijwaard et al., 1998; Zhao, 2009]. In East Asia, the crust and lithospheric mantle is considerably thinner than in Inner Asia. Under the central part of the Songliao basin, the lithosphere is as thin as 80 km, getting thicker towards its northern part [Zhang et al., 1995].
It is noteworthy also that potassic rocks are located almost in front of the currently active directly-sub-
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Fig. 1. Location of the Wudalianchi volcanic zone relative to (a) crustal structural elements of the Late Mesozoic - Early Ce-nozoic Songliao basin, (b) the Nenjiang-Balihan strike-slip fault (NBF) bounded the Songliao basin in the west, and (c) the currently subducted Pacific slab.
In (a), the Wudalianchi volcanic zone corresponds to the Xiaogulihe-Wudalianchi fault (X-WFZ), stretching from the Greater Hinggan Frontal fault (GHFF) and Nenjiang fault (NF) to the Central Fault of the Songliao Basin (CFSB). Scheme (b) shows the Erenhot-Hegenshan Suture Belt (EHSB) and Sonidzuoqi-Xilinhot Magmatic Arc (SXMA). In (c), contour lines of the depths in increments of 50 km to the top of seismically active subducting Pacific slab [Gudmundsson, Sambridge, 1998] show the Hokkaido-Amur slab flexure [Rasskazov, Chuvashova, 2016] and the Marginal Gravitational Step of East Asia [Romanovsky, 1999; Li et al., 2006]. (a) and (b) are modified after [Wang, Chen, 2005] and [Han et al., 2012a, 2012b], respectively.
Рис. 1. Расположение вулканической зоны Удаляньчи относительно (а) структурных элементов коры позднемезо-зойско-раннекайнозойского бассейна Сунляо, (b) сдвигового разлома Нэньцзян-Балихан (NBF - Nenjiang-Balihan), ограничивающего с запада бассейн Сунляо и (с) ныне субдуцирующего Тихоокеанского слэба.
На схеме (а) вулканическая зона Удаляньчи соответствует разломной зоне Сяогулихе-Удаляньчи (X-WFZ), протягивающейся от Фронтального разлома Большого Хингана (DMFF) и разлома Нэньцзян (NF) до Центрального разлома бассейна Сунляо (CFSB). На схеме (b) показаны: шовный пояс Эренхот-Хегеншан (EHSB - Erenhot-Hegenshan Suture Belt) и магматическая дуга Сонид-зуоки-Силинхот (SXMA - Sonidzuoqi-Xilinhot Magmatic Arc). На схеме (с) по изолиниям глубин с шагом 50 км кровли субдуцирующего Тихоокеанского слэба [Gudmundsson, Sambridge, 1998] показаны Хоккайдо-Амурская слэбовая флексура [Rasskazov, Chuvashova, 2016] и краевая гравитационная ступень Восточной Азии [Romanovsky, 1999; Li et al., 2006]. Схемы (а) и (b) приведены, соответственно, по работам [Wang, Chen, 2005] и [Han et al., 2012a, 2012b] с изменениями.
ducted Hokkaido-Amur flexure of the Pacific slab (Fig. 1, c). Under the Wudalianchi area, the Heilon-gjiang S-wave low-velocity anomaly was defined in tomographic models of Yanovskaya and Kozhevnikov [2003] and Wei et al. [2012] at depth of about 100 km.
4. Timing of volcanism and variations of K2O
CONTENTS IN ROCKS FROM THE WUDALIANCHI ZONE
Volcanism of the Wudalianchi zone was concentrated mainly in the Keluo and Wudalianchi fields. In the
former, eruptions occurred since 4.6 Ma, in the latter since 2.5 Ma. The older ages of rocks from Keluo [Zhang et al., 1991, 1995] were not confirmed [Zhao et al., 2014; unpublished data of the authors]. On the small Erkeshan field, located at the southern end of the zone, volcanic eruptions of three cones (two twin volcanoes) occurred in the time interval from 0.56 to 0.36 Ma [Wang et al., 1996; Wang, Chen, 2005; Guide book..., 2010]. On the Xiaogulihe-Menlu field, situated at the northern end of the zone, eruptions were referred to the Holocene and compositions of volcanic rocks were described as ultrapotassic [Wang, Chen, 2005]. Such
rocks with K2O=8.7-9.5 wt % and K2O/Na2O=4.3-4.9 [Wee, 2002] were found in the northwestern part of the Xiaogulihe-Menlu field, whereas rocks with foO=1.96-2.14 wt %, dated at 1.00-1.28 Ma, were located in its southeastern part [Zhao et al., 2014]. In rocks from the Wudalianchi zone, these K2O contents are the lowest.
Histograms in Fig. 2 demonstrate a change of K2O contents in rocks from a narrow range of 5.6-5.8 wt % in the Erkeshan field to wider intervals in the Wuda-lianchi and Keluo fields that show decreasing values to 3.2 wt %. In rocks from the Keluo field, this interval is expanded also due to increasing K2O to 7.0 wt %. In the Xiaogulihe-Menlu field, the least and most potassic rocks yield a total range from 2.0 to 9.5 wt % K2O. In this paper, we focus on causes of decreasing K2O contents in rocks from the Wudalianchi volcanic field. Variations of K2O contents in the Keluo and Xiaogulihe-Menlu rocks will be considered elsewhere.
Volcanic rocks from the Xunke field to the east of the Wudalianchi zone are dated back to the Pliocene and Quaternary [Liu et al., 2001; Zhao et al., 2014].
5. Spatial-temporal clustering of volcanoes in the Wudalianchi field
The main Wudalianchi field of 14 volcanoes [Zhang et al., 1995; Wang et al., 1996; Guide book..., 2010] (Fig. 3) is supplemented to the west and southwest with three small volcanic areas: (1) Lianhuashan edifices, (2) Jianshan, Jianshanzi edifices, and (3) Laoshantou lava flow [Zhao et al., 2014]. Most of the volcanoes have lava pedestals, crowned by slag or slag-lava cones. Diameters at the bases of such cones are 500-1200 m, and heights range from 40 to170 m. As a rule, the cones contain craters. The spatial distribution of the cones was described in [Wang et al., 1996; Wang, Chen, 2005; Guide book..., 2010; Zhao et al., 2014] with respect to the northeastern or both northeastern and northwestern volcanic lines.
Potassium-argon ages were obtained for both the main Wudalianchi field and satellite volcanic areas. Seven volcanic phases were defined [Wang et al., 1996; Liu, Taniguchi, 2001; Liu et al., 2001; Wang, Chen, 2005; Guide book..., 2010; Zhao et al., 2014]. Considering the general Quaternary evolution of volcanism in Asia [Rasskazov et al., 2012], spatial-temporal distribution and compositional variations of volcanic products, we distinguish three time intervals of the volcanic evolution: (1) 2.5-2.0 Ma, (2) 1.3-0.8 Ma, and (3) <0.6 Ma.
The lower limit of the initial eruptive interval was constrained by the K-Ar age of 2.52±0.06 Ma for the Laoshantou flow [Zhao et al., 2014], and the upper limit was designated by the age of 2.076±0.054 Ma for the Old Gelaqiushan flow [Wang et al., 1996]. In the 1.3-0.8 Ma interval, eruptions occurred in the Wohushan and
West Longmenshan volcanoes at about 1.3 Ma, in the Yaoquanshan volcano at about 0.93 Ma, and in the Lianhuashan, West Jiaodebushan, South Gelaqiushan volcanoes at about 0.8 Ma. Volcanoes of this interval distributed along a west-eastern bend, with the exception of the South Gelaqiushan volcano, activation of which together with the Lianhuashan volcano marked the northeastern line of eruptions. In the last 0.6 Ma, three groups of volcanoes erupted: Western (North and South Gelaqiushan, Lianhuashan, Jianshan-Jianshanzi), Central (Wohushan, Bijiashan, Laoheishan, Huoshao-shan), and Eastern (Weishan, East Jiaodebushan, Xiao-gushan, West and East Longmenshan, Molabushan). As compared to the edifices of the 1.3-0.8 Ma interval, those of the last 0.6 Ma shifted northeastwards in the Eastern group and southwestwards in the Western group (Fig. 4). The final eruptions took place in the Central group on the Laoheishan and Huoshaoshan volcanoes in 1720-1721 AD [Liu and Taniguchi, 2001], in the Western group on the Jianshan volcano at about 0.12 Ma, and in the Eastern group on the East Longmenshan and Xiaogushan volcanoes at about 0.19-0.17 Ma or 0.32 Ma [Zhao et al., 2014].
The Central group of volcanoes showed persistent shift of eruptions from Wohushan (1.33-0.42 Ma) to Bijiashan (0.45-0.28 Ma) to Laoheishan (1720-1721, possibly earlier) to Huoshaoshan (1721). No spatial-temporal regularity of eruptions in volcanoes of the Erkeshan field and Western and Eastern groups of the Wudalianchi field reflected background activity.
Fig. 5 illustrates a jumping character of volcanic shift along the Wohushan-Huoshaoshan line. Activity of the first (Wohushan) volcano ceased simultaneously with the beginning activity of the second (Bijiashan) volcano (within error of K-Ar dating). Further, eruptions at the second volcano stopped simultaneously with transfer of activity to the third (Laoheishan) volcano. Likewise, volcanic activity jumped from the third to the fourth volcano (from Laoheishan to Huoshao-shan) in 1720-1721.
6. Sampling
Representative sampling of rocks from the Wohu-shan-Huoshaoshan volcanic line was aimed to identify changing geochemical signatures along the whole volcanic line and in the course of eruptions in each volcano. For comparisons, the background volcanoes were also sampled with a priority to the Yaoquanshan and Weishan volcanoes, located in close proximity to the Wohushan-Huoshaoshan line. Temporal inconsistency of eruptions assumes that these two volcanoes belong to the Eastern group.
Data on chemical compositions of rocks from the Wudalianchi area have been published mostly with
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Fig. 2. Structural setting of the Pliocene-Quaternary volcanic fields with potassic, ultrapotassic, and potassic-sodic rocks between the Songliao and Amur-Zeya sedimentary basins.
Histograms (b-g) demonstrate variations of K2O contents in lavas from different volcanic fields (explanations in the text). The colors of rock groups with different contents of potassium in the histograms match the coloring of the volcanic fields in the scheme (a). Volcanic rocks of K series are located along the NS trending Wudalianchi zone and those of K-Na series along the WNW-ESE Nuominhe-Xunke zone. Data sources: Zhang et al., 1995; Liu et al., 2001; Wang, Chen, 2005; Chuvashova et al., 2009; Rasskazov et al., 2011 and unpublished data of the authors.
Рис. 2. Структурное положение плиоцен-четвертичных вулканических полей с калиевыми, ультракалиевыми и ка-линатровыми породами между осадочными бассейнами Сунляо и Амуро-Зейским.
Гистограммы (b-g) демонстрируют вариации содержаний К2О в лавах из разных вулканических полей (объяснения в тексте). Цвета групп пород с разным содержанием калия на гистограммах соответствуют раскраске вулканических полей на схеме (а). Вулканические породы калиевой серии расположены вдоль субмеридиональной зоны Удаляньчи, калинатровой серии - вдоль зоны Нуоминхе-Сюнке с простиранием запад-северо-запад - восток-юго-восток. Источники данных: Zhang et al., 1995; Liu et al., 2001; Wang, Chen, 2005; Chuvashova et al., 2009; Rasskazov et al., 2011 и неопубликованные данные авторов.
48.75°
48.65°
126.00
126.20°
r
Initial eruptions, lava flow
Background / ^ eruptions, ^ / Western and —> Eastern groups of volcanoes
126.40°
Propagating eruptions, Laoshantou flow -Central group of volcanoes
Fig. 3. Spatial distribution of volcanoes in the main Wudalianchi field (a), and sampling sites of the Huoshaoshan volcanic cone (Ь).
Рис. 3. Пространственное распределение вулканов на основном поле Удаляньчи (a) и распределение образцов в опробовании конуса Хуошаошан (Ь).
105° 35'
105° 55'
106° 15'
48° 45'
48° 35'-
10 km
,// ^TV-shape
O 2.5-2.0 Ma initial flow ( NE locus # 1.3-0.8 Ma cinder cone i™*
<0.6 Ma background volcano and / NF . Relative temporal
flow, Western and Eastern arouDS * shift of volcanoes
<0.6 Ma volcano and flow, Central group
flow, Western and Eastern groups
^ Stepwise northeastward shift of volcanism
Fig. 4. Spatial-temporal distribution of volcanic eruptions of the main Wudalianchi volcanic field and satellite volcanic areas.
I Рис. 4. Пространственно-временное распределение извержений на основном вулканическом поле Удаляньчи и са-теллитных вулканических проявлениях.
Background eruptions, Western group of volcanoes
Propagating eruptions, Central group of volcanoes
Background eruptions, Eastern group of volcanoes
50'
100-
150'
200'
250-
И 0)
ST 2
о го ïl
P О à. °
■
II
о от
С го -С
о
го -С
I с <0
X m
■ I
□ 2
i
I I I I
■с
го
2 ! ï of«
О
о
I
а
■С (О с
to
я_
Timing, Ka
Fig. 5. Timing of activity in the Western, Central, and Eastern groups of volcanoes in the Wudalianchi field. The sequence of eruptions in the line of the Central group of volcanoes Wohushan-Huoshaoshan is marked by a blue colour. Arrows show jumps of volcanic eruptions. The scheme is modified after [Guide Book..., 2010] with additions (see text).
Рис. 5. Время активности Западной, Центральной и Восточной групп вулканов на поле Удаляньчи. Последовательность извержений в линии Центральной группы вулканов Уохушан-Хуошаошан выделена голубым цветом. Стрелки обозначают скачкообразное смещение извержений. Схема составлена по работе [Guide Book..., 2010] с дополнениями и изменениями (см. текст).
references to volcanoes without indications their exact locations. For example, major and trace-element contents for eight samples from the Laoheishan volcano were reported by Hsu and Chen [1998]. Phases of eruptions in this volcano were compositionally different, however. Without information on the locations, these data cannot be properly interpreted. Sr, Nd, and Pb iso-topic ratios have more pronounced characteristics of rock groups than the major oxides or trace elements. These analytical results from [Zhang et al., 1995; Zou et al., 2003; Chu et al., 2013] are used here to extend the data sets for individual volcanoes (Table).
Wohushan - the first edifice in the sequence of volcanoes - was sampled in cones 1 and 2. Four samples of slag and porous basalts were selected from cone 1 (W-14-6 - W-14-9) and five samples were taken from cone 2 (W-14-1 - W-14-5) (Fig. 6). The previously published composition of sample W1-32 from the volcano [Zhang et al., 1995] is similar to the one of rocks from cone 1.
Bijiashan - the second volcano in the sequence - is a vast lava shield crowned with a cinder cone. Samples Bd-14-1 and Bd-14-2 were taken from the lava pedes-
tal of the volcano on its southern periphery. These rocks are compositionally comparable to sample B-1a [Zhang et al., 1995]. Samples Ch-07-31, Ch-07-32, and Ch-07-33 were selected from the southern and southeastern parts of the cinder cone and the adjacent lava flow.
Laoheishan - the third volcano in the sequence - revealed two phases of activity. An early pyroclastic cone (not sampled) was partially destroyed by the explosion that created a somma-crater occupied by a new pyro-clastic cone with a new crater (Fig. 7). Agglutinate and vesicular glassy basalt samples Lo-11-5 - Lo-11-7 were taken from the late cone. Compositionally, these rocks are similar to lava samples Lo-11-11, Lo-11-12, Lao-09 that were picked up from the northern bocca of the edifice. This group is comparable to samples A-2 [Zhang et al., 1995], LE 6-4, and LHK-1 [Zou et al., 2003]. Volcanic bomb sample Lo-11-8, taken from pyroclastic debris deposited on the southwestern edge of the late crater, is compositionally similar to samples A-65 [Zhang et al., 1995] and LW5-2 [Zou et al., 2003]. Other samples related to the final crater of the edifice show a compact group of compositions: a piece of black glass from the
Representative major-oxide (wt %) and trace-element (ppm) compositions, and Sr isotopic ratios in rocks sampled from volcanoes in the Wudalianchi field, China
Представительные содержания петрогенных оксидов (мае. %), микроэлементов (мкг/г) и Sr изотопные отношения в породах вулканов поля Удаляньчи
Volcanoes Wohushan Bijiashan Laoheishan Huoshaoshan Weishan Yaoquanshan
Samples W-14-2 W-14-8 Ch-07-32 Bd-14-1 LO-11-9 LO-11-8 LO-11-5 H-07-11 H-ll-14 Wei-14-20 Wei-14-21 Ch-07-34 Ch-07-35
SiCh 52.40 52.19 51.87 53.41 53.88 50.90 51.64 52.94 52.16 51.89 53.47 51.59 51.95
Ti02 2.27 2.42 2.32 2.32 2.35 2.91 2.32 2.26 2.34 2.64 2.48 2.96 2.87
AI2O3 14.06 14.27 13.20 14.39 13.69 13.60 13.31 12.80 13.39 14.39 14.81 13.78 13.60
РегОз 3.65 6.45 3.63 1.85 1.73 2.60 2.68 2.45 2.06 4.77 2.55 5.89 2.19
FeO 4.09 1.99 4.71 5.59 5.90 6.36 5.71 6.11 6.15 3.17 4.78 2.48 5.74
MnO 0.11 0.11 0.12 0.10 0.10 0.10 0.11 0.11 0.11 0.10 0.09 0.11 0.11
MgO 6.55 6.27 7.31 6.47 6.30 6.55 7.69 7.43 7.46 5.79 5.42 5.74 5.71
CaO 6.13 6.02 7.02 5.80 5.19 6.04 6.42 7.50 5.86 5.98 5.44 6.17 5.96
Na20 3.96 3.53 3.72 3.49 3.86 3.53 3.61 3.72 3.80 4.09 4.30 3.25 3.86
K2O 5.12 5.10 4.43 5.17 5.46 5.09 4.91 3.15 4.91 5.45 5.39 5.89 5.68
P2O5 0.90 0.96 0.89 0.86 0.91 1.07 0.89 0.89 0.87 1.25 1.10 1.22 1.19
LOI 0.81 0.97 0.93 0.72 0.68 1.17 0.84 0.81 0.82 0.57 0.40 0.59 1.24
Total 100.05 100.28 100.15 100.18 100.05 99.92 100.13 100.17 99.93 100.09 100.24 99.67 100.10
Sc, ppm 19.8 24.4 13.8 15.0 12.1 24.4 14.6 16.6 13.8 7.1 6.0 13.3 20.5
V 99 104 124 125 114 142 138 139 134 111 99 116 128
Cr 184 150 226 191 165 126 178 208 174 155 140 166 176
Co 30 32 32 32 26 32 33 34 31 29 27 29 35
Ni 148 148 157 162 149 119 183 150 175 121 122 137 150
Rb 88 75 104 95 102 103 94 82 92 90 90 98 115
Sr 1274 1393 1218 1374 1365 1318 1286 1259 1242 1771 1712 1564 1670
Y 18.8 21.1 20.5 17.9 22.9 20.8 22.8 22.2 22.1 21.6 19.2 23.8 25.7
Zr 379 446 322 350 553 468 419 338 411 366 350 487 549
Nb 51.2 57.1 48.5 47.8 67.8 56.1 65.1 54.5 62.7 69.7 66.2 60.9 70.3
End of Table Окончание таблицы
Volcanoes Wohushan Bijiashan Laoheishan Huoshaoshan Weishan Yaoquanshan
Samples W-14-2 W-14-8 Ch-07-32 Bd-14-1 LO-11-9 LO-11-8 LO-11-5 H-07-11 H-ll-14 Wei-14-20 Wei-14-21 Ch-07-34 Ch-07-35
Cs 0.88 0.91 0.80 0.58 0.92 0.87 0.90 0.74 0.87 0.86 0.85 0.75 0.99
Ba 1602 1821 1519 1770 1563 1937 1570 1504 1535 1860 1827 1868 1956
La 76.4 88.1 70.4 73.1 90.5 81.4 80.5 76.4 78.2 108 99.8 95.5 96.5
Ce 145 166 130 135 170 154 152 144 148 207 190 185 185
Pr 15.9 18.3 14.7 15.1 19.1 17.5 16.9 15.9 16.7 22.4 20.3 20.8 21.0
Nd 57.9 66.9 58.1 58.2 71.8 67.5 64.3 62.4 63.2 80.1 72.3 81.2 81.1
Sm 9.96 11.4 9.56 9.74 11.5 11.1 10.72 10.14 10.52 13.3 12.1 12.3 12.2
Eu 2.90 3.28 2.77 2.66 3.23 3.13 3.06 2.97 2.91 3.68 3.27 3.58 3.39
Gd 8.12 8.88 7.38 7.41 9.14 8.95 8.82 8.03 8.54 10.08 8.98 9.23 9.35
Tb 1.08 1.19 0.99 0.95 1.07 1.03 1.05 1.06 1.00 1.30 1.18 1.20 1.11
Dy 4.32 4.89 4.45 4.09 4.90 4.66 4.74 4.84 4.69 4.98 4.40 5.21 5.20
Ho 0.72 0.80 0.74 0.69 0.86 0.77 0.83 0.78 0.81 0.83 0.73 0.85 0.87
Er 1.74 1.87 1.78 1.54 1.96 1.80 1.91 1.86 1.90 1.88 1.73 1.99 2.03
Yb 1.21 1.36 1.25 1.07 1.33 1.21 1.38 1.32 1.35 1.20 1.10 1.35 1.41
Lu 0.13 0.16 0.15 0.16 0.17 0.15 0.17 0.16 0.17 0.12 0.10 0.17 0.18
Hf 9.59 11.2 7.87 9.05 12.8 11.1 10.14 8.24 9.99 9.02 8.60 12.3 13.3
Та 2.52 2.89 2.84 2.34 4.09 3.48 3.94 3.20 3.83 3.37 3.10 3.61 3.78
Pb 18.7 21.6 9.7 11.7 16.4 13.5 56.9 11.5 13.3 12.8 12.9 13.1 15.8
Th 7.13 7.99 6.49 5.65 7.23 6.50 7.06 7.35 6.94 7.94 7.97 6.88 6.51
U 1.51 1.59 1.21 0.78 1.62 1.30 1.57 1.53 1.58 1.60 1.34 1.59 1.51
(87Sr/86Sr)0 0.705175 0.705308 0.705221 N.d. N.d. 0.705260 0.705254 0.705115 N.d. 0.705178 0.705189 0.705411 0.705393
26 0.000008 0.000008 0.000011 0.000011 0.000014 0.000010 0.000008 0.000009 0.000012 0.000011
Note. Initial 87Sr/86Sr ratios are given relative to NBS 987=0.710250. N.d. - no data.
Примечание. Начальные 87Sr/86Sr приведены относительно значения NBS 987=0.710250. N.d. - не определялось.
E 126.035°
E126.045°
N48.655°-
N48.650°-
Lavas
l-Г
Pyroclastic rocks
<- cone 1 cone 2
cone 3
Crater bottom
Sample locations
А Л™"9
cone 1 cone 2
I Fig. 6. A sketch-map of the Wohushan volcano showing sampling sites. For simplification, index "14" is skipped in the sample numbers shown in the figures (W-1 means W-14-1 etc.).
I Рис. 6. Точки отбора образцов на схематической карте вулкана Уохушан. Для упрощения номера образцов приведены без «14» (W-1 вместо W-14-1 и т.д.).
northwestern edge of the crater (Lo-11-9, Lo-11-10), red slag, lapilli and bombs 1.2 km to the south of the crater (samples Lo-11-13A, B, C). Similar compositions were reported for samples L-13, A-4 [Zhang et al., 1995], LS3-1, HL9720, and NDW2-2b [Zou et al., 2003].
Huoshaoshan - the fourth volcano in the sequence -unlike other edifices, is composed of porous basalts that formed a volcanic cone and surrounding lava flows. Samples H-07-1 - H-07-20 were taken from the cone and adjacent lava flow (Fig. 3, b). Sample H-11-14 was picked up from a flow at a distance of 1 km south of the cone, and sample Lo-11-15 at a distance of 2 km east-northeast of it (in the garnito site). The previously published compositions of rocks from the cone of this volcano were: H-38 [Zhang et al., 1995], HS2-2, HK-2, HN6-1 [Zou et al., 2003]. Samples HSS2, HSS5 were
taken from a flow to the northeast of the cone, and samples HSS6, HSS8 from a flow to the east of it [Chu et al., 2013].
7. Silica and alkalis oxides
On the total alkalis-silica (TAS) diagram (Fig. 8, a), data points of background rocks are distributed along the dividing lines between highly and moderately alkaline series mainly in tephriphonolite and trachyande-site fields with minor samples in the phonotephrite field. Background rocks from some volcanoes (e.g. Yaoquanshan and Weishan) are highly alkaline (pho-notephrites and tephriphonolites). Background rocks from other volcanoes (Longmenshan, Jiaodebushan
126.110 _L
Lo-12
126.130°
48.720°-
48.715°-
Lo-11-12 Lao-09
Lavas
->4 Early cone 'X
Pyroclastic rocks
Late cone
Late crater
Bocca
Edge of crater Somma Late
Sample locations L g
Д ДАг "
Late Late cone crater
Bocca
I Fig. 7. Locations of samples on a sketch-map (a) and cross-section AB (b) of the Laoheishan volcano. Scheme of the edifice is modified after [Xiao, Wang, 2009; Guide Book..., 2010].
I Рис. 7. Местоположение образцов на карте-схеме (а) и разрезе AB (b) вулкана Лаохейшан. Схема постройки [Xiao, Wang, 2009; Guide Book., 2010] с уточнениями.
etc.) are moderately alkaline (mostly trachyandesites). In background rocks, Na2O+K2O range from 8.6 to 9.7 wt %, SÍO2 from 51.6 to 55.0 wt %. Phonotephrites from the Erkeshan field are comparable with the Wu-dalianchi background rocks of this type (Table).
Data points of rocks from the Central group of volcanoes are also distributed along the discriminating line of highly and moderately alkaline series, mainly in the phonotephrite and trachyandesite fields. Almost all samples from the first volcano (Wohushan) fall within the data field of background rocks. Rock compositions of the second and third volcanoes (Bijiashan and Laoheishan) changed on each of them from similar to the background to the ones distinguished by the lower silica and alkalis contents. On the Bijiashan volcano, eruptions were exhibited by trachyandesites of a lava shield
and by basaltic trachyandesites and phonotephrites of a volcanic cone. The trachyandesites were comparable to the background rocks, the basaltic trachyandesites and phonotephrites differed from them. On the Laoheishan volcano, rocks of the second phase of eruption were subdivided into three groups: (1) basaltic trachyandesites and phonotephrites, (2) trachyandesites, and (3) phonotephrites. The first group was recorded in pyroclastic material from the late volcanic cone and lavas from the northern bocca, the second group in py-roclastic material from the northwestern edge of the late crater, and the third group in bombs from its southwestern edge.
On the fourth volcano (Huoshaoshan), rocks are basaltic trachyandesites and phonotephrites. In the peripheral parts of the lava fan, samples HUO-11-11 and
Na20+K20, wt % 10-,
Tephri-phonolite
K20/Na20
Si02, wt %
Fig. 8. Diagrams Na2O+K2O vs. SiO2 (a) and K2O/Na2O vs. K2O (b) for volcanic rocks.
Symbols in the ellipses: BC - background compositions; WO1, WO2 - compositions of pyroclastic rocks from cones 1 and 2 of the Wohu-shan volcano; BCB, LCB - close to background compositions in the Bijiashan peripheral flows and bombs of the late Laoheishan volcanic crater (eruption phase 2); BT, LT - transitional compositions in the cone of the Bijiashan volcano and in the Laoheishan volcano (eruption phase 2); LF - final pyroclastic material related to the late crater of the Laoheishan volcano (eruption phase 2); HI - Huoshaoshan initial compositions of peripheral lava flows; HF - Huoshaoshan final compositions of the cone. Analyses are recalculated to 100 % without loss on ignition.
Рис. 8. Диаграммы Na2O+K2O - SiO2 (a) и K2O/Na2O - SiO2 (b) вулканических пород.
Обозначения в эллипсах: BC (background compositions) - фоновые составы; WO1, WO2 - составы пирокластических пород конусов 1 и 2 вулкана Уохушан; BCB, LCB (Bijiashan, Laoheishan close to background) - близкие к фоновым составы периферийного потока вулкана Бидзиашан и бомб позднего кратера вулкана Лаохейшан (вторая фаза извержений); BT, LT (transitional) - переходные составы на конусе вулкана Бидзиашан и на вулкане Лаохейшан (вторая фаза извержений); LF (Laoheishan final) - составы пирокластических пород позднего кратера вулкана Лаохейшан (вторая фаза извержений); HI (Huoshaoshan initial) - начальные составы периферических лавовых потоков вулкана Хуошаошан; HF (Huoshaoshan final) финальные составы извержений конуса этого же вулкана. Анализы приведены к 100 % с исключением из суммы потерь при прокаливании.
K20, wt %
Na20, wt %
Fig. 9. The diagram of K2O vs. Na2O for volcanic rocks. The symbols are given in Fig. 8.
Рис. 9. Диаграмма K2O - Na2O вулканических пород. Условные обозначения см. на рис. 8.
HUO-11-14 overlap with the background rocks under the elevated silica and alkalis contents (about 52-53 and 9 wt %, respectively). The rocks of the cone considerably differ from the background rocks. The pho-notephrites show a decreasing SiO2 content to 49 wt % at the alkalis content about 9 wt %. The trachyan-desites demonstrate a decreasing alkalis content to 7 wt % at SiO2 content about 53 wt %.
In background rocks, K2O/Na2O vary from 1.1 to 1.8 (Fig. 8, b). Rocks from the Wohushan, Bijiashan, and Laoheishan volcanoes have comparable ratios. Flows of peripheral parts of the lava fan in the Huoshaoshan volcano have elevated K2O/Na2O ratios (1.26-1.29) within the ratio range of background rocks. The rocks from the Huoshaoshan volcanic cone display a trend to the K2O/Na2O interval of 0.82-1.20 with transition from potassic to potassic-sodic series.
Background rocks show K2O and Na2O contents in the ranges of 4.8-6.0 and 3.3-4.5 wt %, respectively (Fig. 9). Among rocks from the Central group of volcanoes, the K2O contents decrease from the near background values in rocks from the Bijiashan and Laohei-shan volcanoes through concentrations in the Wohu-shan rocks, transitional compositions in the Bijiashan and Laoheishan volcanoes together with peripheral lavas from the Huoshaoshan volcano to the final compositions of the Huoshaoshan volcanic cone. The final
trachyandesite pyroclastics of the Laoheishan volcano reveals the K2O contents as high as 5.5-5.8 wt % (i.e. within the background level), whereas the initial Lao-shantou flow shows the K2O content as low as 4 wt % [Zhao et al., 2014].
We infer that, in terms of Na2O, K2O, and SiO2 contents, peripheral lavas of volcanic fans in the Bijiashan, Laoheishan, and Huoshaoshan volcanoes were close to background rocks. The contents of these major oxides differ from the background signatures and characterize rocks from volcanic cones in a linear progression that demonstrates the transition from compositions of the Wohushan volcano, close to the background ones, through the intermediate values in the Bijiashan and Laoheishan volcanoes to the final compositions in the Huoshaoshan volcanic cone.
8. Other major oxides
Changes of rock compositions along the Wohushan-Huoshaoshan line from the close to background signatures at the first volcano (Wohushan) through the contrast major oxide contents at the Bijiashan and Laohei-shan edifices to notably different from the background ones at the Huoshaoshan cone are illustrated further by diagrams of SiO2 vs. MgO, A^3 vs. MgO, CaO vs. MgO, and P2O5 vs. MgO (Figs 10, 11).
The MgO contents of the background rocks vary from 5.4 to 7.0 wt %. As contents of this oxide increase to 8.2 wt %, the transitional compositions of the Bijiashan and Laoheishan volcanoes, initial and final compositions of the Huoshaoshan volcano show decreasing Al2O3 concentrations up to 12 wt % with increasing CaO to 8 wt %. The SiO2 and Al2O3 contents of the transitional groups of rocks overlap with both the initial and final compositions of rocks from the Huoshaoshan volcano, unlike the CaO contents that differ from the final compositions. The transitional compositions reveal CaO scattering between 6 and 8 wt %. In the initial lavas from the Huoshaoshan volcano, the CaO contents coincide with the lowest values of this interval, while the contents in the final lavas of the volcano have a narrow range of 7-8 wt %. At maximal concentrations of CaO and MgO, trends of transitional compositions in the Laoheishan volcano and final compositions in the Huoshaoshan volcano intersect.
Plots of SiO2 vs. MgO and A^O3 vs. MgO emphasize similarities of rock compositions from the Wohushan volcano and low-Mg groups of rocks from the Bijiashan and Laoheishan volcanoes to background compositions and their differences to compositions of the transitional groups with elevated MgO contents from these volcanoes. The diagram of CaO vs. MgO demonstrates CaO scattering of background rocks from 5 to 6 wt %. The CaO contents in the final pyroclastic material from the
MgO, wt %
MgO, wt %
Fig. 10. Diagrams of SiO2 vs. MgO (a) and AbO3 vs. MgO (b) for volcanic rocks. The symbols are given in Fig. 8. Рис. 10. Диаграммы SiO2 -MgO (a) и AbO3 -MgO (b) вулканических пород. Условные обозначения см. на рис. 8.
Laoheishan volcano correspond to the lower values of this range (5.0-5.4 wt %), while the contents in the Huoshaoshan rocks and compositionally similar Bijiashan and Laoheishan rocks rise to the range of 5.9-6.6 wt %.
A plot of P2O5 vs. MgO (Fig. 11, b) reveals a wide range of P2O5 concentrations in the background rocks (0.9-1.3 wt %). Concentrations of this oxide as low as 0.90-0.95 wt % are characteristic of rocks with a limi-
ted range of MgO contents, 5.4-6.1 wt %. The elevated values between 1.2 and 1.3 wt % are observed in rocks with the full range of MgO, 5.5-6.8 wt %. Rocks from the Central group of volcanoes reveal opposite P2O5 trends for the low- and moderate-Mg compositions. The lower P2O5 concentrations of 0.85-1.00 wt % are defined for the full MgO ranges of 5.3-6.6 and 6.8-8.1 wt %, the elevated ones of 1.02-1.21 wt % are limited to the MgO ranges of 7.6-8.2 and 5.5-6.8 wt %. Data
CaO, wt %
4 -I-1-1-г
5 6 7 8
MgO, wt %
MgO, wt %
Fig. 11. Diagrams of CaO vs. MgO (a) and P2O5 vs. MgO (b) for volcanic rocks. The symbols are given in Fig. 8. Рис. 11. Диаграммы CaO - MgO (a) и P2O5 - MgO (b) вулканических пород. Условные обозначения см. на рис. 8.
points of the final pyroclastics from the late crater of the Laoheishan volcano show P2O5 concentrations of 0.9-1.1 wt % within the background range. On the diagrams of CaO vs. MgO and P2O5 vs. MgO, data points of the final pyroclastic material from the Laoheishan volcano occupy the lower parts of the data fields for the background rocks.
9. Trace elements
Background rocks show relatively low concentrations of compatible trace elements Ni and Cr (respec-
tively, 120-156 and 120-180 ppm). In rocks from the Central group of volcanoes, which overlap with the background ones in terms of MgO contents, concentrations of Ni and Cr vary within the same ranges and partially exceed the background concentrations. In the moderate-Mg transitional rocks from the Bijiashan and Laoheishan volcanoes, Ni contents increase to a range of 160-220 ppm and Cr contents to a range of 150-280 ppm. The Ni and Cr concentrations for peripheral flows of the initial Huoshaoshan eruptions fall in these intervals, but Ni and Cr contents for the final cone of the volcano differ markedly. As MgO decreases, Ni and Cr contents in these Huoshaoshan rocks
Cr, ppm
MgO, wt %
Fig. 12. Diagrams of Ni vs. MgO (a) and Cr vs. MgO (b) for volcanic rocks. The symbols are given in Fig. 8. Рис. 12. Диаграммы Ni - MgO (a) и Cr - MgO (b) вулканических пород. Условные обозначения см. на рис. 8.
decrease, respectively, from 210 to 120 ppm and from 260 to 180 ppm. The start of the downward trend coincides with the Ni data field of transitional rocks from the Bijiashan and Laoheishan volcanoes. The reduction trend of Cr concentrations in rocks from the Huosha-oshan volcano coincides with the data field of transitional rocks from the Bijiashan volcano and with a start of a trend of transitional rocks from the Laoheishan volcano. In transitional rocks from the Laoheishan volcano and peripheral lava flows of the Huoshaoshan volcano, Cr concentrations are relatively low (Fig. 12).
In diagrams of trace elements, normalized to the primitive mantle composition, rocks from the Central
volcanic group demonstrate uniform spectra, as it was shown by previous studies [Hsu, Chen, 1998; Chu et al., 2013]. The presented patterns for volcanic rocks from the Wohushan and Huoshaoshan volcanoes show Ba, K, La, and Pb peaks, Th-U, Nb-Ta, and Ce troughs, and no significant anomalies from Pr to Yb (Fig. 13).
No significant differences are found also in terms of rare earth elements between the rocks from the Central volcanic group. Nevertheless, their specific features, similar to the background signatures and different from them, are emphasized on plots of Rb vs. MgO, Zr vs. MgO, Ba vs. MgO, Th vs. MgO, Sr vs. MgO, and La/Yb vs. MgO (Figs 14, 15). A similar behavior is
Rock/Primitive mantle
I Fig. 13. Primitive mantle-normalized trace-element patterns for rocks from the Wohushan (a) and Huoshaoshan (b) volcanoes. Normalizing values after [McDonough, Sun, 1995].
Рис. 13. Сопоставление спектров несовместимых элементов, нормированных к примитивной мантии, для пород вулканов Уохушан (а) и Хуошаошан (b). Для нормирования использован состав [McDonough, Sun, 1995].
shown, on the one hand, for Rb and Zr, and on the other hand, for Ba, Th, Sr, and La/Yb.
In rocks from the Central group of volcanoes, which are compositionally close to the background ones, Rb concentrations increase from the first volcano (Wohushan) through the second (Bijiashan) to the third (Laoheishan). In rocks, which differ from the background ones, Rb concentrations increase from the second to the fourth volcano and decrease in its final edifice (Fig. 14, a). In rocks close to the background ones, Zr concentrations decrease from the first to the second volcano and increase to the third volcano. In rocks distinguished from background ones, relatively low concentrations of Zr at the first volcano change to elevated concentrations at the third and fourth volcanoes with a relative decrease at the final Huoshaoshan edifice (Fig. 14, b). While analyzing similar behavior of Rb and Zr, one can notice anomalously low concentrations of the latter element in rocks from the second volcano (Bijiashan).
On diagrams of Ba vs. MgO and Th vs. MgO (Fig. 15, a, b), data fields of low-Mg volcanic rocks from the Central volcanic group are distributed randomly within the data field of the background rocks. Meanwhile, mode-rate-Mg rocks (distinguished from background ones) show repeated configurations of data fields. Similarly, rock compositions vary in plots of Sr vs. MgO and La/Yb vs. MgO (Fig. 16, a, b).
10. Discussion
When interpreting the potassic volcanism as a derivative of mainly lithospheric sources, we rely on its distinct dependence on the lithosphere structure formed during the subsidence of the Songliao basin, and suggest that spatial-temporal distribution of volcanic activity and rock compositional variations are controlled by transtension developed along the Wuda-lianchi zone. While searching for regularities of magma
Rb, ppm
120 n
110 ■
100 ■
90 ■
80 ■
70
Zr, ppm
600 T
550 ■
MgO, wt %
500 ■
450 ■
400 ■
350 ■
300
MgO, wt %
Fig. 14. Diagrams of Rb vs. MgO (a) and Zr vs. MgO (b) for volcanic rocks. The symbols are given in Fig. 8. Рис. 14. Диаграммы Rb - MgO (a) и Zr - MgO (b) вулканических пород. Условные обозначения см. на рис. 8.
generation beneath the Wudalianchi field, we compare rock sources along the Wohushan-Huoshaoshan line and in background volcanoes and link a local stepwise compositional change of eruption products along the line with a crack that propagated in a layer of the lithosphere base in the central part of the transtensional segment.
10.1. Components of potassic rock sources
Sub-lithospheric continuum of components under East Asia comprises a material from the convective mantle domain with subducted slab (paleoslab) frag-
ments of the oceanic (paleooceanic) crust, as well as delaminated lithospheric blocks of orogens. Volcanic rocks from the Wudalianchi field show a sub-lithosphe-ric end-member that belongs to this continuum, but lithospheric components of these rocks, however, have no connection with other sub-lithospheric components. We refer the Wudalianchi rocks to a sub-lithospheric-lithospheric cluster of components from the boundary between the lithosphere and sub-lithospheric convec-tive mantle (Fig. 17).
An enriched mantle source begins to melt at lower temperature than a depleted source due to elevated concentrations of fluids. Incompatible fusible compo-
MgO, wt %
MgO, wt %
Fig. 15. Diagrams of Ba vs. MgO (a) and Th vs. MgO (b) for volcanic rocks. The symbols are given in Fig. 8. Рис. 15. Диаграммы Ba - MgO (a) и Th - MgO (b) вулканических пород. Условные обозначения см. на рис. 8.
nents are concentrated in low-volume liquids, whereas compatible elements of slightly melted medium are held in a restite. As the degree of melting increases, concentrations of incompatible elements in melts decrease and concentrations of compatible elements, on the contrary, increase. Such a change of liquid compositions occurs in the enriched mantle under rising temperatures. In a study of melting enriched sources, one should trace a relative role of compatible and incompatible elements that have isomorphic relationships in crystal structures.
We examine a behavior of strontium that isomor-phically replaces calcium in apatite, clinopyroxene,
garnet, amphibole, plagioclase, calcite, and possibly in other calcium-bearing minerals. In terms of a compatible oxide CaO, rocks from the Wohushan, Bijiashan, and Laoheishan volcanoes (except final pyroclastics of the latter volcano) fall between the background rocks (CaO=5.0-6.3 wt %) and rocks from the Huoshaoshan volcanic cone (CaO=7.1-8.1 wt %). Wide ranges of Sr concentrations in the background and final Huoshao-shan rocks are comparable with the range of this element in transitional rocks from the Laoheishan volcano. Similarly, narrow ranges of Sr concentrations in rocks from the Wohushan, Bijiashan, and Laoheishan volcanoes close to background, are comparable with
Sr, ppm
MgO, wt %
La/Yb
Fig. 16. Diagrams of Sr vs. MgO (a) and La/Yb vs. MgO (b) for volcanic rocks. The symbols are given in Fig. 8. Рис. 16. Диаграммы Sr - MgO (a) и La/Yb - MgO (b) вулканических пород. Условные обозначения см. на рис. 8.
the range of this element in transitional rocks from the Bijiashan volcano and initial Huoshaoshan flows (Fig. 18, a).
In transitional rocks from the Laoheishan volcano, increasing CaO contents are accompanied with increasing Sr concentrations from values of the initial eruptions at the Huoshaoshan volcano to the maximal values of its final eruptions. Thus, in terms of CaO and Sr contents, a common source material is defined in these two volcanoes.
In rocks from the early to late volcanoes of the Central group, CaO contents were increasing with expanding ranges of Sr concentrations. This is indicative of
the temporal intensification of melting processes. Strontium concentrations in the Wohushan rocks, close to the background values, decrease slightly to the transitional ones from the Bijiashan volcano and reach a minimum in rocks from the final Huoshaoshan cone. Taking into account the decreasing low limits of Sr concentrations in the material erupted at the Laoheishan and Huoshaoshan volcanoes, one has to admit this trend as an indication of a possible change of sources.
On the diagram of CaO/Sr vs. MgO (Fig. 18, b), transitional rocks from the Laoheishan cone are comparable to the initial Huoshaoshan rocks and, consequently, belong to the same source. Each of these volcanoes
I Fig. 17. Possible relationships between a common sub-lithospheric component of convective mantle and components of lithospheric versus sub-lithospheric sources for volcanic rocks.
Рис. 17. Возможные соотношения общего подлитосферного компонента конвектирующей мантии с компонентами литосферных и подлитосферных источников вулканических пород.
evolved with the changing erupted material due to its outcome from an additional source. On the one hand, a source for bombs from the late Laoheishan crater was similar to the background one, while most of its erupted material was affected with crustal contamination, which resulted in a remarkable decrease in CaO/Sr, as well as CaO and MgO contents (see the next section). On the other hand, a source of rocks from the final Huo-shaoshan cone produced the material with a higher degree of melting, expressed in an increasing CaO/Sr ratio at lower Sr concentrations and elevated CaO contents.
Low CaO/Sr ratios at low MgO contents in the background rocks and higher CaO/Sr ratios at elevated MgO contents in the transitional compositions of the Bijia-shan and Laoheishan volcanoes, as well as in advanced compositions of the Huoshaoshan volcano (Fig. 18, b), reflect activity of two contrasting types of sources, respectively, with low and higher temperatures of melts reflected in low and higher degrees of melting. In fact, all rocks from the Central group of volcanoes show CaO/Sr ratios in excess of those in the background rocks. This difference demonstrates a general change of magma generating processes from margins of the Wu-dalianchi field to its central part.
A common P2O5-Sr trend and individual P2O5-CaO trends for different groups of rocks (Fig. 19, a, b) indicate that Sr concentrations in liquids are governed by
melting of an apatite-bearing source, whereas CaO contents depend on involvement of other Ca-bearing minerals in background sources of the Western and Eastern groups of volcanoes, on the one hand, and in sources of the Central group of volcanoes, on the other hand.
In apatites from metasomatized mantle xenoliths, Sr contents reach 4.4 wt % at Zr concentrations of 22 ppm and less [Exley, Smith, 1982; O'Reilly, Griffin, 2000]. Therefore, small-volume melt fractions from an apatite-bearing source can be traced by high Sr concentrations and Sr/Zr ratios. Such a high-Sr component with Sr/Zr>5 indicates initial isotopic ratio 87Sr/86Sr= =0.70491 defined at intersecting trends of transitional compositions of the Laoheishan volcano and final compositions of the Huoshaoshan cone (Fig. 20, a, b). The final rocks from the Huoshaoshan volcano demonstrate a trend of mixing between the high-Sr component of a mantle lithospheric source and a common component of sub-lithospheric convective mantle with the parameters [Chuvashova et al., 2009; Rasskazov et al., 2011]: 103/Sr=~1.4 1/ppm (Sr=~700 ppm), Sr/Zr=~2, initial 87Sr/86Sr=0.7052. Transitional compositions of the Laoheishan volcano show another mixing between the same high-Sr component and less-Sr one (Sr=1300 ppm, 103/Sr=0.77 1/ppm, Sr/Zr=~3) with strontium isotope signature similar to the value of the common sub-lithospheric source. It is likely that the less-Sr
Sr, ppm
MgO, wt %
Fig. 18. Diagrams of Sr vs. CaO (a) and CaO/Sr vs. MgO (b) for volcanic rocks. The symbols are given in Fig. 8. Рис. 18. Диаграммы Sr - CaO (a) и CaO/Sr - MgO (b) вулканических пород. Условные обозначения см. на рис. 8.
component originated from the small-degree (as compared to sub-lithospheric material) melting lithosphe-ric medium that experienced transformation by fluids, rising from the sub-lithospheric convective mantle. Components with the same Sr-isotope ratio and varying concentrations of Sr characterize transitional rocks from the Bijiashan volcano as well as the background rocks. The latter show also the higher Sr-isotope ratios at relatively low Sr/Zr values (2.2-3.2).
On the diagram in Fig. 20, b, additionally plotted data points of flows sampled near the Laoheishan volcano [Chu et al., 2013] are subdivided into two groups. One group falls on the trend of transitional rocks from the
late volcanic cone of this volcano, another group shifts to the field of background compositions, enriched by radiogenic strontium.
Isotopic heterogeneity of sources for the lava flows related to the Huoshaoshan volcano is obvious from the data on samples taken from lava flows to the northeast and east of the volcanic cone by Chu et al. [2013]. Data points of these samples, shifted from the data field of the volcanic cone, demonstrate notably increasing 87Sr/86Sr ratios. Points of both the northeastern (gar-nito site) and eastern samples fall on the trend of transitional rocks from the Laoheishan volcano and, therefore, demonstrate Sr-isotope heterogeneity typical for
PA, wt %
1.2 ■
1.0
0.8 ■
4.5
P205, wt %
1.3 n
5.5
6.5
CaO, wt %
7.5
LH common source
8.5
1.1 ■
0.9 ■
0.7
1100
1300
1500
Sr, ppm
1700
Fig. 19. Diagrams of P2O5 vs. CaO (a) and P2O5 vs. Sr (b) for volcanic rocks. The symbols are given in Fig. 8. Рис. 19. Диаграммы P2O5 - CaO (a) и P2O5 - Sr (b) вулканических пород. Условные обозначения см. на рис. 8.
this volcano. The trend of mixing between low-Sr sub-lithospheric and high-Sr lithospheric components characterizes the rocks sampled only from the final Huoshaoshan cone (see Fig. 3, b).
10.2. Timing of processes in lithospheric and
sub-lithospheric sources
There were some attempts to constrain timing of enrichment events for the WEK source by model iso-topic ages. The estimate of 2.4 Ga was obtained from the two-stage Pb evolution model and the estimate of 800-900 Ma from the Nd (Tdm) evolution model. It was
inferred that "...the WEK lithospheric mantle source was modified during an early Proterozoic metasomatic event and was then isolated until the onset of the potassic magmatism" [Zhang et al., 1995, p. 1293]. A similar model approach was applied to explain the geochemical characteristics of the Wudalianchi basalts as derivatives from a deep stagnant slab. It was proposed that both the potassic- and EMl-like signatures of the basalts originated from the mantle transition zone, metasomatized by K-rich sediment fluids ~1.5 Ga ago through a stagnation of an ancient slab [Kuritani et al., 2013]. These and other Pre-Cambrian estimates assume a complex nature of protoliths involved in
Initial e7Sr/"Sr
0.70565
0.7054 -
0.7053 -
0.7052 - ,
0.7051 -
0.7050
LH common source
Initial Sr/ Sr
0.7054
0.7053
Common sub-lithospheric component
a Yaokuanshan Huoshaoshan □ Laoheishan
103/Sr, 1/ppm
^ Erkeshan
Fig. 20. Diagrams of initial 87Sr/86Sr vs. Sr/Zr (a) and initial 87Sr/86Sr vs. 103/Sr (b) for volcanic rocks.
The symbols are given in Fig. 8. Additionally shown are contrasting Sr-isotope compositions of lavas related to the Huoshaoshan volcano taken to the northeast of the cone (HNE - Huoshaoshan, northeast), to the east of it (HE - Huoshaoshan, east), and also Sr-isotope compositions of rocks from the Laoheishan and Yaoquanshan volcanoes and the Erkeshan volcanic field [Chu et al., 2013]. The value of initial 87Sr/86Sr=0.7052 of the sub-lithospheric component is obtained with lower inverse Sr concentrations for olivine leucitite from the Dangzishan volcano and trachyandesites from the Nanshan volcano in the Keluo field.
Рис. 20. Диаграммы начального 87Sr/86Sr - Sr/Zr (a) и начального 87Sr/86Sr - 103/Sr (b) вулканических пород.
Усл. обозн. см. на рис. 8. Дополнительно показаны контрастные составы изотопов Sr в породах вулкана Хуошаошан, отобранных северо-восточнее конуса (HNE - Huoshaoshan, northeast) и восточнее его (HE - Huoshaoshan, east), в породах вулканов Лаохейшан и Йаоцюаншан, а также поля Еркешан [Chu et al., 2013]. Значению начального 87Sr/86Sr=0.7052 подлитосферного компонента с более низкими обратными концентрациями Sr соответствуют изотопные отношения Sr оливинового лейцитита и пород вулкана Наншан поля Келуо.
"SiTSr Sub-lithospheric
"RbTSr
Fig. 21. The diagram of 87Sr/86Sr vs. 87Rb/86Sr for volcanic rocks from the Huoshaoshan cone.
Four data points (red squares) are distributed along the 98 Ma reference line. This age interpretation assumes equal bulk mineral-melt partition coefficients in a source region for Rb and Sr. Other points (blue rhombs) are plotted above the line.
Рис. 21. Диаграмма 87Sr/86Sr - 87Rb/86Sr вулканических пород конуса Хуошаошан.
Четыре фигуративные точки (красные квадраты) распределены вдоль линии 98 млн лет. Такая возрастная интерпретация предполагает равенство валовых коэффициентов распределения минерал-расплав в источнике для Rb и Sr. Другие точки расположены выше этой линии (синие ромбы).
melting without any geologically reasonable grounds related to the Late Phanerozoic geological setting of potassic series.
It is noteworthy that the character of geological evolution of East Asia such as Mesozoic inland migrating volcanic arcs, multiple magmatic intrusions, subsidence of the Songliao and other basins etc. is indicative of significant Late Phanerozoic sub-lithospheric activity of the upper mantle. On the background of the complex protoliths signatures in a source region, one should find a way to register these processes.
Because of the short period of radiogenic 143Nd accumulation, time-integrated 143Nd/144Nd values might yield only slight variations related to Late Phanerozoic geological events on the background of significant isotopic heterogeneity created by long-term accumulations of this isotopes in older protoliths. Similarly, an uranium-lead isotopic system is also insensitive for age definitions of a younger lithospheric material because of strong older isotopic heterogeneity of Pb in a source region (i.e. older heterogeneous lithospheric
substrate for potassic series). Meanwhile, a significant effect of radiogenic isotope accumulation after homo-genization and differentiation can be expected in a Rb-Sr isotopic system due to anomalous enrichment of a source region for potassic series with radioactive isotope 87Rb. The inferred change of melts from lithospheric to sub-lithospheric sources along the Wohu-shan-Huoshaoshan volcanic line makes the most probable Rb-Sr isotopic registration of timing for convec-tive homogenization of the sub-lithospheric material in rocks from the Huoshaoshan volcanic cone.
We suggest that a rubidium-free mineral phase (i.e. apatite) yields an initial Sr-isotope ratio in a source of the lithosphere base, while the common convective mantle component characterizes the time-integrated 87Sr/86Sr at a homogeneous reservoir of the evolved Rb-Sr isotopic system. Through the extremely low row of four data points for rocks from the Huoshaoshan volcano in the diagram of 87Sr/86Sr vs. 87Rb/86Sr, we get an estimate of about 98 Ma for the isotopic system closure at the base of the lithosphere with the initial 87Sr/86Sr apatite-related value of 0.70485 and the underlying convective mantle domain with Rb/Sr=0.092 (Fig. 21). This event corresponds to the transition from the syn-rift to post-rift stage of the Songliao basin evolution, according to Wang et al. [2016]. According to the estimates, the thermal influence of the convective mantle material on the lithosphere base decreased in the Mid-Cretaceous, at the time of a "superplume" state of the Earth's mantle [Larson, 1991], when avalanche-like collapses of paleoslab fragments from the transition layer into the lower mantle could provide reverse hot flows from the lower to the upper mantle beneath suture zones and adjacent areas of the Phanerozoic paleooceans closed in Central and East Asia [Rasskazov, Chuvashova, 2016].
10.3. Crustal contamination
The anomalous velocity structure of the crust implying the currently existing shallow magma chambers was identified through a local reduction of S-wave velocity in a depth range of 15-22 km under the Huo-shaoshan, Laoheishan volcanic edifices and adjacent areas of the Wudalianchi field, using the receiver function method [He et al., 2003]. This anomalous structural heterogeneity of the crust beneath the volcanoes was confirmed also by results of magnetotelluric sounding [Zhan et al., 2006].
Chemical parameters of crustal contamination are obscure in the Wudalianchi eruption products due to enriched compositions of mantle magmatic liquids [Zhang et al., 1995]. Crustal xenoliths, often partially molten, were detected in all edifices of the volcanic field and were specifically emphasized in the Huoshao-shan volcanic rocks [McGee et al., 2015]. A trend of in-
creasing Sr-isotope ratios with increasing inverse concentrations of Sr in rocks from the Huoshaoshan cone (see Fig. 20, b) is interpreted here as an evidence for mixing between components of the lithosphere base and the sub-lithospheric convective mantle. The trend was not significantly distorted by any crustal admixture. A small admixture of the lower crustal material (2-8 %) to the Yaoquanshan, Laoheishan, Huoshaoshan, and Erkeshan rocks was assumed from variations of 187Os/188Os ratios [Chu et al., 2013].
It is noteworthy, however, that sufficient contamination by the crustal material is quite obvious in the final pyroclastics related to the late crater of the Laoheishan volcano. Multiple small quartz-feldspar xenoliths are dispersed in the material that has low-Mg content (MgO=5.5-6.4 wt %) and, as compared to other low-Mg rocks from the Wudalianchi field, show elevated contents of SiO2 (52.3-54.0 wt %), K2O (5.3-5.8 wt %), Na2O+K2O (9.2-9.5 wt %), Ni (145-180 ppm), Zr (500560 ppm) at lower contents of M2O3 (13.5-14.2 wt %), CaO (5.0-5.4 wt %), and Ba (1500-1680 ppm) (Fig. 812, 14, 15). On the plot of initial 87Sr/86Sr vs. 103/Sr, three samples of this unite demonstrate elevated 87Sr/86Sr (0.705365-0.705410) and yield a typical trend of crustal contamination - increasing Sr-isotope ratios with increasing inverse concentrations of this element (see Fig. 20, b). No compositionally similar rocks occur in other volcanoes of the Central group.
Major element contents similar to those of contaminated rocks from the Laoheishan volcano were determined in samples taken by Chu et al. [2013] from the Huoshaoshan lava flows. The reported trace-element concentrations of the samples and Sr-isotope ratios differ, however, from the parameters obtained for the crust-contaminated rocks. For instance, Zr concentrations of the Huoshaoshan lava flows were defined in a range of 382-418 ppm.
10.4. Control of potassic volcanism by developing transtension zone
The transtensional nature of magma-controlling processes was well pronounced in the Keluo and Wudalianchi volcanic fields; extensional structures were expressed with northeastern volcanic loci under the general north-south direction of the zone (azimuth 353°). However, the fact that magmatism was not uniform along the zone is demonstrated by both different timing of volcanism and northward widening ranges of potassium contents in rocks (see Fig. 2).
Eruptions of olivine-leucitite magmas on the Keluo volcanic field with MgO contents as high as 14 % reflected the adiabatic ascent of hot material in the sub-lithospheric convective mantle, focused in the oblique lithospheric fault of the Wudalianchi zone. These high-temperature magmas erupted along the northeastern
(azimuth 51°) 42-km long line extended from the Dangzishan volcano in the southwest to the Xishan volcano in the northeast with a lateral widening up to 20 km in the field's center. An angle between the orientation of the Dangzishan-Xishan volcanic line and the general strike of the Wudalianchi zone is 58°.
In rocks from the Wudalianchi field, MgO contents do not exceed 8.2 wt %. Unlike Keluo, this field shows magmatic processes that might develop without any additional heat supply by the adiabatically upwelling convective mantle. At the beginning of volcanism at 2.5 Ma, the thermal regime of the sub-lithospheric material under this area was probably close to the state of the ambient mantle. Such a material had no excess buoyancy flax and did not intrude into the lithosphere base.
On the Wudalianchi field, the initial lava eruptions of 2.5-2.0 Ma were displayed along the oblique locus of the main transtensional zone. A line of lava flows was directed at azimuth 7-8°. A small deviation of the oblique magma-controlling crack relative to the general strike of the zone (approximately 14-15°) is considered as an evidence for local decompression that initiated melting of the low-viscous sub-lithospheric material in the central part of this zone, which was only 7-10-km wide and extended within an active 20-km wide transtensional segment (Fig. 22, a).
After the volcanic lull of 2.0-1.3 Ma (Fig. 22, b), the less viscous lithosphere began melting. The 40-km wide magmatically active transtensional zone and the 20-km wide Wudalianchi volcanic segment were comparable with these parameters of the Keluo volcanic field. The melting northeastern fragments deviated from the general strike of the zone by angles from 47° to 52°, i.e. less than 58°, which is the angle characteristic of the deviation of the Dangzishan-Xishan line in the Keluo volcanic field. Reducing angles are indicative of the development of magmatic processes beneath the Wudalianchi area under conditions of the less viscous lithosphere than beneath the Keluo field. Unlike concentrated high-temperature reactivation of a single melting locus under the Keluo field, the lower-temperature processes in the mantle portion of the lithosphere beneath the Wudalianchi field provided for the development of a couple of sub-parallel oblique melting loci comprising a transtensional segment (Fig. 22, c).
In the last 0.6 Ma, the transtensional zone expanded to 55 km, although the width of the segment (about 20 km) remained relatively stable (Fig. 22, d). In the marginal parts of the segment, melting spread from the initial north-south magmatic locus in the axial part of the transtensional zone in opposite directions - from the southwest to northeast at the eastern margin and from northeast to southwest at the western margin (see Fig. 4).
125° 35'
48° 45' -
48° 35' -
10 km
126° 15'
a
2.5-2.0 Ma
а' Lithosphère ' <=i
Sub-lithospheric mantle
b
2.0-1.3 Ma
Volcanic gap, reactivation of processes in the transtensional zone resulted in transfer of heat by fluids through the boundary shielding layer
48° 45' -
48° 35' -
48° 45' -
48° 35' -
0 Lava flows, active volcanoes A of different age intervals Melting at foot Melting at top
of shielding layer of shielding layer
Volcanic remnants
/
1\ }
in
Axis and \ i............I в
boundaries of / Boundaries of
the Wudallanchi f transtensional
transtensional zone segment
Active volcanoes (in sections a'-d')
Shielding layer at base of the llthosphere
Fluides from convective mantle affected shielding layer
Fig. 22. Control of volcanism by a developing transtensional structure at a shielding layer of the lithospheric base in the Wudalianchi field (a-d) and below (a'-d) (explanations in the text).
Рис. 22. Контроль вулканизма развивающейся структурой транстенсии в экранирующем слое основания литосферы на поле Удаляньчи (а-d) и под ним (а'-d) (объяснения в тексте).
Along the Wohushan-Huoshaoshan line, volcanic activity progressively shifted in the central part of the transtensional segment from the initial melting locus in the axial part of the transtensional zone northeastwards and was generally consistent with the temporal shift of activity in the Western group of volcanoes. The rate of volcanic shifting from the Laoshantou flow through the first entries of volcanic eruptions in the Wohushan and Bijiashan volcanoes to the Laoheishan one was about 0.8 cmxyear-1 on average. This rate could correspond to the lateral convective propagation of the sub-lithospheric material from the initially reactivated axial part of the transtensional zone. It is likely that the further rapid shift of volcanic activity from the Laoheishan to Huoshaoshan cones over a distance of 3.5 km was provided by the crack propagation in the lithosphere.
10.5. Changes of sources
An estimated temperature at the lithosphere-asthenosphere boundary under cratons at a depth of 200 km is as high as 1300 °C [O'Reilly, Griffin, 2010]. Under the off-craton regions of continents, a heat flux (that is generally higher than under cratons and the lithosphere-asthenosphere boundary) is recorded at shallower depths. The reference of the Wudalianchi erupted magmatic melts of 1720-1721 to the transition layer source was based on the assumed craton-like conductive heat flow of 40 mW/m2 in the lithosphere [Kuritani et al., 2013]. This estimate of the current convective heat flow in the lithosphere can hardly be applied to describe the thermal state of the lithosphere base within the Wudalianchi segment as a part of the volcanically-active transtensional zone developed in the past 2.5 Ma. The lithosphere base was affected by warming up and should be characterized by elevated temperatures that ultimately might approach the estimate of the rift geotherm of Mercier [1980].
Taking into account the hypothesis on space-time control of volcanism by the developing Wudalianchi transtensional segment, we speculate that a primary heterogeneity of the transtensional segment was created at the lithosphere base in the course of the Late Mesozoic - Early Cenozoic deformations of the lithosphere when the Songliao basin subsided. This assumption is consistent with the Mid-Cretaceous (98 Ma) closure of the Rb-Sr isotope system at the base of the lithosphere and the underlying convective mantle. In the Quaternary, the lithosphere base, that underwent transtensional deformations, was partly affected by fluids rising from the underlying convective mantle. A boundary layer of the lithosphere base shielded an overlying region of background melting from the sub-lithospheric convective mantle. The revealed spatial-temporal changes of components in melts that erupted
in the Wudalianchi field mark deformations in the lower part of the shielding layer bordering the convective mantle domain, as well as deformations in its top that bourdered the enriched region of the lithosphere.
We suggest the following sequence of the processes:
1. Melting was initiated at the boundary between the shielding layer and the underlying convective mantle in the time interval of 2.5-2.0 Ma. Magma-generating decompression effects were concentrated along the narrow north-south melting locus and resulted in effusions of the Laoshantou and Old Gelaqiushan flows marked the initial low-angle oblique fragment of the transtensional zone.
2. The subsequent lack of efficiency of the magmagenerating processes resulted in a lull of volcanic activity in the time interval of 2.0-1.3 Ma. The initial reactivation of the transtensional segment could provide, however, the separation of fluids from the sub-litho-spheric mantle and their penetration into the deformed parts of the shielding layer and partly into the overlying enriched region. The mass-and-heat transfer distorted the conductive lithospheric geotherm through increasing temperatures in the enriched region above the asthenosphere-lithosphere boundary layer.
3. At about 1.3 Ma, the temperature of the lithosphere base was elevated enough to initiate background melting of the lithosphere in the transtensional segment. By 0.8 Ma, the entire segment was enveloped by melting. The source of the first volcano of the Central group (Wohushan), which occurred in the background melting region, was followed by the material from cone 2 of this volcano, characterized by the initial 87Sr/86Sr values about 0.7052, which are indicative of the occurrence of the source material modified by the sub-lithospheric mantle fluids.
4. In the past 0.6 Ma, background melting of the lithosphere advanced into the northeastern sector of the transtensional segment. This melting areal was firstly accompanied with magma generating processes beneath the second volcano of the line (Bijiashan), eruptions of which began with the material from the background region and continued with the material from the shielding layer modified by the sub-lithosphe-ric mantle fluids. Later on, the sources of the third and fourth volcanoes of the line (Laoheishan and Huoshao-shan) were activated. The Laoheishan volcano could have erupted before 1720. The initial phase of the short (1720-1721) activity in the Laoheishan volcano was exhibited by the pyroclastic material of the late cone and, afterwards, in a half of year after the end of its activity, by lava flows of the Huoshaoshan volcano. In both cases, sources were reactivated in the shielding boundary layer at the lithosphere base that had been modified by fluids that ascended from the sub-litho-spheric convective mantle in the Mid-Cretaceous and Quaternary. The final explosive phase of the Laohei-
shan volcano, which resulted in formation of the late crater, was provided by eruptions of liquids derived from the background lithospheric source and contaminated by the crustal material, whereas the final phase of the Huoshaoshan volcano was supplied by eruptions of liquids from the boundary shielding layer mixed with the sub-lithospheric material.
11. Conclusion
The Wudalianchi zone, traced with the Pliocene-Quaternary potassic volcanic rocks, is strictly controlled by the lithospheric structure in the northern circuit of the Songliao basin that subsided in the Late Mesozo-ic - Early Cenozoic. The rock compositions vary along this zone in a stepwise pattern from the Erkeshan field (K2O content of 5.6-5.8 wt %) to the Xiaogulihe-Menlu field (contrasting K2O contents of about 2.0 wt % and 7.2-9.5 wt %). This study has been focused on clarifying the character and nature of spatial-temporal variations of K2O contents in rocks from the Wudalianchi field, which range from 6.0 to 3.2 wt %.
Considering the spatial-temporal compositional variations of rocks, we distinguish three time intervals of the volcanic evolution in the Wudalianchi field: (1) 2.5-2.0 Ma, (2) 1.3-0.8 Ma, and (3) <0.6 Ma. At the beginning, lava flows erupted along the north-south Laoshantou - Old Gelaqiushan locus, then in the South Gelaqiushan volcano and along the west-east band of the Lianhuashan, Wohushan, Yaoquanshan, West Jiao-debushan, West Longmenshan volcanoes, and, finally, in three groups of volcanoes: Western (North Gelaqiushan, Lianhuashan, Jianshan-Jianshanzi), Central (Wohushan, Bijiashan, Laoheishan, Huoshaoshan) and Eastern (Weishan, East Jiaodebushan, Xiaogushan, West and East Longmenshan, Molabushan). The Western and Eastern groups exhibited irregular background eruptions of the last 1.3 Ma, whereas the Central group showed the stepwise northeastward shift of volcanic activity with an average rate of about 0.8 cm per year. On the one hand, the background erupted liquids displayed the limited range of K2O concentrations (4.8-6.0 wt %), on the other hand, the migrated erupted melts demonstrated relatively reduced K2O contents at the beginning and at the end of the volcanic sequence.
The representative sampling revealed the stepwise changes of rock compositions in the Central group of volcanoes. On the first volcano (Wohushan), the rocks were compositionally close to the background ones. On the second and third volcanoes (Bijiashan and Laoheishan), the rocks were compositionally both close to the background rocks and different from them. On the final cone of the fourth volcano (Huoshaoshan), the rock compositions differed significantly from the back-
ground ones. The background rocks showed specific compositions: SiO2 51-55 wt %, CaO 5.3-6.8 wt %, MgO 5.3-7.0 wt %, K2O/Na2O=1.1-1.8 wt %, and CaO/Sr 31-45. The final Huoshaoshan rocks demonstrated decreesing SiO2 to 49 wt % and increasing CaO to 8.1 wt %, MgO to 8.3 wt %, CaO/Sr to 65.
From the analyses of the major, trace-element and Sr-isotope data on the volcanic rocks, we suggest that the stepwise propagation and jumps of volcanic activity along the Wohushan-Huoshaoshan line were accompanied by both the introduction of sub-lithospheric component into the melting region and melting of the crustal material. Extremely contrasting sources were defined in the 1720-1721 eruptive products, that included final pyroclastics from the late crater of the Laoheishan volcano contaminated with the crustal material, and the final lavas from the Huoshaoshan cone containing the admixture of the sub-lithospheric mantle component.
The established spatial-temporal variations in the rocks compositions are explained as a result of the magma generation control by the north-south trans-tensional zone in the layer of the lithospheric base that shielded the underlying sub-lithospheric convective mantle from the overlying more enriched lithosphere. Sub-lithospheric liquids were distinct due to the initial 87Sr/86Sr ratio of about 0.7052, melts from the boundary shielding layer due to the same and lower ratios, and those from the overlying enriched lithosphere due to the same and higher ratios.
We proposed that the local venue of the convective mantle material from below the shielding layer and the melting enriched background material above it was governed by transtensional deformations. The eruptions of sub-lithospheric melts from the axial part of the main transtensional zone, which took place at 2.5-2.0 Ma, were followed by the propagation of the background liquids from the wider segment of the enriched lithospheric region at 1.3-0.8 Ma. In the past 0.6 Ma, background melting progressed at the margins of the transtensional segment simultaneously with local melting along the crack that propagated in the boundary shielding layer under the central part of the background melting region.
12. Acknowledgments
We thank Prof. Jiaqi Liu who arranged a survey of Cenozoic volcanic rocks in Northeast China in 20072013. The results obtained in these workshops served as a basis for creating the Chinese-Russian Wudalian-chi-Baikal Research Center on volcanism and environment, in the framework of which this study was developed - grants No. P162011012 and of the Scientific Research Fund of the Heilongjiang Academy of Sciences
(2016). We are grateful to Prof. Yogjiang Liu for his introduction into regional geology of Northeast China, which improved our understanding of the structural control of potassic volcanic rocks. We analyzed the trace-elements and isotopes using the equipment of the Microanalysis (Limnological Institute of SB RAS) and Geodynamics and Geochronology (Institute of the Earth's Crust SB RAS) Collective Use Centers. Samples
for isotopic analysis of Sr were prepared by E.V. Sara-nina, and mass-spectrometer measurements were performed by N.N. Fefelov. Samples for trace-element analysis were prepared by M.E. Markova, and mass-spectrometer measurements and mass-corrections were performed by A.P. Chebykin. The "wet chemistry" analytical work was conducted by G.V. Bondareva and M.M. Samoilenko.
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Rasskazov, Sergei V., Doctor of Geology and Mineralogy, Professor, Head of Laboratory Institute of the Earth's Crust, Siberian Branch of RAS
128 Lermontov street, Irkutsk 664033, Russia Irkutsk State University, Geological Faculty
3 Lenin street, Irkutsk 664003, Russia Tel. +7(3952)511659; И e-mail: [email protected]
Рассказов Сергей Васильевич, докт. геол.-мин. наук, профессор, зав. лабораторией Институт земной коры СО РАН
664033, Иркутск, ул. Лермонтова, 128, Россия Иркутский государственный университет, геологический факультет
664003, Иркутск, ул. Ленина, 3, Россия Тел. (3952)511659; И e-mail: [email protected]
Chuvashova, Irina S., Candidate of Geology and Mineralogy, Senior Researcher Institute of the Earth's Crust, Siberian Branch of RAS
128 Lermontov street, Irkutsk 664033, Russia Irkutsk State University, Geological Faculty
3 Lenin street, Irkutsk 664003, Russia Tel. +7(3952)511659; e-mail: [email protected]
Чувашова Ирина Сергеевна, канд. геол.-мин. наук, с.н.с. Институт земной коры СО РАН
664033, Иркутск, ул. Лермонтова, 128, Россия Иркутский государственный университет, геологический факультет
664003, Иркутск, ул. Ленина, 3, Россия Тел. +7(3952)511659; e-mail: [email protected]
Yi-min Sun, Research assistant
Institute of Volcano and Mineral Spring, Heilongjiang Academy of Science Wudalianchi 164155, Heilongjiang, China e-mail: [email protected]
Йи-минь Сунь, научный сотрудник
Институт вулканов и минеральных источников Хэйлунцзянской академии наук 164155, Удаляньчи, Хэйлунцзян, Китай e-mail: [email protected]
Chen Yang, Deputy director
Institute of Volcano and Mineral Spring, Heilongjiang Academy of Science Wudalianchi 164155, Heilongjiang, China
Чэнь Янг, заместитель директора
Институт вулканов и минеральных источников Хэйлунцзянской академии наук 164155, Удаляньчи, Хэйлунцзян, Китай
Zhenhua Xie, Director
Institute of Volcano and Mineral Spring, Heilongjiang Academy of Science Wudalianchi 164155, Heilongjiang, China
Чжэньхуа Сие, директор
Институт вулканов и минеральных источников Хэйлунцзянской академии наук 164155, Удаляньчи, Хэйлунцзян, Китай
Yasnygina, Tatiana A., Candidate of Geology and Mineralogy, Senior Researcher Institute of the Earth's Crust, Siberian Branch of RAS 128 Lermontov street, Irkutsk 664033, Russia Tel. 8(3952)511659; e-mail: [email protected]
Ясныгина Татьяна Александровна, канд. геол.-мин. наук, с.н.с. Институт земной коры СО РАН 664033, Иркутск, ул. Лермонтова, 128, Россия Тел. 8(3952)511659; e-mail: [email protected]
Saranina, Elena V., Candidate of Geology and Mineralogy, Senior Researcher Institute of the Earth's Crust, Siberian Branch of RAS 128 Lermontov street, Irkutsk 664033, Russia Tel. +7(3952)511659; e-mail: [email protected]
Саранина Елена Владимировна, канд. геол.-мин. наук, с.н.с. Институт земной коры СО РАН 664033, Иркутск, ул. Лермонтова, 128, Россия Тел. +7(3952)511659; e-mail: [email protected]
Zhenxing Fang, Deputy director
Institute of Volcano and Mineral Spring, Heilongjiang Academy of Science Wudalianchi 164155, Heilongjiang, China
Чжэньсин Фан, заместитель директора
Институт вулканов и минеральных источников Хэйлунцзянской академии наук 164155, Удаляньчи, Хэйлунцзян, Китай
