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GEODYNAMICS & TECTONOPHYSICS
PUBLISHED BY THE INSTITUTE OF THE EARTH'S CRUST SIBERIAN BRANCH OF RUSSIAN ACADEMY OF SCIENCES
2017 VOLUME 8 ISSUE 3 PAGES 499-501 ISSN 2078-502X
https://doi.org/10.5800/GT-2017-8-3-0272
Proceedings of the Second Russia-China International Meeting on the Central Asian Orogenic Belt (September 6-12, 2017, Irkutsk, Russia)
Early Neoproterozoic crust formaton in the Dzabkhan
MICROCONTINENT, CENTRAL ASIAN OROGENIC BELT
V. P. Kovach1, I. K. Kozakov1, A. Kroner2, 3, E. B. Salnikova1, K.-L. Wang4, 5, H.-Y. Lee4, S.-L. Chung4, 5
1 Institute of Precambrian Geology and Geochronology of RAS, St. Petersburg, Russia
2 Beijing SHRIMP Centre, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
3 Institute of Geosciences, University of Mainz, 55099 Mainz, Germany
4 Institute of Earth Sciences, Academia Sinica, Taipei 11529, Taiwan
5 Department of Geosciences, National Taiwan University, Taipei 106, Taiwan
For citation: Kovach V.P., Kozakov I.K., Kröner A., Salnikova E.B., Wang K.-L., Lee H.-Y., Chung S.-L., 2017. Early Neoproterozoic crust formaton in the Dzabkhan microcontinent, Central Asian Orogenic Belt. Geodynamics & Tectonophysics 8 (3), 499-501. doi:10.5800/GT-2017-8-3-0272.
The Dzabkhan microcontinent was defined by [Mossakovsky et al., 1994] as a cratonic terrane with an early Precambrian basement that combines highgrade metamorphic complexes of the Songino, Dzabkhan, Otgon, Baidarik, Ider and Jargalant Blocks. However, early Precambrian ages have so far only been recognized in the Baidarik and Ider blocks [Kozakov et al., 2007,2011; Kroner et al., 2015].
The high-grade metamorphic rocks of the Songino Block consist of variably migmatized biotite trondhje-mitic gneisses and biotite±garnet±sillimanite gneiss with rare interlayers of hornblende gneiss. An age of 802±6 Ma, obtained from zircons of a granite-gneiss, is close to the timing of regional amphibolite-facies me-
tamorphism [Kozakov et al., 2013]. The lower-grade rocks are made up of volcanogenic, clastic-volcano-genic, and clastic rocks, variably metamorphosed from greenschist- to upper amphibolite-facies. Moderate-and high-Ti basalts of the volcanogenic sequences are comparable to modern N-MORB and E-MORB-OIB oceanic rocks, whereas a differentiated basalt-andesite-rhyolite volcanic sequence and associated clastic sediments are interpreted as analogues of modern island arcs and accretionary wedges [Kovach et al., 2013; Yarmolyuk et al., 2015]. Zircon ages and Nd isotopic data suggest early Neoproterozoic (ca. 890-860 Ma) juvenile oceanic and island arc crust for the lower-grade association and late Meso- to early Neoprotero-
V.P. Kovach et al.: Early Neoproterozoic crust formaton in the Dzabkhan microcontinent.
zoic (ca. 1.30-0.79 Ga) continental crust for the highgrade complex. These units were tectonically juxtaposed in the early Neoproterozoic (ca. 0.80-0.79 Ga).
The northwestern part of the Dzabkhan block is composed of three zones, which differ in rock associations and their structural-metamorphic evolution [Kozakov et al., 2014]. One zone consists of mostly sedimentary rocks, metamorphosed to the amphibolite-facies, and detrital zircons from a garnet-muscovite-biotite paragneiss yielded ages of 820-900 Ma and a whole-rock s Nd(0.86Ga) value of -6.7, corresponding to a Nd model age of ca. 2.2 Ga.
The second zone consists of intensely deformed and migmatized biotite paragneisses, hornblende±clino-pyroxene mafic gneisses, amphibolites, and granite-gneisses that experienced two amphibolite-facies me-tamorphic events. Detrital zircons from a garnet-biotite paragneiss vary in age from 850 to 962 Ma. Tabular and lens-shaped granite-gneiss bodies occurring within the migmatized paragneisses were emplaced between the above metamorphic episodes at 856±2 Ma. Various gneisses, probably derived from andesite and felsic volcanic rocks or tonalite, trondhjemite, and grey-wacke, and granite-gneisses of this have variable whole-rock sNd(t) values from -2.6 to +3.6 and Nd model ages of 1.8-1.2 Ga. Synmetamorphic hornblende gabbro and gabbrodiorite cut the paragneiss/granite-gneiss association and crystallized at 860±3 Ma. They have sNd(t) values from +2.1 to +5.6, suggesting the involvement of juvenile and some crustal material in their generation. Post-tectonic TTGs dated at 786±6 Ma have SNd(t) values ranging from -2.1 to +1.5 and late Palaeo- to Mesoproterozoic Nd model ages of 1.71.3 Ga. These granitoids probably formed in an active continental margin setting involving juvenile early Neoproterozoic and older crustal sources.
The third zone is composed of tectonic slabs of migmatized gneiss, granite-gneiss and granitoids and slices of greenschist-facies metavolcanic rocks (basalt, andesite, rhyolite), clastic sediments as well as slabs of hornblende schists, trondhjemitic gneiss, marble, conglomerate and quartzite, all metamorphosed to
upper amphibolites-facies conditions. Trondhjemites and a gabbro dyke dated at 959±8, 944±6, and 930±6 Ma and yielded positive s Nd(t) values from +4.4 to +6.1. Zircons from trondhjemites are also characterized by strongly positive sHf(t) values. These data suggest that the plutonic rocks formed in an intra-oceanic arc setting with only minor contributions from recycled crustal material.
In contrast to the northwestern part of the Dzabkhan block, zircon ages and whole-rock Nd isotopic data for the southern part indicate extensive early Neoproterozoic crustal reworking [Kozakov et al., 2015; Bold et al., 2016].
The Jargalant block consists of biotite gneiss, inter-layered with marble and thin layers of amphibolite as well as biotite and hornblende gneiss. The rocks of the complex were metamorphosed at ca. 810 Ma to the high temperature amphibolite-facies [Kozakov et al., 2011]. Nd model ages of 1.9-1.3 Ga and sNd(0.81) values from -1.6 to +1.1 define a maximum age of proto-lith formation at ca. 1.3 Ga, presumably in an active continental margin setting.
The biotite gneisses (metatrondhjemite) from the Otgon block are characterized by a whole-rock Nd model age of 1.2 Ga and low positive SNd(0.8Ga) values from +2.8 to +3.0 that suggest an active continental margin setting for the generation of the granitoid pro-toliths [Kozakov et al., 2014].
Combining all ages and isotopic data for the Dzab-khan microcontinent suggests that both crustal reworking and juvenile crustal additions in the Neopro-terozoic played significant role in its generation and evolution, and we therefore consider it to be of mixed origin. Furthermore, it is apparent from field geology and the above data that the Dzabkhan microcontinent does not represent a unified large early Precambrian tract of continental crust, overlain by Ediacaran shelf deposits but represents an early Neoproterozoic composite terrane composed of palaeo-oceanic, island arc, active continental margin complexes and reworked early Precambrian continental crust.
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