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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 583-587 ISSN 2078-502X
https://doi.org/10.5800/GT-2017-8-3-0298
Proceedings of the Second Russia-China International Meeting on the Central Asian Orogenic Belt (September 6-12, 2017, Irkutsk, Russia)
Depleted SSZ type mantle peridotites in Proterozoic Eastern Sayan ophiolites in Siberia
Kuo-Lung Wang1- 2, Zhuyin Chu3, Marina A. Gornova4, Sergei Dril4, Vasilii A. Belyaev1- 4, Kuan-Yu Lin1- 2, Suzanne Y. O'Reilly5
1 Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan
2 Department of Geosciences, National Taiwan University, Taipei, Taiwan
3 Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China
4 A.P. Vinogradov Institute of Geochemistry, Siberian Branch of RAS, Irkutsk, Russia
5 CCFS/GEMOCKey Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney, Australia
For citation: Wang K.-L., Chu Z., Gornova M.A., Dril S., Belyaev V.A., Lin K.-Y., O'Reilly S. Y., 2017. Depleted SSZ type mantle peridotites in Proterozoic Eastern Sayan ophiolites in Siberia. Geodynamics & Tectonophysics 8 (3), 583-587. doi:10.5800/GT-2017-8-3-0298.
N.L. Dobretsov et al. [1985] first described the rock complexes in Eastern Sayan as ophiolites. Ophiolites formed in Dunzhugur island arc and were obducted onto Gargan block, a Neoarchean crystalline basement of the Tuva-Mongolian Massif (TMM), as a single nappe [Khain et al., 2002; Kuzmichev, 2004]. Zircons from pla-giogranite were dated at 1021 ±5 Ma by multigrain TIMS and 1020±1 Ma by Pb-Pb single-grains evaporation method [Khain et al., 2002]. Later [Kuzmichev, Lari-onov, 2013] analysed 12 grains of detrital zircons from gravelstone of the Dunzhugur formation and obtained 206Pb/238U ages from 844±8 to 1048±12 Ma. Careful examination of these data shows that 206Pb/238U ages
for concordant zircons only vary from 962±11 to 1048±12 Ma. Two groups of data give Concordia ages of 974±11 and 1028±10 Ma. Rocks of the Dunzhugur complex are characterized by slightly negative to slightly positive sNd(t) values from -1.0 to +1.5 and Late Palaeoproterozoic depleted mantle Nd model ages of 1.8-1.6 Ga [Sklyarov et al., 2016]. The Dunzhugur complex was intruded by tonalite plutons of the Sumsunur complex dated at 785±11 Ma [Kuzmichev et al., 2001] and 811±7 Ma [Kovach et al., 2012]. These tonalites have strongly negative sNd(t) values from -13.2 to -12.3 and Nd model ages of 2.5-2.4 Ga, suggesting formation of these melts from a mixture of
K.-L. Wang et al.: Depleted SSZ type mantle peridotites in Proterozoic Eastern Sayan Ophiolites.
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| Fig. 1. Diagram of Cr# vs. Mg# of spinel (modified from
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Neoproterozoic juvenile and Archaean crustal sources [Kovach et al., 2012].
The Eastern Sayan ophiolites, also named Dunzhu-gur ophiolites, are composed of several tectonic units/ massifs include (1) Ospin (Ospin-Kitoy), (2) Ilchir, (3)
Ulan-Sardag, (4) Hara-Nur, and (5) Dunzhugur from east to west (Fig. 1). Authochton rocks are represented by terrigenous-carbonate deposits of Irkut and Ilchir formations, metamorphosed in green schist to epidote-amphibolite-facies. Ophiolitic peridotites from two localities: Ulan-Sardag (US; one dunite, two harzburgites and one orthopyroxenite) and Hara-Nur (HN; five harzburgites) are studied here.
Geochemistry and geodynamic implications. Two harzburgites from Hara-Nur massif did not undergo metamorphism, whereas metaperidotites and serpen-tinites underwent metamorphism in lower amphibolite facies. Ulan-Sardag massif is composed of metaperido-tites, which formed after serpentinites as a result of prograde metamorphism up to mid-amphibolite facies. Composition of both their relic Sp cores is primary and allows to make petrogenetic investigation. On the diagram of Cr# vs. Mg# of spinel [Dick, Bullen, 1984; Fig. 1 ], some HN harzburites locate in abyssal peridotite field, but a few are close to the supra-subduction zone (SSZ) field. All US harzburgites are in the field of SSZ.
Both HN and US harzburgites have negative correlations between Cr# in Sp. and WR AhO3 and CaO indicating degrees of partial melting/melt extraction play a main role in controlling their geochemical features (Fig. 2). US orthopyroxenite has the lowest MgO and La/Yb but highest A^3 and CaO. Although US dunite has the lowest AhO3, its high CaO and La/Yb suggest that this dunite was readily modified by later metasomatism.
The harzburgites and dunites, resembling subduction zone peridotites, are represented by depletion of MREE with respect to HREE (Fig. 3). In addition, significant enrichments of LILE, LREE and some HFSE are
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| Fig. 2. Diagrams of Cr# in Sp. vs. WR MgO, WR A^3, WR CaO and WR La/Yb.
Hara-Nur
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La Ca Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
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I Fig. 3. Chondrite/Primitive mantle-normalized WR trace-element patterns of HN (upper) and US (lower) peridotites.
consistent with a possible interaction of these peridotites with fluids or melts derived from the subducted slab [e.g. Parkinson, Pearce, 1998]. Partial enrichment of LREE and incompatible elements (Ba, Nb and Pb) is believed to result from the interaction of boninitic magmas with peridotites in a mantle wedge. US harz-burgites are characterized by more sharper decrease from HREE to MREE with respect to those of US dunite, orthpyroxenite and HN harzburgites. The unmetamor-phosed harzburgites and dunite show pronounced positive Ti anomalies. In addition, all rock types show pronounced peaks in Nb and Pb.
The HSE abundances exhibit strongly fractionated patterns for both US and HN harzburgites (Fig. 4). US dunite has enrichments in Os, Ir and Ru and the highest abundance among all. Another US harzburgite (S12-113) also show typical residual pattern. All other harzburgites show the intermidiate patterns between residual and melt indicating they had been experienced interaction with later metasomatic melts/fluid. Such melts/fluid metasomatism also caused fractionation among the HSEs. Among them, Os-Ir (both I-PGE), Pt-Ir and Re-Ir covariation in US dunite/harzburgite, Pd-Ir (P-PGE/I-PGE) co-variation for both US and HN perido-tites had been modified (Fig. 5).
Dunite, harzburgite and chromitite showing residual PGE patterns (Pd/Ir=0.07-0.62) with low i87Re/*880s ratios (0.0037-0.0632) yield tMA model ages of 2.38, 1.84, 1.44 and 1.22 Ga (1870s/1880s=0.11353-0.11978). A low 187Re/1880s harzburgite with IPGE-depleted PGE
pattern and one sulfide in dunite yield tMA model ages of 1.38 and 1.53 Ga (Table). The Re-Os isotope compositions of HN and US peridotites yield model age peaks at ~2.38 Ga, ~1.84 Ga and ~1.44-1.22 Ga, which may
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record major tectonic events prior to and associated with closure of paleo-Asian ocean at the southern margin of the Siberian craton since the late Mesoprotero-zoic time. The pristine residual signatures, along with the ancient Os model ages older than the Dunzhugur ophiolites cannot be ascribed to recycled (subducted) ancient crustal materials in the lithospheric mantle. Similar crustal event age peaks occur in the South China block/Laurentia, suggesting a connection with the southern Siberian craton before the opening of the Paleo-Asian ocean. Thus, remnant lithospheric fragments stranded by rifting of the ancient continental regions are inferred to occur in the Neoproterozoic Dunzhugur ophiolites as observed in modern ocean basins [O'Reilly et al., 2009].
Conclusion. Mineral chemistry and whole-rock trace and PGE data indicate that formation of the Hara-Nur and Ulan-Sardag peridotites cannot be explained
by a single stage melting event but at least two-stages of melting and re-enrichment processes are needed to explain their geochemical characteristics. Their trace-element patterns are similar to residual peridotites melted in a SSZ environment indicating these depleted harzburgites and dunites are the product of melting and related re-enrichment took place in SSZ. The pristine residual signatures, along with the ancient Os model ages older than the Dunzhugur ophiolites cannot be ascribed to recycled (subducted) ancient crustal materials in the lithospheric mantle. Similar crustal event age peaks occur in the South China block/Laurentia, suggesting a connection with the southern Siberian craton before the opening of the Paleo-Asian ocean. Thus, remnant lithospheric fragments stranded by rifting of the ancient continental regions are inferred to occur in the Neoproterozoic Dunzhugur ophiolites as observed in modern ocean basins.
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