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Nature and(in-)coherent metamorphic evolution of subducted continental crust in the Neoproterozoic accretionary collage of SW Mongolia

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Geoscience Frontiers 2021年第3期12卷 78-103页

作者： sergei skuzovatov Vinogradov Institute of Geochemistry SB RAS Favorskogo str.1AIrkutsk 664033Russia

In continental subduction complexes minor volumes of high-pressure mafic rocks(eclogites)often co-exist with much more abundant felsic(granitic)and metasedimentary rocks,which are vital for resolving the origin and metamorphic evolution of subducted continental *** SW Mongolia,the Alag Khadny eclogite-bearing accretionary complex(AKC)is assumed to represent either a remnant of oceanic slab,or a continental margin,subducted in the Early *** we present geochronological,geochemical and petrological evidence of subduction records for the three major types of lithologies that host mafic eclogites,including Mesoproterozoic and Neoproterozoic granitic basement and overlying Neoproterozoic continental-margin *** deformed,ferroan and peraluminous metagranitoids compose a major part of AKC and are interlayered with eclogites in its southern and eastern *** have geochemical features of post-collisional/intraplate high-K calc-alkaline ***-ICP-MS U-Pb zircon geochronology of three distinct metagranite samples show uniform protolith crystallization ages of ca.0.96 Ga and uncertain re-crystallization in the Late Neoproterozoic or Early Paleozoic metamorphic event,whereas abundant zircon inheritance indicates older,Mesoproterozoic to Paleoproterozoic crustal substrate during granite *** existence of Mesoproterozoic crust is highlighted by finding of distinct metagranitoids with the U-Pb zircon crystallization age of ca.1.6 *** isotope signatures(T_(DM)^(C)2.88-1.85 Ga)of zircons from all lithologies preserved the evidence of reworked Neoarchean to Paleoproterozoic crust,similar to that of the Baidrag block(southern Mongolia),for both Mesoproterozoic and Neoproterozoic *** of the specific lithology,the rocks display indicators of high-pressure metamorphic re-equilibration,including garnet(X_(Ca)up to 0.65)+epidote+phengite(Si *** to 3.56)±rutile assemblage in metagranitoids,garnet+phengite(Si ***

关键词： Central Asian Orogenic Belt SW Mongolia Subduction Metasedimentary rocks Orthogneisses Trace elements Zircon U-Pb age Hf isotopes

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The evolution of refertilized lithospheric mantle beneath the northeastern Siberian craton:Links between mantle metasomatism,thermal state and diamond potential

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Geoscience Frontiers 2022年第6期13卷 153-169页

作者： sergei skuzovatov Vladislav S.Shatsky Alexey L.Ragozin Alexander P.Smelov Vinogradov Institute of Geochemistry Russian Academy of SciencesSiberian Branch1AFavorskogo Str.Irkutsk 664033Russia Sobolev Institute of Geology and Mineralogy Russian Academy of SciencesSiberian Branch3Koptyug Ave.Novosibirsk 630090Russia Geology&Geophysics Department Novosibirsk State University2 Pirogova Str.Novosibirsk 630090Russia Diamond and Precious Metal Geology Institute Russian Academy of SciencesSiberian Branch39Lenina Ave.Yakutsk 677000Russia

Although the diamond potential of cratons is linked mainly to thick and depleted Archean lithospheric keels, there are examples of craton-edge locations and circum-cratonic Proterozoic terranes underlain by diamondiferous mantle. Here, we use the results of comprehensive major and trace-element studies of detrital garnets from diamond-rich Late Triassic(Carnian) sedimentary rocks in the northeastern Siberia to constrain the thermal and chemical state of the pre-Triassic mantle and its ability to sustain the diamond storage. The studied detrital mantle-derived garnets are dominated by low-to mediumCr lherzolitic(~45%) and low-Cr megacrystic(~39%) chemistries, with a significant proportion of eclogitic garnets(~11%), and only subordinate contribution from harzburgitic garnets(~5%) with variable CrOcontents(1.2–8.4 wt.%). Low-Cr megacrysts display uniform, “normal” rare-earth element(REE)patterns with no Eu/Eu* anomalies, systematic Zr and Ti enrichment(mainly within 2.5–5), which are evidence of their crystallization from deep metasomatic melts. Lherzolitic(G9) garnets exhibit normal or humped to MREE-depleted sinusoidal REE patterns and elevated Nd/Y(up to 0.33–0.41) and Zr/Y ratios(up to 7.62). Rare low-to high-Cr harzburgitic(G10) garnets have primarily “depleted”, sinusoidal REEpatterns, low Ti, Y and HREE, but vary significantly in Zr-Hf, Ti and MREE-HREE contents, Nd/Y(within 0.1–2.4) and Zr/Y(1.53–19.9) ratios. The observed trends of chemical enrichment from the most depleted,harzburgitic garnets towards lherzolitic(including high-Ti high-Cr G11-type) garnets and megacrysts result from either voluminous high-temperature metasomatism by plume-derived silicate melts or recurrent mobilization of less voluminous kimberlitic or related carbonated mantle melts, rather than the initially primitive, fertile nature of the Proterozoic SCLM. Calculated Ni-in-garnet temperatures(primarily within ~1150–1250 ℃) indicate their derivation from at least ~220 km thick Cr-undersa

关键词： Siberian craton Kimberlite Diamond placers Peridotites Eclogites Megacrysts

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