PETROLOGICAL AND ISOTOPE-GEOCHEMICAL INDICATORS OF CONTINENTAL MARGIN SUBDUCTION IN THE MOUNT SULU-TYUBE AREA MELANGE (KOKCHETAV SUBDUCTION-COLLISION ZONE, KAZAKHSTAN)
S.Yu. Skuzovatov1, V.S. Shatsky1-2, A.L. Ragozin2
1Vinogradov Institute of Geochemistry, Russian Academy of Sciences, Siberian Branch, Irkutsk, Russia
2Sobolev Institute of Geology and Mineralogy, Russian Academy of Sciences, Siberian Branch, Novosibirsk, Russia
Keywords: Central Asian Orogenic belt, Kokchetav massif, subduction, collision, high-pressure metamorphism, eclogites, trace elements, Nd–Sr isotopes, P–T–trends
Abstract
Among fine- and medium-grained eclogites, which are predominant within the Kokchetav subduction-collision zone (Kazakhstan), metasedimentary mélange in the Sulu-Tyube area bears rare porphyroblastic eclogites, which are juxtaposed with host rocks of the Zerenda series. The assemblage of garnet cores граната (XAlm
up to 0.59, XPrp from 0.08) with epidote–clinozoisite (XCzo=0.36–0.90), amphibole, ilmenite, titanite and rutile, prograde garnet zonation imply nearly isothermal burial of rocks down to 11.5–20.5 kbar and 535–570°С. The growth of high-Mg garnet rims (XPrp up to 0.31) in paragenetic relations with omphacite (XJd up to 0.35) stem the second stage with nearly isobaric heating up to 645°С at 21.5 kbar, which, for individual samples, reach 620–690°С. Fine- and medium-grained eclogites with uniform textures and weakly zones garnet, which contains inclusions of omphacite (XJd=0.20÷0.40), rutile and quartz, were formed during heating and limited pressure increase (580–660°С and 16–19 kbar). Variations of equilibrium temperatures (620–730°С) indicate their different position within the structure of subducted lithosphere. MORB-like signatures of eclogites correspond to those of low-Ti tholeitic rocks, but fine-grained rocks are depleted in Nb-Ta and Eu and enriched with Th at radiogenic Sr isotope composition Sr (87Sr/86SrI 0.71181–0.72935) and moderately depleted Nd isotopes (εNd(530) +0.5÷+4.2), whereas porphyroblastic rocks, rehydrated during exhumation, reveal juvenile Nd-Sr signatures (87Sr/86SrI 0.70212–0.70426, εNd(530) +6.6÷+7.2). Contrasting features of eclogites result from subduction burial of rifted margin of the Kokchetav microcontinent or more ancient continental unit, which included variably contaminated N-MORB an E-MORB basites, and involvement of oceanic or eroded Cambrian island-arc rocks into the same process. Principal differences in fluid regimes of subduction and exhumation were governed by proximity of hydrated subduction channel for porphyroblastic rocks and weak fluid permeability of continental lithosphere – for their fine- and medium-grained counterparts.
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