PETROLOGICAL AND ISOTOPE-GEOCHEMICAL INDICATORS OF CONTINENTAL-MARGIN SUBDUCTION IN THE MT. SULU-TYUBE AREA MELANGE ( Kokchetav subduction–collision zone , Kazakhstan )

Metasedimentary melange in the Mt. Sulu-Tyube area bears fine- and medium-grained eclogites, which are predominant within the Kokchetav subduction–collision zone (Kazakhstan), along with scarce porphyroblastic eclogites juxtaposed with rocks of the Zerenda Group. The assemblage of garnet cores (XAlm ≤ 0.59, XPrp 0.08) with epidote–clinozoisite (XCzo = 0.36–0.90), amphibole, ilmenite, titanite, and rutile and the prograde zoning of garnet suggest the nearly isothermal burial of rocks at 11.5–20.5 kbar and 535–570 °С. The growth of high-Mg garnet rims (XPrp ≤ 0.31) in paragenesis with omphacite (XJd ≤ 0.35) marks the second stage with nearly isobaric heating up to 645 °С at 21.5 kbar and to 620–690 °С for some samples. Fine- and medium-grained eclogites with uniform structures and with weak zoning of garnet containing omphacite (XJd = 0.20–0.40), rutile, and quartz inclusions formed during heating and limited pressure increase This article presents new data on ore mineral microinclusions and the ore element content of Kamchatka adakites associated with melting of oceanic lithosphere of varying ages in the transition zone from the Pacific Ocean to the Asian continent. Based on these data, hypotheses are made regarding the generation patterns of metalliferous adakites and their role in the formation of copper-gold-silver mineralization in the region. (580–660 °С and 16–19 kbar). Variations in equilibrium temperatures (620–730 °С) indicate their different positions within the subducted lithosphere. The eclogites are similar in signatures to MORB and correspond to low-Ti tholeiitic rocks, but the fine-grained rocks are depleted in Nb, Ta, and Eu and enriched in Th and are characterized by a radiogenic isotopic composition of Sr (87Sr/86SrI = 0.71181–0.72935) and moderately depleted Nd isotopes (εNd(530) = 0.5–4.2), whereas porphyroblastic rocks rehydrated during exhumation show juvenile Nd–Sr signatures (87Sr/86SrI = 0.70212–0.70426, εNd(530) = 6.6–7.2). The contrasting features of eclogites result from the subduction of the rift-related margin of the Kokchetav microcontinent or a more ancient continental structure including variably contaminated N-MORB- and E-MORB-type basites and from the involvement of oceanic or eroded Cambrian island arc rocks in the same process. The differences in the fluid regimes of subduction and exhumation were due to the proximity of a hydrated subduction channel for porphyroblastic eclogites and to the weak fluid permeability of continental lithosphere for fine- and mediumgrained eclogites.