Petrotectonic Evolution of the Maksyutov Complex,Southern Urals,Russia: Implications for Ultrahigh-Pressure Metamorphism

The Maksyutov Complex consists of two juxtaposed lithotectonic units—Unit #1 of probable Late Proterozoic formation age, and Unit #2, apparently generated in Cambro-Ordovician time. The eclogite-facies metamorphism of Unit #1 occurred prior to 370-380 Ma, when this unit was subjected to blueschist-facies overprinting. Unit #2 displays the effects of a somewhat similar blueschist- or high-pressure greenschist-facies recrystallization, indicating that it may have been metamorphosed contemporaneously with Unit #1. Our field work and geochemical studies have focused on the Sakmara River area. Preliminary conclusions are as follows: (1) Unit #1 was subjected to metamorphic temperatures of 620 ± 70° C and minimum pressures of 1.5 GPa, or 2.7 GPa if the previously reported interpretation of coesite pseudomorphs from similar rocks exposed near the village of Shubino, 75 km to the south (Chesnokov and Popov, 1965), is correct. Peak metamorphic pressures would have reached at least 3.2 GPa if blocky graphite described in this report from a Sakmara River eclogitic mica schist has replaced neoblastic diamond; (2) Unit #2 experienced much lower maximum metamorphic pressures, on the order of 0.5 to 0.6 GPa; (3) Unit #2 was variably but intensely metasomatized, indicating the presence of an aqueous fluid during the Early Devonian blueschist/greenschist-facies metamorphism; (4) tectonic parallelism of the lithostratigraphic units and their bounding sutures, combined with P-T conditions of recrystallization, suggest assembly of the Maksyutov Complex in an intra-oceanic subduction zone. This process was followed by exhumation and suturing against the more easterly Middle Paleozoic unmetamorphosed ophiolitic (oceanic) basement and superjacent calc-alkaline Magnitogorsk island arc. The Late Proterozoic-Ordovician Mugodzhar and Ilmen microcontinents subsequently were thrust beneath the eastern edge of the Devonian Magnitogorsk Arc. Collision of the entire complex with the Ordovician-Lower Carboniferous continentalmargin Suvanjak-Sakmara accretionary complex, lying to the west on the Russian Platform, also occurred during Middle Paleozoic time. Finally, (5), the tectonic imbrication of the several units within and adjoining the Maksyutov Complex was itself truncated and deformed into N-S parallelism by postulated Late Paleozoic postcollisional strike-slip movement (Dobretsov et al., in review).