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Russian Geology and Geophysics

2005 year, number 8


N.L. Dobretsov, M.M. Buslov, and F.I. Zhimulev
Institute of Geology, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
Keywords: Subduction, collision, diamond- and coesite-bearing gneisses, exhumation, olistostromes, overthrust, nappe, Kokchetav microcontinent
Pages: 785-795


The Cambrian-Ordovician tectonic evolution of the Kokchetav metamorphic belt containing UHP-HP rocks proceeded through several stages. The subduction of the Paleoasian Ocean lithosphere, containing blocks of continental crust, and collision of the Kokchetav microcontinent with island arcs are responsible for the formation and exhumation of UHP-HP rocks. Initially, in the Early Cambrian (535-523 Ma), the microcontinent was subsided into the subduction zone to depths of 150-200 km, which led to metamorphism as well as to partial melting of rocks. In next substage (523-513 Ma), the produced acid melts including blocks of UHP-HP rocks were exhumated quickly, at a velocity of up to 100-10 cm/year, to depths of 90 km, which permitted the preservation of the high-pressure associations. Then, the UHP-HP rocks were exhumated at a velocity of 0.5 cm/year along faulting zones of the accretionary wedge to depths of 30 km. In the Middle and Late Cambrian (513-480 Ma), the continuing subduction of the Kokchetav microcontinent led to the outwedging of the subduction zone, extrusion of UHP-HP rocks along retro-overthrust zones, heaping and overthrust in the accretionary prism. The formation of a new zone of subduction and the Late Arenigian-Early Caradocian collisional processes (480-450 Ma) led to the thrusting of allochthones, made up of the rocks of the Kokchetav microcontinent, and accretionary prism over the fore-arc trough of the Stepnyak island arc. The formation of an imbricated structure was completed with granitoid magmatism and accumulation of molasses in the Late Ordovician.