THE EARLY ORDOVICIAN AGE OF DEFORMATIONS IN THE KOKCHETAV SUBDUCTION-COLLISION ZONE: NEW STRUCTURAL AND 40Ar/39Ar DATA
J. De Grave, M.M. Buslov*, F. Zhimulev*, P. Vermeesch**, M.O. McWilliams**, and J. Metcalf**
Ghent University, Ghent, Belgium
* Institute of Geology and Mineralogy, Siberian Branch of the RAS,
3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia
** Stanford University, Stanford, USA
Keywords: Kokchetav metamorphic belt, Berlyk Formation, granite-gneisses, eclogites, garnet amphibolites, phengite, geochronology
Pages: 441-450
Abstract
It has been shown by extensive mapping and sampling in the central Kokchetav subduction-collision zone that it is a terrane of a Vendian-Cambrian paleosubduction zone made up of tectonic sheets of Precambrian granite gneisses, Late Precambrian garnet-kyanite-sillimanite-biotite schists with boudins of garnet and garnet-plagioclase amphibolites, and Cambrian mylonitized granite gneisses with boudins of eclogites. The fault zones between tectonic sheets are formed by garnet-quartz-muscovite and quartz-muscovite schists. Along with the central Kokchetav belt, they are isoclinally overfolded southwestward. 40Ar/39Ar dating of muscovite from five mica-schist samples yielded close plateau and isochron ages of 480-485 Ma (Early Ordovician). The geological and geochronological data confirm the Early Ordovician formation of the nappe-imbricated structure of the Kokchetav subduction-collision
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