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UHP-05 General topics of geology, tectonics and petrology of collisional orogenic belts: Implication to UHP-HP metamorphic rocks
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Tectonic and exhumation structure of the Kokchetav HP - UHP metamorphic belt (Northern Kazakhstan): Constraints from 40Ar/39Ar geochronology
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Fedor Zhimulev, Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences (Russian Federation)
Aleksey Travin, Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences (Russian Federation)
Johan De Grave, Ghent University (Belgium)
Mikhail Buslov, Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences (Russian Federation)
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The Kokchetav HP - UHP metamorphic belt (Northern Kazakhstan) consists of gneisses and eclogites that were formed by Cambrian continental subduction and associated metamorphism of the Precambrian Kokchetav microcontinent and subsequent exhumation of some fragments of this metamorphosed continental crust. 40Ar/39Ar dating of mica from fault zones that delineate the metamorphic belt and that divide its different composing tectonic nappes was performed. All 40Ar/39Ar stepwise heating experiments yield well-defined plateau and isochron ages.
To the north, the Kokchetav HP - UHP metamorphic belt is bounded by the Northern Kokchetav tectonic zone. This zone includes thin nappes of (1) Paleo-Mesoproterozoic gneisses of the metamorphic basement of the Kokchetav microcontinent and the Neoproterozoic meta-sandstones and dolomites of its deformed cover, (2) pre-Ordovician island-arcs volcanic rocks, (3) Early Ordovician turbidities with olistostrome lenses, and (4) gneisses with eclogite boudins. These nappes are tectonically juxtaposed with unmetamorphosed rocks. The fault zones between the different tectonic units are formed by quart-muscovite schists. The 40Ar/39Ar ages of the muscovite from these schists cluster between 492 - 476 Ma (mainly about 490 Ma). The described Northern Kokchetav tectonic zone was formed by the Early Ordovician collision between the Kokchetav microcontinent, including the Kokchetav metamorphic belt, and the Stepnyak island-arc.
The Kokchetav metamorphic belt itself is composed of folded nappes consisting of gneisses with eclogite boudins and garnet-biotite-kyanite schists. The fault zones between the nappes here are formed by quartz-garnet-muscovite schists. The 40Ar/39Ar ages of the muscovite from these fault zones lies in the range between 490 and 475 Ma (mainly 480-485 Ma). The southern boundary of the Kokchetav metamorphic belt is marked by the LP biotite-andalusite-cordierite schists of the Daulet formation. The 40Ar/39Ar ages of the biotite from these schists are 514 Ma in the western part of the Kokchetav metamorphic belt, and 496 Ma in the eastern part.
Therefore Late Cambrian - Early Ordovican deformation is revealed everywhere in the Kokchetav region. The Late Cambrian - Early Ordovician tectonic events that were dated in this study led to the juxtaposition of heterogeneous nappes, including nappes that consist of HP - UHP metamorphic rocks, to the upper crustal level.
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