Geochemical data and zircon ages for rocks in a high-pressure belt of Chu-Yili Mountains, southern Kazakhstan: Implications for the earliest stages of accretion in Kazakhstan and the Tianshan

摘要

The mechanism and age of Palaeozoic accretion in the Central Asian Orogenic Belt remain poorly constrained. One of the most complex belts extends from the Kokchetav area in northern Kazakhstan to the Kyrgyz northern Tianshan. It represents an assemblage of small blocks with Palaeoproterozoic continental crust, intervening slivers containing early Palaeozoic ophiolites and/or deep-marine sediments, and a number of HP and UHP metamorphic complexes. The HP-LT metamorphic rocks provide important clues for reconstructions of the overall structure and evolution of the accretionary collage.This study is aimed to constrain the metamorphic age and tectonic implications of HP garnet pyroxenites and enclosing mica schists in the Anrakhai area of the Chu-Yili Mountains of southern Kazakhstan. The HP belt is located in the central part of the Anrakhai metamorphic block and extends NW-SE between Neoproterozoic gneisses in the SW and undated ophiolites and granodiorites in the NE. Garnet pyroxenites and their retrograded equivalents form tectonic boudins and blocks in garnet-muscovite and muscovite schist of sedimentary origin. Metamorphic zircons from two garnet pyroxenite samples yielded a SHRIMP mean ~(206)Pb/~(238)U age of 489.9 ± 3.1 Ma. This age is interpreted to reflect the time of Early Ordovician peak metamorphism and ongoing subduction in the area as constrained by geological data and suggests that HP metamorphism was related to subduction. Exhumation of the HP rocks occurred between 490 and ~475 Ma, based on the Early Ordovician age of overlap assemblages. Detrital zircons from a garnet-muscovite schist enclosing the pyroxenites were dated by LA-ICP-MS and range in age from 694 to 2557 Ma. They suggest a maximum late Neoproterozoic age for deposition of the sedimentary protolith and derivation from a continental source including Neoproterozoic to Archaean crustal components. Granodiorites with chemical characteristics of island arc rocks intruded into Proterozoic gneisses in the NE of the HP belt, and magmatic zircons from one sample yielded a SHRIMP ~(206)Pb/~(238)U mean age of 508.3 ± 3 Ma. This signifies the existence of a Cambrian magmatic arc. The early Palaeozoic age of the HP garnet pyroxenites indicates that the Anrakhai block is not part of a extensive Precambrian microcon-tinent, as previously thought, but represents a package of tectonically interlayered slivers, made up of Precambrian basement and early Palaeozoic rocks. Stacking of these heterogeneous domains may be due to subduction of continental crust, mutual underthrusting of continental and ophiolitic rocks, wedge extrusion of HP rocks, and strike-slip deformation in a subduction and/or collision setting. The Anrakhai collision zone is part of larger accretionary belt which formed by the end of the Early Ordovician and may extend from the Kyrgyz northern Tianshan to the Kokchetav area in northern Kazakhstan.