Introduction

Abstract

The monograph describes the evolution of granitoid magmatism at convergent continental margins during transition from subduction to continental collision in the Hercynian belt of western Tien Shan. The objects of the study comprised Hercynian granitoids of western Tien Shan. Postcollisional granitoids were studied in the Kyzylkum, Gissar, Alai and Kokshaal Segments of the South Tien Shan, which formed at a former passive margin of the Turkestan Ocean. Supra-subduction granitoids and the transition from subduction-related to postcollisional magmatism were studied in Sultan-Uvais mountains and in the Chatkal-Kurama terrane of the Middle Tien Shan that formed at the northern active margin of the Turkestan Ocean. Introductory sections provide a review of current understanding of geological structure and history of western Tien Shan and spatial distribution and characteristic features of the Paleozoic granitoid series. Because the main volume of granitoid series of western Tien Shan is represented by postcollisional magmatic series, models for postcollisional magmatism are specifically described in these sections. Principles of isotope mapping and interpretation of genesis of granitoid series utilizing Nd-Sr-Pb-Hf isotopic data, which were applied in this work, are described in the introductory sections. Postcollisional granites of the Kokshaal Segment of South Tien Shan comprise about 20 postkinematic intrusions composed of biotite-amphibole granites, biotite granites and Li-F topaz-bearing leucogranites. The granites were emplaced coevally with tholeiitic mafic rocks and alkaline syenites. Geochemically the granites are classified as A-type and characterized by elevated Fe/(Fe+Mg) and K2O/Na2O values and high concentrations of Na2O+K2O, Rb, HFSE. On the discrimination diagrams Y-Nb and Rb-(Y+Nb) compositions of the granites plot into the field of intra-plate granites. On a regional scale, the compositional variations of the Kokshaal granites can be explained by fractionation of potassium feldspar and amphibole. The granites were probably derived from the crustal protoloiths represented by Precambrian metamorphic rocks of the Tarim microcontinent. Crystallization ages of the Kokshaal granites, established utilizing U-Pb zircon dating, fall in the relatively narrow range between 280 and 295 Ma corresponding to the early Permian, which is in agreement with ages of postcollisional granites elsewhere in Tien Shan. Genesis of granites was related to trans-crustal shear zones. The early Permian intrusions of the Alai segment of the South Tien Shan comprise four geochemically contrasting intrusive series, including (1) I-type and (2) shoshonitic granitoids, (3) peraluminous granitoids including S-type leucogranites and (4) alkaline rocks and carbonatites, closely associated in space and time. New geochronological data indicate that diverse magmatic series of the Alai segment formed virtually coevally in a post-collisional setting. Five U-Pb zircon rock ages in the range 287 – 281 Ma, in combination with published ages, define the main post-collisional magmatic pulse at 290 - 280, which is similar to ages of post-collisional intrusions elsewhere in the South Tien Shan. The Sr-Nd-Pb-Hf isotopic compositions of the studied intrusions are consistent with the reworking of crustal material with 1.6 – 1.1 Ga average crustal residence times indicating the formation of the Alai segment of the South Tien Shan on a continental basement with Mesoproterozoic or older crust. It is suggested that position of the Alai microcontinent between two major sutures enabled delamination of its lithospheric mantle, which triggered the diverse post-collisional magmatism as a result of interaction of ascending asthenospheric material with lithospheric mantle and various crustal protoliths. The Paleozoic evolution of the Southern and Middle Tien Shan terranes in the territory of Uzbekistan is generally associated with the history of two ocean basins - the Turkestan and Paleotethys. Ages of ophiolites indicate the opening of the oceans in Cambrian – Ordovician, and partial closure with formation of an island arc in the northern part of the basin in Ordovician - Silurian. At the northern margin of the Turkestan ocean, the northward subduction under the Middle Tien Shan continued until Devonian, which led to formation of an active margin with granitoids emplaced between 429 and 416 Ma. In the late Devonian, subduction-related magmtism terminated and the whole region developed as passive margin. Northward subduction resumed in the early Carboniferous and formed magmatic Andean-type belt exposed in the Chatkal-Kurama terrane. Late Carboniferous collision resulted in crust thickening and emplacement of postcollisional granitoids. Formation of postcollisional intrusions in different terranes took place in various tectonic settings. Shoshonitic granitoids of the Chatkal-Kurama terrane formed as a result of slab break off at postcollisional stage. Voluminous postcollisional magmatism of Kyzylkum can be explained by delamination of lower crust and its replacement by the material of astenospheric mantle. Coeval emplacement of geochemically contrasting granitoids in the North Nuratau fault zone could result from contemporaneous melting of different protoliths at different depths in a translithospheric shear zone. Thus, in western Tien Shan there established 4 stages of Paleozoic magmatism related to the evolution of the Turkestan paleo-ocean: 1) plagiogranites of ophiolites with ages 506 – 438 Ma; 2) supra-subduction granitoids with ages in the ranges 429 – 414 and 320 – 300 Ma; 3) postcollisional granitoids with ages 295 – 280 Ma. Magmatic series of former active margins, exposed in the Chatkal-Kurama, Gissar and Sultan-Uvais, are represented by supra-subduction granitoids of calc-alkaline series (I-type granites) and by postcollisional shoshonitic granitoids. Postcollisional magmatism at former passive margins was controlled by the composition of the basement: homogeneous A-type granites predominate in the Kokshaal Segment, built up on Precambrian basement, while varied I- S- and А-type granites formed in the Alai and Kyzylkum Segments with more heterogeneous basements. Most of the granitoids of western Tien Shan were derived from Neoproterozoic or older continental crust. Granitoids derived from juvenile island arc crust were established only in northern Kyzylkum and Sultan-Uvais. Postcollisional granotoids formed in various geodynamic settings. Shoshonitic series of the Chatkal-Kurama were produced from enriched mantle wedge as a result of slab break-off. Origin and emplacement of intrusions in the Nuratau mountains and in the Kokshaal Segment was controlled by uplift of asthenospheric material in regional shear zones. Heterogeneous basement structures of the Kyzylkum and Alai Segments enabled delamination of the lower crust and its replacement by asthenospheric material.