Geological development and Phanerozoic crustal accretion in the western segment of the southern Tien Shan (Kyrgyzstan, Uzbekistan and Tajikistan)

The Tien Shan form a high intracontinental mountain belt, lying north of th e main India-Asia collision mountains, and consist of re-activated Paleozoi c orogens. The western segment of the southern Tien Shan lies northwest of the Pamir and west of the Talas-Fergana fault. The stratigraphy, lithology, igneous and metamorphic petrology and geochemistry of this segment indicat e that it was formed by the assembly of Lower Paleozoic area which develope d into microcontinents with Upper Paleozoic mature shelf and slope elastic and carbonate sediments. Precambrian continental crust is confined to two s mall blocks along its southern margin. The bulk of the southern Tien Shan consists of ?Vendian to Silurian oceanic and slope elastic rocks, resting on oceanic lithosphere, and overlain by t hick passive margin Devonian to mid-Carboniferous mature shelf clastics and carbonates. These are unconformably overlain by syn- and post-orogenic imm ature elastic sediments derived from mountains on the north formed by closu re of a Calboniferus southern Tajik and a northern Vendian to Carboniferous Turkestan ocean with the southern Tien Shan microcontinent sandwiched betw een. Associated with these collisions an late Carboniferous to Permian intr usives, which form three south to north (though overlapping) suites; a sout hern calc-alkaline granodiorite-granite suite, an intermediate gabbro-monzo diorite-granite suite, and a northern alkaline monzodiorite-granite-alaskit e suite. The gabbro-monzodiorite-granite suite forms the earliest subductio n-related magmatism of the southern Tien Shan: rare earth element patterns are consistent with derivation from a primitive or slightly enriched mantle . The other suites show more crustal contamination. Rb and Sr vary with dep th and degree of partial melting and are consistent with progressive involv ement of crustal material in partial melts during collision. The gradual ch ange in composition within each complex, lasting in some cases from 295 to 250 Ma (the entire Permian), may be explained by a consecutive shift in the melting sedimentary cover of the subducting plate from oceanic crust throu gh transitional crust to marginal continental crust. Like the Central Asian orogenic belt (the main focus of IGCP 420), the Tien Shan represent a net addition of continental crust during the Phanerozoic. Very little of the be lt has any Precambrian precursor. (C) 2000 Elsevier Science B.V. All rights reserved.