POSTCOLLISIONAL MAGMATISM IN THE SOUTH TIEN SHAN VARISCAN OROGENIC BELT, KYRGYZSTAN: EVIDENCE FOR HIGH-TEMPERATURE AND HIGH-PRESSURE COLLISION

Abstract

The South Tien Shan Collisional belt consists of two segments divided by the largest in the Central Asia Talas-Fergan diagonal dextral strike-slip fault. The narrow eastern (Kokshal) segment, where the Late Carboniferous collision was followed by continental subduction of the Precambrian Tarim platform beneath the Caledonian Kazakh continent, is characterized by overthickened (60–65km) crust and ultrahigh-pressure (coesite eclogite) regional metamorphism. The Permian postcollisional magmatism is represented there by a small volume of relatively ‘cool’ two-mica peraluminous leucogranites, moderate volumes of K-rich calk-alkaline (transitional from I- to S-type) granites and several large plutons of A-type rapakivi granites. The latest could have formed by a two-step process involving: (1) syncollisional submersion of the Tarim platform granulitic basement into the mantle accompanied by considerable heating of the source rocks, (2) ‘rapid’ postcollisional exhumation accompanied by decompression and extensive high-degree of melting. In the western (Alay) segment, which is much wider, the collision was not followed by continental subduction. The thickness of crust decreases westward dramatically (from 60 to 45km). The Alay segment is characterized by the high-temperature/low-pressure regional metamorphism and significant volume of Permian shoshonitic/ultrapotassic magmatism, connected with postcollisional strike-slip tectonics. The shoshonitic magmatism is indicative of lithosphere delamination and input of mantle-derived melts and heat into the crust. In contrast to the Kokshal segment, the postcollisional granitic magmatism in the Alay segment is represented by relatively ‘hot’ cordierite- and sillimanite-bearing strongly peraluminous (S-type) granites, locally rooted in the high-temperature/low-pressure metamorphic rocks and large volume of calc-alkaline (I-type) granitoides of mixed (crust-mantle) origin.