Hercynian post-collisional A-type granites of the Kokshaal Range, Southern Tien Shan, Kyrgyzstan

The Tien Shan (Tianshan) orogen formed during Late Palaeozoic collision between the Karakum-Tarim continent and the Paleo-Kazakhstan continent, a Caledonian component of the Altaid Collage. The Southern Tien Shan terrane represents an intensely deformed fold and thrust belt formed after the final closure of the Paleo-Turkestan ocean. In the Late Carboniferous-Early Permian the Tien Shan was affected by regionally developed diverse post-collisional magmatism. Post-collisional granites of the Kokshaal Segment (3 5 0 x 100 km), the easternmost segment of Southern Tien Shan in Kyrgyzstan, include some 20 post-tectonic intrusions varying in composition from biotite-hornblene rapakivi granite to biotite and topaz-bearing leucogranite. The granites are associated with coeval tholeiitic mafic rocks and with alkaline undersaturated syenites. The granites have a pronounced A-type affinity: they are metaluminous with high concentrations of Na2O + K2O, Rb, HFSE and high K2O/Na2O and Fe/(Fe + Mg) ratios, and comprise a single geochemical trend explained by fractionation of K-feldspar and homblende. The U-Pb zircon SIMS ages of the Djangart, Uch-Koshkon, Mudryum and Kok-Kiya intrusions of the Kokshaal are 296 4 Ma, 279 8 Ma, 281 2 Ma and 281 3 Ma, respectively. The oldest age was obtained for rapakivi granites in the eastern part of Kokshaal while the leucogranites in its western part were dated at 280 Ma. This time span closely corresponds to known ages of post-collisional granitoids and related gold deposits of the Tien Shan. The granites have negative initial epsilon:(Nd) values (- 1.6 to - 6.9 at 280 Ma) and depleted mantle model ages of 1.05-1.43 Ga indicating a mixing of asthenospheric mantle and Precambrian crustal components. Precambrian Mesoproterozoic crust comprises the basement of the Tarim microcontinent underthrusted to the north beneath the sedimentary piles of Kokshaal. Mafic rocks associated with the granites may represent juvenile asthenospheric melts underplating the Tarim margin at the post-collisional stage and providing heat for generation of A-type granites from its Precambrian crust. Lithospheric plate-scale strike-slip shear zones that formed in the region subsequent to collision provided suitable conduits for ascending asthenospheric material. On a regional scale, available Nd data for the Tien Shan show that most Paleozoic felsic rocks originated from Precambrian crustal sources. The Northern Tarim rocks, exposed in the Kokshaal Range, and rocks of the southern Paleo-Kazakhstan margin have similar Proterozoic depleted mantle model ages. (c) 2007 Elsevier B.V. All rights reserved.