Paleozoic tectonism and magmatism of Kekesu-Qiongkushitai section in southwestern Chinese Tianshan and their constraints on the age of the orogeny
Abstract
The Tianshan is a polyphase collision-accretion orogenic belt, its ages of orogeny is an issue in hot debate. The Kekesu-Qiongkushitai section of southwestern Chinese Tianshan experienced complicated deformations including mainly top-to-the-north thrust and NE-SW extending ductile strike-slip faulting. This section can be subdivided into three lithotectonic units, namely Yili magmatic arc, Yili Proterozoic basement and HP metamorphic complex. In the field, granodiorite of Yili are intrudes in greenschist facies metamorphic rocks cutting foliations of greenschists. Zircon U-Pb LA-ICPMS dating yields 341 +/- 6 Ma and 338 +/- 8 Ma for K-granite from Kekesu River, and 313 +/- 4 Ma for biotite-bearing granodiorite from Qiongkushitai, respectively. Biotite from granodiorite yields a Ar-40/Ar-39 plateau age at 263.4 +/- 0.6Ma, revealing an important thermal overprint in Permian. Geochemistry of granodiorite indicates that it belongs to calc-alkaline series, with significant low compositions in Nb and Ta, and enrichment in Rb, Ba and Th. These features are consitent with the geochemical components of are-type magmatic rocks. Taking into account the previous results on HP metamorphic and magmatic rocks in west Tianshan areas, this paper proposes that the subduction-collision events in southwestern Chinese Tianshan occurred before Late Carboniferous, post-collisional ductile strike-slip faulting confirms that the collisional orogenic processes ended in Permian for the southwestern Chinese Tianshan belts.