摘要

We examined the petrography, litho-geochemistry, and geochronology of granite plutons in the Altyn orogenic belt, northern margin of the Qinghai-Tibet Plateau, to investigate the geodynamic history of the belt. The granites are peraluminous, with variable chondrite-normalized rare earth element patterns but generally similar trace element compositions; all are depleted in Ba, Nb, Sr, P, and Ti, and enriched in Rb, Th, Ta, Zr, and Hf. Their U-Pb zircon ages record multiple magmatic episodes between 500 and 404Ma (north Altyn block (NAB)), 522 and 432Ma (central Altyn block (CAB)), and 483 and 226Ma (south Altyn block (SAB)). We propose a geodynamic model for the Altyn orogenic belt. Prior to 467Ma, the north Altyn Ocean subducted southward below the NAB. The north Altyn region saw a change from subduction to collision between the Dunhuang block and the NAB during 467-450Ma. The collision lasted until 430Ma and was followed by post-collisional extension until 400Ma. Farther south, the central Altyn Ocean subducted northwards beneath the CAB, resulting in 520-500Ma granitic magmatism and leading to collision of the south Altyn micro-block (SAMB) with the CAB. Deep subduction of SAMB crust below the CAB resulted in granite intrusions during 490-455Ma. The SAMB-CAB collision gave way to post-collisional extension, slab break-off, and delamination during 450-430Ma. Bidirectional subduction of the south Altyn Ocean after similar to 485Ma, dominated by southward subduction, continued until 456Ma. Closure of the south Altyn Ocean led to continent-continent collision between the SAMB and the Qaidam block at 450-430Ma, followed after similar to 410Ma by repeated break-offs of slab segments until 337Ma. Subsequent intrusions formed at 264-226Ma in a setting of either post-collisional extension or sinistral strike-slip.