Age, composition, and tectonic significance of Palaeozoic granites in the Altyn orogenic belt, China

ABSTRACTWe 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 404 Ma (north Altyn block (NAB)), 522 and 432 Ma (central Altyn block (CAB)), and 483 and 226 Ma (south Altyn block (SAB)). We propose a geodynamic model for the Altyn orogenic belt. Prior to 467 Ma, 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 467–450 Ma. The collision lasted until 430 Ma and was followed by post-collisional extension until 400 Ma. Farther south, the central Altyn Ocean subducted northwards beneath the CAB, resulting in 520–500 Ma 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 490–455 Ma. The SAMB–CAB collision gave way to post-collisional extension, slab break-off, and delamination during 450–430 Ma. Bidirectional subduction of the south Altyn Ocean after ~485 Ma, dominated by southward subduction, continued until 456 Ma. Closure of the south Altyn Ocean led to continent–continent collision between the SAMB and the Qaidam block at 450–430 Ma, followed after ~410 Ma by repeated break-offs of slab segments until 337 Ma. Subsequent intrusions formed at 264–226 Ma in a setting of either post-collisional extension or sinistral strike-slip.

摘要 我们针对青藏高原北缘阿尔金造山带内的花岗岩岩体开展了岩石学、岩石地球化学以及地质年代学研究，旨在揭示该造山带的地球动力学演化历史。这些花岗岩属于过铝质岩石，球粒陨石标准化稀土元素配分模式存在一定差异，但微量元素组成整体相似；所有样品均亏损Ba、Nb、Sr、P和Ti，且富集Rb、Th、Ta、Zr和Hf。其锆石U-Pb年龄记录了多期岩浆事件：北阿尔金地块（NAB）对应的年龄区间为500~404 Ma，中阿尔金地块（CAB）为522~432 Ma，南阿尔金地块（SAB）为483~226 Ma。我们为阿尔金造山带构建了一套地球动力学演化模型：在467 Ma之前，北阿尔金洋在北阿尔金地块下方向南俯冲；467~450 Ma期间，北阿尔金地区经历了从俯冲作用到敦煌地块与北阿尔金地块碰撞的构造转换；该碰撞过程持续至430 Ma，之后进入碰撞后伸展阶段，直至400 Ma。在更靠南的区域，中阿尔金洋在中阿尔金地块下方向北俯冲，引发了520~500 Ma的花岗岩岩浆作用，并导致南阿尔金微地块（SAMB）与中阿尔金地块发生碰撞；南阿尔金微地壳在中阿尔金地块下方发生深俯冲，进而在490~455 Ma形成了花岗岩侵入体。450~430 Ma期间，南阿尔金微地块与中阿尔金地块的碰撞后过渡为碰撞后伸展、板片断离及拆沉作用。约485 Ma之后，南阿尔金洋以向南俯冲为主导的双向俯冲作用持续至456 Ma；南阿尔金洋闭合后，南阿尔金微地块与柴达木地块在450~430 Ma发生陆陆碰撞，约410 Ma之后，板片片段持续发生断离，直至337 Ma；264~226 Ma形成的后期侵入体，其形成背景为碰撞后伸展或左旋走滑构造环境。