西天山巴仑台地区晚石炭世岩浆岩的岩石成因及其构造背景

Petrogenesis and Tectonic Setting of the Late Carboniferous Igneous Rocks in the Baluntai Region of the Chinese Western Tianshan

位于中亚造山带西段和塔里木克拉通之间的天山造山带的古生代构造演化历史目前还存在很大争议,其广泛发育的古生代岩浆岩则是揭示俯冲增生过程和构造体制转换的重要岩石探针.本文对我国西天山巴仑台地区的7个古生代岩浆岩进行了系统的年代学和地球化学研究.LA-ICP-MS锆石U-Pb定年限定它们的结晶年龄在319~307 Ma之间,均形成于晚石炭世.地球化学特征显示晚石炭世的镁铁质岩浆岩主要起源于软流圈地幔或者受俯冲交代富集的岩石圈地幔;而同期花岗质岩石总体上均属于准铝质-弱过铝质的中钾钙碱性和高钾钙碱性I型花岗岩,主要起源于下地壳基性岩的部分熔融.根据西天山地区古生代岩浆岩的时空分布规律及变质岩、蛇绿岩和沉积岩的研究成果,本文提出320~310 Ma的岩浆岩形成于板片断离的构造背景,标志着由大陆碰撞向后碰撞的构造体制转换;而310~307 Ma的岩浆岩形成于后碰撞伸展的构造背景.

The Paleozoic tectonic evolution of the Tianshan orogenic belt, which is situated between the western Central Asian Orogenic Belt and the Tarim Craton, is still in heated debate. The widespread Paleozoic igneous rocks in the Tianshan orogen are a powerful tool to reveal the subduction and accretion processes as well as the tectonic switching. In this paper,detailed geochronological and geochemical studies were presented for seven suites of Paleozoic igneous rocks in the Baluntai domain of the Chinese western Tianshan. Zircon LA-ICP-MS U-Pb dating reveals that their crystallization ages are in the range of ca. 319-307 Ma, i.e., Late Carboniferous. Geochemical characteristics indicate that the Late Carboniferous mafic igneous rocks were mainly derived from asthenosphere mantle or subduction-modified lithospheric mantle;while the coeval granitoid rocks are generally metaluminous to weak peraluminous medium-and high-K calc-alkaline I-type granites that were mainly originated from partial melting of the lower crustal meta-basic rocks. In combination with the tempo-spatial distribution of the Paleozoic igneous rocks and published results of the metamorphic, ophiolitic and sedimentary rocks in the western Tianshan, we propose that the ca. 320-310 Ma igneous rocks were generated during slab breakoff, marking the tectonic transition from continental collision to post-collision settings. By inference, the subsequent 310-307 Ma magmatism was formed in a post-collisional extensional setting caused by lithospheric delamination.