The Early Paleozoic intrusive magmatism and tectonic thermal evolution in the Hala Lake area, southern Qilian, NW China

Abstract

南祁连增生杂岩带作为祁连造山带的构造单元之一，是研究祁连造山带与柴达木地块构造演化及二者耦合关系的关键地区，得到了国内外学者的广泛关注。前人对南祁连增生杂岩早古生代以来的构造热演化历史研究相对较少，且缺少相对准确的年代学数据约束。本文通过对南祁连增生杂岩带哈拉湖地区阿腊郭勒岩体二长花岗岩开展LA-ICP-MS锆石U-Pb定年、岩石地球化学特征、锆石和磷灰石裂变径迹年龄测试及热历史模拟，并结合野外地质调查和构造演化特征，揭示南祁连增生杂岩带哈拉湖地区的构造热演化历史和山脉隆升过程。结果显示：（1）南祁连哈拉湖地区在中志留世发生一期岩浆侵入事件（425~429Ma），其形成的岩体具有壳源花岗岩特征，产出于同碰撞的构造环境，说明该期岩浆事件是与祁连洋洋壳俯冲结束后的柴北缘地区大陆碰撞过程中的岩浆活动有关；（2）该岩体经历了中志留世至晚泥盆世的岩浆侵位与快速冷却阶段以及晚泥盆世-侏罗纪的构造平稳与缓慢冷却阶段；（3）早白垩世以来的中低温冷却和快速隆升的构造热演化历史。此外，祁连山地区自始新世以来经历了多期与印度欧亚-板块碰撞有关的构造变形。;As an important tectonic unit of the Qilian orogenic belt, the southern Qilian accretionary belt plays a significant role in studying the tectonic evolution and their coupling relationship between the Qilian orogenic belt and the Qaidam block. Although wide attention has been paid to this belt by geologists all over the world, the tectono-thermal evolution of the southern Qilian accretionary belt since Early Paleozoic is still unclear due to the lack of relatively precise geochronological data constraints. In this paper, we integrated field obervations, LA-ICP-MS zircon U-Pb age, geochemical characteristics, low-temperature themochronometry (apatite- and zircon-fission track ages) and thermal history modeling to provide constraints on the tectono-thermal history and range growth of the Hala Lake area in the southern Qilian accretionary belt. The results suggest that the southern Qilian accretionary belt experienced a multi-phase tectono-thermal evolution history, including:(1) Early Paleozoic intrusive magmatism and rapid cooling due to the Qilian orogeny from Middle Silurian (425~429Ma) to Late Devonian, with continental crust resource characters and generating from syn-collisional tectonic setting of the monzonitic granites, which indicates that this period of magmatism was related to the continental's collision of southern Qilian accretionary belt and the Qaidam block after the ocean crust subduction of Qilian Ocean; (2) tectonic quiescence and slow cooling during Late Devonian to Jurassic; (3) mid-low temperature cooling and rapid uplift since Early Cretaceous. Furthermore, the Cenozoic Qilian Shan thrust belt also experienced multi-phase structural deformations since Eocene as a result of the India-Asia collision.