东准噶尔晚古生代火山岩的成因机制及其对构造演化的约束

The northern Xinjiang region occupies a key part of the Central Asian gigantic accretionary Orogenic Belt and metallogenic domain, and represents one of the most important sites for studying orogenic tectonic evolution and crustal growth (vertical or lateral) during the late Paleozoic. The volcanic-sedimentary strata from early Devonian to early Permian are widely exposed in the eastern Junggar terrane. These late Paleozoic volcanic rocks may record important constraints on the subduction and closure evolution of the Paleo-Junggar Ocean and the processes of continental crustal formation. However, obvious less geochemical and geochronological research has been done on them, compared with the volcanic rocks in the western Junggar terrane. In this project, the late Paleozoic volcanic rocks from three typical volcanic-sedimentary sections including Zhaheba, Beitashan and Xidagou in the eastern Junggar terrane are selected as main objects. On the basis of detailed field investigation, and in-house petrography and mineral internal structure observation, the lithosfacies of volcanic rocks and interbedded sedimentary rocks are analyzed in detail. The ages of the volcanic rocks from different formations will be constrained by magmatic zircon U-Pb or fresh matrix 40Ar-39Ar dating methods. The systematic studies on mineral chemistry, whole-rock major-element, trace-element and Sr-Nd-Pb isotopes, and zircon Hf-O isotopes will also be carried out in order to clarify the petrogenesis and tectonic implication of late Paleozoic volcanic rocks. Combined with other regional works such as ophiolites, we will constrain the closure time and subduction direction of the Paleo-Junggar Ocean, and then discuss the tectonic entities connection between the eastern and western Junggar terrane, and finally unravel the basement nature of the Junggar basin and the growth mechanism of continental crust.

新疆北部是中亚巨型增生型造山带和成矿域的重要位置，是研究造山带晚古生代构造演化和地壳增生机制的重要地区之一。东准噶尔出露有从早泥盆世到早二叠世的火山-沉积地层，这套完整的晚古生代火山岩可能记录着古洋盆俯冲闭合演化和大陆地壳形成的重要信息，但其研究程度明显较西准噶尔火山岩低。项目拟重点选择东准噶尔扎河坝、北塔山和西大沟3条晚古生代火山-沉积地层出露完整的典型剖面，在对互层沉积岩系统的岩相分析基础上，重点对火山岩进行详细的野外地质调查、岩相学和矿物内部结构观察，利用岩浆锆石U-Pb和新鲜基质40Ar-39Ar法精确限定各组火山岩的形成时代；通过矿物化学、全岩主量微量元素及Sr-Nd-Pb同位素和锆石Hf-O同位素的配套研究，厘清各组火山岩的岩石成因过程和构造背景演变。结合区域蛇绿岩研究成果，限定古准噶尔洋闭合时限及俯冲方向，探讨东、西准噶尔地体的构造属性关系，揭示盆地基底属性和地壳增生机制。

新疆北部位于中亚巨型增生型造山带和成矿域的重要位置，是研究造山带晚古生代构造演化和地壳增生机制的重要地区之一。东准噶尔出露有从早泥盆世到早二叠世的火山-沉积地层，这套完整的晚古生代火山岩可能记录着古洋盆俯冲闭合演化和大陆地壳形成的重要信息。项目对东准噶尔火山岩开展了详细的野外地质调查、岩相学和矿物内部结构，利用岩浆锆石U-Pb和新鲜基质40Ar-39Ar法精确限定各组火山岩的形成时代；通过矿物化学、全岩主量微量元素及Sr-Nd-Pb同位素和锆石Hf-O同位素的配套研究，厘清各组火山岩的岩石成因过程和构造背景演变。项目主要获得了以下几点认识：（1）东准噶尔广泛存在早二叠世的双峰式火山活动，基性岩石来源于受俯冲流/熔体交代的岩石圈地幔源区，酸性岩石来源于新近底侵岩石的部分熔融，二叠纪火山活动与塔里木地幔柱具有密切成因联系；（2）对东准噶尔早晚石炭世的火山岩深入对比后认为，早石炭世火山岩形成于俯冲环境，晚石炭世火山岩形成于后碰撞伸展环境，板片断离模式可以用于解释石炭纪快速的构造转换；（3）东准噶尔基性岩墙群的40Ar/39Ar地质年龄为321.4±3.4 Ma，西准噶尔侏罗纪玄武岩的40Ar/39Ar年龄为189.3~193.3 Ma。两者具有完全不同的矿物学和地球化学特征，揭示了不同的成因过程和构造背景；（4）新疆二叠纪哈拉峻碱长花岗岩侵位于268-275Ma，与区域基性-超基性杂岩体和正长岩体几乎同时形成。它们来源于由软流圈上涌导致的新底侵岩石的部分熔融，然后发生强烈的分异结晶。哈拉峻花岗岩形成于非造山环境，对应于可能持续超过30Ma的塔里木大火成岩省的晚期阶段。