伊犁盆地尼勒克地区中二叠世陆相地层碎屑物源分析及其对西天山构造演化过程的约束

西天山在二叠纪时期的大地构造环境有岛弧成因和大陆裂谷成因两种不同的观点,伊犁盆地尼勒克地区中二叠统陆相红层的研究是解决上述争议的关键之一。该段地层自下而上是一套由冲洪积扇相、河流相、浅湖相和半深湖相组成的湖侵序列。本文综合该套地层的古水流、砾岩砾石成分与砂岩碎屑组分、重矿物组合、碎屑岩地球化学以及石榴子石化学特征等方面的研究,认为尼勒克盆地中二叠统陆相红层的物源复杂多样,包括超镁铁质-镁铁质岩、安山岩、英安岩、流纹岩、矽卡岩、片麻岩、灰岩和碎屑岩等不同岩石类型,记录了西天山造山带的构造剥蚀与再沉积过程。结合下二叠统乌郎组双峰式火山岩以及区域性角度不整合等资料的综合分析,本研究认为尼勒克地区中二叠世盆地形成于造山后伸展的大地构造背景。

Detrital provenances of Middle Permian continental strata in the Yili basin (Nileke area), Xinjiang: Constraints on the tectonic evolution of the western Tianshan Orogenic Belt during the Middle Permian

There are two different viewpoints about the Permian tectonic settings of the western Tianshan Orogenic Belt, referred as to arc or continental rift respectively. The determination of tectonic settings of the Permian Yili basin is one of the key points to tackle this issue. The provenance analysis shows that the Middle Permian Yili basin in the Nileke area is dominated by a lacustrine transgressive sequence, which is characterised by alluvial fan facies, fluvial facies, and shallow to semi-depth lacustrine facies. In this paper, the gravel and detritus compositions, heavy mineral assemblages, geochemistry of clastic rocks and garnet detritus of Middle Permian strata are systematically analysized. It provides that the sources of the Middle Permian in the Nileke area contain ultramafic and mafic rocks, andesite, dacite, rhyolite, skarn, gneiss, limestone and clastic rocks, indicating that the Middle Permian in the Yili basin is a suite of proximal and rapid deposits which recorded tectonic denudation in the post-orogeny stage. Combined with the bimodal volcanic suite of the Lower Permian Wulang Formation and regional unconformity, this study suggests that the Middle Permian Yili basin in the western Tianshan Orogenic Belt was an extensional basin during the post-orogeny stage.