新疆库车盆地晚三叠世-早侏罗世沉积碎屑矿物成分及其物源区示踪与构造意义

通过同构造沉积物质记录了解物源区剥蚀作用过程进而探索盆山构造相互关系,是大陆构造动力学研究的一个重要方面。砂岩骨架成分分析和重矿物及部分标型矿物分析方法,已被较多地用来探索新疆天山造山带与其南部库车盆地晚中生代以来的相互关系和构造演变过程,但是关于中生代较早时期库车盆地构造属性及北部造山带隆升剥蚀过程的认识仍存在较大分歧。本文运用电子探针微区成分分析方法,对库车盆地北缘中段晚三叠世和早侏罗世砂岩中具代表性的15个砂岩样品中的38颗碎屑长石和26颗碎屑白云母矿物进行了矿物化学成分分析。结果表明,长石主要来自变质岩物源区,25颗碎屑白云母均属多硅白云母,表明其源区岩石曾经历了高压变质作用。这些多硅白云母中Si原子含量显示它们比目前保留在天山造山带高压变质带蓝片岩和榴辉岩中的多硅白云母形成的压力要低,可能反映了源区高压变质岩的正常剥露顺序。这项研究结果表明,至少在晚三叠世-早侏罗世时期,库车盆地北部天山造山带中的高压变质岩已经剥露于地表并遭受剥蚀成为物源供给区。具体是中天山南缘早古生代高压变质带还是晚古生代南天山高压变质带作为重要物源区,尚需进一步的研究工作。

Chemical compositions of detrital feldspars and micas within the Late Triassic and Early Jurassic sequences of northern Kuqa basin, Xinjiang and their implications to provenance tectonics

Synkinematic sedimentation analysis is one of the effective approaches to understand the tectonic unroofing process of their provenances and the interrelations of the mountains and related adjacent basins. Sandstone framework analyses have been employed together with the investigations of selected detrital heavy minerals and special minerals such as phengites in previous research to study the interrelationships between Tianshan Mountains and Kuqa basin during Late Mesozoic and Cenozoic. It remains controversial at present that the tectonic uplift and exhumation process of paleo Tianshan Mountains and its contributions to the sedimentation of Kuqa basin in early Mesozoic. 38 detrital feldspars and 26 micas were separated from 15 sandstone samples collected from middle segment of northern part of Kuqa basin for microprobe analysis in order to explore the clues regarding to the provenance features. Chemical compositions of feldspars suggest that they derived dominantly from a provenance mainly composed of metamorphic rocks. 25 micas are phengites and imply that their provenance rocks experienced high-pressure metamorphism. The high-pressure metamorphic belt composed of blue schists and eclogite developed within the Tianshan orogenic belt probably serve as the provenance. The volume of Si within the detrital phengites of this study are lower than those of phengite preserved in blue schists and eclogites in current Tianshan Mountains, which suggest a normal exhumation process of the high-pressure metamorphic belts. This research reveals that at least as early as in Late Triassic and Early Jurassic the high-pressure metamorphic belt have been exhumed at surface and served as the provenance for Kuqa basin instead of the Late Mesozoic. It deserves further study to clarify that the high-pressure belt is Early Paleozoic one in southern Middle Tianshan or the Late Paleozoic one in southern Tianshan Mountains.