柴北缘北乌兰变质杂岩早古生代低压高温变质作用及其构造意义

The early Paleozoic North Qaidam tectonic belt (NQB) records an evolution history of oceanic subduction followed by continental collision. However, the relationship between the different metamorphic terranes within this belt is still poorly constrained. Most previous studies have focused on high to ultrahigh pressure (HP-UHP) metamorphic belt, whereas the newly-identified low-pressure high-temperature (LP-HT) metamorphic belt has received little in-depth study, which limits the comprehensive understanding of the early Paleozoic tectonic evolution in the NQB. In this study, an integrated study of field investigation, petrology, mineralogy, geochronology and phase equilibrium modelling will be conducted on the LP-HT metamorphic rocks in the North Wulan metamorphic complex so as to reveal the range, spatial distribution, P-T conditions, timing and P-T-t trajectory of LP-HT metamorphism. Combined with previous studies, this project will discuss the heat-generating mechanism and tectonic setting of the LP-HT metamorphic belt, as well as its relationship with the HP-UHP metamorphic belt. This project can provide important evidences for restricting the subduction polarity of the South Qilian oceanic slab, and the reconstruction of early Paleozoic tectonic process in the NQB, and further understanding the coupled relationship between the South Qilian tectonic belt and the NQB.

柴北缘构造带记录了早古生代大洋俯冲到大陆碰撞的演化历史，但其内部不同变质地体间的关系尚不明确。前人在变质作用方面的研究主要集中于高压-超高压变质带，而对新近识别出的低压高温变质带缺乏深入研究，这制约了对柴北缘早古生代构造演化过程的全面理解。本项目将重点选择北乌兰变质杂岩内低压高温变质岩为研究对象，通过系统野外地质调查、岩相学、矿物学、年代学和变质相平衡模拟等综合研究，揭示低压高温变质作用的范围、空间展布、温压条件、时代和P-T-t轨迹。结合前人研究成果，探讨柴北缘低压高温变质带形成的热源机制和构造环境，及其与高压-超高压变质带的相互关系。该项目可为柴北缘南祁连洋俯冲极性的厘定和早古生代构造过程的反演提供证据，同时，也对理解南祁连构造带和柴北缘构造带之间的耦合关系具有重要意义。

柴北缘构造带记录了早古生代大洋俯冲到大陆碰撞的演化历史，但其内部不同变质地体间的关系尚不明确。本项目以北乌兰变质杂岩内低压高温变质岩为主要研究对象，通过野外地质调查、岩相学、矿物学、年代学和变质相平衡模拟等综合研究，揭示了低压高温变质岩的空间展布、温压条件、时代和P-T轨迹。研究表明北乌兰变质杂岩经历了中-新元古代和早古生代两期高温变质作用。中-新元古代中低压高温变质作用的年龄为1.2-1.0Ga，其峰期变质温压条件为~800℃/8-10kbar，该期变质作用可能发生在Rodinia超大陆汇聚过程中的大陆弧环境，暗示了北乌兰变质杂岩周缘与Rodinia超大陆汇聚相关的大洋俯冲作用应早于1.2Ga，并且持续到新元古代早期。早古生代低压高温麻粒岩相变质作用的年龄为467-448Ma，具有顺时针的变质P-T轨迹，涉及早阶段埋藏和加热、高温减压和晚阶段冷却过程，其峰期变质温压条件为T=800-860℃，P=5-8.5kbar。结合同时代中-基性侵入岩的地球化学特征和前人研究成果，揭示了北乌兰变质杂岩中早古生代高温低压麻粒岩相变质作用发生的主要热机制为大陆弧-弧后体系中的软流圈地幔热流传递，同时代的中-基性岩浆岩可能也提供了少量的热源。以北乌兰变质杂岩为代表的低压高温变质带与柴北缘高压-超高压变质杂岩带共同构成了双变质带，进一步证明了柴北缘地区原特提斯洋俯冲具有向北的俯冲极性。欧龙布鲁克微陆块在早古生代位于俯冲带上盘，其作为大陆弧基底参与了柴北缘早古生代的构造演化过程，在中-下地壳（弧根）的高地温梯度环境下形成了麻粒岩相变质岩。该项研究为柴北缘大洋俯冲极性的厘定和早古生代构造过程的反演提供了重要证据。