班公湖-怒江缝合带西部晚二叠世-三叠纪洋壳的俯冲-折返过程——以改则榴辉岩相变质带为例

A high-pressure matemorphic slice was found in the Dongco- Gaize area of the western Bangong-Nujiang suture. Our preliminary studies indicate that the metamorphic belt is composed of complicated rocks and minerals, including two types of eclogitic rocks. The eclogitic rocks include typical minerals of eclogitic facies, such as C-type garnets, clinopyroxenes and rutiles. The type I eclogitic rocks are massive, coase-grained and the prolith is dated at 260 Ma using zircon SHRIMP U-Pb method, geochemically with MORB affinity. In contrast, zircons collected from the type II eclogitic rocks are well dated at 242 Ma, with a protolith of oceanic island basalt. Therefore, the Dongco oceanic eclogitic slice provides critical materials to constrain the tectonic evolution of the Bangong-Nujiang Meso-Tethyan ocean. We will conduct detailed field mapping and multidisciplinary studies on the Dongco eclogitic slice, including petrology, mineralogy, elemental-isotopical geochemistry, and isotopic dates. We will determine the occurrences and geometric features of the Dongco eclogitic slice, identify the typical lithic and mineral assemblages, restore the protoliths and their tectonic backgrounds, and reveal the metamorphic pressure and temperature conditions and metamorphic geochronology. Then, we will reconstruct the pressure-temperature-time (P-T-t) path, and process and mechanism of the formation and exhumation of the oceanic eclogitic rocks. Finally, we will discuss the implications of the eclogitic slice to the tectonic evolution of the Late Permian-Triassic Bangong-Nujiang Meso-Tethyan ocean.

班公湖-怒江缝合带西部改则洞错地区出露一套高压变质带。该变质带的岩石-矿物组合复杂，其中包含两类榴辉岩相岩石，峰期矿物组合为C型石榴石+单斜辉石+金红石等，达到榴辉岩相高压变质。第I类为块状粗粒榴辉岩相岩石，其原岩为晚二叠世（260 Ma）正常洋壳型的亚碱性基性岩，第II类为透镜状细粒榴辉岩相岩石，其原岩为中三叠世（242 Ma）的洋岛型碱性玄武岩。初步认为这是一套晚二叠世-三叠纪大洋型榴辉岩相变质带，这为深入认识中特提斯洋的演化提供了宝贵素材。拟针对该变质带开展精细岩性-构造填图、岩石矿物学分析、全岩/单矿物元素-同位素测试和多手段同位素测年等研究，厘定变质带的组成与几何学和运动学特征，查明典型岩石-矿物组合和变质压力-温度条件，恢复原岩组成与大地构造背景，确定成岩-变质时代，重建P-T-t轨迹，揭示其俯冲-折返过程与机制，探讨晚二叠世-三叠纪中特提斯洋的性质与演化历史。

班公湖-怒江（简称班怒）带西部改则地区出露一套榴辉岩相变质带，以这套高压变质带为关键地区，对班怒带西部及邻区开展野外精细岩性-构造填图、路线地质、重要样品采集和室内综合研究。改则退变质榴辉岩的峰期矿物组合为C型石榴石+单斜辉石+硅金红石+金红石+绿帘石等，形成压力达25–23 kbar，后期同行研究发现了绿辉石包体并证实榴辉岩经历麻粒岩相叠加改造；改则退变质榴辉岩为大洋型，一类榴辉岩为块状粗粒榴辉岩相岩石，其原岩为中二叠世（260 Ma）正常洋壳型的亚碱性基性岩，另一类榴辉岩为透镜状细粒榴辉岩相岩石，其原岩为中三叠世（242 Ma）的洋岛型碱性玄武岩，中二叠世-三叠纪班怒中特提斯洋经历了洋内俯冲，榴辉岩相变质与折返，班怒带的变质事件比经历了大陆俯冲的双湖古特提斯带至少晚20–30 Ma；榴辉岩的原岩时代显示班怒中特提斯洋的形成不晚于中二叠世，早二叠世羌塘地幔柱导致冈瓦纳超级大陆北缘裂解，包括班怒中特提斯洋在内的特提斯洋可能同时形成。班怒带西部的蛇绿岩碎片主要为洋底高原型，中特提斯洋底高原的岩浆活动可划分为两个阶段，分别为早中侏罗世（193–173 Ma，平均184.6 Ma）和早白垩世（127–104 Ma，平均121.3 Ma），这两个时间段代表了班怒中特提斯洋中存在的两个洋底高原的形成时期，第一期中特提斯洋底高原导致羌塘地块南缘早侏罗世（~175 Ma）洋底高原增生相关的造山事件；班怒大洋岩石圈于晚侏罗世-早白垩世早期（161–142 Ma）向北陡角度俯冲，因洋底高原阻挡大洋岩石圈于早白垩世中期（142–128 Ma）缓角度俯冲，因大洋岩石圈回撤又于早白垩晚期（128–106 Ma）陡角度俯冲，洋陆俯冲在羌塘南缘形成两套岩浆岩组合，并在两套岩浆岩组合中部形成早白垩世弧后裂谷沉积。中二叠世既已形成的班怒中特提斯洋西部经历了洋内俯冲、榴辉岩相变质与折返，早中侏罗世、早白垩世两期洋底高原，晚侏罗世-早白垩世早期、早白垩世晚期阶段性两期陡角度洋陆俯冲，中白垩世闭合的复杂演化过程。