新疆西南天山榴辉岩高压-超高压峰期变质作用时代:Lu-Hf同位素年代学证据  

Ages of peak HP-UHP metamorphism of eclogites from Chinese southwestern Tianshan:Evidence from Lu-Hf geochronology

作　　者：龚庭楠 杜瑾雪[1] 张立飞[2] 沈晓洁 GONG TingNan;DU JinXue;ZHANG LiFei;SHEN XiaoJie(School of Earth Sciences and Resources,China University of Geosciences,Beijing 100083,China;Key Laboratory of Orogenic Belts and Crustal Evolution,MOE,School of Earth and Space Sciences,Peking University,Beijing 100871,China)

机构地区：[1]中国地质大学地球科学与资源学院,北京100083 [2]造山带与地壳演化教育部重点实验室,北京大学地球与空间科学学院,北京100871

出　　处：《岩石学报》2023年第3期689-700,共12页Acta Petrologica Sinica

基　　金：国家自然科学基金指南引导类原创探索项目(42150104)资助。

摘　　要：新疆西南天山蓝片岩-榴辉岩带是全球少有的几个经历超高压变质作用的洋壳俯冲带之一,近年来的同位素年代学研究表明其变质作用主要发生于石炭纪。然而,该蓝片岩-榴辉岩带峰期变质作用,特别是超高压变质作用的时代还未精确限定。本文选取来自该带的典型钠云母黝帘石榴辉岩(样品211-3和H76-10),在详细的岩石学和相平衡研究基础上,开展了Lu-Hf同位素年代学研究,以期获得其峰期变质作用时代。相平衡模拟结果表明两榴辉岩中石榴石均记录了升温降压型折返P-T(温度-压力)轨迹。其中,样品211-3经历了超高压变质作用,其压力峰期P-T条件为~540℃、~2.9GPa,石榴石-绿辉石-全岩Lu-Hf等时线年龄为326.9±1.3Ma;样品H76-10仅经历了高压变质作用,其压力峰期P-T条件为~490℃、~2.4GPa,石榴石-绿辉石-全岩Lu-Hf等时线年龄为306±11Ma。结合前人年代学数据,获得新疆西南天山超高压榴辉岩峰期变质作用时代为327~326Ma,高压榴辉岩峰期变质作用时代为316-306Ma。本文获得的超高压变质作用确切年龄326.9±1.3Ma,对于揭示古南天山洋深俯冲和闭合的时间具有重要意义。The blueschist-eclogite belt in Chinese southwestern Tianshan is one of the rarely exposed occurrences of ultrahigh pressure(UHP) metamorphic rocks derived from subducted oceanic crust in the world. Recently, numerous isotope geochronology studies indicate that the metamorphism occurred at the Carboniferous. However, the age of peak metamorphism in this belt, especially the UHP metamorphism, has not been precisely constrained. In this paper, Lu-Hf isotope geochronology was carried out based on detailed petrological study and phase equilibria modeling for typical paragonite-zoisite eclogites(samples 211-3 and H76-10). Phase equilibria modeling reveals that garnets in both samples record P-T paths characterized by heating decompression. Sample 211-3 experienced UHP metamorphism with the peak pressure conditions of ~540℃, ~2.9 GPa. Sample H76-10 experienced only high pressure(HP) metamorphism with the peak pressure conditions of ~490℃, ~2.4 GPa. The garnet-omphacite-whole rock Lu-Hf isochrons for samples 211-3 and H76-10 yield ages of 326.9±1.3 Ma and 306±11 Ma, respectively. Combined with previous geochronological data, we propose that the UHP metamorphism in Chinese southwestern Tianshan occurred at 327~326 Ma, and the HP metamorphism at 315~306 Ma. The age of 326.9±1.3 Ma for UHP metamorphism in Chinese southwestern Tianshan is important for revealing the timing of deep subduction and closure of South Tianshan paleo-ocean.

关 键 词：西南天山 超高压变质作用 榴辉岩 相平衡 Lu-Hf年代学 

分 类 号：P588.348[天文地球—岩石学] P597.3[天文地球—地质学]