一致的石榴石Lu–Hf和Sm–Nd年龄表明海洋俯冲带中存在短暂的高压变质作用和快速俯冲:以青藏高原东南部长宁–孟连造山带为例

IF 3.5 2区 地球科学 Q1 GEOLOGY Journal of Metamorphic Geology Pub Date : 2023-05-26 DOI:10.1111/jmg.12735
Ying Zhou, Hao Cheng, Zhi-min Peng, Besim Dragovic, Yu-zhen Fu, Kai-Yang Du
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引用次数: 0

摘要

榴辉岩和蓝片岩中基于石榴石的岩石学约束与多矿物地质年代数据的结合使得俯冲带变质作用的时间和速率受到限制。我们对西藏东南部新发现的超高压变质带长宁-孟连造山带的三个榴辉岩、一个含石榴石蓝片岩和一个云母片岩进行了石榴石Lu–Hf/Sm–Nd和锆石/金红石U–Pb的联合地质年代学研究,以表征构造变质事件,并确定古特提斯洋俯冲的持续时间。榴辉岩的相平衡建模和传统温压测量的结合确定了一条顺时针P–T路径,从约1.4 GPa和约505–530°C的蓝片岩相条件演化到约2.8 GPa和630–640°C的峰值榴辉岩相条件,然后在约1.0 GPa和~630–650°C的等温减压到角闪岩相。公元239-236年的鲁时代 从榴辉岩和蓝片岩中获得的Ma与c.239的金红石U–Pb年龄无法区分 从榴辉岩中获得的Ma,结合对石榴石中保存完好的瑞利分馏型Mn和Lu分带剖面的观察,反映了早期前进石榴石生长的时间。约242–236年的Sm–Nd年龄 Ma反映了石榴石生长的后期,平坦和/或M形Sm分区剖面证明了这一点。每个Sm–Nd年龄在不确定性范围内与其对应的Lu–Hf年龄重叠(即,来自同一石榴石部分)。Lu–Hf和Sm–Nd年龄的一致性表明,从蓝片岩到榴辉岩相变质作用,石榴石生长的总体持续时间很短,反映了大洋板块的快速俯冲。岩浆锆石U–Pb的年代约为247年 Ma限制了这些变质玄武岩的原岩时代。紧密的原岩和高压变质年龄,以及一致的石榴石Lu–Hf和Sm–Nd年龄,以及海洋型和大陆型榴辉岩的最年轻和最古老的重叠变质年龄,分别表明了从发散到会聚的快速构造转变,突出表现为快速的海洋俯冲,从海洋俯冲到大陆俯冲的持续过渡,以及俯冲界面的快速冷却。
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Consistent garnet Lu–Hf and Sm–Nd ages indicate short-lived high-pressure metamorphism and rapid subduction in oceanic subduction belt: An example from the Changning–Menglian orogenic belt, southeastern Tibetan Plateau

The integration of garnet-based petrologic constraints with multimineral geochronologic data in eclogites and blueschists allows the timing and rate of subduction zone metamorphism to be constrained. We present a combined garnet Lu–Hf/Sm–Nd and zircon/rutile U–Pb geochronology study on three eclogites, a garnet-bearing blueschist, and a micaschist from the Changning–Menglian orogenic belt, a newly discovered ultrahigh-pressure metamorphic belt in southeast Tibet, in order to characterize tectono-metamorphic events and determine the duration of Paleo-Tethys oceanic subduction. Integration of phase equilibrium modelling and conventional thermobarometry for the eclogites defines a clockwise P–T path evolving from blueschist facies conditions at ~1.4 GPa and ~505–530°C to peak eclogite facies conditions at ~2.8 GPa and ~630–640°C, followed by isothermal decompression to amphibolite facies at ~1.0 GPa and ~630–650°C. The Lu–Hf ages of c. 239–236 Ma obtained for the eclogites and the blueschist are indistinguishable from the rutile U–Pb age of c. 239 Ma obtained for the eclogites and, combined with the observation of well-preserved Rayleigh-fractionation-style Mn and Lu zoning profiles in garnet, reflect the timing of early prograde garnet growth. The Sm–Nd ages of c. 242–236 Ma reflect a later period of garnet growth, evidenced by flat and/or M-shaped Sm zoning profiles. Each of the Sm–Nd ages overlaps, within uncertainty, with its corresponding Lu–Hf age (i.e., from the same garnet fraction). The consistency of the Lu–Hf and Sm–Nd ages indicates a short overall duration of garnet growth from blueschist to eclogite facies metamorphism, reflecting rapid subduction of the oceanic slab. The magmatic zircon U–Pb dates of c. 247 Ma constrain the protolith age of these metabasaltic rocks. The close protolith and the high-pressure metamorphic ages, together with the consistent garnet Lu–Hf and Sm–Nd ages and the overlapping youngest and oldest metamorphic ages of the oceanic-type and continental-type eclogites, respectively, suggest a fast tectonic transition from divergence to convergence highlighted by rapid oceanic subduction, continuous transition from oceanic to continental subduction, and a rapid cooling of the subduction interface.

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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
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