阿伦特U-Pb年代测定为喜马拉雅深部地壳从高压到高温的演化提供了新的制约因素

IF 3.5 2区 地球科学 Q1 GEOLOGY
Eleni Wood, Clare J. Warren, Barbara E. Kunz, Tom W. Argles, Anna Bidgood, Alison Halton, Samantha J. Hammond, Ian L. Millar, Nick M. W. Roberts
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引用次数: 0

摘要

在大陆碰撞过程中,地壳岩石被掩埋、变形、转化和排出。这些过程的速率、时间尺度和对构造的影响是通过主相和附属相变质反应的顺序和条件来确定的。喜马拉雅山脉东部不丹西北部偏闪长岩样本的岩相、同位素和元素数据表明,最初是在高压(无斜长石、有金红石)条件下发生平衡,随后在高温条件下减压至较低的压力,从而稳定了斜长石、正长石和钛铁矿。实地观察和化学指标表明,低压条件下的平衡很可能与主基岩的熔体渗入有关。变质岩保留了两个变质生长阶段,在化学和岩石学上与锆石生长的两个阶段不同,在时间上重叠。奥氏体核心和锆石幔分别生长于约 19±2 Ma 和 17-15.5 Ma,在质地和化学上与高压演化有关。凹陷的绿帘石核心上的交辉边缘、完全交辉的Aln-Ilm和Aln-Cpx晶粒以及高U锆石边缘生长于约15.5-14.5 Ma,在化学上与熔体的存在和低压高温条件有关。单一的石榴石Lu-Hf日期被解释为没有地质意义,因为在石榴石形成之后,熔体的渗入改变了大块岩石的成分。这些岩石的开放系统演化排除了对变质演化过程中反应性块体成分的精确测定,因此也排除了绝对条件,尤其是早期高压演化过程中的绝对条件。尽管存在这些局限性,但我们的研究表明,结合地球化学和岩石学数据集,仍然能够深入了解深部造山过程的速率和时间尺度。这些数据表明,与喜马拉雅山脉中部和东部的类似岩石相比,不丹西北部的玄武岩演化更年轻、更浅。
本文章由计算机程序翻译,如有差异,请以英文原文为准。

Allanite U–Pb dating places new constraints on the high-pressure to high-temperature evolution of the deep Himalayan crust

Allanite U–Pb dating places new constraints on the high-pressure to high-temperature evolution of the deep Himalayan crust

During continental collision, crustal rocks are buried, deformed, transformed and exhumed. The rates, timescales and tectonic implications of these processes are constrained through the sequence and conditions of metamorphic reactions in major and accessory phases. Petrographic, isotopic and elemental data from metabasite samples in NW Bhutan, eastern Himalaya, suggest initial equilibration under high-pressure (plagioclase-absent and rutile-present) conditions, followed by decompression to lower pressure conditions at high-temperatures that stabilized plagioclase, orthopyroxene and ilmenite. Field observations and chemical indicators suggest equilibration under the lower pressure conditions is likely linked to the infiltration of melt from the host metasedimentary rocks. The metabasites preserve two metamorphic growth stages of chemically-and petrographically distinct allanite that temporally overlap two stages of zircon growth. Allanite cores and zircon mantles grew at c. 19 ± 2 and 17–15.5 Ma respectively, linked texturally and chemically to the high-pressure evolution. Symplectitic rims on embayed allanite cores, wholly symplectized Aln–Ilm and Aln–Cpx grains, and high U zircon rims grew at c. 15.5–14.5 Ma, linked chemically to the presence of melt and lower pressure, high-temperature conditions. A single garnet Lu–Hf date is interpreted as geologically meaningless, with the bulk rock composition modified by melt infiltration after garnet formation. The open system evolution of these rocks precludes precise determination of the reactive bulk composition during metamorphic evolution and thus absolute conditions, especially during the early high-pressure evolution. Despite these limitations, we show that combined geochemical and petrographic datasets are still able to provide insights into the rates and timescales of deep orogenic processes. The data suggest a younger and shallower evolution for the NW Bhutan metabasites compared to similar rocks in the central and eastern Himalayas.

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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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