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首页> 外文期刊>Contributions to Mineralogy and Petrology >Timescales of crustal melting in the Higher Himalayan Crystallines (Sikkim, Eastern Himalaya) inferred from trace element-constrained monazite and zircon chronology
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Timescales of crustal melting in the Higher Himalayan Crystallines (Sikkim, Eastern Himalaya) inferred from trace element-constrained monazite and zircon chronology

机译：微量元素约束独居石和锆石年代学推论出喜马拉雅山高级结晶（锡金，喜马拉雅东部）地壳融化的时标

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摘要

The petrology and timing of crustal melting has been investigated in the migmatites of the Higher Himalayan Crystalline (HHC) exposed in Sikkim, India. The metapelites underwent pervasive partial melting through hydrous as well as dehydration melting reactions involving muscovite and biotite to produce a main assemblage of quartz, K-feldspar, plagioclase, biotite, garnet ± sillimanite. Peak metamorphic conditions were 8–9 kbar and ~800 °C. Monazite and zircon crystals in several migmatites collected along a N–S transect show multiple growth domains. The domains were analyzed by microbeam techniques for age (SHRIMP) and trace element composition (LA-ICP-MS) to relate ages to conditions of formation. Monazite preserves the best record of metamorphism with domains that have different zoning pattern, composition and age. Zircon was generally less reactive than monazite, with metamorphic growth zones preserved in only a few samples. The growth of accessory minerals in the presence of melt was episodic in the interval between 31 and 17 Ma, but widespread and diachronous across samples. Systematic variations in the chemical composition of the dated mineral zones (HREE content and negative Eu anomaly) are related to the variation in garnet and K-feldspar abundances, respectively, and thus to metamorphic reactions and P–T stages. In turn, this allows prograde versus decompressional and retrograde melt production to be timed. A hierarchy of timescales characterizes melting which occurred over a period of ~15 Ma (31–17 Ma): a given block within this region traversed the field of melting in 5–7 Ma, whereas individual melting reactions lasted for time durations below, or approaching, the resolution of microbeam dating techniques (~0.6 Ma). An older ~36 Ma high-grade event is recorded in an allocthonous relict related to mafic lenses. We identify two sections of the HHC in Sikkim that traversed similar P–T conditions at different times, separated by a tectonic discontinuity. The higher structural levels reached melting and peak conditions later (~26–23 Ma) than the lower structural levels (~31–27 Ma). Diachronicity across the HHC cannot be reconciled with channel flow models in their simplest form, as it requires two similar high-grade sections to move independently during collision.
机译：在印度锡金暴露的高级喜马拉雅结晶（HHC）的辉钼矿中研究了地壳的岩石学和时机。变质岩通过含水和熔融熔化反应进行广泛的部分熔化，涉及白云母和黑云母，以产生石英，钾长石，斜长石，黑云母，石榴石±硅线石的主要组合。峰值变质条件为8–9 kbar和〜800°C。沿NS断面收集的几种辉锰矿中的独居石和锆石晶体显示出多个生长域。通过微束技术分析域的年龄（SHRIMP）和微量元素组成（LA-ICP-MS），以将年龄与形成条件相关联。独居石保留了具有不同分区模式，组成和年龄的域的最佳变质记录。锆石通常比独居石反应性差，仅在少数样品中保留了变质生长区。在熔体存在的情况下，辅助矿物的生长在31和17Ma之间的时间间隔内是偶发的，但在整个样本中分布广泛且历时性。过时矿物区化学成分的系统变化（HREE含量和负Eu异常）分别与石榴石和钾长石丰度的变化有关，因此与变质反应和P–T阶段有关。反过来，这使得定时生产熔融与减压和逆行熔体成为可能。时间尺度的层次表征了发生在〜15 Ma（31-17 Ma）期间的熔化：此区域中的给定块穿越了5-7 Ma的熔化场，而单个熔化反应持续的持续时间低于或小于接近，微束测年技术的分辨率（〜0.6 Ma）。在与黑手性晶状体有关的全相遗迹中记录了一个年龄较大的〜36 Ma的高级事件。我们在锡金确定了HHC的两个部分，它们在不同的时间穿越了相似的P–T条件，并被构造不连续性所分隔。较高的结构水平比较低的结构水平（〜31–27 Ma）达到熔化和峰值条件的时间晚（〜26-23 Ma）。 HHC的历时性无法与最简单形式的通道流模型进行协调，因为它需要两个相似的高品位段在碰撞过程中独立移动。

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