Evolution of the melt source during protracted crustal anatexis: An example from the Bhutan Himalaya
Evolution of the melt source during protracted crustal anatexis: An example from the Bhutan Himalaya
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长期地壳深熔过程中熔体源的演化：以不丹喜马拉雅山为例
作者：
Hopkinson T
岩浆锆石生长带的化学成分能够记录时间和当地的化学环境，为了解岩浆成分的演化提供了有力的依据。通过对不丹-喜马拉雅渐新世-中新世浅花岗岩锆石边缘的原位U-Pb和Hf同位素分析，首次揭示了32 ~ 12 Ma之间的熔体成分演化。数据表明，从32 Ma到17 Ma的熔体源是均匀的，并且至少从17 Ma开始，一个较旧的源成分逐渐加入到熔体中。年龄校正后的h比值在12 Ma时从−10到−15下降到−23。补充的全岩Nd同位素数据证实了Hf数据，从约18 Ma到12 Ma， Nd(t)逐渐减少。已发表的喜马拉雅地区不同岩石构造单元的锆石和全岩Nd数据表明，年轻的大喜马拉雅系列（GHS）和较老的小喜马拉雅系列（LHS）存在化学差异。浅花岗岩中显示的随时间变化的同位素演化表明，较老岩性的熔融贡献逐渐增加，表明随着时间的推移，LHS对喜马拉雅熔融的参与越来越多。时间分辨数据与LHS物质从约17 Ma开始逐渐被吸积到GHS底部一致，这是由于沿中央主逆冲的变形所促进的。从17 Ma开始，古近纪以来的减压引发了GHS的深熔作用，使得来自被吸积的LHS的较老源熔融，现在形成了冲断的最下端上壁。
The chemical compositions of magmatic zircon growth zones provide powerful insight into evolving magma compositions due to their ability to record both time and the local chemical environment. In situ U-Pb and Hf isotope analyses of zircon rims from Oligocene–Miocene leucogranites of the Bhutan Himalaya reveal, for the first time, an evolution in melt composition between 32 and 12 Ma. The data indicate a uniform melt source from 32 Ma to 17 Ma, and the progressive addition of an older source component to the melt from at least ca. 17 Ma. Age-corrected ɛHfratios decrease from between −10 and −15 down to values as low as −23 by 12 Ma. Complementary whole-rock Nd isotope data corroborate the Hf data, with a progressive decrease in ɛNd(t)from ca. 18 to 12 Ma. Published zircon and whole-rock Nd data from different lithotectonic units in the Himalaya suggest a chemical distinction between the younger Greater Himalayan Series (GHS) and the older Lesser Himalayan Series (LHS). The time-dependent isotopic evolution shown in the leucogranites demonstrates a progressive increase in melt contribution from older lithologies, suggestive of increasing LHS involvement in Himalayan melting over time. The time-resolved data are consistent with LHS material being progressively accreted to the base of the GHS from ca. 17 Ma, facilitated by deformation along the Main Central thrust. From 17 Ma, decompression, which had triggered anatexis in the GHS since the Paleogene, enabled melting in older sources from the accreted LHS, now forming the lowermost hanging wall of the thrust.
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影响因子：
3.9
作者：
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影响因子：
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作者：
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DOI：
--
发表时间：
2017
期刊：
影响因子：
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作者：
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