1.74 Ga crustal melting after rifting at the northern Indian margin: investigation of mylonitic orthogneisses in the Kathmandu area, central Nepal.
1.74 Ga crustal melting after rifting at the northern Indian margin: investigation of mylonitic orthogneisses in the Kathmandu area, central Nepal.
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1.74 Ga 印度北部边缘裂谷后的地壳熔融：尼泊尔中部加德满都地区糜棱岩正片麻岩的调查。
DOI：
10.1080/00206814.2018.1504329
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发表时间：
2018
影响因子：
2.6
通讯作者：
Kawabata Ryoichi
中科院分区：
文献类型：
--
作者：
Imayama Takeshi;Arita Kazunori;Fukuyama Mayuko;Yi Keewook;Kawabata Ryoichi
利用全岩和矿物化学、Rb-Sr同位素和锆石U-Pb年龄测定，对尼泊尔中部加德满都地区的糜棱岩正长岩进行了研究。正长岩确定的锆石REE模式以富集的HREE模式和明显的Eu异常为特征，符合岩浆成因。U-Pb锆石年龄定年和ti -in-锆石温度测定结果表明，结晶发生在约1.74 Ga，温度为705 ~ 765℃；地壳中典型的长英质岩浆活动。本研究中大部分正射界的全岩数据以及以往研究中类似岩石的全岩数据在各种构造判别图上跨越了“同碰撞”和“后碰撞”场，而一些数据也绘制了与裂谷相关的岩浆活动场。糜棱质正长岩的过铝组成、较高的Sr同位素比值（0.865 ~ 3.585）和较高的Th、U浓度表明糜棱质正长岩主要为s型地壳成因。本文提出的新资料，结合前人的研究成果，勾勒出至少两个古元古代岩浆活动时期：1)约1.92-1.90 Ga源自地幔熔融的裂谷相关岩浆活动，2)1.84-1.74 Ga源自裂谷后热沉降过程中印度基底埋藏的地壳熔融活动。尼泊尔的两期古元古代岩浆活动发生在哥伦比亚超大陆分裂期间和/或之后，沿印度基底的北被动边缘。
Mylonitic orthogneisses in the Kathmandu area, central Nepal have been investigated using whole-rock and mineral chemistry, Rb-Sr isotopes, and zircon U-Pb age dating. Zircon REE patterns determined from orthogneisses are characterized by enriched HREE patterns and the prominent Eu anomalies, consistent with a magmatic origin. The U-Pb zircon age dating and Ti-in-zircon thermometry revealed crystallization took place ca. 1.74 Ga at temperatures of 705–765℃; typical of felsic magmatism in the crust. Whole-rock data from most orthogneisses in this study and from similar rocks in previous studies span the ‘syn-collisional’ and ‘post-collisional’ fields on various tectonic discrimination diagrams, while some data also plot in rift-related magmatism fields. The peraluminous compositions, very high Sr isotopic ratios (0.865–3.585) and high Th and U concentrations for all orthogneisses in this study indicate that mylonitic orthogneisses are largely of S-type crustal origins. The new data presented herein, combined with that of previous studies, outline at least two Palaeoproterozoic magmatic episodes: 1) ca. 1.92–1.90 Ga rift-related magmatism derived from mantle melting and 2) 1.84–1.74 Ga crustal melting, resulting from burial of the Indian basement during thermal subsidence after rifting. This two-stage Palaeoproterozoic magmatism in Nepal occurred along the northern passive margin of the Indian basement during and/or after the breakup of the Columbia supercontinent.