The anomalous lithium isotopic signature of Himalayan collisional zone carbonatites in western Sichuan, SW China: Enriched mantle source and petrogenesis
The anomalous lithium isotopic signature of Himalayan collisional zone carbonatites in western Sichuan, SW China: Enriched mantle source and petrogenesis
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四川西部喜马拉雅碰撞带碳酸盐岩的异常锂同位素特征：丰富的地幔来源与岩石成因
DOI：
10.1016/j.gca.2015.03.016
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发表时间：
2015
影响因子：
5
通讯作者：
Yang Zhusen
中科院分区：
文献类型：
--
作者：
Tian Shihong;Hou Zengqian;Su Aina;Qiu Lin;Mo Xuanxue;Hou Kejun;Zhao Yue;Hu Wenjie;Yang Zhusen
利用川西毛牛坪、李庄、大鹿草地区38种碳酸盐岩及其伴生正长岩的锂离子浓度和同位素组成，结合前人和新发表的Pb-Sr-Nd-C-O同位素数据和全岩分析，对其地幔源和成因进行了研究。碳酸盐岩和正长岩的特征是锂浓度变化极大（0.8 ~ 120ppm），锂同位素组成变化很大（−4.5‰~ +10.8‰）。其中，大部分碳酸盐岩和正长岩的δ7Li值在+0.2‰~ +5.8‰之间，与MORB和OIB的报告值重叠；3碳酸盐岩δ7Li值较高，介于+8.7‰~ +10.8‰之间；5种碳酸岩和4种正长岩的δ7Li值较轻，在−4.5‰~−0.3‰之间。这些高度变化的δ7Li组成不可能是由Li的扩散驱动同位素分馏产生的，因此可能记录了晚元古代次大陆岩石圈地幔（SCLM）的同位素特征。本文证实了晚元古代次大陆岩石圈地幔内存在异常δ7Li，表明川西地区古岩石圈受俯冲洋壳和海相沉积物流体的作用而发生了变质。具有代表性地幔成分的脱水平板流体模型曲线（比值：aoc80 - sed20 ~ AOC40-SED60）可以解释大部分锂成分的变化。有些样品的Pb-Sr-Nd-O同位素组成异常，δ7Li组成变化很大，这些样品可能受到源区海相沉积物的影响，而没有受到地壳物质的污染。川西地区碳酸盐岩和正长岩是由次大陆岩石圈地幔的部分熔融作用和俯冲洋壳和海相沉积物的富锂流体的交代作用形成的。这次融化很可能是由新生代软流圈地幔底辟引起的，这与印度-亚洲大陆碰撞和渐新世碰撞后或碰撞后期应力松弛有关。
Lithium concentrations and isotopic compositions of 38 carbonatites and associated syenites from the Maoniuping, Lizhuang, and Dalucao in western Sichuan, along with previously published and new Pb–Sr–Nd–C–O isotope data and whole-rock analyses, are used to constrain their mantle source and genesis. Carbonatites and syenites are characterized by extremely varying Li concentrations (0.8–120 ppm) and highly variable Li isotopic compositions (−4.5‰ to +10.8‰). Among them, the majority of the carbonatites and syenites have δ7Li values between +0.2‰ and +5.8‰, which overlap with the reported values for MORB and OIB; 3 carbonatites have higher δ7Li values between +8.7‰ and +10.8‰; 5 carbonatites and 4 syenites have lighter δ7Li values between −4.5‰ and −0.3‰. These highly variable δ7Li compositions could not have been produced by diffusive-driven isotopic fractionation of Li and thus may record the isotopic signature of the late Proterozoic subcontinental lithospheric mantle (SCLM). This paper demonstrates the existence of anomalous δ7Li within the late Proterozoic subcontinental lithospheric mantle, suggesting that the ancient SCLM beneath western Sichuan was modified by interaction with fluids derived from the subducted oceanic crust and marine sediments.The modeling curves of fluids derived from a dehydrated slab (ratios: AOC80–SED20to AOC40–SED60) with a representative mantle composition can account for the majority of lithium compositional variations. Some samples with unusual Pb–Sr–Nd–O isotopic compositions and highly variable δ7Li compositions are affected by significant involvement of marine sediments in their source region, not contaminated by crustal materials. The carbonatites and syenites in western Sichuan were generated by the partial melting of subcontinental lithospheric mantle, which was metasomatized by the Li-rich fluids derived from the subducted oceanic crust and marine sediments. This melting was most likely triggered by a Cenozoic asthenospheric mantle diapir related to Indian-Asian continental collision and post- or late-collisional stress relaxation in the Oligocene.