Recycled oceanic crust in the form of pyroxenite contributing to the Cenozoic continental basalts in central Asia: new perspectives from olivine chemistry and whole‑rock B–Mo isotopes
Recycled oceanic crust in the form of pyroxenite contributing to the Cenozoic continental basalts in central Asia: new perspectives from olivine chemistry and whole‑rock B–Mo isotopes
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辉石岩形式的再生海洋地壳对中亚新生代大陆玄武岩的贡献：橄榄石化学和全岩 B-Mo 同位素的新视角
DOI：
10.1007/s00410-019-1620-4
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发表时间：
2019
影响因子：
3.5
通讯作者：
Lin Zhengfan
中科院分区：
文献类型：
--
作者：
Zhang Yunying;Yuan Chao;Sun Min;Chen Ming;Hong Lubing;Li Jie;Long Xiaoping;Li Pengfei;Lin Zhengfan
亚洲中部广泛分布着新生代大陆玄武岩。为探讨其物源性质和成因，本文综合研究了中新世（约2000年）橄榄岩化学、岩石40 Ar/39 Ar年龄、岩石地球化学和Sr-Nd-Pb-B-Mo同位素特征。15.5 Ma）哈拉乔拉玄武岩。哈拉乔拉玄武岩多为高碱（Na_2 O + K_2 O = 6.89- 8.01wt%）和高K_2 O/Na_2 O（0.87-1.39）的碧玄岩。与地幔橄榄岩部分熔融产物相比，玄武岩样品具有较低的CaO和CaO/Al 2 O3，而较高的TiO 2、Zn/Mn和Zn/Fe值。同时，这些玄武岩中的橄榄石斑晶的特征是较低的Ca，Ni和Mn的含量，但较高的Fe/Mn比比在橄榄岩熔体中的对应物，这表明辉石岩丰富的来源。此外，这些岩石显示出OIB样的微量元素模式（例如，Ba、Sr、Nb和Ta的峰值和Th、U的谷值），Nd同位素组成恒定，Sr和EM 1类Pb同位素组成变化较大，δ11B和δ 98 Mo值较轻（δ11B =-11.0 ~-8.1 ‰，δ 98 Mo =-0.40 ~-0.06 ‰）。上述地球化学数据表明，次生辉石岩可能是由再循环洋壳与环境橄榄岩反应生成的，并随后成为碧玄岩的主要来源。δ 98 Mo值的变化和轻，可能反映了参与辉石岩的再循环洋壳受到不同程度的蚀变。结合邻区已有资料，我们认为印度-欧亚大陆碰撞的远场效应是中亚地区分散的软流圈地幔上涌的一级因素，软流圈地幔的熔融导致了广泛的新生代火山活动。
Cenozoic continental basalts are widespread in central Asia. To explore their source nature and petrogenesis, this study presents an integrated study of olivine chemistry, bulk-rock40Ar/39Ar age and geochemistry as well as Sr–Nd–Pb–B–Mo isotopes for the Miocene (ca. 15.5 Ma) Halaqiaola basalts in the Chinese Altai, central Asia. The Halaqiaola basalts mostly have basanite compositions with high total alkali (Na2O + K2O = 6.89–8.01 wt%) contents and high K2O/Na2O (0.87–1.39) ratios. Compared with partial melting products of mantle peridotite, the basaltic samples possess lower CaO and CaO/Al2O3but higher TiO2, Zn/Mn and Zn/Fe values. Meanwhile, olivine phenocrysts from these basalts are characterized by lower Ca, Ni and Mn contents but higher Fe/Mn ratios than their counterparts in the peridotitic melts, suggesting a pyroxenite-rich source. Moreover, these rocks show OIB-like trace element patterns (e.g., spikes of Ba, Sr, Nb and Ta and troughs of Th and U), and constant Nd but variable Sr and EM1-like Pb isotopic compositions, and yield light δ11B (– 11.0 to – 8.1‰) and δ98Mo (– 0.40 to – 0.06‰) values. The above geochemical data suggest that secondary pyroxenite was likely produced by reaction of recycled oceanic crust with its ambient peridotite and subsequently became the main source for the basanite. Furthermore, their light and variable δ98Mo values probably reflect that recycled oceanic crust involved in such pyroxenite was altered with different degrees. In combination with available data from adjacent regions, we propose that the far-field effect of India–Eurasia collision was the first-order factor for the upwelling of dispersive asthenospheric mantle beneath central Asia, subsequent melting of which gave rise to the widespread Cenozoic volcanism.