Magma mixing origin for the Quxu intrusive complex in southern Tibet: insights into the early Eocene magmatism and geodynamics of the southern Lhasa subterrane
Magma mixing origin for the Quxu intrusive complex in southern Tibet: insights into the early Eocene magmatism and geodynamics of the southern Lhasa subterrane
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藏南曲须侵入杂岩的岩浆混合起源：拉萨南部地下早始新世岩浆作用和地球动力学的见解
DOI：
10.1016/j.lithos.2019.01.019
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发表时间：
2019-03
期刊：
影响因子：
3.5
通讯作者：
Xu Hang
中科院分区：
文献类型：
--
作者：
Wang Rui Qiang;Qiu Jian Sheng;Yu Si Bin;Lin Lei;Xu Hang
具有成分多样性的侵入杂岩通常记录了岩浆混合过程，并提供了对区域岩浆作用和构造的见解。本文通过对西藏南部曲水杂岩体的野外调查、岩石学、矿物化学、全岩地球化学、Sr、Nd同位素组成和锆石U-Pb-Hf同位素组成的综合研究，旨在确定该杂岩体的成因，并对拉萨南部早始新世岩浆活动和地球动力学进行深入探讨。曲水侵入杂岩主要由英云闪长岩和少量辉石角闪辉长岩组成，在这两种岩石的接触带内有少量的石英闪长岩。锆石LA-ICP-MS U单键Pb定年结果表明，该杂岩体中所有岩石的结晶年龄为51 Ma。辉石-角闪石辉长岩显示辉石被角闪石和/或黑云母取代，并含有成分可变的斜长石晶体（An 38 -74）和石英，这些特征与岩浆混合的反应过程一致。石英闪长岩和英云闪长岩中保存有复杂的振荡分带和重复的吸收面的斜长石晶体，指示岩浆混合成因。地球化学，辉石角闪石辉长岩，石英闪长岩，英云闪长岩表现出连续的变化，在大多数的主要和微量元素的组合物，与发表的数据一致的镁铁质微粒包体（MME）和主机花岗岩，导致岩浆混合。曲水杂岩岩石的同位素组成均质亏损，全岩87 Sr/86 Sr初始比值（ISr）为0.7038-0.7041，εNd（t）值为+5.5 ~+6.3，锆石εHf（t）值为+8.5 ~+12.9。这些地球化学和同位素数据，结合矿物学特征，表明曲水侵入杂岩起源于镁铁质和长英质端元之间的岩浆混合以不同的比例。基性端元来自上涌软流圈的减压熔融，长英质端元来自基性端元底侵作用触发的幼年下地壳部分熔融。我们的研究结果还表明，新特提斯洋板的断裂负责曲水侵入杂岩的产生，更普遍的是，在拉萨subtocene南部的早始新世岩浆活动。
Intrusive complexes with compositional diversity commonly document magma mixing processes and provide insights into regional magmatism and tectonics. Here, we present an integrated study involving detailed field investigations, petrography, mineral chemistry, whole-rock geochemistry, Srsingle bondNd isotopic compositions, and zircon U–Pb–Hf isotopes for the Quxu intrusive complex in southern Tibet, with the aim of constraining the origin of the complex and gaining insights into the early Eocene magmatism and geodynamics of the southern Lhasa subterrane. The Quxu intrusive complex consists predominantly of tonalites and minor pyroxene–hornblende gabbronorites, with lesser amounts of quartz diorites within the contact zones of these two types of rock. Zircon LA–ICP–MS Usingle bondPb dating yields crystallization ages of ~51 Ma for all lithologies in this complex. The pyroxene–hornblende gabbronorites show replacement of pyroxene by amphibole and/or biotite and contain plagioclase crystals of variable composition (An38–74) and quartz, characteristics that are consistent with a reactive process via magma mixing. The quartz diorites and tonalites preserve plagioclase crystals with complex oscillatory zoning and repeated resorption surfaces, indicating a magma mixing origin. Geochemically, the pyroxene–hornblende gabbronorites, quartz diorites, and tonalites exhibit continuous variations in most of their major- and trace-element compositions, consistent with published data for mafic microgranular enclaves (MMEs) and host granitoids that resulted from magma mixing. All the lithologies in the Quxu intrusive complex have homogeneous and depleted isotopic compositions, with whole-rock initial87Sr/86Sr ratios (ISr) of 0.7038–0.7041, εNd(t) values of +5.5 to +6.3, and zircon εHf(t) values of +8.5 to +12.9. These geochemical and isotopic data, combined with the mineralogical features, indicate that the Quxu intrusive complex originated from magma mixing between mafic and felsic end-members in varying proportions. The mafic end-member was derived from the decompression melting of upwelling asthenosphere, and the felsic end-member was derived from the partial melting of juvenile lower crust triggered by underplating of a mafic end-member. Our results also indicate that breakoff of the Neo-Tethyan oceanic slab was responsible for the generation of the Quxu intrusive complex and, more generally, for the early Eocene magmatism in the southern Lhasa subterrane.
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