Slab Breakoff of the Neo‐Tethys Ocean in the Lhasa Terrane Inferred From Contemporaneous Melting of the Mantle and Crust
Slab Breakoff of the Neo‐Tethys Ocean in the Lhasa Terrane Inferred From Contemporaneous Melting of the Mantle and Crust
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DOI：
10.1002/2017gc007039
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发表时间：
2017-11
期刊：
影响因子：
3.7
通讯作者：
F. Huang;Ji‐Feng Xu;Yunchuan Zeng;Jian‐Lin Chen;Bao-di Wang;Hongxia Yu;Ling-Kang Chen;Wenlong Huang
中科院分区：
文献类型：
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作者：
F. Huang;Ji‐Feng Xu;Yunchuan Zeng;Jian‐Lin Chen;Bao-di Wang;Hongxia Yu;Ling-Kang Chen;Wenlong Huang
大洋板片的断裂对大陆碰撞过程中岩石圈的热状态有重要影响。这往往会引发与伸展相关的镁铁质岩浆活动和地壳熔融。一般认为，新特提斯岩石圈俯冲到拉萨次大陆南部之下，导致了冈底斯岩浆弧的形成。然而，由于缺乏与伸展相关的镁铁质岩浆作用的证据，板片断裂的时间仍然存在争议。本研究提供了拉萨地核南部大巨地区基性岩脉、长英质侵入体和包体的全面年龄、元素和Sr-Nd-Hf同位素数据。基性岩脉和花岗岩类的形成时间大约在57 Ma左右。基性岩脉具有高Th/U、Zr/Y比值的特征，其地球化学特征显示为板内亲合岩浆而非弧岩浆。此外，基性岩脉显示出强烈变化的火成岩锆石Hf（t）和低于花岗岩类的全岩Nd（t）。这一证据表明，基性岩脉代表了由不同程度的古岩石圈污染的软流圈来源的熔体。而花岗岩类则是直接来源于年轻的下地壳。由于印度和亚洲之间的收敛速度的突然下降，以及在拉萨Submucae南部的地表隆起和沉积停止在新生代早期，最好的板片断裂模式的发生的基性岩脉和花岗岩类的解释。板内型岩浆的出现可能对应于新特提斯板块断裂初期的岩浆表现。板片断裂概念也解释了57 Ma后岩浆“爆发”和地壳生长的开始。
Oceanic slab breakoff significantly affects the thermal regime of the lithosphere during continental collision. This often triggers extension‐related mafic magmatism and crustal melting. It is generally accepted that the Neo‐Tethyan lithosphere subducted beneath the southern Lhasa Subterrane, resulting in the formation of the Gangdese magmatic arc. However, the timing of slab breakoff is still disputed, due to a lack of evidence for extension‐related mafic magmatism. In this study, we provide comprehensive age, element and Sr–Nd–Hf isotopic data of mafic dikes, felsic intrusions, and enclaves from the Daju area, southern Lhasa Subterrane. The timing of mafic dikes and granitoids are contemporaneous at circa 57 Ma. The mafic dikes are characterized by high Th/U, and Zr/Y ratios, their geochemistry indicates an intraplate affinity rather than arc magmas. Furthermore, the mafic dikes show strongly variable igneous zircon ɛHf(t), and lower whole‐rock ɛNd(t) than granitoids. This evidence suggests that the mafic dikes represent asthenosphere‐derived melts contaminated by various degrees of ancient lithosphere. However, the granitoids were directly derived from the juvenile lower crust. Given the abrupt decrease in the convergence rate between India and Asia, and the surface uplift and sedimentation cessation in the southern Lhasa Subterrane in the early Cenozoic, the occurrence of synchronous mafic dikes and granitoids is best explained by a slab breakoff model. The occurrence of intraplate‐type magmas likely corresponds to the magmatic expression of the initial stage of Neo‐Tethyan slab breakoff. The slab breakoff concept also explains the onset of the magmatic “flare‐up” and crustal growth after 57 Ma.