Episodic continental extension in eastern Gondwana during the mid-late mesozoic: insights from geochronology and geochemistry of mafic rocks in the Tethyan Himalaya

Mesozoic extension and rifting processes of the Gondwana continent are critical for understanding the opening and formation of the Indian Ocean. Here, we report petrological, geochemical, zircon U–Pb age, and Lu–Hf isotopic data of mafic dikes in the central Tethyan Himalaya to reveal the timing and mechanism of the eastern Gondwana rifting. These mafic rocks exhibit two groups in terms of TiO₂ and MgO contents (Group I with TiO₂ < 2.0 wt.%, whereas Group II with TiO₂ > 2.0 wt.%), but (1) they have similar trace elements patterns and exhibit enriched Light Rare Earth Element (LREE) patterns without Eu anomalies and (2) they have a wide range of zircon ε_Hf(t) values. Both the above geochemical characteristics are similar to those of the Oceanic Island Basalt (OIB). However, they also show typical features of continental crust input. Compared to Group II, Group I has higher MgO, Cr, and Ni abundances (more primitive) and more depleted Nb-Ta contents, which are similar to those of lower TiO₂ Dala mafic rocks, suggesting that the continental crust signature was mainly inherited from the Greater India subcontinental lithospheric mantle (GI-SCLM). Therefore, the mafic rocks here were mainly derived from a hybrid source from both OIB-like enriched mantle and GI-SCLM. Furthermore, two groups of zircon U–Pb ages have been identified: the first group with a weight mean age of 139.9 ± 0.2 Ma, whereas the second group with a weight mean age of 163.2 ± 0.9 Ma. The Middle Jurassic zircons show similar characteristics to those of intermediate to acidic igneous rocks in the continental setting, and thus they might record a stage of magmatism associated with a tectonic extension event in the Indian passive continental margin. The Early Cretaceous ages represent the crystallization timing of the mafic dikes, which are coeval with most mafic rocks in the Tethyan Himalaya. Based on these observations and the literature data in the Tethyan Himalaya, we proposed that the Kerguelen plume was incubating underneath the Tethyan Himalaya at ca. 140 Ma, but it was not located in the triple junction among the Antarctic, Australian and Indian plates. Therefore, the Kerguelen plume might play a synergistic role in the break-up of eastern Gondwana, which had experienced at least two stages of tectonic extension during the Mid-Late Mesozoic.

冈瓦纳大陆的中生代伸展与裂谷作用，是理解印度洋张开与形成的关键科学问题。本文报道了采自特提斯喜马拉雅中部的镁铁质岩脉的岩石学、地球化学、锆石U-Pb年龄及Lu-Hf同位素数据，以揭示东冈瓦纳裂谷作用的时限与机制。这些镁铁质岩石依据TiO₂与MgO含量可划分为两组：第一组TiO₂含量<2.0 wt.%，第二组TiO₂含量>2.0 wt.%。（1）二者具有相似的微量元素配分模式，均呈现轻稀土元素（Light Rare Earth Element，LREE）富集型配分特征且无铕异常；（2）锆石ε_Hf(t)值分布范围较广。上述地球化学特征均与洋岛玄武岩（Oceanic Island Basalt，OIB）相似，但同时也表现出典型的陆壳物质加入的地球化学属性。相较于第二组，第一组具有更高的MgO、Cr及Ni丰度（更原始），且Nb-Ta元素亏损程度更强，这与低TiO₂含量的达拉镁铁质岩的地球化学特征一致，指示陆壳信号主要继承自大印度次大陆岩石圈地幔（Greater India subcontinental lithospheric mantle，GI-SCLM）。因此，研究区的镁铁质岩石主要起源于洋岛玄武岩型富集地幔与大印度次大陆岩石圈地幔的混合源区。此外，本次研究识别出两组锆石U-Pb年龄：第一组加权平均年龄为139.9 ± 0.2 Ma，第二组加权平均年龄为163.2 ± 0.9 Ma。中侏罗世锆石的地球化学特征与大陆环境下形成的中酸性火成岩相似，因此其可能记录了印度被动大陆边缘与构造伸展事件相关的一期岩浆作用。早白垩世年龄代表了本次研究的镁铁质岩脉的结晶时限，该时限与特提斯喜马拉雅地区多数镁铁质岩的形成时代一致。基于上述研究结果并结合特提斯喜马拉雅地区的已有文献数据，本文提出：约140 Ma时凯尔盖朗地幔柱处于特提斯喜马拉雅区域之下，但并未位于南极板块、澳大利亚板块与印度板块的三联点位置。因此，凯尔盖朗地幔柱可能在东冈瓦纳大陆裂解过程中起到了协同促进作用，而东冈瓦纳大陆在中生代中晚期至少经历了两期构造伸展事件。