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

Abstract

ABSTRACT 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 εHf(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.