Magmatic records of Gondwana assembly and break-up in the eastern Himalayan syntaxis, northeast India

Abstract

The Indian sub-continent was an integral part of the Gondwana supercontinent with multiple magmatic episodes during the Gondwana assembly and break-up events. However, most of these vital records to understand the past magmatism were obliterated during the Himalayan orogeny due to the subduction of the Indian plate. In this contribution, we attempt to tackle this issue by investigating the Abor magmatic rocks from the eastern Himalayan syntaxis, which are likely to represent the leftover fragments of the eastern Gondwana continental margin. This study uses zircon U-Pb dating, whole-rock geochemistry, and Sr-Nd isotopic ratios data of the mafic intrusive and felsic volcanic rocks of the Abor magmatism. The mafic intrusive rocks have zircon ages of 500–473 Ma, while the felsic rocks yield ages of 145–132 Ma, indicating two temporally separated episodes of magmatism. The mafic intrusives are sub-alkaline/tholeiitic (Nb/Y < 0.65), with high TiO2 (1.63–3.42 wt%) and ocean island basalt to enriched-mid oceanic ridge basalt affinities. A relatively narrow range of initial 87Sr/86Sr (0.703887–0.705513), 143Nd/144Nd (0.511978–0.512118), and εNd(t) (-0.323–+2.43) of the mafic intrusives suggest fractional crystallization with negligible crustal contamination, generated by low degree (∼3–13 %) partial melting of a primitive mantle (garnet and spinel lherzolite). The felsic rocks display low MgO (0.38–1.17 wt%), CaO (1.06–5.31 wt%), LREE and LILE (Rb, K, Pb) enrichment, depletion in HREE, Sr, Nb, Ti, with strong negative Eu-anomaly (0.48–0.73), high initial 87Sr/86Sr (0.707878–0.717650), and negative εNd(t) (-14.35 to −9.21), suggesting A-type felsic magmatism. The older mafic intrusions were thus attributed to the events of the Gondwana assembly and were inferred to form in an extensional passive margin during the early Paleozoic. However, the younger felsic rocks were likely to have been generated by the interaction of the upwelling Kerguelen mantle plume and the pre-existing crust during the initiation of the eastern Gondwana break-up during the early Cretaceous. Our new findings reveal that two episodic magmatic events related to the eastern Gondwana assembly and the subsequent Gondwana break-up are responsible for the magmatism in the Siang window of eastern Himalayan syntaxis, northeast India.