The Nagaland‐Manipur Ophiolites (NMO) in northeast India is known for its complex geological history. Tough terrain, thick vegetation, and dismembered exposure of ophiolitic suite of rocks in the region made uneasy for geological investigation and put it in a deadlock for a long time. Only in the last decade has seen an appreciable amount of publications but the results boil down to a hot debate between two opposite schools of thoughts of subduction origin versus non‐subduction origin of the NMO. In this article, we revisit the literature data and compare it with our new geochemical data with an attempt to provide fresh insight into the long‐standing debate on the geodynamic evolution of the NMO. Our investigation arrives at the conclusion that the NMO as a whole cannot be considered as 100% subduction or 100% non‐subduction origin. It is indeed a combination of both. The non‐subduction group of mafic rocks shows a high ratio of incompatible elements (Nb/Yb >1), high‐Ti, enriched LILE, and HFSE with primitive mantle normalized values >5. Their bulk‐rock geochemical composition is equivalent to mid‐ocean ridge basalt (MORB) and ocean island basalt (OIB). The subduction group of rocks, on the other hand, shows a low ratio of incompatible elements (Nb/Yb <1), low‐Ti, depleted LILE, and HFSE with primitive mantle normalized values <1, affinity to the fore‐arc depleted N‐MORB type. Similarly, spinels present in subduction‐influenced mantle rocks show high chromium content (Cr# >50) but it is lower (Cr# <50) in non‐subduction abyssal peridotites of the NMO. Such geochemical variations cannot simply be explained by fractional crystallization or variable degree of partial melting of a single source, but rather signifies derivation from different tectonic settings of subduction and non‐subduction magma factories. We further conclude that the primary compressional force of India‐Myanmar Plate collision and secondary strike‐slip faults running along this ophiolite belt jeopardized the accretionary process which led to distortion and dismembering of the rocks like a scrambled bread.
This paper reports new zircon U–Pb ages and Hf isotopic compositions of felsic units of the Abor volcanic rocks (AVR) of Eastern Himalayan Syntaxis (EHS), Northeast India, and discusses their relationship to the Kerguelen plume activity. The AVR are bimodal and predominantly constituted by mafic rocks with minor felsic units. Mafic volcanics are identified as basalt and basaltic andesite with light rare earth elements (LREE) enriched and slightly depleted heavy rare earth elements (HREE) pattern without Eu anomalies. Low concentrations of LILE, high contents of Fe2O3, and other incompatible trace elements ratios reflect enriched nature of these mafic volcanics. Felsic volcanic rocks are dacitic to rhyolitic in composition, which have high REE content, high LREE/HREE, and pronounced negative Eu anomalies. Enriched LREE, high Th/Nb, Ce/Nb ratios, and variations in Rb/Zr, K/Rb, La/Sm ratios with negative anomalies of Ba, Nb, Sr, P, Ti in felsic rocks suggest substantial contribution of crustal contamination at the time of eruption. Zircons from felsic units yield an average U–Pb age of ~132 Ma and unradiogenic (ƐHf(t) < 0) Hf isotope values of −7.0 to −13.3 with model ages between 1.5 and 2.1 Ga, suggesting old crustal assimilation in their genesis. The AVR were emplaced in the continental rift tectonic setting, and depth of the magma source is confirmed as near spinel stability zone. The AVR are positively comparable with other flood basalts that were formed due to the Kerguelen plume activity. Therefore, our combined new geochemical and geochronological data show that the AVR were emplaced at early stage (~132 Ma) of eastern Gondwana breakup due to outbreak of the Kerguelen plume. This study thus supports the idea of the Kerguelen plume affecting a large area of Eastern India, Western Australia, and Antarctica during early stage of Gondwana breakup.
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