Textural Fingerprints of Magmatic, Metamorphic and Sedimentary Rocks Associated with the Naga Hills Ophiolite, Northeast India

Abstract

The Naga Hills Ophiolite (NHO) in the northern sector of Indo-Myanmar Ranges (IMR) constitutes a long (200 km), narrow (2–15 km wide), NNE-SSW trending, westerly convex sigmoidal belt of Maestrichtian–Paleocene age. It comprises peridotite tectonite, serpentinite, garnet lherzolite xenolith, layered sequence of ultramafic–mafic cumulates, plagiogranite, minor dolerite dykes, basalts, pyroclastic rocks, and pelagic sediments containing radiolaria. The litho-assemblages are highly tectonised and dismembered and occur as imbricated sheets. Presence of a suite of low-grade prehnite-clinochlore schist and greenschist, and high-pressure rocks, viz., glaucophane schist and metachert containing glaucophane and eclogite, gives evidence of their development in convergent lithospheric plate junctions. The rocks have undergone three phases of deformations correlatable with the Himalayan orogeny accompanied by closure of the Tethyan Sea. Petrographic study of magmatic and metamorphic rocks from the ophiolite suite reveals several new textural and mineralogical features that are reported for the first time in this work. Notable amongst these are quenched and vitrophyric textures in basalts, interaction of pelitic xenolith with basalt, and presence of a wide range of volcaniclastic rocks. Occurrences of perovskite in crystal–lithic tuff and welded tuff suggest their derivation at high temperature. The quenched texture in basalt characterized by elongated, radiating and curved crystals of hollow core plagioclase filled with clinopyroxene indicates rapid cooling in aqueous environment. Veins of plagiogranite in such basalts strongly support their derivation due to partial melting at high temperature. These features together with other supporting evidence like chemistry (from published works) of basalts reflect close similarity with those formed at the present day mid-ocean ridge (MOR) of divergent plate boundaries. Absence of olivine in basalts and presence of K-feldspar in pyroclastics (lithic tuff) indicates pronounced fractionation of lava prior to eruption. Micro-structural features like ‘mica-fish’ in glaucophane schist and ‘S-C mylonite’ in garnet lherzolite xenolith support emplacement of ophiolite in ductile shear zone, contradicting advocates of NHO representing an overthrusted klippe. This lends strong evidence to the idea that the IMR defines the eastern suture of the Indian plate and its emplacement is due to the result of eastward convergence of the Indian plate colliding with the Eurasian plate (Sino-Myanmar microplate).