RESEARCH COMMUNICATIONS CURRENT SCIENCE, VOL. 121, NO. 5, 10 SEPTEMBER 2021 685 *For correspondence. (e-mail: nayak.ranjit213@gmail.com) Low-titanium clinopyroxene composition of Nidar ophiolite gabbros, southeastern Ladakh Himalaya, India: implications to geotectonic setting Ranjit Nayak 1,2, * and Debasis Pal 1 1 Department of Earth and Atmospheric Sciences, National Institute of Technology, Rourkela 769 008, India 2 Department of Applied Geology, School of Applied Natural Science, Adama Science and Technology University, 18888 Adama, Ethiopia The Nidar ophiolite complex is one of the well- preserved ophiolite sequences of the Indus Tsangpo Suture Zone (ITSZ) towards the southeastern part of Ladakh Himalaya, India. This study presents petro- graphy and clinopyroxene mineral chemistry of gab- broic rocks from the Nidar ophiolite. These gabbros are massive, essentially composed of plagioclase and clinopyroxene with minor amounts of olivine, ortho- pyroxene, hornblende and magnetite. The clinopyrox- enes are very low in TiO 2 (0.05–0.77 wt%) and Na 2 O (0.12–0.85 wt%) but rich in SiO 2 (52–55 wt%). It is observed that there is a wide variation of CaO (12.26– 23.88 wt%) and in the Wo–En–Fs ternary diagram, clinopyroxene shows augitic to diopside compositional variation. These low-titanium clinopyroxenes are in- ferred to be tholeiitic in nature with an island-arc bo- ninitic affinities. Keywords: Gabbro, island-arc tholeiite, Ladakh Hima- laya, low-Ti clinopyroxene, Nidar ophiolite. THE compositional variation of clinopyroxene depends on the magma types related to the tectonic setting of the ophiolite sequences 1 . The high-titanium clinopyroxenes are associated with the mid-ocean ridge and back-arc/ marginal sea magmatism. The low-titanium (Ti) clinopy- roxenes are associated with the island arc and boninite magmatism above the subduction zone. The aluminium (Al) and Ti concentration in clinopyroxenes generally increases from tholeiitic to per-alkaline magma series, and the activity of silica controls such elemental distribu- tion during the crystallization of magma 2,3 . Clinopyro- xenes crystallized from iron (Fe)-depleted magma will have higher Ti content 4 . Earlier studies have shown that the clinopyroxene mineral chemistry can be used as an alternative tool for interpreting magmatic affinities and palaeo-geodynamic setting of ophiolites 1,4–16 . The magmatic signatures of clinopyroxenes usually remain unaltered even in metasomatized rocks 1,4,17 . Such behaviour of clinopyroxenes allows us to characterize the magmatic affinities in different tectonic settings based on statistical studies of mineral chemistry along with differ- ent discrimination plots 1,18 . Though extensive geochemi- cal data are available on gabbros and associated rocks of the Nidar ophiolite 19,20 , less emphasis was given on min- eral chemistry to understand the genesis and tectonic set- ting of the host mafic rocks. Clinopyroxene is a common mineral found in mafic rocks, and preserves vital infor- mation about the parental magma and mantle source compositions 21–23 . Therefore, the main objective of the present study was to decipher the petrogenesis of gabbros and interpret the geodynamic setting of Nidar ophiolite based on the mineral chemistry of clinopyroxenes. Fur- ther, we compared our results with the available data of other Neo-Tethyan ophiolites. The Nidar ophiolite is sandwiched between Tso Morari crystalline complex towards the south and Indus Forma- tion towards the north (Figure 1). The total thickness of Nidar ophiolite varies from 3 to 12 km (ref. 24) with a maximum width of 12 km along the Nidar valley 25 . The Nidar ophiolite is a part of the Indus Suture Zone (Figure 1 c), characterized by ophiolites and ophiolite mélanges all along its length from Nanga Parbat to Namche Barwa 26 . The Nidar ophiolite complex can be subdivided into three litho-units 25,27–29 . The ultramafics at the bot- tom, followed by mafic and subsequently the volcanic units and pelagic sediments, including chert and lime- stone at the top. The ultramafic unit mainly consists of peridotites (harzburgite, lherzolite and dunite) with sub- ordinate amount of chromitite and pyroxenite veins. Chromitites occur as thin parallel veins with variable thickness (1–100 cm) within the basal dunite 24,29 . Mas- sive and isotropic gabbros are well exposed in the north- western part of Nidar village. Well-preserved sills and dykes of plagiogranites within the gabbro unit are ob- served near the Shyrok stream (Figure 2 a). Occasionally, the gabbros also exhibit compositional layering (Figure 2 b). Pillow lavas from the topmost unit display a concor- dant association with the gabbros and are superimposed by a volcano-sedimentary unit comprising basaltic to andesitic flows, limestone, shale, green chert, siltstone and jasperite 20,29,30 . The Nidar ophiolite gabbros are fine- to medium- grained, mesocratic and massive in hand specimen. They are essentially composed of plagioclase and clinopyro- xene with minor amounts of olivine, orthopyroxene, hornblende and magnetite, with an intergranular and ophitic to subophitic textural relationship. The gabbro samples were metamorphosed to some extent, but the primary magmatic phases are still preserved (Figure 2 c). Plagioclase is mostly subhedral lath-shaped and occurs as phenocrysts set within the groundmass of plagioclase, pyro- xene, hornblende and opaque minerals. Saussuritization is more commonly observed along the grain boundary. Occasionally, phenocrysts of plagioclase exhibit albite twinning and zoning (Figure 2 c). Clinopyroxene in gabbros is subhedral to anhedral in nature and displays panidiomorphic to allotriomorphic texture (Figure 2 d). It