J. Earth Syst. Sci. (2020) 129 47 Ó Indian Academy of Sciences https://doi.org/10.1007/s12040-019-1308-1 (0123456789().,-volV)(0123456789().,-volV) Petrological study of spinel peridotites of Nidar ophiolite, Ladakh Himalaya, India 1,2, 3 RANJIT NAYAK * and BIDYANANDA MAIBAM 1 Department of Earth and Atmospheric Sciences, NIT Rourkela, Rourkela 769 008, India. 2 Department of Geology, Addis Ababa Science and Technology University, Addis Ababa, Ethiopia. 3 Department of Earth Sciences, Manipur University, Canchipur, Imphal 795 003, India. *Corresponding author. e-mail: [email protected] MS received 12 March 2019; revised 24 September 2019; accepted 9 October 2019 Petrological study of the ultramaBc rocks from the Nidar Ophiolite Complex (NOC) of the Indus Suture Zone is carried out. The study of Cr-spinels along with olivine and pyroxenes emphasizes the genesis and tectonic setting of the ultramaBtes. Olivine from the harzburgite is Mg-rich, with the molar ratio Mg# [Mg/(Mg + Fe2+)] varying between 0.91 and 0.94 and olivine in dunite between 0.92 and 0.94. Clinopyroxene from the harzburgite is TiO2 and Na2O-poor diopside (Wo47–50En47–50Fs2–4). Spinel in harzburgite shows wide Cr#, molar ratio varied between 0.26 and 0.72, and significantly higher in dunites with Cr# ranges from 0.69–0.85. Cr# of the peridotite spinel follow a depletion trend. Calculated equilibrium conditions of the samples are 800–900°C temperature, 32 and 40 kbar pressure, oxygen fugacity À0.09 to 0.55 log units above the FMQ buAer. Residual nature of the harzburgites and the presence of high and low Cr# spinels may be due to the genetic artifact of the different ultramaBc units. Keywords. Spinel peridotites; Nidar Ophiolite Complex (NOC); supra-subduction zone (SSZ); partial melting. 1. Introduction petrological information of the upper mantle. Phase composition of the ophiolitic mantle ultra- The ophiolitic complexes of the world provide us maBc rocks is dependent on the degree and condi- vital clues in understanding magmatic and tectonic tions of partial melting and the eAects of process for the formation of oceanic lithosphere magma–rock interactions (Uysal et al. 2012). Well (Ahmed et al. 2005) and their emplacement on the preserved and fresh peridotite sequence in ophiolite margins of continental lithosphere. Ophiolite gen- complexes, can be a good source to examine the esis is in a variety of tectonic settings, either at the compositional variations and chemical geodynam- suprasubduction zone (SSZ) or mid-ocean ridge ics of upper mantle. (MOR) settings, as shown by refractory mantle The Indus Suture Zone (ISZ) are the remnants of character and Cr-rich spinel in peridotites (Pearce the Neo Tethyan ocean which closed during the et al. 1981; Dick and Bullen 1984). Peridotites are Early Cretaceous (110–130 Ma: Maheo et al. 2004: the most abundant rock type in ophiolitic com- cooling ages; *120–110 Ma: Pudsey 1986). It was plexes and are the key sources of geochemical and followed by obduction in Late Cretaceous in Supplementary materials pertaining to this article are available on the Journal of Earth Science Website (http://www.ias.ac.in/ Journals/Journal˙of˙Earth˙System˙Science). 47 Page 2 of 15 J. Earth Syst. Sci. (2020) 129 47 Kohistan–Ladakh terranes (Searle et al. 1999)asthe well exposed from Nidar village to Kyun Tso, the Indian plate wandered towards the Asian plate total thickness of the whole sequence is about 12 (Ahmed et al. 2008). An island arc origin was infer- km. The ophiolite complex can be divided into red by Thakur and Bhatt (1983) for the volcanic three main units from north to south. The ultra- sequence but Ahmed et al.(2008)proposedthatthe maBc unit is the basal unit of about 8–10 km in maBc unit of Nidar Ophiolite Complex (NOC) is thickness (Sachan 2001), gabbros in the middle and intra-oceanic arc magmatism. Previous studies at the top are the volcanic rocks consisting of (Sachan 2001;SiddaiahandMasuda2001) sug- sheeted dykes and volcanogenic sediments with gested that the formation of NOC is in a supra chert (Ravikant et al. 2004; Das et al. 2015). subduction zone environment. NOC is a well- Dunite, peridotite (harzburgite and lherzolite) and exposed Indus Suture Zone Ophiolite complex. The pyroxenite veins form the ultramaBcunit.Harzbur- occurrence of high-pressure mineral like clinoen- gite is the dominant peridotite. The samples have statite, coesite shows that the peridotites were undergone varying degrees of serpentinization. Dunite formed in the mantle transition zone (Das et al. is associated with the harzburgite and occurs as len- 2015). Diamond inclusions were also reported within soidal bodies at few places. The peridotites from NOC enstatite and olivine from peridotites of ISZ ophio- is cutting across by discordant dunite sequences in lite indicating the origin of peridotites at much great some places and peridotites are also encountered in the depth (*410 km) (Das et al. 2017). The ultramaBc Beld within the channelized dunite (Siddaiah 2001; rocks of the Nidar ophiolites are remarkably fresh, Sachan and Mukherjee 2003;Daset al. 2015). The and the relict primary minerals could be used to infer basal dunite body host podiform chromitite of about the original characteristics. In the present study, we 1–1.5 m thick and thin bends of chromitite (Sachan have analyzed primary silicate mineral phases of and Mukherjee 2003;Sachanet al. 2007). Pyroxenite mantle rocks of the Nidar ophiolite to Bnd out the veins are present in dunite and harzburgite as sills and equilibrium condition and related intensive param- dykes (Sachan 2001). The gabbroic unit of NOC shows eter of the mantle-sequence. The study is expected to variations from Bne grain to coarse grain without any give insights into the mantle processes (such as compositional layering. Pillow lava displays a con- partial melting and metasomatism) as well as the cordant association with the gabbros of the sequence tectonic setting of the Nidar ophiolite. and it is superimposed by volcano-sedimentary unit comprising basaltic to andesitic Cows. The lower stratigraphic part of the topmost unit has pillow lavas 2. Geology of the area and it consists of volcanogenic derived intercalated sediments like conglomerates, shale, chert, siltstone The ISZ is one of the main tectonic units of the and jasperite (Sharma and Gupta 1982). northern Himalayas, covering nearly 2500 km from Main Mantle Thrust at Pakistan (Tahirkheli et al. 1979; Ahmed et al. 2008) in the west to the Lhasa 3. Sampling and Beld observations block in the east (Le Fort 1975; Molnar and Tap- ponnier 1975). The sedimentary and ophiolite Fresh samples of harzburgite and dunite were sequences are the remnants of the Neo-Tethys collected along the Nidar valley to Kyun Tso. In seaCoor (Thakur and Mishra 1984; Ahmed et al. the study area, the dominant lithology is massive 2008). NOC is a block, thrust over the Indus Group and partially serpentinized harzburgites (*60%), and Ladakh batholith (Bgure 1) to the north and it followed by dunite (*20%), lherzolite (*5%) and has a thrust contact with the Zildat ophiolite pyroxenite veins. Undeformed coarse-grained melange in the south (Maheo et al. 2004). Further pyroxenite dykes (Bgure 2a) and dunite veins south the Zildat ophiolite melange has a thrust crosscut the harzburgite (Bgure 2b) at a few places. contact with the Tso Morari Crystalline Complex Channelized networks of dunite trending NE–SW bearing ultra-high pressure minerals (Mukherjee with disseminated chromite are also encountered et al. 2003). The general trend of Zildat ophiolite (Bgure 2c). The basal part of the mantle section is melange is NW-SE, sandwiched between Tso- marked by the presence of spinel-bearing dunite Morari Crystalline in the south and the NOC in the with moderate serpentinization (Das et al. 2015). north (Sen et al. 2013). It consists of serpentine, Here, we attempt to illustrate the petrogenetic gabbro, chert, exotic blocks of limestone and clasts processes that could explain the peridotite–dunite of blueschists. The sequences of Nidar ophiolite are association in the studied area. J. Earth Syst. Sci. (2020) 129 47 Page 3 of 15 47 Figure 1. SimpliBed geological map of Nidar Ophiolite Complex (after Thakur and Misra 1984). 4. Petrography euhedral and contain olivine inclusions and also occurs as euhedral to anhedral crystals 4.1 Harzburgite (Bgure 3e–g). In some of the sections, spinel occurs as dark red vermicular crystals and occasionally Modal composition of harzburgites ranges between magnetite rim surrounded spinel are also observed 69 and 78% olivine, orthopyroxene varying between (Bgure 3h). 15 and 21%, clinopyroxene from 1 to 3%, and spinel ranging between 1 and 2% and mainly shows pro- togranular and porphyroclastic texture. In pro- 5. Analytical techniques togranular texture, olivine and orthopyroxene grains display deformation features like kink bands and Selected samples of harzburgite and dunite were distorted lamellae in orthopyroxene (Bgure 3aand analyzed by a CAMECA SX100 electron micro- b). Recrystallization of olivine around orthopyrox- probe at the Department of Geology and Geo- ene is observed in harzburgites (Bgure 3b). physics, Indian Institute of Technology, Kharagpur Orthopyroxene occurs as large prophyroclasts with (India). Major element compositions of the mineral exsolutions of clinopyroxene (Bgure 3b). In some phases were measured with an accelerating voltage samples, orthopyroxene clasts host olivine inclusions of 15 kV with a beam diameter of 2 lm and probe (Bgure 3c). In porphyroclastic type of texture, large current of 20 nA. Natural and synthetic standards orthopyroxene grains form resorbed shapes and were used during the analysis. Standards are small clinopyroxene crystals with lobate boundaries. jadeite (Na-Ka), Cuor-apatite (F-Ka, P-Ka), NaCl The occurrence of spinels in harzburgite is in the (Cl-Ka), Fe O (Fe-Ka), diopside (Ca-Ka), MgO B 2 3 form of subhedral ( gure 3d).