Alteration of primary Cr-spinel mineral composition from the Suru Valley ophiolitic peridotites, Ladakh Himalaya: Their low-temperature metamorphic implications

Abstract

The present study focuses on the chemical modification of a Cr-spinel from the Suru Valley ophiolitic peridotites exposed near Trespone Village of Kargil district, Ladakh Himalaya. The Suru Valley peridotite is partially serpentinised with the preservation of the spinel and relics of olivine and pyroxene. These peridotites contain characteristic red-brown spinel grains of corroded grain boundaries. While analysing these grains for mineral chemistry, compositional variation was observed with Cr-rich cores rimmed by Cr-poor compositions. Secondary spinel compositions, i.e., ferritchromite and magnetite were observed along the margins and cracks of primary Cr-spinel grains. The primary Cr-spinel cores are identified as Cr-rich and are characterised by higher values of Cr³⁺# (0.5–0.6) and lower values of Al³⁺# (0.42–0.54). From primary spinel cores to altered rims it was observed that Cr³⁺# and Fe³⁺# increase while Mg²⁺# decreases due to Mg²⁺–Fe²⁺ and Al³⁺–Fe³⁺ exchange with surrounding silicates of host peridotite during alteration. On the basis of present spinel mineral chemistry, metamorphic alteration conditions were transitional between greenschist and lower amphibolite similar to most of the Neo-Tethyan ophiolite peridotites.

1 Introduction

The mineral assemblage of ophiolite peridotites cropped out at the surface of the Earth provides imperative information regarding the geochemical and geodynamic evolution of the upper mantle (Dai et al. 2011, 2013; Uysal et al. 2012; Khedr et al. 2014). The compositional variation of a chromium spinel [Cr-spinel; (Mg, Fe²⁺) (Cr, Al, Fe³⁺)₂O₄] is poorly known from these low-temperature metamorphosed ophiolite mafic–ultramafic rocks (Sack and Ghiorso 1991; Barnes 2000).

Primary minerals such as olivine and pyroxene in ophiolite mantle peridotites are usually altered to secondary low-temperature minerals (e.g., serpentine, chlorite); however, the Cr-spinel is the only preserved primary mineral even in completely serpentinised peridotites (Arai 1994a). Therefore, the study of its mineral chemistry is useful to understand the petrogenetic processes of host peridotites (Irvine 1965, 1967; Dick and Bullen 1984; Arai 1992, 1994a, b; Ahmed et al. 2001, 2005; Proenza et al. 2004; Farahat 2008; Aswad et al. 2011; Singh and Singh 2011; Colas et al. 2014, 2016; Bhat et al. 2017b). During low-temperature alteration, the change in chemistry of the primary Cr-spinel results in secondary mineral formation, i.e., Fe–Cr spinels also called ferritchromite and/or magnetite depends on the degree of alteration (Arai 1978). Despite secondary origin these ferritchromites actually have chemical characteristics of primary mantle Cr-spinels (Arai et al. 2006).

The purpose of the present paper is to describe the chemical variation of accessory primary Cr-spinels from Suru Valley ophiolitic peridotites and their low-temperature alteration implications.

2 Geology and petrography

The Ladakh Himalaya, marks the northern position in the Alpine–Himalayan Orogen, was formed due to collision between two major continental landmasses India on south and Eurasia on north during Cenozoic Era (Gansser 1964, 1980; Frank et al. 1977; Srikantia and Razdan 1980; Brookfield and Reynolds 1981; Honegger et al. 1982; Deitrich et al. 1983; Searle et al. 1987; Aitchison et al. 2000; Maheo et al. 2004; Ahmad et al. 2008 and references therein). The Neo-Tethys Ocean once separating Indian and Eurasian landmasses during Mesozoic start closing by the initiation of intra-oceanic island arcs thereby collision between continental margins and created suture zones outlined by ophiolites (Gansser 1980; Searle et al. 1987; Aitchison et al. 2000, 2002, 2007; Clift et al. 2002; Maheo et al. 2004, 2006; Ahmad et al. 2008; Buckman et al. 2018). These ophiolite slices are distributed along the Indus Suture Zone (ISZ) as dismembered ophiolite slices and from south-east to north-west are Nidar ophiolite sequence (Maheo et al. 2004; Ahmad et al. 2008), Spongtang ophiolite (Clift et al. 2002; Maheo et al. 2004; Buckman et al. 2018 and reference therein), Shergol ophiolitic slice (Honegger et al. 1989; Sinha and Mishra 1992, 1994; Maheo et al. 2006; Bhat et al. 2017a, b, 2018), Suru Valley ophiolitic slice (Reuber 1989; Robertson 2000; Bhat et al. 2019a) and Dras ophiolite (Radhakrishna et al. 1984, 1987). The Indus ophiolite belt extends westward to Pakistan, Iraq, Iran up to Turkey and eastward to Tibet, Nagaland-Manipur up to Indonesia (Hall 1997; Bortolotti and Principi 2005; Dilek and Furnes 2009) and these ophiolites are classified as Tethyan type ophiolites by Moores et al. (2000). Based on the radiometric age data and fossil record these ophiolitic slices are Mesozoic in age (Reuber et al. 1990; Aitchison et al. 2000; Maheo et al. 2004; Ahmad et al. 2008; Buckman et al. 2018). A supra-subduction zone setting of these ophiolites is commonly recognised based on the bulk rock geochemistry and mineral chemistry (Robertson 2000; Sachan 2001; Maheo et al. 2004, 2006; Ahmad et al. 2008; Buckman et al. 2018; Bhat et al. 2019a).

The regional geology of the study area is described in detail in our published papers (Bhat et al. 2017a, b, 2018, 2019a, b and references therein), here we explain the geological setup of the Suru Valley ophiolitic slice. The present peridotite samples were obtained from the Suru Valley ophiolitic slice along ISZ, outcropped at Trespone village of Kargil district Ladakh Himalaya (figure 1). The Suru Valley ophiolitic slice is mainly composed of peridotite of mantle origin serpentinised to various degrees and shows affinity towards alpine-type peridotites as compared to layered-igneous complexes (Bhat et al. 2019a).

Figure 1

Geological map of Kargil district, NW Himalaya after Reuber (1989) showing location of the study area.

These ophiolitic peridotites contain serpentine pseudomorphs after olivine, bastites after pyroxene (figure 2a) and subhedral–anhedral primary spinel grains (figure 2b). They show pseudomorphic and interpenetrating textures besides, mesh and hourglass texture were also observed. The serpentine pseudomorph after olivine is mainly chrysotile and/or lizardite in studied serpentinised peridotites (figure 2a). The presence of the bastite pseudomorph after orthopyroxene reflects harzburgite as protolith (Arai et al. 2006) which is also confirmed by their major and trace element geochemistry (Bhat et al. 2019a). Also the anhedral shape of the Cr-spinel and the stout shape of bastites reflect proto-granular texture of the protolith. Serpentine and magnetite are the main metamorphic byproducts after primary silicates (figure 2a). The primary red-brown Cr-spinels are characterised by zoning with darker cores of low reflectance compared to a light grey outer rim with high reflectance (figure 2c). The Cr-spinel polished surfaces under a scanning electron microscope (SEM) show two different reflectivities with a sharp optical boundary; the core region has low reflectivity while the rim region has high reflectivity (figure 2c and d). These secondary spinels with high reflectivity not only rim the primary Cr-spinel grains but also fill their cracks (figure 2d).

Figure 2

Photomicrograph and back-scattered electron (BSE) images of Suru Valley ophiolitic peridotites, western Ladakh Himalaya: (a) photomicrograph (under crossed polarised light) showing serpentine after primary olivine, orthopyroxene bastite, accessory spinels and secondary magnetite; (b) photomicrograph (under reflected light) showing serpentine after olivine and fractured subhedral spinel grain; (c) BSE-image showing zoned Cr-spinel grain with a low-reflectance core of Cr-spinel composition and high-reflectance rim of magnetite composition; and (d) BSE-image showing partly altered primary Cr-spinel grain with a metamorphic spinel of ferritchromite composition.

3 Analytical technique

A total of 18 thin sections from 10 samples of Suru Valley ophiolitic peridotites were examined under a polarising light microscope equipped with a digital camera at the department of Earth Sciences, University of Kashmir, Srinagar. A total of 36 mineral chemical analyses (three to four analyses from each sample) of the Cr-spinel were performed using an electron probe micro analyser (EPMA) CAMECA SX-Five instrument at the Banaras Hindu University, Varanasi, India. The CAMECA SX-Five Instrument was operating on a 15 kV accelerating voltage and 10–20 nA probe beam current at a diameter of 1 µm. Quality control was maintained by analysing well calibrated natural mineral standards (chromite for Cr, haematite for Fe and NiO for Ni) during each electron microprobe session. The proportion of ferrous and ferric iron in spinels was calculated by assuming spinel stoichiometry. Routine X-ray spectral analysis (calibration, acquisition and quantification) and data processing were carried out using SxSAB version 6.1 and SX-Results software of CAMECA and replicate analyses of individual points show an analytical error of <2%.

4 Results

Electron microprobe data of the secondary spinel minerals i.e., ferritchromite and magnetite from the Suru Valley ophiolitic peridotites are given in tables 1 and 2, respectively. For the primary Cr-spinel composition we have used our published data from Bhat et al. (2019a). In accessory spinel grains from studied peridotites, three different compositional zones from the core to rim were observed by EPMA studies, which are: (i) core zones of primary chromite (i.e., Cr-spinel) composition, (ii) the intermediate ferritchromite (or ferrian chromite) zone and (iii) the rim of magnetite (figure 2). These zones from the core to rim are characterised by a significant increase in Fe³⁺ and Fe²⁺, and a decrease in Mg and Al. The Cr-spinel core composition of the studied peridotites is characterised by high Cr₂O₃ (35.11–43.43 wt%), low Al₂O₃ (22.42–30.41 wt%), FeO (14.33–19.83 wt%) and MgO (9.28–13.51 wt%). Their Cr number [Cr³⁺#: cationic ratio of Cr³⁺/(Cr³⁺ + Al³⁺)], Mg number [Mg²⁺#: cationic ratio of Mg²⁺/(Mg²⁺ + Fe²⁺)] and Al number [Al³⁺#: cationic ratio of Al³⁺/(Cr³⁺ + Al³⁺ + Fe³⁺)] ranges from 0.5 to 0.6, 0.4 to 0.5 and 0.5 to 0.6, respectively (Bhat et al. 2019a). Also, the Al₂O₃ concentration ranges from 1.01 to 1.75 whereas TiO₂ is <0.06 wt% (Bhat et al. 2019a).

Table 1 Representative electron-microprobe analyses of ferritchromite (rim composition) from Suru Valley peridotites along ISZ, NW Ladakh Himalaya (calculation based on 32 oxygen).

Table 2 Representative electron-microprobe analyses of magnetite (rim composition) from Suru Valley peridotites along ISZ, NW Ladakh Himalaya (calculation based on 32 oxygen).

In the studied peridotites, the relic olivine grains range in composition from Fo₉₀–Fo₉₂ and are similar to the primary olivine relics of ophiolitic peridotites (meta-ultramafics) from the south Eastern Desert (SED), Egypt (Fo_89–96 after Khudeir 1995; Khalil and Azer 2007; Khedr and Arai 2013; Obeid et al. 2016), Fawakhir area of Central Eastern Desert (CED), Egypt (Fo_89–92 after Abdel-Karim et al. 2018) and Spongtang ophiolite peridotites (Fo_87–92 after Jonnalagadda et al. 2019). This olivine chemistry corresponds to chrysotile and/or forsterite composition (Dick and Bullen 1984). Also, the Mg number [Mg²⁺#: cationic ratio of Mg²⁺/(Mg²⁺ + Fe²⁺)] of olivine is high and ranges from 0.90–0.92 (Bhat et al. 2019a). The relic orthopyroxene of enstatite composition (En_91–93Fs_7–9) is highly magnesian in nature with Mg²⁺# varies from 0.91 to 0.93 similar to Spongtang peridotites (En_89–90, Fs_9–10 after Jonnalagadda et al. 2019).

The studied Cr-spinel compositions from the Suru Valley peridotites when plotted in the Al³⁺–Cr³⁺–Fe³⁺ ternary diagram characterise three compositional varieties i.e., Cr-spinel, ferritchromite and magnetite (figure 3). The ferritchromite and magnetite rims mantling the Cr-spinel cores are the alteration/metamorphic products of primary Cr-spinel and are highly depleted in Mg²⁺ and Al³⁺ as compared to Cr-spinel cores. Similarly, relative to Cr-spinel core composition, ferritchromite has slight depletion of Cr³⁺ compared with that of magnetite which shows more depletion. The only cations i.e., Fe²⁺ and Fe³⁺ are significantly enriched at the rim (i.e., magnetite) compared to the unaltered core (i.e., Cr-spinel). Also relative to the Cr-spinel core and magnetite rim, Mn and Si show enrichment in ferritchromite (table 1). These compositional discrepancies reflect diffusion of Al, Mg and Cr and infusion of Fe, Mn and Si with respect to the Cr-spinel core composition during alteration. This chemical variability in studied spinels further reflects the existence of two compositional miscibility gaps, one between the Cr-spinel core and ferritchromite zone and other between the ferritchromite zone and magnetite rim (figure 3).

Figure 3

Triangular trivalent cation classification diagram showing the compositional variation of spinel grains from the Suru Valley ophiolitic peridotites, Ladakh Himalaya.

The extensive Mg²⁺–Fe²⁺ divalent exchange between the coexisting spinel and silicate phases (e.g., olivine) in mantle peridotites results in decreasing Mg²⁺# from the Cr-spinel core through the ferritchromite to magnetite rim (figure 4a). Whereas due to Al³⁺ loss and Fe³⁺ enrichment during alteration of mantle peridotites, Fe³⁺# [cationic ratio of Fe³⁺/(Fe³⁺ + Cr³⁺ + Al³⁺)] increases with decreasing Mg²⁺# from the Cr-spinel core to the magnetite rim (figure 4b).

Figure 4

Suru Valley peridotite spinel composition plotted in (a) Cr³⁺/(Cr³⁺ + Al³⁺) against Mg²⁺/(Mg²⁺ + Fe²⁺) and (b) Fe³⁺/(Fe³⁺ + Al³⁺ + Cr³⁺) against Mg²⁺/(Mg²⁺ + Fe²⁺).

The Cr-spinel in the studied peridotites has higher Cr³⁺# (>40) akin to that of ophiolitic peridotites of Bela, Muslimbagh-Zhob and Waziristan from Pakistan (Arif and Jan 2006), SED and CED from Egypt (Farahat et al. 2011; Ahmed et al. 2012; Abdel-Karim et al. 2016; Obeid et al. 2016) and Spongtang ophiolite from Ladakh Himalaya (Jonnalagadda et al. 2019). However, the Cr-spinel has lower Cr³⁺ than Tidding Suture Zone serpentinites from eastern Himalaya (Singh and Singh 2011) and higher than Manipur ophiolitic peridotites from Indo-Myanmar orogenic belt (Singh 2009), Shergol ophiolitic peridotites from western Ladakh (Bhat et al. 2017b) and Kamyaran ophiolitic peridotites from Iran (Veisinia et al. 2018). Also, the studied Cr-spinel TiO₂ and Al₂O₃ contents are comparable with supra-subduction zone peridotites and overlap with Spongtang and Nidar ophiolitic peridotites of Ladakh Himalaya (Maheo et al. 2004; Jonnalagadda et al. 2019) and Izu–Mariana arcs (Kamenetsky et al. 2001).

5 Discussion and conclusion

5.1 Geochemical consequences due to alteration

The Suru Valley ophiolitic peridotites from western Ladakh Himalaya have experienced serpentinisation – a low-grade hydrothermal metamorphism in the presence of serpentine and magnetite as a secondary mineral assemblage (Bhat et al. 2019a). Despite mineralogical modifications due to the serpentinisation process of mantle peridotites, geochemical studies on global peridotites show insignificant changes in the major elements (except for Ca, Na, K and to some extent Mg) at the hand-specimen scale (Mevel 2003; Niu 2004; Deschamps et al. 2010, 2013). Similarly, serpentinised peridotites specify insignificant modifications in the trace element compositions except Rb, Ba, U, Pb and Sr (Niu 2004; Paulick et al. 2006; Deschamps et al. 2013). Recently, Bhat et al. (2019a) reported 5–8 wt% MgO loss, the mobility of Fe, Ca, Na, K, Rb, Ba, U, Pb, Sr, etc., the pristine characters of Si, Al, Ti and highly incompatible rare earth and high-field strength elements in Suru Valley serpentinised with peridotites on the basis of bulk-rock major and trace element geochemistry.

Generally, in ophiolite peridotites the serpentinisation process leads to the alteration of a primary silicate mineral assemblage (Khudeir et al. 1992; Khalil and Azer 2007). Primary minerals like olivine and pyroxene may completely alter to serpentine, whereas the spinel is the only primary mineral that can preserve its composition even in completely serpentinised peridotites and may show zoning from the core to rim depending on the extent of the serpentinisation process and post-serpentinisation metamorphism (Arai et al. 2006). According to Arai (1994b), the Cr³⁺# of the spinel in mantle peridotites appears to remain unchanged during the sub-solidus stage. Chromite alteration caused by fluid infiltration along cracks and grain boundaries during low-temperature metamorphism leads to the growth of Fe-enriched chromite i.e., ferritchromite (Barnes 2000; Farahat 2008). Further at higher temperature, fluid entrance gives rise to extensive magnetite replacement of chromite (Barnes 2000). In figure 3 the magnetite plots along the line joining Cr³⁺–Fe³⁺ closer to Fe³⁺ apex reflect Fe₂O₃ increase and loss of Cr₂O₃ and Al₂O₃ during alteration. Their high-ferric iron (i.e., Fe³⁺) reflects oxidation conditions during alteration (Anzil et al. 2012). The ferritchromites preserve high Cr₂O₃ (21–29 wt%) close to those of unaltered Cr-spinel cores but have lower Mg²⁺# (0.01–0.1).

The secondary alteration/metamorphism typically imposes a chemical zoning on the primary spinel grains in the Suru Valley peridotites. The alteration of Cr-spinel composition (represented by 1–10 profile points in figure 2d) to ferritchromite (represented by 11–20 profile points in figure 2d) is characterised by a progressive enrichment in the total iron content (Fe³⁺ + Fe²⁺) and Mn (figure 5). However, the alterations are characterised by a strong depletion in Al, Cr and Mg as compared with the relic Cr-spinel cores (figure 5a) and a negligible change in Ti (figure 5b). Similarly, the magnetite rims (represented by 21–30 profile points in figure 2d) show a strong systematic enrichment in Fe³⁺ and Fe²⁺ (figure 5a), whereas strong depletion in Al, Cr, Mg and Mn with respect to ferritchromite (figure 5). Therefore, compositional profiling across the studied spinel grains proves chemical variation from the core to rim as observed in other global peridotite spinels (e.g., Proenza et al. 2004; Karipi et al. 2007; Farahat 2008; Aswad et al. 2011; Singh and Singh 2011; Colas et al. 2014; Bhat et al. 2017b).

Figure 5

Compositional profile of electron-microprobe data across a compositionally zoned accessory Cr-spinel grain in Suru Valley peridotites: (a) Cr₂O₃, Fe₂O₃, FeO, Al₂O₃ and MgO chemical variation along profile and (b) MnO and TiO₂ chemical variation along profile.

5.2 Low-temperature metamorphic implications on spinel composition

The compositional heterogeneity of the Cr-spinel is a function of the fluid-rock ratio and metamorphic grade (Barnes 2000; Farahat 2008). Therefore, post-magmatic hydrothermal metamorphism of host peridotites may cause chemical inconsistency in spinel mineral composition (Barnes 2000; Farahat 2008; Aswad et al. 2011). The Cr-spinel is the only reliable petrogenetic indicator in altered peridotites where all primary silicates were altered due to metamorphism (Ahmed et al. 2001, 2005). The alteration of the primary Cr-spinel results in the modification of its mineral chemistry in relation to the alteration of surrounding silicate minerals (Barnes 2000). The Cr-spinel in altered peridotites enhances its Cr³⁺# as well as magnetite component (both Fe²⁺ and Fe³⁺) to become so-called ferritchromite (Arai et al. 2006). This secondary spinel with high Cr³⁺# and low Fe³⁺# can be achieved by the removal of Al without Fe addition (Barnes 2000; Arai et al. 2006).

As described, the studied spinel grains show systematic compositional variation along their rims or through cracks from the Cr-spinel to ferritchromite or from the Cr-spinel to magnetite. However, the ferritchromite is the most common alteration phase observed in spinels elsewhere (Kimball 1990; Barnes 2000; Proenza et al. 2004; Farahat 2008; Aswad et al. 2011; Gervilla et al. 2012; Colas et al. 2014, 2016; Bhat et al. 2017b). Based on the earlier studies, the origin of ferritchromite is related to metamorphic and/or hydrothermal processes. According to Bliss and MacLean (1975) and Barnes (2000) ferritchromite is produced by the reaction of pristine spinel cores with magnetite rims during prograde metamorphism of the host serpentinised ultramafic rocks. Likewise, Evans and Frost (1975) concluded that the Cr content of spinels increases with an increase in grade of the amphibolite facies. On the other hand, the results obtained by Burkhard (1993) suggest the opposite trend, and Roeder (1994), Gervilla et al. (2012) and Colas et al. (2014) showed ferritchromite to be the product of a reaction between Cr-spinels and chlorite from host rocks. Hence, the origin of ferritchromite remains an unsolved problem, the resolution of which is beyond the scope of this paper. However, both the models discretely assume that the Cr-spinel cores represent relic primary chromite that remained unaffected by the metamorphism.

Magnetite formation along fractures and rims (figure 2c and d) is the result of Mg²⁺ and Fe²⁺ cation exchange between spinels and coexisting silicates, particularly olivine (Barnes 2000; Karipi et al. 2007; Farahat 2008) which in turn changes into serpentine (Bhat et al. 2019a). In studied peridotites, the olivine and pyroxene minerals are highly magnesian i.e., Fo_90–92 and En_91–93, respectively (Bhat et al. 2019a); therefore, limited amounts of Fe are released on their hydration. The activity of the low-fluid-rock ratio during metamorphism may be inferred because of the common preservation of Cr-spinel core compositions in the studied peridotites. By the diffusive replacement of Mg²⁺ with Fe²⁺ in the spinel structure, the ferritchromite and magnetite in studied peridotites show higher Cr³⁺# and Fe³⁺# but lower Mg²⁺# as compared to Cr-spinel core compositions (figure 4). This chemical discrepancy is due to preferential retention of Cr in unaltered Cr-spinel cores while its alteration provides Mg for secondary spinels (Kimball 1990; Karipi et al. 2007). Although the compositional differences between the primary Cr-spinel and secondary spinel, i.e., ferritchromite are small (figure 3) their prominent optical/chemical boundary advocates are controlled by a miscibility gap (Barnes 2000; Proenza et al. 2004; Arai et al. 2006; Farahat 2008).

The possibility of alteration of the primary spinel composition of the mantle origin is believed to be recognised easily by their marked increase of Fe³⁺# (Arai et al. 2006). In the present study, the primary Cr-spinels are characterised by Cr³⁺# (0.5–0.6) and Fe³⁺# (0.01–0.13), whereas secondary spinels i.e., ferritchromite have Cr³⁺# (0.8–0.9) and Fe³⁺# (0.5–0.6). These secondary spinels have chemical composition within the range of spinels from mantle peridotites and podiform chromites (Arai and Yurimoto 1994; Roeder 1994). With respect to the global Cr–Al spinel trends, their compositions plot at the high Cr³⁺# and low Mg²⁺# ends in Mg²⁺# vs. Cr³⁺# and Mg²⁺# vs. Fe³⁺# spaces (figure 4), respectively (Barnes and Roeder 2001). The same results were observed by Arai et al. (2006) while studying Cr-spinels of serpentinised harzburgites and chromitites from Rayat area of northeastern Iraq. Thus, the present study suggests that while interpreting high Cr³⁺# Cr-spinels of mantle composition in altered rocks, caution is needed because for petrogenetic interpretation only primary Cr-spinels give paramount results.

Metamorphism of greenschist to lower amphibolite facies substantially modifies the primary spinel composition through infiltrating metamorphic fluids (Wylie et al. 1987; Barnes 2000). In spinels, the Cr-spinel core compositions equilibrated with ferritchromite and/or magnetite are usually attributed to greenschist compared to lower amphibolite facies metamorphism (Barnes 2000; Kapsiotis et al. 2007; Farahat 2008; Aswad et al. 2011). The relative proportion of trivalent ions i.e., Cr³⁺, Al³⁺ and Fe³⁺ in spinels is not modified up to lower amphibolite metamorphic grade and therefore can be used to estimate the metamorphic grade experienced by the host peridotites (Barnes 2000). Barnes (2000) also suggested that the metamorphism effectively controls Mg²⁺# of spinels. Peridotites experienced greenschist to lower amphibolite facies metamorphism possess spinels with Mg²⁺# ranging from 0.4 to 0.7; however, higher amphibolite grade rocks have Mg²⁺# <0.35 (Barnes 2000; Farahat 2008). Mg²⁺# in the studied Cr-spinel ranges from 0.5 to 0.6, reflecting green schist grade of metamorphism.

Also, the primary Cr-content is retained in Cr-spinels equilibrated below ~500–550°C stability limit but their Mg²⁺# is substantially lowered by Fe²⁺–Mg²⁺ exchange with surrounding silicates (Farahat 2008). The chemical composition of analysed spinels (i.e., Cr-spinel, ferritchromite and magnetite) from Suru Valley peridotites in relation to metamorphic temperature conditions is plotted on a triangular Fe³⁺–Cr³⁺–Al³⁺ diagram (figure 6a). In this diagram the Cr-spinel core composition plots outside these stability limits reflecting primary spinel compositions not being affected by metamorphism; however, ferritchromite and magnetite plots below 600°C stability limit indicate secondary compositions. Along grain boundaries and through cracks, magnetite was formed at lower temperature because of the readily exchange of Fe from host peridotite silicates e.g., olivine during serpentinisation (Gahlan et al. 2006). In mantle peridotites pure magnetite rims with limited Cr solubility, implying magnetite growth well below 500°C (Barnes 2000). The studied magnetite rims have substantial amounts of Cr concentration (0.05–1.66 wt%; table 2) and thus reflecting magnetite growth well below 500°C stability limit as shown in figure 6(a).

Figure 6

Compositional changes in spinels from Suru Valley peridotites (Cr-spinel, ferritchromite and magnetite) expressed in a triangular Cr³⁺–Fe³⁺–Al³⁺ plot after Barnes (2000): (a) studied spinel compositions compared with spinel stability fields after Sack and Ghiorso (1991) and (b) studied spinel compositions compared with spinel compositional fields from different metamorphic facies after Evans and Frost (1975).

Also, the analysed spinel chemical composition is plotted on a triangular Fe³⁺–Cr³⁺–Al³⁺ diagram, together with the spinel compositional fields from different metamorphic facies (figure 6b), where spinel compositional changes have been recorded with increasing metamorphic grade (Evans and Frost 1975; Frost 1991; Barnes and Roeder 2001). In this diagram, the altered spinel compositions characterised by lower Al content plot along the Cr–Fe³⁺ joint and are typical of spinels altered at lower temperatures (Roeder 1994; Proenza et al. 2004). As shown in figure 6(b), the altered spinel compositions lie outside the upper-amphibolite to granulite facies spinel compositional field; however, the plot within the lower-amphibolite to the greenschist facies compositional field is similar to peridotites that have experienced low-PT metamorphism (Proenza et al. 2004; Colas et al. 2014; Bhat et al. 2017b). This interpretation is also favoured by the chemical composition of relic primary Cr-spinels and the whole-rock chondrite normalised rare earth element distribution patterns of the host peridotites (Bhat et al. 2019a). Similar geochemical characteristics have been observed in other Tethyan ophiolites that have not experienced eclogite facies metamorphism e.g., Iti and Kallidromon (Pindos) ophiolite, central Greece (Karipi et al. 2007), ophiolites of CED of Egypt (Farahat 2008), Serpentinites of Tidding Suture Zone, eastern Himalaya (Singh and Singh 2011) and Shergol ophiolitic peridotites, western Ladakh (Bhat et al. 2017b).

According to Barnes (2000) and Kapsiotis et al. (2007), due to greenschist to lower amphibolite grade metamorphism, Cr-spinels of mantle peridotites alter to magnetite and/or ferritchromite along grain margins and fractures as these are the only sites for metamorphic fluid interaction. The complete replacement of primary Cr-spinel grains with magnetite commonly occurs at mid to higher amphibolite grade metamorphism of host peridotites (Farahat 2008). In studied peridotites, the primary Cr-spinel composition is mantled by magnetite rims and/or ferritchromite, thus provides the textural indication for transitional greenschist to lower amphibolite facies metamorphism experienced by the host peridotites. The higher metamorphic conditions such as mid-amphibolite or granulite grade have not been achieved by the host peridotites because of the absence of complete replacement of Cr-spinel grains with magnetite compositions. Therefore, on the basis of the present spinel mineral chemistry the metamorphic conditions experienced by Suru Valley ophiolite peridotites are transitional greenschist to lower amphibolite facies consistent with an estimated metamorphic equilibration temperature of ~500–600°C (figure 6a). Further, in a high-pressure metamorphic environment, the degree of fluid–rock interaction is not pervasive (Cartwright and Barnicoat 1999). Also, the high-pressure metamorphism induces a local fluid flow at low water–rock ratios, therefore displays a closed-system behaviour (Fruh-Green et al. 2001). Accordingly, we hesitantly suggest that under high-pressure conditions there is no resetting of the primary igneous composition of Cr-spinels. Therefore, the main alteration products observed in the Suru Valley peridotite spinels are the consequences of a low-P and T metamorphic alteration.

The chemical changes in primary Cr-spinel grains of Suru Valley peridotites during metamorphism with element exchange noted with arrows are shown in figure 7. The compositional data show that Fe and to a less extent Mn are introduced into the Cr-spinel, whereas Al, Mg and Cr of the Cr-spinel show outward diffusion during metamorphism. This results in a decrease in Mg/(Mg + Fe²⁺) and an increase in Cr/(Cr + Al) of the magnetite compared to the unaltered Cr-spinel core. Although, Mn²⁺ usually follows Fe²⁺, the ionic radius of Mn²⁺ (0.91 Å) is much larger than that of Fe²⁺ (0.83 Å) and Mg²⁺ (0.78 Å), its ionic potential is relatively low and hence it is more susceptible to leaching by weakly acidic solutions (Economou-Elipoulos 2003). The studied ferritchromite with high-Mn content (MnO ranges from 1.77 to 4.37 wt%) is due to the ability of the Cr-spinel to act as a favourable Mn-receptor during metamorphism as the secondary silicates (e.g., serpentine) accommodate less Mn than their igneous precursors (Deer et al. 1992). Therefore, the availability of Mn²⁺ in solution may be the major controlling factor for the incorporation of Mn in the lattice of ferritchromite and to some extent Cr-spinel (Sack and Ghiorso 1991; Farahat 2008). This is the reason for the significant Mn enrichment up to 4.37 and 2.23 wt%, respectively, in ferritchromite and the Cr-spinel in the Suru Valley ophiolite peridotites. Similar explanation was given by Singh and Singh (2011) for the spinels from the Tidding Suture Zone, eastern Himalaya.

Figure 7

Schematic illustration of chemical changes in the primary Cr-spinel of Suru Valley peridotites during secondary alteration and metamorphism. The extent of element exchange is noted with arrows.