1 Introduction

Chromite [(Mg, Fe2+) Cr2O4] is one of the end members of the spinel group, represented by the general formula AB2O4. While it frequently occurs as an accessory mineral in mafic to ultramafic rocks, it only appears as a dominant mineral in chromitite (Irvine 1965; Rollinson et al. 2002). Because it is the first mineral to crystallise from ultramafic magma and a highly sensitive indicator of the primary magma composition, it is frequently utilised as a petrogenetic indicator (Irvine 1965; Cameron 1975; Zhou et al. 2014; Gonzalez-Jimenez et al. 2015; Ishwar-Kumar et al. 2016; Kang et al. 2022). Due to this, many researchers have focused on how the degree of mantle melting, the content of the magma, the order of crystallisation, pressure–temperature parameters, and tectonic context all affect the composition of chromite crystallisation from magma (Barnes and Roeder 2001; Rollinson 2005; Karipi et al. 2007; Boudier et al. 2021).

The composition of the chromite is controlled by the different processes operating in the magma chamber like magma differentiation, magma mixing, disequilibrium crystallisation assimilation and cooling rate of magma (Thy 1983; Scowen et al. 1991; Yang et al. 2021). Dick and Bullen (1984) proposed that C# increases with the increase of the degree of melting. Bannister et al. (1998) demonstrated that continued growth of magnetite on chromite and diffusion in the spinel during cooling of the lava flow gave rise to magnetite, which result in Cr/Fe3+ > 1 in magnetite while Fe3+# (Fe3+/(Cr + Al + Fe3+) > 0.3 in chromite. Takahashi (1987) recorded that Fo content of the olivine was inversely related to the Ti and Fe3+# (Fe3+/(Cr + Al + Fe3+) of coexisting chromian spinel phase, while Cr# (Cr/Cr + Al) of chromian spinel increases or decreases or is unaffected with a decrease of Fo content of the olivine. However, the composition of the chromite also changes with the grade of metamorphism and hydrothermal alteration (Evans and Frost 1976; Zakrzewski 1989; Kimball 1990).

Many researchers have conducted research on the HHC unit at the regional scale in previous decades. However, the structure, metamorphism, and geochronology were the key areas of attention in their work (Pognante et al. 1990; Inger and Harris 1992; Reddy et al. 1993; Carosi et al. 2010; Webb et al. 2011; Thöni et al. 2012; Bartoli et al. 2019; Carosi et al. 2019). Chromite is reported from the ophiolite sequence of Himalayan orogen like Manipur Ophiolite, Nidar Ophiolite, Karzok Ophiolite, Spontang Ophiolite (Nayak and Maibam 2020; Jonnalagadda et al. 2022), while chromite chemistry from HHC is rarely reported in published literature along the entire length of Himalayan orogen. Due to the petrogenetic significance of chromite, the present study is an attempt to explore further the importance of detailed petrography and chromite geochemistry in understanding the petrogenesis of ultramafic rocks. Furthermore, we intend to relate the petrogenetic conclusions from investigated chromite. The chromites from the altered ultramafic rocks are investigated for the first time from the Higher Himalayan Crystalline Unit of northwestern Himalaya. The detailed petrological character and geochemistry of chromite are utilised to understand the tectonic setting of parent magma and its post-crystallisation effect.

2 Geological setting

The Himalayas is a typical example of a continent–continent collision orogenic mountain system formed during the Cenozoic period by the collision between Indian and Eurasian plates that began 55–50 Ma ago (Dewey and Bird 1970; Thakur 1987). The Himalayan orogen is characterised by continuous south-directed thrusting and piling up of tectonic units, traditionally divided into tectonostratigraphic zones (figure 1a), each having characteristic stratigraphy and bounded by major thrust that extends the full length of the orogen (DiPietro and Pogue 2004). Le Fort (1996) divided it from south to north as Neogene Sub-Himalaya (Siwalik Group), the Proterozoic Lesser Himalayan Sequence (LHS), the Proterozoic to Ordovician Greater (or Higher) Himalayan Crystalline Complex (HHC), and the Proterozoic to Eocene Tethyan Himalayan Sequence (THS). But out of these zones, the Higher Himalayan Crystalline (HHC) zone, of which the present area of study is a part, represents the central metamorphic belt and axis of maximum surface uplift of the orogen (Thakur 1980). The HHC is also known as Higher Himalaya/Greater Himalayan Crystalline Complex/Central Himalaya/Central Crystallines. This zone is considered as the backbone of the Himalayan orogen (Saxena 1971) and is characterised by the highest topographical relief. It is commonly represented by a thick Paleoproterozoic to Ordovician (1800–480 Ma) high-grade metamorphic belt and root zone of the allochthonous klippen of the Lesser Himalaya (Parrish and Hodges 1996; DeCelles et al. 2000). These metasediments are frequently intruded by granites of Ordovician (~500 Ma) and Lower Miocene (~22 Ma) age.

Figure 1
figure 1

Geological setup of the study area. (a) Tectonic subdivision of Himalaya marked by thrust system and (b) regional geological map of the study area (after Sorkhabi et al. 1997). ITSZ: Indo-Tsangpo Suture Zone, STD: South Tibetan Detachment, MCT: Main Central Thrust, MBT: Main Boundary Thrust, HFT: Himalayan Frontal Thrust. (c) Geological map of Paddar sapphire mine area showing the distribution of pegmatites (after Srivastava and Singh 2022).

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Geologically, the study area is part of the Kishtwar region, which shows a very complex litho-tectonic setup as it is surrounded by Kashmir Nappe in the west; where the THS directly overlies the LHS along the MCT (Frank et al. 1995; Pogue et al. 1999). The present investigation is confined to the Paddar Sapphire mine area, exposing the high-grade metamorphic rock of the HHC unit of Himalayan orogen. In general, the stones of HHC rocks show thrusted contact with Lesser Himalaya marked by the well-known Main Central Thrust (MCT, figure 1b). The low-grade metamorphic rocks of Lesser Himalayan form window structure (Kishtwar Window) within HHC along the Kishtwar Fault (Fuchs 1975; Stephenson et al. 2000; Singh 2010). The Zanskar Shear Zone (ZSZ) marks the boundary between HHC and Tethys Himalaya along the northern edge of the HHC (Herren 1987; Singh 2010). Precambrian HHC occurs at the base of Tethys Himalaya, forming the basement (Thakur 1987).

In the present study area, HHC is composed of migmatites, gneisses of sedimentary protolith associated with gneisses of magmatic protolith, pelitic schists, intruded by porphyritic granites and leucogranites (Stephenson et al. 2000; Singh 2010). The Tethys Himalaya of the Paleozoic–Mesozoic age is represented by a thick marine sedimentary sequence intruded by Permian volcanic (e.g., Frank et al. 1995). In the Paddar area, the Lesser Himalayan unit is represented by quartzites, phyllites, slate, schist, mylonitic augen gneiss, volcanics, and gneissose granite. The western and southern boundaries of the Kishtwar Window are represented by the sericitic-phyllites and quartzites and show a gentle dip towards N to NE (Fuchs 1975; Honegger 1983; Staubli 1988). Tewari (1981) proposed gneissic rocks intercalated with marble occur as exotic thrust mass placed over migmatitic complex, as a klippe along Sumjum Thrust. Singh (2007) classified the stones of the proterozoic rocks of Lesser Himalaya and HHC under the Kishtwar Group. In the Kishtwar region, the Phe Formation, Haimanta Group, and the Lilang Group are exposed in the north of the HHC.

The Kishtwar region has undergone three phases of metamorphism represented by M1 (amphibolites grade metamorphism with moderate pressure), M2 (development of syn-metamorphic dome in the Higher Himalaya), and M3 (retrogressive mineral assemblage quartz–muscovite–chlorite and the upliftment of the Kishtwar window; Kundig 1989; Staubli 1989; Frank et al. 1995; Stephenson et al. 2000). The M2 phase is associated with the migmatisation and large-scale crustal melting in the HHC (Noble and Searle 1995). Kundig (1989) observed several gneisses domes, such as the Chishoti dome and the Bhazun dome, at the lower structural level within the HHC unit just north of the Kishtwar window. These domes are syn-metamorphic in origin and consist of pelitic schists and gneisses folded around granite and granitic gneiss core. Migmatisation commonly occurs along the contacts within the granites and overlying schists and gneisses. In Bhat Nala, folding of the main foliation into the adjacent Chisoti and Bhazun domes creates foliation triple junctions. These suggest regions of high strain and are characterised by isoclinal folded compositional banding into upright-facing synforms. Stephenson et al. (2001) interpreted it as synchronous with thrusting on the Main Central Thrust and local extension on the Zanskar Shear Zone. Searle et al. (2007) also suggested that the domes were synchronous with movement on the Main Central Thrust and could be thrust-related culminations. Fission track data, however, from the cores of the Chisoti dome suggests that these structures have experienced relatively fast exhumation (Kumar et al. 1995).

3 Sampling and analytical technique

The author carried out detailed fieldwork in the HHC of the Paddar area along the Bhat Nala. About 30 samples of ultramafic rock were selected for detailed petrographic examination. Based on the petrography, three samples were selected for the EPMA investigation to better understand the compositions of the opaque mineral phase. At the Department of Geology, Banaras Hindu University, the chosen chromite grains are examined using the Electron Probe Micro Analyzer (EPMA) CAMECA SXFive. For performing the analysis, the polished thin sections were coated with a 20 nm thin layer of carbon by using the LEICA-EM ACE200 instrument. The instrument was operated at a voltage of 15 kV with 40 nA and set with a focused beam of diameter 1 μm with counting time of 120 to 300 seconds. B–K, Na–K, Mg–K, Al–K, Si–K, K–K, Ca–K, Ti–K, Cr–K, and Fe–K X-ray lines were employed in the analysis. For calibration, a variety of synthetic and natural standards were used. For major element oxides, the analysis accuracy is better than 1%. The representative compositions of chromite grains along with the calculated mineral formula based on 32 oxygen atoms, are given in table 1.

Table 1 Representative electron microprobe analysis (in wt%), cation proportion and composition of chromite from altered ultramafic rocks of Paddar area.
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4 Field description and petrography

The ultramafic rocks are the conspicuous rock in the study area. The fresh rock shows an olive-green hue, while the weathered surface of these rocks has an oxidised brown staining. They are intruded in the gneissic rocks in the form of small bodies of laccolith, lenses, and bulbous (figure 2). They contain olivine and pyroxenes predominately, along with varying amounts of opaque minerals. They are intensely altered to serpentine and talc, giving a soapy feel while touching with a hand. They are enclosed within the garnetiferous biotite gneiss and shows both concordant and discordant relationship. They show forceful emplacement by the up arching of the strata is clearly visible in the main sapphire mine ridge (figure 2).

Figure 2
figure 2

Regional view of sapphire mine ridge showing ultramafic rock intruded into garnetiferous biotite gneiss, up arched marble bed at the top of the ridge.

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The ultramafic rocks are the most interesting rock types well exposed in the Paddar sapphire mine area. They are medium to coarse-grained, characterised by radiating crystals of pyroxenes and amphiboles. Mineralogical composed of olivine and pyroxene as main mineral phases while phlogopite and opaque minerals occur as minor mineral phases (figure 3). The main mineral phases (olivine + pyroxene) show alteration to talc, serpentine, anthophyllite, and tremolite by the effect hydrothermal fluids (figure 3).

Figure 3
figure 3

(a) Relict olivine grain associated with opaque mineral and surrounded by talc alteration. (b) Serpentinised olivine grain sharing grain boundary with clinopyroxene, phlogopite, opaque minerals are also present. (c) Clinopyroxene altered to anthophyllite. (d) Pyroxene and olivine altered to tremolite and anthophyllite.

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Fe-oxide and chromite are the opaque phases identified in EPMA studies. Chromite is one of the most important accessory phases in these rocks. They occur as isolated subhedral to anhedral grains and scattered fine dust in altered parts (figure 4a, b). They vary in size from <5 to 800 µm in diameter. They occupy the intergranular space between olivine, pyroxene, and altered domain (figure 4c). Some subhedral grains have sharp boundaries with phlogopite and pyroxenes. Most of the grains are unaltered and homogenous, except the grain boundary is corroded (figure 4c, d). However, some grains show alteration along the fractures (figure 4d).

Figure 4
figure 4

(a) Chromite grain sharing boundary with phlogopite and pyroxene, grain boundary of chromite is corroded due to alteration, (b) chromite showing variation in the grain size, (c) chromite grains associated with serpentinised olivine grain, and (d) subhedral chromite grain altered along the fracture and grain boundary.

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5 Chromite chemistry

The chromite from the ultramafics of the study area shows enrichment and a wide range of Cr2O3 and Al2O3 ranging from 47.89 to 60.18 wt%, 5.57–13.40 wt% with an average of 56.38 wt%, 8.53 wt%, respectively. The FeO is also higher in concentration, but it is more consistent ranging from 22.10 to 28.98 wt% with an average of 25.65 wt%. However, the MgO is lower in concentration, varying from 2.58–5.49 wt% with an average of 4.36 wt%. The Na2O and K2O occur in traces <0.1%. The CaO is higher than the alkali elements ranging from 0.002 to 0.21 wt% with an average of 0.06 wt%.

The TiO2 varies from 0.03 to 0.72 wt%, which is higher in concentration, while MnO is detected in a few analysed points ranging from 0.43 to 0.66 wt%. The P2O5 varies from 0.01 to 0.13 wt% and chlorine content ranges from 0.0010 to 0.02 wt%.

The chromites show very higher Cr# (100*Cr/Cr + Al), ranging from 71.10 to 87.80 with an average of 81.55. However, they show wider ranges of the Mg# (100*Mg/Mg + Fe2+) ranging between 13.95 and 28.16 with an average of 23.36, while Fe3+# (100*Fe3+/Cr + Al) varies in a range of <0.1–2.15.

6 Discussion

6.1 Classification of chromite

Based on structure, Palache et al. (1944) divided the spinel group into three series: the spinel series, magnetite series, and chromite series. According to guidelines of nomenclature and Commission on New Minerals, Nomenclature and Classification (CNMNC), Mills et al. (2009) classified the spinel supergroup into three groups on the basis of the X species given in table 2. The chromite mineral series are classified under the spinel subgroup (2–3) represented by the general formula A2+(B3+)2O4 under the Oxyspinel group. Naturally, occurring chromites are essentially solid solutions mainly of the Mg and Fe members of the spinel and chromite series, viz., magnesiochromite and (ferro) chromite, with minor amounts of the magnetite series. On plotting the composition of analysed mineral grain in Cr–Al–Fe3+ ternary classification diagram (after Stevens 1944), all grains occupy the Al-chromite field.

Table 2 The classification of the spinel supergroup into group-subgroup hierarchy and general formulae of the spinels (Mills et al. 2009).
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The spinel prism (figure 5a) diagram, presented by Haggerty (1991), represents a solid solution between spinel, hercynite, chromite, magnesiochromite, magnesioferrite, and magnetite. Gargiulo et al. (2013) projected the face ‘B’ and ‘C’ of the prism diagram into compositional variation diagrams based on the previously published diagrams of Stevens (1944) and Deer et al. (1992).

Figure 5
figure 5

Chemical classification diagrams for the chromite from Paddar area. (a) Spinel prism for the multi-component system: spinel–hercynite–chromite–magnesiochromite–magnesioferrite–magnetite (after Deer et al. 1992); (b) Triangular classification diagram (Cr3+–Fe3+–Al3+): Y(Cr3+) = Cr/(Cr + Fe3+ + Al); Y(Fe3+) = Fe3+/(Cr + Fe3+ + Al); Y(Al3+) = Al/(Cr + Fe3+ + Al). Spinel gap field from Barnes and Roeder (2001). (c) Binary classification diagram considering the Mg2+–Fe2+ exchange in the structural site ‘X’: Fe2+/(Mg2+ + Fe2+). (d) Classification diagram for chromites in terms of Mg number [Mg2+/(Mg2+ + Fe2+)] vs. Cr-number [Cr/(Cr + Al)]. Fields are after Kapsiotis (2009).

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On plotting the composition of analysed chromite in ternary Y(Cr3+) = Cr/(Cr + Fe3+ + Al); Y(Fe3+) = Fe3+/(Cr + Fe3+ + Al); Y(Al3+) = Al/(Cr + Fe3+ + Al) diagram the analysed points occupy the Al-chromite to chromite field (figure 5b). Further, in both classification diagrams, the chromite displays Cr–Al trend indicating Cr ↔ Al substitution. On the projected ‘C’ face of the spinel prism diagram in binary Mg2+/Mg2+ + Fe2+ vs. Al3+/Fe3+ + Al3+ plot, most of the analysed chromites fall in the spinel field while few fall in the pleonaste field (figure 5c). From this plot, it is evident that in the X-site shows, there was lower Mg2+–Fe2+ and Al3+–Fe3+ exchange during the crystallisation. Further on plotting the Cr number and Mg number of the studied chromite in the Mg2+/Mg2+ + Fe2+ vs. Cr/Al + Cr, all the analysed points are occupying chromite field, indicating higher chromium and iron concentration as compared to the magnesia (figure 5d).

6.2 Substitution mechanism

The spinel group is represented by the general formula AB2X4, here A and B represent the constituents instead of the crystallographic site. The octahedral site in the chromite is occupied by Cr, which is substituted by trivalent and tetravalent cations such as Al3+, Fe3+, Ga3+, Sc3+, V3+, and Ti4+. The tetrahedral site is mainly occupied by Mg and Fe2+ that may be substituted by divalent cations Zn, Co, Mn, and Ni. The correlation matrix of the major and minor elements is presented in table 3. The binary variation diagram of the cations per unit formula shows significant positive (+ve R-value) and negative relation (–ve R-value) between the elements. The Cr shows a strong negative relationship with the Al with a very high R-value of –0.98, while it shows significant positive relation with Ti (R = 0.46). However, the Al shows negative relation with the Ti with a –0.46 R-value. The Fe3+ shows negative relation with the Cr (R = –0.34) and Ti (R = –0.39; figure 6). Based on these relations, it is suggested that Ti + Cr ↔ Al and Cr ↔ Fe substitution mechanism operated in the octahedral site. The binary variation diagram (figure 7) shows a strong negative relation between the Fe2+ and Mg (R = –0.99) while positive relation of Mg with Ti (R = 0.41). However, Al shows negative relation with both Fe2+ and Mg. Thus it is evident that Fe ↔ Mg and Al substitute both Fe and Mg in the tetrahedral site.

Table 3 Pearson correlation matrix of major and minor elements for investigated chromite.
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Figure 6
figure 6

Binary variation diagram of major and minor elements to understand the substitution mechanism in the chromite during its crystallisation: (a) Al vs. Cr; (b) Ti vs. Cr; (c) Fe3+ vs. Cr; (d) Fe2+ vs. Cr; (e) Al vs. Ti; (f) Al vs. Fe2+. Note: Al, Fe3+ and Fe2+ show antipathic relationship with Cr, while Ti shows sympathetic relationship (a–d). Further, antipathic relationship is observed between Al and Ti, while Al shows sympathetic relationship with Fe2+. Scattering of the elements is due to the hydrothermal alteration.

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Figure 7
figure 7

Binary variation diagram of major and minor elements to understand the substitution mechanism in the chromite during its crystallisation: (a) Fe2+ vs. Mg; (b) Ti vs. Mg; (c) Al vs. Mg and (d) Al vs. Fe2+. Note: Fe2+ and Al show antipathic relationship with Mg, while Ti shows sympathetic relationship and a sympathetic relationship is observed between Ti and Mg.

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6.3 Metamorphism and alteration effect

Chromite re-equilibration, hydrothermal alteration, and metamorphic modification have all been extensively covered in previous research (Onyeagocha 1974; Ulmer 1974; Evans and Frost 1976; Hoffman and Walker 1978; Loferski and Lipin 1983; Kimball 1990; Burkhard 1993). These investigations show that chromite alteration results in the overgrowth of magnetite rim and is gradually replaced by chromian magnetite, also known as ‘ferritchromite’. By exchanging components with nearby silicate minerals, the prograde metamorphism (green-schist to amphibolte facies) modifies the core-to-rim compositional zoning (Evans and Frost 1976; Abzalov 1998; Barnes 2000; Gonzalez Jimenez et al. 2009; Waterton et al. 2020). Previous investigations of chromite chemistry suggest that alteration and post-crystallisation re-equilibration leads to the complexity of the interpretation (Evans and Frost 1976; Burkhard 1993).

The metamorphic transformation is well recovered as optical and compositional zoning of chromite. However, some authors believe that optical zoning is completely eradicated during the prograde metamorphism at a higher grade of metamorphism that gives rise to ferrian chromite or Cr-magnetite and/or with a homogenous chemistry (Ulmer 1974; Barnes 2000; González Jiménez et al. 2009).

The petrographic study of the chromite from the study area reveals that chromites are homogenous and do not show any optical zoning. However, the alteration of these chromites is confirmed by the alteration observed along the fractures and corroded grain boundary. This is well reflected in the BSE images (figure 4). To observe the effect of metamorphism/alteration on chromite, different plots are used. On plotting the chromite chemistry in Fe# vs. Mg# diagram (figure 8a), the investigation chromite falls near the partly altered chromite field. From this plot, it is revealed that these chromite grains have depleted Fe3+ and Mg content. On plotting the chemistry of the chromite in Mg# vs. Cr# diagram (figure 8b), they are showing Cr# within range of the partly altered chromite while depleted Mg content. Further on plotting the chromite chemistry in the ternary Cr–Al–Fe3+ diagram (figure 8c), it is confirmed that chromite from the Paddar area has undergone the effect of metamorphism/alteration. Most of the investigated chromite falls near the spinel core field, and some occupy the spinel core field in Cr# vs. Fe3+# diagram (figure 8d), which does not follow the typical alteration trend of Fe3+ enrichment. From the above discussion, it is inferred that chromite grains are suitable for petrogenetic investigation. Liipo et al. (1994) observed low XMg (0.0–0.37) and revealed that Mg and Al are more mobile than Cr during the hydrothermal alteration. Similar results were recorded by Kimball (1990). The depletion of Mg and Al content of the chromite from the present study area can be related to the hydrothermal alteration.

Figure 8
figure 8

(a) Fe# vs. Mg# diagram, (b) Mg# vs. Cr# diagram, (c) ternary Cr–Al–Fe3+ diagram, and (d) the Cr# [Cr/(Cr + Al)] vs. Fe3+# [Fe3+/(Cr + Al + Fe3+)] plot defining the unaltered nature chromites from the study area (modified after Ahmed et al. 2009).

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6.4 Composition and environment of the parent magma

Primary chromite phases' composition is closely related to the composition of the parental melt, the extent of partial melting, and the fractional crystallisation, thus effectively used by many workers to measure parental melt composition (Irvine 1977; Barnes and Roeder 2001). The aluminum concentration in chromite is fruitful for the study of the tectonic environment and the melt composition (Zhou et al. 1996; Rollinson 2008; Zaccarini et al. 2011). In 1982, Maurel and Maurel proposed the equation [Al2O3,spinel = 0.035 × (Al2O3,melt)2.42] for the estimation of the alumina in the parent magma by considering the equilibrium between melt and chromite at 1 Kb. According to the equation, the calculated Al2O3 of the parent melt in which the chromite of the study area varies from 8.11 to 11.67 wt% (table 1).

In order to interpret the crystallisation pressure of ultramafic rocks, Basu and McGregor (1976) observed significant variation in the Mg# and Cr# in the chromite from alkali-basalt and kimberlite xenoliths. As a result, he proposed that this relationship between the Mg# and Cr# in the chromite is appropriate. Using these parameters by plotting the studied chromites in the Mg# (Mg2+/Mg2+ + Fe2+) vs. Cr# (Cr/Al + Cr) plot, the chromites from the study area show higher Cr# while lower Mg# thus fall near the field of chromites from the Kimberlite xenoliths suggesting that these chromites crystallised at high pressure (figure 9a). The Fe3+/Fe2+ ratio is <0.05, suggesting chromite crystallisation at low oxygen fugacity.

Figure 9
figure 9

Discrimination diagrams (a) Cr# vs. Mg# diagram showing the discrimination between alkali-basalt and kimberlite xenoliths (after Basu and McGregor 1975). The chromites from the Paddar area falls near the kimberlite xenolith field, suggesting a high-pressure origin; (b) Plot based on the estimated composition of parental magma, Al2O3 in melts vs. Al2O3 in spinel (after Rollinson 2008), based on melt calculations of Maurel and Maurel (1982); (c) Cr# vs. Mg# plot showing discrimination between chromites from Komatiites and layered intrusion (fields after Barnes and Roeder 2001), studied chromites shows affinity towards komatiite; (d) Tectonic discrimination diagram based on TiO2 vs. Al2O3 for chromite from Paddar area (fields after Kamenetsky et al. 2001) indicating arc to supra-subdution tectonic setting; (e) Plot of Mg# against Cr# showing compositions of chromite from Paddar ultramafics in comparison to various ophiolite rocks. Cr-spinel compositional fields of Zildat and Nidar peridotites are from Guillot et al. (2001), Shergol and Suru valley peridotites are from Bhat et al. (2019), abyssal peridotite is from Dick and Bullen (1984) and supra-subduction zone peridotite is from Ishii (1992); (f) Ternary Cr–Al–Fe3+ + 2Ti plot, the investigated chromite occupy field of alpine peridotite complex (field after Jan and Windley 1990; Zhou and Kerrich 1992).

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6.5 Petrogenetic implications

Due to chromite's sensitivity to the parent magma/melt composition and its early crystallisation from mafic–ultramafic magma, it has been employed extensively to study the petrogenesis of its host rocks (Irvine 1965; Cameron 1975; Barnes and Hill 1995; Zhou et al. 2014; Ishwar-Kumar et al. 2016). In podiform chromitites, a wide variety of compositions were noted by Saleh (2006). They are characterised by high Cr (0.62–0.88) to high Al varieties. The chromites from ophiolites are high-Cr and high-Al. High-Al chromitites evolve from basaltic melts in a back-arc basin or MORB environment, whereas high-Cr chromitites develop from andesitic or boninitic melts that are Mg-rich (e.g., Zhou and Robinson 1994; Arai 1997; Zhou et al. 1998; Akmaz et al. 2014; Moghadam et al. 2015). Alaskan-type complexes are distinguished from ophiolitic or stratiform complexes by their higher Fe3+ and Ti contents and lower Al contents (Bird and Clark 1976). Arc environments are characterised by Cr-rich spinels (Cr# > 0.7), whereas MOR settings are characterised by Cr-poor spinels (Cr# 0.6; e.g., Zhou et al. 1998; Kamenetsky et al. 2001). In order to understand the petrogenesis of the ultramafic of Paddar area, different discrimination plots have been discussed below.

According to Zhou et al. (2005), the higher-Al, lower-Cr spinels found in peridotites are produced in a MORB setting by low degrees of partial melting of the fertile upper mantle, whereas the higher-Cr, lower Al-Cr spinel are distinctive to a subduction zone tectonic setting and are produced by higher degrees of partial melting of a depleted mantle. The investigated chromite's Cr# (0.71–0.87) is similar to that of the layered igneous intrusions from Bushveld and Stillwater (Irvine 1967), higher than that of the mid-ocean ridge tholeiitic source magma (Allen et al. 1988), and lower than that of the boninites (Cr# = 0.80–0.90; Roeder and Reynolds 1991). On plotting the calculated alumina percentage of parent melt and chromite in binary Al2O3 in melt vs. Al2O3 spinel plot (Kamenetsky et al. 2001; Rollinson 2008), the analysed chromites plot very close to the evolutionary trend of an arc system (figure 9b). They exhibit affinity towards the komatiites with higher Cr number, while lower Mg number in the Cr# vs. Mg# plot (figure 9c). Dick and Bullen (1984) noted that spinels with high Cr# have parent magmas of the boninitic type that are richer in magnesium. According to Kostopoulos (1991), spinels with Cr# > 0.70 are thought to be the product of partial melting at high temperatures that produce dunites free of pyroxene. Similar considerations have been made by other authors (Arai 1992; Zhou et al. 1994). The higher Cr# (0.71–0.87) from investigated chromite indicates that parent magma was Mg enriched and generated at higher temperatures.

On plotting the chromite composition in TiO2 vs. Al2O3 tectonic discrimination diagram (Kamenetsky et al. 2001), the studied chromite occupies the high-Ti arc to supra-subduction field with wide TiO2 and less Al2O3 variation (figure 9d). Dick and Bullen (1984) claimed that spinels with a greater Cr# (>60%) are typical of volcanic arcs, stratiform complexes, and oceanic plateau basalts, whereas spinels with a lower Cr# (60%) are indicative of oceanic crust. Therefore the arc-related tectonic environment is suggested for the studied ultramafic rocks. On plotting the chromite in Cr# against Mg# (figure 9e), studied chromites plot in and near the supra-subduction peridotites field showing the geochemical characteristics more similar to the spinel-chromites from Zildat peridotites. According to Singh's (2009) research, the Manipur Ophiolite Complex's peridotites contain chromian spinels (Cr-spinels) that have low Cr# (10.75–26.37) values and high Al# (72.71–88.36) and Mg# (70.00–73.43) values. They exhibit affinities with MORB at low partial melt rates, which are tholeiitic melt-derived abyssal and alpine-type peridotites. In ternary Cr–Al–Fe3+ + 2Ti, chromite from Paddar area occupy the Alpine peridotite field towards the Cr-rich end away from the Manipur Ophiolite Complex (figure 9f).

The geochemical characteristics of the chromite suggest a supra-subduction zone (SSZ) ophiolite geodynamic setting for the chromite origin (figure 10). Ultramafic magma was generated from the upper mantle and intruded into HHC unit along the deep-seated shear zone (Darlang Shear Zone). During the exhumation, deep seated ultramafic were brought near the surface while partial melting of HHC resulted in the migmatisation and due to movement along the MCT and DSZ, hydrothermal fluid was generated, which interacted with the chromite resulting in chromite alteration.

Figure 10
figure 10

Schematic tectonomagmatic model for the formation of chromites in ultramafics of Higher Himayan Crystalline Unit (HHC) in the NW Hiamalyan orogen belt and alteration of chromite by hydrothermal fluids generated along the shear zones. Abbreviation: MCT: Main Central Thrust, DSZ: Darlang Shear Zone, ZSZ: Zanskar Shear Zone, SP: Spotang Ophiolite, ITSZ: Indus-Tsangpo Suture Zone, DV: Dras Volcanics (Kishtwar-Western Zanskar cross-section after Searle et al. 2007).

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7 Conclusion

The ultramafic rocks of the study area are hosted by the gneissic rocks of HHC, occurring in the form of laccolith, lenses, and bulbous bodies. They are mineralogically characterised by olivine, pyroxene (major phase), phlogopite, chromite, and Fe-oxide (minor phase). The presence of serpentine, talc, anthophyllite, and tremolite indicates the interaction with hydrothermal fluids. Chromite undergoes alteration along fracture and grain boundaries, which is also supported by chromite geochemistry. It is geochemically characterised by higher Cr/Cr + Al and Fe2+/Fe2+ + Mg, while Mg/Mg + Fe2+ and Fe3+/Cr + Al ratios are lower. Calculated Al2O3 of the parent melt ranges between 8.11 and 11.67 wt%. The Ti + Cr ↔ Al, Cr ↔ Fe, Fe ↔ Mg, Fe + Mg ↔ Al substitution mechanism operated during the crystallisation of chromite. It crystallised under high temperature, pressure conditions, and lower oxygen fugacity. The geochemistry of chromite suggests an arc to supra-subduction tectonic for the chromite genesis. Moreover, the proposed model is the first attempt to elucidate the crystallisation of chromite from ultramafic rock of HHC.