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Article

Petrogenesis of Mafic–Ultramafic Cumulates in the Mayudia Ophiolite Complex, NE Himalaya: Evidence of an Island Arc Root in Eastern Neo-Tethys

by
Sapneswar Sahoo
1,
Alik S. Majumdar
1,*,
Rajagopal Anand
1,
Dwijesh Ray
2 and
José M. Fuenlabrada
3
1
Department of Applied Geology, Indian Institute of Technology (Indian School of Mines), Dhanbad 826004, Jharkhand, India
2
Planetary Sciences Division, Physical Research Laboratory, Ahmedabad 380009, Gujarat, India
3
Geochronology Laboratory, Universidad Complutense de Madrid, Calle José Antonio Novais, 228040 Madrid, Spain
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(6), 572; https://doi.org/10.3390/min15060572
Submission received: 18 April 2025 / Revised: 19 May 2025 / Accepted: 20 May 2025 / Published: 27 May 2025
(This article belongs to the Special Issue Tectonic Evolution of the Tethys Ocean in the Qinghai–Tibet Plateau)
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Versions Notes

Abstract

:
Amphibole-rich cumulates provide crucial information pertaining to the petrogenetic history of suprasubduction zone ophiolites and are, therefore, helpful in constraining the evolution and closure of the Neo-Tethys during the late Cretaceous to the early Tertiary period. Following this, the present contribution examines the meta-hornblendite and meta-hornblende-gabbro lithologies in the Mayudia ophiolite complex (MdOC), NE Himalaya, based on their field and petrographic relations, constituent mineral compositions, whole rock major and trace element chemistry and bulk strontium (Sr)—neodymium (Nd) isotope systematics. MdOC cumulates potentially represent the fossilized record of an island arc root, where amphibole + titanite + magnetite was fractionally crystallized from a super hydrous magma (10.56–13.61 wt.% melt water content) prior to plagioclase in a stable physico-chemical condition (T: 865–940 °C, P: 0.8–1.4 GPa, logfO2: −8.59–−11.19 unit) at lower crustal depths (30–38 km). Such extreme hydrous nature in the parental magma was generated by the flux melting of the sub-arc mantle wedge with aqueous inputs from the dehydrating slab. A super hydrous magmatic reservoir was, therefore, extant at sub-arc mantle depths in the eastern Neo-Tethys, which has likely modulated the composition of the oceanic crust during intraoceanic subduction.

1. Introduction

Mafic–ultramafic cumulate rocks are common worldwide in various tectonic settings, which provide important insights into the mantle melting process, magmatic differentiation and geodynamic evolution at plate boundaries [1,2,3]. They are commonly associated with ophiolites formed in mid-ocean ridges, back-arc, island arc and fore-arc basin environments [4], magma chambers produced within subduction-related upper-plate crust [5] and/or xenoliths introduced by alkaline basalt in intracontinental settings [6]. Compared to rift-related cumulates, which are typically linked to asthenopheric upwelling and/or mantle plumes, subduction-related cumulates typically exhibit geochemical signatures indicating hydrous melting, with rare stability of magmatic hydrous amphibole [7,8]. Compositional characteristics of such unique igneous amphiboles (commonly hornblende) are pivotal in understanding various aspects of the physico-chemical conditions (e.g., pressure, temperature, fluid activity, and oxygen fugacity) within sub-arc magma chambers [8,9]. Primary amphibole also imparts a major influence on arc magma compositions, which in turn controls arc crust formation and differentiation processes [10]. Amphibole-rich intrusives may also act as an important sink for subduction-derived fluids, which affects the aqueous budget and fluid-mobile element recycling in the convergent plate boundaries [11]. Studies on amphibole-rich cumulates (e.g., hornblendite, hornblende-gabbro) have, therefore, received significant attention over the years to unravel various aspects of sub-arc processes and geodynamic evolution in orogenic belts [1,2,11,12,13].
The Indus Tsangpo Suture Zone (ITSZ) in the Himalayan orogenic belt records the subduction of the Neo-Tethyan oceanic and Indian continental lithospheres beneath the Eurasia [14,15]. The Neo-Tethys Ocean opened up to the south of the Paleo-Tethys in the mid-Permian as an ocean or its complexes [16]. A spreading ridge was extant in the Neo-Tethys by the mid-Jurassic [16], which subsequently initiated at least two subduction systems to its north during the Cretaceous to the early Tertiary period, namely—(a) the ‘Trans-Tethyan’ intraoceanic subduction and (b) the ‘Andean-type’ subduction of the Neo-Tethyan oceanic slab beneath Eurasia [15]. The collision between India and the Trans-Tethyan subduction zone began in early Palaeocene (ca. 50–55 Ma) and continued until the final continental collision between India and Eurasia in the Middle Eocene (~40 Ma) [14]. This multistage collision consumed the Neo-Tethys, placed the oceanic remnants onto the crystalline rocks of the Greater Himalayan Sequence (GHS) and produced ophiolite complexes and/or mélanges with diversified lithologies, geochemistry, modal mineralogy, and age [17]. Within these ITSZ ophiolites, exposure of amphibole-rich cumulate rocks (e.g., hornblendite, hornblende-gabbro) is extremely rare although they are considered to be an important constituent of suprasubduction zone ophiolites worldwide [18]. In fact, limited occurrences of hornblendite/hornblende-gabbro have only been reported from the Jijal ophiolite complex, western Himalaya and the Mayudia ophiolite complex, eastern Himalaya (Figure 1a), where they are associated with garnetites and peridotite-tectonites, respectively [1,19,20]. Unravelling the petrogenetic history of these unique cumulates is, therefore, crucial to understand the sub-arc processes operating in the eastern and western flank of the ITSZ and the evolution of the Neo-Tethyan oceanic lithosphere during its closure.
To better constrain the sub-arc processes in the eastern Neo-Tethys during Mesozoic subduction, the present contribution focuses on the hornblende-rich cumulate lithologies in the far eastern margin of the ITSZ. These lithologies are exposed within the Mayudia ophiolite complex, NE Himalaya (Figure 1) and include meta-hornblendite and meta-hornblende-gabbro [19,21]. Combining mineral and whole-rock geochemistry with strontium (Sr) and neodymium (Nd) isotope systematics, this study aims to elucidate the magmatic history, geodynamic settings and physico-chemical conditions of the crystallization of the Mayudia meta-hornblendite and meta-hornblende-gabbro protoliths. The unique characteristics of the sub-arc Neo-Tethyan lithosphere have parallelly been examined in relevance to such typical occurrences of hydrous cumulates in the eastern Himalaya, which may have implications to better constrain the evolution in arc magma and crust compositions in the eastern Neo-Tethys during its final closure.
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Figure 1. Geological map of (a) northeastern Himalaya, modified after [22], (b) Eastern Himalayan Syntaxis, following [23], and (c) Mayudia Klippe (modified after [24]), demarcating major tectonic and lithological units. The inset map in (a) shows the Indus Tsangpo Suture Zone (ITSZ) between India and the Tibetan plateau, modified after [25]. ITSZ ophiolite locations are shown as yellow stars in (a,b) along with their ages (see text for references). (a) elaborates on the area marked by the dotted black box in (a) (inset map). Similarly, (b,c) amplify the area outlined by dotted black boxes in (a) and (b), respectively. In (c), white stars demarcate sample locations, whereas opposing black arrowheads and white dotted lines represent the Mayudia synclinorium and its fold axis, respectively. White rectangular labels as the ”view point” and “65 km” are the reference locations along the Roing–Hunli road in the same figure. MdOC: Mayudia ophiolite complex; TOC: Tidding ophiolite complex; NOC: Nagaland ophiolite complex; MpOC: Manipur ophiolite complex; KOC: Kalaymyo ophiolite complex; MkOC: Myitkyina ophiolite complex; STD: South Tibetan Detachment fault; MCT: Main Central Thrust; MBT: Main Boundary Thrust; MFT: Main Frontal Thrust; LHS: Lesser Himalayan Sequence; SHS: Siwalik Himalayan Sequence. M-Hblite: Meta-hornblendite; M-Hbl-gabbro: Meta-hornblende-gabbro. Other abbreviations are according to [22].
Figure 1. Geological map of (a) northeastern Himalaya, modified after [22], (b) Eastern Himalayan Syntaxis, following [23], and (c) Mayudia Klippe (modified after [24]), demarcating major tectonic and lithological units. The inset map in (a) shows the Indus Tsangpo Suture Zone (ITSZ) between India and the Tibetan plateau, modified after [25]. ITSZ ophiolite locations are shown as yellow stars in (a,b) along with their ages (see text for references). (a) elaborates on the area marked by the dotted black box in (a) (inset map). Similarly, (b,c) amplify the area outlined by dotted black boxes in (a) and (b), respectively. In (c), white stars demarcate sample locations, whereas opposing black arrowheads and white dotted lines represent the Mayudia synclinorium and its fold axis, respectively. White rectangular labels as the ”view point” and “65 km” are the reference locations along the Roing–Hunli road in the same figure. MdOC: Mayudia ophiolite complex; TOC: Tidding ophiolite complex; NOC: Nagaland ophiolite complex; MpOC: Manipur ophiolite complex; KOC: Kalaymyo ophiolite complex; MkOC: Myitkyina ophiolite complex; STD: South Tibetan Detachment fault; MCT: Main Central Thrust; MBT: Main Boundary Thrust; MFT: Main Frontal Thrust; LHS: Lesser Himalayan Sequence; SHS: Siwalik Himalayan Sequence. M-Hblite: Meta-hornblendite; M-Hbl-gabbro: Meta-hornblende-gabbro. Other abbreviations are according to [22].
Minerals 15 00572 g001

2. Geological Background and Field Relationships

In the NE Himalaya, the regional strikes of the major tectonic units bend from ENE-WSW to NW-SE around Namcha Barwa (NB), which is described as the Eastern Himalayan Syntaxis (EHS) (Figure 1a) [26]. Several ophiolite complexes and/or mélange rocks (e.g., Mayudia, Tidding, Nagaland, Manipur, Kalaymyo and Myitkyina) are sporadically exposed on the eastern limb of the EHS (Figure 1a) [21,27,28,29,30]. Of these, the Mayudia ophiolite complex (MdOC) provides a near-complete record of an ophiolite succession [19,21,31], bounded by the Lohit and Tidding thrusts on either side (Figure 1b, Table 1) [23]. The occurrence of metaperidotite, dunite, hornblendite, hornblende-zoisite schist, amphibolite, and metabasalt is evident in the MdOC lithospheric sequence (Table 1) [24,31]. These lithologies have originated in different sub-settings (back-arc, fore-arc and/or intra-arc) within a supra-subduction zone environment [21,32]. Lithological comparison of the Mayudia ophiolite rocks and their structural relations with adjacent tectonic units suggests that the Indus-Tsangpo Suture (ITS) is possibly represented by the highly imbricated Tidding Thrust (TT) (Figure 1b) [26]. Multi-generation ductile contractional and lateral-slip deformations modified the geometry of the complex, causing highly tectonized oceanic rocks to occur as slices at different structural levels [23,33]. Three stages of deformation (D1, D2, and D3) were identified in the MdOC rocks [34], where D2 deformation folded the Tidding thrust and produced the ophiolite-bearing klippe in its frontal portion near Mayudia (i.e., Mayudia Klippe, Figure 1b) [33]. These rocks have also experienced at least three stages of metamorphism under different pressure–temperature conditions (Stage-1: greenschist facies; Stage-2: eclogite facies; Stage-3: amphibolite facies) [26,35].
This study is based on the hornblende-dominated lithologies with conspicuous cumulus textures in the MdOC sequence that are solely restricted within peridotite tectonites as discordant bodies (Figure 2). These hornblende-dominated cumulates can be categorised into two distinct lithotypes based on their modal mineralogy: (1) meta-hornblendite (Hblite)—predominantly (>90 modal%) comprised of very-coarse grained (up to 10 cm) euhedral crystals of cumulus amphiboles (Figure 2a), and (2) meta-hornblende-gabbro (M-Hbl-gabbro)—consisting of large cumulus crystals of melanocratic amphibole and intercumulus grains of leucocratic epidote (Figure 2b). Notably, this meta-hornblende-gabbro lithotype in the MdOC sequence has been described as a hornblende-zoisite schist by previous authors [19,21]. Frequently, meta-hornblendite grades into meta-hornblende-gabbro (Figure 2c), where the leucocratic regions occur as pods (Figure 2d), veins (Figure 2e) and selvages (Figure 2f). Evidence of cross-cutting relations between these cumulate lithologies and adjacent metaperidotite is common, where their lithological contact is sharp without any evidence of chilled margins (Figure 2d,e). Towards the contact, meta-hornblende-gabbro is more common than meta-hornblendite (Figure 2d,e). The interpretation of this study is based on three meta-hornblendite and six meta-hornblende-gabbro samples, which were collected from different structural levels within the Mayudia Klippe (Figure 1c).

3. Analytical Techniques

Bulk rock mineral constituents were identified by powdered X-ray diffraction (XRD) analysis, whereas textural characteristics across grain interfaces were examined using visible-light microscopy on polished thin sections. Individual mineral compositions were obtained through Electron Probe Micro Analysis (EPMA), whereas whole-rock major and trace element concentrations were determined by employing Inductively Coupled Plasma–Optical Emission Spectrometry (ICP–OES) and Quadrupole-Inductively Coupled Plasma–Mass Spectrometry (Q-ICP–MS) techniques, respectively. In parallel, bulk 143Nd/144Nd and 87Sr/86Sr isotope ratios on selected samples were measured using Isotope Dilution–Thermal Ionization Mass Spectrometry (ID–TIMS).
For whole-rock powder preparation, samples were first crushed to <5 mm rock chips, repeatedly washed with ultrapure water and alcohol in an ultrasonic bath and then dried overnight at 60 °C to remove surface moisture. Visibly unweathered rock chips were subsequently powdered using a motorized tungsten carbide mill for 5 min and again left at 60 °C for overnight drying.
For XRD analyses, a monochromatic High-resolution HyPix-3000 semiconductor detector (Manufacturer: Rigaku Corporation, Tokyo, Japan), attached to the Rigaku SmartLab powder X-ray diffractometer (Manufacturer: Rigaku Corporation, Tokyo, Japan) and operated with a Cu anode (source energy 3 KW), was used to collect X-ray peak intensities at the Central Research Facility, IIT (ISM) Dhanbad, India. Here, powdered samples were scanned over a 5–80° 2θ range with a step-size of 0.02° and a counting time of 1s per step. After linear background correction, each XRD spectrum was analyzed in “PANalytical Highscore Plus” software (version 5.1) to identify the phases while comparing with a reference database and to perform the Rietveld analysis for modal mineralogy determination.
The EPMA analyses were performed at the Planetary Sciences Division, Physical Research Laboratory, Ahmedabad (India) using a Jeol JXA-8530F Plus Hyper probe (Manufacturer: JEOL Ltd., Tokyo, Japan). Silicates and oxides were analyzed with five wavelength dispersive spectrometers with a pointed probe (diameter zero) at an acceleration voltage of 15 kV and a probe current of 20 nA. Different spectrometers were calibrated using diopside (Si, Mg, Ca), rutile (Ti), corundum (Al), hematite (Fe), synthetic NiO (Ni), albite (Na), orthoclase (K), synthetic Cr2O3 (Cr) and rhodonite (Mn). For Na and K, a peak count time of 10 s and a background count time of 5 s were used. For all other elements analyzed, the peak and background intensities were measured for 20 s and 10 s, respectively. The average detection limits were 0.01 wt.% for Si, Al, Fe, Cr, Ca, Mn, and K and 0.02 wt.% for Ti, Mg, Ni and Na. Average constituent mineral compositions and the complete data are shown in Table S1.
The ICP-OES and Q-ICP-MS analyses were carried out at the Australian Laboratory Service (ALS) Geochemistry laboratory, Loughrea, Ireland, following the ME-MS81D method (www.alsglobal.com, accessed on 24 February 2020). For ICP-OES analysis, ~0.2 g of the rock powders was mixed thoroughly with ~0.9 g of lithium metaborate/lithium tetraborate flux and fused at 1050 °C in a muffle furnace. The resulting melt was cooled and dissolved in 100 mL of 4 vol% HNO3 + 2 vol% HCl. The solutions thus prepared were analyzed and corrected for spectral inter-element interferences to obtain concentrations of SiO2, TiO2, Al2O3, Fe2O3T, CaO, MgO, MnO, Na2O, K2O and P2O5 in wt.% and Cr in ppm. For trace element analysis, ~0.1 g of the rock powders was fused with 0.9 g of lithium metaborate-tetraborate flux at 1025 °C and then dissolving the melt in an acid mixture of HCl, HNO3 and HF. The resulting solutions were then analyzed using the Q-ICP-MS technique to measure concentrations (in ppm) of Sc, Ti, V, Ni, Co, Ba, Cs, Rb, Sr, U, Th, Pb, high field strength elements (HFSE: Nb, Hf, Zr), rare earth elements (REE: La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu), and Y. The external precision and accuracy were assessed by repeated analysis of international standards: OREAS 102a, AMIS0304, SY-4, JP-1, UB-N [36]. The average blank level is below detection limits for respective elements. Average analytical uncertainty is commonly better than 5%, 10% and 20% for element concentrations above 1 wt.%, up to 1 ppm and <1 ppm, respectively. In addition, ~1.0 g of the rock powders was heated in the muffle furnace at 1050 °C for one hour to calculate the percent of loss on ignition from the difference in weight. Complete bulk rock data of this study are tabulated in Table S2.
Strontium-neodymium (Sr-Nd) isotope ratios of six selected samples were analyzed at the Geochronology Laboratory, Complutense University of Madrid, Spain following the sample preparation and analytical procedure described in [37]. In short, bulk sample powders were dissolved in ultra-pure reagents, followed by chromatographic separation of Rb, Sr, Sm and Nd isotopes. Isotope ratio analyses were performed using an IsotopX-Phoenix Multicollector Thermal Ionization Mass Spectrometer with data acquired in multidynamic mode. Repeated analyses on NBS-987 standard [36] (87Sr/86Sr = 0.710248 ± 0.000003) yielded 87Sr/86Sr = 0.710242 ± 0.00014 (2σ, n = 8) and that for Nd isotopic reference material JNdi-1 (143Nd/144Nd = 0.512115 ± 0.000002 [38]) provided 143Nd/144Nd values of 0.512101 ± 0.000008 (2σ, n = 9). The 2σ analytical errors were <0.008% for 87Sr/86Sr and <0.004% for 143Nd/144Nd. The procedural blanks were <5 ng and <1 ng for Rb-Sr and Sm-Nd, respectively. The analytical results are shown in Table S3.

4. Results

Powder XRD analyses on bulk-rock samples have identified hornblende (hbl) as the dominant (>90 modal%) constituent mineral in meta-hornblendites, whereas meta-hornblende-gabbro is comprised of hornblende (60–70 modal%) and epidote (ep, 20–30 modal%). Chlorite (chl, 5–10 modal%), titanite (ttn, 2–4 modal%) and magnetite (mag, up to 2 modal%) are present in significant proportions in both lithotypes, although their abundances are relatively higher in the meta-hornblendites as compared to the meta-hornblende-gabbros. Any presence of clinopyroxene (cpx), plagioclase (plag) and/or olivine (ol) has not been detected by XRD analysis. In the following sections, the textural–compositional characteristics in bulk-rock samples and individual minerals are presented in detail.

4.1. Petrography and Mineral Chemistry

In meta-hornblendites and meta-hornblende-gabbros, hornblende grains retained adcumulate and orthocumulate textures, respectively, of its precursors (Figure 3a,b) along with the presence of intercumulus epidotes in the latter (Figure 3c). In both lithotypes, hornblende grains preserved a relict interlocking texture, idiomorphic to hypidiomorphic shapes, straight to curved but sharp grain boundaries and evidence of triple junctions (Figure 3a,b). Chlorite always developed after replacing hornblendes either transgressing through the latter and/or at grain boundaries (Figure 3a,c–e). Titanite is detected either as interstitial grains and/or as inclusions (Figure 3d–f). Along with modal proportions, its crystal size is also often higher in the meta-hornblendites than the meta-hornblende-gabbros. Magnetite was identified as inclusion trails within hornblende in both lithologies (Figure 3a,f). Any presence of clinopyroxene, olivine and/or plagioclase was also not identified in the studied samples during the petrographic studies.
Compositionally, all hornblendes contain significant proportions of CaO (12.41–13.05 wt.% in M-Hblite and 11.96–12.49 wt.% in M-Hbl-gabbro) and TiO2 (0.37–0.61 wt.% and 0.25–0.54 wt.%, respectively) with lesser concentrations of Na2O (1.80–2.66 and 2.22–2.57 wt.%, respectively) and K2O (0.21–0.33 and 0.27–0.34 wt.%, respectively) (Table S1). Based on stoichiometry, they are classified as pargasite (Figure 4a, [40]), that exhibit strongly linear positive correlations in IVAl vs. Ti and A(Na + K) (calculated based on 24 oxygen atoms per formula unit, a.p.f.u.) content (Figure 4b,c). This is opposed to the negative trend identified in their FeTotal vs. Mg (per 24 O a.p.f.u.) and Si vs. (Ca + Na + K) proportions (Figure 4d,e). No correlation is present in their IVAl vs. VIAl, Ca, and Mg/(Mg + Fe2+) (per 24 O a.p.f.u.) concentrations (Figure 4f–h). However, the pargasites in the meta-hornblendites contain lower VIAl but higher Ca in their octahedral sites as compared to that in meta-hornblende-gabbros (Figure 4f,g). Intercumulus epidotes in M-Hbl-gabbro possess significant Al2O3 (27.60–32.84 wt.%), low Fe2O3calc (up to 7.57 wt.%) and are of zoisitic composition with low pistacite content (Ps = Fe3+/(Fe3+ + Al) ≤ 0.15) (Table S1). Secondary chlorites, after replacing amphiboles in both lithotypes (Figure 3a–c), exhibit clinochlore compositions with significant SiO2 (M-Hblite: 28.31–29.29 wt.% and M-Hbl-gabbro: 28.32–30.00 wt.%, respectively) and Al2O3 (19.93–22.07 wt.% and 20.03–21.65 wt.%, respectively) contents and Mg# values (80.50 ± 1.03) (Table S1). Titanite in both lithologies shows narrow-ranging SiO2 (31.25 ± 0.18 and 31.32 ± 0.36, respectively), TiO2 (37.55 ± 0.39 and 38.14 ± 0.62, respectively), CaO (29.74 ± 0.20 and 29.74 ± 0.23, respectively) and Al2O3 (1.71 ± 0.09 and 1.39 ± 0.32, respectively) contents (Table S1).

4.2. Whole-Rock Geochemistry

The investigated samples are characterized by variable CaO (6.46–19.85 wt.%), MgO (5.61–14.40 wt.%), Fe2O3T (5.68–12.50 wt.%) contents, and Mg# values (65.03–78.80), with lower SiO2 (37.20–42.50 wt.%), Na2O + K2O (0.78–2.59 wt.%), Co (27–82 ppm), and Sc (22–82 ppm) concentrations (Figure 5 and Figure 6, Table S2). Significant proportions of TiO2 (0.56–1.21 wt.%) are evident in all the investigated rocks (Table S2). Furthermore, the meta-hornblendites exhibit higher MgO (17.60–20.40 wt.%), Fe2O3T (10.50–12.50 wt.%), Sm (3.00–3.62 ppm), Y (33.20–41.90 ppm), MREE (16.97–21.97 ppm), HREE (8.75–11.21 ppm) and Mg# concentrations (74.39–78.80) than the meta-hornblende-gabbros (5.61–14.40 wt.%, 5.68–10.35 wt.%, 1.31–2.21 ppm, 19.67–34.12 ppm, 8.51–14.84 ppm, 4.38–6.87 ppm and 65.03–74.28, respectively), which is compensated by opposite trends in their SiO2 (M-Hblite: 37.20–38.30 wt.%; M-Hbl-gabbro: 40.20–42.50 wt.%), CaO (M-Hblite: 6.46–8.03 wt.%; M-Hbl-gabbro: 11.25–19.85 wt.%), and Sr (M-Hblite: 5.20–18.20 ppm; M-Hbl-gabbro: 31.90–501 ppm) contents (Figure 5 and Figure 6, Table S2). High field strength elements (HFSE: Nb, Zr, Hf) and Ti concentrations are comparable in both lithotypes (Nb: 0.30–0.70 ppm and 0.30–0.80 ppm, respectively, Zr: 14.00–17.00 ppm and 11.00–18.00 ppm, respectively, Hf: 0.60–0.80 ppm and 0.60–0.90 ppm, respectively, Ti: 5995–7254 ppm and 3357–6774 ppm, respectively) (Table S2). Strongly negative linear correlations in MgO vs. Al2O3 and CaO concentrations are conspicuous across the samples, which contrasts with their positive linear trends in MgO vs. Fe2O3T and TiO2 variations (Figure 6). In the SiO2 vs. Mg# diagram, their compositions are akin to the cumulate line of descent (Figure 7a), which has also been observed in other Himalayan hornblende-bearing intrusives [19,20]. Furthermore, all rocks show a clear trend of positive linear correlation in the SiO2 vs. Sm/Zr plot, where the M-Hblites are characterized by higher Sm/Zr (0.20–0.21) but lower SiO2 (37.20–38.30 wt.%) as compared to the M-Hbl-gabbros (0.10–0.16; 40.20–42.50 wt.%) (Figure 7b, Table S2). This is opposite to the trend observed in the Eu/Eu* (M-Hblite: 0.61–0.79; M-Hbl-gabbro: 1.15–2.59) ratio against SiO2 concentrations. (Figure 7c, Table S2). All samples further display low Nb/La ratios (0.10–0.83) and LaN/SmN values (0.12–0.62) (Table S2), which has previously been observed in oceanic arc-related rocks [44].
In the CI chondrite normalized REE diagrams, all samples display a LREE-depleted pattern (CeN/YbN: 0.20–0.59), where a greater degree of MREE to LREE depletion (CeN/SmN: 0.26–0.62) is evident as compared to HREE to MREE fractionation (SmN/YbN: 0.75–1.02) (Figure 8a,b, Table S2). Furthermore, meta-hornblendites exhibit strong negative Eu and Sr anomalies (Eu/Eu* = 0.61–0.79; Sr/Sr* = 0.04–0.13) in contrast to positive Eu-Sr anomalies in the meta-hornblende-gabbros (Eu/Eu* = 1.15–2.59; Sr/Sr* = 2.13–6.62 except M1831D, 0.46) (Figure 8, Table S2). Strong HFSE (Nb, Zr, Hf) and Ti depletion are also evident across the samples in primitive PM-normalized multi-element diagrams, which contrasts with significant enrichment in incompatible elements, such as Ba, Cs, U, and Th (Figure 8c,d).

4.3. Sr-Nd Isotope Signatures

Measured 87Sr/86Sr and 143Nd/144Nd ratios in selected meta-hornblendite and meta-hornblende-gabbro samples vary between 0.703826 ± 3–0.706314 ± 60 and 0.513116 ± 11–0.513303 ± 1, respectively (Table S3). In the 87Sr/86Sr(t) and 143Nd/144Nd(t) diagram (calculated considering crystallization age (t) of ca. 127 Ma [25]), the samples exhibit compositional consistency with other ITSZ hornblende-bearing cumulate lithologies, while variably diverging from the N-MORB and mantle array compositions (Figure 9a) [1,19]. They further yield εNd(t) (t = 127 Ma) and fSm/Nd values ranging from +7.71–+12.29–+0.2172–+0.6816, respectively (Figure 9b, Table S3). Interestingly, εNd(t) values of the meta-hornblende-gabbro samples (+7.71–+10.76) are progressively lower as compared to those of the meta-hornblendites (+10.16–+12.29) (Table S3, Figure 9b). In the La/Yb vs. Ba/La diagram, all investigated lithologies further exhibit a greater extent of Ba/La variation and negligible La/Yb change, in a similar manner to that observed in other Himalayan Hbl-cumulates (Figure 10).

5. Discussion

5.1. Protolith Signatures

Bulk-rock XRD analyses combined with petrographic studies identify the presence of hornblende, chlorite, titanite, and magnetite ± epidote in the studied lithologies (Figure 2 and Figure 3). The formation of amphibole in mafic–ultramafic rocks has previously been described as the consequence of three distinct processes: (a) direct crystallization from a hydrous silicate melt [48,54], (b) peritectic reaction between clinopyroxene (cpx) ± olivine (ol) and melt [55], and (c) retrograde metamorphism of granulite facies rocks [56]. In the case of amphiboles produced by the melt-induced reaction-replacement process, the presence of cpx ± ol relicts, embedded within an amphibole matrix, and the development of granoblastic and/or poikilitic textures with indented grain boundaries (attesting disequilibrium reactions) are common [43,57]. Major element compositions of peritectic amphiboles also exhibit low IVAl (a.p.f.u.) and high Mg# values in proximity to clinopyroxene chemistry [43]. A similar compositional distinction for metamorphic amphiboles can be made based on low total alkali (A(Na + K)) (a.p.f.u.) but elevated Si (a.p.f.u.) contents [41,58]. Neither of these characteristics is consistent with the textural–compositional observations of the studied pargasites, nor is there any evidence of cpx ± ol in the studied assemblages (Figure 3a–e and Figure 4h) [43]. Rather, the idiomorphic to hypidiomorphic shape of relict pargasitic hornblendes with interlocking textures and sharp straight-to-curved grain boundaries with evidence of triple junctions (Figure 3a–c) clearly indicate an equilibrium condition during amphibole formation, which may only result from magmatic crystallization [59]. Higher proportions of A(Na + K), IVAl (a.p.f.u.) and lower concentrations of Si (a.p.f.u.) in pargasites are also more akin to global primary hornblende compositions (Figure 4c,e) [41,43]. An igneous origin of the studied pargasite is, therefore, plausible in both lithotypes, as previously observed in mafic–ultramafic intrusives worldwide [43]. The massive euhedral grain size of amphibole crystals (up to 10 cm) further points to a relatively stable magmatic environment during their crystallization, which might have provided a prolonged residence time for crystal growth [60]. Evidence of titanite and magnetite, either as interstitial phases and/or inclusion trails within the amphibole grains, suggest their comagmatic origin with igneous amphiboles [61]. The epidote grains within the meta-hornblende-gabbro samples, however, exhibit compositional consistency (Ps ≤ 0.15, Table S1) with the secondary zoisites identified in other high P/T metamorphic terranes [62] and have previously been interpreted as the consequence of plagioclase breakdown [21]. Likewise, the development of chlorite after replacing pargasites, either along grain boundaries and/or transgressing through the hornblendes, is indicative of a metamorphic origin [63]. Hence, the igneous minerals preserved within the studied assemblages include pargasitic hornblende, titanite and magnetite.
However, the investigated lithologies are variably metamorphosed to chlorite and/or epidote (Figure 3a,c–f). An a-priori assessment of protolith signatures is, therefore, important before further petrogenetic inferences. In this regard, any interpretation (e.g., melt composition, water content) based on the igneous mineral chemistry should be accurate. Given the meta-hornblendites are modally dominated (>92 modal%) by igneous minerals (Hbl + Ttn + Mag), the bulk geochemical characteristics of these ultramafic assemblages should also reflect their magmatic signatures. In the case of the meta-hornblende-gabbros, metamorphic chlorite and epidote constitute up to 38 modal%. Chloritization, however, commonly conserves the bulk geochemical attributes of the protolith assemblages, as revealed in previous studies [64]. Likewise, epidotization tends to retain the bulk elemental characteristics of the parental rock, except for CaO, Sr, and Eu [65]. The possibility of post-magmatic Sr enrichment in meta-hornblende-gabbros cannot be ruled out in the present case. However, the strong linear correlation of CaO (against MgO) and Eu/Eu* (against SiO2) in the meta-hornblende-gabbros to that in the meta-hornblendites (in contrast to their scattered behaviour) suggests that CaO and Eu possibly have also retained the bulk magmatic signatures. Nevertheless, it is important to emphasize that any interpretation in this study is not based on a particular textural, mineralogical and/or geochemical attribute; rather different observations are combined to support any inference.
Following this, a cumulate igneous origin of these rocks is evident from—(a) the compositional similarities of the meta-hornblendites and the meta-hornblende-gabbros with arc-related ultramafic and mafic cumulates in MgO vs. FeOT vs. (Na2O + K2O) diagram (Figure 5) [45], as observed in previous studies on Himalayan mafic–ultramafic intrusives [1,19,20,21]; (b) evidence of interlocking cumulate textures, defined by idiomorphic hornblende crystals, in the field and petrographic observations (Figure 2a,b and Figure 3a,b); and (c) concurrent variation in bulk Mg# with SiO2 (in wt.%) along the cumulate line of descent (Figure 7a) [66,67]. The intercumulus phase in the precursor to the meta-hornblende-gabbros is likely to be plagioclase (before zoisite formation), owing to the covariation in CaO and Al2O3 (against MgO) (Figure 6a,b) and positive Eu anomaly in these samples (Table S2, Figure 7c and Figure 8b,d) [21]. The magmatic mineralogy of the meta-hornblendite may, therefore, have been dominated by hornblende (>90 modal %), indicating a hornblendite protolith. In contrast, the precursor of the meta-hornblende-gabbro would have consisted of pargasites (60–70 modal %) + plagioclase (~20–30 modal %) apart from minor titanite and magnetite, which is consistent with hornblende-gabbro mineralogy.
Different elemental behaviours in protolith assemblages are commonly governed by the modal variation in magmatic minerals. For instance, the meta-hornblendites exhibit higher concentrations of MgO, TiO2, Fe2O3T, Sc, Ni, Co, MREE (except Eu) and HREE + Y and Mg# values relative to meta-hornblende-gabbro (Figure 6 and Figure 8, Table S2), as these elements are more compatible in amphiboles than other MdOC cumulate minerals [68]. However, greater enrichment in TiO2 and Fe2O3T in the meta-hornblendites than in the meta-hornblende-gabbros (Figure 6c,d, Table S2) might also have been aided by a higher abundance of titanite and magnetite in the former (Figure 3a,d–f) [69]. This contrasts with the higher CaO, Al2O3, SiO2, and Eu contents in the meta-hornblende-gabbros as compared to the meta-hornblendites (Figure 6a,b, Figure 7 and Figure 8). that implies governing role of plagioclase abundance in controlling their behaviours in bulk rock [70]. Comparable concentrations of HFSE (Nb, Zr, Hf) in the studied samples (Table S2), however, suggest that parental source composition rather than modal mineralogy has controlled their abundances in MdOC cumulates [70]. A decreasing trend in MgO, Fe2O3T, and TiO2 contents, Sm/Zr ratios, and Mg# values compensated by an opposite trend in CaO, Al2O3, and SiO2 contents, Eu/Eu* ratio in the meta-hornblendites relative to the meta-hornblende-gabbros (Figure 6a,b and Figure 7 and Table S2) further suggest that the protolith of the meta-hornblendite has crystallized earlier from a more primitive magma than that of the meta-hornblende-gabbro [68]. This is also evident from the lack of any correlation between the IVAl vs. Ca (a.p.f.u.) contents in amphibole, indicating the insignificant role of plagioclase during amphibole formation (Figure 4g) [71,72]. Hence, plagioclase possibly crystallized at a later stage than amphibole, which is supported by textural occurrences of epidote (signifying earlier plagioclase) only within intercumulus spaces (Figure 3c). However, Fe-Ti bearing phases (e.g., titanite, magnetite) might have been crystallized simultaneously along with amphiboles, given the covariation in Fe2O3T, and TiO2 along with MgO (Figure 6c,d). Greater modal abundance of titanite and magnetite in the meta-hornblendites (compared to the meta-hornblende-gabbros) also corroborates this interpretation (Figure 3a,d–f). Mineral crystallization sequence in studied cumulates may, therefore, be interpreted as hornblende + titanite + magnetite → plagioclase, which has also been previously observed in mafic–ultramafic cumulate rocks worldwide [54,73,74]. Thus, earlier fractionation of pargasites solely constituted the precursor hornblendites, and later fractionation of intercumulus plagioclase within amphibole orthocumulate together formed the hornblende-gabbro protoliths within Mayudia ophiolite complex.

5.2. Physico-Chemical Conditions

Amphibole major element compositions are sensitive to various physico-chemical parameters, which include the pressure, temperature, oxygen fugacity and melt water content of the parental magma [8]. Understanding the relative dominance of these intensive variables by using amphibole stoichiometry is, therefore, crucial before any thermodynamic interpretation [71]. Consequently, previous studies have demonstrated that substitution mechanisms are often useful in identifying the controlling parameters during amphibole crystallization [72,75]. In the investigated amphiboles, a simultaneous increase in A(Na + K) and Ti (a.p.f.u.) with IVAl (a.p.f.u.) (Figure 4b,c) thus suggest that temperature-sensitive edenite-exchange (IVSi + A□ = IVAl + A(Na + K)) and Ti–tschermakite substitution (IVSi + VIMg = IVAl + VITi) have possibly dictated their composition during crystallization [72,75]. Similar control of oxygen fugacity is evident from a linear increase in pargasite FeT with decreasing CMg (a.p.f.u.) content (Figure 4d) [71,76]. However, pressure-sensitive Al–tschermakite substitution (IVSi + VIMg = IVAl + VIAl) likely has not played any significant role during the formation of the amphiboles in the MdOC cumulates, as documented by the lack of correlation between IVAl and VIAl (Figure 4f). MdOC cumulate pargasites were, therefore, more sensitive to temperature and oxygen fugacity in the parental magma as compared to pressure.
Following this, the application of single-phase geothermobarometers are commonly found to be advantageous to estimate different physico-chemical conditions, owing to better preservation of the subvolcanic records in the minerals produced from hybridized magmas [77,78]. Late-stage formation of plagioclase after amphibole, as proposed earlier, also restricts the suitability of amphibole–plagioclase geothermobarometers to this study [79,80,81,82]. Empirical barometers calibrated for amphiboles either forming at shallow crustal depths and/or not genetically linked to calc-alkaline magmas may also not be relevant here [19]. Therefore, this study estimates amphibole crystallization temperatures based on the thermometers calibrated by Putirka [83] and Ridolfi [84], whereas pressure is determined using barometers developed by Krawczynski [8], modified after Larocque and Canil [85] (Table S5). Pressure estimates are converted to depth following Anderson [86], whereas the oxygen fugacity of and H2O fraction in the MdOC melt (s) are calculated based on Ridolfi [84] (Table S5). Our calculations yield comparable ranges in crystallization temperatures, oxygen fugacities and melt water fractions for the meta-hornblendite (865–940 °C, −8.59 to −11.19 log units, and 10.56–13.58 wt.%, respectively) and meta-hornblende-gabbro amphiboles (902–928 °C, −9.48 to −10.49 log units and 11.14–13.61 wt.%, respectively), but higher-pressure conditions (1.2–1.4 GPa) and depth estimates (~40–48 km) of the magmatic hornblendes in the meta-hornblende-gabbros as compared to the former (0.8–1.1 GPa and ~30–38 km, respectively) (Figure 11). These results are consistent with earlier available data on Himalayan (including MdOC) cumulates (Figure 11) [19,87,88], and suggest a stable physicochemical condition within the magma chamber during meta-hornblendite precursor followed by meta-hornblende-gabbro protolith crystallization. However, the parental rock of the meta-hornblende-gabbro has crystallized at higher pressure conditions than that of the meta-hornblendites (Figure 11a), which is most likely related to a reduction in effective volume within the magma chamber due to the accumulation of amphibole at earlier stages. Thus, evaluation of the depth of precursor hornblende-gabbro formation based on amphibole crystallization pressure data tends to be fallacious. This study, therefore, only relies on the meta-hornblendite amphibole pressure data (0.8–1.1 GPa) for the estimation of cumulate crystallization within the MdOC at a depth of approx. 30–38 km, which implies lower crustal pressures and depths near Mohorovičiċ discontinuity [89].
Notably, MdOC cumulate amphiboles record a highly oxidizing environment (logfO2: −8.59 to −11.19) above the Fayalite–Magnetite–Quartz (FMQ) buffer at elevated pressure conditions (0.8–1.4 GPa) during crystallization, which corresponds to Ni–NiO (NNO) buffer values of +2.09–+ 3.69 (Figure 11b, Table S5). At such high oxidation and pressure conditions, divergence in Mg# values between bulk rock samples and amphibole grains becomes minimal and magnetite saturates at an early stage of magmatic crystallization [69]. This is corroborated by the similarity in Mg# values in MdOC bulk cumulates (65.03–78.80) and constituent amphiboles (69.39–78.53) and co-magmatic precipitation of magnetite with amphiboles as inclusion trails (Figure 3a,f, Tables S1 and S2). However, such an extremely oxidizing environment at high pressure (0.8–1.4 GPa) can only be sustained when the water content in the parental magma is very high (~10–20 wt.%) (Figure 11a) [8], which correlates well with the inferred H2O% of the parental melt to the MdOC cumulates (10.56–13.61 wt.% at 0.8–1.4 GPa, Figure 11c, Table S1). Similarly high H2O contents and oxygen fugacities at high pressures have been reported for the cumulate rocks in the Jijal ophiolite complex (up to 20 wt.% at 1.5 GPa, [87]). Under such circumstances, suppression of plagioclase saturation towards lower temperatures is anticipated [90] because depolymerization in the liquid structure would favour the crystallization of less polymerized phases over highly polymerized structures [91]. The same is reflected by the inferred sequence of plagioclase crystallization post-dating amphibole in MdOC cumulates.
Minerals 15 00572 g011
Figure 11. Plot of physico-chemical conditions of amphibole crystallization in (a) T (°C) vs. P (GPa), (b) T (°C) vs. logfO2, and (c) H2O in melt (wt.%) vs. T (°C) space. Different fields and univariant lines in (a) are after [92] and that in (b) are after [84]. Reference data for Himalayan hornblendite and hornblende-gabbro are listed in Table S5.
Figure 11. Plot of physico-chemical conditions of amphibole crystallization in (a) T (°C) vs. P (GPa), (b) T (°C) vs. logfO2, and (c) H2O in melt (wt.%) vs. T (°C) space. Different fields and univariant lines in (a) are after [92] and that in (b) are after [84]. Reference data for Himalayan hornblendite and hornblende-gabbro are listed in Table S5.
Minerals 15 00572 g011

5.3. Parental Magma Characteristics

In MdOC cumulates, linear covariance in CaO, Al2O3, Fe2O3T, and TiO2 contents, Sm/Zr ratio with MgO and/or SiO2 contents (Figure 6 and Figure 7b), identical LREE-depleted patterns in the CI-normalized REE diagrams (Figure 8a,b), and comparable HFSE (Nb, Zr, Hf) contents, and εNd(t) values (Tables S2 and S3, and Figure 9b) indicate that their parental magmas were all derived from a single source [68]. This is supported by the gradational contact between hornblendite and meta-hornblende-gabbro in the field (Figure 2c). Positive εNd(t) values in the MdOC cumulates (+7.71–+12.29), coupled with LREE-depleted REE patterns and positive fSm/Nd values (+0.2172–+0.6816), further suggest that they represent the crystallization products of a depleted magma composition (Figure 8a,b and Figure 9b) [93]. However, a greater extent of depletion relative to the mid-oceanic ridge environment is observed from their negative Ti, and HFSE (Nb, Zr, Hf) anomalies and lower LREE concentrations in PM-normalized plots compared to N-MORB (Figure 8c,d). In the Sr–Nd plot, they also exhibit progressive deviation from the N-MORB and mantle array compositions (Figure 9a). A mid-oceanic ridge-related origin of the MdOC parental magma is, therefore, not tenable; rather it suggests an arc-related suprasubduction zone environment for their formation, which is further supported by the following observations: (a) they show compositional resemblance with arc-related cumulates in the triangular MgO vs. FeOT vs. (Na2O + K2O) diagram (Figure 5), as observed earlier in the MdOC cumulates [19], (b) inferred high oxygen fugacities (NNO: +2.09–+3.69) vis-à-vis water contents (10.56–13.61 wt.%) in the parental magma are consistent with an arc-related environment in a subduction zone (Figure 11b), as documented in other Himalayan cumulates [87], (c) all samples are selectively enriched in large ion lithophile elements like Cs and Ba relative to PM and N-MORB values (Figure 8c,d), implying potential subduction input in the parental magma [4]. However, low LaN/SmN (0.12–0.62) and Nb/La (0.10–0.83) ratios (Table S2), coupled with positive eNd(t) (+7.71–+12.29) and fSm/Nd values (+0.2172–+0.6816) (Figure 9b), in studied samples reveal that the MdOC cumulates are genetically linked to an oceanic arc setting [44]. The absence of any felsic rock and the presence of metaperidotite in lithological contact with these cumulates also corroborates this interpretation (Figure 2d,e). Highly variable Ba/La ratios and consistent La/Yb values in the studied samples (Figure 10) further imply that a metasomatized aqueous fluid in the parental magma was derived from the dehydrating altered oceanic crust (AOC) in a suprasubduction zone [94].
To reconcile the crystallization process of the MdOC cumulates, a modal fractional crystallization modelling on fore-arc basaltic magma was performed (Figure 12) [70,95]. A fore-arc basalt composition [95] has categorically been considered for parental magma because it represents a relatively primitive melt composition in a suprasubduction zone environment [96]. Whereas the hornblendite composition is modelled assuming 100 modal % amphibole fractionation, the amphibole vs. plagioclase modal proportions is considered at a ratio of 70:30 for the hornblende-gabbro, which are consistent with XRD analysis and petrographic observations. Amphibole/melt and plagioclase/melt partitioning coefficients are taken from Tiepolo [97] and White [70], respectively. Our calculations yield that the meta-hornblendite compositions, particularly their HREE pattern, could be obtained by ~ 30%–60% fractional crystallization from the parental magma (Figure 12a). However, the observed REE pattern of the meta-hornblende-gabbros could not be reproduced by direct crystallization from the residual melt left after the crystallization of the meta-hornblendite protolith. The probable petrogenetic process that led to the formation of the observed REE patterns of the meta-hornblende-gabbros could be modelled by the fractionation of amphibole + plagioclase crystals from a hybridized magma that was generated by the mixing of residual liquid (70%) and a fresh pulse of the parental fore-arc basalt (30%) (Figure 12b). Hybridization of the residual magma composition with fresh parental melt could, therefore, explain the emplacement of the hornblende-gabbros after the hornblendites. Hydrous magma derived by the melting of the sub-arc mantle, metasomatized by slab-derived fluid in a suprasubduction zone and the successive crystallization of the hornblendites and hornblende-gabbros by dynamic fractional crystallization, thus better explains the mineralogical and chemical compositions obtained from the mafic–ultramafic cumulates of the Mayudia ophiolite complex.

5.4. Geodynamic Implications

The closure of the Neo-Tethys preceding the India-Eurasia collision has resulted from two different types of subductions, namely the intraoceanic subduction within the Neo-Tethys and the “Andean type” subduction of the Indian plate beneath Eurasia (Figure 13) [14,66]. Evidence of the “Andean type” subduction of India under Eurasia is well documented by the continental arc-related intermediate to felsic rocks (hornblende- and/or biotite-bearing granites, granodiorites, and diorites of calc-alkaline affinity) in the southern parts of the Karakoram and Lhasa terranes, which are defined as the parts of the Trans-Himalayan batholiths (Gangdese arc) [98,99,100,101]. In contrast, fossilized records of intraoceanic Neo-Tethyan subduction are preserved within the ultrabasic to basic cumulate rocks (hornblendite, hornblende-gabbro, pyroxenite, and wehrlite) in the Kohistan, Dras, Spong, Zedong and Pai arcs in the Himalaya [1,102,103,104,105]. Whether one or more Trans-Tethyan subduction zones were active during the late Cretaceous to early Tertiary period is, however, debated in the context of the evolution and closure of the Neo-Tethys Ocean. Whereas some authors suggested the presence of only one north-dipping subduction zone within the Neo-Tethys Ocean, others argued for the existence of at least two NNE-dipping intraoceanic subduction systems along the southern margin of the Eurasian plate [102,105,106]. Such ambiguity is even more persistent during the final closure of the eastern Neo-Tethys Ocean, either because of the absence or controversial nature of key evidence from the rock records (arc-type magmatic rocks) in the eastern Himalayas. In fact, the arc-related magmatic rocks bearing signatures of intraoceanic subduction have only been reported to date from the Zedong terrane, and Pai and Mayudia areas in eastern Himalaya [19,103,104]. Such information is further limited from the eastern limb of the EHS [19,107], although geodynamic reconstructions of this region are crucial to understanding the final stage of the evolution and closure of the Neo-Tethys Ocean.
In this context, the geochemical and isotopic affinities of the MdOC cumulates with island arc-related rocks (Figure 8, Figure 9, Figure 10 and Figure 12, Table S2), along with their lithological association with ultrabasic metaperidotites (Figure 2d,e), imply that they formed during intraoceanic subduction and were not influenced by any crustal inputs [89]. Furthermore, the physico-chemical conditions of constituent amphibole formation (Figure 11), coupled with the REE modelling of the crystallization process (Figure 12), suggest the origin of these cumulates through dynamic fractional crystallization of a super hydrous (~10.56–13.61 wt.% H2O) magma at high pressures (0.8–1.1 GPa) and lower crustal levels (~30–38 km) near the Moho transition zone. The extremely hydrous nature of the sub-arc magma likely resulted in the suppression of clinopyroxene crystallization and the formation of amphibole before plagioclase in this part of the Himalayas. This is common worldwide at deeper crustal levels within a suprasubduction zone [2,18,43,68,87] and is consistent with the root of an island arc complex [1]. It may, therefore, be rational to interpret that the studied pargasitic hornblende-bearing cumulates within the Mayudia ophiolite complex possibly represent the root of a fossilized island arc system at the eastern fringe of the ITSZ, as observed earlier in the Zedong and Pai area in the eastern Himalayas [103,104]. If so, these cumulates potentially have regulated the composition of the ascending magma at sub-arc depths beneath the Mayudia area [68]. For instance, it is expected that amphibole crystallization would cause an increase in SiO2 and an accompanied decrease in MREE–HREE and water content in the differentiated magma, making it more andesitic and anhydrous [3,108], and contribute to the production of low-K and calc-alkaline granitoid suites [109]. Exposure of diorite-rich granitoid suites has also been previously documented within the eastern Lohit plutonic complex in contact with the MdOC ophiolite complex (Figure 1) [110]. Whether these granitoid suites are genetically linked with the MdOC cumulate is, therefore, a topic of future research.
The MdOC cumulates also signify the presence of a super hydrous magmatic reservoir in the sub-arc mantle, regulating volatile cycling during intraoceanic Neo-Tethyan subduction [11]. Globally, magma with hydrous behaviour (>10 wt.% H2O) at sub-arc depth is common, as demonstrated by the Gangdese arc [111], Alpine records [92], and global convergence margins [112,113]. However, the occurrence of super hydrous magmas at sub-arc depths during intra-oceanic Neo-Tethyan subduction is relatively rare in the Himalayas and has only been recorded from the Jijal ophiolite complex (up to 20 wt.% at 1.5 GPa) beneath the Kohistan arc, western Himalaya [87]. Given that the different geochemical and isotopic signatures observed in the studied MdOC cumulates are consistent with the cumulate lithologies identified in the Jijal ophiolite complex (Figure 5, Figure 6, Figure 7, Figure 8, Figure 9, Figure 10 and Figure 11), this may imply that the downgoing sub-arc oceanic lithosphere beneath the MdOC complex may have undergone a similar geodynamic evolution to that of the Kohistan arc during the final closure of the Neo-Tethys. It would, therefore, be rational to conclude that the MdOC cumulates preserve the records of a large-scale intraoceanic subduction system that extended over more than 2000 km from the western fringe of the Nanga Parbat to the eastern extension of the Namcha Barwa.

6. Conclusions

Based on various mineralogical and bulk-rock geochemical and isotopic signatures, in combination with field and petrographic evidence, in the studied MdOC cumulates, the following conclusions can be made:
  • The cumulate lithologies in the MdOC complex varies from hornblendite to hornblende-gabbro, where hornblendite originated prior to hornblende-gabbro due to earlier crystallization of amphibole than plagioclase.
  • Constituent amphiboles have crystallized in a stable physico-chemical condition (T: 865–940 °C, P: 0.8–1.4 GPa, logfO2: −8.59–−11.19 unit) at a lower crustal depth (30–38 km) from a super hydrous (H2O in melt: 10.56–13.61 wt.%) parental magma, which was, in turn, produced due to the flux melting of sub-arc mantle with aqueous inputs from the dehydrating subducting slab.
  • The MdOC cumulates preserve the root of an island arc complex and signify the presence of super hydrous sub-arc mantle reservoirs in the eastern Neo-Tethys Ocean, which have potentially controlled the composition of arc magmas and crust during intraoceanic subduction.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15060572/s1, Table S1: Microprobe analysis data; Table S2: Bulk rock geochemical data; Table S3: Isotope analysis data; Table S4: Reference bulk rock data for hornblende-bearing cumulates from the Himalaya and worldwide locations; Table S5: Estimates of physico-chemical conditions of amphibole crystallization. References [12,13,74,103,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134] are cited in the Supplementary Materials.

Author Contributions

Conceptualization, S.S. and A.S.M.; methodology, S.S., A.S.M., D.R. and J.M.F.; software, S.S. and A.S.M.; validation, S.S., A.S.M., R.A., D.R. and J.M.F.; formal analysis, S.S., A.S.M., D.R. and J.M.F.; investigation, S.S., A.S.M. and R.A.; resources, S.S. and A.S.M.; data curation, S.S. and A.S.M.; writing—original draft preparation, S.S. and A.S.M.; writing—review and editing, S.S., A.S.M., R.A., D.R. and J.M.F.; visualization, S.S., A.S.M. and R.A.; supervision, A.S.M. and R.A.; project administration, A.S.M.; funding acquisition, A.S.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was jointly funded by the IIT (ISM) Dhanbad doctoral fellowship, grant number 17DR000510 and the DST-INSPIRE Faculty Award, grant number IFA-17-EAS-056, to S.S. and A.S.M., respectively.

Data Availability Statement

The raw data supporting the findings of this paper will be made available by the corresponding author upon request.

Acknowledgments

This contribution is part of the doctoral thesis of the first author, funded by the IIT (ISM) Dhanbad doctoral fellowship. We express our gratitude to Piyush Sriwastava, IIT Bombay for field assistance. Roing district administration (Arunachal Pradesh), Mustak Ali (transport), and Mayudia guest house caretaker are thanked for their logistic support during the field. Continental Instrument (Lucknow, India) and ALS Geochemistry laboratory (Loughrea, Ireland) are acknowledged for thin section preparation, and bulk-rock analyses, respectively. S.S. and A.S.M. are indebted to the Director, IIT (ISM) Dhanbad for his constant support and encouragement. Constructive comments by four anonymous reviewers have greatly improved the quality of the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 2. Field photographs of (a) meta-hornblendite (M-Hblite), consisting of pegmatitic hornblende and (b) meta-hornblende-gabbro (M-Hbl-gabbro), comprised of cumulus hornblende and intercumulus epidote, within the Mayudia ophiolite complex. (c) The gradational contact between the two lithotypes. (d–f) display the occurrence of epidotes as pods, vein and selvages, respectively, with the meta-hornblende-gabbro. The sharp lithological contacts without any evidence of chilled margins are also visible in (d,e). MP: Metaperidotite.
Figure 2. Field photographs of (a) meta-hornblendite (M-Hblite), consisting of pegmatitic hornblende and (b) meta-hornblende-gabbro (M-Hbl-gabbro), comprised of cumulus hornblende and intercumulus epidote, within the Mayudia ophiolite complex. (c) The gradational contact between the two lithotypes. (d–f) display the occurrence of epidotes as pods, vein and selvages, respectively, with the meta-hornblende-gabbro. The sharp lithological contacts without any evidence of chilled margins are also visible in (d,e). MP: Metaperidotite.
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Figure 3. Representative visible-light photomicrographs of (a) meta-hornblendite (M-Hblite), exhibiting adcumulate textures and (b,c) meta-hornblende-gabbro (M-Hbl-gabbro) samples, displaying relict orthocumulate interlocking textures. Red and yellow arrowheads indicate straight to curved but sharp grain boundaries and triple junctions at hornblende (Hbl) interfaces. High dihedral angles at grain interfaces are denoted by yellow dashed lines. White arrowheads point out magnetite inclusion trails within hornblende crystals. (c) Occurrence of epidote (Ep) as intercumulus phase within amphibole orthocumulates devoid of any plagioclase feldspar. Titanite (Ttn, indicated by red arrow) occurs as (d) interstitial grains, and/or (e) inclusions within meta-hornblendite, whereas both titanite (Ttn) and magnetite (Mag) are present as inclusions within amphibole grains in meta-hornblende-gabbro as demarcated in (f) Chlorite (Chl) is evident in both lithotypes replacing amphibole in (c–e). Mineral abbreviations are after [39].
Figure 3. Representative visible-light photomicrographs of (a) meta-hornblendite (M-Hblite), exhibiting adcumulate textures and (b,c) meta-hornblende-gabbro (M-Hbl-gabbro) samples, displaying relict orthocumulate interlocking textures. Red and yellow arrowheads indicate straight to curved but sharp grain boundaries and triple junctions at hornblende (Hbl) interfaces. High dihedral angles at grain interfaces are denoted by yellow dashed lines. White arrowheads point out magnetite inclusion trails within hornblende crystals. (c) Occurrence of epidote (Ep) as intercumulus phase within amphibole orthocumulates devoid of any plagioclase feldspar. Titanite (Ttn, indicated by red arrow) occurs as (d) interstitial grains, and/or (e) inclusions within meta-hornblendite, whereas both titanite (Ttn) and magnetite (Mag) are present as inclusions within amphibole grains in meta-hornblende-gabbro as demarcated in (f) Chlorite (Chl) is evident in both lithotypes replacing amphibole in (c–e). Mineral abbreviations are after [39].
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Figure 4. (a) Amphibole classification diagram with mineral stoichiometry calculated on 24 (O, OH) basis after [40]. Cation substitution diagrams for amphibole compositions, showing relative variations in (b) IVAl vs. Ti (a.p.f.u.), (c) IVAl vs. A(Na + K) (a.p.f.u.), (d) CMg vs. FeT (a.p.f.u.), (e) Si vs. (Ca + Na + K) (a.p.f.u.), (f) IVAl vs. VIAl (a.p.f.u.), (g) IVAl vs. Ca (a.p.f.u.) and (h) IVAl vs. Mg/(Mg + Fe2+) (a.p.f.u.). Primary and secondary amphibole fields with boundary arrow positions are after [41] in (c), the magmatic and post-magmatic amphibole boundary in (e) are after [42], whereas primary and peritectic amphibole fields in (h) are following [43].
Figure 4. (a) Amphibole classification diagram with mineral stoichiometry calculated on 24 (O, OH) basis after [40]. Cation substitution diagrams for amphibole compositions, showing relative variations in (b) IVAl vs. Ti (a.p.f.u.), (c) IVAl vs. A(Na + K) (a.p.f.u.), (d) CMg vs. FeT (a.p.f.u.), (e) Si vs. (Ca + Na + K) (a.p.f.u.), (f) IVAl vs. VIAl (a.p.f.u.), (g) IVAl vs. Ca (a.p.f.u.) and (h) IVAl vs. Mg/(Mg + Fe2+) (a.p.f.u.). Primary and secondary amphibole fields with boundary arrow positions are after [41] in (c), the magmatic and post-magmatic amphibole boundary in (e) are after [42], whereas primary and peritectic amphibole fields in (h) are following [43].
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Figure 5. Ternary (Na2O +K2O) vs. MgO vs. FeOT diagram of studied samples, with demarcation of different arc-related and unrelated cumulate fields after [45]. MdOC meta-hornblendite and meta-hornblende-gabbro data are taken from [46] and [19,21], respectively, whereas Himalayan hornblendite compositions are reported from [1,20] as per Table S4.
Figure 5. Ternary (Na2O +K2O) vs. MgO vs. FeOT diagram of studied samples, with demarcation of different arc-related and unrelated cumulate fields after [45]. MdOC meta-hornblendite and meta-hornblende-gabbro data are taken from [46] and [19,21], respectively, whereas Himalayan hornblendite compositions are reported from [1,20] as per Table S4.
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Figure 6. Bulk compositional variations in MgO (wt.%) vs. (a) Al2O3 (wt.%), (b) CaO (wt.%), (c) Fe2O3T (wt.%) and (d) TiO2 (wt.%) in studied samples. Reference data are from Table S4.
Figure 6. Bulk compositional variations in MgO (wt.%) vs. (a) Al2O3 (wt.%), (b) CaO (wt.%), (c) Fe2O3T (wt.%) and (d) TiO2 (wt.%) in studied samples. Reference data are from Table S4.
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Figure 7. Bulk compositional variations in SiO2 (wt.%) vs. (a) Mg#, (b) Sm/Zr, and (c) Eu/Eu* in studied samples. The cumulate line of descent is after [20]. Reference data are taken from Table S4.
Figure 7. Bulk compositional variations in SiO2 (wt.%) vs. (a) Mg#, (b) Sm/Zr, and (c) Eu/Eu* in studied samples. The cumulate line of descent is after [20]. Reference data are taken from Table S4.
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Figure 8. (a,b) CI chondrite-normalized and (c,d) PM-normalized concentration diagrams for meta-hornblendite (a,c) and meta-hornblende-gabbro (b,d) samples. CI chondrite, primitive mantle and Normal-Mid Ocean Ridge Basalt (N-MORB) values are taken from [47]. Reference data for Himalayan and worldwide hornblendite and hornblende-gabbro are from Table S4. Hornblendite data from Fiskenæsset complex and hornblende-gabbro data from the Lesser Antilles Arc are also plotted for reference after [2,48,49], respectively. Light green vertical stripes point out anomalous behaviours of different elements in these diagrams.
Figure 8. (a,b) CI chondrite-normalized and (c,d) PM-normalized concentration diagrams for meta-hornblendite (a,c) and meta-hornblende-gabbro (b,d) samples. CI chondrite, primitive mantle and Normal-Mid Ocean Ridge Basalt (N-MORB) values are taken from [47]. Reference data for Himalayan and worldwide hornblendite and hornblende-gabbro are from Table S4. Hornblendite data from Fiskenæsset complex and hornblende-gabbro data from the Lesser Antilles Arc are also plotted for reference after [2,48,49], respectively. Light green vertical stripes point out anomalous behaviours of different elements in these diagrams.
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Figure 9. (a) Age-corrected 87Sr/86Sr(t) vs. 143Nd/144Nd(t), and (b) εNd(t) vs. fSm/Nd plots of selected samples from this study. Compositions of depleted mantle (DM) [50], mantle array [51] and MORB plus different worldwide island arcs [52] are also plotted for comparison. Reference Himalayan hornblendite data are from Table S4, and altered oceanic crust data are from [53]. LREE: Light Rare Earth Elements; CHUR: Chondritic Uniform Reservoir.
Figure 9. (a) Age-corrected 87Sr/86Sr(t) vs. 143Nd/144Nd(t), and (b) εNd(t) vs. fSm/Nd plots of selected samples from this study. Compositions of depleted mantle (DM) [50], mantle array [51] and MORB plus different worldwide island arcs [52] are also plotted for comparison. Reference Himalayan hornblendite data are from Table S4, and altered oceanic crust data are from [53]. LREE: Light Rare Earth Elements; CHUR: Chondritic Uniform Reservoir.
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Figure 10. La/Yb vs. Ba/La plot of studied samples.
Figure 10. La/Yb vs. Ba/La plot of studied samples.
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Figure 12. REE modelling of the fractional crystallization process in studied MdOC (a) meta-hornblendite protolith, and (b) meta-hornblende-gabbro precursors. Source fore-arc basalt composition is after [95], Himalayan samples after Table S4. See text for other details.
Figure 12. REE modelling of the fractional crystallization process in studied MdOC (a) meta-hornblendite protolith, and (b) meta-hornblende-gabbro precursors. Source fore-arc basalt composition is after [95], Himalayan samples after Table S4. See text for other details.
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Figure 13. Schematic illustrations of the possible tectonomagmatic history of the eastern Neo-Tethyan oceanic lithosphere, depicting flux melting within the sub-arc mantle and associated formation of hornblende-bearing cumulates (based on the double-subduction model [15]. TTS: Trans-Tethyan Subduction, ATS: Andean Type Subduction, IA: Island Arc, LM: Lithospheric mantle, AM: Asthenospheric mantle, IDM: Initial depth of melting, TBL: Thermal Boundary Layer, Plag: plagioclase feldspar, Hbl: hornblende. Figure is not to scale.
Figure 13. Schematic illustrations of the possible tectonomagmatic history of the eastern Neo-Tethyan oceanic lithosphere, depicting flux melting within the sub-arc mantle and associated formation of hornblende-bearing cumulates (based on the double-subduction model [15]. TTS: Trans-Tethyan Subduction, ATS: Andean Type Subduction, IA: Island Arc, LM: Lithospheric mantle, AM: Asthenospheric mantle, IDM: Initial depth of melting, TBL: Thermal Boundary Layer, Plag: plagioclase feldspar, Hbl: hornblende. Figure is not to scale.
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Table 1. General lithostratigraphy in the study area, as modified after [26,31,33].
Table 1. General lithostratigraphy in the study area, as modified after [26,31,33].
Tectonic
Domain		Tectonostratigraphic Unit
and Subunit		Lithology with Mineralogical Characteristics
Trans-Himalayan GranitoidsEastern Lohit
Plutonic Complex		Leucogranite, quartz diorite, granodiorite,
tourmaline-bearing pegmatites
Warlung Thrust (WT)
Western Lohit
Plutonic Complex		Gabbro, intrusive leucogranite, aplite dyke, trondhjemite, mylonite gneiss, hornblende granite, tonalite, meta-diorite, dacite, pegmatite
Lohit Thrust (LT)
Mayudia Ophiolite Complex (MdOC)Tidding formationMetacarbonate (Dolomite + muscovite + quartz)
Metapelite grading into metachert (Quartz + biotite +
muscovite + plagioclase + epidote ± titanite ± carbonate ± garnet ± chlorite)
Metabasalt (Actinolititc amphibole + plagioclase + chlorite)
Amphibolite (Hornblende + plagioclase + opaque (mostly magnetite) ± epidote ± titanite)
Metaperidotite (olivine + clinopyroxene + serpentine +
spinel) with hornblendite–hornblende–zoisite schist
(pargasitic amphibole ± plagioclase ± zoisite) intrusives
Tidding Thrust (TT)
Mayudia GneissMayudia GroupGarnet–biotite schist and gneiss, quartzite, and amphibolite
Mayudia Thrust (MyT)
Lalpani GroupLesser Himalayan Sequence (LHS)Garnet–biotite schist, and paragneiss, micaceous quartzite, marble, carbonaceous schist, carbon-bearing calcareous schist, amphibolite
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Sahoo, S.; Majumdar, A.S.; Anand, R.; Ray, D.; Fuenlabrada, J.M. Petrogenesis of Mafic–Ultramafic Cumulates in the Mayudia Ophiolite Complex, NE Himalaya: Evidence of an Island Arc Root in Eastern Neo-Tethys. Minerals 2025, 15, 572. https://doi.org/10.3390/min15060572

AMA Style

Sahoo S, Majumdar AS, Anand R, Ray D, Fuenlabrada JM. Petrogenesis of Mafic–Ultramafic Cumulates in the Mayudia Ophiolite Complex, NE Himalaya: Evidence of an Island Arc Root in Eastern Neo-Tethys. Minerals. 2025; 15(6):572. https://doi.org/10.3390/min15060572

Chicago/Turabian Style

Sahoo, Sapneswar, Alik S. Majumdar, Rajagopal Anand, Dwijesh Ray, and José M. Fuenlabrada. 2025. "Petrogenesis of Mafic–Ultramafic Cumulates in the Mayudia Ophiolite Complex, NE Himalaya: Evidence of an Island Arc Root in Eastern Neo-Tethys" Minerals 15, no. 6: 572. https://doi.org/10.3390/min15060572

APA Style

Sahoo, S., Majumdar, A. S., Anand, R., Ray, D., & Fuenlabrada, J. M. (2025). Petrogenesis of Mafic–Ultramafic Cumulates in the Mayudia Ophiolite Complex, NE Himalaya: Evidence of an Island Arc Root in Eastern Neo-Tethys. Minerals, 15(6), 572. https://doi.org/10.3390/min15060572

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