The origin of the ultrahigh-pressure Tso Morari complex, NW Himalaya: implication for early Paleozoic rifting

High-pressure (HP) to ultrahigh-pressure (UHP) metamorphic rocks such as the eclogites are ubiquitous at the zones of oceanic plate subduction and continental collision in orogenic belts. They provide important geodynamic constraints on the regional tectono-metamorphic evolution. Numerous HP and UHP metamorphic rocks are present in the Tibet-Himalayan orogen and its vicinity (Fig. 1). The Himalayan eclogites can be broadly grouped as UHP coesite-bearing eclogites from the NW Himalaya (Tso Morari in Ladakh, NW India and Kaghan in NW Pakistan) (Guillot et al.1997; Mukherjee & Sachan, 2001, de Sigoyer et al.2004, St-Onge et al.2013; Rehman et al.2016) and HP eclogites from the central Himalaya Ama Drime Massif and Arun area in eastern Nepal (Lombardo & Rolfo, 2000; Groppo et al.2007; Cottle et al.2009; Corrie et al.2010; Imayama et al.2020). Previous petrological and geochronological works demonstrate that the geothermal gradient of the UHP eclogites is significantly lower than that of their HP counterparts (Kohn, 2014; O’Brien, 2019 and references therein). Although the differences in the origin of the two eclogite types have not received much attention, they are important to understand the pre-Himalayan geological history and the regional geological variations along the orogeny.

Generally, the protoliths of the UHP eclogites in the NW Himalaya are thought to have been basalts associated with the Permian Panjal Traps formed in a continental rift setting (Spencer et al.1995; de Sigoyer et al.2004). In the NW Himalaya, the Panjal volcanics regionally intruded into the High-Tethyan Himalayas (Shellnutt, 2016). The Kaghan eclogites (259±10 Ma) and associated gneisses in NW Pakistan are Early Permian in age (Rehman et al.2016). The Tso Morari eclogites and associated amphibolites also exhibit geochemical signatures of a rift environment (Rao & Rai, 2006; Jonnalagadda et al.2019). On the other hand, recent Nd-Sr isotope data (Ahmad et al.2022) proposed their origin of ca. 289 Ma depleted Panjal volcanics and ca. 140 Ma Ladakh (Nidar) ophiolitic mafic rocks instead of the previously accepted origin from the enriched Panjal (ca. 289 Ma) and Phe volcanics (Zanskar). However, the only pre-Himalayan ages of ca. 479 Ma in the Tso Morari Crystallines (TMC) are reported for the undeformed Polokongka La granite and gneiss (Girard & Bussy, 1999), and the lack of geochronological study hinders our understanding of the origin of the UHP metamorphic rocks in the TMC. Here, we present the whole-rock geochemical analyses and zircon SHRIMP dating of the TMC gneisses and metabasites including eclogites. Our new data indicate that the UHP TMC formed in a continental rift setting during the Early Paleozoic. The results also demonstrate the diversity in the sources and ages of the HP-UHP Himalayan eclogites along the E-W direction of the orogen.

The TMC is located in the northwestern part of Indian Trans-Himalaya and is characterised by a sub-elliptical outline (Fig. 2a). It occurs as a northwesterly trending antiformal dome, separated from the Indus-Tsangpo suture including the Nidar ophiolite to the north and the Tetraogal Nappe to the south by detachment faults that dip away from the core of the dome. Further to the south, lies the Mata Nappe, which consists of two Early Paleozoic granite intrusions viz. Rupshu (ca. 482 Ma) and Nyimaling (ca. 460 Ma) (Epard & Steck, 2008). The Tso Morari Gneiss forms the core of the TMC and is overlain by a metasedimentary sequence (Fig. 2a, (Buchs & Epard, 2019). The metabasites including eclogites occur as foliation-parallel boudins (Fig. 2b, c) within the ductilely deformed gneisses (Fig. 2d). Eclogites have been widely retrogressed to amphibolites (Fig. 2e). The tectono-thermal evolution of the TMC can be broadly classified into four stages: (i) prograde metamorphism during Neo-Tethys subduction, (ii) (U)HP metamorphism under eclogite-facies condition (>2 GPa, 500–760°C) at ca. 55 Ma, (iii) near-isothermal decompression under granulite-facies condition at the mid-crustal level at ca. 48–45 Ma, and, finally, (iv) retrograde amphibolite-facies conditions at ca. 31–29 Ma (Guillot et al.1997; St-Onge et al.2013; O’Brien, 2019 and references therein). The Early Eocene Indo-Eurasia collision facilitated TMC extrusion along a northeasterly dipping channel (Dutta & Mukherjee, 2021). At least three deformation phases affected the TMC during extrusion and produced the gently dipping gneissic foliations (de Sigoyer et al.2004; Epard & Steck, 2008).

Gneisses and metabasites samples were collected for petrographic, geochemical and geochronogical analyses (see Supplementary Materials Table S1 for sample details). Recently, Pan et al. (2023) investigated the fluid evolution of the TMC since the Eocene, and they sampled a traverse from the centre of an eclogite boudin out into the host orthogneiss. In this study, we focus on the protoliths of the TMC, and thus samples were not collected from the reaction zone to avoid the effects due to the metasomatism. Quartz and calcite veins are found in the outcrops but fist-sized samples without the veins were selected for whole-rock analyses to avoid the effect of veins. Gneisses consist mainly of Ph + Bt + Pl + Qz ± Mc ± Grt ± Zo (Fig. 2f–j, mineral abbreviations are as per Whitney & Evans, 2010). They are subdivided into Ph-Bt gneiss, Grt-Ph gneiss, Ph-rich gneiss and quartzofeldspathic gneiss depending on the modal amount of minerals and presence of garnet. Most of the gneisses are orthogneisses whereas the quartzofeldspathic gneisses may be paragneisses in part. The metabasites include (retrograded) eclogites, Grt amphibolites and K-rich metabasites. Eclogite-facies metamorphism is characterised by the mineral assemblage of Grt + Omp + Rt + Coe/Qz ± Ph ± Zo (Fig. 2k–m) with later phases of Amp + Pl ±Di. The eclogites have zoning patterns in garnets associated with prograde and peak metamorphism but compositions and textures related to metasomatism stages are rare (e.g., St-Onge et al.2013). The Grt amphibolites and K-rich metabasites mainly consist of Grt + Amp + Pl + Qz + Ilm + Ttn (Fig. 2n) and Grt + Bt + Ph + Amp + Pl + Qz + Rt + Zo ± Ttn, respectively.

The ten metabasite samples have low SiO₂ (44.09–51.20 wt%) and MgO (5.23–9.40 wt%), high TiO₂ (1.09–3.12 wt%) relative to FeO/MgO ratio and variable alkali contents (1.61–7.21 wt%) (Supplementary Materials Table S2 and Figure S1). Unlike the retrograded eclogites and K-rich metabasites, the Grt amphibolites and eclogite boulder have low alkali contents. The analysed samples plot on the field of subalkaline basalts in the Zr/TiO₂ vs. Nb/Y diagram, and the eclogite boulder has a low Nb/Y ratio (Supplementary Materials Figure S1).

The large ion lithophile elements (e.g., Rb and Ba) concentrations are variable (Fig. 3a, b) due to their mobility during surface weathering and alteration processes (Xia & Li, 2019). Thus it is important to check element mobility before applying tectonic discrimination diagrams (Polat & Hofmann, 2003; Imayama et al.2021). The eclogites and amphibolites lack large Ce anomalies as shown by the Ce* values between 0.96 and 1.04 (Ce* = Ce_N/Sqrt(La_N×Pr_N), which is considered as immobile when the range is 0.9<Ce*<1.1. There is also no significant carbonisation or silicification due to the absence of carbonate and sulfide minerals in samples. These occurrences imply that the major chemical compositions of the sampled eclogites and amphibolites have not been strongly affected by hydrothermal alteration (Polat & Hofmann, 2003; Imayama et al.2021).

All the metabasite samples are enriched in high field strength elements (HFSE) with Nb-Ta negative anomalies, and their chondrite-normalised rare earth element (REE) patterns plot in the fields between oceanic island basalt (OIB) and enriched mid-ocean ridge basalt (E-MORB, Fig. 3a–d). The HFSE and REE contents are lowest in the eclogite boulder along with prominent Nb and Zr–Hf negative anomalies. In the Nb–Zr–Y diagram, the metabasites, except for the eclogite boulder, plot in the fields of within-plate tholeiite or volcanic-arc basalts (Fig. 3e). In the Zr/Y–Zr diagram (Fig. 3f), these metabasites have high Zr/Y representing the within-plate basalts and are thus distinguished from the island-arc origin. In contrast, the eclogite boulder plots in the fields where island-arc basalt and MORB overlap on both diagrams (Fig. 3e, f).

We separated the zircons from eight gneisses and one Grt amphibolite (Fig. 2a, Supplementary Materials Table S1) to obtain cathodoluminescence (CL) images and U–Pb ages (see Supplementary Materials Text S1). The CL images of the zircon grains from the gneisses show well-developed prismatic faces and internal oscillatory zoning, which is typical igneous-type zircons (Fig. 4). Some grains have modified grey-CL rims showing faint oscillatory zoning or patchy pattern of dark and bright colours in CL-image (Supplementary Materials Figure S2). Thin metamorphic rims showing dark-CL domains are observed in 19-2 and 19-4, but they are too thin to analyse. The U–Pb analyses of two Ph-Bt gneisses (15-3B and 9g) concentrated at a single population of concordant to near-concordant grains with a weighted mean ²⁰⁶Pb/²³⁸U date of 481.2±4.4 Ma (n = 14, mean squared weighted deviation or MSWD = 2.5, Fig. 4a) and 484.3±5.7 Ma (n = 14, MSWD = 2.1, Supplementary Materials Figure S3a), respectively. Some grains were possibly affected by Pb-loss resulting in young ²⁰⁶Pb/²³⁸U ages. The inherited grains yielded scattered dates ranging from 1402 Ma to 606 Ma. The zircons from a Grt-Ph gneiss (18-3B) yielded ²⁰⁶Pb/²³⁸U dates ranging from 500 to 470 Ma, but the weighted mean date was not calculated due to the large MSWD value (Supplementary Materials Figure S3b). The U–Pb analyses of concordant grains from the other two Grt-Ph gneisses (16-2A and 18-4) yielded the weighted mean ²⁰⁶Pb/²³⁸U date of 485.9±3.3 Ma (n = 12, MSWD = 1.7, Fig. 4b) and 493.4±5.2 Ma (n = 13, MSWD = 1.6, Supplementary Materials Figure S3c), respectively. Some inherited grains observed in three Grt-Ph gneisses yield ²⁰⁶Pb/²³⁸U dates of 1841 to 796 Ma. The igneous zircons of 15 concordant grains from Ph-rich gneiss (18-5B) yielded the weighted mean ²⁰⁶Pb/²³⁸U date of 493.3±5.4 Ma (n = 15, MSWD = 2.1, Supplementary Materials Figure S3d). The inherited domains gave ²⁰⁶Pb/²³⁸U dates of 1154 to 766 Ma. Some grains from two quartzofeldspathic gneisses (19-2 and 19-4) show discordant behaviour due to the disturbance of common Pb and Pb loss. The U–Pb analyses of the crystals that yielded concordant dates gave weighted mean ²⁰⁶Pb/²³⁸U dates of 480.4±4.4 Ma (n = 18, MSWD = 1.8, Fig. 4c) and 485.6±4.5 Ma (n = 14, MSWD = 3.0, Supplementary Materials Figure S3e), respectively.

The zircon grains from Grt amphibolite (20-2) show well-developed prismatic faces and show bright-CL oscillatory zoning surrounding the inherited core (Fig. 4d). The igneous zircons of 13 concordant grains yielded the weighted mean ²⁰⁶Pb/²³⁸U date of 475.8±6.9 Ma (MSWD = 2.6, Fig. 4d). The inherited domains gave ²⁰⁶Pb/²³⁸U dates of 1039 to 813 Ma.

Previous workers suggested that the protoliths of the Tso Morari eclogites are either the Permian Panjal basalts associated with a mantle plume and crustal contamination (Spencer et al.1995; de Sigoyer et al.2004; Jonnalagadda et al.2019) or the Late Jurassic to Early Cretaceous Ladakh ophiolitic mafic rocks in the supra-subduction zone (Ahmad et al.2022). The metabasites we analysed have enriched light REE contents (Fig. 3c,d), and the mixing line for the lithospheric contamination in the Y/Nb–Zr/Nb diagram (Fig. 3h) starts from the mantle plume source, rather than the depleted MORB on the mantle array, precluding the possibility of N-MORB components in the mantle source. Although the enriched light REE patterns are comparable to both E-MORB and OIB, the concentrations of incompatible trace elements such as the HFSE are much higher than those of the E-MORB and resemble those of OIB (Fig. 3a, b). The signatures of within-plate basalts are marked by the high Zr/Y content in most metabasites (Fig. 3f). Although the Nb-Ta negative anomalies (Fig. 3a, b) and Nb/La<1 (Fig. 3g) in most of the TMC metabasites imply either continental intraplate basalts that experienced crustal contamination or island-arc basalts as their protoliths, the former is supported by high concentrations of incompatible trace elements (Xia and Li, 2019). Crustal contamination leads to low Nb content, increasing Y/Nb and Zr/Nb ratios (Fig. 3h). The trend has a different slope from that of the mantle array line from the plume, through MORB, to the depleted asthenosphere. Based on field occurrences, the contaminant lithology could partially consist of the Early Paleozoic orthogneisses surrounding metabasites. The Zr/Nb ratios from a few orthogneisses reach up to 30 (Ahmad et al.2022), which exceed the average Zr/Nb ratios of around 10 for continental crust compositions (e.g., Wedepohl 1995; Rudnick & Gao, 2003). The crustal contamination is also supported by the presence of the inherited zircon cores in the garnet amphibolite (sample 20-2). Therefore, the TMC metabasites most likely formed in a continental rift setting and experienced lithospheric contamination. In contrast, the eclogite boulder with prominent Nb and Zr–Hf negative anomalies suggests its island-arc origin, which could correspond to the Nidar ophiolite mafic rocks formed by the subduction initiation in the forearc (Ahmad et al.2022).

The U–Pb SHRIMP data from the eight gneisses suggest an Early Paleozoic (ca. 493–480 Ma) protolith, which is consistent with the previously reported U–Pb zircon date of 479±2 Ma from the Tso Morari Gneiss (Girard & Bussy, 1999). The youngest zircon date of 475.8±6.9 Ma was obtained from the Grt amphibolite, which represents a retrograde product of the eclogites. Oscillatory zoning is a common feature in zircons from felsic igneous rocks, but some zircon textures in amphibolites from the orogenic belts also include oscillatory zoning (Oh et al.2017; Kang et al.2020), representing the timing of mafic magmatism. It is unlikely that contamination of zircons from orthogneisses occurred during intrusion because the garnet amphibolite occurs within the paragneisses, not orthogneisses. The zircons may have crystalised from basaltic magmas if the rocks displayed high Zr contents (Shao et al.2019). Therefore, the ca. 476 Ma zircon date from garnet amphibolite is interpreted as the timing of mafic magmatism and is roughly consistent with those of the gneisses within error, indicating the bimodal magmatism during the Early Paleozoic at least for some time. In terms of trace element geochemistry, continental basalt is similar to the OIB but the presence of coherent granites implies the tectonic setting of continental rift. These Early Paleozoic ages indicate that the TMC eclogites were not derived from the Permian Panjal Traps associated with the opening of the Neo-Tethys Ocean. The crystallisation ages (ca. 493 Ma) from the central part of the TMC near Puga are slightly older than those (ca. 486–476 Ma) in surrounding parts, implying multi-stage magmatism. Felsic rocks volumetrically dominate the TMC, whereas the volume of mafic rocks is quite small. This is also in contrast to what is observed in the NW Himalaya where the Panjal Traps are characterised by abundant mafic magmatism (Shellnutt, 2016). We alternatively propose that the protoliths of the UHP TMC formed in the extensional setting at the rifted Indian margin during the Early Paleozoic. During the Pan-African orogeny, the Indian shield was located in the northern East Gondwana (Gray et al.2008, Fig. 5). Epard and Steck (2008) pointed out that the sequences deposited in the north Indian margin in the Tso Morari region are affected by extensional structures, rather than the compressional structures of the Pan-African orogeny. The Early Paleozoic tectonic events are widely known in the Himalayas (Le Fort & Cronin, 1988; Gehrels et al.2006), but the investigation of the detailed tectonic evolution is beyond the scope of this study as here we only discuss the Early Paleozoic igneous events in the NW Himalaya. The report of A-type alkaline Kaghan metagranite of ca. 470 Ma (Trivedi et al.1986) also supports the Early Paleozoic rifting at the passive margin of northern India. The Early Paleozoic felsic magmatism is also exposed in Pakistan (483–476 Ma, Ogasawara et al.2019; ca. 459 Ma, Mughal et al.2022) and in the Garhwal, NW India (495–490 Ma, Imayama et al.2023; ca. 512 Ma, Sen et al.2021), which are considered as S-type and A-type, respectively. Thus, the A-type Early Paleozoic granites in the Garhwal are more closely related to the Early Paleozoic rifting events in this study. Coupled with geochemical results in this study, we posit that the Early Paleozoic rifting after the Neoproterozoic Pan-African orogeny produced the protoliths of the TMC. Recently, a close relationship between North India and South China during the Neoproterozoic to Early Paleozoic has been discussed (Qi et al.2020; Imayama et al.2023), and the Early Paleozoic rifting in NW India led to the separation of South China from North India (Fig. 5, Imayama et al.2023). However, more study is required to confirm this assumption.

Combined with the UHP Nanga Parbat eclogites (Rehman et al.2016), our results suggest multiple sources and ages for the protolith of the UHP metamorphic rocks in the NW Himalaya. Similarly, the protolith of the HP metamorphic rocks in the central Himalaya also originate from multiple sources and ages (Zhang et al.2022). The HP eclogites of the Ama Drime Massif originated from the Ordovician (∼480–430 Ma) rift-related or E-MORB-like magmatism (Wang et al.2017; Dong et al.2022) and the Paleoproterozoic (∼1850 Ma) continental flood basalts (Zhang et al.2022). In Bhutan, the HP metabasite was derived from young Paleoproteorozoic rifting (∼1742 Ma, Chakungal et al.2010). These occurrences indicate the sources and ages of the protoliths of the Himalayan HP–UHP metamorphic rocks are diverse in the NW-SE direction along the orogen.

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The research was supported by the Japan Society for the Promotion of Science (22H01324) and the Korea Basic Science Institute under the R&D programme (C330430). We are grateful to Shinae Lee for the analytical work. We thank Dr. Johannes Pohlner and the anonymous reviewer for constructive and critical reviews that significantly helped to improve the manuscript. We also thank Dr. Simon Schorn for their careful editorial handling.