Magmatic evolution of the Paleoproterozoic A2-type granite along the northern Indian margin: insights from geochemistry and U-Pb geochronology of Baijnath Klippe, NW Himalaya

Abstract

Paleoproterozoic granitoids of the lesser Himalayan belt are keys to understanding the evolution of the northern Indian continental margin and its position in the Columbia supercontinent assembly. We present whole-rock chemistry and zircon U-Pb geochronological data for Gwaldam Biotite Granite (GBGr) from the Baijnath Klippe (BK) in Kumaun Himalaya to elucidate their petrogenesis and geodynamic implications. Granites are characterized by ferroan, weakly peraluminous nature with high SiO₂ and K₂O contents, enrichment in LILE (Rb, Th, K and Pb), and depletion in Ba, Nb, P, Hf and Ti. Granites show enrichment in light rare earth element relative to heavy rare earth elements and pronounced negative Eu anomalies. Such chemistry suggests typical A-type granite with high Y/Nb >2 values that characterize it as A₂-type granite. Zircon U-Pb ages for the granite yield upper intercept at 1900 ± 3 Ma (core) and 1854 ± 2 Ma (rim). Integrating the chemical and geochronological data, we propose a two-stage evolution model for the area. In the GBGr, the ∼1900 Ma date of zircon core is likely the date of crystallization of the melts presumably formed during the first extensional stage at uppermost mantle – lower crust levels caused by slab break-off/rollback, which followed a post-collisional setting. The second incipient rifting stage produced melt that entrained the zircon cores (∼1900 Ma) during its ascendance and crystallized as the GBGr at ∼1854 Ma when the zircon rims crystallized. It is further proposed that the Paleoproterozoic Northern Indian continental margin later underwent at least two crustal extensions during the Columbia supercontinent agglomeration.

1. Introduction

Throughout the Earth’s history, supercontinents accreted, eventually to split, and their fragmentation plays a crucial role in governing mantle dynamics and crustal growth (Santosh et al. 2009; Kaur et al. 2014; Nance et al. 2014; Pirajno and Santosh, 2015). Columbia, one of the earliest known Supercontinents, grew by accretion of most of the present continental blocks between 2.1 and 1.8 Ga. (Wilson, 1963; Nance et al. 1988; Rogers, 2000; Rogers and Santosh, 2002; Zhao et al. 2002; Condie, 2002; Phukon, 2022). Tectonic setting and location of the northern Indian continental margin (NICM) during the assembly of the Columbia supercontinent is increasingly being refined. One school of thought postulates Indian plate to be a passive margin juxtaposed with Australia, Madagascar and East Antarctica during the Paleoproterozoic assembly (Rogers and Santosh, 2002; Zhao et al. 2004), while others suggest a continuous subduction zone or an active Andean-type continental margin along the NICM during the Paleoproterozoic-Mesoproterozoic period (Hou et al. 2008; Kohn et al. 2010; Rao and Sharma, 2011; Mandal et al. 2016; Phukon et al. 2018 and Phukon, 2022).

All along the Himalayan belt, the igneous–metamorphic composite nappes and klippen that overlie the LHS host several granites that are largely sheared. Such granites include the Shang Granodiorite and Iskere gneiss in Pakistan (DiPietro and Isachsen, 2001; Zeitler et al. 1989), Wangtu granite in Himachal Himalaya (Singh 1993; Singh et al. 1994), Bhatwari formation in Kumaun Himalaya (Sen et al. 2013), granites of Toneta in Garhwal (Mishra et al. 2019), Ulleri gneisses in Central Nepal (Le Fort and Rai, 1999), Lingste augen gneiss in Darjeeling-Sikkim Himalaya (Sinha-Roy, 1980), Shumar Formation in Bhutan Himalaya (Dasgupta, 1995), Mayong granitoids of Assam (Doley et al. 2022), Bomdila granite gneiss in Arunachal Himalaya (Pathak & Kumar, 2019) and Salari granite of Arunachal Himalaya (Bikramaditya et al. 2022). The evolution of Himalaya, which commenced during the Early Eocene with the collision of Indian and Eurasian plates, has been the focus of numerous geological investigations that address global and regional plate tectonics. The Precambrian to Eocene rocks that comprise the NICM of the Indian plate were geologically reworked during the Himalayan orogeny (Valdiya, 1980, Joshi 1999).

The NICM, largely developed during the assembly of the Columbia supercontinent, is represented by these Paleoproterozoic rocks. Three-fourths of the continental crust comprises granites, which can be genetically linked to their tectonomagmatic environments. Granite magmas are end products of regional and global thermal events that episodically shape the continental crust over time (Clarke, 1996; Bonin, 2007). Paleoproterozoic witnessed a notable increase in the formation of juvenile continental crust (Condie, 1998, 2000, 2002). An explicit understanding of the tectonic environment of granite genesis in Himalayan nappes and klippen is indispensable to specify the active or passive nature of the NICM during the Paleoproterozoic Columbia supercontinent assembly.

Almora-Jajarkot Nappe, likely the largest nappe in the world (Joshi et al. 2019), is spread over the eastern Kumaun Himalaya and western Nepal Himalaya. The equivalents of Almora-Jajarkot Nappe in the Kumaun Himalaya are represented by the Askot, Baijnath, Lansdowne and Chhiplakot klippen (Valdiya, 1980; Joshi, 1999; Joshi and Tiwari, 2007, 2009 and Mandal et al. 2016). It is essential to understand the tectonic relationship among these klippen, the Almora-Jajarkot Nappe and the Higher Himalayan Crystalline (HHC) to understand their role in the Columbia supercontinent assembly. The paper addresses petrogenesis, emplacement and tectonic environment of the evolution of the Gwaldam granite based on integrated geochemical and geochronological datasets. The study provides further constraints on palaeogeographic reconstructions of the NICM during the Paleoproterozoic.

2. Geological background

Himalayan orogen comprises four major tectonic zones along its ∼2400 km length that are classified from south to north (Fig. 1a) as: (1) the Sub-Himalaya, (2) the Lesser Himalayan sequences, (3) the HHC and (4) the Tethyan Himalaya (Gansser, 1964; Thakur, 2013; Celerier et al. 2009; Carosi et al. 2018). The main central thrust (MCT) and the main boundary thrust delimit the Lesser Himalayan belt in the north and south, respectively. The crustal shortening brought about by continental collision-driven Cenozoic Himalayan orogeny (Dewey and Burke, 1973; Molnar and Tapponnier, 1975; Joshi, 1999; Yin and Harrison, 2000; Joshi et al. 2019), transported tectonically exhumed slices of the NICM southwards over the MCT as thrust sheets. The erosional remnants of this thrust sheet comprise present-day klippen and nappes that sit atop the LHS all along the Himalaya (Fig. 1b). The Gwaldam area (Fig. 1c) is part of one such klippe viz. the Baijnath Klippe (BK). The geology of Kumaun and Garhwal Himalaya has been studied for over a century by Holland (1908); Auden (1935); Valdiya (1980); Joshi and Tiwari (2007, 2009) and Joshi et al. (2019) as it encapsulates the Himalayan geology.

Figure 1. (a) Generalized geological map of the Himalayan mountain belt (after Carosi et al. 2018) showing the distribution of Higher and Lesser Himalaya, (b) Simplified geological map of the Kumaun Lesser Himalaya (after Valdiya 1980; Joshi 1999, and Joshi et al. 2019). (c) Traverse geological map along road section of the southern Baijnath Klippe, showing granites, pelitic schists and gneisses of Ramgarh and Almora groups. Red dots show the important sample (8-sample) locations and Star shows the location of the sample for U-Pb zircon dating. Abbreviations: NAF = North Almora Fault, NRT = North Ramgarh Thrust, NAT = North Almora Thrust, SAT = South Almora Thrust, SRT = South Ramgarh Thrust.

The BK has been considered equivalent to the Almora Nappe (Valdiya, 1980), which is also an equivalent of other major klippen such as Chhiplakot, Askot-Thal and Dharamghar that have been traced back to the Munsiari Group of the HHC (Valdiya, 1980; DeCelles et al. 2001). Covering an area of ∼1020 km², the BK is located immediately north of the Almora Nappe and overlies the Berinag Formation of the Jaunsar Group. In the east, the BK overlies the Bageshwar Formation, which has been considered equivalent to the Berinag Formation (Valdiya, 1962; Ahmad, 1975). Several workers have considered Kausani Thrust as the contact between the BK and the Berinag Formation (Valdiya, 1962, Gansser, 1964, Sarkar and Shrish, 1976, Auden, 1937, Heim and Gansser, 1939). The BK is predominated by granite-granodiorite and gneisses with subordinate metapelites, psammites and psammopelites with schists and gneisses that rest over the arenaceous, argillaceous and calcareous rock formations of the inner sedimentary autochthonous belt. The BK is tectonically separated from the underlying LHS by the Kausani Thrust (Ramgarh Thrust) (Sarkar and Shrish, 1976).

Heim and Gansser (1939) reported a sizable granite body exposed in the Gwaldam area of the BK, which they termed Gneissic Marginal Zones. Pandey et al. (1980) dated Gwaldam granite at 1300±80 Ma using the whole-rock Rb-Sr dating method. The BK is locally intruded by basic dykes at several places close to the Kausani Thrust and also within the Gwaldam Granite. The area comprises a large northwest plunging syncline (sic) (Pandey, 1971). The dark grey Ramgarh granite gneisses (Fig. 1c) largely comprise the BK, which is separated from the underlying LHS by the Kausani Thrust or Ramgarh Thrust. These gneisses have been considered part of the Ramgarh (Chail/Bhatwari) Group. The Ramgarh granite and gneisses can be correlated largely after Valdiya (1980; 1983, 2010) with other gneisses along the Himalayan strike. Thus, the Ramgarh gneisses can be correlated with the Bhatwari Formation farther north in the Kumaun Himalaya, the Himachal Himalaya, the Dhauladhar gneisses in Kashmir, the augen gneisses of the Bhimpedi Group in western Nepal, the Ulleri gneisses in Central Nepal (Le Fort and Rai, 1999) migmatitic gneisses of the Tumlingtar Group in Eastern Nepal, the Bomdila mylonitic gneisses in the western Arunachal Himalaya (Bikramaditya Singh, 2010), the Lingste augen gneisses of the Darjeeling-Sikkim Himalaya (Sinha-Roy, 1980) and with gneisses of the Shumar Formation of Bhutan Himalaya (Dasgupta, 1995). The Ramgarh granite gneisses and their equivalents in the Kumaun Himalaya show shoshonitic signatures (Sen et al. 2018; Chauhan et al. 2024).

3. Field relationships and Petrographic features

Our study is focused on an intrusive mesocratic granitoid stock from the BK, namely the Gwaldam pluton, located about 23 km N of Baijnath (Fig. 1c). We have collected 8 samples from different outcrop locations within the Gwaldam biotite granite (GBGr). Most of the studied samples are medium to coarse-grained and show porphyritic texture defined by plagioclase and K-feldspar phenocrysts embedded in a matrix comprising largely quartz and subordinate K-feldspar. Biotite flakes in the groundmass at times occur as fairly large clusters. Plagioclase, K-feldspar, quartz and biotite are fresh and undeformed in the central parts of the pluton.

The Gwaldam pluton comprises K-feldspar (avg. 34 vol. %), which is largely orthoclase and albite to oligoclase plagioclase (avg. 33 vol. %) in roughly similar modal percentages followed by quartz (avg. 28 vol. %) based on Cross, Iddings, Pirsson and Washington Norm calculations and biotite (∼10 vol. %, estimated visually on petrographic microscope) as the essential mineral phases with zircon, ilmenite, epidote and allanite as accessories. The biotite flakes in the granite body are characterized by random orientation. However, at the margins of the granitic body, they define crude gneissose foliation (Fig. 2c). Rare almandine-grossular garnet is noticed in one of the studied samples (Fig. 3a). Biotite (annite; Fe/(Fe+Mg) = 0.92–0.96) is the dominant ferromagnesian mineral that occurs as flakes of varying sizes (50–300 µm) and shows random orientations and other minerals in the groundmass (Fig. 3b). Biotite shows sharp contacts with quartz (Fig. 3c). The quartz, K-feldspar and plagioclase grains are subhedral to euhedral and plagioclase grains show polysynthetic as well as contact twinning (Fig. 3d). Plagioclase is largely albite to oligoclase [Ab_80−99% An_0−20%]. Microcline displays characteristic cross-hatched twinning (Fig. 3e). Perthite locally contains poikilitic inclusions of plagioclase, quartz and biotite (Fig. 3f), which suggests that perthite was the last to exolve during the sub-solvus cooling. The dominance of perthitic texture in the K-feldspar with cross-hatched twinning in microcline indicates slow cooling under sub-solvus conditions (Shelley, 1993). The exsolution of albite as perthite is accompanied by conspicuous contact twins. This is consistent with the granite’s low-temperature crystallization, further corroborated by the geochemical data showing low degree of fractionation. The presence of accessory minerals including zircon and allanite is reflected in the trace element signatures, particularly high Zr and REE concentrations observed in the geochemical analyses. Allanite’s presence is also reflected in elevated light rare earth elements (LREEs) that contribute to the overall REE profile of the granite. The margins of the granite plutons are characterized by chessboard twinning in plagioclase and undulose extinction in quartz likely due to deformation during the intrusion of the granite body. Sericitization of plagioclase feldspars is restricted to the margins of the GBGr likely due to the interaction of the plagioclase with late hydrothermal fluids that separated from the granite in the last stages of crystallization.

Figure 2. (a–b) Outcrop of Gwaldam Biotie granite. (c) Hand specimen of medium to coarse-grained light pink biotite granite that contains biotite.

Figure 3. Photomicrograph of studied granites (Gwaldam Biotie granite) showing important petrographic features in the investigated samples from the Gwaldam area (a–f): (a) subhedral-anhedral grain of garnet along with muscovite and biotite that shows chloritized margin; (b) Light green biotite flake and associated accessory minerals zircon, allanite and epidote; (c) Biotite flakes show sharp contact with quartz grains; (d) Subhedral to euhedral plagioclase grains showing polysynthetic as well as contact twinning; (e) Microcline crystal showing tartan twining and (f) subhedral-anhedral perthite showing Carlsbad twinning and inclusions of plagioclase, biotite and K-feldspar. Mineral abbreviations are after Whitney DL and Evans BW (2010): Bt = biotite, Ms = muscovite, Pl = plagioclase, Kfs = k-feldspar, Qz = quartz, Zrn = zircon, Chl = chlorite, Mc = microcline, Grt = garnet, Ep = epidote, Aln = allanite.

4. Analytical techniques

4.a. Whole-rock geochemistry

Whole-rock major oxides and trace elements of the representative samples (eight) were determined using a Wavelength Dispersive – X-ray Fluorescence Spectrometer (WD-XRF; Bruker, Tiger S8) at Wadia Institute of Himalayan Geology (WIHG), Dehradun, India. Approximately 0.5 kg of each sample was cut into small chips and then crushed using a steel jaw crusher. Then the samples were pulverized up to 200 mesh size following which pellets were prepared with 6g of each powdered sample. Loss on ignition was measured after 5g of each sample was heated at 1000°C in a muffle furnace. Analytical precision ranges between ±2 to 3 % for major elements and ±5 to 6% for trace elements. Rare earth elements (REEs) and selected trace element (Hf and Ta) concentrations were analyzed using Perkin-Elmer SCIEX ELAN DRC-e Inductively Coupled Plasma Mass Spectrometer (ICP-MS) at WIHG, Dehradun, India. The samples were dissolved following the open-system digestion procedure. Approximately 0.1 g of each powdered sample was mixed with 20 ml of nitric (HNO₃) and Hydrofluoric acids (2:1 ratio) and ∼2 ml of HClO₄ in Teflon crucibles. Then the crucibles were heated over a hot plate until the samples were fully digested and dried to form a paste. This was followed by the addition of 20 ml of 10% HNO₃ to each sample, which was left on a hot plate for 10–15 min until a clear solution was obtained. The final solution was made up to 100 ml volume with milli-Q water. International standards (BHVO-1 and JB-1a) were used for the analysis to monitor the analytical data. The accuracy and precision range between 2–12% and 1–8%, respectively. Whole-rock chemistry of the studied samples is given in Table 1.

Table 1. Major oxide (wt.%) and trace elements (ppm) concentrations including rare earth elements (ppm) data of GBGr, Baijnath Klippe, Northwestern Himalaya, north India

LOI = Loss on ignition; ASI(ACNK) = Molar Al₂O₃/(CaO+Na₂O+K₂O); ANK = Molar Al₂O₃/((Na₂O+K₂O).

Normalizing values (N) from Sun and McDonough (1989); Eu/Eu* = Eu_N/(Sm_N*GdN)^0.5.

4.b. Zircon U-Pb geochronology

A fresh representative granite sample (DWL-1) from the Gwaldam area was selected for zircon U-Pb dating. Zircon grains were separated from ∼ 5 kg of sample by crushing and sieving (up to 60 mesh size), followed by gravity separation using the Holman-Wilfley water table, heavy liquid and magnetic separation methods. The separated zircon grains were handpicked under a binocular microscope at WIHG, Dehradun, India and ∼ 80 zircon grains were mounted on per-fluoro-alkoxy alkane (PFA®) Teflon sheet and then polished with diamond paste up to the mid-section. Cathodoluminescence (CL) images of the zircon grains were obtained using Zeiss EVO 40 extended pressure Scanning Electron Microscope at WIHG, Dehradun, India. In-situ zircon U-Pb geochronology was carried out using Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS; Neptune Plus, Thermo Fisher Scientific, Inc.) coupled with 193 nm excimer laser ablation (UV Laser, Model Analyte G2, Cetec-Photon machine, Inc.) at WIHG, Dehradun, India. Instrumental conditions and analytical procedures were similar to those described by Mukherjee et al. (2017). Spot diameter of 25µm (the spots were positioned in the CL images), repetition rate of 5 hertz, an energy density of 3.5 joules per square centimetre, and laser intensity of 67.5% were applied for the analysis. Zircon standard Z91500 was used for fractionation correction and results calculation (Wiedenbeck et al. 1995) and later validated using zircon standard Plesovice as the external standard (Slama et al. 2008). The standards were analyzed first and then after every ten unknown samples. Isotopic ratios and elemental compositions of zircon were processed using Iolite software (Paton et al. 2011). During data acquisition, the weighted mean ages of ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U are 338.54 ± 1.37 (Mean square weighted deviation, MSWD = 0.79, n = 30) and 335.36 ± 1.77 (MSWD = 0.5, n = 30), respectively,\ for zircon secondary standard Plesovice. Concordia diagrams and U-Pb age calculation were plotted using Isoplot R (Vermeesch, 2018). Common Pb was corrected by the method of Stacey and Kramers (1975). The uncertainty for a single analytical spot is presented at 2σ. Zircon U–Pb analytical results are listed in Supplementary Table S1.

5. Results

5.a. Major and trace elements geochemistry

The studied samples show high concentration of SiO₂ (72.15–72.96 wt. %), Al₂O₃ (14.45–15.05 wt. %), total alkali (Na₂O+K₂O = 9.11–9.59 wt. %) with low content of CaO (0.52–0.95 wt. %), MgO (0.09–0.28 wt. %), P₂O₅ (0.01–0.02 wt. %), TiO₂ (0.17–0.19 wt. %) and Fe₂O₃ (2.31–2.69 wt. %). They are classified as granite in the total alkali-silica diagram (Fig. 4a). High FeO^t/ (FeO^t+MgO) ratio (>0.89) of the samples suggests the presence of ferroan pluton (Fig. 4b) and belong to the shoshonitic series (Fig. 4c). Moreover, their A/CNK and A/NK vary between 1.05 to 1.14 and 1.19 to 1.25, respectively, that shows weakly peraluminous character (Fig. 4d). Further, they have an alkali-calcic character, as apparent by the modified alkali-lime index (MALI= Na₂O+K₂O-CaO) value (8.44–8.91 wt. %) (Fig. 4e). Granites with ferroan, alkalic-calcic and peraluminous characteristics are classified as A-type granites (Bonin, 2007). Studied biotite granite has 10000*Ga/Al ratio ranging from 2.69 to 2.92, similar to that of other A-type granites (>2.6), and has a close match with the global average of 3.75 (Whalen et.al. 1987). This is further substantiated by the discrimination diagrams of Whalen et al. (1987) where all the samples plot in A-type granitoids field (Fig. 5a and 5b).

Figure 4. (a) Total alkali (Na₂O+K₂O) vs. SiO₂ content diagram (Middlemost 1994), and the sub-alkaline and alkaline division (after Irvine and Baragar 1971). (b) Fe₂O₃ ^t/(Fe₂O₃ ^t+MgO) vs SiO₂ diagram, Magnesian and Ferroan division line is from (Frost et al. 2001). (c) K₂O vs SiO₂ diagram (after Peccerillo and Taylor 1976). (d) Alkalinity index A/NK = (Al₂O₃/Na₂O+K₂O)_molar vs. Aluminum Saturation Index A/CNK=(Al₂O₃/CaO+Na₂O+K₂O)_molar diagram showing weakly peraluminous nature of the rocks (after Maniar and Piccoli 1989). (e) modified alkali-lime index (MALI = Na₂O+K₂O-CaO) vs. SiO₂ diagram (fields after Frost et al. 2001).

Figure 5. Discrimination diagram for A-type granites (after Whalen et al. 1987) (a) Plot of Nb vs 10000*Ga/Al (b) Na₂O+K₂O/CaO vs 10000*Ga/Al (c) Y-Nb-Ce and (d) Ce/Nb vs Y/Nb sub-discrimination diagrams for A-type granites (Eby 1992). A₁-type is generally referred to as mantle-derived, anorogenic A-type granites, A₂-type granites are continental crust rocks emplaced in a variety of tectonic settings (collisional or arc-type sources). Abbreviation: OIB- ocean island basalt; IAB-island arc basalt.

The total rare earth elements (∑REE) content in the granites range from 130 to 258 ppm. In the chondrite-normalized REE diagram, the samples are characterized by enrichment in LREE {(La/Sm)_N = 2–4; N indicates chondrite normalized}, depletion in heavy REE {(La/Yb)_N = 3–8} with negative Eu anomalies (Eu/Eu*= 0.26–0.36) (Fig. 6a). On the primitive mantle-normalized multi-element diagram (Fig. 6b), the studied biotite granites exhibit enrichment in large-ion lithophile elements (LILEs; Rb, K, Pb and Th) and depletion in Ba and high field-strength elements (HFSEs) such as Nb, Ta, P, Hf and Ti.

Figure 6. (a) Chondrite-normalized rare earth element pattern (b) Primitive mantle-normalized multi-element spider diagram of trace elements of studied granites. Normalization values are from Sun and McDonough (1989).

5.b. Zircon U-Pb geochronology

For zircon U-Pb geochronology, a fresh representative granite sample viz. DWL-1 was selected. Representative CL images for the zircon grains and concordia plots of the sample DWL-1 are shown in Fig. 7. Zircon grains are transparent, colourless to light brown, mostly euhedral, bipyramidal tetragonal-shaped, 80–200 μm long with aspect ratios (length/width) of 3:1–1:1. Most of the zircon grains are weakly zoned and display relatively bright rims and dark-grey cores in CL images that indicate oscillatory zoning. Th, U and Pb concentrations of the analyzed zircons range from 133 to 709 ppm, 452 to 2534 ppm and 65 to 259 ppm respectively, with Th/U ratios of 0.18–0.51 that suggest magmatic origin (Hoskin and Black, 2000) (Supplementary Table S1). We have chosen 85 grains for the analysis, out of these sixty-four spot analyses produced discordant ages with very few concordant ages that indicate significant radiogenic Pb loss (Fig. 8b), with measured ²⁰⁷Pb/²⁰⁶Pb ages that range from 1857.9 to 2028 Ma. Out of the 64 spots, 21 spots analyzed on rims of the zircons yielded an upper intercept age of 1854.45 ± 2.20 Ma (Mean square weighted deviation, MSWD, = 2.1, n = 21), whereas 15 spots in the core yielded an upper intercept age of 1900.23±3.26 Ma (MSWD = 2.3, n = 15), which is consistent with the observed peak ages in the relative probability of the zircon U-Pb analysis (Fig. 8c and 8d). These ages are interpreted as the two stages of crystallization of the studied granite.

Figure 7. Cathodoluminescence images of representative zircon from the studied samples. The solid circle indicates U-Pb spot with diameter of 25μm.

Figure 8. Wetherill U/Pb Concordia diagram for biotite Granite specimen (DWL-1): Diagram constructed using Isoplot R (Vermeesch, 2018). (a) Relative age probability histogram of the same sample. b) U/Pb Concordia plot for all the selected 64 spots. (C) U/Pb Concordia plot for 21 spots out of 64 conducted on rim. (d) U/Pb Concordia plot for 15 spots out of 64 from the same sample analyzed on core.

6. Discussion

6.a. Petrogenesis

The studied granites have high SiO₂, alkali (K₂O+Na₂O), FeO^t/(FeO^t+MgO and (10000*Ga/Al), and low MgO, TiO₂, CaO and P₂O₅ concentrations. In addition, they display enrichment in LREE, significant negative Eu anomalies, no obvious Nb-Ta anomalies and extreme depletion in Ba, P, Sr, Hf and Ti with high contents of HFSE (Nb+Ce+Y+Zr = 317.72–381.14) (Fig. 6a and 6b). These geochemical observations suggest that GBGr is an A-type granite. Existing models for the generation of such granites involve three mechanisms, viz. (i) fractionation of mantle-derived basaltic magmas with or without the contribution of crustal rocks (Turner et al. 1992), (ii) partial melting of the crustal rocks (Clemens et al. 1986) and (iii) mixing of the crustal rocks and the mantle-derived magmas (Bonin, 2007). The peraluminous character of the studied granite suggests that the parental magma was generated by the partial melting of crustal rocks (Frost and Frost, 2008; 2013). Further, the enrichment of Pb and Th in the primitive mantle-normalized multi-element diagram indicates the mixing of crustal and mantle sources during the emplacement of granite. As a result, some trace-element ratios are employed to differentiate the relative involvement of the crustal and mantle sources in the granite genesis. Eby (1992) classified A-type granite into A₁ and A₂, where the A₁-type granites represent trace-element ratios that are similar to the oceanic island, intraplate, and rift zone magmas, whereas, A₂-type granite is characterized by similar trace-element ratios to those of crustal and island arc magmas that are derived from the underplated crustal rocks in continent-continent collisional setting. In the Nb-Y-Ce ternary and Ce/Nb vs Y/Nb diagrams, the studied samples plot in the A₂-type granite field, which suggests that the GBGr originated from the crustal magmas (Fig. 5c and 5d).

The geochemical characteristics of the studied granite are characterized by low MgO, Cr and Ni contents; LREE enrichment and negative anomalies of Ba, Nb, Ti, Sr and P indicate that the ascending melt was contaminated by assimilation of crustal material (Fig. 6b, Kamber et al. 2002; J Chen et al. 2024). In addition, higher Rb abundance than the Sr in these granites (Rb/Sr = 5.31 to 7.37) also indicates a crustal enrichment of elements. All samples have a shoshonitic affinity (Fig. 4c) and indicate a post-collisional setting (Liegeois et al. 1998; Phukon et al. 2018). As an additional possibility, they can be generated from metasomatised sub-continental lithospheric mantle (Aldanmaz et al. 2000; Seghedi et al. 2004), or assimilation of crustal rocks (Meen 1987; Feeley and Cosca, 2003). Moreover, peraluminous character and positive anomalies in LILEs such as U, Th and Pb also suggest that they likely originated from the crustal source. Furthermore, the ratio of Nb/Ta (7.56–10.81) falls close to the average crust value (11) (Taylor and McLennan, 1985). The positive correlation between La/Sm vs. La and La/Yb vs. La (Fig. 9a and 9b) suggests partial melting was a major controlling factor for the generation of the studied granite. Apart from the partial melting, the fractionation of Ti-bearing phases such as ilmenite is observed from the negative anomalies of Ti and the negative anomalies of P likely caused by the apatite fractionation (Fig. 6b).

Figure 9. (a) La/Sm vs. La and (b) La/Yb vs. La plots of the Gwaldam biotite granite from the Baijnath Klippe, Kumaun Himalaya, NW Himalaya.

Based on the whole-rock geochemistry of the studied granite, it can be concluded that the GBGr were generated by partial melting of the crustal rocks followed by crustal contamination/assimilation in an extensional environment.

6.b. Geodynamic implications

The petrogenesis of the Lesser Himalayan Paleoproterozoic granites is crucial to understanding the tectonic setting of the NICM during the assembly of the Columbia supercontinent. It is believed that the Indian plate was situated between North China and North America during Paleoproterozoic to Mesoproterozoic and it is deduced that there was a continuous subduction zone running along the northern boundary of the Indian plate at that time (Hou et al. 2008) (Fig. 10a and 10b). The extensive distribution of 1.8 Ga magmatic rocks throughout the nappes and klippen over the entire LHS indicates a significant magmatic event during Paleoproterozoic time (Phukon et al. 2018). The detailed mapping by several researchers has shown multiple Paleoproterozoic plutons emplaced as nappe and klippen that overlie the LHS sequences with their equivalents in Higher Himalaya with varied geological contexts and tectono-metamorphic histories (Valdiya, 1980; Trivedi et al. 1984; Joshi, 1999; Mandal et al. 2016; Joshi et al. 2019). In the Kumaun Lesser Himalaya, NW India, the Munsiari augen gneisses are known to have crystallized from 1970 Ma to 1950 Ma, which are associated with active subduction and magmatism along the northern Indian margin while the crystallization of the Chhiplakot crystalline may have been a later part of the continental arc magmatism at ca. 1920 Ma, which can be associated with extension likely driven by slab break-off and/or slab rollback (Phukon et al. 2018). Moreover, Sen et al. (2013) divided the Bhatwari gneisses into two groups: the Lower Bhatwari Gneisses, which have an alkaline I-type granite protolith with crystallization age of 1988 ± 12 Ma, whereas the Upper Bhatwari Gneisses are dated at 1895 ± 22 Ma and characterized by calc-alkaline S-type granites that are more fractionated and associated with back-arc rifting. The Paleoproterozoic (ca. 1860 Ma) magmatic rocks, which are likely equivalents of their Higher Himalayan counterparts, are widespread in the basal part of the LHS, which is apparently due to the remobilization of older crustal material that was tectonically exhumed by thrusting as a result of collisional tectonics during the Himalayan orogeny (Joshi, 1999; Joshi and Tiwari, 2009; Singh et al. 2009, Joshi et al. 2019). A-type granites are commonly accepted as products of lithospheric extension environments in various tectonic settings viz. intra-plate, post-collisional or back-arc (Whalen et al. 1987; Eby, 1992). In Nb vs. Y and Rb vs. Y + Nb tectonic discrimination diagrams, all the samples from GBGr plot either on within-plate or on the post-collisional setting fields (Fig. 11a and 11b).

Figure 10. Dynamic model showing different stages of Paleoproterozoic tectonic evolution of the North Indian Continental Margin (present lesser Himalaya) of the Indian continental crust: Fig. (a–b) Configuration and spatiality of the Indian continent during Paleoproterozoic and arc-magmatism along its northern margin (after Hou et al. 2008).

Figure 11. Discrimination diagrams for tectonic settings: (a) Nb vs. Y and (b) Rb vs. Y+Nb are after Pearce et al. (1984). Abbreviation: Syn-COLG = syn collisional granites; post-COLG = post-collisional granites; ORG = ocean ridges granites; VAG = volcanic arc granites; WPG = within plate granites.

As discussed earlier, partial melting of the crustal rocks is a plausible mechanism for the genesis of the studied A₂-type biotite granite from the Kumaun Himalaya. The heat from K-rich shoshonitic magma during the later stages of slab rollback/break-off led to the genesis of the studied post-collisional granites. Moreover, in tectonic discrimination diagrams, the samples cluster at and around the triple point dividing the VAG-SynCOLG-WPG fields that suggest post-collisional settings (Fig. 11b). Zircon U-Pb studies of the GBGr yielded two episodes of magmatism, viz. ca 1900 and 1854 Ma, which are consistent with the Paleoproterozoic magmatism reported from the granite and gneissose-granite of the LHS. These ages are also comparable to those of the Debguru granitic gneisses (Celerier et al. 2009), Upper Bhatwari gneiss (Sen et al. 2013), Bandal orthogneiss (Singh et al. 2009) and Askot Klippe (Mandal et al. 2016) from the NW Himalaya and the eastern Himalaya, viz. Salari granite (Bikramaditya et al. 2022), and A-type Mayong granitoids of Assam generated by the recycling of early Paleoproterozoic crust in extensional settings linkage with the magmatic event of the Columbia supercontinent (Doley et al. 2022). The widespread geographic distribution and presence of rocks dated between 1.9 and 1.8 Ga that tectonically rest over the LHS indicates large magmatic activity that occurred during the Paleoproterozoic (Pandey, 2022). There are two subduction-related magmatic events linked to an active continental margin during the Paleoproterozoic Columbia supercontinent assembly genetically linked to continental accretionary processes, one at ca 1988–1950 Ma that was subduction-related arc magmatism (Fig. 12a) and a later 1920–1800 Ma, which followed the slab rollback/ break-off (Phukon et al. 2018, Sen et al. 2013 and Mandal et al. 2016).

Figure 12. (a) Subduction magmatism circa 1988–1950 ma (after Phukon et al. 2018; Sen et al. 2013. (b) Slab rollback/break off and partial melting of crust resulting in felsic, shoshonitic magmatism at circa 1900–1854 Ma.

Based on the field, geochemical and geochronological observations, we propose a tectonic model (Fig. 12b) that begins with an initial stage of post-collisional (slab rollback) setting ca. 1900 Ma that was likely part of the large magmatic activity in the Himalaya and a later stage incipient rift setting that triggered partial melting in the lower to mid crust at ca 1854 Ma. Interestingly, the zircon core and rim ages are separated by a good 46 Ma, which indicates that the core and rim formed in two distinct magmatic stages. Slab break-off or delamination of thickened crust induced upwelling of asthenosphere, which caused partial melting in initial stages at the interface between lower crust and upper mantle, likely responsible for the post-collisional magmatism, as these processes provide enough heat for crustal melting (Bonin, 2004, Luo et al. 2015, Phukon et al. 2018, Sen et al. 2013) up to mid-crustal level, producing granitic magmatism (Davies and von Blanckenburg, 1995).

Integrating the geochemistry and crystallization ages of the studied biotite granite, we infer that the parental magma for the Paleoproterozoic mesocratic GBGr intruded into the dark grey Ramgarh granite gneisses that comprise the host rock. Both these rocks belong to the shoshonitic series. The Ramgarh granite gneisses formed primarily through the partial melting of the lower to mid crust, during or synchronous to the post-collisional extension (Fig. 12b), which followed the subduction phase (Kohn et al. 2010, Rao and Sharma 2011, Sen et al. 2013, Mandal et al. 2016). It can therefore be inferred that the GBGr represents the Paleoproterozoic active margin of the north Indian continental region, genetically linked to the Paleoproterozoic magmatism during the assembly of the Columbia supercontinent that was a consequence of at least two episodes of crustal rifting, one at ca 1900 Ma, which was a widespread magmatic event that likely produced the dark grey Ramgarh granite gneisses and the other, a relatively feeble one, at 1854 Ma that triggered incipient rifting which in turn gave rise to the melt that later crystallized as the GBGr.

7. Conclusions

1. 1. The GBGrs of the BK in Kumaun Himalaya have high SiO₂, total-alkali contents, Y/Nb and enrichment in LREE and LILE that classify it as A₂-type granite, which are derived from partial melting of the underplated crustal rocks in post-collisional rift settings.

2. 2. The older magmatic event at ∼1900 Ma was a likely consequence of the partial melting of the basement rocks which generated the granite melts, which were emplaced in an extensional tectonic environment that followed the post-collisional setting during the formation of the Columbia supercontinent.

3. 3. Zircon U-Pb results indicate that the 1854 Ma GBGr crystallized as a consequence of partial melting at mid-crust levels during the Paleoproterozoic incipient rifting within the 1900 Ma Ramgarh gneisses during the later stages of the Columbia supercontinent accretion.

4. 4. Geochemistry and zircon U-Pb geochronology results of the GBGr indicate that the Paleoproterozoic crust in the western Himalaya witnessed at least two events of crustal extension viz. (1) a widespread one at ∼ 1900 Ma and (2) another at ∼ 1854 Ma during the Columbia Supercontinent assembly.