Petrogenesis of Late Stenian Syn-Orogenic A-Type Granites in the Chhotanagpur Gneissic Complex and Eastern Indian Shield

Abstract

We report the petrogenesis of arfvedsonite granites from the Dimra Pahar pluton in the Chhotanagpur Gneissic Complex based on petrology, whole-rock chemistry, mineral chemistry, and La-ICP-MS zircon U-Pb ages and Hf-Lu isotopic analyses. These granites are dominantly peralkaline, occasionally peraluminous, and demonstrate features of A1-type granites. The magma was emplaced at a shallow depth and had a high liquidus temperature, fO₂ (>NNO), and water saturation. The zircons exhibit three distinct U-Pb isotopic ages. The oldest (1324 ± 6 Ma), large-sized inherited zircons (εHf(t) = +1.65 to +7.64), show complex zoning and signs of partial resorption. The euhedral, prismatic-bipyramidal zircons displaying oscillatory zoning (εHf(t) = −3.43 to +1.43) reveal a crystallization age of 1046 ± 7 Ma. Their thin periphery (εHf(t) = −3.23 to +0.27) grew during retrograde metamorphism (995 ± 6 Ma). The whole-rock geochemistry and the Hf-isotope values imply that the parental magma of these granites resulted from the anatexis of metasomatized lithospheric mantle sources. These granites intruded in a syn-orogenic (syn-collisional exhumation) stage of the orogeny.

1. Introduction

Peralkaline granites display molar (Na₂O + K₂O)/Al₂O₃ >1) and may contain alkali-rich mafic minerals such as aegirine, arfvedsonite, annite, etc. [1,2,3], which usually crystallize after feldspar [4]. The peralkaline granites contain elevated levels of SiO₂, typically >70 wt%, and sometimes >75 wt%, volatile elements, Zr, Nb, Ta, Ga, and rare earth elements [5,6,7]. The peralkaline granites occur in anorogenic or post-collision settings, categorized as A1 and A2 subtypes [8,9]. However, recent discoveries have indicated their genesis in subduction-related environments, as well [10,11,12]. Understanding the formation of peralkaline granite is significant for achieving insights into tectonic settings and continental evolution (e.g., [2,13,14,15,16,17]). It also pertains to rare-metal mineralization, including Nb, Ta, Sn, Zr, and REEs [18,19,20,21,22]. The origin of peralkaline granites remains a debatable topic. At least four accepted models can explain their formation, as follows: (1) Extensive differentiation of a mantle-derived alkaline mafic magma [3,14,22,23,24]; (2) partial melting of K-rich mafic rocks to form syenitic magma, followed by fractional crystallization to form peralkaline granites [14,25,26,27,28]; (3) low-degree partial melting of dry, halogen (F⁻ and/or Cl⁻)-enriched lower crustal granulitic residue (e.g., [13,16,17,29,30,31]); and (4) partial melting of enriched, fluid-fluxed crustal sources [11,32,33]. The uncertainty about these characteristics makes it challenging to interpret the initial magmatic features and evolution processes, which hinders our understanding of petrogenesis and tectonic settings of peralkaline granites.

The E–W trending Satpura Orogenic Belt (SOB) is believed to be a collisional zone where the northern Indian and southern Indian cratons collided and then experienced a break-off repeatedly between 2.1 and 0.9 billion years [34,35]. The SOB comprises the Chhotanagpur Gneissic Complex (CGC) in the central parts. The Central Indian Tectonic Zone and the Shillong plateau are located at the western and eastern extensions of the SOB [36]. The extensive coverage of CGC by granitic and mafic rocks makes studying them essential for understanding the significant growth of the continental crust and constraining the tectonic evolution of the SOB. The North Purulia Shear Zone (NPSZ) of the CGC is important in this context because it contains numerous occurrences of Proterozoic peralkaline and alkaline granitic intrusions, nepheline syenites, and carbonatites (Figure 1b) [37,38,39,40].

This study focuses on the peralkaline granites from the Dimra Pahar lying on the NPSZ [37,38] in the central part of the CGC (Figure 1c). We present new whole-rock major- trace analysis, including rare earth elements, LA-ICP-MS U-Pb ages, and Hf-Lu isotopic compositions of zircons. This paper aims to constrain the timing of emplacement, reconstruct the petrogenetic evolution, identify magma sources, and determine the tectonic setting of the peralkaline A-type granitic rocks from the Dimra Pahar pluton.

2. Geological Setting

The arfvedsonite granite pluton lies along the North Purulia Shear Zone of the CGC, which belongs to the ENE-WSW trending 1500 km long Satpura Orogenic Belt, evolved during the Grenvillian period around 900–1000 Ma ago (Figure 1a). The CGC covers about 100,000 square kilometers of the eastern Indian shield ([41], Figure 1b). It records a history of polyphase deformation, metamorphism, and magmatism during the Proterozoic [42,43,44,45]. The South Purulia Shear Zone (SPSZ) separates the CGC from the North Singhbum Mobile (NSMB) belt in the south. The Permo-Jurassic continental sediments of the Gondwana rift basins burry the western boundary of the CGC. The Quaternary alluvium of the Ganga-Brahmaputra river system covers the northern and eastern parts of the CGC. Rajmahal basalt traps (Cretaceous) cover the north-eastern part of the CGC (Figure 1b). The unclassified felsic gneisses and migmatites with older enclaves of metasedimentary and metaigneous rocks (not dated yet) form the basement of the CGC. The younger intrusive suite within the CGC is represented by porphyritic granitoids, syenite, nepheline syenite, anorthosite, mafic and ultramafic rocks, etc. [1,45,46,47,48,49]. The strike of foliation and elongation of the younger intrusives and older enclaves in CGC are dominantly oriented E-W to ENE-WSW (Figure 1b).

The regional metamorphism (M₁) in the area has been classified as reaching granulite facies conditions, occurring synchronously or after the first folding event (D₁). This process culminated in anatectic melting, leading to the formation of migmatites [44]. During M1, the peak metamorphic conditions achieved were approximately 7 ± 0.5 kbar in pressure, with temperatures ranging from 700 to 820 °C [44].

The Bengal Anorthosite pluton is located at the center of a doubly plunging fold that exhibits a pronounced E-W fabric and contains granulite xenoliths. Its emplacement is believed to have taken place after the D₁ event but before the onset of D₂ [45]. The subsequent pervasive D₂ deformation has been identified as late- to post-kinematic in nature [35].

The third folding deformation phase (D₃) resulted in the formation of open folds featuring axial surfaces that strike roughly E-W. Nepheline syenites have intruded prior to this third deformation [46], approximately 1012 Ma ago (unpublished data from the authors). Zircon U-Pb dating indicates that the porphyritic granite batholiths, which have ages of 998 Ma [50], fall within the time frame of the tectonothermal event associated with D3 deformation [35]. Terrain exhumation and retrograde metamorphism took place between 995 and 940 Ma [45].

During the fourth phase of regional shear deformation, pegmatites and pink homophanous granites (dated to 950 Ma) were emplaced along ENE-WSW striking shear zones (unpublished data of the present authors). Additionally, E-MORB-like mafic dykes and lamprophyric dykes also appeared around the same age of 950 Ma [47,51].

Geology of the Study Area

The arfvedsonite granite pluton of Dimra Pahar (Figure 1c) lies in the Hazaribagh District, Jharkhand, India, at the east–central part of the CGC (Figure 1b). The gray granitoid gneisses forms the basement of the study area (Figure 1b,c). The supracrustal rocks comprising mica schists (biotite-muscovite-quartz, rare garnet and fibrolite bearing), micaceous quartzites, quartzites, calc-silicate gneisses (amphibole + plagioclase + quartz ± epidote ± calcite), minor amphibolites (amphibole + plagioclase ± quartz ± epidote ± sphene), and ferruginous quartzites, structurally overlie the gray granitoid gneisses (Figure 1c).

The Dimra Pahar arfvedsonite granite pluton forms a series of east–west trending hillocks that stretch approximately 8 km from Perehendo Pahar in the east to Birhaddih village in the west. The average width of the rock body is about 1200 m (Figure 1c). These granites intrude into the fibrolite-bearing mica schist, while the latter is intricately associated with the migmatitic gray granitoid gneisses, calc-silicate rocks, and amphibolites.

Four deformation events (D₁–D₄) are recorded in the rocks of the study area. The S₁ is the earliest recognizable secondary planar fabric related to the D₁-deformation (Figure 2a). The S₁-fabric is prominant in gray granitoid gneisses, quartzites, and calc-silicate gneisses. The S₁ gneissosity is defined by alternate biotite-rich and quartzo-feldspathic mineral-rich layers in the medium-grained gray gneisses. This early gneissosity (S₁) in grey gneisses is folded into open, upright to overturned (the southern limb of antiforms being overturned) D₂-folds with low plunge of fold-axes and steep northerly dipping E-W axial planes (S₂) (Figure 2a). In fibrolite-garnet mica schists, the dominant schistosity (S₂) is puckered during D₃-deformation with the development of reclined D₃-folds with E-W steeply dipping crenulation cleavage (S₃) (Figure 2b). On the west of Dimra village and southwest of the Sokla railway crossing, tight to isoclinal folds with steep easterly plunging fold axes are encountered in calc-silicate rocks. The gneissosity of the arfvedsonite granites is parallel to the S₃-cleavage, suggesting the intrusion of the arfvedsonite granite magma before the end of D₃-deformation.

The D₄ deformation produces a steeply dipping E-W shear nearly parallel to the S₃-cleavage. The northern margin of the arfvedsonite granite pluton is highly mylonized. The sense of shear is sinistral (Figure 2c,d). Additionally, thin mylonized zones are occasionally present in the granite body. Basak and Goswami [38] have demonstrated several shear sense criteria, such as intrafolial sub-vertical drag folds, mineral fishes, domino-type structures, rotation of cleavage set in porphyroclasts of K-feldspars in pegmatitic veins, and parallel sets of fractures in vein quartz indicating a sinistral sense of shearing in the granite body.

3. Material and Methods

3.1. Sampling and Optical Petrography

The authors collected samples weighing at least 5 kg each during fieldwork in 2015–2017. Using a water-cooled diamond blade, billets of approximately 25 × 45 × 15 mm were cut from inherently fresh pieces that showed no signs of weathering. In order to produce a mirror polish, billets were planed using 1000 mesh carborundum slurry, mounted with Araldite epoxy on a 27 × 47 mm standard petrographic slide glass, and then the mounted sections were lapped down to a thickness of around 35 μm with diamond pastes 4–2–1–0.25 μm on cloth to achieve a mirror polish. Sections that were finalized were examined using a Nikon LV100 POL optical petrographic microscope (Nikon Instruments Inc., Melville, NY, USA) in the University of Calcutta.

3.2. Major and Trace Elements

Every sample was mechanically crushed and milled to a granulometric fraction of less than 100 mesh. The steps listed below were used to prepare pressed pellets for XRF analysis.

The major oxides of 5 samples were analyzed employing an AXIOS PANalytical wavelength dispersive XRF with a flow Scintillation Detector (Malvern Panalytical Ltd., Great Malvern, Worcestershire, UK) at the Department of Geology, Presidency University, Kolkata. This analysis involved using pressed pellets. Additionally, major oxides of 5 more samples were analyzed at Geochemical Division, NGRI, India, using the Philips MagiX PRO model PW 2440 wavelength dispersive X-ray fluorescence spectrometer coupled with automatic sample changer PW 2540 (Malvern Panalytical Ltd., Great Malvern, Worcestershire, UK) and also using pressed pellets. Certified reference material G-2 was used to validate the results in both laboratories. Furthermore, major oxides of DPAG 1 and DPAG 10, as well as major and trace elements of sample DPAG 12, were analyzed using a WDXRF S8 Tiger (4 kW) from Bruker-AXS (Bruker Corporation, Billerica, MA, USA) at the Department of Earth Sciences, IISER, Kolkata, India, using the fused bead technique. The results were validated using certified reference material JG-2.

Trace elements contents including REEs of eight samples of the arfvedsonite granites were analyzed by HR-ICPMS at the Geochemical Division of NGRI, Hyderabad. Digested samples were fed into a Perkin Elmer SCIEX ELAN DRC II HR-ICPMS instrument (PerkinElmer^® GC Instruments Ltd., Waltham, MA, USA). Certified reference material JG-1a was used for calibration and correction of the analytical data. Instrument specifications, lower detection limit, and accuracy of the analyses are given in detail in Satyanarayanan et al. [52].

3.3. Mineral Separation and Sample Preparation

Over 1 kg of arfvedsonite granite sample were crushed using a toggle jaw crusher at Macquarie University, Sydney, Australia. After crushing, the rocks were processed through a Wilfley table for preliminary heavy mineral concentration, and subsequently passed through magnetic separation using a Frantz^® isodynamic separator (SG Frantz Co Inc., Tullytown, PA, USA) in order to remove magnetic and paramagnetic minerals. Non-magnetic separates underwent heavy liquid separation using methylene iodide which allows the concentration of minerals with specific gravity > 3.32 (e.g., zircon 4.6–4.7). The granite returned an excellent zircon yield with over a 1000 zircon grains. 110 grains were handpicked under a binocular microscope and mounted on a 1-inch polished epoxy round for subsequent analytical work.

3.4. Cathodoluminescence and BSE Imaging

Cathodoluminescence (CL) and backscattered electron (BSE) images of the polished zircon grains were performed to provide growth and microstructural information, and to be used as a tool to select the best crystal domains for subsequent U-Pb dating. The CL images were obtained at Macquarie University using a Zeiss EVO MA 15 Scanning Electron Microscope (SEM) (Zeiss, Oberkochen, Germany). Operating conditions consisted of an accelerating voltage of 20 kV and a working distance (WD) of 12 mm.

3.5. In Situ U-Pb Dating of Zircon

In situ U-Pb dating of the zircons of the studied granites was performed by Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS) using a Teledyne-Photon Machines Analyte Excite 193 nm Ar-F excimer laser system (Teledyne Photon Machines, Bozeman, MT, USA) coupled to an Agilent 7700X quadrupole ICP-MS (Agilent Technologies, Santa Clara, CA, USA) (MQ GeoAnalytcal—Macquarie University, Sydney). Ablation was performed in a tunable 2-volume HelEx cell under helium atmosphere. He flow rates of 0.45 and 0.40 LPM (litre per minute) were applied to the inner and outer cells, respectively. Ablation was carried out in He for better transport efficiency of the analyte, but also as it offered more stable signals and Pb/U fractionation (Pb/U ≈ 1). The laser ablation conditions consisted of a beam diameter of 40 µm, a repetition rate of 5 Hz, and a laser energy density of 7.56 J/cm². Each analysis consisted of 60 s background acquisition (gas blank) followed by 120 s of ablation. The following masses were measured: ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²³²Th, and ²³⁸U. GEMOC GJ-1 zircon reference material [53] was used as calibrating reference material. Zircon 91500 [54], Mud Tank [55,56], and TEMORA-II [57] were used as secondary reference materials to monitor reproducibility and instrument stability (see supplementary reference material table, Supplementary Table S1). A typical analytical run consists of two sets of GJ-1 analyses bracketing two analyses of 91,500 and Mud Tank, one analysis of Temora-II, and 12 analyses of unknowns. Raw signals are reduced and uncorrected U-Pb dates are calculated using the online software package GLITTER^® (http://www.glitter-gemoc.com/ (accessed on 30 Ocotber 2024)) [58]. GLITTER^® allows isotopically homogeneous segments of the signal to be selected for integration. GLITTER^® subsequently corrects the integrated ratios for downhole fractionation and instrumental bias by calibrating the selected segments against the identical time segments of the calibrating primary reference material.

The common-Pb correction of Andersen [59] was applied to analyses which were non-concordant within 2σ analytical uncertainty in ²⁰⁷Pb/²³⁵U and ²⁰⁶Pb/²³⁸U or which had more than 0.2% common Pb. When applied the correction assumed recent lead-loss with a common-Pb composition of present-day average orogenic Pb as given by the two-stage growth curve of Stacey and Kramers [60] for ²³⁸U/²⁰⁴Pb = 9.74.

3.6. In Situ Lu-Hf Systematics of Zircon

In Situ Lu-Hf isotopes characterisation of zircons of these granites was performed at Macquarie University GeoAnalytcal (MQGA, Sydney, Australia) by laser ablation multicollector ICP-MS (LA-MC-ICP-MS), using a Teledyne-Photon Machines Analyte G2 193 nm Ar-F excimer laser system (Teledyne Photon Machines, Bozeman, MT, USA) coupled to a Nu Instruments Plasma II MC-ICP-MS (Nu Instruments, North Wales, UK). Laser ablation conditions were identical to the above U-Pb methods, but with a laser spot size of 50 µm. Typical ablation times are 60–100 s and are preceded by a 30 s “zero” measurement. Measurements were carried out in static collection mode using faraday cups only. Masses measured included 171 (Yb), 172 (Yb), 173 (Yb), 174 (Hf, Yb), 175 (Lu), 176 (Hf, Lu, Yb), 177 (Hf), 178 (Hf), 179 (Hf), 180 (Hf), and 182 (W). Mass bias was corrected using an exponential law and using ¹⁷⁹Hf/¹⁷⁷Hf = 0.7325 [61]. As shown above, measurement of the ¹⁷⁶Hf is hindered by 2 isobaric interferences ¹⁷⁶Lu and ¹⁷⁶Yb. The ¹⁷⁶Lu interference is corrected by measuring the ¹⁷⁵Lu which is interference-free and using ¹⁷⁶Lu/¹⁷⁵Lu = 0.2668 [62]. The ¹⁷⁶Yb interference is corrected in a similar manner by measuring the interference-free ¹⁷³Yb and ¹⁷¹Yb, allowing first to measure the ¹⁷³Yb/¹⁷¹Yb ratio to estimate the Yb mass bias in each analysis and then measure the ¹⁷⁶Yb/¹⁷³Yb ratio to calculate the amount of interfering ¹⁷⁶Yb to subtract from the 176 signal. Zircons Mud Tank and Temora-II were used as secondary reference materials to monitor instrument stability and reproducibility, including the efficiency of the ¹⁷⁶Lu and ¹⁷⁶Yb corrections on Temora-II (¹⁷⁶Yb/¹⁷⁷Hf ca. 0.04), and were analysed every 10–15 unknowns (see supplementary reference material table; Supplementary Table S2).

Hf initial (Hf_i) and εHf were obtained using the ¹⁷⁶Lu decay constant of Sherer et al. (2001) [61] (l¹⁷⁶Lu = 1.865 × 10⁻¹¹ y⁻¹) and the chondritic values of Blichert-Toft and Albarede (1997) [62] ¹⁷⁶Hf/¹⁷⁷Hf (CHUR, today) = 0.282772 and ¹⁷⁶Lu/¹⁷⁷Hf (CHUR, today) = 0.0332. Model ages (T_DM) are assumed to be based on a depleted mantle source and that the depleted mantle reservoir developed from an initially chondritic mantle and is complementary to the crust extracted over time. “Crustal” modal ages T_DM^C ages (or two-stage model ages) were calculated assuming that the parental magma was produced from an average continental crust (¹⁷⁶Lu/¹⁷⁷Hf = 0.015 [63]; Geochemical Earth Reference Model database, http://www.earthref.org/ (accessed on 30 Ocotber 2024)).

4. Results

4.1. Optical Petrography

The granites of the Dimra Pahar pluton are light green, medium-grained, weakly banded, and, at places, strongly mylonitic. They consist mainly of quartz, K-feldspar (perthitic in some samples), Na-feldspar, and arfvedsonite, with minor aegirine, biotite, zircon, ilmenite, and secondary hydrous minerals. The weak magmatic banding, when present, is defined by thin, impersistent layers of ferromagnesian minerals that alternate with thicker bands composed of quartz, K-feldspar, and albite (Figure 3a). The quartz grains in these bands are subidiomorphic to xenomorphic, ellipsoidal, lenticular, and ribbon-shaped in mylonitic bands. K-feldspars are typically microcline and sporadically mesoperthitic. In places, subidiomorphic K-feldspar porphyroclasts athwart the foliation, defined by the surrounding fine-grained quartzo-feldspathic mass. Undulose extinction and subgrain formation are present within these quartzo-feldspathic minerals. Quartz may occur as an inclusion within both K-feldspar and albite. Arfvedsonite (yellowish green to opaque dark blue pleochroism) is prismatic to wedge-shaped. Locally, it may contain inclusions of K-feldspar and quartz. Aegirine is elongated, prismatic, lenticular, subidiomorphic, or xenomorphic. In some places, inclusions of arfvedsonite and quartz may be present in aegirine (Figure 3b). Rarely do subidiomorphic biotites occur in clusters and follow the general banding of the rock. Zircon and ilmenite are the main accessory phases.

Paper-thin veins formed by the granulation of quartzo-feldspathic mineral to fine-sized aggregates of grains representing shearing cut across the aegirine-alkali amphibole granite (Figure 3c). Thicker quartz veins often intrude into arfedsonite granite. These veins display a prominent signature of shearing (Figure 3d). In such cases, poor but distinct orientation of the minerals of the granite may be noted parallel to the boundary with sheared veins (Figure 3d). These textural changes in granite suggest that the crystallization of granite magma predates shearing.

4.2. Zircon Morphology

The zircons retrieved display light pink to light brown colours, and the latter are a bit cloudy. Size is varying but the grains are several hundreds of micrometres and up to 2 mm. Morphologically, the habits are either elongated with relatively flat bipyramidal terminations or almost lack the middle section between the bipyramidal ends (Figure 4). Overall, most grains are euhedral or subhedral with a slight rounding of the bipyramidal tips, however many grains are fully rounded, and fragments are also frequent.

Cathodoluminescence revealed very complex and rare microstructures, with either very bright patchy CL response or very dull structureless response. Both CL responses are commonly present within a given grain, but grains with only dull response are also common. Moreover, it is noteworthy that these dark CL grains are also frequently associated with a high level of porosity (Figure 4).

4.3. Zircon Geochronology

Fifty-three LA-ICP-MS spots were performed for U-Pb dating on 37 zircon grains from the Arfvedsonite granite (

Table 1. Zircon LA-ICPMS U–Pb data of arfvedsonite granite from North Purulia Shear Zone, Chhotanagpur Gneissic Complex, Eastern India.

Analysis No.	Th (ppm)	U (ppm)	Th/U	R A T I O S (Common-Pb Corrected)							DATES (Common-Pb Corrected, Ma)
				207Pb/206Pb	±1s	207Pb/235U	±1s	206Pb/238U	±1s		207Pb/206Pb	±1s	207Pb/235U	±1s	206Pb/238U	±1s	Disc.	Correction Type
																	%
190-1_029	27	34	0.79	0.071	0.007	1.563	0.150	0.160	0.006		956	213	955	59	956	32	0	None
190-1_015	38	214	0.18	0.077	0.004	1.712	0.083	0.162	0.002		1109	104	1013	31	969	14	13.6	Disc
190-1_040	73	1006	0.07	0.074	0.009	1.672	0.191	0.164	0.007		1035	255	998	73	981	37	5.6	None
190-1_079r	106	1198	0.09	0.072	0.001	1.639	0.016	0.165	0.001		988	20	985	6	984	6	0.5	None
190-1_039	27	727	0.04	0.073	0.008	1.679	0.172	0.167	0.006		1015	229	1001	65	995	34	2.2	None
190-1_022	142	1353	0.10	0.072	0.003	1.658	0.070	0.167	0.003		981	91	992	27	997	15	−1.8	None
190-1_085r	101	889	0.11	0.073	0.001	1.676	0.027	0.167	0.001		1004	35	1000	10	998	7	0.8	None
190-1_035	203	344	0.59	0.073	0.006	1.689	0.124	0.168	0.004		1016	160	1004	47	999	25	1.8	None
190-1_031	42	42	1.01	0.074	0.005	1.710	0.111	0.168	0.004		1032	140	1012	41	1003	22	3.1	None
190-1_028	120	880	0.14	0.072	0.008	1.685	0.179	0.169	0.006		999	237	1003	68	1005	35	−0.7	None
190-1_014c	221	2045	0.11	0.073	0.001	1.695	0.025	0.169	0.001		1004	32	1007	9	1008	7	−0.4	None
190-1_034	15	314	0.05	0.072	0.005	1.682	0.115	0.169	0.004		988	149	1002	43	1008	23	−2.3	None
190-1_005	37	219	0.17	0.074	0.002	1.742	0.048	0.170	0.002		1054	60	1024	18	1011	11	4.5	None
190-1_040pit	2825	2380	1.19	0.074	0.018	1.724	0.407	0.170	0.016		1030	541	1017	152	1012	85	1.9	None
190-1_006	213	1482	0.14	0.073	0.002	1.724	0.053	0.172	0.002		1003	66	1018	20	1024	12	−2.3	None
190-1_014	193	1542	0.13	0.073	0.003	1.741	0.072	0.173	0.003		1018	89	1024	27	1027	15	−0.9	None
190-1_016	21	35	0.61	0.071	0.006	1.694	0.137	0.174	0.005		950	181	1006	52	1032	29	−9.4	None
190-1_079c	94	1058	0.09	0.075	0.001	1.785	0.030	0.175	0.001		1069	36	1040	11	1040	8	4.4	None
190-1_036	215	543	0.40	0.078	0.006	1.897	0.148	0.176	0.005		1156	166	1080	52	1044	27	10.5	None
190-1_060r	85	634	0.13	0.074	0.001	1.796	0.028	0.177	0.001		1055	34	1044	10	1053	8	1.6	None
190-1_022c	57	407	0.14	0.074	0.002	1.817	0.036	0.178	0.001		1043	45	1051	13	1056	8	−1.3	Disc
190-1_021	72	597	0.12	0.078	0.010	1.924	0.223	0.179	0.008		1144	262	1089	77	1063	43	7.8	None
190-1_090r	1202	969	1.24	0.075	0.002	1.864	0.053	0.180	0.002		1074	61	1068	19	1066	12	0.8	None
190-1_017	50	693	0.07	0.076	0.007	1.884	0.171	0.181	0.006		1087	200	1076	60	1071	33	1.8	None
190-1_026	482	672	0.72	0.098	0.010	3.036	0.310	0.224	0.004		1591	202	1417	78	1304	22	19.9	Disc
190-1_007	598	773	0.77	0.086	0.002	2.677	0.047	0.225	0.002		1341	36	1322	13	1311	10	2.5	None
190-1_080	355	675	0.53	0.085	0.001	2.639	0.028	0.225	0.001		1313	22	1312	8	1311	8	0.2	None
190-1_034.2	471	603	0.78	0.085	0.001	2.650	0.040	0.226	0.002		1318	31	1315	11	1313	9	0.4	None
190-1_009	181	729	0.25	0.085	0.002	2.662	0.067	0.226	0.002		1322	52	1318	19	1315	12	0.6	None
190-1_066	138	385	0.36	0.086	0.002	2.679	0.065	0.227	0.002		1333	51	1323	18	1317	13	1.4	None
190-1_002c	855	694	1.23	0.085	0.003	2.672	0.082	0.227	0.003		1324	64	1321	23	1319	15	0.5	None
190-1_030	407	772	0.53	0.085	0.010	2.672	0.299	0.227	0.009		1326	239	1321	83	1319	47	0.7	None
190-1_027	623	1067	0.58	0.086	0.006	2.687	0.179	0.227	0.006		1332	138	1325	49	1320	29	1	None
190-1_008	1381	432	3.19	0.086	0.003	2.705	0.077	0.228	0.003		1344	59	1330	21	1322	14	1.8	None
190-1_013	425	720	0.59	0.085	0.003	2.694	0.081	0.229	0.003		1325	62	1327	22	1328	14	−0.3	None
190-1_043	319	970	0.33	0.087	0.001	2.769	0.034	0.232	0.002		1349	25	1347	9	1346	8	0.3	None
190-1_013r	91	192	0.47	0.086	0.001	2.769	0.043	0.233	0.002		1345	32	1347	12	1349	10	−0.4	None

Laser ablation spot size 40 µm. Ratio and date uncertainties are displayed at 1s level (standard deviation).

). Fourty-four concordant dates (²⁰⁷Pb/²³⁵U − ²⁰⁶Pb/²³⁸U) were obtained, spreading between 956 Ma and 1349 Ma (Table 1; Figure 5). It was recognised early in the analytical campaign that the bright CL domains were returning almost systematically discordant dates and were therefore subsequently avoided. Three broad age populations can be identified at ca. 1324 Ma, and ca. 1046 and 995 Ma (Figure 5).

The oldest population consists of 13 concordant analyses giving a ²⁰⁶Pb/²³⁸U weighted mean age of 1324.6 ± 9.5 Ma (95% conf. − MSWD = 1.8) and a ²⁰⁷Pb/²³⁵U − ²⁰⁶Pb/²³⁸U Concordia age of 1324.8 ± 3 Ma (Figure 5a,b). Eleven zircons belong to this population: five rounded grains, four subhedral/subrounded grains and two euhedral grains (Figure 4).

The bulk of the analysis (24 analyses—59% of all concordant analyses) groups were around ca. 1000 Ma, spreading from 956 Ma to 1071 Ma. It is difficult to reconcile such a large spread within a single population. This could be due to either Pb-loss, mixing, or two discrete geological processes/events. We assume two distinct events generated two separate populations (1046 Ma and 995 Ma). A first population consisting of 12 concordant analyses giving a ²⁰⁶Pb/²³⁸U weighted mean age of 1046.2 ± 7.4 Ma (95% conf − MSWD = 1.17) and a ²⁰⁷Pb/²³⁵U − ²⁰⁶Pb/²³⁸U Concordia age of 1046.2 ± 3.6 Ma (Figure 5c,d). Ten individual zircons belong to this population: three rounded grains, three subhedral/subrounded grains and four euhedral grains (Figure 4). A second and youngest population consisting of 13 concordant analyses giving a ²⁰⁶Pb/²³⁸U weighted mean age of 993.1 ± 8.9 Ma (95% conf. − MSWD = 1.7) and a ²⁰⁷Pb/²³⁵U − ²⁰⁶Pb/²³⁸U Concordia age of 995.2 ± 3.0 Ma (Figure 5e,f). This latter population consists of 13 individual zircons of which 9 are euhedral, 2 are subhedral and 2 are rounded (Figure 4).

Uranium and thorium contents are high and very variable in this sample. Uranium ranges from 34 to 2380 ppm (mean = 675 ppm) and thorium varies between 15 and 2825 ppm (mean = 359 ppm).

4.4. Zircon Hf-Isotopic Compositions

Fifty-three LA-MC-ICP-MS spots were performed for Lu-Hf systematics of 39 zircon grains from the arfvedsonite granite. To ensure comparability of the U-Pb and Lu-Hf datasets, the Lu-Hf spots (50 µm ø) were placed over the U-Pb spots (40 µm ø). The measured ¹⁷⁶Hf/¹⁷⁷Hf ranges from 0.282028 to 0.282199 (Table 2). The initial ¹⁷⁶Hf/¹⁷⁷Hf (¹⁷⁶Hf/¹⁷⁷Hf_i) is recalculated at the age recorded by U-Pb dating (correcting for radiogenic ingrowth) using the ¹⁷⁶Lu decay constant of Scherer et al. (2001) [61]. ¹⁷⁶Hf/¹⁷⁷Hf_i ranges from 0.281993 to 0.282182 (Table 2) and epsilon Hf(t) (εHf(t)) ranging between −3.43 and +7.64 (spots with discordant U-Pb were not considered) (Table 2; Figure 6a).

Figure 6. εHf evolution plot of zircons from (a): arfvedsonite granite; insets (b,c): possible interpretations for the nature of the inherited zircons of the arfvedsonite granite, metasomatism of the subcontinental lithospheric mantle (SCLM) and magma mixing, respectively. CHUR = Chondritic Uniform Reservoir model, DM = Depleted Mantle model. Grey lines with arrows are shown for references and define crustal evolution trajectories for hypothetical juvenile protoliths extracted from the Depleted Mantle at 3.0, 2.5, 2.0, 1.5, and 1.0 Ga using a ¹⁷⁶Lu/¹⁷⁷Hf of average continental crust of 0.015 (Griffin et al., 2002) [64]. Blue array in a corresponds to the εHf evolution trajectory of the inherited zircon population considering a crustal ¹⁷⁶Lu/¹⁷⁷Hf = 0.015. Yellow array in a corresponds to the εHf evolution trajectory of the inherited zircon population considering a mafic ¹⁷⁶Lu/¹⁷⁷Hf = 0.023.

Figure 6. εHf evolution plot of zircons from (a): arfvedsonite granite; insets (b,c): possible interpretations for the nature of the inherited zircons of the arfvedsonite granite, metasomatism of the subcontinental lithospheric mantle (SCLM) and magma mixing, respectively. CHUR = Chondritic Uniform Reservoir model, DM = Depleted Mantle model. Grey lines with arrows are shown for references and define crustal evolution trajectories for hypothetical juvenile protoliths extracted from the Depleted Mantle at 3.0, 2.5, 2.0, 1.5, and 1.0 Ga using a ¹⁷⁶Lu/¹⁷⁷Hf of average continental crust of 0.015 (Griffin et al., 2002) [64]. Blue array in a corresponds to the εHf evolution trajectory of the inherited zircon population considering a crustal ¹⁷⁶Lu/¹⁷⁷Hf = 0.015. Yellow array in a corresponds to the εHf evolution trajectory of the inherited zircon population considering a mafic ¹⁷⁶Lu/¹⁷⁷Hf = 0.023.

Table 2. Hf isotopic compositions of zircons from arfvedsonite granites from the North Purulia Shear Zone, Chhotanagpur Gneissic Complex, Eastern India.

Analysis No.	Age Population	206Pb/238U Date (Ma)	±1 s	Measurements				Initial			Model Ages (Ga)
				176Hf/177Hf	±1SE	176Lu/177Hf	176Yb/177Hf	176Hf/177Hf Initial (t = o)	εHf Initial (t = o)	±1SE	TDM	TDMC
190-1_029	995.2 Ma	956	32	0.282188	0.000011	0.0003	0.017	0.282182	1.01	0.38	1.42	1.73
190-1_015		969	14	0.282154	0.000011	0.0006	0.032	0.282142	−0.13	0.37	1.48	1.81
190-1_040		981	37	0.282151	0.000015	0.0043	0.196	0.282069	−2.44	0.53	1.64	1.96
190-1_079r		984	6	0.282147	0.000008	0.0012	0.055	0.282125	−0.38	0.29	1.51	1.84
190-1_039		995	34	0.282136	0.000010	0.0005	0.024	0.282125	−0.10	0.36	1.50	1.83
190-1_022		997	15	0.28216	0.000009	0.0007	0.033	0.282146	0.67	0.32	1.48	1.79
190-1_085r		998	7	0.282179	0.000008	0.0013	0.06	0.282154	0.99	0.28	1.47	1.77
190-1_035		999	25	0.282097	0.000011	0.0015	0.085	0.282067	−2.09	0.38	1.60	1.96
190-1_031		1003	22	0.282156	0.000011	0.0006	0.028	0.282145	0.77	0.39	1.48	1.78
190-1_028		1005	35	0.282115	0.000011	0.0014	0.078	0.282087	−1.22	0.39	1.57	1.91
190-1_014c		1008	7	0.28212	0.000012	0.0016	0.078	0.282088	−1.12	0.41	1.57	1.90
190-1_034		1008	23	0.282178	0.000010	0.0006	0.029	0.282167	1.68	0.34	1.45	1.73
190-1_005		1011	11	0.282173	0.000014	0.0006	0.028	0.282162	1.58	0.47	1.45	1.74
190-1_040pit	1046.2 Ma	1012	85	0.28213	0.000016	0.0046	0.191	0.282040	−2.75	0.57	1.69	2.01
190-1_006		1024	12	0.282127	0.000013	0.0015	0.066	0.282097	−0.43	0.47	1.55	1.87
190-1_014		1027	15	0.282137	0.000009	0.001	0.047	0.282118	0.37	0.31	1.52	1.83
190-1_016		1032	29	0.282109	0.000014	0.0008	0.044	0.282093	−0.38	0.48	1.55	1.88
190-1_079c		1040	8	0.282153	0.000010	0.0011	0.052	0.282130	1.11	0.36	1.50	1.79
190-1_036		1044	27	0.282096	0.000012	0.0018	0.090	0.282060	−1.29	0.43	1.61	1.94
190-1_060r		1053	8	0.282167	0.000009	0.0007	0.032	0.282153	2.23	0.31	1.47	1.73
190-1_022c		1056	8	0.282108	0.000013	0.0011	0.054	0.282084	−0.15	0.44	1.56	1.88
190-1_021		1063	43	0.282141	0.000010	0.0009	0.043	0.282123	1.40	0.35	1.51	1.79
190-1_090r		1066	12	0.282107	0.000011	0.0011	0.061	0.282083	0.04	0.37	1.57	1.88
190-1_017		1071	33	0.282141	0.000012	0.001	0.041	0.282120	1.47	0.42	1.51	1.79
190-1_026	1323.8 Ma	1304	22	0.282073	0.000009	0.0003	0.017	0.282065	4.92	0.33	1.58	1.76
190-1_007		1311	10	0.282152	0.000012	0.001	0.046	0.282128	7.32	0.41	1.50	1.62
190-1_080		1311	8	0.282103	0.000012	0.0004	0.023	0.282092	6.07	0.41	1.54	1.70
190-1_034.2		1313	9	0.28207	0.000011	0.001	0.049	0.282044	4.40	0.39	1.61	1.80
190-1_009		1315	12	0.282059	0.000019	0.0026	0.104	0.281992	2.61	0.68	1.70	1.91
190-1_066		1317	13	0.282155	0.000011	0.0011	0.053	0.282127	7.42	0.39	1.50	1.62
190-1_002c		1319	15	0.282086	0.000010	0.0004	0.021	0.282075	5.65	0.35	1.56	1.73
190-1_030		1319	47	0.282064	0.000012	0.0019	0.112	0.282014	3.49	0.41	1.66	1.86
190-1_027		1320	29	0.282072	0.000009	0.0006	0.026	0.282056	5.00	0.32	1.59	1.77
190-1_008		1322	14	0.282031	0.000015	0.0007	0.032	0.282013	3.50	0.51	1.65	1.86
190-1_013		1328	14	0.28213	0.000013	0.0025	0.108	0.282066	5.54	0.44	1.59	1.74
190-1_043		1346	8	0.28207	0.000009	0.0003	0.013	0.282063	5.84	0.32	1.58	1.74
190-1_013r		1349	10	0.282122	0.000013	0.0017	0.072	0.282078	6.45	0.47	1.57	1.70

Laser ablation spot size 50 µm overlapping pre-existing U-Pb ablation pits. Ratio uncertainties are displayed at 1SE level (standard error). Initial Hf composition calculated assuming the ¹⁷⁶Lu decay constant of 1.865 × 10⁻¹¹ a⁻¹ of Scherer et al. (2001) [61]. εHf calculated assuming the Chondritic Uniform Reservoir (CHUR) compositions ¹⁷⁶Hf/¹⁷⁷Hf = 0.282772 − ¹⁷⁶Lu/¹⁷⁷Hf = 0.0332 of Blichert-Toft and Albarede (1997) [62]. T_DM model ages calculated assuming the Depleted Mantle (DM) compositions ¹⁷⁶Hf/¹⁷⁷Hf = 0.283251 − ¹⁷⁶Lu/¹⁷⁷Hf = 0.0384 of Vervoort and Blichert-Toft (1999) [63]. T_DM^C two-stage model ages calculated assuming the Depleted Mantle (DM) compositions ¹⁷⁶Hf/¹⁷⁷Hf = 0.283251 − ¹⁷⁶Lu/¹⁷⁷Hf = 0.0384 of Vervoort and Blichert-Toft (1999) [65] and average crustal ¹⁷⁶Lu/¹⁷⁷Hf = 0.015 of Griffin et al. (2002) [64].

Table 2. Hf isotopic compositions of zircons from arfvedsonite granites from the North Purulia Shear Zone, Chhotanagpur Gneissic Complex, Eastern India.

Analysis No.	Age Population	206Pb/238U Date (Ma)	±1 s	Measurements				Initial			Model Ages (Ga)
				176Hf/177Hf	±1SE	176Lu/177Hf	176Yb/177Hf	176Hf/177Hf Initial (t = o)	εHf Initial (t = o)	±1SE	TDM	TDMC
190-1_029	995.2 Ma	956	32	0.282188	0.000011	0.0003	0.017	0.282182	1.01	0.38	1.42	1.73
190-1_015		969	14	0.282154	0.000011	0.0006	0.032	0.282142	−0.13	0.37	1.48	1.81
190-1_040		981	37	0.282151	0.000015	0.0043	0.196	0.282069	−2.44	0.53	1.64	1.96
190-1_079r		984	6	0.282147	0.000008	0.0012	0.055	0.282125	−0.38	0.29	1.51	1.84
190-1_039		995	34	0.282136	0.000010	0.0005	0.024	0.282125	−0.10	0.36	1.50	1.83
190-1_022		997	15	0.28216	0.000009	0.0007	0.033	0.282146	0.67	0.32	1.48	1.79
190-1_085r		998	7	0.282179	0.000008	0.0013	0.06	0.282154	0.99	0.28	1.47	1.77
190-1_035		999	25	0.282097	0.000011	0.0015	0.085	0.282067	−2.09	0.38	1.60	1.96
190-1_031		1003	22	0.282156	0.000011	0.0006	0.028	0.282145	0.77	0.39	1.48	1.78
190-1_028		1005	35	0.282115	0.000011	0.0014	0.078	0.282087	−1.22	0.39	1.57	1.91
190-1_014c		1008	7	0.28212	0.000012	0.0016	0.078	0.282088	−1.12	0.41	1.57	1.90
190-1_034		1008	23	0.282178	0.000010	0.0006	0.029	0.282167	1.68	0.34	1.45	1.73
190-1_005		1011	11	0.282173	0.000014	0.0006	0.028	0.282162	1.58	0.47	1.45	1.74
190-1_040pit	1046.2 Ma	1012	85	0.28213	0.000016	0.0046	0.191	0.282040	−2.75	0.57	1.69	2.01
190-1_006		1024	12	0.282127	0.000013	0.0015	0.066	0.282097	−0.43	0.47	1.55	1.87
190-1_014		1027	15	0.282137	0.000009	0.001	0.047	0.282118	0.37	0.31	1.52	1.83
190-1_016		1032	29	0.282109	0.000014	0.0008	0.044	0.282093	−0.38	0.48	1.55	1.88
190-1_079c		1040	8	0.282153	0.000010	0.0011	0.052	0.282130	1.11	0.36	1.50	1.79
190-1_036		1044	27	0.282096	0.000012	0.0018	0.090	0.282060	−1.29	0.43	1.61	1.94
190-1_060r		1053	8	0.282167	0.000009	0.0007	0.032	0.282153	2.23	0.31	1.47	1.73
190-1_022c		1056	8	0.282108	0.000013	0.0011	0.054	0.282084	−0.15	0.44	1.56	1.88
190-1_021		1063	43	0.282141	0.000010	0.0009	0.043	0.282123	1.40	0.35	1.51	1.79
190-1_090r		1066	12	0.282107	0.000011	0.0011	0.061	0.282083	0.04	0.37	1.57	1.88
190-1_017		1071	33	0.282141	0.000012	0.001	0.041	0.282120	1.47	0.42	1.51	1.79
190-1_026	1323.8 Ma	1304	22	0.282073	0.000009	0.0003	0.017	0.282065	4.92	0.33	1.58	1.76
190-1_007		1311	10	0.282152	0.000012	0.001	0.046	0.282128	7.32	0.41	1.50	1.62
190-1_080		1311	8	0.282103	0.000012	0.0004	0.023	0.282092	6.07	0.41	1.54	1.70
190-1_034.2		1313	9	0.28207	0.000011	0.001	0.049	0.282044	4.40	0.39	1.61	1.80
190-1_009		1315	12	0.282059	0.000019	0.0026	0.104	0.281992	2.61	0.68	1.70	1.91
190-1_066		1317	13	0.282155	0.000011	0.0011	0.053	0.282127	7.42	0.39	1.50	1.62
190-1_002c		1319	15	0.282086	0.000010	0.0004	0.021	0.282075	5.65	0.35	1.56	1.73
190-1_030		1319	47	0.282064	0.000012	0.0019	0.112	0.282014	3.49	0.41	1.66	1.86
190-1_027		1320	29	0.282072	0.000009	0.0006	0.026	0.282056	5.00	0.32	1.59	1.77
190-1_008		1322	14	0.282031	0.000015	0.0007	0.032	0.282013	3.50	0.51	1.65	1.86
190-1_013		1328	14	0.28213	0.000013	0.0025	0.108	0.282066	5.54	0.44	1.59	1.74
190-1_043		1346	8	0.28207	0.000009	0.0003	0.013	0.282063	5.84	0.32	1.58	1.74
190-1_013r		1349	10	0.282122	0.000013	0.0017	0.072	0.282078	6.45	0.47	1.57	1.70

Laser ablation spot size 50 µm overlapping pre-existing U-Pb ablation pits. Ratio uncertainties are displayed at 1SE level (standard error). Initial Hf composition calculated assuming the ¹⁷⁶Lu decay constant of 1.865 × 10⁻¹¹ a⁻¹ of Scherer et al. (2001) [61]. εHf calculated assuming the Chondritic Uniform Reservoir (CHUR) compositions ¹⁷⁶Hf/¹⁷⁷Hf = 0.282772 − ¹⁷⁶Lu/¹⁷⁷Hf = 0.0332 of Blichert-Toft and Albarede (1997) [62]. T_DM model ages calculated assuming the Depleted Mantle (DM) compositions ¹⁷⁶Hf/¹⁷⁷Hf = 0.283251 − ¹⁷⁶Lu/¹⁷⁷Hf = 0.0384 of Vervoort and Blichert-Toft (1999) [63]. T_DM^C two-stage model ages calculated assuming the Depleted Mantle (DM) compositions ¹⁷⁶Hf/¹⁷⁷Hf = 0.283251 − ¹⁷⁶Lu/¹⁷⁷Hf = 0.0384 of Vervoort and Blichert-Toft (1999) [65] and average crustal ¹⁷⁶Lu/¹⁷⁷Hf = 0.015 of Griffin et al. (2002) [64].

The εHf(t) for the 1324 Ma population sit above the CHUR line (εHf = 0), ranging between +1.65 and +7.64 (Figure 6b,c; Table 2), and therefore encompassing 5.99 εHf units (Figure 6a). Finally, the combined populations at ca. 1000 Ma sit mostly below the CHUR line with εHf between −3.43 and +1.43, for a total of 4.86 εHf units. The two-stage model ages (T_DM^C) for this dataset range between 1.62 and 2.09 Ga (mean = 1.90 ± 0.11 Ga; Table 2).

4.5. Major and Trace Elements Chemistry

Table 3. Major (wt.%) and trace (ppm) elements of the arfvedsonite granites from the North Purulia Shear Zone, Chhotanagpur Gneissic Complex.

Sample	DPAG 731A	DPAG 727A	DPAG 728A	DPAG 735	DPAG 735E	DPAG 736C	DPAG 1	DPAG 10	DPAG 735C	DPAG 736 D	DPAG 727	DPAG 730A	DPAG 732C	DPAG 12	Average
SiO2	70.92	74.75	74.36	77.03	77	74.71	76.7	78.63	73.64	73.76	74.21	74.05	74.54	76.7	75.07
TiO2	0.28	0.29	0.29	0.27	0.29	0.25	0.23	0.19	0.28	0.25	0.24	0.24	0.24	0.2	0.25
Al2O3	11.82	10.75	11.61	11.36	11.69	10.73	11.64	11.63	11.77	11.31	11.53	12.03	11.11	11.68	11.48
Fe2O3T	5.05	3.56	3.82	2.69	2.48	3.53	3.65	3.1	3.64	4.85	3.84	3.66	4.84	3.24	3.71
MnO	0.05	0.04	0.04	0	0.01	0.03	0.05	0.04	0.04	0.05	0.05	0.04	0.02	0.04	0.04
MgO	0.2	0.05	0.07	0.01	0.02	0.02	0	0	0.57	0.53	0.54	0.52	0.53	0	0.22
CaO	0.11	0.37	0.14	0.06	0.08	0.07	0.27	0.06	0.08	0.04	0.32	0.08	0.04	0.42	0.15
Na2O	4.31	4.15	3.82	3.54	3.46	4.06	4.18	3.99	4.52	4.06	4.21	4.13	3.32	4.02	3.98
K2O	5.56	4.67	4.69	4.61	4.71	4.74	4.78	4.77	4.73	4.64	4.73	4.83	4.68	4.89	4.79
P2O5	0.03	0.02	0.03	0.03	0.04	0.02	0	0	0.01	0.03	0.01	0.03	0.02	0	0.02
LOI	1.17	0.46	0.68	0.64	0.71	1.02	0	0	0.6	0.64	0.47	0.55	2.02	0	0.64
Total	99.5	99.11	99.55	100.24	100.49	99.18	101.5	102.41	99.88	100.16	100.15	100.16	101.36	101.19	100.35
Na2O + K2O	9.87	8.82	8.51	8.15	8.17	8.8	8.96	8.76	9.25	8.7	8.94	8.96	8	8.91	8.77
Sc	1.76	1.87	1.66	2.1	1.21	2.02	2.11	1.28							1.75
V	7.11	6.63	5.13	8.31	7.95	1.35	5.15	3.03						0.6	5.03
Cr	11.28	8.78	7.32	9.36	12.16	4.61	7.38	7.37						1.1	7.71
Ni	6.1	4.26	4.1	4.85	4.28	4.39	6.18	6.67							5.10
Ga	27.76	31.48	21.21	38.46	29.82	29.3	45.26	43.25						32.8	33.26
Rb	188.04	201.21	139.28	323.9	167.8	220.89	256.4	216.1	232	227	246	272	279	316	234.69
Sr	162.99	11	13.52	12.94	10.9	8.47	14.56	7.71	12	11	17	13	15		23.85
Y	5.57	76.79	15.32	8.19	12.14	11.08	103.1	48.21	66	65	51	45	55	60	44.46
Zr	914.14	761.48	478.87	893.18	562.56	630.6	1360	876.48	741	623	543	773	2011	996.46	868.94
Nb	43.04	81.04	42.83	69.33	40.36	75.48	127	44.58	123	108	109	142	192	68.9	90.47
Cs	2.08	2.58	1.75	2.13	10.77	3.47	2.87	1.81							3.43
Ba	62.61	21.27	29.42	40.21	24.53	18.85	128.9	108.24						17.9	50.22
La	19.85	77.33	23.19	26.2	17.98	9.36	106.2	74.59							44.34
Ce	41.28	177.45	55.77	60.43	42.17	22.73	246	181.2						210	115.22
Pr	4.67	23.09	7.68	7.92	5.64	3.16	32.37	25.24							13.72
Nd	14.52	81.82	27.62	27.34	19.35	11.79	112.8	91.45							48.34
Sm	2.86	17.1	6.21	5.26	4.05	3.19	24	18.49							10.15
Eu	0.13	0.69	0.27	0.21	0.17	0.15	0.87	0.64							0.39
Gd	1.61	12.75	4.27	3.12	2.76	2.46	16.3	12.22							6.94
Tb	0.3	2.38	0.85	0.52	0.55	0.63	3.18	2.17							1.32
Dy	1.51	11.41	4.12	2.36	2.7	3.45	15.85	9.82							6.40
Ho	0.32	2.25	0.82	0.53	0.57	0.73	2.71	1.5							1.18
Er	0.83	5.33	1.95	1.49	1.47	1.87	7.92	4.15							3.13
Tm	0.17	0.79	0.32	0.26	0.26	0.33	1.23	0.64							0.50
Yb	1.38	5.27	2.51	1.96	1.95	2.61	8.36	4.71						8.3	4.12
Lu	0.22	0.85	0.47	0.31	0.33	0.51	1.36	0.82							0.61
Hf	18.61	16.84	10.71	20.53	11.28	13.46	31.05	18.35						20	17.87
Ta	2.23	5.76	2.5	4.81	2.65	4.42	8.47	3.32						1.5	3.96
Pb	30.83	13.99	22.18	17.02	25.19	17.06	13.46	9.08						3.8	16.96
Th	12.63	11.81	10.17	15.58	9.56	10.68	32.15	14.16						15.5	14.69
U	3.81	5.9	2.68	2.53	3.43	4.54	5.23	2.58						4.2	3.88
10,000 Ga/Al	1.24	1.55	0.97	1.79	1.35	1.45	2.06	1.97						1.49	1.54
Ce + Zr + Nb + Y	1004.03	1096.8	592.79	1031.13	657.23	739.89	1836	1150.5						1335	1049.34
Nb/Y	7.73	1.06	2.80	8.47	3.32	6.81	1.23	0.92	1.86	1.66	2.14	3.16	3.49	1.15	3.27
Nb/Ta	19.30	14.07	17.13	14.41	15.23	17.08	14.99	13.43						45.93	19.06
Th/Ta	5.66	2.05	4.07	3.24	3.61	2.42	3.80	4.27						10.33	4.38
Zircon saturation temperature (°C)	888.6	923.2	910.3	1070.5	918.6	960.5	968.2	918.3	905.9	999	936.5	938	889.5	956.8	941.7

presents the major and trace element data for 14 rock samples from arfvedsonite granites. These granites are silica-saturated with high abundances of total alkalis (Na₂O + K₂O = 8.0–9.87 wt.%) (Table 3 and Figure 7a) [66]. They display a peralkaline to peraluminous chemical affinity (Figure 7b) [67]. These rocks are shoshonitic to high-K calc-alkaline rocks (Figure 7c,d) with Na₂O/K₂O ratios less than 1 (0.71–0.96). Since the granites of this study are shallow-level plutons [38], we have used the Na₂O versus K₂O (proposed by Turner et al., 1996) [68] and SiO₂ versus K₂O (proposed by Peccerillo and Taylor, 1976) [69] diagrams, which were originally created to distinguish the magmatic series of volcanic rocks. The arfvedsonite granites show ferroan characteristics in the SiO₂ vs. FeO_T/(FeO_T + MgO) diagram (Figure 7e) and alkalic affinity the in SiO₂ vs. MALI (Na₂O + K₂O − CaO) diagram (Figure 7f) [70].

These samples have low MgO (0.01–0.57 wt%), total Fe₂O₃ (2.48–5.05 wt%), Cr (4.61–12.16 ppm), and Ni (4.10–6.67 ppm) contents related to the low concentration of mafic minerals (Table 3). They have high Rb (139.28–323.90 ppm), but relatively low Sr (7.71–162.99 ppm) and Ba (18.85–128.93 ppm) abundances, with high Rb/Sr ratios (1.15–28.03) pointing to the low abundances of plagioclase (Table 3). The covariance between the SiO₂ (wt%) and other major and trace elements for the arfvedsonite granites is poor (Figure 8).

In the chondrite-normalized REE diagram (Figure 9a), they exhibit strong enrichment in LREE relative to HREE, with (La/Yb)_N and (Gd/Yb)_N ratios of 2.42–10.68 and 0.76–2.09 and strong negative Eu anomalies [(Eu/Eu*)_N = 0.13–0.19]. In the primitive mantle-normalized (normalization values from Sun and McDonough, 1989) [71] trace element diagram (Figure 9b), the arfvedsonite granites show enrichment of Rb, K, LREEs, Th, U, Zr, and Hf and have moderate to strong negative anomalies in Ba, Nb, Ta, Sr, P, Eu, and Ti.

5. Discussion

5.1. Zircon U-Pb Ages

The zircons retrieved from the arfvedsonite granite are relatively unusual both texturally (dark CL, porosity) and chemically with their very high U and Th content. There is no obvious age-related pattern in the morphologies or internal structures, except that the older (1324 ± 6 Ma) population has, overall, more rounded or subrounded (resorbed) grains, and the youngest population (995 ± 6 Ma) has more euhedral grains. It would be reasonable to assume that 1324 ± 6 Ma old resorbed, rounded, and subrounded grains of zircon represent the xenocrystic zircon of an older lithology. The 1046 ± 7 Ma age occurs at the core, while the 995 ± 6 Ma ages are found at the boundaries of the bipyramidal zircons. The 1046 ± 7 Ma age is considered the age of emplacement. The emplacement was followed by a recrystallization age of 995 ± 6 Ma. The northern part of the rock body is overprinted by shearing movement of the shear zone at about 995 ± 6 Ma.

5.2. Geochemical Affinity of Magma

The arfvedsonite granites of this study include alkaline amphiboles like arfvedsonite, along with their significant enrichment in SiO₂, total alkali, Nb, Zr, and Y, and REE (except Eu), as well as high FeO_T/MgO and Rb/Sr ratios. All of these mineralogical and geochemical characteristics suggest that these granitoids are typically A-type [9,30,73,74]. The granite samples lie in the A-type granite field in the SiO₂ vs. Fe# and SiO₂ vs. MALI diagrams (Figure 7e,f) [70]. Additionally, all samples of the studied granites plot in the A-type granite field in the FeO^T/MgO and (Na₂O + K₂O)/CaO vs. Zr + Nb + Ce + Y ratio diagrams (Figure 10a,b) [30].

Eby [8,9] classified the A-type granites further into A1 and A2 subtypes to distinguish between magma sources and tectonic environments. The arfvedsonite granites of this study are similar to the A1-type granite (Y/Nb ratio < 1.2), with their Y/Nb ratios varying from 0.11 to 1.08. The arfvedsonite granites plot in the A1 subfield according to the Y-Nb-Ce and Y-Nb-3*Ga discrimination diagrams of A-type granites proposed by Eby [8] (Figure 10c,d). Furthermore, the arfvedsonite granite samples plot in the domain of A1 subtype and overlap with the OIB field in the Yb/Ta vs. Y/Nb diagram (Figure 10e) of Eby [8,9]. Low Y/Nb ratios are typical in oceanic island, intraplate and rift zone magmas derived from OIB-type mantle sources (Eby, 1992) [9]. A high Y/Nb ratio is common in magmas derived from arc-type sources (lithospheric mantle modified by subduction-related fluids) or in magmas derived dominantly from the continental crust, as continental crust has a high Y/Nb ratio [9]. Consequently, the A1-type suites with lower Y/Nb ratios are thought to represent differentiates of OIB-like mantle-derived melts. The Th/Yb and Nb/Yb ratios remain unchanged during fractional crystallization of mantle-derived magmas. In contrast, magmas with a subduction component or those that have assimilated continental crust during ascent display higher Th/Yb values [75]. Consequently, crustal contamination in the magmas is detected using the Th/Yb versus Nb/Yb diagram. The diagonal MORB-OIB array in this diagram includes OIB, E-MORB, and N-MORB end members. The diagram (Figure 10f) shows that the arfvedsonite granite samples were plotted close to the mantle array field and the OIB composition. Plots of the granite samples in the Nb/Yb vs. Th/Yb [75] suggest that their parental magma could evolve from OIB-type magma through fractionation and assimilation with the crust (Figure 10f).

Figure 10. Discrimination diagrams for granites with data from this study: (a) FeOT/MgO vs. Zr + Nb + Ce + Y and (b) (Na₂O + K₂O)/CaO Zr + Nb + Ce + Y diagram (after Whalen et al., 1987) [30]; FG = Fractionated granite; OGT = Other Granite Types (unfractionated M-, I-, and S-type granites); (c,d) A1- and A2-type granite discrimination diagrams after Eby (1990, 1992) [8,9]. Dashed lines in (c,d) corresponds to Y/Nb = 1.2 (Eby, 1992) [9]; (e) Yb/Ta vs. Y/Nb diagram in which fields with dashed lines represent A1- and A2-type granites of Eby (1990) [8]; oceanic island basalt, OIB; island arc basalt, IAB. The A1 group (Y/Nb ratios < 1.2) are differentiates derived from mantle-derived parent magmas similar to oceanic island, intra-plate, and rift-zone basaltic magmas. The A2-type granitoids (Y/Nb ratios > 1.2) are related to processes that form continental crust and island-arc basalts. (f) Variation diagram of Th/Yb vs. Nb/Yb after Pearce (2008) [76]. High Th/Yb values are indicative of crustal input, whereas high Nb/Yb values are characteristics of alkali basalts [75]).

Figure 10. Discrimination diagrams for granites with data from this study: (a) FeOT/MgO vs. Zr + Nb + Ce + Y and (b) (Na₂O + K₂O)/CaO Zr + Nb + Ce + Y diagram (after Whalen et al., 1987) [30]; FG = Fractionated granite; OGT = Other Granite Types (unfractionated M-, I-, and S-type granites); (c,d) A1- and A2-type granite discrimination diagrams after Eby (1990, 1992) [8,9]. Dashed lines in (c,d) corresponds to Y/Nb = 1.2 (Eby, 1992) [9]; (e) Yb/Ta vs. Y/Nb diagram in which fields with dashed lines represent A1- and A2-type granites of Eby (1990) [8]; oceanic island basalt, OIB; island arc basalt, IAB. The A1 group (Y/Nb ratios < 1.2) are differentiates derived from mantle-derived parent magmas similar to oceanic island, intra-plate, and rift-zone basaltic magmas. The A2-type granitoids (Y/Nb ratios > 1.2) are related to processes that form continental crust and island-arc basalts. (f) Variation diagram of Th/Yb vs. Nb/Yb after Pearce (2008) [76]. High Th/Yb values are indicative of crustal input, whereas high Nb/Yb values are characteristics of alkali basalts [75]).

5.3. Physicochemical Parameters of the Arfvedsonite Granite Magma

Basak and Goswami [38] determined the physicochemical parameters of magma during crystallization using conventional thermo-, oxy-, and hygro-barometry and pseudosection modeling. The intrusion occurred at a relatively shallow depth, at approximately 200–300 MPa pressure [38].

Zircon saturation temperatures give a good indication of the liquidus temperatures of felsic magmas [77,78]. The investigated samples contain inherited (xenocrystic) zircons, which suggest that the parental melts have been zircon-saturated. The zircon saturation temperatures of the arfvedsonite granite samples range from 888 to 1070 °C, with an average of 942 °C (Table 3). This temperature is consistent with the high temperatures of crystallization of zircons (822–836 °C), as determined by the Ti-in-zircon thermometer [38]. The peralkaline magma experienced elevated crystallization temperatures of feldspar, ranging from 647 to 705 °C, and arfvedsonite and aegirine at around 670 °C, suggesting near solidus temperature of the magma [38].

The FeOt/(FeOt + Mg) ratio, according to Frost et al. [70], can serve as an indicator of oxygen fugacity in magmatic systems, where higher ratios indicate more reducing conditions. Dall’Agnol and Oliveira [79] utilized a similar approach to differentiate between reduced and oxidized A-type granitoids, noting that reduced A-type granitoids exhibit higher FeOt/(FeOt + MgO) ratios. It is mention-worthy that at low MgO concentrations one would yeild spurious results for the FeOt/(FeOt + MgO) ratio. The studied granites have very low MgO concentrations, down to 0.01%, which cannot be measured precisely by XRF. Moreover, using this ratio as an indicator of relative oxygen fugacity in peralkaline melts is questionable. In peralkaline melts, during the late magmatic stage, when Mg concentrations of the magma reach near exhaustion, high oxygen fugacity can lead to the substitution of Fe²⁺ + Mg with Fe³⁺ + Li in late magmatic alkali amphiboles [80,81]. This substitution effect challenges the efficacy of the FeOt/(FeOt + MgO) ratio in accurately reflecting oxidation states in peralkaline systems, making its use as an indicator of oxygen fugacity contentious in such contexts.

Discrete zircon crystals and the Zr-free nature of arfvedsonite and aegirine in the studied granites suggest high oxygen fugacity of the magma [82,83]. The Li content of the arfvedsonite in the Dimra Pahar granite is high (1258 ppm) and higher than the concentration of Li noted in the alkali amphibole of peralkaline granite and Kakortokite of the Ilimaussaq intrusion [81]. The significant enrichment of Li in the alkali amphiboles reflects the rising oxygen fugacity [80,81].

Additionally, Basak and Goswami’s research delved into the water content of the granitic melt, estimating it to be around 6 wt% at 200–300 MPa, which is consistent with the water concentration (5.8 wt%) for a peralkaline (pantelleritic) melt at 200 MPa and 850 °C [84].

5.4. Petrogenetic Model

Various petrogenetic models are proposed for the peralkaline granites including anatexis of different middle or lower crustal source rocks [29,31,85], magma mixing between mantle-derived mafic magmas and more felsic crustal melts [64,86], silicate liquid immiscibility [87,88,89], and extreme fractionation of mantle-derived mafic magmas [22,90,91]. We assess these models based on geological, geochronological, and geochemical results.

Partial melting of chemically depleted (granulitic) lower crustal sources or fusion of modified crustal materials by LILE and volatiles can produce A-type granites [29,92]. These granites are generally peraluminous [93]. The peralkaline arfvedsonite granites of this study cannot be produced from the partial melting of the lower crustal granulitic source.

Magma mixing between mafic magmas and felsic crustal melts is suggested for producing granitoid [64,94], but this interaction lowers SiO₂ content,failing to explain the highly siliceous nature of arfvedsonite granites. Field (absence of mafic microgranular enclaves) and petrological evidence (lack of acicular apatite) contradicts this hypothesis for the studied granites, suggesting magma mixing may not be the primary process.

The model of silicate liquid immiscibility suggests that mafic and peralkaline felsic plutons in large igneous provinces form layered complexes [88,95,96]. The arfvedsonite granitic pluton of this study is not part of a layered complex, suggesting liquid immiscibility may not influence the generation of arfvedsonite granite magma.

Peralkaline granites, which form at higher temperatures, suggest that fractional crystallization of mantle-derived magmas plays a significant role in their genesis [22]. These granites exhibit anomalous enrichment of HFSE and REE, a hallmark of rocks derived from metasomatized mantle [2,8,9,97,98]. We favor this mechanism for the genesis of arfvedsonite granite magma.

The arfvedsonite granites of this study have high SiO₂ (70.92–78.63 wt.%) and low abundances of MgO (0.01–0.57 wt.%), TiO₂ (0.19–0.29 wt.%), Cr (4.61–12.16 ppm), and Ni (4.10–6.67 ppm), indicating fractionation of mafic minerals (e.g., clinopyroxene, amphibole and or olivine). In the PM-normalized trace element diagrams (Figure 9b), the arfvedsonite granites show significant depletions in Sr, Ba, and Eu, indicating extensive fractionation of feldspars from parental melts.

The arfvedsonite granites have high Rb/Sr ratios (av. 18.14) with low Ba (av. 54.26 ppm) and Sr (av. 23.85 ppm) contents. These geochemical features imply partial melting processes involving mica breakdown at source or late plagioclase fractionation. D_Nb/D_Ta for pargasite/fluid is >1, so the separation of pargasite from magma will show a decrease in Nb/Ta [99]. A negative correlation between SiO₂ and Nb/Ta implies the fractionation of amphibole (Figure 8). A deep negative P anomaly in the PM-normalized spider diagram (Figure 9b) and very low abundances of P₂O₅ in these granites imply apatite fractionation from the parental magma. In addition to principal phases, accessory minerals would have controlled much of the REE variation. Zircon strongly fractionates the REE by having partition coefficients much higher than one for the heavy REE and having rather low partition coefficients for the light REE. The increase in HREE (Table 3) suggests an accumulation of zircon with high partition coefficients (Kd) [100].

Fractional crystallization and batch partial melting processes can be distinguished using plots of incompatible trace elements in the C_H1 vs. C_H2 graph, where C is the measured concentration of a trace element in the liquid and H1 and H2 are two highly incompatible elements [101]. Samples related to fractional crystallization have straight lines, while partial melting and mixing samples have hyperbolic curves and linear trends [101]. Good linear correlations between the samples of arfvedsonite granites with constant ratios are observed for incompatible element pairs such as Th–Rb (see figure in Section 5.6) and La–Ce (see figure in Section 5.6), indicating that fractional crystallisation was the dominant petrogenetic process.

5.5. Nature of the Source Rocks

The peralkaline granites of this study are rich in incompatible elements and are located in a continental setting. In Section 5.3, we demonstrated that the parental melts of the studied pluton were derived from mantle sources. Peralkaline granite magmas are suggested to be members of the transitional to alkaline magma series ([102], p. 366). The partial melting of the asthenosphere due to decompression can produce alkaline magmas at greater depths and transitional basalts at intermediate depths ([102], p. 373). Furthermore, the K-rich alkaline and peralkaline rock series can be derived from the partial melting of the SCLM. Numerous authors suggest that metasomatism of the SCLM is a consequential factor in the elevated incompatible element concentrations of highly alkaline magmas. Metasomatism of SCLM may be caused by subduction zone fluids rising from the dehydrating slab into the overlying SCLM or via melt infiltration, underplating, or supercritical volatile fluids from the deep asthenosphere ([102], pp. 398–399).

In the Section 5.5.1 and Section 5.5.2, we shall discuss the characteristics of the mantle source of the parental magma of the studied granites from trace elements and Hf-isotope ratios.

5.5.1. Evidences from Trace Elements

Intracontinental alkaline rocks derived from melting of the SCLM exhibit Nb/Ta ratios 15.0–19.1, lower than the chondritic value (19.9 ± 0.6) but overlap the values of ocean-island basalts (15–16) and are distinctly higher than Nb/Ta ratios in the continental crust (12–13) and in the bulk-silicate Earth (BSE: ∼14) [103]. The Nb/Ta values for the arfvedsonite granites (Av. 15.71; range 13.43–19.3) are similar to those of ocean-island basalts and alkaline volcanic rocks derived from SCLM [103].

The Th/Ta ratio of granitic rocks is noteworthy in revealing magma–crust interaction since mantle-derived rocks exhibit Th/Ta ratios of ≈2, lower than the ratios of the upper crust (Th/Ta ≈ 6.9) and lower crust (Th/Ta ≈ 7.9) [75]. The average Th/Ta values obtained in the samples of peralkaline granites are approximately 3.64 (Table 3), suggesting two possibilities. Either the peralkaline granitoids were derived from mantle-generated magma and contaminated by crust or the source magma was derived from a metasomatized mantle.

Arfvedsonite granite samples exhibit negative Nb anomalies (i.e., Nb_PM/Th_PM < 1; Figure 11a). Negative Nb and Ta anomalies in the arfvedsonite granites (Figure 9b) reflect inheritance from metasomatism of the overlying mantle wedge by a subducting slab or crustal contamination [104]. Negative Ti (±other HFSE) anomaly (Figure 9b) can also indicate residual rutile in the source region. Rutile in the mantle could be a metasomatic phase [105,106,107].

Melting of subduction-related metasomatized mantle produces Nb/La ratios mostly below 0.3 [108,109]. The variable Nb/La ratios of the arfvedsonite granites (0.6–8.06) indicate an asthenospheric or a mixed asthenosphere-lithospheric mantle source (Figure 11b).

Owing to the distinctive REE partition coefficients between garnet and spinel, magmas from garnet-bearing and spinel-bearing peridotites exhibit very different REE patterns, particularly for HREE [110]. In general, low Tb/Yb ratios reflect a melting regime dominated by a larger melt fraction, or spinel, as the predominant residual phase, whereas high Tb/Yb ratios indicate smaller melt fractions, or garnet, as a residual phase [111]. The low [Tb/Yb]_N ratios (Figure 11c) indicate that the magmas parental to the arfvedsonite granites were derived from a spinel peridotite source region at depths above 60–80 km [112].

The water content of the arfvedsonite granite magma was around 6 wt% [38]. It is more likely that the abundance of H₂O in magmas resulted from the melting of the subcontinental lithospheric mantle metasomatized by subducted sediments and water.

Hydrous pyroxenites with mica and alkali amphibole are likely the source of a variety of potassic magmas [113,114]. Considering the high-K composition of the arfvedsonite granites, we postulate that a K-bearing phase, such as amphibole (K-richterite) or phlogopite, was present in the mantle source. The melting of phlogopite can generate K- and Al-rich melts (similar to those of nepheline syenites). In contrast, the melting of K-richterite will produce melts with high SiO₂ and K₂O but low CaO and Al₂O₃ (as we observe in arfvedsonite granites) [114]. Moreover, phlogopite and K-richterite are robust reservoirs in the mantle for K, Rb, Ba, and volatiles [115,116]. Therefore, a K-richterite- or phlogopite-bearing source can be considered the source of these arfvedsonite granites.

Figure 11. Incompatible trace element ratios in the arfvedsonite granites and undersaturated rocks from the North Purulia Shear Zone, Chhotanagpur Gneissic Complex. (a) (Nb/Th)_N vs. (Ta/U)_N diagram [104]. Normalizations are by the primitive mantle values [71]. High (Nb/Th)_N and (Ta/U)_N values in undersaturated rocks are interpreted to indicate that their sources are recycled oceanic lithosphere that had previously undergone subduction zone dehydration which preferentially transferred the Th and U (vs. Nb and Ta) to the mantle wedge above with the “residual” lithosphere enriched in Nb and Ta. The low (Nb/Th)_N and (Ta/U)_N values in arfvedsonite granites are due to crustal contamination of the fractionating magma. (b) Nb/La vs. La/Yb diagram to distinguish between the lithospheric and asthenospheric mantle of the source [108]. (c) (La/Sm)_N vs. (Tb/Yb)_N diagram. The horizontal line separates the fields for melting of garnet-bearing peridotite and of spinel-bearing peridotite [112]. Normalizations are by the primitive mantle values [71].

Figure 11. Incompatible trace element ratios in the arfvedsonite granites and undersaturated rocks from the North Purulia Shear Zone, Chhotanagpur Gneissic Complex. (a) (Nb/Th)_N vs. (Ta/U)_N diagram [104]. Normalizations are by the primitive mantle values [71]. High (Nb/Th)_N and (Ta/U)_N values in undersaturated rocks are interpreted to indicate that their sources are recycled oceanic lithosphere that had previously undergone subduction zone dehydration which preferentially transferred the Th and U (vs. Nb and Ta) to the mantle wedge above with the “residual” lithosphere enriched in Nb and Ta. The low (Nb/Th)_N and (Ta/U)_N values in arfvedsonite granites are due to crustal contamination of the fractionating magma. (b) Nb/La vs. La/Yb diagram to distinguish between the lithospheric and asthenospheric mantle of the source [108]. (c) (La/Sm)_N vs. (Tb/Yb)_N diagram. The horizontal line separates the fields for melting of garnet-bearing peridotite and of spinel-bearing peridotite [112]. Normalizations are by the primitive mantle values [71].

5.5.2. Evidence from Radiogenic Isotopes

We have demonstrated in the preceding section (Section 5.1) that the arfvedsonite granites exhibit three distinct age populations in zircons. Among these, 1324 Ma old zircons show a large spread in εHf_(t) (+1.65 and +7.64), suggesting a combination of two sources. We consider two possibilities: (1) The first possibility is that zircons were crystallized in I-type granitoids. The I-type granitoid melts evolved in a subduction setting. Melting of a crustal component and mixing with depleted mantle melts (juvenile mantle input) formed the parental magma of the I-type granitoids (Figure 6c). Zircons were later incorporated as xenocrysts in the arfvedsonite granite at 1046 Ma. (2) Alternately, these 1324 Ma old zircons were crystallized during modal (or patent) mantle metasomatism or fertilization by recycled enriched fluids from the dehydrating subducted plate in a subduction zone setting (Figure 6b). In modal metasomatism of peridotite, new minerals (such as amphibole, phlogopite, apatite, clinopyroxene, etc.) form in the veins or matrix ([102], p. 398). As a result of modal metasomatism, veins of “mafic” lithologies such as pyroxenites, amphibolites, MARID (mica-amphibole-rutile-ilmenite-diopside rocks) [116], or PIC (phlogopite-ilmenite-clinopyroxene) [117] were formed. The large amount and size of the inherited 1324 Ma zircons recovered in arfvedsonite granite suggest that the parental magma of the studied granites could be derived from the melting of a veined metasomatized mantle (Figure 6b). Considering the geochemical signature of the arfvedsonite granite, this would be our preferred hypothesis.

The zircons of around 1046 Ma age crystallized in arfvedsonite granite magma. The parental magma of the arvedsonite granite may have been generated from one of two sources: (1) Straight remelting of the source rock of the 1324 Ma inherited zircons, that is, veined metasomatized mantle. This seems to be supported by an almost perfect match of the εHf evolution for the 1324 Ma zircons (assuming a ¹⁷⁶Lu/¹⁷⁷Hf = 0.015 − average continental crust) and the 1046 Ma zircons (Figure 6b). Alternately, (2) unradiogenic (OIB-like?) input melting and mixing with a metasomatised or veined mantle (Figure 6c). However, if we consider a metasomatised mantle, perhaps a ¹⁷⁶Lu/¹⁷⁷Hf = 0.015 is not very appropriate, considering the mafic nature of such lithologies. Therefore, a higher (i.e., more mafic, 0.018–0.029, typically 0.023) ¹⁷⁶Lu/¹⁷⁷Hf ratio could be envisaged. In this case the expected εHf evolution trajectory of the source of the inherited zircons would propagate more horizontally (Figure 6a), and the current range of negative εHf_(t) of the 1046 Ma population would extend below this newly defined evolution field, which in turn would mean a mixing with another enriched source (OIB-like) would be needed to explain these lower εHf_(t). This hypothesis supports the conclusions that the studied rock suites were derived from an asthenospheric or a mixed asthenosphere-lithospheric mantle source (Figure 11b), evidenced by several geochemical signatures including the high Nb/La ratios of the arfvedsonite granites (0.6–8.06).

5.6. Major and Trace Element Modeling of Fractional Crystallization

As we discussed in Section 5.4, fractional crystallization processes involving mantle-derived shoshonitic parental magma can provide a conceivable mechanism to yield the arfvedsonite granite rocks. For the parental melt composition, we assume the (OIB-like) shoshonitic alkaline metamafic rocks [118] of a similar age as the relevant option. These shoshonitic mafic rocks were emplaced during the orogeny [118]). As major and trace element composition of the parental melt, we used compositions of the sample SD8b of the metamafic rock suite (see Supplementary Tables S3 and S4 of this paper for the composition of the sample SD 8b) described by Goswami et al. [119]).

To evaluate the fractional crystallization process quantitatively, we conducted numerical major element modeling using the OPTIMASBA program [119]. We performed multiple FC models to derive the residual melt composition for a specific parental magma. The entire output data of the major element modeling is given in the Electronic Supplementary Table S3.

The OPTIMASBA program uses the mass balance principle, which can be represented as follows: m₀ = m_f ± m_i1 ± … ± m_in, where m₀ is the initial mass, m_f is the final mass, and m_i are the masses added to or removed (from 1 to n terms) from the initial mass.

OPTIMASBA employs multiple linear regression statistics to execute the mass balance calculations utilizing the concentration of up to 11 major oxides.

Trace element modeling was undertaken to assess the petrogenesis of the arfvedsonite granites. Selected trace elements (Rb, Ba, Sr, and Th) and rare earth elements (La, Ce, Sm, Eu, and Gd) in the arfvedsonite granites have been modeled by assuming a shoshonitic parental melt (sample SD 8b in [119]) and fractionating phases obtained from the major element modeling. The Rayleigh crystal fractionation equation ([102], p. 160) was employed:

C_L/C_O = F^(D−1)

where C_O is the concentration of the trace element in the parental liquid, C_L is the concentration of the same element in the residual liquid, D_i is the bulk distribution coefficient, and F is the fraction of melt remaining after the removal of crystal from the parental magma.

Proportions of fractionating mineral phases have been obtained from major element modeling of the FC process using OPTIMASBA software (see Supplementary Table S3). Here, we assume that crystal fractionation occurred in a closed reservoir (magma chamber), and that crystals formed and accumulated on the reservoir floor, removed and isolated from further reaction with the remaining liquid (i.e., arfvedsonite granites in this case).

The graphical representation of the results of trace element modeling is shown in Figure 12. These diagrams (Figure 12) illustrate that the compositional changes between an alkaline basalt (shoshonitic) magma (representative Sample 8b) and the arfvedsonite granite samples can be explained by fractionation of a Plag-Cpx-Olv-Amp-Sph-Ap fractionation (in proportions 38.75:1.31:3.14:54.05:2.47:0.28).

Both major oxide mass balance calculations and trace element modeling indicate that a residual melt with a composition akin to arfvedsonite granites can be achieved when approximately 10 to 12 wt% of liquid remains after extensive fractionation from a parental shoshonitic mafic magma.

5.7. Tectonic Implications

Alkaline silicate magmatism is found in continental rifts and syn-collisional and post-collisional stages of continental collision [120,121]. Syn-collisional alkaline magmas are predominantly derived from the Earth’s crust, although there are some examples of mantle-derived alkaline igneous rocks [122]. On the other hand, post-collisional magmatism occurs in collisional belts and is typically associated with transcurrent displacement along major trans-crustal shear zones [123]. Post-collisional magmatism usually occurs millions to tens of millions of years after the peak of the main collisional event. Alkaline mafic magma can originate from the lower crust, metasomatized lithospheric, and asthenospheric mantle [50].

The ~1046 Ma old peralkaline arfvedsonite granites of this study are significant because these have been emplaced during the regional shortening (syn-collisional stage of the Satpura Orogeny) and are about 50 Ma older than the post-collisional granitoids (~998 Ma old) from the North Purulia Shear Zone of the CGC [124]. Moreover, the studied granites show characteristics of A1-type granites (Figure 10), which are commonly thought to be the product of anorogenic magmatism [8,9]. However, several works have challenged the idea of correlating A1- and A2-type granites of Eby [8,9] with the anorogenic and post-orogenic settings. Recent studies have described A1-type granites from a post-collisional orogenic setting [11] and a syn-collisional setting [122].

Several studies have shown that potassic, peralkaline magma originates in exceptional conditions from an enriched lithospheric mantle source under the following conditions: plume impact [125,126], decompression melting under extensional tectonics associated with lithospheric delamination [127], lithospheric extension [128], and transtensional strike-slip shear zones [129,130].

Two contrasting hypotheses can explain the origin of syn-collisional alkaline magmatism:

• Zhao et al. [131] have suggested that the parental magma of the alkaline magmatism can be derived from the partial melting of the orogenic lithospheric mantle and the subducted continental crust during the exhumation stage of the syn-collisional phase of orogeny. These authors have explained the origin of late Triassic (~201 to 212 Ma old) syn-exhumation (syn-collisional) alkaline intrusives during the continental collision from the Sulu orogeny. The alkaline rocks are characterized by the arc-like patterns in primitive-mantle-normalized trace element diagrams, with relative enrichment of LILE and LREE but relative depletion of HFSE, suggesting a fertile mantle source. Enriched radiogenic Sr–Nd–Hf isotopic characteristics [radiogenic Sr and unradiogenic Nd and Hf isotopes (indicating long-term depletion Sr relative to Rb, Sm relative to Nd, and Lu relative to Hf—in comparison to primitive mantle) in the alkaline rocks indicate their derivation from an enriched mantle source.
• Jung et al. [122] have shown that alkaline rocks from the Otjimbingwe alkaline complex (Damara orogen) intruded through a large-scale shear zone. Monzodiorites to syenites and granites of the alkaline suite are interpreted to be the result of fractional crystallization and crustal assimilation of a parental alkaline magma that was derived by small degrees of partial melting from a phlogopite-bearing SCLM during a continent–continent collision. Mantle-normalized multi-element diagrams of these rocks show depletion in Nb (Ta), P, and Ti. However, these show strongly fractionated REE patterns, and enriched radiogenic Sr-Nd isotopic characteristics. These alkaline rocks display features similar to the anorogenic within-plate magmatism. The mantle melted due to the rise of SCLM to shallow depths after the rupture of the cold lithospheric plate. According to Jung et al. [122], the absence of arc rocks, blueschists, and eclogites and the occurrence of syn-collisional A-type granites in the Damara Orogen can be explained by a model of flat subduction of an oceanic plate below a continent.

The peralkaline granites of this study intruded before the end of regional compression and along the North Purulia Shear Zone (NPSZ, Figure 1), a main tectonic feature within the Chhotanagpur Gneissic Complex. Several authors [1,38,44,49] documented several pieces of structural evidence for transtensional strike-slip shear movement along the NPSZ. The peralkaline granites of this study show chemical characteristics of A1-type granites with elevated concentrations of Nb, Ta, Zr, and Hf (Figure 10). The slightly negative εHf values of the 1046 Ma old zircons of the arfvedsonite granites of this study argue in favor of an enriched source (Figure 6). However, the absence of voluminous mafic and intermediate rocks of a similar age refutes the possibility that the parental magma of the arfvedsonite granites was derived due to lithospheric delamination and lithospheric extension. Moreover, blueschists and eclogites have not been described from the CGC and North Singhbhum Mobile Belt (Figure 1) of the Satpura Orogenic Belt. The absence of blueschists and calc-alkaline arc granitoids occurs in a subduction zone when a low-angle subduction (flat subduction) of an oceanic plate occurs below a continental lithosphere instead of a typical subduction of an oceanic lithosphere with about 40° angles [122]. The blueschist facies metamorphic rocks rarely preserve in Proterozoic subduction zone. However, the absence of voluminous calc-alkaline arc granites from the CGC during the Satpura Orogeny suggests that dehydration of the subducted oceanic plate did not lower the solidus of the asthenospheric mantle (as it is typical for subduction zones with dips of 40°) that potentially leads to typical calc-alkaline igneous activity. Instead, fluids resulting from dehydration of the subducting layer infiltrate the lithospheric mantle and the crustal part of the overriding plate. Therefore, in line with the model of Jung et al. [122], we propose that the parental mafic magma of the studied granites was derived from the SCLM that underwent melting when transtensional strike-slip shear movement along the shear zone allowed the SCLM to move to shallower levels. The SCLM below the CGC was metasomatized by the previous subduction of an oceanic plate [129]. The mafic magma, parental to the arfvedsonite granite generated from the melting of the K-richterite- or phlogopite-bearing SCLM or a mixed asthenosphere-lithospheric mantle source (Figure 6b,c) during the syn-collisional exhumation stage of the Satpura orogeny.

6. Conclusions

The Dimra Pahar pluton belonging to the Chhotanagpur Gneissic Complex of the Eastern Indian Shield was emplaced into the fibrolite-bearing micaschist, while the latter is intricately associated with the migmatitic gray granitoid gneisses, calc-silicate rocks, and amphibolites. The foliation of the pluton is parallel to the crenulation cleavage of the micaschist, suggesting emplacement during regional compression. Mylonitic deformational structures are overprinted on the northern boundary of the pluton.

The alkali feldspar granite pluton contains arfvedsonite ± aegirine. Mesoperthites and discrete K-feldspars and albite are present in different samples. Arfvedsonite and aegirine crystallized after quartz and alkali feldspar, with aegirine being the last mineral to form.

Four deformation stages (D₁, D₂, D₃ and D₄) are dominant in the area, with D₁, D_2, and D₃ relating to regional folding and D₄ coupling to shearing. Arfvedsonite granites were emplaced parallel to the axial plane of the D₃-fold. Later on, the peralkaline rocks underwent minor metamorphism due to shearing (D₄).

These granites are dominantly peralkaline, occasionally peraluminous, and demonstrate features of A-type granites, with elevated values of silica (average 75.07 wt%), total alkali (average 8.77 wt%), Ce + Zr + Nb + Y (average 1049 ppm) and K₂O/Na₂O (>1), FeO_tot/(FeO_tot + MgO) (average 0.95), and low alumina (average 11.48 wt%), Ba, Sr, and Eu abundances. Their high Nb/Y ratios are consistent with A1-type granites.

The zircon saturation temperatures of the arfvedsonite granite samples range from 888 to 1070 °C, with an average of 942 °C which is consistent with the liquidus temperature of a water-saturated peralkaline granite magma. The magma was emplaced at a shallow depth (200–300 MPa) and had a high liquidus temperature, fO₂ (>NNO), and water saturation. The water content of the granitic melt, estimated to be around 6 wt% at 200–300 MPa, is consistent with the water content (5.8 wt%) for a peralkaline (pantelleritic) melt at 200 MPa and 850 °C.

The zircons exhibit three distinct U-Pb isotopic ages. The oldest (1324 ± 6 Ma), large-sized inherited zircons (εHf(t) = +1.65 to +7.64), show complex zoning and signs of partial resorption. The euhedral, prismatic-bipyramidal zircons displaying oscillatory zoning (εHf(t) = −3.43 to +1.43) reveal a crystallization age 1046 ± 7 Ma. Their thin periphery (εHf(t) = −3.23 to +0.27) grew during retrograde metamorphism (995 ± 6 Ma).

The arfvedsonite granites formed from mafic melt through substantial clinopyroxene and plagioclase-dominated crystallization (at ~1046 Ma). The εHf(t) values from the 1046 ± 7 Ma arfvedsonite granite population range from −3.43 to +1.43, and the two-stage model T_DM^C ages range from 1.6 to 2.04 Ga. The Hf(t) values of 1046 Ma old zircons and trace element ratios from arfvedsonite granites are consistent with the characteristics of parental magmas derived from the metasomatized (K-richterite- or phlogopite-bearing) subcontinental lithospheric mantle.

The parental mafic magma of the studied granites was derived from the SCLM that underwent melting when transtensional strike-slip shear movements along the regional shear zone allowed the SCLM to move to shallower levels. The SCLM below the CGC was metasomatized by the previous subduction of an oceanic plate. The shoshonitic mafic magma, parental to the arfvedsonite granite, was generated from the melting of the SCLM during the syn-collisional exhumation stage of the Satpura orogeny.