1. Introduction
Continental crustal evolution during the Proterozoic included extensive amalgamation of crustal blocks by collisional and accretionary orogenesis (Cawood et al. Reference Cawood, Kröner, Collins, Kusky, Mooney and Windley2009). Crustal thickening during such orogenic episodes led to high-pressure granulite facies metamorphism and associated granitic magmatism due to melting of subducting slabs or asthenospheric upwelling during slab breakoff (Vielzeuf et al. Reference Vielzeuf, Clemens, Pin and Moinet1990; Cawood et al. Reference Cawood, Kröner, Collins, Kusky, Mooney and Windley2009). The geochemistry of granitoids provides valuable information on their probable sources (mantle vs. crustal) and, thus, the origin (crustal growth vs. crustal reworking) (e.g., Rudnick, Reference Rudnick1995; Hawkesworth and Kemp, 2006a). On the other hand, the outcrop structures and microstructures of granitic plutons provide key information on their emplacement and post-crystallisation deformation mechanisms, particularly in orogenic belts (e.g., Paterson et al., Reference Paterson, Vernon and Tobisch1989, Reference Paterson, Ardill, Vernon and Žák2019). Most Precambrian-age orogenic belts have experienced polyphase tectonic deformation, metamorphism and magmatism (e.g., Tobisch et al., Reference Tobisch, Collerson, Bhattacharyya and Mukhopadhyay1994; Dasgupta et al., Reference Dasgupta, Guha, Sengupta, Miura and Ehl1997), and granitic plutons emplaced within such belts preserve the records of multiple geological events that can be deciphered using field and laboratory studies for an improved understanding of continental crustal evolution.
The Aravalli Craton of northwestern India (Fig. 1a) is one of the ancient continental nuclei (Roy & Jakhar, Reference Roy and Jakhar2002). It comprises an Archaean basement (the Mewar Gneiss), the Palaeoproterozoic Sandmata Complex (representing high-grade reworked cratonic basement) and Palaeo- to Neoproterozoic metasedimentary sequences of the Aravalli and Delhi Supergroups (Gupta, Reference Gupta1934; Heron, Reference Heron1953; Ghosh et al. Reference Ghosh, D’Souza, Goud and Prabhakar2022 and references therein). The Aravalli and Delhi Supergroups together constitute the Aravalli-Delhi Mobile Belt (ADMB). Existing studies suggest that the ADMB has a complex evolutionary history and preserves records of three major orogenic cycles, namely the Bhilwara (∼3.3–2.5 Ga), the Aravalli (∼2.2–1.7 Ga) and the Delhi (∼1.7–0.7 Ga) orogenies (Biswal et al. Reference Biswal, Pradhan, Sharma, Tiwari, Beniest, Behera, Singh, Saraswati, Bhardwaj, Umasankar, Singh, Sarkar, Mahadani and Saha2022; Ghosh et al. Reference Ghosh, D’Souza, Goud and Prabhakar2022 and references therein). Episodes of large-scale granitic magmatism in the Aravalli Craton (the Mewar Gneiss and the Sandmata Complex) and the ADMB (the Aravalli and Delhi Supergroups) appear to correspond to the above-mentioned orogenic cycles. Thus, ∼2.6–2.4 Ga granitoids in the craton (e.g., Roy & Jakhar, Reference Roy and Jakhar2002; Kaur et al. Reference Kaur, Zeh and Chaudhri2019; D’Souza et al. Reference D’Souza, Sheth, Xu, Wegner, Prabhakar, Sharma and Koeberl2020) mark magmatism during the Bhilwara orogeny and ∼1.8–1.6 Ga granitoids correspond to the Aravalli and North Delhi orogenies (e.g., Sarkar et al. Reference Sarkar, Barman and Corfu1989; Wiedenbeck et al. Reference Wiedenbeck, Goswami and Roy1996; Gupta et al. Reference Gupta, Guha and Chattopadhyay1998; Kaur et al. Reference Kaur, Chaudhri, Raczek, Kröner and Hofmann2009, Reference Kaur, Chaudhri, Raczek, Kröner, Hofmann and Okrusch2011, Reference Kaur, Chaudhri and Hofmann2015, Reference Kaur, Zeh, Chaudhri and Eliyas2017; Pandit et al. Reference Pandit, Kumar and Wang2021). In contrast, ∼1.0–0.7 Ga granitoids (e.g., de Wall et al. Reference De Wall, Schöbel, Pandit, Sharma and Just2010; Just et al. Reference Just, Schulz, de Wall, Jourdan and Pandit2011; Chatterjee et al. Reference Chatterjee, Sarkar, Roy and Manna2020) are associated with the South Delhi orogeny.
(a) Simplified geological map of the Aravalli Craton and the Aravalli-Delhi Mobile Belt of northwestern India showing its major lithostratigraphic units, namely the Mewar Gneiss basement (Banded Gneissic Complex-I), the Sandmata Complex and the Aravalli and Delhi Supergroups after Roy and Jakhar (Reference Roy and Jakhar2002). The area of the present study (shown by the square) is enlarged in (b). Blue lines are the Phulad shear zone (PSZ) and Kaliguman shear zone (KSZ). The inset map of peninsular India shows its major geological-geodynamic provinces. Abbreviations used are as follows: AC - Aravalli Craton; BuC - Bundelkhand Craton; SMB - Satpura Mobile Belt; DVP - Deccan Volcanic Province; BC - Bastar Craton; SC - Singhbhum Craton; DC - Dharwar Craton; EGMB - Eastern Ghats Mobile Belt, SGT - Southern Granulite Terrane. (b) Geological map of the study area showing the Gyangarh and Anjana plutons within the Sandmata Complex (modified after Gupta et al. Reference Gupta, Arora, Mathur, Iqballuddin, Sahai and Sharma1997). Outcrops and samples selected for structural study, petrography and mineral chemistry, whole-rock major and trace element geochemistry, LA-ICP-MS U-Pb zircon dating and EPMA U-Th-total Pb in situ monazite dating are shown on the map. Sample numbers with different prefixes were collected during different field seasons (SND = 2016, BSG = 2018, PC = 2019, MS = 2021, MC = 2022).
Late Palaeoproterozoic granitoids are widely distributed in the Sandmata Complex including, from north to south, the Rupahali, Bhinai, Ramgarh, Gyangarh, Anjana, Amet and Darwal granitoids (Sarkar et al. Reference Sarkar, Barman and Corfu1989; Wiedenbeck et al. Reference Wiedenbeck, Goswami and Roy1996; Rao et al. Reference Rao, Santosh, Purohit, Wang, Jiang and Kusky2011; Kaur et al. Reference Kaur, Zeh and Chaudhri2021). Previous studies of granitoids in the Sandmata Complex (Sarkar et al. Reference Sarkar, Barman and Corfu1989; Wiedenbeck et al. Reference Wiedenbeck, Goswami and Roy1996; Guha et al. Reference Guha, Neogi, Raza and Mondal2019; Raza et al. Reference Raza, Guha and Neogi2021) provide valuable geochemical and geochronological (mainly U-Pb zircon and some Rb-Sr isochron) data. These studies suggest that the granitoids were emplaced in a post-collisional setting after the closure of the Aravalli Basin at ∼1.9–1.8 Ga (Roy & Jakhar, Reference Roy and Jakhar2002). The Sandmata Complex is juxtaposed against the Delhi Supergroup along the Kaliguman shear zone (KSZ), which is characterised by a SW- to NW-dipping thrust system (Heron, Reference Heron1953; Bhattacharya et al. Reference Bhattacharyya, Sengupta and Mukhopadhyay1995; Ruj & Dasgupta, Reference Ruj and Dasgupta2014; Biswal et al. Reference Biswal, Pradhan, Sharma, Tiwari, Beniest, Behera, Singh, Saraswati, Bhardwaj, Umasankar, Singh, Sarkar, Mahadani and Saha2022). However, existing studies provide little or no structural data, though the granitoid plutons are notably deformed (Figs. 2, 3; see also Gangopadhyay & Lahiri, Reference Gangopadhyay and Lahiri1988). Further, the age of the KSZ is poorly constrained. According to Sinha-Roy (Reference Sinha-Roy2004), the dislocation along KSZ occurred during ∼0.8 Ga and facilitated upthrusting of the Sandmata granulites over the younger Delhi Supergroup of rocks.
Field photographs showing the general features and structures of the Gyangarh granitoids. Outcrop and sample numbers and the geographic zones to which they belong (see text) are indicated. (a) Porphyritic granitoid with mafic microgranular enclaves (MMEs). Vertical section view. (b) Well-deformed granitoid at the southern margin of the Gyangarh pluton, showing ENE–WSW-striking and gently SE-dipping mylonitic fabric. Vertical section view, coin 2.5 cm wide. The Asymmetric feldspar augen shows east-down shear movement along the mylonitic foliation. (c) NE-SW-striking and gently-dipping mylonitic foliation in the southern parts of the Gyangarh pluton. Oblique plan view, hammer 33 cm long. (d) An ∼E-W-striking S1 foliation (dashed line) folded to develop a ∼N-S-striking S2 axial planar fabric (continuous line) in the northern parts of the Gyangarh pluton. Oblique plan view, pen 15 cm long. (e) An ∼E-W-striking steeply-dipping S2 mylonitic foliation. Plan view, pen 15 cm long. (f) The stretched mafic microgranular enclave in the central part of the Gyangarh pluton shows the normal movement of the northwestern block. Vertical section view. (g) One of many local, ∼NNE-SSW-striking ultramylonite zones in the northwestern parts of the Gyangarh pluton. Note the sharp contact between the ultramylonite (foreground) and the protomylonite parts of the granitoid (with a thin quartz vein). Oblique plan view, hammer 33 cm long. (h) Mylonitic granite from the southwestern parts of the Gyangarh pluton, showing rotated feldspar augen and S-C fabrics that indicate a sinistral sense of shear, vertical section view. (i) The western margin of the Gyangarh pluton showing well-developed NNW-SSE-striking and steeply dipping mylonitic foliation. Oblique plan view, hammer 33 cm long. (j) Displacement on quartz veins in granitoid showing antithetic Riedel (dextral) shears associated with the S2 mylonitization.
(a–f) Field photographs showing the general features and structures of the Anjana granitoids. (a, b) Well-foliated blastoporphyritic granitoid showing the preferred alignment of feldspar phenocrysts. Vertical section view in (a), hammer 33 cm long. In (b), the back face is a vertical section and the foreground is a subhorizontal face; pen is 15 cm long. The asymmetric feldspar augen define a dextral-normal (west-vergent) shear sense. (c) Strongly stretched feldspar megacrysts showing spaced shears with a SE-vergent thrust. Vertical section view, pen 15 cm long. (d) Rotation of plagioclase augen and S-C fabrics showing SE-vergent thrusting. Vertical section view, pen 15 cm long. (e, f) Protomylonite (MS37) and ultramylonite (BSG19) exposures show significant grain size reduction within the pluton. Note pegmatite veins in (f), and extreme grain size reduction and stretching of quartz-feldspar-biotite minerals in the ultramylonite. The stretching lineations associated with the (g) steeply dipping S2 foliation and (h) moderately dipping S1 foliation are indicated on the respective planes.
Here, we present a study of the Gyangarh and Anjana granitoid plutons, located in the south-central part of the Sandmata Complex, to the east of the Kaliguman shear zone (Fig. 1a, b). This study includes data on outcrop-scale and micro-scale structures, petrography, geochemistry (mineral, whole-rock and zircon trace element geochemistry) and U-Pb zircon and U-Th-total Pb monazite geochronology. We use this integrated dataset to understand the tectonomagmatic framework for these granitoids and the host Sandmata Complex and discuss its implications for the Precambrian crustal evolution of northwestern India.
2. Geological setting
The Palaeoproterozoic-age Sandmata Complex is the reworked Archaean basement of the Aravalli Craton (e.g., Sharma, Reference Sharma2009; Roy et al. Reference Roy, Kröner, Rathore, Laul and Purohit2012 and references therein). The NNE-SSW-trending, ∼400 km long Sandmata Complex (Fig. 1a) is located between the Palaeoproterozoic-age Aravalli Supergroup (2.2–1.7 Ga) to the east and the Palaeo- to Neoproterozoic-age (1.7–0.7 Ga) Delhi Supergroup to the west (Roy & Jakhar, Reference Roy and Jakhar2002; Ghosh et al. Reference Ghosh, Tomson, Prabhakar and Sheth2023). The eastern parts of the Sandmata Complex comprise amphibolite facies rocks, including grey granite gneisses, migmatitic gneisses and pelitic schists (Heron, Reference Heron1953; Sharma, Reference Sharma2009). The western parts of the Complex comprise granulite-facies migmatitic gneisses, pelitic granulites, calc-granulites and mafic granulites (Guha & Bhattacharya, Reference Guha, Bhattacharya, Sinha-Roy and Gupta1995; Bhowmik et al. Reference Bhowmik, Bernhardt and Dasgupta2010; Roy et al. Reference Roy, Kröner, Rathore, Laul and Purohit2012; Ghosh et al. Reference Ghosh, Prabhakar and D’Souza2021). Numerous Palaeoproterozoic (∼1.8–1.6 Ga) granitoids, gabbronorites and charno-enderbites have been emplaced into the upper amphibolite to granulite facies rocks of the Sandmata Complex (Gangopadhyay & Lahiri, Reference Gangopadhyay and Lahiri1988; Sarkar et al. Reference Sarkar, Barman and Corfu1989; Wiedenbeck et al. Reference Wiedenbeck, Goswami and Roy1996; Buick et al. Reference Buick, Clark, Rubatto, Hermann, Pandit and Hand2010; Rao et al. Reference Rao, Santosh, Purohit, Wang, Jiang and Kusky2011; Guha et al. Reference Guha, Neogi, Raza and Mondal2019; Kaur et al. Reference Kaur, Zeh and Chaudhri2021).
The largest intrusions in the Sandmata Complex are the Gyangarh and Anjana granitoid plutons (Sarkar et al. Reference Sarkar, Barman and Corfu1989). Choudhary et al. (Reference Choudhary, Gopalan and Sastry1984) reported a whole-rock Rb-Sr isochron age of 1870 ± 200 Ma (2σ) for the Gyangarh pluton (Fig. 1b). Sarkar et al. (Reference Sarkar, Barman and Corfu1989) obtained a U-Pb zircon crystallisation age of 1723 ± 14 Ma (2σ) for the same pluton, based on the upper intercept age of discordant points. Wiedenbeck et al. (Reference Wiedenbeck, Goswami and Roy1996) obtained U-Pb zircon ages of 1766 ± 16 (2σ) Ma and 1646 ± 19 Ma (2σ) for the Anjana pluton and an aplite dyke. According to Gangopadhyay & Lahiri (Reference Gangopadhyay and Lahiri1988), the Anjana pluton intruded into the Sandmata high-grade gneisses during their second deformation phase. Recently, Ghosh et al. (Reference Ghosh, Tomson, Prabhakar and Sheth2023) presented structural and geochronological data on quartzofeldspathic gneisses located to the east and south of the Gyangarh pluton. They showed that these gneisses preserve the records of tectonothermal events at 1743–1647 Ma (2σ) and 901–783 Ma (2σ), which they correlated with the emplacement of the Gyangarh and Anjana plutons and the South Delhi orogeny, respectively. Based on geochemical studies, Raza et al. (Reference Raza, Guha and Neogi2021) identified the Anjana granitoids as sanukitoids, produced in an arc setting. However, the crystallisation conditions of these granitoids remain unknown.
3. Field relationships and structures
The Gyangarh and Anjana granitoid plutons intrude high-grade migmatitic quartzofeldspathic gneisses of the Sandmata Complex (Ghosh et al. Reference Ghosh, Tomson, Prabhakar and Sheth2023, Fig. 1b). Ghosh et al. (Reference Ghosh, Tomson, Prabhakar and Sheth2023) showed that these gneisses have undergone three deformation events. The earliest deformation event (D1) is represented by gneissic layering comprising the S0//S1 fabric. A subsequent deformation event (D2) is marked by the development of tight- to isoclinal folds on S0//S1 and the formation of a NNE- to NNW-striking, gently to moderately dipping S2 axial planar fabric. The formation of NE-SW-trending asymmetrical folds with spaced S3 foliation marks the third deformation event (D3) in the gneisses (Ghosh et al. Reference Ghosh, Tomson, Prabhakar and Sheth2023).
The Gyangarh pluton exposed over ∼470 km2 is composed essentially of granitoids but includes a few mafic rocks (Fig. 1b); however, the genetic relationships between these rocks are unclear. The Gyangarh granitoids are grey, medium- to coarse-grained and contain mafic microgranular enclaves (MMEs) at several places (Fig. 2a). They range from weakly foliated (Fig. 2a) to strongly foliated (mylonitic) and ultramylonitic types (Fig. 2b–e, g–h). The Anjana pluton covers ∼60 km2. It is a feldspar-megacrystic granitoid (Fig. 3a, b), which has been deformed in certain parts to produce mylonitic (Fig. 3c–e) and locally ultramylonitic (Fig. 3f) structures. To document the structural deformation of the Gyangarh and Anjana plutons (Fig. 1b), we divided them into four different zones based on variations in the intensity of mylonitization as perceived in outcrop (Fig. 4a). Zones Z1, Z2 and Z3 represent the eastern, northwestern and southwestern parts of the Gyangarh pluton, whereas zone Z4 represents the Anjana pluton (Fig. 4a). Stereograms were plotted for the visualization of foliation and stretching lineation data collected from the different zones (Fig. 4b–e).
Generalized geological map showing planar and linear structures in the Gyangarh and Anjana plutons. Due to variations in fabric orientation, structural data for the Gyangarh pluton are presented separately for the eastern (Z1), northwestern (Z2) and southwestern (Z3) zones. Equal-area projections for the Gyangarh pluton (zones Z1, Z2 and Z3) and the Anjana pluton (zone Z4) show the pole distribution of gently-dipping (S1) and steeply-dipping (S2) mylonitic foliations, as well as associated stretching lineations SL1 (red arrows) and SL2 (blue arrows). The high-strain zones and corresponding shear senses in the various zones of the batholith are indicated on the map. Abbreviations: SR - Sinistral Reverse; SN - Sinistral Normal; SL - stretching lineation.
3.a. The Gyangarh pluton
Zone Z1, constituting ∼50% of the Gyangarh pluton, exposes variably deformed granitoids. The granitoids locally preserve a weak foliation (Fig. 2a, f), with a major part of the pluton showing pervasive mylonitic (with feldspar augen) to gneissic foliations (Fig. 2b–e, g). These structures are characterized by ductile and brittle-ductile deformation features, as indicated by lenticular recrystallised feldspar augen with S-C fabrics (Fig. 2b, c, g–i), microfaults (Fig. 2f) and Riedel shears (Fig. 2j), respectively. Shallow to moderately dipping mylonitic foliations (mean S1: 014/26°E) with shallow (< 30°) NE- to NW-plunging SL1 stretching lineations are observed along the southern and eastern parts of this zone (Fig. 2b–c, 4b). In the northern parts of the pluton, the shallow-dipping mylonitic foliation (S1) has produced asymmetric folds with a NNE-SSW-striking axial planar fabric (S2; Fig. 2d), which has locally developed a pervasive foliation (010/80°W; Fig. 2d). The ~N-S and ~E-W steeply dipping mylonitic foliations (S2C and S2S) in this zone show a mean orientation of 221/87° NW with moderately plunging (largely < 50°) SL2 stretching lineations towards NE direction (Fig. 2e, 4a, b). The variation in pole distribution of the shallow-dipping mylonitic foliation (S1) and corresponding lineation data (SL1) indicate that the shallow-dipping S1 mylonitic foliation has been overprinted by a steep S2 mylonitic foliation (Fig. 4b).
Zone Z2 represents ∼20% area of the Gyangarh pluton (Fig. 4a). The western boundary of Z2 has been affected by the NE-SW-striking Kaliguman shear zone (KSZ, Fig. 1a), which is reflected in the intense mylonitization, cross-cutting shears and low-temperature solid-state deformation microstructures (Fig. 2f, g, j, 4a). This zone is dominated by a NE-SW-striking mylonitic foliation (S2), which dips steeply to the northwest or the southeast with an average orientation of 028/82° W (Fig. 2f, g, j, 4a, c). The mesoscale S-C fabrics suggest a sinistral sense of shearing during mylonitization (Fig. 4a). The shallow-dipping mylonitic foliation (S1) is weakly preserved and strikes NNW-SSE to NNE-SSW with gentle dips to the northeast or southeast (mean S1: 359/29°E, Fig. 4c). The variation in the pole distribution of shallow-dipping mylonitic foliation (S1) suggests that the S1 foliation was affected by steeply dipping S2 mylonitic fabric (Fig. 4c). The limited stretching lineation data from Z2 suggest that SL2 is moderately plunging towards NNE.
Zone Z3 represents ∼30% area of the Gyangarh pluton (Fig. 4a). The granitoids in this zone are intensely sheared due to the effect of the KSZ and have developed ductile deformation structures (Fig. 2h). The S1 fabrics are poorly preserved in Z3 and represented by NE-SW-striking foliations with variable dips to the northwest (mean S1: 031/19°W; Fig. 4a, d). The well-developed S2 mylonitic foliation strikes predominantly NE-SW with variable steep dips to the northwest and southeast (mean S2: 045/82°W, Fig. 4a, d). The SL2 stretching lineation plunges moderately to steeply to the northeast and east. S-C fabrics show sinistral shearing along the western margin of Z3 (Figs. 2h, 4a).
Overall, it is evident that the shallow dipping S1 mylonitic foliation in the granitoids is equivalent to the shallow dipping S2 foliation observed in the surrounding quartzofeldspathic gneisses (cf. Ghosh et al. Reference Ghosh, Tomson, Prabhakar and Sheth2023). The steep ENE-striking foliation in the central parts of the pluton and NNE-striking foliation in the marginal parts of the pluton are synchronous during D2 deformation and considered as S2S and S2C fabrics, respectively. These S2S and S2C fabrics constitute S-C structures developed during D2 deformation (Fig. 2e, i). The ESE-trending Riedel shears associated with the D2 deformation show a dextral sense of movement on the quartz veins (Fig. 2i).
3.b. The Anjana pluton
Zone Z4, represented by the Anjana pluton, is composed of feldspar-megacrystic and foliated blastoporphyritic monzogranites (Fig. 3a–e, h). The pluton shows megacrysts (up to ∼10 cm in length) of plagioclase and K-feldspar in outcrop (Fig. 3a–b). As with the Gyangarh pluton, marginal parts of the Anjana pluton have undergone strong grain size reduction, flattening and lenticular augen formation through the development of NE-SW-striking S2 mylonitic foliations with variable steep dips to the northwest or southeast (mean S2: 029/83°W; Fig. 3c–f, 4a, e). The SL2 stretching lineation mostly plunges moderately to steeply to the northeast, closer to the strike of the S2 mylonitic foliation (Fig. 3g; 4a, e). The pluton locally preserves shallow-dipping mylonitic foliations (S1; Fig. 4a, e). These foliations have variable strike (N-S to NE-SW) with moderate dips to the east or to the southeast (mean S1: 002/32°E; Fig. 4e), and the corresponding SL1 lineation plunges moderately towards NW and SE (Fig. 3h; 4a, e). The asymmetric feldspar augen and S-C fabrics associated with S2 mylonitic foliation indicate sinistral shearing (Fig. 3e) with a reverse (Fig. 3c, d) movement in the Anjana pluton.
4. Samples and analytical methods
We carried out geological and structural mapping of the Gyangarh and Anjana granitoids, and collected structural data (on planar and linear structures) from 60 outcrop locations covering the area shown in Fig. 1b. We also collected oriented rock samples from five locations for kinematic studies. Petrographic study was done on 22 representative samples of the Gyangarh and Anjana granitoids collected from zones Z1 to Z4. Eight rock samples were analyzed for mineral chemistry using a CAMECA SX-FIVE Electron Probe Microanalyser (EPMA) at the Department of Earth Sciences, IIT Bombay (India). Twenty-two whole-rock samples (14 from the Gyangarh pluton and 8 from the Anjana pluton) were analyzed for the major and trace elements (Fig. 1b) using Inductively Coupled Plasma Atomic Emission Spectrometry (ICPAES) at the Sophisticated Analytical Instrument Facility (SAIF) at IIT Bombay, and Inductively Coupled Plasma Mass Spectrometry (ICPMS) at the Department of Earth Sciences, IIT Bombay, respectively.
Two representative samples from each pluton were selected for zircon U-Pb geochronology and zircon trace element analyses. In-situ LA-ICP-MS U-Pb zircon analyses and trace element characterization of the zircon grains were performed at the Isotope Geochemistry Facility, National Centre for Earth Science Studies (NCESS, Thiruvananthapuram) using the Teledyne CETAC, Nd:YAG (213nm) solid-state laser coupled to an Agilent 7800 quadrupole ICPMS. Two samples from the Gyangarh pluton were also selected for U-Th-total Pb in situ monazite geochronology; monazites are rare in the Gyangarh granitoids and were not found in the Anjana granitoids. Monazite dating was performed using a CAMECA SX-FIVE Electron Probe Microanalyser at the Department of Earth Sciences, IIT Bombay. Additionally, pyroxene exsolution thermometry, zircon saturation thermometry and Ti-in-zircon thermometry were carried out. The sample preparation methodology, analytical conditions and data reduction procedures are described in Appendix A.
5. Results
5.a. Petrography
Medium- to coarse-grained porphyritic varieties of the Gyangarh granitoids are composed of plagioclase (30%–40%), K-feldspar (20%–25%) and quartz (25%–30%), with minor biotite (5%–10%) and amphibole (2%–5%). In mylonitic zones, the combined abundance of biotite and amphibole reaches up to ∼20%. The K-feldspars are microcline with minor perthite, and show tartan twinning (MS01, Fig. 5a). Plagioclase shows polysynthetic twinning. Quartz grains are generally serrated and show undulose extinction, and rarely occur as blocky subgrains surrounded by fine-grained quartz aggregates (MS08, Fig. 5b). Small charnockitic bodies containing orthopyroxene (∼5%) with exsolved clinopyroxene are occasionally seen in the central parts of the Gyangarh pluton (PC3, Fig. 5c). Other accessory minerals include titanite, epidote, apatite, magnetite, ilmenite, muscovite, chlorite and zircon (Table 1), with minor garnet being found in only one sample (MS12).
Cross-polarized light (XPL) microphotographs showing petrographic characteristics of the Gyangarh granitoids (a–c) and the Anjana granitoids (d–e). All mineral abbreviations are after Whitney and Evans (Reference Whitney and Evans2010). (a) Weakly-deformed granitoid showing perthitic texture in K-feldspar and lamellar twinning in plagioclase grains. (b) Subgrains and chessboard twinning in quartz-rich domains from the central part of the pluton (c) Occasionally, Gyangarh granitoids are composed of orthopyroxene with clinopyroxene exsolutions. (d) The preferred orientation of biotite grains and deformed quartz grains define the orientation of mylonitic foliation in Anjana pluton. (e) Deformation twinning in locally kinked and deformed plagioclase phenocryst, surrounded by subgrains of quartz showing grain boundary migration in the Anjana pluton. (f) Ribbon quartz stretched along the foliation defined by mica.
Sample locations and mineralogy of the Gyangarh-Anjana granitoids studied for whole-rock geochemistry
$ Mineral abbreviations after Whitney and Evans (Reference Whitney and Evans2010); #Samples selected for zircon dating.
The Anjana granitoids are coarser-grained than the Gyangarh granitoids and show blastoporphyritic texture. They commonly contain K-feldspar (30%–50%) and plagioclase (5%–10%) megacrysts (up to 10 cm in size). The groundmass is composed of quartz (25%–30%), plagioclase and K-feldspar with minor biotite (∼10%) and amphibole (2%–5%). The plagioclase megacrysts show deformation twins and bending of lenticular twin lamellae (BSG19, Fig. 5d). The K-feldspar megacrysts are microcline; they show tartan twinning and the presence of flame perthites (BSG19, MS40, Fig. 5d, e). Stretched quartz ribbons show undulose extinction, with polygonised subgrains forming triple junctions (BSG19 and MS40; Fig. 5d–f). Biotite grains define the pervasive foliation in mylonitic zones (BSG19 and MS32; Fig. 5d, f). Accessory minerals are zircon, apatite, ilmenite, magnetite and sphene (Table 1).
5.b. Deformation microstructures
Oriented mylonite samples were collected from different zones of the Gyangarh-Anjana plutons, including zones Z1 (MS4), Z2 (MS18), Z3 (MS25 and MS27) and Z4 (MS33). Sample MS4 is dominated by a shallow- to moderately dipping S1 mylonitic foliation, whereas samples MS18, MS25, MS27 and MS33 are characterized by a steeply-dipping S2 mylonitic foliation. The oriented samples were collected by marking the north direction, foliation strike and dip and trace of the stretching lineation on the outcrop faces (Fig. 6). Thin sections were prepared by cutting the oriented samples perpendicular to foliation and parallel to lineation (i.e., XZ sections or L sections) and perpendicular to both foliation and lineation (i.e., YZ sections or T sections). The polished slabs showing shear senses (i.e., S-C fabrics and rotated phenocrysts) on XZ and YZ sections, from different zones of the Gyangarh and Anjana plutons, are shown in Fig. 6a–d. The deformation microstructures from these samples are summarized in Fig. 7a–e.
Schematic diagrams and polished slabs of XZ and YZ sections showing shear sense indicators from (a–c) Gyangarh (Z1, Z2 and Z3) and (d) Anjana (Z4) mylonitic granitoids. Block diagrams in (a–d) show the orientation of foliation and lineation from their respective locations. Oriented thin sections were prepared by cutting the rock samples perpendicular to foliation and parallel to lineation (i.e., XZ sections or L sections) and perpendicular to both foliation and lineation (i.e., YZ sections or T sections). (a) Asymmetric feldspar porphyroclasts in granite mylonite (MS4; Z1) from the southern part of the Gyangarh pluton showing dextral normal shearing with top-down-to-NE along the shallow-dipping S1 mylonitic foliation. (b–c) Deformed σ-type plagioclase porphyroclasts and S-C-C′ fabrics showing sinistral reverse shearing along S2 mylonitic foliation in the northwestern (Z2) and southwestern (Z3) parts of the Gyangarh pluton. (d) σ-type K-feldspar porphyroclasts within fine-grained quartz and biotite matrix indicating sinistral reverse shearing along S2 mylonitic foliation from the Anjana pluton (Z4).
Plane-polarized light (PPL) and cross-polarized light (XPL) microphotographs showing deformation microstructures in various zones of the Gyangarh granitoids (a–d) and the Anjana granitoids. (e). All mineral abbreviations are after Whitney and Evans (Reference Whitney and Evans2010). Yellow, red and dashed lines indicate C, C′ and S shear planes, respectively. The triangular pink, blue, yellow and peach-coloured arrowheads represent subgrain rotation recrystallisation (SGR), grain boundary migration recrystallization (GBM), SGR-GBM transition and GBM-GBAR (grain boundary area reduction) transition, respectively. The details of deformation microstructures are summarized in Table 2. (a) XZ (L section) of southern margin granite (MS4) shows S-C-C′ fabrics, elongated grains of quartz, plagioclase, K-feldspar and biotite, suggesting dextral shearing (S1), XPL. (b) YZ (T section) view of granite mylonite from northwestern parts of the pluton showing feldspar porphyroclasts within fine-grained recrystallized quartz grains and elongated biotite aggregates enveloping feldspar porphyroclasts with the development of S-C-C′ fabrics. These fabrics suggest solid-state deformation with sinistral sense of shearing (S2), XPL. (c) Dynamically recrystallized plagioclase porphyroclasts of plagioclase showing core-mantle structure with quartz ribbons and biotite aggregates wrapping around the porphyroclasts. The S-C-C′ fabrics developed around plagioclase indicate a sinistral sense of shearing (S2) on XZ (L section). (d) The δ-type plagioclase porphyroclasts with mantle and winged tails indicate top-to-east movement in the intensely mylonitized zone (Z3) of the Gyangarh pluton, PPL. (e) Dynamically recrystallized quartz and biotite aggregates showing shape-preferred orientation, defining mylonitic foliation (S2) with sinistral shearing in the Anjana pluton, XPL.
Zone 1: The shallow- to moderately dipping S1 mylonitic foliation in the southern part of the Gyangarh pluton (Z1) dips towards NW with gently plunging stretching lineation (Fig. 6a). The S-C-C′ fabrics and lensoidal feldspar augen on XZ and YZ sections show dextral normal shearing with top-down-to-the-NE movement (Fig. 6a, 7a). The quartz grains show ribbon and polycrystalline aggregates with irregular grain boundaries. The kinked plagioclase grains show inhomogeneous extinction with tapered twin lamellae (Fig. 7a; Table 2). These microstructures indicate a transition from subgrain rotation recrystallisation (SGR) to grain boundary migration (GBM) in the quartz and feldspar grains (Fig. 7a; Table 2).
Summary of deformation microtextures observed in quartz and feldspar minerals of Gyangarh and Anjana granitoids
Abbreviations: P-Porphyroclast; M-Matrix; SGR-Subgrain Rotation Recrystallization; GBM-Grain Boundary Migration recrystallization; GBAR-Grain Boundary Area Reduction.
Zone 2: The steeply NW-dipping S2 mylonitic foliation in the northwestern part of the Gyangarh pluton (Z2) shows a moderately plunging stretching lineation (Fig. 6b). The S-C fabrics and stretched feldspar augen show a sinistral reverse sense of shearing (Fig. 6b). The S-C-C′ foliation, defined by elongated quartz, biotite and fine-grained feldspar aggregates (S2), envelops the feldspar porphyroclasts and show a sinistral sense of shearing (Fig. 7b; Table 2). Further, feldspar grains show intragranular brittle fractures and deformation twin lamellae (Fig. 7b). These microstructures indicate a transition from SGR to GBM in the quartz and feldspar grains (Fig. 7b; Table 2).
Zone 3: The granitoids in the southwestern part (Z3) of the Gyangarh pluton are characterized by steeply NW-dipping S2 mylonitic foliation (Fig. 6c), preserving σ-type, δ-type feldspar porphyroclasts and S-C fabrics (Fig. 6c and 7c–d), showing a sinistral reverse sense of shearing (Fig. 6c). The structures are similar to those observed in Z2 (Fig. 7b). The oriented feldspar porphyroclasts are surrounded by quartz ribbons with polycrystalline aggregates, indicating core-mantle structures (Fig. 7c; Table 2). This texture is overprinted by intragranular fractures and deformation twin lamellae in feldspar (Fig. 7c), suggesting that dynamic recrystallisation of quartz and feldspar occurred between SGR-GBM transition and GBM zones (Fig. 7c; Table 2).
Zone 4: The mylonitized Anjana granitoids in the marginal zones show lensoidal feldspar augen with inhomogeneous extinction. Quartz shows ribbon (SGR-GBM) (Fig. 7e, Table 2) and polygonal aggregate (Grain Boundary Area Reduction, GBAR) (MS40, Fig. 5e, Table 2) structures. These structures are overprinted by S-C-C′ fabrics, indicating a sinistral reverse sense of shearing along the S2 mylonitic foliation on XZ and YZ sections (Fig. 6d).
5.c. Mineral chemistry
Representative mineral analyses of Gyangarh (PC3, BSG64, MS12 and MS13) and Anjana granitoids (BSG19, BSG20, SND3 and MS40) are given in Table S1. Phenocrysts of plagioclase (An1.7-53Ab45-98Or0.0-2.1) and K-feldspar (An0.0-1.2Ab4.5-18Or76-95) in the Gyangarh granitoids show wide compositional variation (Fig. 8a). Orthopyroxenes show ferrosilite to pigeonite compositions with a variation in XFe (0.51–0.61) and XCa (0.01–0.08) values (Fig. 8b). Exsolved lamellae of clinopyroxene within orthopyroxene show diopside composition with XFe and XCa values of 0.19–0.24 and 0.44–0.47, respectively (Fig. 8b). Biotites are annite with XFe ranging widely between 0.50 and 0.79 (Fig. 8c). Amphibole compositions mainly plot within the ferro-tschermakite hornblende field with XMg values varying between 0.22 and 0.46 (Fig. 8d). Rarely found garnet (Alm60-63Pyp8-10Spss5-8Grs20-26) in sample MS12 shows a uniform composition (Table S1).
Mineral chemistry plots showing compositional variations in minerals in the Gyangarh pluton (samples BSG64, PC3, MS12 and MS13) and the Anjana pluton (BSG19, BSG20, SND3 and MS40). (a) Ternary feldspar plot showing that feldspar compositions vary between orthoclase and plagioclase feldspars. (b) Pyroxene quadrilateral plot showing diopside and pigeonite-ferrosilite compositions for the exsolved and host pyroxenes, respectively. (c) Fe/(Fe+Mg) versus Al (total) plot for biotites, and (d) XMg versus Si in formula plot for amphiboles.
The Anjana granitoids are mineralogically similar to the Gyangarh granitoids but lack pyroxenes, and are notably coarser-grained to megacrystic. They are rich in alkali feldspar with the composition An0.0-3.0Ab4.7-18.9Or81-95, whereas plagioclase compositions range between andesine and albite (An0.5-45Ab56-98Or0.3-8.7) (Fig. 8a). Biotites are compositionally similar to those in the Gyangarh granitoids, with XFe values between 0.60 and 0.79 (Fig. 8c). Amphibole compositions plot within the ferro-tschermakite hornblende to ferro-tschermakite fields, and the amphiboles show lower XMg values of 0.17–0.26 as compared to those in the Gyangarh granitoids (Fig. 8d).
5.d. Whole-rock major and trace element geochemistry
Whole-rock compositional data for the Gyangarh and Anjana granitoids are tabulated in Tables 3 and 4 and the rocks are classified in Fig. 9a–e. The modal abundances of quartz, K-feldspar and plagioclase in the Gyangarh and Anjana granitoids show that most of them are monzogranites (Fig. 9a). Two samples from the Gyangarh pluton plot in the quartz monzonite and quartz monzodiorite fields in the QAP diagram (Fig. 9a). The Gyangarh monzogranites have intermediate to high contents of SiO2 (57.98–73.96 wt.%, calculated on an anhydrous basis) and Na2O + K2O (5.70–8.03 wt.%), whereas the Anjana monzogranites show narrower ranges of SiO2 (62.69–68.07 wt.%) and Na2O + K2O (6.42–7.38 wt.%). All rocks are metaluminous based on their combined A/CNK values of 0.80–0.99 and A/NK values > 1.00 (Fig. 9b, Shand, Reference Shand1943), and they belong to the calc-alkaline series on the modified alkali-lime index (MALI) plot (Fig. 9c, Frost and Frost, Reference Frost and Frost2008). Binary Harker plots show negative correlations of Al2O3, CaO, TiO2, MgO, P2O5 and FeOtotal with SiO2 and a positive correlation between K2O and SiO2 (Fig. S1).
Whole-rock major oxide compositions (in wt.%) of the Gyangarh-Anjana granitoids
FeOT = total FeO; Fe# = FeOT/(FeOT + MgO).
Whole-rock trace element compositions (in ppm) of the Gyangarh-Anjana granitoids
bdl: below detection limit.
(a) The modal QAP (Quartz-Alkali feldspar-Plagioclase feldspar) ternary plot (Streckeisen et al., Reference Streckeisen, Zanettin, Le Bas, Bonin, Bateman, Bellieni, Dudek, Efremova, Keller, Lamere and Sabine2002) showing the nomenclature of the Gyangarh-Anjana granitoids. (b) (A/NK) versus (A/CNK) plot (after Shand, Reference Shand1943) for the Gyangarh-Anjana granitoids. (c) Modified alkali-lime index plot (after Frost and Frost, Reference Frost and Frost2008). (d, e) Chondrite-normalized rare earth element plot and primitive mantle-normalized multielement plot (normalizing values after McDonough and Sun, Reference McDonough and Sun1995).
Total rare earth element (ΣREE) contents of the Gyangarh and Anjana granitoids are variable in the ranges of 288–785 ppm and 124–868 ppm, respectively (Table 4). Chondrite-normalized REE patterns of these granitoids are characterized by enrichment of the light REE with the nearly flat patterns of the heavy REE (Fig. 9d). The (La/Yb)CN ratios of the Gyangarh and Anjana granitoids range between 11.2–17.8 and 6.1–16.9, respectively. Most samples show negative Eu anomalies (calculated as EuCN/√SmCN*GdCN) with values ranging from 0.42 to 0.79 for the Gyangarh granitoids and 0.55 to 0.80 for the Anjana granitoids. Primitive mantle-normalized trace element patterns of the Gyangarh and Anjana granitoids are similar to each other and show negative Nb, Ce, Sr, Zr and Ti anomalies (Fig. 9e). However, all the Gyangarh and Anjana granitoids show enrichment in the large ion lithophile elements (LILE) and depletion in the high-field strength elements (HFSE) (Fig. 9e).
5.e. In situ U-Pb zircon geochronology
In-situ LA-ICP-MS U-Pb zircon analyses and trace element characterization of the zircon grains were performed at the Isotope Geochemistry Facility, NCESS (Thiruvananthapuram) using the Teledyne CETAC, Nd:YAG (213nm) solid-state laser coupled to an Agilent 7800 quadrupole ICPMS. Details of the analytical procedure, instrument operation conditions, and data reduction methodology are provided in Dev et al. (Reference Dev, Tomson, Sorcar and Francis2022). All data were plotted using Isoplot 3.75 software (Ludwig, Reference Ludwig2012). All errors on isotope ratios and ages are reported at the 2σ confidence level. The weighted mean of the calculated 206Pb/238U dates of the zircon standards yielded 1062 ± 9 Ma, 564 ± 4 Ma and 338 ± 3 Ma for 91500, BB11, and Plêsovice zircon standards, respectively (Table S3). These values are consistent with published values of 91500 (1066 ± 3 Ma, Wiedenbeck et al., Reference Wiedenbeck, Alle, Corfu, Griffin, Meier, Oberli, Quadt, Roddick and Spiegel1995), BB11 (558 ± 2 Ma; Santos et al., Reference Santos, Lana, Scholz, Buick, Schmitz, Kamo, Gerdes, Corfu, Tapster, Lancaster and Storey2017), and Plêsovice (337 ± 0.4 Ma, Sláma et al., Reference Sláma, Košler, Condon, Crowley, Gerdes, Hanchar, Horstwood, Morris, Nasdala, Norberg and Schaltegger2008) zircon standards.
Two monzogranite samples each from the Gyangarh pluton (BSG63 and PC6) and the Anjana pluton (PC9 and SND3, Fig. 1b) were selected for in-situ LA-ICP-MS zircon U-Pb geochronology. Sample PC6 collected from zone Z1 is weakly foliated, whereas sample BSG63, also from Z1, has a dominant shallow-dipping S1 mylonitic foliation. Samples PC9 and SND3, collected from Z4, are characterized by S1 and S2 mylonitic foliations, respectively. Polished thin sections of the four samples were subjected to back-scattered electron (BSE) and cathodoluminescence (CL) imaging to locate zircon grains and to study rock textures and the internal compositional zoning of the zircons. The U-Pb isotope ratios and ages of analyzed zircon grains from the samples are given in Table S4. Representative zircon images (BSE and CL), the locations of analyzed spots and corresponding dates are shown in Fig. 10(a-h). Age vs. Th/U ratios, Wetherill concordia plots and their corresponding 207Pb/206Pb weighted mean dates are shown in Fig. 11(a-h).
Back-scattered electron (BSE) images and cathodoluminescence (CL) images of representative zircon grains in the Gyangarh monzogranites PC6 and BSG63 (a–d) and the Anjana monzogranites PC9 and SND3 (e–h). Minerals in the BSE images (a, c, e and g) are abbreviated following Whitney and Evans (Reference Whitney and Evans2010). Locations of the spot analyses (30 µm beam diameter) and corresponding 207Pb/206Pb dates (see text for details) are shown on the zircon CL images (b, d, f and h).
Zircon Th/U versus 207Pb/206Pb plot, U–Pb concordia plots and 207Pb/206Pb weighted mean age diagrams for (a–d) Gyangarh monzogranites PC6 and BSG63, and (e–h) Anjana monzogranites PC9 and SND3. For the 207Pb/206Pb weighted mean age diagrams (d, h), data from all samples from the same pluton have been combined.
Monzogranite PC6 was collected from the south-central part of the Gyangarh pluton (Z1; Fig. 1b, 4). Zircons in this sample are mostly found as inclusions within amphibole (Fig. 10a). The zircon grains are mostly elongated and prismatic, with long axes measuring 100–500 μm (Fig. 10a–b). Under CL imaging, zircons show weakly oscillatory-zoned dark cores that are surrounded by bright, oscillatory-zoned overgrowths (Fig. 10b). The Th/U ratios of the analyzed spots lie between 1.34 and 2.58, indicating a magmatic origin for the zircons (Fig. 11a, Table S3; cf. Rubatto, Reference Rubatto2002). Nine concordant spots analysed in six grains define a weighted mean 207Pb/206Pb date of 1744 ± 42 Ma (MSWD = 1.07, Fig. 11b).
Monzogranite BSG63 was collected from the northern margin of the Gyangarh pluton (Z1; Fig. 1b, 4). Zircons in this sample occur in both quartzofeldspathic and biotite-rich domains (Fig. 10c). The zircon grains are mostly elongated and prismatic, with their long axes measuring 50–500 μm, and show well-defined oscillatory growth zoning characteristic of their magmatic origin (Fig. 10d). Thirteen spot analyses on zircon grains yielded a 207Pb/206Pb age distribution between 1662 ± 118 Ma and 1924 Ma ± 126 Ma (Table S3). The analyzed spots have Th/U ratios between 0.90 and 2.44, indicating a magmatic origin for the zircons (Fig. 11a, Table S3; cf. Rubatto, Reference Rubatto2002). The 207Pb/206Pb dates of these thirteen spots define a weighted mean 207Pb/206Pb date of 1794 ± 49 Ma (MSWD = 2.0, Fig. 11c). The combined zircon ages for Gyangarh monzogranites PC6 and BSG63 yield a weighted mean 207Pb/206Pb date of 1776 ± 35 Ma for the Gyangarh pluton (MSWD = 1.7, n = 22; Fig. 11d, Table S3).
Monzogranite PC9 was collected from the central part of the Anjana pluton (Z4; Fig. 1b, 4). The zircon grains occur in the quartzofeldspathic matrix and biotite-rich domains, invariably associated with titanite (Fig. 10e). The zircon grains are mostly short (long axes of 80–150 μm) with poorly developed prismatic shapes, whereas a few grains are elongated (long axes > 250 μm), irregular and wedge-shaped. CL imaging of the zircon grains exhibits well-developed oscillatory-zoned cores that are occasionally surrounded by homogenous CL-bright overgrowths (Fig. 10f). In the wedge-shaped grains, faint traces of patchy zoning were also observed. Fifteen spot analyses on zircon grains yielded Th/U ratios between 0.22 and 2.11 with a weighted mean 207Pb/206Pb date of 1727 ± 53 Ma (MSWD = 2.3; Fig. 11f, Table S3).
Monzogranite SND3 was collected from the central part of the Anjana pluton (Z4; Fig. 1b, 4). Zircons dominantly occur within the biotite-rich domains (Fig. 10g) and are elongated (long axes varying from 100–600μm) with poorly developed prismatic faces. A few grains are short (long axes < 100 μm), rhombic, or wedge-shaped (Fig. 10g). In the CL images most zircons show well-developed oscillatory zoning, although some grains comprise CL-dark cores surrounded by planar overgrowths (Fig. 10h). Eighteen spots were analyzed in zircon grains from different domains, yielding Th/U ratios between 0.71 and 1.83 with a weighted mean 207Pb/206Pb date of 1684 ± 35 Ma (MSWD = 0.62; Fig. 11g, Table S3). The combined zircon ages for Anjana monzogranites PC9 and SND3 yield a weighted mean date of 1706 ± 29 Ma for the Anjana pluton (MSWD = 1.4, n = 33; Fig. 11h).
5.f. In situ U-Th-total Pb Monazite geochronology
Monazite dating was performed for two granitoids (BSG77 and BSG41) collected from the southeastern part (Z1) of the Gyangarh pluton (Fig. 1b). Based on outcrop-scale structures and porphyroclast-matrix relationships, BSG77 is a protomylonite, showing a shallow-dipping S1 mylonitic foliation (Fig. 12a). In contrast, BSG41 is an ultramylonite with a pervasive S2 mylonitic foliation (Fig. 12b). The results of monazite dating are presented in Fig. 12c–g and Table S5.
Field photographs, Back-scattered electron images, X-ray element maps (Y L a and Th M a ) and probability distribution plots showing the mylonitic characteristics, textural occurrence and monazite compositional variation, respectively from (a–e) Gyangarh protomylonite BSG77 and (f–g) Gyangarh ultramylonite BSG41. On outcrop scale, protomylonite (BSG77) and ultramylonite (BSG41) samples are characterized by shallow-dipping (S1; Section view, hammer 33 cm long) and steeply dipping (S2; Plan view, marker 13.8 cm long) mylonitic foliations, respectively. The monazite dates (in Ma ± 2σ) are indicated on the X-ray element maps. Probability density plots of the monazite age data yielded unmixed age populations of 1776 ± 15 Ma, 1708 ± 19 Ma and 1653 ± 30 Ma (n = 31; relative misfit = 0.808) and 933 ± 11 Ma to 897 ± 9 Ma (n = 37; relative misfit = 0.952) for BSG77 and BSG41, respectively.
In protomylonite BSG77 the monazite grains occur as inclusions in quartz, feldspar and biotite and align along the major foliation defined by quartz-feldspar-biotite aggregates (Fig. 12c-d). X-ray element mapping of texturally constrained monazite grains shows three distinct domains (Fig. 12e). The first domain type is characterized by high Th and low Y contents (ThO2 = 6.03–7.98 wt.% and Y2O3 = up to 0.50 wt.%), with an age range of 1739 ± 43 Ma to 1812 ± 44 Ma. The second domain type has high Th and high Y contents (ThO2 = 6.22–7.34 wt.%, Y2O3 = 0.25–2.41 wt%) and an age range of 1721 ± 46 Ma to 1709 ± 41 Ma (Table S5). However, a few recrystallised rims of matrix monazites that are aligned parallel to the major foliation, show slightly younger dates from 1698 ± 46 Ma to 1629 ± 43 Ma. The monazite dates for these domains yield unmixed ages of 1776 ± 15 Ma, 1708 ± 19 Ma and 1653 ± 30 Ma for the protomylonite BSG77 (n = 31; Fig. 12e).
Monazite grains in the ultramylonite BSG41 occur as inclusions in quartz and muscovite-biotite aggregates and are aligned parallel to the mylonitic foliation (Fig. 12f). Monazite compositional maps exhibit two distinct domain types (Fig. 12g), one characterized by high Th and low Y concentrations (ThO2 = 4.96–9.27 wt.%, Y2O3 up to 0.85 wt.%) and with ages between 883 ± 32 Ma and 962 ± 27 Ma. The other domain type, with high Th and high Y (ThO2 = 4.95–7.05 wt.%, Y2O3 = 1.37–2.12 wt.%), yields ages between 868 ± 37 Ma and 917 ± 30 Ma (Table S5). The combined monazite dates for both these domains yield unmixed ages of 897 ± 9 Ma and 933 ± 11 Ma for the ultramylonite BSG41 (n = 37; Fig. 12g).
5.g. Trace element geochemistry of zircon
For in-situ trace element determination, NIST 612 and NIST 610 (Pearce et al., Reference Pearce, Perkins, Westgate, Gorton, Jackson, Neal and Chenery1997) were used as reference standards for time-drift correction and quality monitoring, and the 29Si (14.98%) was used as the internal standard. Zircon trace element variation diagrams for Gyangarh and Anjana granitoids are shown in Fig. 13a–d.
Chondrite-normalized REE patterns of zircons from Gyangarh monzogranites BSG63 and PC6 are shown in Fig. 13a. The zircon REE patterns for both monzogranites show depletion in the LREE and enrichment in the HREE. The zircon ∑REE for BSG63 ranges from 405 to 1060 ppm, with ∑HREE variable between 385 and 862 ppm. Similarly, the zircon ∑REE and ∑HREE contents for PC6 are in the ranges of 268–762 ppm and 253–732 ppm, respectively. The analyzed spots in both samples are characterized by positive Ce anomalies (Ce/Ce* = CeN/√LaN*PrN) and negative Eu anomalies (Eu/Eu*= EuN/√SmN*GdN). The Ce/Ce* values for BSG63 zircons vary between 6 and 337, whereas for PC6 zircons they vary from 123 to 139 (Fig. 13c). The Eu/Eu* values are variable in the ranges of 0.03–0.13 and 0.03–0.07 for samples BSG63 and PC6, respectively. The ratio Zr/Hf for these zircon spots ranges between 40–53 for BSG63 and 43–50 for PC6 (Table S6).
(a–b) Chondrite-normalized plots showing REE variations in zircon grains and their host whole-rock samples of the Gyangarh (a) and Anjana (b) granitoids. Chondritic values used in normalization are from McDonough and Sun (Reference McDonough and Sun1995). (c) Ce/Ce* versus Hf plot and Eu/Eu* versus Ce/Ce* plot (inset) showing the oxidation state of the magmas from which the zircons crystallized (Ayonta Kenne et al. Reference Ayonta Kenne, Tanko Njiosseu, Ganno, Ngnotue, Fossi, Hamdja Ngoniri, Nga Essomba and Nzenti2021). (d) Zircon Th/Nb versus Hf/Th (Yang et al. Reference Yang, Jiang, Zhao, Jiang, Ling and Luo2012) and Th/U versus Nb/Hf (Hawkesworth and Kemp, Reference Hawkesworth and Kemp2006) tectonic discrimination diagrams depicting an arc-related or orogenic setting for the Gyangarh and Anjana granitoids.
Chondrite-normalized REE patterns for zircons from Anjana monzogranites PC9 and SND3 are shown in Fig. 13b. The zircon patterns show depletion of LREE and enrichment of HREE. The PC9 zircons show ∑REE and ∑HREE values in the range of 317–1006 ppm and 303–960 ppm, respectively. The SND3 zircons show ∑REE (271–528 ppm) and ∑HREE (258–505 ppm) values similar to those of the Anjana monzogranite PC9 and the two Gyangarh monzogranites. The analyzed spots on zircon grains show positive Ce anomalies with values between 11–547 and 15–372 for the Anjana monzogranites PC9 and SND3, respectively (Fig. 13c). In contrast, negative Eu anomalies are observed for PC9 zircons (Eu/Eu* = 0.03–0.16) and SND3 zircons (Eu/Eu* = 0.02–0.11) with Zr/Hf ratios of 32–52 and 42–51, respectively (Table S6).
6. Discussion
6.a. Petrogenetic evolution of the Gyangarh and Anjana granitoids
The Gyangarh granitoids are commonly porphyritic (Fig. 2a), whereas the Anjana granitoids are megacrystic (Fig. 3a, b). Following Vernon (Reference Vernon1986) and Moore and Sisson (Reference Moore and Sisson2008), we consider the feldspar megacrysts as phenocrysts, with little or no role for subsolidus textural coarsening in their production (see Vernon and Patterson Reference Vernon and Paterson2008). Thus, we interpret the Anjana megacrystic monzogranite pluton as essentially a cumulate that has been tectonically deformed (cf. McCurry Reference McCurry2001).
The Gyangarh and Anjana granitoids (mostly monzogranites) show variable SiO2 (57.48–73.43 wt.%) contents and are metaluminous (A/CNK = 1.26–1.37). The distinct changes in the SiO2 concentration are consistent with the accumulation of early crystallized pyroxene and amphibole, followed by later crystallized biotite-rich granitoids. The low Sr (118–403 ppm) contents, low Sr/Y ratios (1–8) and negative Eu anomalies of the granitoids (Fig. 9d) suggest plagioclase fractionation, whereas Sr and Ti depletions (Fig. 9e) suggest fractionation of plagioclase and Fe-Ti oxides from their parental magmas. The fractionation of plagioclase is consistent with the change in composition from andesine-oligoclase to albite (Fig. 8a). The fractionation of plagioclase and Fe-Ti oxides is supported by the strong negative correlations of Al2O3, CaO, FeOT and TiO2 with SiO2 in the whole-rock compositional plots (Fig. S1). The positive Eu anomalies shown by a few samples (Fig. 9d) may reflect plagioclase accumulation, which is evident from the high abundance of plagioclase in a few samples of Gyangarh pluton (e.g., BSG48).
The Gyangarh and Anjana granitoids are calc-alkalic (Fig. 9b). Calc-alkalic compositions, as often perceived with inherently unsuitable geochemical plots, are commonly argued to provide evidence for an arc-related or subduction zone setting, which has been duly questioned (Sheth et al. Reference Sheth, Torres-Alvarado and Verma2002). Compounding the problem here is the cumulus-enriched nature of many of the rocks, which thus do not represent liquid compositions. In fact, the megacrystic granitoids represent a cumulate mush, as indicated by the common occurrence of MMEs (composed of biotite and amphibole) (Fig. 2a) and the cumulus textures (Fig. 2a, f and 3a, b, e). Although dry granitic magmas are highly viscous, and there is too little density contrast between their major minerals and matrix melt to permit crystal accumulation, the presence of small amounts of dissolved H2O greatly lowers the magma viscosities and facilitates crystal accumulation (cf. Vernon & Paterson, Reference Vernon and Paterson2008; Vernon & Collins, Reference Vernon and Collins2011).
Detailed modern accounts of geochemistry of granitoid rocks do not even mention megacrystic types, and the geochemical analyses and interpretations of megacrystic granitoids are far from straightforward (R. H. Vernon, pers. comm., 2022). In this situation, zircons provide helpful information on the crystallization conditions and likely tectonic setting of the Gyangarh-Anjana granitoids. The oscillatory zoning, high Th/U ratios (> 1, Fig. 10, 11a, d) and the overall trace element patterns of the zircons are typical of unaltered magmatic zircons (e.g., Claiborne et al. Reference Claiborne, Miller and Wooden2010), with negligible chemical variation across samples (Fig. 13a–b, Tables S3 and S6). High temperatures (800–900 °C) determined for oscillatory-zoned zircon cores using Ti-in-zircon thermometry also indicate magmatic crystallisation. The estimated high temperatures are consistent with pyroxene exsolution thermometry (800–900 °C; Fig. S3, Appendix B), and represent nearly the liquidus temperatures for granitic rocks (Watson & Harrison, Reference Watson and Harrison1983). The lower range of temperatures (620–750 °C) obtained from zircon saturation thermometry and Ti-in-zircon thermometry correspond to crystallisation of zircon rims. The zircons show strong negative Eu anomalies that are similar to those of their host granitoids (Fig. 9d, 13a–b). Eu2+ and Ce4+ can exist in upper crustal sources (Burnham and Berry, Reference Burnham and Berry2014, Burnham et al., Reference Burnham, Berry, Halse, Schofield, Cibin and Mosselmans2015). Eu2+ is less compatible with the zircon structure than Ce4+ due to the similar ionic radii and charge of Zr4+ and Ce4+. This explains the negative Eu anomaly in the zircon REE pattern, whereas Ce4+ shows a higher positive anomaly (Fig. 13a, b) (cf. Loader et al. Reference Loader, Nathwani, Wilkinson and Armstrong2022). The positive Ce anomaly also serves as a proxy for oxidation conditions of magma. Loader et al. (Reference Loader, Nathwani, Wilkinson and Armstrong2022) suggested that a positive Ce anomaly indicates oxidized melts during the cooling of magma and crystallisation of zircon. Based on plots of Ce/Ce* versus Hf (ppm) (Claiborne et al. Reference Claiborne, Miller and Wooden2010) and Eu/Eu* versus Ce/Ce* (Trail et al., Reference Trail, Watson and Tailby2012) (Fig. 13c), the zircons appear to have crystallised under variable-fO 2 conditions. Furthermore, Ce accumulating in the early forming phases, like zircon, depletes the remaining melt in Ce (Loader et al. Reference Loader, Nathwani, Wilkinson and Armstrong2022), thus resulting in the negative Ce anomaly in the whole-rock geochemistry of granitoids (Fig. 9d). These anomalies, therefore, reflect zircon fractionation.
Zircon trace element tectonic discrimination diagrams (Fig. 13d, Grimes et al. Reference Grimes, Wooden, Cheadle and John2015 and references therein; Ayonta Kenne et al., Reference Ayonta Kenne, Tanko Njiosseu, Ganno, Ngnotue, Fossi, Hamdja Ngoniri, Nga Essomba and Nzenti2021) suggest that the Gyangarh-Anjana granitoids formed in an arc-related or collisional setting. The formation processes of monzogranites include fractional crystallisation of mantle-derived mafic magmas (Frost & Frost, Reference Frost and Frost2011), melting of thickened continental crust (Barbarin, Reference Barbarin1996), and mixing between crust-derived and mantle-derived magmas (Kerr & Fryer, Reference Kerr and Fryer1993; Barbarin, Reference Barbarin1996; Johansson, Reference Johansson2023 and references therein). We find that the whole-rock and zircon geochemical characteristics of the Gyangarh-Anjana monzogranites are consistent with an origin by fractional crystallisation of mantle-derived mafic magmas in an arc setting. Further, the primary Fe-rich garnet seen in the monzogranite sample MS12 is comparable to those crystallising from andesite and dacite magmas in the deeper levels of continental arcs (Blatter et al. Reference Blatter, Sisson and Hankins2023).
6.b. Structural evolution of the Gyangarh and Anjana granitoid plutons
The structural data from the Gyangarh (Z1, Z2 and Z3) and Anjana (Z4) plutons suggest that both plutons experienced two episodes of deformation, indicated by the development of shallow- to moderate-dipping early mylonitic fabric (S1) and moderate- to steeply-dipping late mylonitic fabric (S2). Stretching lineations SL1, measured on S1 planes, show moderate plunges to NW or SE. In contrast, the SL2 stretching lineations measured on S2 planes show moderate to steep plunges to the NE (Fig. 4). Moreover, the SL1 stretching lineations were reoriented by the later D2 deformation episode as can be seen from the spread of the stretching lineations on the stereoplots (Fig. 4a, b). This demonstrates the non-coaxial nature of the two deformation events. The reorientation of early structures by later deformation episodes is common in polydeformed terranes and therefore complicates their interpretation (Duebendorfer, Reference Duebendorfer2003). For example, Ghosh et al. (Reference Ghosh, Tomson, Prabhakar and Sheth2023) documented the tight isoclinal folds related to the D1 deformation preserved only in the Sandmata quartzofeldspathic gneisses. However, the neighbouring granitoids lack such folds but preserve shallow-dipping S1 mylonitic foliation (this study), which is shared with the Sandmata gneisses (Ghosh et al. Reference Ghosh, Tomson, Prabhakar and Sheth2023). In addition, the deformational imprints of D2 in granitoids become more pronounced to the west, as is evidenced by the well-developed steep NNE-SSW mylonitic fabrics documented from Z2, Z3 and Z4 in the present study area. The broad folds on the S1 fabric demonstrated in Z1, showing high angular relations with S2, are thus compressed and transposed to nearly limb-parallel fabrics (S1//S2) in Z2, Z3 and Z4 of the pluton (Fig. 4).
The S-C-C′ fabrics and lensoidal feldspar augen indicate right-lateral (dextral) and oblique-slip motion during the development of early S1 mylonitic foliation, which is largely documented from Z1. This oblique slip was top-down-to-the-NE, i.e., normal in character (Figs. 4, 6, 14). Normal faulting and the formation of detachment faults are common during regional-scale extension related to orogenic collapse; these structures act as conduits for ascending granitic magmas and also act as pathways for the exhumation of deep crustal rocks (Lister and Baldwin, Reference Lister and Baldwin1993; Parsons & Thompson, Reference Parsons and Thompson1993). The co-existence of the Sandmata granulites and granitoids with shared shallow-dipping fabrics suggests a similar tectonic setting. The later mylonitic foliation is characterised by a NE-SW-directed left-lateral (sinistral) shear sense with oblique slip and reverse kinematics (Figs. 4, 6, 14). This oblique-slip character is corroborated by the moderately-to-steeply plunging stretching lineations on the S2 planes, suggesting triclinic shear geometry. The D2 deformation is thus largely a compressional event, and the sinistral-reverse shearing resulted in the pervasive development of NE-SW-striking S2 mylonitic foliations throughout both plutons (Figs. 4a, 14).
The outcrop-scale and microscopic-scale structural evidence indicate the superimposition of two episodes of mylonitic events. Due to these mylonitic events, the plutons do not show any evidence of mesoscopic magmatic flow structures. However, the granitoids display heterogeneous strain patterns marked by the local preservation of blocky subgrains (chessboard twinning) and lobate grain boundaries in recrystallized quartz (Fig. 5b), indicating high-temperature solid-state deformation (550–700 °C; Kruhl, Reference Kruhl1996; Passchier & Trouw, Reference Passchier and Trouw2005). The deformation microstructures of quartz and feldspar in the mylonitic zones suggest that these minerals were dynamically recrystallised under conditions intermediate between subgrain rotation recrystallisation (SGR) and grain boundary migration recrystallisation (GBM) (Fig. 7a; Table 2). This SGR-GBM transition usually occurs at ∼500–525 ºC (Stipp et al., Reference Stipp, Stünitz, Heilbronner, Schmid, de Meer, Drury, de Bresser and Pennock2002). Further, the onset of brittle-ductile, solid-state deformation was associated with grain-size reduction of feldspar grains with the development of core-mantle structures (Fig. 7c), dynamic recrystallisation of quartz with preferred orientation of subgrains (Fig. 5e, 7b, c) and microfaulting of MMEs (Fig. 2f). These solid-state microstructures develop relatively at lower temperatures (400–500 ºC; Stipp et al. Reference Stipp, Stünitz, Heilbronner, Schmid, de Meer, Drury, de Bresser and Pennock2002), indicating superimposition of brittle-ductile deformation during NE-SW-directed sinistral shearing (D2) on relatively high-temperature ductile deformation (Fig. 7c; Table 2). These later deformational imprints are most pronounced in the quartz and feldspar minerals in the rock, whereas pyroxenes and zircons retain signatures of the magmatic history as is evidenced by the high-temperature sub-magmatic conditions obtained from pyroxene exsolution thermometry, zircon saturation thermometry and Ti-in-zircon thermometry (∼650–900°C; Appendix B). The Gyangarh and Anjana plutons thus preserve a complete record of the entire sequence of events, starting with magmatic emplacement, high-T solid-state deformation (D1) and low-T solid-state deformation (D2) conditions.
6.c. Timing of granitoid magmatism and mylonitization in the Sandmata Complex
In this study, four representative samples of the Gyangarh-Anjana monzogranites yielded U-Pb age of 1776 ± 35 Ma (MSWD =1.7, n = 22) to 1706 ± 29 Ma (MSWD =1.4, n = 33) for magmatic zircons with oscillatory zoning and high Th/U ratios (Figs. 10, 11, Table S3). Thus, the Gyangarh and Anjana plutons were emplaced during 1.78–1.71 Ga. This age interval overlaps with previously available U-Pb zircon ages for granitoids within the Sandmata Complex (1.72–1.64 Ga, Sarkar et al. Reference Sarkar, Barman and Corfu1989; Wiedenbeck et al. Reference Wiedenbeck, Goswami and Roy1996) and for the timing of granulite facies metamorphism in the Complex (1.89–1.69 Ga, Buick et al. Reference Buick, Clark, Rubatto, Hermann, Pandit and Hand2010; Ghosh et al. Reference Ghosh, Tomson, Prabhakar and Sheth2023). Monazite geochronology, which has not previously been attempted on these granitoids, complements the zircon geochronology. The monazite age population of 1776 ± 15 Ma to 1708 ± 19 Ma for the protomylonite BSG77 (Fig. 12c-e) is identical with the U-Pb zircon age (1776 ± 35 Ma to 1706 ± 29 Ma) of the Gyangarh pluton within analytical error (Fig. 11). The timing of granitoid plutonism (1.78–1.71 Ga; this study) and the granulite facies metamorphism in the Sandmata Complex (∼1.9–1.8 Ga; Ghosh et al. Reference Ghosh, Tomson, Prabhakar and Sheth2023) reveals that these two events were broadly contemporaneous and together formed a granite-granulite terrane during the closure of Aravalli orogeny. This inference is further supported by previous studies, which suggest that the Aravalli Basin developed along the western margin of the Aravalli-Bundelkhand cratonic block by ∼2.2–2.1 Ga (Deb and Thorpe, Reference Deb, Thorpe, Deb and Goodfellow2004) and closed at ∼1.8–1.7 Ga (Sharma, Reference Sharma2009). Therefore, the Gyangarh-Anjana granitoid magmatism indicates collision-related continental-arc magmatism during the waning stages of the Aravalli orogeny (Ghosh et al. Reference Ghosh, Tomson, Prabhakar and Sheth2023 and references therein).
Schematic diagrams showing a two-stage model for the emplacement and deformation of the Gyangarh and Anjana plutons. (a) The granitoids were emplaced during 1.78–1.71 Ga and subsequently deformed to develop an early mylonitic foliation (S1) at ∼1.65 Ga. This episode was associated with the late stages of the Aravalli orogeny. (b) The granitoids experienced a second episode of mylonitization (S2), involving sinistral transpressional deformation during the late stages of the Delhi orogeny (0.93–0.90 Ga).
The interpretative value of the new geochronological data provided in the present study is considerably enhanced by the data on outcrop structures and microstructures (Fig. 2–7). As mentioned earlier, the Gyangarh pluton experienced two episodes of solid-state deformation events (D1-D2; Fig. 14a, b). The monazite dates obtained from the rims of matrix monazites (1653 ± 30 Ma; BSG77; Fig. 12d, e), which were developed as recrystallised domains parallel to the S1 mylonitic foliations, mark the timing of early mylonitization in the granitoids. This interpretation is consistent with the timing of D2 deformation identified in the adjacent quartzofeldspathic gneisses in the Sandmata Complex (1.74–1.65 Ga; Ghosh et al. Reference Ghosh, Tomson, Prabhakar and Sheth2023). The simultaneous development of shallow-dipping fabrics in the granitoids (S1; this study) and gneisses (S2; Ghosh et al. Reference Ghosh, Tomson, Prabhakar and Sheth2023) and associated normal sense of shears are indicative of deformation of the granitoids during late-stage extensional collapse of the Aravalli orogen. This interpretation is supported by the angular relationship of S1 mylonitic foliation observed in the central parts of the Gyangarh pluton with those observed in the marginal zones of the pluton (Fig. 4, 14a; cf. Ramsay and Huber, Reference Ramsay and Huber1987).
The monazites aligned parallel to the later mylonitic foliation constrain the timing of S2 foliation at 933 ± 11 Ma to 897 ± 9 Ma (BSG41). The structural fabrics, together with chemical zones, indicate the growth of new monazites or recrystallisation of pre-existing monazites during sinistral shearing and low-temperature solid-state deformation in the Sandmata Complex. This tectonic imprint is also distinctive in the quartzofeldspathic gneisses, which were affected by a strong D3 event at 901–783 Ma (Ghosh et al. Reference Ghosh, Tomson, Prabhakar and Sheth2023). We consider this 933 ± 11 Ma to 897 ± 9 Ma solid-state deformation fabric in the Gyangarh-Anjana granitoids (represented by intense mylonitization) as having been generated due to the collision between the putative Marwar Craton and Aravalli Craton (including Sandmata Complex) (Ghosh et al. Reference Ghosh, D’Souza, Goud and Prabhakar2022 and references therein). This collision has resulted in the development of Delhi orogen, which is demarcated by the Phulad shear zone to the west (Chatterjee et al. Reference Chatterjee, Sarkar, Roy and Manna2020) and the Kaliguman shear zone (Sinha-Roy et al. Reference Sinha-Roy, Malhotra, Guha, Sinha-Roy and Gupta1995) to the east.
The combined structural-microstructural, U-Pb zircon and U-Th-total Pb monazite geochronological data and zircon trace element geochemistry of the Gyangarh-Anjana granitoid plutons (Sarkar et al. Reference Sarkar, Barman and Corfu1989; Wiedenbeck et al. Reference Wiedenbeck, Goswami and Roy1996; present study) indicate their formation and ductile deformation during the late stages of the Aravalli orogeny (∼1.65 Ga), overprinted by intense brittle-ductile deformation during the late stages of the Delhi orogeny (0.93–0.90 Ga).
6.d. Tectonic significance of widespread Late Palaeoproterozoic granitic magmatism in the ADMB
The findings of this study and previous studies (references) indicate that Late Palaeoproterozoic (∼1.8–1.6 Ga) granitoids are widespread in the Aravalli Craton and define a ∼700 km-long, NNE-SSW-trending magmatic front, extending from the Darwal area in the south to the Khetri-Alwar areas to the north (Fig. 15). These include the granitoids of Tehara (207Pb–206Pb age of 1711–1600 Ma, Kaur et al. Reference Kaur, Chaudhri, Raczek, Kröner and Hofmann2006), Khetri (1671–1530 Ma, Kaur et al., Reference Kaur, Chaudhri, Raczek, Kröner, Hofmann and Okrusch2011), Ajitgarh (1700–1500 Ma, Pandit & Khatatneh, Reference Pandit and Khatatneh1998), Harsora-Dadikar (1780–1726 Ma, Kaur et al. Reference Kaur, Zeh, Chaudhri and Eliyas2017a), Sakun-Ladera (1721 ± 9 Ma, Pandit et al. Reference Pandit, Kumar and Wang2021), Anasagar (1809 ± 9 Ma, Kaur et al. Reference Kaur, Zeh and Chaudhri2017b), Ramgarh (1729 ± 14 Ma, Rao et al. Reference Rao, Santosh, Purohit, Wang, Jiang and Kusky2011) and Bhinai-Darwal-Rupahali (1716–1692 Ma, Kaur et al. Reference Kaur, Zeh and Chaudhri2021). Geochronological (Fig. 15) and geochemical data (Figs. S1 and S2) for these plutons have been compiled for comparison with the Gyangarh and Anjana plutons. The compiled data suggest that although this granitoid magmatism collectively spans over ∼200 Myr, the majority of the plutons were emplaced in a smaller temporal window of 1.75–1.64 Ga (Fig. 15), coeval with ∼1.74–1.62 Ga high pressure-high temperature (HP-HT) peak metamorphism in the pelitic gneisses and metagranitoids of the Sandmata Complex (Sarkar et al. Reference Sarkar, Barman and Corfu1989; Buick et al. Reference Buick, Allen, Pandit, Rubatto and Hermann2006; Bhowmik et al. Reference Bhowmik, Bernhardt and Dasgupta2010; Roy et al. Reference Roy, Kröner, Rathore, Laul and Purohit2012).
Geological map of the Aravalli Craton and the Aravalli-Delhi Mobile Belt showing a regional-scale magmatic front defined by various Palaeoproterozoic (∼1.8–1.6 Ga) granitoids.
The above observations and age data from various granitoids suggest that intense Palaeoproterozoic (1.75–1.64 Ga) granitic magmatism, which occurred in the Sandmata Complex is the product of syn- to late-orogenic processes associated with the Aravalli Orogeny. The Aravalli Basin was formed at ∼2.2–2.1 Ga along the western margin of the Aravalli cratonic nucleus (Roy and Jakhar, Reference Roy and Jakhar2002). The closure of this basin during Aravalli orogeny and fractional crystallisation of associated mantle-derived mafic magmas formed the Gyangarh and Anjana plutons. This large-scale granitic magmatism was associated with migmatisation in quartzofeldspathic gneisses, emplacement of mafic intrusions (Hamidullah et al. Reference Hamidullah, Mondal, Ahmad, Dash and Rahaman2022) within the Gyangarh and Anjana granitoids and high-grade (granulite facies) metamorphism in the Sandmata Complex at 1.74–1.62 Ga (Bhowmik et al. Reference Bhowmik, Bernhardt and Dasgupta2010; Roy et al. Reference Roy, Kröner, Rathore, Laul and Purohit2012). Overall, the structural relationships and geochronological data for the Gyangarh and Anjana plutons suggest its emplacement during the Aravalli Orogeny (Fig. 14). The intense deformation during the late stages of the Aravalli and Delhi orogenies resulted in the development of mylonitic to ultramylonitic fabrics (S1 to S2) in the Gyangarh and Anjana plutons.
Several continents preserve records of 1.8–1.6 Ga granitic magmatism associated with the final amalgamation of the Palaeoproterozoic supercontinent Columbia (Rogers & Santosh, Reference Rogers and Santosh2002; Zhao et al. Reference Zhao, Sun, Wilde and Li2004; Cawood et al. Reference Cawood, Zhao, Yao, Wang, Xu and Wang2018; Han et al. Reference Han, Peng, Polat and Kusky2019). The closure of the Aravalli Basin and the emplacement of voluminous 1.8–1.6 Ga granitoids were coeval with the global-scale Palaeoproterozoic supercontinent episodes (Ghosh et al. Reference Ghosh, D’Souza, Goud and Prabhakar2022, Reference Ghosh, Tomson, Prabhakar and Sheth2023 and references therein). Therefore, the granitic magmatism and associated geological events indicate that the Sandmata Complex (reworked Archaean basement) and the Aravalli Supergroup (a supracrustal sequence) of the Aravalli Craton were involved in the amalgamation of the long-lived Columbia supercontinent. Further, global records of younger (1.3–1.0 Ga) orogenic belts provide critical clues regarding the assembly of the Rodinia supercontinent. The NE-SW-directed 0.93–0.90 Ga shearing recorded in the Gyangarh and Anjana granitoids can be attributed to tectonic deformation related to the assembly of Rodinia.
7. Conclusions
The Gyangarh and Anjana plutons in the Palaeoproterozoic Sandmata Complex, Aravalli Craton, are largely monzogranites and include feldspar-megacrystic types that represent cumulates. U-Pb zircon dating of magmatic zircon grains and in situ U–Th–total Pb monazite dating suggests that these granitoids formed during 1.78–1.71 Ga, a time period of extensive granitoid magmatism in the Sandmata Complex and the larger Aravalli-Delhi fold belt. The whole-rock and zircon geochemical characteristics suggest that the Gyangarh and Anjana monzogranites were the products of fractional crystallization of mantle-derived mafic magmas. This scenario can explain the regional-scale Late Palaeoproterozoic (1.75–1.64 Ga) granitoid magmatism, including the Gyangarh and Anjana plutons. The closely similar zircon U-Pb emplacement ages (Sarkar et al. Reference Sarkar, Barman and Corfu1989; Wiedenbeck et al. Reference Wiedenbeck, Goswami and Roy1996; this study), mineral assemblages (Table 1) and structural features (Figs. 2, 3) indicate a common shared history for the Gyangarh and Anjana plutons (Fig. 1b). The two plutons may be viewed as parts of a larger, likely composite, intrusive body, termed here as the Gyangarh-Anjana batholith. The batholith is variably deformed (D1-D2), and outcrop-scale structures, microstructures and zircon trace element geochemical characteristics imply that the batholith was emplaced during the Aravalli orogeny (1.78–1.71 Ga), marking the assembly of the supercontinent Columbia. Monazite dating also suggests that the Gyangarh-Anjana batholith experienced early dextral shearing at ∼1.65 Ga, followed by sinistral, NE–SW-directed shearing at 0.93–0.90 Ga. These events correspond to continental convergence during the late stages of Aravalli and Delhi orogenies, marking the assemblies of the supercontinents Columbia and Rodinia, respectively. The Late Palaeoproterozoic Gyangarh-Anjana batholith in the Sandmata Complex thus records the imprints of two major orogenies that have shaped the Aravalli Craton.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0016756825000068
Acknowledgements
This research is part of the doctoral research work of Manisha Sahu at the Indian Institute of Technology Bombay. We acknowledge the financial support provided by the Science and Engineering Research Board (SERB, Govt. of India) through the Core Research Grant (Sanction No. CRG/2019/000812) for performing geological field work and analytical studies. The SERB-funded EPMA National Facility at the Department of Earth Sciences, IIT Bombay (IRPHA grant No. IR/S4/ESF-16/2009) was used for carrying out mineral chemical analyses and monazite dating. Joseph D’Souza, Abhishek Singh, Piyush Kumar and Manisha Gupta are thanked for their assistance during different field sessions. Discussions with Malay Mukul helped to improve the parts of the manuscript related to the structural deformation. Javed M. Shaikh and Prem Kumar Verma are thanked for their help with EPMA and whole-rock analyses, respectively. We thank Kamal Kant Sharma and Taija Torvela for their suggestions on an initial version of this manuscript. We appreciate two anonymous journal reviewers and the Editor Tim Johnson for their constructive and helpful comments on this work.
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