1. Introduction
Globally, A-type granites occur in Precambrian and Phanerozoic terrains, mainly in extensional tectonic settings (e.g. Loiselle & Wones, Reference Loiselle and Wones1979; Eby, Reference Eby1990, Reference Eby1992; Black & Liégeois, Reference Black and Liégeois1993). A-type granites are distinct from other granite types and are defined as relatively anhydrous with high SiO2, high Na2O + K2O, low MgO and high incompatible trace element contents including REE, Zr, Y, Nb and Ta (e.g. Eby, Reference Eby1992; Frost et al. Reference Frost, Frost, Chamberlain and Edwards1999; Bonin, Reference Bonin2007; Dall’Agnol et al. Reference Dall’Agnol, Frost and Rämö2012; Li et al. Reference Li, Watanabe and Yonezu2014). A-type granites predominantly range from single-feldspar (hypersolvus) to two-feldspar (subsolvus) granites. In the hypersolvus varieties, the granitic rocks are composed of quartz and alkali feldspar; the latter yields coarse perthitic exsolution (e.g. Turner et al. Reference Turner, Foden and Morrison1992).
Several mechanisms have been proposed to explain the origin of A-type magmas: (1) direct fractionation of mantle-derived alkaline basalts (e.g. Turner et al. Reference Turner, Foden and Morrison1992; Litvinovsky et al. Reference Litvinovsky, Jahn, Zanvilevich, Saunders, Poulain, Kuzmin, Reichow and Titov2002; Mushkin et al. Reference Mushkin, Navon, Halicz, Hartmann and Stein2003); (2) low degrees of partial melting of F−- and Cl−-enriched dry, lower-crustal granulitic residue from which a granite melt was previously extracted (e.g. Collins et al. Reference Collins, Beams, White and Chappell1982; Clemens et al. Reference Clemens, Holloway and White1986; King et al. Reference King, White, Chappell and Allen1997); (3) low-pressure melting of calc-alkaline rocks at upper crustal levels (Skjerlie & Johnston, Reference Skjerlie and Johnston1993; Patiño Douce, Reference Patiño Douce1997); and (4) hybridization between anatectic granitic and mantle-derived mafic magmas (Bedard, Reference Bedard1990; Kerr & Fryer, Reference Kerr and Fryer1993; Wickham et al. Reference Wickham, Albertz, Zanvilevich, Litvinovsky, Bindeman and Schauble1996; Mingram et al. Reference Mingram, Trumbull, Littman and Gerstenberger2000).
Although fine-grained, dark-coloured microgranular enclaves are most common in calc-alkaline, metaluminous (I-type) granites (e.g. Didier & Barbarin, Reference Didier and Barbarin1991; Kumar and Rino, Reference Kumar and Rino2006), they also occur in A-type granites and provide vital clues on the processes that have acted at different levels during the magma evolutionary history (Bonin, Reference Bonin, Didier and Barbarin1991). Further, microgranular enclaves and their distribution in felsic plutons, along with their textural and contact relationships with felsic host, may provide insights into the magma chamber processes and dynamics (e.g. Vernon, Reference Vernon1984; Vernon et al. Reference Vernon, Etheridge and Wall1988; Kumar, Reference Kumar1995, Reference Kumar2010, Reference Kumar2020; Perugini et al. Reference Perugini, Valentini and Poli2007).
In Peninsular India, A-type granites are widely distributed along the marginal zone of the Eastern Ghat Granulite Belt (EGGB), in the Southern Granulite Terrain, Mahakoshal Supracrustal Belt, Central India; the Mayurbhanj area, north of the Bastar Craton; and the Malani Igneous Province (e.g. Eby & Kochhar, Reference Eby and Kochhar1990; Meert et al. Reference Meert, Pandit, Pradhan, Banks, Sirianni, Stroud, Newstead and Gifford2010; Sarvothman & Sesha Sai, Reference Sarvothman and Sesha Sai2010; Kumar et al. Reference Kumar, Gupta, Sensarma and Bhutani2020; Yadav et al. Reference Yadav, Ahmad, Kaulina, Bayanova and Bhutani2020). In Southern India, the western margin of the Neoarchaean Nellore Schist Belt (NSB) is characterized by the emplacement of several granite plutons of Proterozoic age (Gupta et al. Reference Gupta, Pandey, Chabria, Banerjee and Jayaram1984; Dobmeier et al. Reference Dobmeier, Lutke, Hammerschmidt and Mezger2006; Sesha Sai, Reference Sesha Sai2013). These granite plutons form part of the Proterozoic granite emplacement zone located close to a major terrain boundary in the Eastern Dharwar Craton (EDC), i.e. the eastern margin of the Proterozoic Cuddapah Basin and Neoarchaean NSB. The Punugodu granite (PG) is one among these granite plutons spatially associated with other granites, gabbros, felsic volcanics and ophiolite mélange rocks. The origin and evolution of PG have not yet been described. The present study therefore aimed to carry out a detailed investigation involving the field relation, petrography, geochemistry and geochronology of the PG occurring to the east of Cuddapah Basin, in order to characterize and discuss its petrogenesis, which may contribute to our understanding of the Proterozoic accretionary and rifting history of this region.
2. Geological setting
The Neoarchaean NSB constituted by metavolcanics and metasediments is characterized by emplacement of mafic and felsic magmatic rocks of Proterozoic age (e.g. Vasudevan & Rao, Reference Vasudevan and Rao1976; Gupta et al. Reference Gupta, Pandey, Chabria, Banerjee and Jayaram1984; Srinivasan & Roop Kumar, Reference Srinivasan and Roop Kumar1995; Raman & Murty, Reference Raman and Murthy1997; Dharma Rao & Reddy, Reference Dharma Rao and Reddy2007; Ravikant, Reference Ravikant2010; Sesha Sai et al. Reference Sesha Sai2013; Saha et al. Reference Saha, Sain, Kundu, Mazumder and Kar2015). The two domains that constitute the NSB are the upper western Udaigiri and the lower eastern Vinjamuru domains (Dobmeier & Raith, Reference Dobmeier and Raith2003; Saha et al. Reference Saha, Sain, Kundu, Mazumder and Kar2015). The mafic and felsic alkaline plutons of the NSB fall within the southern continuity of the Prakasam Alkaline Province (PAkP) (Ratnakar & Leelanandam Reference Ratnakar and Leelanandam1985; Rao et al. Reference Rao, Rao and Murthy1988; Ratnakar & Vijaya Kumar, Reference Ratnakar and Vijaya Kumar1995; Leelanandam et al. Reference Leelanandam, Burke, Ashwal and Webb2006). Several collision- and extension-related Proterozoic magmatic events played a significant role in the crustal accretion and growth to the east of Cuddapah Basin in the EDC (Divakara Rao et al. Reference Divakara Rao, Rama Rao and Subba Rao1999; French et al. Reference French, Heaman, Chacko and Srivastava2008; Dharma Rao et al. Reference Dharma Rao, Santosh and Yuan-Bao2011; Sain et al. Reference Sain, Saha, Joy, Jelsma and Armstrong2017; Sesha Sai, Reference Sesha Sai2019).
The occurrence of ophiolite affinity rocks at Kandra (Kandra Ophiolite Complex) in the southern part of the NSB indicates that subduction would have initiated ∼1.85 Ga (Sesha Sai, Reference Sesha Sai2009; Vijaya Kumar et al. Reference Vijaya Kumar, Ernst, Leelanandam, Wooden and Grove2010). The eastern margin of the EDC evolved as the site for collision with the Napier Complex forming the 1.7–1.55 Ga old orogen (Chaudhuri et al. Reference Chaudhuri, Saha, Deb, Deb, Mukherjee and Ghosh2002; Dobmeier & Raith, Reference Dobmeier and Raith2003; Ravikant Reference Ravikant2010; Vijaya Kumar et al. Reference Vijaya Kumar, Ernst, Leelanandam, Wooden and Grove2010), resulting in the formation of granulites (1700 Ma), anorthosite (1690–1630 Ma) and granites (1650–1450 Ma) in the Ongole domain (Kovach et al. Reference Kovach, Simmat, Rickers, Berezhnaya, Salnikova, Dobmeier, Raith, Yakovleva and Kotov2001; Simmat & Raith, Reference Simmat and Raith2008; Dharma Rao et al. Reference Dharma Rao, Santosh and Dong2012). The overlapping ages of these rocks indicates that the domain was magmatically active between at least c. 1720 and 1570 Ma (Henderson et al. Reference Henderson, Collins, Payne, Forbes and Saha2014).
The plutons emplaced in the northern part of the NSB in the PAkP yield the zircon U–Pb Mesoproterozoic ages of Elchuru nepheline syenite (1321 ± 17 Ma; Upadhyay et al. Reference Upadhyay, Raith, Mezger and Hammer Schmidt2006), Purimetla gabbro (1334 ± 15 Ma; Subramanyam et al. Reference Subramanyam, Santosh, Yang, Zhang, Balaram and Reddy2016), Uppalapadu nepheline syenite (1356 ± 41 Ma; Vijaya Kumar et al. Reference Vijaya Kumar, Frost, Frost and Chamberlain2007), Errakonda ferrosyenite (1352 ± 2 Ma; Vijaya Kumar et al. Reference Vijaya Kumar, Frost, Frost and Chamberlain2007) and Kanigiri gabbro (1338 ± 27 Ma; Subramanyam et al. Reference Subramanyam, Santosh, Yang, Zhang, Balaram and Reddy2016), which are emplaced between the Udaigiri and Vinjamuru domains. The Kanigiri granite (1284 ± 4 Ma; Sain et al. Reference Sain, Saha, Joy, Jelsma and Armstrong2017), Punugodu granite (PG) (1256 ± 6 Ma; present study) and Peddacharlopalle gabbro (1251.2 ± 9.4 Ma; Subramanyam et al. Reference Subramanyam, Santosh, Yang, Zhang, Balaram and Reddy2016) are ∼100 Ma younger than the PAk rocks. The 1334 ± 20 Ma age of Kanigiri Ophiolite Mélange and the age of the plutons emplaced in its vicinity show a gap of ∼80 Ma. However, further studies are required on those undated southern and northern plutons of the NSB and PAkP to shed light on the tectono-magmatic evolution of the NSB.
3. Field relationships and petrography
The PG pluton, covering an area of 7 km2, occurs as a N–S-trending oval-shaped body commonly exposed to the east of Kanigiri granite and south of the Podili granite plutons (Fig. 1). Xenoliths of the pre-existing NSB volcanics are found hosted in the PG. These xenoliths are felsic, angular and fractured, and show sharp to partly reactive contacts with the host PG (Fig. 2a). In places, calc-silicate bands (Fig. 2b) of the pre-existing NSB are also observed. Reddy & Sesha Sai (Reference Reddy and Sesha Sai2003) reported a rhyodacitic (metavolcanic) rock having intimate contact with the PG. Along the contact with PG the hornfelsic textures are developed (Fig. 2c), which clearly indicate intrusive contact formed by thermal metamorphism. Therefore, the rhyodacite represents pre-existing supracrustal NSB rock, now engulfed into the PG, and formed the large xenolith. In the southern margin of PG, contact with the chlorite schist of NSB is observed (Fig. 2d). Due to soil development and the weathered contact of PG and chlorite schist of NSB, the actual nature of the contact is unknown.
Fig. 1. Geological map of Nellore Schist Belt and Prakasam Alkaline Province, after Srinivasan & Roop Kumar (Reference Srinivasan and Roop Kumar1995) and Sesha Sai (Reference Sesha Sai2013). Map of Punugodu granite pluton is given in the inset. NSB: Nellore Schist Belt; PAkP: Prakasam Alkaline Province; ENS: Elchuru nepheline syenite; PGb: Purimetla gabbro; UNS: Uppalapadu nepheline syenite; EFS: Errakonda ferosyenite; PdG: Podili granite; PG: Punugodu granite; KGb: Kanigiri gabbro; KG: Kanigiri granite; PcGb: Peddacharlapalle gabbro; AC: Aravali Craton; MIP: Malani Igneous Province; BuC: Bundelkhand Craton; SC: Singhhbum Craton; BC: Bastar Craton; DVP: Deccan Volcanic Province; EGGB: Eastern Ghats Granulite Belt; CB: Cuddapah Basin; WDC: Western Dharwar Craton; EDC: Eastern Dharwar Craton; SGT: Southern Granulite Terrain; MSB: Mahakoshal Supracrustal Belt.
Fig. 2. Field photographs showing the field relationship of the PG pluton. (a) Xenolith with fractures showing sharp contact with host rock. (b) Occurrence of a calc-silicate xenolith of NSB in PG. (c) Presence of hornfelsic rock at the contact with pre-existing metavolcanic rock. (d) Field photograph showing contact between PG and chlorite schist of NSB country rock. (e) Fine-grained, mafic microgranular enclave surrounded by mafic clusters. (f) Hybrid microgranular enclave with ellipsoidal shape. (g) Field photograph showing oriented nature of magmatic enclaves at the pluton contact indicating magma mingling. (h) Fine-grained felsic microgranular enclave with rounded shape.
The PG hosts abundant microgranular enclaves (ME). These ME are fine-grained, mafic–melanocratic (MME) to porphyritic (hybrid) mesocratic (HME), and vary in size from a few mm to metres, with rounded, elliptical, spherical and lenticular shapes commonly having sharp, crenulated and occasionally diffuse contacts with the host PG (Fig. 2e–h). These field features of ME indicate ME magma mingling and undercooling within the partly crystalline PG melt (e.g. Kumar et al. Reference Kumar, Rino and Paul2004). The HME exhibit characteristics that are intermediate between MME and felsic microgranular enclaves (FME) (Fig. 2f). The N–S-oriented nature of ME perhaps indicates the phenomenon of magmatic flowage attained during the emplacement and dynamics of coexisting ME and PG magmas (Fig. 2g) prior to solidification of the entire system as described by Kumar (Reference Kumar2020). The FME are fine-grained, linear to spherical to lenticular (Fig. 2h), most likely representing fragments of early, chilled margin facies of PG pluton and disrupted as FME due to prevailing convective force in the PG chamber (e.g. Didier & Barbarin, Reference Didier and Barbarin1991; Kumar, Reference Kumar2010). The depth of emplacement is one factor that plays a vital role in the crystallization of A-type alkali feldspar granites. Felsic magmas of A-type affinity are products of residual liquids resulting from the differentiation phenomenon in the magma chambers with varying depths (Bonin, Reference Bonin1998).
A total of 29 rock samples, comprising 14 PG, four xenoliths, four FME, three HME and four MME samples, were collected during fieldwork covering the outcrops of PG pluton. Sample locations are shown in Figure 1.
3.a Petrography of PG
Detailed petrographic studies reveal that the PG is essentially composed of relatively large subhedral laths of perthitic K-feldspar and quartz, while the mafic phases noticed are biotite and amphibole, with zircon, titanite, fluorite, apatite and opaques as the accessories. Relict clinopyroxene is also noticed in places in the PG. Amphibole is pleochroic in shades of brown to blue. The presence of single feldspar in the form of microcline perthite indicates the hypersolvus nature of PG (Fig. 3a and b). Petrographic studies of the PG samples collected along the margins of the pluton reveal that quartz is anhedral and exhibits wavy extinction and recrystallization indicating a deformational regime (Fig. 3c). Perthitic K-feldspars are characteristically twinned (Fig. 3d). Perthite occurs as string, vein and patchy types (Anderson, Reference Anderson1928; Smithson, Reference Smithson1963). String perthites are formed due to exsolution, as evident from small, well-oriented blebs perthites, whereas the patchy and coarse vein types of perthites are formed due to replacement evident from irregular, redistributed and enlarged albite blebs, which occur close to and on the margin of the grain (e.g. Alling, Reference Alling1938). Fluorite is observed, associated with biotite (Fig. 3e). Bluish amphibole occurs as subhedral prismatic grains in places (Fig. 3f). Clusters of mafic phases represented mainly by amphiboles and associated opaques are observed in parts of the quartzo-feldspathic material (Fig. 3g). Fine-grained skeletal opaques are observed in the vicinity of biotite and amphibole. Zircon is euhedral and exhibits zoning (Fig. 3h).
Fig. 3. (a) Photomicrograph under crossed Nicols showing coarse-grained perthitic feldspar. (b) Photomicrograph of microcline microperthite with biotite cluster. (c) Admixture of recrystallized quartz along with the feldspar grain boundary indicating recrystallization due to deformation. (d) Photomicrograph under crossed Nicols showing Carlsbad twinning in perthitic K-feldspar. (e) Photomicrograph in plane-polarized light showing fluorite associated with biotite. (f) Photomicrograph in plane-polarized light showing relict clinopyroxene surrounded by alkali amphibole. (g) Photomicrograph in plane-polarized light showing skeletal opaques in the vicinity of mafic minerals. (h) Zoned euhedral zircon (plane polarized light). Mineral abbreviations used as in Whitney & Evans (Reference Whitney and Evans2010).
3.b. Petrography of enclaves
Petrographic studies indicated variation in the mineralogy of three types of magmatic enclaves observed in PG, namely FME, HME and MME. The FMEs are mineralogically similar to host granite and mainly consist of quartz, microcline, plagioclase and biotite (Fig. 4a and b). The HMEs are fine-grained and porphyritic, exhibiting typical igneous texture with felsic phenocrysts (xenocrysts). Quartz xenocrysts (ocelli) with biotite and hornblende-rich mantle (Fig. 4c), perthites and opaques with biotite rim and clusters are common (Fig. 4d). Mineralogically, the MMEs consist of plagioclase and pyroxene as essential minerals and exhibit subophitic texture (Fig. 4e). Twinned plagioclase crystals occur as elongated laths in places in the MMEs, a textural feature suggesting magmatic origin (Fig. 4 f). Biotite appears as interstitial flakes in the MMEs. Subhedral grains of Fe-oxides occur as accessory minerals in the MMEs (Fig. 4g). Most MMEs show diffusive and cuspate contacts with host PG, indicating their magmatic nature. This type of enclave contact mainly appears in some particular cases where large volumes of mafic magma dominate over felsic magma (Barbarin & Didier, Reference Barbarin, Didier, Didier and Barbarin1991). The enclaves as xenoliths represent the caught-up patches of the NSB metavolcanics and hence are different from the observed ME types. Xenoliths show feeble deformation, which is evident from the recrystallized quartz and distorted twin lamellae in plagioclase. Petrographic studies show the presence in the metavolcanic xenolith of quartz porphyroclasts along with feldspar, hornblende and biotite (Fig. 4h).
Fig. 4. (a, b) Photomicrographs under crossed Nicols showing quartz, plagioclase, microcline and biotite minerals in FME (c) Photomicrograph under plain polarized light showing quartz xenocrysts with biotite- and hornblende-rich mantle in HME. (d) Photomicrographs under crossed Nicols showing perthites and clusters of biotite in HME. (e, f) Photomicrograph showing plagioclase and pyroxene with sub-ophitic texture in MME. (g) Photomicrograph showing the cluster of opaques in MME. (h) Photomicrograph under crossed Nicols showing quartz porphyroclast in NSB metavolcanic xenolith.
4. Sampling and analytical techniques
Relatively fresh and unweathered samples were collected during the fieldwork. High-quality thin-sections were made for the systematic petrographic studies. Rock samples for major, trace and rare earth elemental analysis were crushed and pulverized to fine powders manually using the agate mortar and pestle.
4.a. Major oxides and trace elements
Geochemical analyses were carried out at the Geochemistry Division, CSIR – National Geophysical Research Institute (NGRI), Hyderabad, India. Major-element abundances were determined on pressed powder pellets using X-Ray Fluorescence (XRF) (Phillips MAGIX PRO Model 2440) following the methods outlined by Krishna et al. (Reference Krishna, Murthy and Govil2007). Trace and rare earth elements were determined by high-resolution inductively coupled mass spectrometer (HR-ICP-MS; Nu Instruments Attom, UK) in jump wiggle mode. The analytical procedure, precision and accuracy for HR-ICP-MS are reported in Manikyamba et al. (Reference Manikyamba, Santosh, Chandan Kumar, Rambabu, Tang, Saha, Khelen, Ganguly, Dhanakumar Singh and Subba Rao2016). 103Rh was used as an internal standard, and external drift was corrected by repeated analyses of standards used for calibration.
4.b. Zircon separation and U–Pb analysis
Zircon grain extractions were carried out after crushing the rock samples into ∼80–250 μm sieve fractions and were processed on the Wilfley shake table to separate heavy mineral fractions. The heavies were then subjected to isodynamic magnetic separation before handpicking zircons under a binocular microscope. Zircon grains were mounted in 25 mm epoxy resin blocks, diamond-polished, carbon-coated and cathodoluminescence (CL)-imaged on a TESCAN LM3H SEM mounted with a Rainbow colour CL detector. Zircon U–Pb dating was performed using Thermo X SeriesII ICP-MS coupled to a New Wave Universal Platform 213 nm Nd–YAG laser ablation system with a large-format ablation cell operated with a combination of He and Ar gases as detailed in Babu et al. (Reference Babu, Bhaskar Rao and Vijaya Kumar2009). The data were processed offline using ‘IsoplotR’ (Vermeesh, Reference Vermeesh2018) and ‘Glitter software’ to compute the ages and corrected for common Pb using Andersen’s scheme (Andersen, Reference Andersen2002).
Results
The major, trace and rare earth element data of the granite samples, along with those of the xenoliths, FME, HME and MME analysed in the present study, are presented in Table 1.
Table 1. Major oxide analysis (%), trace and rare earth element analyses (ppm) of Punugodu granite pluton and enclaves
5.a. Geochemistry of PG and enclaves
Major-oxide geochemistry of PG reveals high SiO2 (avg. 70.41 wt %) and high Na2O + K2O (avg, 9.40 wt %) contents, a feature that corresponds to the anorogenic, alkali (A-type) granites (Whalen et al. Reference Whalen, Currie and Chappell1987). The SiO2 concentrations in enclaves are as follows: in xenoliths from 61.2 to 64.9 wt %, FME from 66.5 to 67.4 wt %, HME from 49.4 to 62.9 wt % and MME from 45.2 to 49 wt %. The K2O + Na2O content in xenoliths, FME, HME and MME ranges from 8.58 to 9.48 wt %, 8.27 to 9.5 wt %, 8.34 to 9.25 wt % and 3.74 to 5.24 wt % respectively. A detailed account of major oxide, trace element and rare earth element geochemical characteristics of PG pluton was presented previously (Narshimha et al. Reference Narshimha, Reddy, Sesha Sai and Subramanyam2018). In the MALI (Na2O + K2O–CaO) vs silica diagram (Frost et al. Reference Frost, Barnes, Collins, Arculus, Ellis and Frost2001), the PG granite, FME and xenoliths exhibit an alkaline to alkali-calcic trend (Fig. 5a). The MMEs in the FeO* vs silica diagram (Frost & Frost, Reference Frost and Frost2008) occupy the tholeiitic and alkaline fields (Fig. 5b). The Fe number of 13 PG samples varies from 0.76 to 0.97, indicating its strong ferron character. In the A/NK–A/CNK alumina saturation diagram (Fig. 5c), the majority of PG samples fall in the metaluminous domain, while a few fall close to the boundary of the metaluminous and peralkaline fields (Shand, Reference Shand1943). In the normative anorthite (An) – albite (Ab) – orthoclase (Or) diagram (O’Connor, Reference O’Connor1965), the samples fall in the granite field (Fig. 5d). In the R1–R2 multicationic parameters (De la Roche et al. Reference De la Roche, Leterrier, Grandclaude and Marshall1980)-based tectonic classification scheme (after Batchelor & Bowden, Reference Batchelor and Bowden1985), the PG shows affinity with syn-collision to late orogenic tectonic settings (Fig. 5e). Pearce et al. (Reference Pearce, Harris and Tindle1984) classified the granites into a within-plate, collisional, volcanic arc and oceanic-ridge granites mostly based on the content of high-field-strength elements (HFSE). The within-plate origin of the PG samples is substantiated by the Rb vs Y + Nb plot (Fig. 5f). the total-alkali silica (TAS) diagram (Le Maitre, Reference Le Maitre2002) of PG and enclaves shows that MMEs are alkaline occupying the basaltic field (Fig. 5g). The HMEs show mafic–felsic intermediate compositions occupying the trachyte–trachyandesite and phonotephrite fields.
Fig. 5. (a) MALI vs silica diagram (Frost et al. Reference Frost, Barnes, Collins, Arculus, Ellis and Frost2001) showing alkali-calcic nature of Punugodu granite. (b) FeO* vs silica diagram (Frost & Frost, Reference Frost and Frost2008) showing MME occupy the tholeiitic/alkaline field. (c) A/NK vs A/CNK diagram (Shand, Reference Shand1943; Maniar & Piccoli, Reference Maniar and Piccoli1989) for granites and enclaves. (d) Normative anorthite (An)–albite (Ab)–orthoclase (Or) diagram (O’Connor, Reference O’Connor1965). (e) R1–R2 tectonic classification diagram (after Batchelor & Bowden, Reference Batchelor and Bowden1985) showing the position of Punugodu granite in syn-collision to late orogenic tectonic setting, R1 = 4Si − 11(Na + K) − 2(Fe + Ti); R2 = 6Ca + 2Mg + Al. (f) Rb vs (Y + Nb) diagram (after Pearce et al. Reference Pearce, Harris and Tindle1984, Reference Pearce, Westgate and Perkins1996) showing the position of Punugodu granite in WPG. (g) Total-alkali silica (TAS) diagram (Le Maitre, Reference Le Maitre2002) of Punugodu samples. Note: The symbols denoting granite, xenoliths, FME. HME and FME are applicable for all geochemical plots.
The Harker variation diagrams (Fig. 6) show the variation of major element oxides with silica as the magma differentiate in which Na2O and K2O increase, indicating a positive correlation during the evolution of PG pluton. The MgO, FeO, TiO2 and CaO decrease with increasing silica. Al2O3 remains almost constant or slightly increases with differentiation. The K2O content in FME ranges from 4.3 to 3.6 wt %, HME from 4.9 to 3.2 wt %, and MME from 2.2 to 1.4 wt %, indicating an overall decrease in K2O content from FMEs to MMEs. The high Fe2O3 content in MME ranges from 10.95 to 11.48 wt %, which can be ascribed to the presence of mafic minerals. The Rb concentrations in the PG samples are generally high and vary from 136 to 278 ppm. This is concomitant with the high abundance of K2O in the granite. Ba, which can also substitute to K, is less abundant, ranging from 32 to 132 ppm. The decrease in Ba content with increasing SiO2 (Fig. 6) indicates the fractionation of K-feldspar during the crystallization of PG. The Rb contents are moderate, varying between 136 and 278 ppm. The Ca and Sr contents show decreasing trends with increasing SiO2. The granite samples are also characterized by moderately high concentrations of U (4.2 to 20.5 ppm) and Th (35 to 55.5 ppm) along with HFSE such as Zr (85.5 to 356 ppm), Nb (99 to 216 ppm) and Y (79.6 to 201.8 ppm), and low concentrations of transition metal trace elements Ni (1.1 to 4 ppm), Cr (7.8 to 8.2 ppm) and Co (0.3 to 1.1 ppm). Trace element geochemistry indicates that the MME consist of relatively high Ni (10.6 to 88 ppm), Cr (63 to 283 ppm) and Co (45 to 115 ppm). The HFSE contents of Zr (144 to 466 ppm) and Y (28 to 79 ppm) in MME show moderate abundances.
Fig. 6. Harker’s variation diagram showing the variation of major oxides and trace elements with SiO2.
The PG samples are characterized by low to moderately fractionated REE patterns (Fig. 7a) with (La/Yb)N varying between 9.05 and 19.26. The PG samples are characterized by strong LREE fractionation ((La/Sm)N = 3.91–5.0) with flat HREE ((Gd/Yb)N = 0.7–3.3) patterns. The PG samples also exhibit negative Eu anomalies (Eu/Eu)*N ranging from 0.04 to 0.07. The chondrite-normalized (after Nakamura, Reference Nakamura1974) REE patterns of xenolith, FME and HME are enriched in LREE and show prominent negative Eu anomalies (Fig. 7b, c and d), indicating a significant role of plagioclase fractionation from the parent magma. The chondrite-normalized REE patterns of the MME exhibit marginally enriched LREE and relatively depleted HREE (Fig. 7e). Low-amplitude negative to no Eu anomaly is observed in the MME, with the (Eu/Eu)*N ranging from 0.3 to 1.0. The primitive-mantle-normalized (after Sun & McDonough, Reference Sun and McDonough1989) multi-element diagram (Fig. 7f) of the PG samples shows prominent negative anomalies in Ba, Sr, P, Eu and Ti, while Th, U, Ce, Pr, Nb, Sm, Dy, Y and Nd show positive anomalies. The negative anomalies of Sr and Eu are related to the fractionation of plagioclase in the source region (Hanson, Reference Hanson1978). The negative anomalies of P and Ti reflect the fractionation of apatite and Fe–Ti oxides. The overall geochemical characteristics of the granites from PG pluton indicate its anorogenic, alkaline (A-type) nature (Whalen et al. Reference Whalen, Currie and Chappell1987). The incompatible trace-element-normalized (after Sun & McDonough, Reference Sun and McDonough1989) multi-element diagrams (Fig. 7g, h and i) for the xenolith, FME and HME depict negative Ba, Sr and Ti anomalies and positive Rb anomalies, wheareas the MMEs show negative Ba with constant Sr and Ti and positive Rb anomaly (Fig. 7j).
Fig. 7. Chondrite-normalized (normalized values after Nakamura, Reference Nakamura1974) REE and primitive-mantle-normalized (values after Sun & McDonough, Reference Sun and McDonough1989) multi-element diagrams of granite and enclaves.
5.b. Zircon U–Pb geochronology
The PG-8 granite sample with abundant zircons is chosen for U–Pb geochronology. Zircon grains extracted from this sample are well-developed euhedral grains, typically 80–200 μm long with aspect ratios of 2:1 to 3:1. They show concentric growth zoning typical of magmatic zircons (Corfu et al. Reference Corfu, Hanchar, Hoskin and Kinny2003), and, in a few cases, convolute zoning suggests crystallization from melts. The CL images of representative zircon grains are given in Figure 8. The zircon grains show well-preserved prismatic terminations suggesting limited magmatic resorption. Most of the zircons have systematic growth zoning from core to rim. Luminescent outer rims are rare, suggesting a general lack of metamorphic overgrowth. Rarely, an effect of alteration is evident in the development of bright luminescent patches across growth zoning.
Fig. 8. Cathodoluminescence images of zircon grains, Punugodu granite.
In general, the laser spots were placed in homogeneous interiors of zircons, avoiding older (inherited) domains and visibly altered segments. In situ laser-ablation ICP-MS U–Th–Pb isotopic data for individual laser spots in zircons from the sample are given in Table 2. Age calculations are discussed based on a 207Pb/235U vs 206Pb/238U concordia diagram (Fig. 9). The concordant zircon grains have U content between 7.6 and 384 ppm with an average of 95.4 ppm, Th ranging from 5.6 ppm to 427.7 ppm with an average 75.6 ppm, and Th/U = 0.43 to 1.49, attesting to their typical magmatic character (Rubatto, Reference Rubatto2002). The zircons yield a concordia upper intercept U–Pb age of 1257 ± 6 Ma (2σ errors), MSWD = 7.5. We interpret 1257 ± 6 Ma concordia upper intercept age as the time of emplacement and magmatic crystallization of the Punugodu granite pluton.
Table 2. LA-ICP-MS U–Th–Pb isotopic data and calculated ages for zircons from the Punugodu granite pluton
Fig. 9. Concordia age diagram of zircon using LA-ICP-MS U–Th–Pb isotopic data on the Punugodu granite.
6. Discussion
6.a. Genetic type and tectonic setting
The Punugodu granite is mineralogically an alkali feldspar granite and is characterized by high SiO2 and Na2O + K2O. The average Fe number for PG is 0.83, indicating its ferron nature. In the A/NK vs A/CNK diagram (Fig. 5c), the sample of PG granite falls at the juncture of metaluminous and peralkaline fields. Based on the major oxide geochemistry, alkali feldspar granites of ferroan affinity, metaluminous and peralkaline by virtue of their alumina saturation, have been recognized as alkaline (A-type) granites (e.g. Frost & Frost, Reference Frost and Frost2011). Whalen et al. (Reference Whalen, Currie and Chappell1987) proposed several discrimination diagrams to distinguish the A-type granite from other granite types such as I-, S- and M-types. The 10 000 * Ga/Al values vary with Zr, Nb and FeO/MgO content, and Na2O + K2O can discriminate A-type granites. The separating line in the discrimination diagram is at Ga/Al > 2.6. All the samples of the PG are positioned exclusively in the field of A-type granite in the Zr and Nb vs Ga/Al diagrams (Fig. 10a and b). The high concentration of Na2O + K2O, high Fe number and enriched HFSE (Zr, Nb and Y) contents along with enriched LREE in the PG correspond with A-type granites (Whalen et al. Reference Whalen, Currie and Chappell1987; Eby, Reference Eby1992).
Fig. 10. (a) Zr and (b) Nb vs 10 000 * Ga/Al discrimination diagrams of Whalen et al. (Reference Whalen, Currie and Chappell1987) showing the A-type nature of the Punugodu granite. (c, d) Chemical classification diagrams of A-type granite (after Eby, Reference Eby1992): (c) Nb–Y–3Ga ternary diagram and (d) Nb–Y–Zr/4 ternary diagram. (e) Zirconium saturation level (ppm) as a function of cationic ratio M = (Na + K + 2Ca)/(Al. Si) (Watson & Harrison Reference Watson and Harrison1983) showing crystallization temperature of granite. (f) The normative Qz–Ab–Or ternary diagram shows the pressure of crystallization of PG (Johannes & Holtz Reference Johannes and Holtz1996).
Eby (Reference Eby1992) classified the A-type granites into subtypes A1 and A2, based on the Y/Nb ratio. The A1 group (Y/Nb < 1.2) was interpreted as alkaline magma derived from an oceanic island basalt (OIB)-like source in a proper anorogenic within-plate setting. The PG shows Y/Nb ratios of <1.2, ranging from 0.6 to 0.91, indicating its A1-type granite, commonly emplaced in an extensional regime in an anorogenic within-plate setting. This is further corroborated by ternary Y–Nb–3Ga and Y–Nb–Zr/4 plots (Fig. 10c and d) for the PG samples (Eby, Reference Eby1992).
Pearce et al. (Reference Pearce, Harris and Tindle1984), based on the compilation of >600 analyses of felsic rocks, divided the granites into four major tectonic groups: ocean ridge granite (ORG), volcanic arc granite (VAG), within-plate granite (WPG) and collisional granite (COLG). In the Rb vs Y + Nb graph, the PG clearly plots within the WPG field (Fig. 5f). This setting for some A-type granites has also been described from occurrences elsewhere in the world (e.g. Eby, Reference Eby1992). The prominent positive anomalies in granite and enclave samples observed for Rb, Th, U, Ce, Pr, Nb, Sm, Dy, Y and Nd are typical of anorogenic-related granite magmatism.
6.b. Magma mingling and mixing
The presence of oval-, sub-oval- and elliptical-shaped ME hosted in the PG indicates the hybridizing magma system (Kumar et al. Reference Kumar, Rino and Paul2004). The host PG and the enclaves such as xenoliths and FME show a continuous variation in SiO2 concentrations, producing tightly linear to slightly curved trends on Harker variation diagrams (Fig. 6), strong evidence for a genetic link between them. However, the observed wide composition of HME may be due to the chemical mixing between two contrasting magma end-members in varying proportions (Kumar & Rino, Reference Kumar and Rino2006). The fine-grained, igneous-textured mafic to hybrid ME from I- and A-type granites has been considered to represent mafic and/or hybrid magma components added to intermediate or felsic magma chambers (e.g. Holden et al. Reference Holden, Halliday and Stephens1987; Vernon et al. Reference Vernon, Etheridge and Wall1988; Bedard, Reference Bedard1990; Didier & Barbarin, Reference Didier and Barbarin1991; Bonin, Reference Bonin2004; Yang et al. Reference Yang, Wu, Chung, Chu and Wilde2004; Kumar, Reference Kumar2020). Silica contents in the MEs indicate a broad bimodal distribution. The SiO2 contents in MME and in some HME range from 45 to 53 %, while the SiO2 contents in FME and granite samples show a relatively wide variation from 61 to 73 %, separated by a silica gap of c. 8 wt %. The relative enrichment of Ni and Cr in the MME and HME samples suggests that they might have formed from mantle-derived mafic and mafic–felsic (hybrid) magmas respectively, injected into the crystallizing host magma chamber as described elsewhere (Kumar et al. Reference Kumar, Pieru, Rino and Hayasaka2017). Notable variation in the Ba content in some of the MME samples is perhaps due to geochemical heterogeneities in the feldspars during magma mixing (e.g. Bonin, Reference Bonin2004; Słaby et al. Reference Slaby, Smigielski and Kronz2011).
6.c. Temperature and pressure
Zirconium saturation level (ppm) as a function of cationic ratio M = (Na + K + 2Ca)/(Al. Si) (Watson & Harrison Reference Watson and Harrison1983) shows two different ranges of crystallization temperatures for granite, 750–800° C and 850–900° C (Fig. 10e). This may be because of crystallization of zircon right from liquidus to solidus. Based on the Cross, Iddings, Pirsson and Washington norm, the normative Qz–Ab–Or ternary diagram (Johannes & Holtz Reference Johannes and Holtz1996), the granite’s crystallization pressure varies from 1 to 2 GPa (Fig. 10f).
6.d. Age constraints
The southeastern margin of India has witnessed several extensional as well as subduction events. The first extension- and rift-related Palaeoproterozoic anorogenic magmatism occurred at 1.9 Ga due to mantle diapir or plume. This rifting would have allowed an open ocean to form on the eastern margin of the EDC. The occurrence of 1.85 Ga Kandra and 1.33 Ga Kanigiri ophiolite affinity rocks in the NSB indicates the repeated subduction-related tectonic convergence. The Mesoproterozoic rift-related magmatism occurred in PAkP, resulting in the emplacement of 1321 Ma Elchuru nepheline syenite, 1334 Ma Purimetla gabbro, 1352 Ma Errakonda ferrosyenite and 1356 Ma Uppalapadu nepheline syenite. We find that the plutons emplaced around the Kanigiri area, such as 1284 Ma Kanigiri granite, 1256 ± 6 Ma Punugodu granite (present study) and 1251 Ma Peddacharlopalle gabbro, are relatively younger (∼ 80 Ma) than the PAkP alkaline rocks. There is a sort of trend of emplacement age of plutons ending towards the south. Sain & Saha (Reference Sain and Saha2018) have opined that the 1284 Ma Kanigiri granite intruded the 1334 Ma Kanigiri Ophiolite and thus affirmed their post-tectonic emplacement. In line with the above argument, the studied PG with age 1257 ± 6 Ma is so far the youngest member of felsic plutons in NSB, formed and emplaced in an extensional environment. The nearly 30 Ma gap between Kanigiri and Punugodu granites has further refined the age of felsic magmatism, thus indicating the existence of a plutonic trinity (alkaline/felsic/mafic) in this belt and implications on the crustal growth during the waning stage of Columbia supercontinent fragmentation in the NSB of the EDC.
7. Conclusions
A new age of 1257 ± 6 Ma for the Punugodu granite located in the NSB of Southern India is reported. The geochemical characters of the PG broadly align its nature with the metaluminous, alkali-calcic, within-plate A-type granite. Xenoliths hosted in PG are pre-existing felsic metavolcanic rock fragments. The MME and HME represent distinct batches of mantle-derived mafic and mafic–felsic (hybrid) magmas respectively that injected, mingled and undercooled within the felsic host PG magma. The FME appears to be early crystallized cogenetic facies of the PG pluton. Y/Nb ratios (Y/Nb < 1.2) indicate an oceanic island basalt (OIB)-like source for the PG melts formed in an anorogenic within-plate setting. Field observations suggests that PG (c. 1257 Ma) has intrusive relationships with the Mesoproterozoic (c. 1334 Ma) KO of oceanic arc supra-subduction affinity. The PG probably emplaced during an extensional tectonic environment along the western margin of the Neoarchaean NSB in the eastern Dharwar Craton.
Acknowledgements
The authors are grateful to the Head Department of Applied Geochemistry, Osmania University, and the Director, CSIR – National Geophysical Research Institute, Hyderabad, for encouragement. CHN acknowledges the funding provided by the University Grant Commission under the UGC-BSR-RFSMS scheme (539/APG/2013). Dr Bernard Bonin and an anonymous reviewer are profusely thanked for their critical review and helpful suggestions. Prof. Santosh Kumar is thanked for extending constant guidance, encouragement and help in discussion during the revision of this paper. Dr Kathryn Goodenough is gratefully acknowledged for her patient and thorough editorial remarks, and constructive suggestions on technical and scientific aspects.
Conflict of interest
None.
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