1 Introduction

The temporal changes in the mechanism(s) of continental crust extraction from the mantle are a consequence of Earth’s cooling and differentiation (Laurent et al. 2014). The genesis of continental material started very early in the Earth’s history, as demonstrated by the Hadean ages (4.0–4.4 Ga) of the Acasta gneisses, Canada (Bowring and Williams 1999) and the Jack Hills, Australia (Wilde et al. 2001). Continental crust has been formed continuously since then, but the growth rates and mechanisms have changed over time (Armstrong 1981; Belousova et al. 2010). The most important changes occurred between 3.0 and 2.5 Ga, at the Archaean–Proterozoic transition (Dhuime et al. 2012). Available detrital zircon age data suggest that at least ~60–70% of the present volume of the continental crust had been generated by 3.0 Ga (Belousova et al. 2010; Dhuime et al. 2012). The widespread occurrence of granitoids in continental setting and their origin is important to understand the crustal evolution and geodynamic processes operated during the Earth's history. The generation of the huge volume of granitoids on the continental crust is due to orogenic processes either by subduction or collision (Laurent et al. 2014). The genetic classification of granitoids is mainly based on the amount of crust or mantle components or crust–mantle interactions involved in their genesis (Barbarin and Didier 1992; Chappell and White 1992; Chappell 1999; Altherr et al. 2000; Chen et al. 2002). The temporal and secular geochemical variations of granitoids provide a strong basis for understanding the nature, source, and physico-chemical conditions of felsic magmatism and the tectonic environment in which they were formed.

Amphibole and biotite are the commonly used ferromagnesian minerals to evaluate the mineral chemical variation in granites to understand the source characteristics and tectonic setting (Elliott 2001; Kumar and Pathak 2010). The composition and texture of amphibole are sensitive to pressure, temperature, and bulk composition and can be used as potential proxies for the qualitative and quantitative estimation of magmatic processes and crystallization parameters (Bachmann and Dungan 2002; Mutch et al. 2016). The Al-in amphibole (hornblende) is widely used to estimate the depth of emplacement of calc-alkaline plutons (Hammarstrom and Zen 1986; Ghent et al. 1991; Vyhnal et al. 1991). The chemical composition of amphibole controls the bulk composition, temperature, pressure, oxygen fugacity, and water content of the source magma (Schmidt 1992; de Oliveira et al. 2010; Ridolfi et al. 2010). The crystallization stability of amphibole is dependent on the water content of the melt and has been used to estimate the actual H2O content of parental magma (Naney 1983; Dall’Agnol et al. 1999; Parat et al. 2008; de Oliveira et al. 2010).

Biotite, another hydrous ferromagnesian rock-forming silicate mineral is structurally and chemically highly sensitive and can accommodate most of the major elements (Mg, Fe, Al, K, Si, O, Ti, and F) in the host magma depending upon the prevailing physico-chemical conditions such as pressure, temperature, oxygen fugacity, and composition (Wones and Eugster 1965; Czamanske and Wones 1973; Czamanske et al. 1977; Speer 1987). Biotite has the potential crystal structure to reflect the nature and physico-chemical conditions of host felsic magma from which it is formed by accommodating and substituting chemical elements from the felsic magma (Abdel-Rahman 1994; Machev et al. 2004; Kumar and Pathak 2010). The annite (Fe2+) and oxynite (Fe3+) components in biotite strongly suggest redox conditions of host felsic magma. Also, they indicate magnetite- and ilmenite-series granites (Ishihara 1977) corresponding to bimodal I- and S-type granites, respectively (Chappell and White 1974). Due to the structural and chemical sensitivity of amphibole and biotite, they can be used as potential proxies to understand the physico-chemical conditions and tectono-magmatic evolution of granitic magma (Wones and Eugster 1965; Ishihara 1977; Abdel-Rahman 1994; Ridolfi et al. 2010). The northeastern part of the Eastern Dharwar Craton (EDC) hosts abundant unclassified granitoids of the Neoarchean age. Granites from the Nizamabad region constituting the northeastern part of EDC are relatively less known, mostly limited to field and petrographic observations. This study aims to assess physico-chemical conditions (i.e., temperature, pressure, oxygen fugacity (log-fO2), H2O content), magma evolution, and tectonic setting.

2 Geology of the Dharwar Craton and field relationships

The Dharwar Craton (DC) (figure 1a) (Geological Survey of India 1996) is one of the well-studied cratonic blocks of southern India (Chadwick et al. 2000; Moyen et al. 2003; Jayananda et al. 2018). Deccan basalts overlie the northern boundary of the DC, and the southern margin is marked by high-grade Southern Granulite Terrain (SGT). The western and eastern margins are bordered by the Western and Eastern Ghats, respectively. Based on lithological associations, age, and crustal thickness, the DC is divided into two cratonic blocks, namely the Western Dharwar Craton (WDC) and the Eastern Dharwar Craton (EDC) (Swaminath et al. 1976; Chardon et al. 2008). The steep mylonitic zone along the eastern margin of the Chitradurga Greenstone Belt marks the boundary between the EDC and WDC. Based on the age of basement rocks and grade of metamorphism, and the isotopic data, the EDC is further divided into the central and eastern provinces (Peucat et al. 2013).

Figure 1
figure 1

(a) Generalized geological map of the Dharwar Craton (after Geological Survey of India 1996) showing two cratonic blocks; older Western Dharwar Craton and younger Eastern Dharwar Craton, the study area Nizamabad is located in the northeastern part of the Eastern Dharwar Craton (marked in the map). (b) Generalized geological map of Nizamabad district (after Geological Survey of India 1971) showing the undifferentiated granite gneisses and granitoids of Neoarchean age, along with Cretaceous Deccan lava flows. Sample locations are marked in figure 1(b).

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The Dharwar Craton constitutes Paleo to Neoarchean crustal blocks that record complex and diverse tectonic processes (Jayananda et al. 2018). The DC is primarily made up of different types of gneisses, two generations of greenstone successions, and calc-alkaline to potassic granitoid plutons. The WDC comprises tonalite–trondhjemite–granodiorite gneiss that was emplaced episodically between 3.44 and 3.10 Ga and are variably deformed (Jayananda et al. 2015; Ranjan et al. 2020). The greenstone successions of the DC are divided into the older Sargur Group and the younger Dharwar Supergroup (Ramakrishnan and Vaidyanathan 2008). The Sargur Group greenstones and interlayered TTG gneisses form the basement for the younger Dharwar Supergroup (Jayananda 2013). The older Sargur Group is mostly composed of supracrustals, i.e., Paleo- to Mesoarchean bimodal volcanics interspersed with meta-sedimentary units of quartz–pelite–carbonate-banded iron formation (Peucat et al. 1995; Maya et al. 2017; Jayananda et al. 2018). The younger Dharwar Supergroup comprises Meso to Neoarchean greenstone successions that unconformably overlies the basement gneisses (Chadwick et al. 1981; Srinivasan and Ojakangas 1986; Kumar et al. 1996; Jayananda et al. 2013). The WDC has undergone multiple crustal reworking events at 3.0 and 2.60 Ga, resulting in younger granitic magmatism (Jayananda et al. 2006, 2020).

The Eastern Dharwar Craton is dominated by voluminous granitoids and linear greenstone belts of the Neoarchean age (Manikyamba et al. 2017; Jayananda et al. 2018, 2020; Mohan et al. 2020). Compared to the WDC, the greenstone successions of the EDC are volcanic-dominated and reveal a bimodal age distribution at ca. 2.75–2.67 Ga and 2.58–2.54 Ga (Manikyamba and Kerrich 2012; Jayananda et al. 2013). However, Meso- to Paleoarchean continental crust in the EDC is revealed by the detrital zircon ages (Maibam et al. 2011). Similar to other Archean cratons, the EDC evolution and assembly has been a matter of debate, with the main hypothesis supportive of a plume growth model (Jayananda et al. 2000). In contrast, the mutually exclusive subduction model (Chadwick et al. 2000) is equally popular. Models involving lateral and vertical tectonics are also envisaged (Harish Kumar et al. 2003; Jayananda et al. 2013). The southern part of the EDC is relatively well-studied. Based on the isotopic and geochemical characteristics of these rocks, a convergent margin setting has been proposed for its evolution (Balakrishnan et al. 1999; Mohan et al. 2013). The Neoarchean granitoids from the EDC display great compositional diversity and are divided into TTG, sanukitoids, biotite- and two mica granites, and hybrid granites (Mohan et al. 2020). The available U–Pb zircon ages indicate a minor 2.67–2.60 Ga episode of TTG magmatism and a major 2.57–2.52 Ga episode of calc-alkaline granitic emplacement (Jayananda et al. 2020; Mohan et al. 2020). After the stabilization of the DC, the Archean basement was intruded by numerous mafic dyke swarms of Paleoproterozoic to Mesoproterozoic age (Sarma et al. 2020; Yadav et al. 2020).

The study area in the Nizamabad region forms the northeastern part of the EDC, and granitoids are the most dominant lithology (figure 1b) (Geological Survey of India 1971). Based on the areal extent and megascopic features, these granitoids are classified into five distinct plutons namely, fine to medium-grained monzogranite (Mamidipally pluton), medium- to coarse-grained syenogranite (Jankampet–Binola pluton), medium-grained leucosyenogranite (Morthad pluton), and Argul and Chillergi plutons (Geological Survey of India 1971).

However, based on field observations (colour, grain size and texture), the most common granite variants from the study area are pink to grey coloured medium- to coarse-grained inequigranular granites, with a variable amount of ferromagnesian minerals such as amphibole and biotite (figure 2a and b). The medium- to coarse-grained pink and grey granites host the microgranular enclaves (ME). Coarse-grained K-feldspar porphyritic granite and fine- to medium-grained monzogranites are also observed (figure 2c and d). The MEs are fine-grained and dominant in mafic phases, vary in size between 10 cm and 1 m, and few beyond 10 m (figure 2e and f). These MEs are globular to sub-rounded in shape, and few occur as disintegrated fragments of synplutonic mafic dyke intrusions within the host (figure 2e). The coarse-grained K-feldspar porphyritic granite and fine- to medium-grained monzogranites are devoid of MEs and synplutonic mafic dykes. The MEs share sharp boundaries with host granites (pink and grey granites) (figure 2f).

Figure 2
figure 2

Field photographs of granite variants from the study area. (a) Medium to coarse-grained pink granite; (b) Medium- to coarse-grained grey granite; (c) Coarse-grained K-feldspar porphyritic pink granite; (d) Fine-grained pink granite; (e) Disintegrated fragments of syn-plutonic mafic dyke intrusions within the host; (f) ME displays sharp contact to the host granite.

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3 Petrography

Based on field observations and detailed petrographic studies, we have divided the studied granitoids into four types, namely, hornblende biotite granites (HBG), biotite granites (BTG), monzogranites (MG), and microgranular enclave (ME).

3.1 Hornblende biotite granite

The hornblende biotite granite is essentially composed of quartz, alkali feldspar and plagioclase; the accessory mineral constituents are hornblende, biotite, titanite, epidote, apatite, and opaque minerals (figure 3a). The HBG is texturally inequigranular, and the euhedral alkali feldspars form perthite exsolutions and subhedral plagioclase feldspar exhibits polysynthetic twinning. Hornblende contains inclusions of alkali feldspar, plagioclase, and quartz (figure 3b and c). Apatites are mostly acicular, and few prismatic scraps are also observed. Secondary epidotes are common, whereas primary epidotes are rare and titanites are most commonly associated with mafic minerals like biotite and hornblende, and few allanite crystals are also observed.

Figure 3
figure 3

Microphotographs of studied granites depicting the mineral associations and textural features. (a) Hornblende biotite granite consists of sericitized plagioclase, alkali feldspar, twinned hornblende, biotite, titanite, epidote, and opaques; (b) Hornblende biotite granite shows the perthite exsolution texture, and constitutes of quartz, plagioclase, microcline, hornblende, biotite, opaques. Hornblende contains inclusions of plagioclase, alkali feldspar, and quartz; (c) Hornblende biotite granite is composed of quartz, sericitized plagioclase, alkali feldspar, and biotite. Hornblende exhibits conspicuous rhombohedral cleavage; (d) Biotite granite shows perthite exsolution texture and essentially constitutes of quartz, alkali feldspar, twinned and sericitized plagioclase, biotite, opaques; (e) Biotite granite with myrmekitic texture, microcline display cross-hatched twining, and constitutes of plagioclase, quartz, and titanite; (f) Monzogranite comprise plagioclase, quartz, and alkali feldspar with minor biotite; (g) The essential minerals in monzogranite are quartz, altered plagioclase, alkali feldspar, and biotite forms the accessory phase; (h) ME depicts the clusters of amphiboles and biotites as essential minerals with tabular and prismatic shapes, respectively. Plagioclase, alkali feldspar, quartz, and opaques are accessory phases. Abbreviations. Qtz: quartz, Afs: alkali feldspar, Pl: plagioclase, Mc: Microcline, Hbl: hornblende, Bt: biotite, Per: perthite, Myr: myrmekite, Tnt: Titanite, Ep: epidote, Chl: chlorite, Op: opaque (magnetite).

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3.2 Biotite granite

Biotite granites essentially constitute quartz, alkali feldspar, and plagioclase. The accessory phases are biotite, titanite, apatite, zircon, allanite, and opaques (figure 3d). These rocks possess an inequigranular texture with myrmekitic intergrowth and perthite exsolution. Vermicular quartz intergrowth in plagioclase forms myrmekite, and exsolution of albite from microcline results in perthitic exsolution texture (figure 3e). Plagioclase has undergone sericitization and saussuritization, and biotite is altered to chlorite.

3.3 Monzogranite

The monzogranite contains quartz, alkali feldspar, and plagioclase as essential minerals, and epidote, apatite, biotite, and titanite are accessory minerals (figure 3f). The monzogranites exhibit inequigranular texture with subhedral plagioclase showing polysynthetic twinning, and the cross-hatched twinning indicates alkali feldspars are microcline. The epidote is found to be confined to altered plagioclases due to saussuritization. Thin biotite prisms occur parallel to the twin lamellae of plagioclase. Biotite, epidote, and opaques form intergranular space between plagioclase grains and alkali-feldspar (figure 3g). The ferromagnesian component of biotite is present as relicts in plagioclase.

3.4 Microgranular enclave

The microgranular enclave (MEs) and syn-plutonic mafic dykes are fine-grained with hornblende, biotite, and plagioclase as the major minerals (figure 3h). The accessory phases include alkali feldspar, apatite, epidote, and opaques. The early formed amphiboles occur as clusters within the fine-grained groundmass. The groundmass is composed of biotite, amphibole, plagioclase, and quartz. The amphibole and biotite phenocrysts are subhedral and tabular in shape, whereas they are prismatic and acicular in the groundmass (figure 3h). Plagioclase and microcline display lamellar and cross-hatched twining, respectively. Alteration of plagioclase resulted in the formation of sericite, and chlorite is formed by alteration of biotite. MEs are fine-grained and essentially composed of ferromagnesian minerals (amphibole and biotite) and occur as rounded to sub-rounded. Few occur as syn-plutonic mafic dykes within the host granites. The MEs vary from a few centimeters to >10 m in size and show sharp contact with the host granites. From the contact relationship and relative proportion of mafic minerals, these microgranular enclaves are characterized as mafic microgranular enclaves (MME).

4 Analytical techniques

Fresh representative granitic samples devoid of alteration and weathering rinds were selected for estimating mineral compositions. For the microprobe analysis, thin sections were well polished using diamond paste (Metatech – 1 µm and ¼ µm) and were then carbon coated. The mineral compositions were analyzed using CAMECA SX Five electron microprobe at the Department of Geology, Banaras Hindu University. The analysis was carried out using wavelength dispersive spectrometry with TAP, LiF, LPET, LTAP, and PET crystals. Standards (GeoStd/Di, GeoStd/Prdt, GeoStd/Alm, GeoStd/Ab, GeoStd/Or) are used to calibrate the instrument at a voltage of 15 kV and a current 10 nA with a LaB6 source. SxSAB version 6.1 and SX-Results software were used to carry out routine calibration, acquisition, quantification, and data processing. The precision of the analysis of major element oxides was estimated to be better than 1% from the repeated analysis of standards, and the details are provided elsewhere (Pandey et al. 2017).

5 Results

5.1 Mineral chemistry

5.1.1 Plagioclase

A total of 227 plagioclase compositions were analyzed from representative samples of hornblende biotite granite (HBG), biotite granite (BTG), monzogranite (MG) and microgranular enclave (ME) (Supplementary table S1). The plagioclase composition within the studied granites varies from albite to oligoclase (figure 4). The plagioclase composition in HBG and BTG ranges from (Ab98–Ab62 to An2 –An28) and (Ab98–Ab70 to An5–An25), respectively. Likewise, in MG (Ab96–Ab78 to An2–An21) and ME (Ab97–Ab77 to An0.5–An5.6).

Figure 4
figure 4

An–Ab–Or ternary feldspar diagram (after Deer et al. 1992) depicts crystallization temperature of granitic magma using plagioclase thermometry and compositions of plagioclase, alkali-feldspars from HBG, BTG, ME, and MG.

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5.1.2 Alkali feldspar

A total of 142 alkali feldspar compositions were analyzed, representing HBG, BTG, MG, and ME (Supplementary table S2). The composition of alkali feldspar from studied granites is mostly confined to the field of orthoclase (figure 4). The composition of alkali feldspar in HBG, BTG, MG, and ME ranges from Or99 to Or95, Ab1 to Ab4, Or95 to Or99, Ab0.1 to Ab2, Or97 to Or95, Ab2 to Ab4, and Or95 to Or93, Ab1.2 to Ab6.2, respectively.

5.1.3 Amphibole

A total of 77 amphibole compositions were analyzed from HBG and ME, and only those analyses with totals exceeding 95 wt.% are considered further (Supplementary table S3). The amphibole compositions were calculated based on 23 oxygens, using Geo-fO2 software (Li et al. 2019) and only those analyses where the Ca + AlIV > 2.5 are considered to be primary amphiboles (Giret et al. 1980). The total octahedral and tetrahedral cations were adjusted to 13, and then Fe3+/Fe2+ ratios were estimated by charge balancing (Leake et al. 1997; Ridolfi et al. 2010; Li et al. 2019). According to the amphibole classification proposed by Leake et al. (1997), the amphiboles from HBG and ME are identified as edenite, Fe-edenite, and a few amphiboles from the HBG and ME are falling in the Mg-hastingsite and pargasite fields, respectively (figure 5). The Fe-rich amphiboles represent the rim compositions and iron-enrichment due to alteration along the amphibole rims. The Mg-number (Mg/(Mg + Fe+2)) of HGB amphiboles ranges from 0.47 to 0.55 with high Si (5.94–6.89). Amphiboles from the ME exhibit higher Si (6.37–7.72) and Mg-number (53–72) when compared to the HGB amphiboles. The Fe-number (Fe/(Fe + Mg)) of amphiboles within HGB and ME range from 0.27 to 0.44 and 0.44 to 0.52, respectively.

Figure 5
figure 5

Classification diagram of amphiboles (Leake et al. 1997) to exhibit HBG and ME amphibole compositions.

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5.1.4 Biotite

A total of 90 spots were analyzed from representative HBG, BTG, MG, and ME samples. The structural formula of biotite was calculated based on 22 oxygens, using an excel program designed by Li et al. (2020), and is provided in Supplementary table S4. The biotites from HBG, BTG, and MEs are Mg-rich compared to the biotites of MG, which are Fe-rich (figure 6a). On the ternary biotite classification diagram, the Mg-rich biotites are identified as magnesio-biotites, and MG biotites are ferro-biotites (figure 6b). As per Nachit et al. (1985) classification, all the studied biotites have re-equilibrated (figure 6c). The negative Al2O3–SiO2 correlations in biotites from HBG, ME, BTG, and MG (figure not shown) infer the accommodation of Al and Si atoms at tetrahedral sites of biotite during magma evolution. This is primarily governed by the change in silica activity \(\left( {{\text{a}}_{{{\text{SiO}}_{{2}} }} } \right)\) of parental magma (Abdel-Rahman 1994; Kumar and Pathak 2010). The Fe/(Fe + Mg) ratios increase from ME (0.37–0.40) and HGB (0.37–0.45), towards BTG (0.47–0.52) and MG (0.58–0.66), respectively. The Fe number of hornblende and biotite are similar in the studied HBG and ME. In contrast, a few HBG samples show a higher Fe number for biotite than those of accompanying amphiboles. The halogen content (Cl and F) in the studied granites reduces from HBG to MG.

Figure 6
figure 6

(a) Fe/(Fe+Mg) vs. Alt(apfu) (Rieder et al. 1998) biotite classification diagram; (b) (AlIV+Fe3++Ti)–Mg–(Fe2++Mn2+) ternary diagram (Foster 1960) showing biotite compositions from HBG, ME, BTG, and MG; and (c) 10×(TiO2)–FeOt + MnO–MgO ternary diagram showing possible origins for biotites from the study area (Nachit et al. 1985).

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5.1.5 Plagioclase thermometry

We have used the ternary feldspar isotherm diagram (Deer et al. 1992) to estimate the crystallization temperature of granitic magma using plagioclase thermometry (figure 4). The estimated temperatures reveal that the studied granites crystallized over a range of temperature conditions. Feldspars from the HBG are limited to the 600–900°C field, and BTG are confined to 600–800°C. Plagioclases from MG and ME are restricted to 600–700°C (figure 4). Plagioclases from the studied granitoids show more or less similar composition but with contrast in crystallization temperatures.

5.1.6 Amphibole thermobarometry and hygrometry

Amphiboles are stable across a wide range of pressure (0.1–25 kbar) and temperature (700–1100°C) conditions; hence, they can be used to effectively estimate the pressure, temperature, water content, and oxygen fugacity of the host magma (Schmidt 1992; Ridolfi et al. 2010; Nandedkar et al. 2014). We have used the Geo-fO2 software (Li et al. 2019) to estimate the above-mentioned physico-chemical parameters of the studied granites. Based on the amphibole thermometer developed by Ridolfi et al. (2010), the estimated crystallization temperature for HBG ranges from 818°C to 859 ± 22°C. Whereas those computed for ME mostly range from 800°C to 855 ± 22°C, the average crystallization temperature indicates that both HBG and ME are crystallized under similar conditions (avg. THBG = 841°C and avg. TME = 831°C), respectively. Similarly, we used an amphibole geobarometer developed by Schmidt (1992) to estimate the pressure. The results reveal that HBG amphiboles crystallized at pressures between 363 and 448 ± 60 MPa (avg. PHBG = 398 MPa). In contrast, the ME (figure 7a) is crystallized at pressures in the range of 313 to 438 ± 60 MPa (avg. PME = 386 MPa).

Figure 7
figure 7

(a) Fe/(Fe+Mg) vs. Altotal diagram (Anderson and Smith 1995) depicts the possible crystallization pressure conditions for amphiboles from HBG and ME; (b) Amphibole Fe/(Fe+Mg) vs. AlIV diagram (Anderson and Smith 1995) display possible oxygen fugacity (fO2) conditions for HBG and ME; and (c) Log fO2–T diagram for HBG and ME (Ridolfi et al. 2010) infer their amphiboles buffered at NNO (Nickel–Nickel–Oxide).

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The estimated pressures from amphibole barometry suggest that these granites crystallized at 14–15 km depths, corresponding to upper to mid-continental crust. It has been demonstrated that the amount of Al in the octahedral site of amphibole is sensitive to the water content and can be used to estimate the amount of water in the crystallizing magma (Ridolfi et al. 2010). The calculated H2O content in HBG and ME varies from 4–5 wt.% and 5–6 wt.%, suggesting high water content during the crystallization of these magmas.

5.1.7 Biotite thermometry

The Ti contents in biotite are strongly dependent on magma temperature. Using this principle, Henry et al. (2005) devised a biotite thermometer to estimate magma temperatures. The application of this thermometer is strictly valid when the XMg = Mg/(Mg + Fe) = 0.275–1.000, Ti = 0.04–0.60 apfu based on 22 oxygens and the upper limit of temperature is 800°C. The biotites from HBG (XMg = 0.65–0.54 and Ti = 0.23–0.06), BTG (XMg = 0.52–0.47 and Ti = 0.15–0.13), MG (XMg = 0.41–0.33 and Ti = 0.25–0.17), and ME (XMg = 0.62–0.59 and Ti = 0.13–0.11) satisfies the above conditions. The estimated temperatures using biotite thermometry shows a higher temperature for HGB and ME (838 to 856 ± 12°C and 844 to 847 ± 12°C) and slightly lower temperature for BTG and MG (829 to 830 ± 12°C and 820 to 829 ± 12°C). The temperature estimates of ME and HBG using amphibole thermometry and biotite thermometry are more or less similar. As stated above, the upper limit for the validity of biotite thermometry is 800°C; these values may not correspond to the actual magma crystallization temperatures.

5.1.8 Oxygen fugacity (fO2)

There are different techniques to estimate the oxygen fugacity of granitic magmas, such as zircon rare earth element systematics (Burnham and Berry 2014), the chemical composition of amphibole and biotite (Wones and Eugster 1965; Anderson and Smith 1995; Ridolfi et al. 2010). The oxygen fugacity (fO2) conditions of amphibole from the HBG and ME are calculated using Geo-fO2 software, based on the technique developed by Ridolfi et al. (2010). The disposition of the HBG and ME amphiboles on the AlIV vs. Fe/(Fe + Mg) diagram (Anderson and Smith 1995) indicates that these granites crystallized under high oxygen fugacity (fO2) conditions (figure 7b) and are confined to the field of NNO buffer (figure 7c). The estimated oxygen fugacity (log fO2) conditions for HBG and ME are –12.93 to –12.36 bar and –12.89 to –12.11 bar, respectively. The mineral assemblage of the studied granites and ME include amphibole, biotite, K-feldspar, plagioclase, magnetite, and titanite. The ternary Fe+2–Fe+3–Mg diagram is widely used to estimate oxygen fugacity (fO2) from biotite compositions (Foster 1960). Here, the field of QFM (Quartz–Fayalite–Magnetite), NNO (Nickel–Nickel–Oxide), and HM (Hematite–Magnetite) buffers correspond to an increase in fO2 from QFM to HM (figure 8a). The biotite compositions from the Nizamabad granites indicate that these magmas are crystallized under NNO buffer. The binary AlT vs. Fe/(Fe + Mg) diagram infers that the biotite compositions from the studied granites are corresponded to magnetite-series granites (oxidised) and exhibit decreasing oxidised trends from HBG to MG (figure 8b). Further, we have estimated the oxygen fugacity with biotite stability at isobaric conditions (P = 2070 bar) using the technique of Wones and Eugster (1965). The HBG and ME reveal higher oxygen fugacity (log fO2) conditions –13.22 to 13.01 bar and –13.12 to 13.09 bar, whereas BTG and MG exhibit slightly lower log fO2 conditions –13.38 to –13.30 bar and –13.57 to –13.39 bar, and display a reducing trend of oxygen fugacity (log fO2) from HBG to MG (figure 8c).

Figure 8
figure 8

(a) Fe+2–Fe+3–Mg ternary diagram (Foster 1960) wherein the biotites indicate NNO buffer field for studied granites (NNO: Nickel–Nickel–Oxide, QFM: Quartz–fayalite–magnetite, HM: Hematite–magnetite); (b) The Altotal vs. Fe/(Fe+Mg) diagram (Foster 1960) displays the oxidized (magnetite series) conditions for HBG, ME, BTG and slightly reduced (low-oxidized) conditions for MG; (c) Temperature (°C) vs. log fO2 (bar) diagram, of biotite stability at isobaric condition (P=2070) (Wones and Eugster 1965) showing oxygen fugacity (fO2) conditions for studied granites (HBG, BTG, and MG) and ME.

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6 Discussion

6.1 Physico-chemical conditions of granitic emplacement

6.1.1 Temperature and pressure

The estimated crystallization temperatures for granitic magma using plagioclase thermometry display a range of crystallization temperatures (figure 4) for the studied granites and MEs. Plagioclase from the HBG shows comparatively higher temperatures, limited to 600–900°C, and BTG are confined to 600–800°C. The plagioclase from MG and ME display slightly lower temperatures and restricted to 600–700°C. The amphibole thermometry of HBG and ME suggest overlapping crystallization temperatures between 818°C and 859 ± 22°C and 800°C to 855 ± 22°C for HBG and ME, respectively (avg. THBG = 841°C and avg. TME = 831°C). The calculated temperatures using biotite thermometry show the higher temperature for HGB and ME (838 to 856 ± 12°C and 844 to 847 ± 12°C) and slightly lower temperatures for BTG and MG (829 to 830 ± 12°C and 820 to 829 ± 12°C). The temperature estimates of ME and HBG using amphibole thermometry and biotite thermometry are more or less similar. The observed similarity in temperatures of HBG and ME suggest the cotectic crystallization temperature range of plagioclase and hornblende (Holland and Blundy 1994; Moazzen and Droop 2005).

The obtained amphibole geobarometry reveals that HGB amphiboles are crystallized at pressures between 363 and 448 ± 60 MPa (avg. PHBG = 398 MPa). In contrast, the MEs are crystallized at pressures between 313 and 438 ± 60 MPa (avg. PME = 386 MPa). The observed lower pressure of ME is possibly due to the undercooling of within-host granite melts (HBG and BTG) at comparatively higher oxidation conditions (Kumar 2010; Bora and Kumar 2015). Alternately, long-term interactions between mafic and felsic end-members and subsequent re-equilibration can also lower the pressure among ME (Kumar and Rino 2006; Kumar 2010; Bora and Kumar 2015). The estimated pressures suggest that these granites have been crystallized at depths of 14–15 km, corresponding to the upper to mid-continental crust.

6.1.2 Oxygen fugacity and water content

The estimated oxygen fugacity (log-fO2) conditions using amphibole compositions for HBG and ME are –12.93 to –12.36 bar and –12.89 to –12.11 bar, respectively. The estimated oxygen fugacity (log-fO2) conditions from biotite stability at isobaric pressure (P = 2070 bar) display higher oxygen fugacity (log fO2) conditions for HBG (–13.22 to –13.01 bar) and ME (–13.12 to 13.09 bar), whereas BTG and MG exhibit slightly lower log-fO2 conditions (–13.38 to –13.30 bar) and (–13.57 to –13.39 bar) (figure 8c).

The presence of opaque (Fe–Ti oxides) and ferromagnesian minerals (amphibole and biotite) account for the high fO2 conditions in granitic rocks (Czamanske and Wones 1973; Czamanske et al. 1977, 1981; López-Moro and López-Plaza 2004). The inferred oxidizing conditions based on biotite stability at isobaric pressure (P = 2070 bar) from the T vs. fO2 diagram (figure 8c) exhibit high oxidizing conditions for HBG, ME compared to BTG, and are consistent with the modal abundance of magnetite, titanite, and ferromagnesian minerals. These granites are confined to the field of NNO buffer (high log-fO2), inferring high oxidizing conditions (figure 8a), and confined to granites of magnetite series (figure 8b). In contrast, the MGs display less log-fO2 compared to HBG, ME, BTG and have evolved under a slightly reduced (low-oxidized) environment with a minor content of ferromagnesian minerals and Fe–Ti oxides and exhibit decreasing oxidised trend from HBG to MG (figure 8b). The absence of amphibole, negligible amount of biotite, and Fe–Ti oxides in MG depict magma evolution’s typical reducing trend (figure 8c).

The calculated H2O content using Al-content in amphibole for HBG and ME varies between 4–5 and 5–6 wt.%, respectively, suggesting high water content during the crystallization of these magmas. It is well established that the crystallization stability of amphibole is highly dependent on the H2O content of a melt (Naney 1983; Dall’Agnol et al. 1999; Parat et al. 2008; de Oliveira et al. 2010). The petrographic studies of HBG and ME reveal the early crystallization of amphibole with euhedral nature containing inclusions of quartz, K-feldspar, biotite, and plagioclase and the absence of relict pyroxenes. The abundant modal proportion of amphibole in the HBG and ME suggests high H2O content during the crystallization of magma. The experimental data infer that 5 wt.% of H2O content required at 400 MPa and/or H2O content of 7–9 wt.% needed at 960 MPa, for amphibole to be in liquidus silicate phase and to disappear the pyroxene (Naney 1983; Prouteau and Scaillet 2003; de Oliveira et al. 2010). This high H2O content is further evident by the pervasive sub-solidus alteration that affected the studied rocks and accounted for the intense saussuritization of plagioclase (figure 3a–d) (de Oliveira et al. 2010). The water-rich and highly oxidizing conditions for HBG, BTG and ME and slightly reducing conditions for MG infer the upper crustal origin for the Nizamabad granites.

6.2 Magma evolution and tectonic setting

Based on the available bulk rock geochemical data, the granitoids of the EDC have been divided into TTG, sanukitoids, biotite- and two mica granite and hybrid granites (Mohan et al. 2020). The modal abundance of alkali feldspar in all the granitoid variants is on par or exceeds the plagioclase, confirming these rocks are not TTGs. The mineral assemblage of the HBG, including the close association of MEs and syn-plutonic mafic dykes, hint towards their sanukitoid affinity and possible mantle contribution. The MGs are poor in ferromagnesian mineral phases and show affinity towards the crustal origin. The I-type granites generally contain calcic amphiboles, whereas, in A-type granites, amphiboles would be sodic rich (Stein and Dietl 2001). The higher Fe/Fe + Mg ratios (>0.65) are often associated with anorogenic granites that crystallize under low oxygen fugacity conditions (Anderson and Smith 1995). In the case of HBG and ME, the avg. Fe/Fe + Mg ratios are 0.47 and 0.41, respectively. The magma composition from which amphiboles have crystallized can be depicted by SiO2 vs. TiO2 plot (Droop 1987). The amphiboles that crystallize from alkaline and ultramafic magmas would be Ti-rich compared to calc-alkaline magmas. The amphiboles from the HBG and ME have been distributed into the field of calc-alkaline magma (figure 9a), supporting the above observations. The SiO2 vs. Na2O diagram of Coltorti et al. (2007) can be used to discriminate amphiboles formed in suprasubduction (S) and intraplate (I) settings. The amphibole from HBG and ME are into the S-amph field, further confirming their calc-alkaline nature (figure 9b), and association of these magmas with subduction zones.

Figure 9
figure 9

(a) SiO2 vs. TiO2 diagram (Droop 1987) depicts calc-alkaline parental magma for HBG and MEs; (b) SiO2 vs. Na2O diagram of Coltorti et al. (2007), show S-amph field for studied granites and ME; (c) TiO2 vs. Al2O3 (Jiang and An 1984), exhibits crust-mantle mixed source for HBG and ME.

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Another important tectono-magmatic discriminator was AlIV = 1.5, where AlIV > 1.5 value denotes the island arc setting, while AlIV < 1.5 denotes an active continental margin (Li et al. 2014). The observed AlIV values for studied amphiboles from Nizamabad rocks are <1.5, which further attests to subduction affinity. Al2O3 and TiO2 compositions of amphiboles confirm the crust-mantle mixed source for studied rocks (figure 9c) (Changyi and Sanyuan 1984).

The Al2O3, FeOt and MgO compositions in biotite are used to discriminate between different magma type(s) and corresponding tectonic settings (Abdel-Rahman 1994), and suggests that the studied rocks are evolved from calc-alkaline orogenic magma (figure 10a and b). On the source discrimination diagram for biotites (Zhou 1986), the HBG, ME, and BTG are confined to the field of crust-mantle interaction (figure 10c), while the MG are purely crustal derived. This observation is further corroborated by comparatively lower P–T estimates and lower fO2 values of MG samples (figure 10c).

Figure 10
figure 10

(a) FeOt vs. MgO and (b) MgO vs. Al2O3 diagrams (Abdel-Rahman 1994) to discriminate the parental magmatic source for studied granitoids and ME (calc-alkaline, peraluminous, and anorogenic). The biotite compositions indicate the calc-alkaline parental source for studied granitoids. (c) MgO vs. FeOt/(FeOt+MgO) source diagram, conforming the crust-mantle mixed source for HBG, ME, and BTG and crustal source for MG (Zhou 1986).

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6.3 Regional implications

The Dharwar Craton preserved the history of crustal accretion from the Paleoarchean and continued up to Neoarchean (Jayananda et al. 2018; Ranjan et al. 2020). Along the west to east cross-section, there is a systematic transition from older to younger lithologies and a decline in the involvement of older felsic crust (Mohan et al. 2013). The EDC is dominated by voluminous 2.5 Ga potassic calc-alkaline granites and minor 2.6 Ga TTGs, whose evolution is ascribed to a convergent margin tectonic setting (Mohan et al. 2013, 2020; Jayananda et al. 2018, 2020). The older 2.6 Ga transitional TTGs or hybrid granites were sourced from lower crustal mafic rocks (Dey et al. 2017) or from a heterogenous source, involving arc crust and subduction modified mantle (Jayananda et al. 2018, 2020), or with the involvement of basaltic source, subduction modified mantle and older felsic crust (Mohan et al. 2020). The sanukitoids are syn- to post-collisional and derived from a depleted mantle reservoir enriched in incompatible elements (Heilimo et al. 2010; Laurent et al. 2014; Jayananda et al. 2018, 2020). The biotite and two-mica granites of the EDC were formed at the last stage by reworking of Neoarchean to Paleoarchean felsic crust (Mohan et al. 2020).

The mineralogy of different granitic variants at Nizamabad suggests these are akin to potassic granites such as sanukitoids, biotite and two-mica granites, and hybrid granites. The amphibole composition indicates that the HBG and ME are crystallized from water-rich magma. The presence of MEs and synplutonic mafic dykes closely associated with the HBG in the study area indicate the role of mantle-derived magmas. The amphibole composition indicates crust-mantle interactions (figure 10c). The biotite compositions from the different granites indicate significant variation in their magma sources. In the case of MG, their Fe/Fe + Mg ratios are higher than the rest of the granites, indicating reducing conditions. Also, the ferromagnesian minerals such as biotite, hornblende, titanite, ilmenite, and magnetite are the least abundant in MG, indicating their crustal origin. Studies have demonstrated that mineral compositions of pre- to post-collisional granitoids do not exhibit significant variation in their chemistry (Zarasvandi et al. 2019). Hence, it may be concluded that the granites of the Nizamabad are emplaced in a convergent margin setting. Compositional variability among these granites is ascribed to the differentiation of mantle-derived magma and the reworking of pre-existing felsic crust. However, this issue can be conclusively resolved with bulk rock geochemistry and radiogenic isotopic data.

7 Conclusions

Following are the main conclusions of this study:

  1. (i)

    Based on mineralogical associations, studied granites are characterized as hornblende biotite granites (HBG), biotite granites (BTG), monzogranites (MG) and microgranular enclaves (ME).

  2. (ii)

    The estimated thermobarometric conditions of HBG and ME are 818 to 859 ± 22°C and 800 to 855 ± 22°C, respectively, with the pressure of 363 to 448 ± 60 MPa (HBG) and 313 to 438 MPa ± 60 (ME).

  3. (iii)

    The estimated temperatures using biotite thermometry shows the higher temperature for HGB and ME (838 to 856 ± 12°C and 844 to 847 ± 12°C), slightly lower temperature for BTG and MG (829 to 830 ± 12°C and 820 to 829 ± 12°C).

  4. (iv)

    The composition of amphibole and biotite suggest their crystallization from calc-alkaline parental magma in subduction setting at high oxygen fugacity (fO2) conditions.

  5. (v)

    The water-rich (>5 wt.% H2O content) and high oxidizing conditions (NNO + 2) of HBG, BTG and ME correspond to magnetite (oxidized) series granites.

  6. (vi)

    The amphibole and biotite compositions from the Nizamabad granites suggest the crust-mantle mixed source for HBG, ME and BTG, while the MG are purely crustal derived.

  7. (vii)

    The estimated pressures suggest that these granites crystallized at depths of 14 to 15 km, corresponding to the upper to mid-continental crust.

  8. (viii)

    The study infers the role of convergent margin tectonics in the emplacement of compositionally variable granites in the NE part of Eastern Dharwar Craton.