geochemical signatures of adoni porphyritic ...

Journal of Applied Geochemistry Vol. 19, No. 2 (2017). pp. 183-199

GEOCHEMICAL SIGNATURES OF ADONI PORPHYRITIC GRANITOIDS, EASTERN DHARWAR CRATON, INDIA: IMPLICATION FOR PARTIAL MELTING OF LOWER CONTINENTAL CRUST Th. Dhanakumar Singh1, C. Manikyamba1, G. Lakshminarayana2, and K.S.V. Subramanyam1 1

CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad, India 2 Midwest Group, Banjara Hills, Hyderabad, India Email: *[email protected]

Abstract The porphyritic granitoids of Adoni area, located ~120 km west of Cuddapah basin, are intrusive into the Peninsular Gneissic Complex (PGC) of Dharwar craton. They contain outliers of Gulcheru sediments and are characterized by well-preserved primary igneous foliation. Petrographically, these granitoids are characterized by distinct porphyritic texture marked by the occurrence of prismatic microcline and lath shaped plagioclase phenocrysts within a groundmass of quartz, microcline, plagioclase, amphibole and biotite. Local development of myrmekitic texture is also noticed. The occurrence of perthites in the form of exsolution lamellae of albite within K-feldspar reflect a sub-solvous cooling of the parent magma during crystallization. Geochemical characteristics suggest that the studied granitoids have alkaline to peralkaline composition with prominent A-type signatures that collectively endorse a magmatic origin for their generation. Chondrite-normalized rare earth elements (REE) patterns exhibit enrichment in LREE and prominent negative Eu anomaly (Eu/Eu* = 0.13 to 0.27) indicating significant plagioclase fractionation from the parent magma. Trace and REE compositions combined with primitive mantle normalized trace element abundance marked by positive Rb and negative K, Nb-Ta, Zr-Hf anomalies suggest derivation of these granitoids by low pressure-high temperature partial melting of a tonalitic-granodioritic crust at mid to shallow crustal levels followed by fractional crystallization. The crustal melting has been induced by asthenospheric upwelling and minor basaltic underplating. Higher concentration of U (4-19 ppm) and Th (16-49 ppm) in these granitoids relative to the high field strength elements (HFSE) may reflect U and Th mineralization. Geochemical signatures corroborate a within plate tectonic realm for the origin and emplacement of these granitoids. Keywords: Eastern Dharwar Craton; Cuddapah Basin; Porphyritic granitoids; Peralkaline; A-type signatures; Within plate.

1. Introduction

Archean-Proterozoic have been identified through detailed studies of the tonalite-trondhjemite-granodiorite (TTG) and granites/granitoids from various cratons, which has led to the identification of sanukitoids that are suggested to reflect the transitional geochemical characteristics that are significant in understanding the Archean-Proterozoic accretionary processes (Jayananda et al., 2000 and references there in). Various aspects controlling the emplacement of granitoid plutons in the Precambrian shield areas and Proterozoic basins of India have been extensively studied to understand the crustal evolution of the respective cratons (Moyen et al., 2003b; Chardon and Jayananda, 2008; Jayananda et al., 2000; Gireesh et al., 2012).

Precambrian crustal components play a vital role in understanding Earth’s crustal growth, as more than 60% of global continental crust was formed during the Archean period suggested to be recycled and mixed with the younger crust through various tectonic processes. Recent studies indicate a large geochemical diversity in Archean granites among which the K-rich granites are linked to late stages of tectonic stabilization. The Proterozoic era represents an inevitable period in the Earth’s magmatic history marked by peak granitoid magmatism, crustal growth and continent generation .Globally, Precambrian granite magmatism has played a significant role in the growth and stabilization of continental crust. In India, it appears to have taken place between Neoarchean to Paleoproterozoic periods(Sesha Sai, 2013). An in depth understanding of their geochemical characterization and petrogenetic processes will throw light on Precambrian crustal evolution processes (Frost and Frost, 2013). Geochemical changes during the

Granitoids are derived by the melting of preexisting metasedimentary or metaigneous rocks and the resultant products are the large- to moderate plutonic intrusions observed within the country rocks in syn- to post- collisional tectonic environments. Granitoids are mostly formed by post-collisional thickening and

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delamination of the lithosphere in an extensional regime. The former covers 80% of granitic rocks worldwide. Granitoids display a large compositional diversity arising from different source compositions, variable melting conditions, complex physical and chemical interactions between mafic and felsic magmas, fractional crystallization and crustal contamination. Some models emphasize fractional crystallization as the dominant process controlling elemental variations observed in granitoid rocks, while in contrast, other processes highlight the importance of source rock composition in the origin of peraluminous, metaluminous, I-type or S-type granitoids. Therefore, compositionally well-characterized granitoids may constrain the evolution and development of continental crust through geological time (Barbarin, 1999).

exposes some of the best crustal sections with exceptionally fresh exposures to study the Precambrian crustal growth processes. The craton is divided into two blocks as the western and eastern Dharwar Cratons based on the nature, abundance and age of greenstone belts, basement gneisses as well as the degree of metamorphism (Jayananda et al., 2000; Chadwick et al., 2007). The Western Dharwar Craton (WDC) encompasses 3.4-3.0 Ga Peninsular gneisses, Sargur Group of (>3.0 Ga) greenstone belts which are overlain by the 3.0 – 2.6 Ga Dharwar Supergroup as supracrustal belts (Nutman et al., 1996). The Eastern Dharwar Craton (EDC) is a collage of N-S trending narrow ~2.7 Ga greenstone belts, gneisses/ migmatites (2.7–2.55 Ga) with minor remnants of an older crust (3.0–3.38 Ga; Peucat et al., 2013), abundant younger TTG gneisses (2.7 – 2.55 Ga), and 2.5-2.2 Ga later granite plutons (Jayananda et al., 2000). Older basement gneisses are present as huge enclaves (within younger TTG gneisses) in the south-eastern part of the EDC (Jayananda et al., 2000), whereas the younger TTG gneisses occupying vast areas, especially in the northern parts were accreted during 2.7–2.55 Ga (Jayananda et al., 2000). The juvenile continental accretion during 2.55-2.51 Ga has led to the emplacement of large magmatic bodies like the Closepet granite and other related granite intrusions during a tectono-metamorphic event that affected the craton at the close of the Archaean (Jayananda et al., 2000; Moyen et al., 2003b). The EDC is overlain by the intra-cratonic Proterozoic Cuddapah Basin –constituted by a thick sequence of sediments and volcanics. The Cuddapah sediments (exposed thickness >7 km) have been interpreted to have been deposited in subtidal to intertidal, fan delta, offshore and carbonate shelf marine environments the migration of basin depocenter with time being reflected through the deposition of the Cuddapah Supergroup in different sub basins. Geophysical studies suggest occurrence of >10 km thick sediments over a felsic crust with a depth of~37 km to Moho, with step faults offsetting the Moho and possible magmatic underplating under the southwestern Papaghni sub basin (Kaila et al., 1987). However, the exact sediment thickness in Cuddapah basin is debatable due to the recent report of Chandrakala et al., (2013) which infers ~4 km. Previous studies on the granitoids in EDC, have been largely confined to the central and southern parts, and studies focusing on the

The Dharwar Craton of southern peninsular India exhibits excellent preservation of TTG, granite and granitoid plutons in and around the greenstone belt sequences that are divided into the western and eastern sectors (WDC and EDC; Naqvi and Rogers, 1987; Ramakrishnan and Vaidyanadhan, 2008; Jayananda et al., 2000). The N-S trending Closepet granitoid batholith situated in the EDC has been extensively studied by many authors and its sanukitoid parental magma composition has been suggested by. Several authors have studied the petrogenesis of small scale granite/granitoid plutons that are present east of Closepet granite. Krogstad et al., (1995) have the reported extension of younger plutons 100 km east of Closepet granite. The area of present study is located in the EDC where an array of granites and gneissic rocks (PGC) and granites similar to the Closepet granites are exposed. Towards the east, PGC is unconformably overlain by the Proterozoic Cuddapah basin. The present study area is located about 10 km NE of Adoni town whose environs expose a monotonous tract of pink to brown granite and gneisses intruded in to grey granites. Here, NW-SE trending, linear and foliated porphyritic granites are exposed in the area of study. In this paper, we document the petrogenesis and tectonic setting of emplacement of porphyritic granitoids of Adoni, west of Cuddapah basin through major, trace and rare earth elemental studies.

2. Geological Setting The Archean Dharwar Craton in peninsular India is bestowed with diverse varieties of granitoids and

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granitoids situated west of the Cuddapah basin are lacking till date.

The EDC is dominated by late Archean granites, granodiorites, monzonites and diorites together named as The Dharwar batholith which also includes the N-S trending Closepet granite consisting of different phases of granitoids (Condie et al., 2009a). The study area is situated ~68 km east of Closepet granite (from Bellary) and ~66 km west of Cuddapah basin (Kurnool). The Kanigiri, Podili and Vinukonda granites present within the Nellore schist belt situated at the eastern margin of Cuddapah basin have been studied in detail who has characterized them as A-type granites have conducted detailed petrological, geochemical and geochronological studies on the porphyritic granites present at the southeastern part of Cuddapah basin, characterizing them as A-type and suggesting to have been derived by partial melting of the cratonic crust due to basaltic underplating. The eastern margin of Cuddapah basin exposes a variety of Proterozoic A-type granite plutons in Vinukonda, Darsi, Podili, Kanigiri, Diguvapalle, Komatigunta, Anumalakonda, Chandrashekarapuram and Pamuru areas among which some of them have been emplaced into the Nellore schist

The present study, focused largely on the porphyritic granitoids occurring in the Adoni area in Kurnool district (Fig. 1), located to the west of Cuddapah basin in the EDC and exposed within late Archaean granitic gneiss, migmatites and granitoids together known as Peninsular Gneissic complex (PGC). Within this PGC terrain, are observed a set of NW-SE dykes and remnants of schist belts, prominent among being the Gadwal and Jonnagiri schist belts. The PGC terrain around the study area exposes different phases of granites and graniticgneiss in which light purple to purplish-red coarse grained, porphyritic granitoids occur as a massive intrusive body into the foliated gneisses, the trend of foliation being NW-SE. These porphyritic granitoids are intruded into the non-porphyritic granites and devoid of dykes. In this region, granitoids occur in batholithic dimensions. The different granite types include tonalite-adamellitegranodiorite along the margins and reddish-pink to purplish- red porphyritic granitoids in the centre (Fig.2).

Fig. 1. (a) Inset map showing the location of the Adoni granites in the eastern Dharwar Craton and (b) geological map of the selected part of Kurnool district showing the sample locations.

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belt are intensely deformed (Sesha Sai 2013). The emplacement of these granitoids is considered to be a significant event of Precambrian crustal growth in Eastern Dharwar Craton (Sesha Sai, 2013) have reported a Pb–Pb baddeleyite age of 2082 Ma for the dykes intruding into the basement gneiss in the EDC in the northern, northwestern and western flanks of the Cuddapah basin and have attributed such an emplacement to large scale crustal extension and thinning followed by thermal relaxation and subsidence that were initially responsible for the formation of the Cuddapah basin. The PGC observed in Adoni area constitute the basement to the early to late Proterozoic Cuddapah Basin (Fig.1). The N-S to NNE-SSW trend of the Cuddapah Basin is which is tangential to the PGC. Outliers of Gulcheru quartzite/ conglomerate occur over these PGC gneisses and granites, thereby indicating that some of the granites particularly porphyritic granitoids of Adoni appear to have emplaced after the deposition of the Cuddapah basin sediments. The Adoni granitoids are well exposed in gently rolling hills aligned NW-SE (Fig. 2a). These are predominantly massive with the preservation of faint, wide spaced primary igneous foliation defined by flaky minerals such as biotite and other ferro-magneisan minerals (amphibole) as observed on weathered surfaces. These granitoids are brown to grayish brown, light pink in colour with feldspar phenocrysts aligned in a NW-SE direction. Size of the phenocrysts ranges from 55-75% threshold of solid fraction . Phenocryst development is a subsurface magma chamber process that requires stable tectonic conditions and the associated plutonic emplacement is correlatable with an post-collisional magmatic episode. 6.3 Geodynamic Implications Numerous studies have shown that A-type granites are formed in a crustal extensional environment either in post-orogenic or anorogenic settings (Eby, 1992; Barbarin , 1999; Bonin, 2007). Therefore, A-type granites can be considered as diagnostic features of intra-plate tectonic realms. A-type granitoids that plot within the VAG field and close to the WPG boundary in the geotectonic discrimination diagrams of Pearce et al., (1984) are mainly post-orogenic, aluminous, sub-solvus types and enriched in Rb, REE, Y and Th that belong to the A2type series (Eby, 1992; Bonin, 2007).

Fig. 12. (a-c) Nb-Y-3*Ga; Y/Nb vs. Rb/Nb; Nb vs. Y plots (Eby, 1992) indicating the studied granites are A2 type. porphyritic Adoni granitoids have been generated through partial melting of a tonalite-granodiorite source at mid to lower crustal levels. The melting was initiated by crustal uplift due to asthenospheric upwelling and basaltic underplating caused by localised mantle perturbations or instability in a localised area due to deep seated thrust faults or low density crustal rocks. After removal of plagioclase and K-feldspar, the residual melt was emplaced in a relatively shallow crust, giving rise to the studied A-type granitoids under lithospheric extensional tectonic setting.

The studied granitoids occupy the A2 field on YNb-3*Ga; Y/Nb vs. Rb/Nb and Nb vs. Y discrimination diagrams (Fig.12 a,b,c) endorsing a post-orogenic extensional tectonic affinity. These granitoids occupy the field of within plate granite (WPG) in the Y+Nb vs. Rb tectonic discrimination diagrams (Fig.13a), confirming their generation in an extensional setting. These observations are also manifested in SiO2 vs. log[CaO/ (K2O/Na2O)] binary diagram depicting their genesis in an extensional tectonic regime (Fig. 13b).

7. Conclusion

On the basis of observed field, petrological and geochemical characteristics it is interpreted that the

The porphyritic granitoids of Adoni occur as plutonic intrusions into the Peninsular Gneissic Complex

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Acknowledgements The authors thank Dr. V.M. Tiwari, Director, CSIRNGRI for permitting to publish this work. CM (C. Manikyamba) acknowledges the funds from Council of Scientific and Industrial Research (CSIR) to National Geophysical Research Institute through the project of MLP 6201-28 (CM) and Ministry of Earth Sciences (No: MoES/PO(Geosci)/8/2014). The authors are grateful to the two anonymous reviewers whose constructive suggestions have strengthened the paper scientifically. Drs. M. Satyanarayanan, S.S. Sawant and A.K. Krishna are acknowledged for providing the geochemical data.

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