Geochemistry of granulitic rocks from the western Madurai Block, Southern Granulite Terrain, India and its Madagascar linkage

Abstract

The Southern Granulite Terrain of peninsular India consists of a wide range of metamorphic rocks with formation ages that span the late Archean Era to the Cambrian Period. It consists of numerous tectonic blocks dissected by deep crustal-scale shear zones. The Madurai Block is the largest crustal block, comprising Neoarchean to Ediacaran-Cambrian gneisses that include charnockite, hornblende-biotite gneiss, mafic granulite and metapelite, amongst other lesser rock types. This study focuses on the geochemistry of granulite-facies rocks from the western part of the Madurai Block, how these rocks correlate with similar types in other tectonic blocks of the Southern Granulite Terrain, and the implication of such correlations for East Gondwana tectonics. The geochemistry of the various granulite-facies rocks from the western Madurai Block reveals metaluminous to slightly peraluminous, calcic to alkalic, and ferroan to magnesian signatures. Geochemical tectonic discrimination diagrams indicate both A-type granitoid and Cordilleran affinities, consistent with petrogenesis in active continental margin and extensional tectonic settings, with chemical variation also generated through magmatic differentiation. Similar lithological, geochronological and geochemical features have been reported from granulites of the Antananarivo Block of Madagascar, based on which a correlation can be made with the western Madurai Block that predates Gondwana assembly.

INTRODUCTION

The southern part of peninsular India, paleogeographically located in Gondwana between Madagascar, Sri Lanka, and East Antarctica (e.g., Katz and Premoli, 1979, Ishwar-Kumar et al., 2013; Ratheesh-Kumar et al., 2014), consists of the Archean Dharwar Craton to the north and the Archean-Proterozoic Southern Granulite Terrain to the south. The Southern Granulite Terrain (SGT) is tectonically and petrologically complex, with a variety of granulite-grade rocks that trace the evolution and tectonic settings of the region. The SGT is dissected by major shear zones that divide it into various crustal blocks (Drury and Holt, 1980). The largest amongst these is the Madurai Block, which is bound to the north by the Palghat Cauvery shear zone (PCSZ), and by the Achankovil shear zone (ASZ) to the south (Fig. 1). The Madurai Block comprises granulite-facies rocks that are mostly metaigneous, with chemistries that vary widely between calcic and alkaline, magnesian and ferroan affinities. Tomson et al. (2006, 2013) suggest that protoliths to granulite-facies rocks were formed by subduction-related arc magmatism during the late Archean and Proterozoic.

Figure 1. (a) Map showing the tectonic framework of Southern Granulite Terrain (Ishwar-Kumar et al., 2013). TB, Trivandrum Block; NB, Nilgiri Block; BR, Billigiri Rangan Block; SB, Salem Block; NaB, Namakkal Block; MB, Madras Block; ASZ, Achankovil Shear Zone; SSZ, Suruli Shear Zone; KKPTSZ, Karur Kambam Painav Trichur Shear Zone; PCSZ, Palghat Cauvery Shear Zone; BSZ, Bhavani Shear Zone; CaSZ, Cauvery Shear Zone; MSZ, Mercara Shear Zone; IB, Isotopic Boundary (after Plavsa et al., 2012). The inset (black rectangular box) shows the study area. (b) Geological map of western Madurai Block, modified after Geological Survey of India (1984, 1992), blue dots mark the locations of rock samples selected for this study.

The SGT has been correlated with paleo-continental fragments of the Gondwana assemblage in Sri Lanka, East Antarctica, and Madagascar (e.g., Plavsa et al., 2012; Li et al., 2020; Durgalakshmi et al., 2021). The Antananarivo Block of Madagascar, adjacent in Gondwana to the western Madurai Block, is also predominantly composed of Neoarchean granulites. Both blocks were later metamorphosed during the Neoproterozoic (700-500 Ma). The rocks from the Antananarivo Block of Madagascar also show geochemical characteristics that vary from calcic to alkali-calcic and ferroan to magnesian (Collins et al., 2003; Ichiki et al., 2015). In this study, an attempt is made to compare and correlate the geochemical features of granulites of the Madurai Block in the SGT and the Antananarivo Block of Madagascar, to better understand the tectonic connections between the two.

Regional geology

The Madurai Block is the most significant crustal fragment of SGT and is located south of the PCSZ and north of the ASZ. The Madurai Block predominantly consists of charnockite, orthogneiss, and metapelites. Recent studies on the Madurai Block (Brandt et al., 2011; Plavsa et al., 2012; Brandt et al., 2014; Collins et al., 2014; Plavsa et al., 2014, 2015) have divided this crustal fragment into crustal domains having varied evolutionary histories based on evidence from geological observations and geochronology. Ghosh et al. (2004) introduced the Karur-Kambam-Painavu-Trichur shear zone (KKPTSZ) as a terrane boundary which divided the Madurai Block into older and younger domains. Subsequently, Plavsa et al. (2012) proposed an isotopic age boundary that separates the Madurai Block into northern and southern parts that parallel the KKPTSZ. The crystallisation of protoliths to metaigneous rocks in the northern part of the Madurai Block occurred in the Neoarchean (∼ 2500 Ma), whereas those in the southern part were emplaced in the Neoproterozoic Era (∼ 750 Ma). In addition, Brandt et al. (2014) proposed that the Madurai Block is divided into an older Archean western Madurai Block and younger Proterozoic eastern Madurai Block (Fig. 1) by the eastern branch of the KKPTSZ and the Suruli shear zone (SSZ). Later, high-temperature metamorphism occurred during the Ediacaran-Cambrian Period during the final stage of Gondwana amalgamation. The chemical composition of metaigneous rocks in the Madurai Block have both magnesian and ferroan; metaluminous to marginally peraluminous; calcic to alkalic characteristics (Rajesh and Santosh, 2004; Tomson et al., 2006, 2013; Kumar et al., 2017; Santosh et al., 2017).

FIELD RELATIONSHIPS

The predominant rock type of the western Madurai Block is charnockite (Fig. 2a). Charnockite is medium-coarse grained and leucocratic to melanocratic with a mostly granoblastic texture. Enclaves of mafic granulite and a high abundance of opaque minerals are particularly prevalent in charnockite north of the KKPTSZ. Some charnockite exposures are intruded by pink granite and younger quartz veins. The melanocratic (mafic) type of charnockite is composed of orthopyroxene, clinopyroxene, amphibole, biotite, and plagioclase. This is typically associated with hornblende-biotite gneiss in the field. In contrast, garnet-bearing charnockite is mostly seen along and to the south of the Plavsa et al. (2012) isotopic boundary (Fig. 2b).

Figure 2. Photographs show field exposures of representative rock types of the western part of Madurai Block. (a) K1B, massive charnockite in quarry. (b) AS-02-01, garnet-bearing charnockite (medium to coarse-grained). (c) AS-12-18-8A, Hbl gneiss with mafic enclave. (d) AS-27-12-1A, pegmatite vein in hornblende-biotite gneiss. Mineral abbreviations after Whitney and Evans (2010).

Hornblende-biotite gneiss commonly contains networks of leucosome, which are attributed to partial melting (Fig. 2c). Veins of felsic pegmatite (Fig. 2d) and intrusive bodies of pink granite are also common. In the vicinity of the KKPTSZ, gneiss is usually found interlayered with hornblende-free clinopyroxene felsic granulite and other felsic gneisses. To the north of the isotopic boundary, hornblende-biotite gneiss is more deformed and is commonly found with mafic granulite instead.

PETROGRAPHY

The petrographic characteristics of all the studied samples are briefly summarised here and representative photomicrographs of samples are shown in Figure 3. Sample locations, rock types, and mineral assemblages are provided in Supplementary Table S1 (Supplementary Tables S1 and S2 are available online from https://doi.org/10.2465/jmps.221212). Charnockite is comprised of orthopyroxene + plagioclase + K-feldspar + quartz ± clinopyroxene ± amphibole ± garnet ± biotite (Fig. 3a). Biotite inclusions are common in plagioclase, and fine plagioclase inclusions are present within the core of resorbed orthopyroxene and garnet. Orthopyroxene and clinopyroxene have idioblastic and granoblastic relationships with adjacent grains of K-feldspar and plagioclase. The presence of numerous antiperthite associated to the grain boundaries of minor perthite grain aggregates in the same samples might indicate the continuous change in the original magma composition.

Figure 3. Photomicrographs of representative rock types of the western part of Madurai Block. (a) Charnockite with granoblastic texture. (b) Hornblende-biotite gneiss. (c) Mafic granulite with retrograde metamorphism demonstrated by hornblende rims on clinopyroxene. (d) Clinopyroxene felsic granulite with mesoperthite (Msp).

Hornblende-biotite gneiss is leucocratic to mesocratic (Fig. 3b) and generally has an assemblage of K-feldspar (40-20%), plagioclase (30-20%), hornblende (30-10%), biotite (20-10%), and quartz (20-10%), with accessory amounts of zircon and Fe-Ti oxides. Mafic/intermediate granulite is composed of K-feldspar (40-20%), plagioclase (20-30%), hornblende (20-30%), clinopyroxene (10-20%) and biotite (10-20%), and lesser quartz and Fe-Ti oxides. Clinopyroxene is commonly replaced by hornblende, whereas biotite defines a weak foliation that predates the hornblende replacement (Fig. 3c).

Clinopyroxene-bearing felsic granulite is composed of mesoperthite (50-60%), clinopyroxene (20-30%), and quartz (10-20%) with lesser plagioclase, magnetite, ilmenite, and apatite (Fig. 3d). Hydrous minerals like hornblende were not identified in the observed sections.

WHOLE ROCK CHEMISTRY

Twenty-eight rock samples from different parts of the western Madurai Block were selected after petrography and analysed for major and trace elements using a WD-X-Ray Fluorescence spectrometer housed at the National Centre for Earth Sciences, Thiruvananthapuram, India. Analysis followed the protocol of Dev et al. (2021). Charnockite and other rock types classified with the QAP diagram (Fig. 4a) have compositions of granite, granodiorite, quartz-monzodiorite, quartz-monzonite, and quartz-diorite. The SiO₂ content of different rock types are as follows: granitic compositions (64.44-77.76 wt%); granodiorite (59.38-69.95 wt%); quartz-monzonite (46.94-60.83 wt%); quartz-monzodiorite (48.46-60.49 wt%); Quartz-diorite/gabbro (49.66-59.20 wt%); and monzodiorite/monzogabbro (50.44-52.44 wt%). The normative An-Ab-Or diagram of Barker (1979) reveals that charnockite from this region shows a wide range of compositions from tonalitic to granitic (Fig. 4b). On the AFM diagram, most samples have a calc-alkaline affinity. In contrast, mafic charnockite, diorite, some samples of charnockite, Grt-charnockite, Hbl-Bt gneiss, and Cpx granulite have tholeiitic affinities (Fig. 4c). The samples are mostly metaluminous, with a few being weakly peraluminous (Fig. 4d). The K₂O content of charnockite varies widely from 0.5 to 6.4 wt%. The Rb-Ba-Sr ternary diagram (Fig. 5) of El Bouseily and El Sokkary (1975) and the QAP diagram yield similar classifications for the samples, i.e., granite to diorite (Fig. 4a). Most of the samples from the study area have magnesian affinity on the SiO₂ versus FeO/FeO+MgO diagram (Fig. 6). On the SiO₂ versus modified alkali lime index (Na₂O + K₂O-CaO) diagram, charnockite has calcic to alkali-calc affinities (Fig. 6). The dioritic granulite is alkalic-calc and ferroan.

Figure 4. (a) Geochemical classification based on QAP diagram after Streckeisen (1979). (b) Normative Ab-An-Or ternary plot and classification of rocks from the study area (excluding mafic rocks) after Barker (1979). (c) Ternary AFM [total alkali (A) - total FeO (F) - MgO (M)] diagram. The discrimination fields of calc-alkaline and tholeiitic rocks are after Irvine and Baragar (1971). (d) Alumina Saturation Index (ASI) diagram after Wilson (1989).

Figure 5. Ternary Rb-Sr-Ba diagram after El Bouseily and El Sokkary (1975).

Figure 6. (a) SiO₂ versus K₂O + Na₂O-CaO (MALI, modified alkali-lime index) with alkalic, alkali-calcic, calc-alkalic, and calcic rock series for samples from the Coorg Block, Nilgiri Block, Biligiri Rangan Block, and Shevaroy Block. (b) SiO₂ versus FeO/(FeO + MgO), the boundary between magnesian and ferroan plutons and the composition fields of leucogranites, A-type granites (= ferroan) and Cordilleran-type granitoids in the Coorg Block, Nilgiri Block, Billigiri Rangan Block, and Shevroy Block. (c) SiO₂ versus K₂O + Na₂O-CaO of the Madurai, Trivandrum, and Nagercoil blocks. (d) SiO₂ versus FeO/(FeO + MgO) of the Coorg Block, Nilgiri Block, Billigiri Rangan Block, and Shevroy Block. (e) SiO₂ versus K₂O + Na₂O-CaO of the Antananarivo Block. (f) SiO₂ versus FeO/(FeO + MgO) of Antananarivo Block (after Frost et al., 2001; Rajesh and Santosh, 2004; Tomson et al., 2006, 2013; Brandt et al., 2014; Ichiki et al., 2015; Ravindra Kumar and Sreejith, 2016; Ratheesh-Kumar et al., 2020, Dhanil-Dev et al., 2022).

DISCUSSION

The Madurai Block comprises granulite-grade charnockites, orthogneisses, and paragneisses. Mineral assemblages are the product of high-temperature metamorphism (Fig. 2), with or without indicators of partial melting (such as that observed in hornblende-biotite gneiss) and hydrous retrogression of charnockite (with hornblende after clinopyroxene, e.g., Fig. 3c).

The geochemical characteristics of charnockite from the study area have wide variations in chemical composition, with SiO₂ contents ranging between 50 to 70 wt%. A similar range of SiO₂ values has been reported from charnockite in the southwestern part of Madurai Block (Rajesh and Santosh, 2004; Tomson et al., 2006; Plavsa et al., 2012; Kumar et al., 2017; Santosh et al., 2017). The wide variation of silica content can be attributed to the fractional crystallisation and varying interaction with pre-existing crust during the ascent of primary magma. Felsic magmas that formed the protoliths to charnockitic granulites can be attributed to a more significant component of crustal melting, whereas more intermediate compositions may derive from melting of more mafic crust or mixing of mafic magmas with felsic magma. In addition, most of the rocks in the study area have metaluminous affinities, with the presence of calcic and aluminous minerals such as hornblende, clinopyroxene, and plagioclase (Fig. 4d). The more mafic samples have stronger metaluminous characteristics, which may be due to the melting of hydrous enriched sources (Frost et al., 2001). In contrast, peraluminous rocks can be formed by the mixing of magmas from sedimentary sources.

Most of the samples from this study show magnesian calcic to alkalic-calc affinities that can be attributed to Cordilleran-type granitoid magmatism associated with subduction on a continental margin (Tomson et al., 2013; Brandt et al., 2014). Fewer samples plot in the A-type granitoid field of Frost et al. (2001) with more ferroan alkalic-calc and ferroan alkalic characteristics and may be attributed to extensional processes (Rajesh and Santosh, 2004).

Comparison with other blocks of the SGT and Madagascar

The available geochemical data of different rock types from the SGT were used to help unravel the geodynamic evolution of various tectonic blocks. Data from other crustal blocks in SGT were compiled and presented in Figure 6 as SiO₂ versus Na₂O + K₂O + CaO and SiO₂ versus FeO/(FeO/MgO) diagrams after Frost et al. (2001). As reported from previous studies, charnockite from the Coorg Block exhibits metaluminous, magnesian, calcic to calc-alkaline characteristics characteristic of volcanic arc magmatism (Dhanil-Dev et al., 2022) at ∼ 3300 Ma with subsequent metamorphism at ∼ 3100 Ma (Santosh et al., 2015; Anoop et al., 2022). Neoarchean charnockite of the Nilgiri Block is magnesian with calcic to calc-alkaline affinities corresponding to Cordilleran-type granitoids. Magmatism occurred in an island arc setting between 2700 and 2500 Ma, with accretion at 2500 Ma (Samuel et al., 2014, 2016). Charnockites of the Billigiri Rangan Block fall within the tonalite and trondhjemite fields and have calcic to calc-alkaline, magnesian to ferroan, metaluminous to marginally peraluminous characteristics that classify as Cordilleran-type granitoids (Fig. 6). A few samples have leucogranitic characteristics (Rajesh and Santosh, 2004; Ratheesh-Kumar et al., 2020). Protoliths were produced by partially melting subducted oceanic crust about ∼ 2700-2633 Ma (Ratheesh-Kumar et al., 2020). Neoarchean charnockite of the Shevaroy Block has the composition of monzodiorite-tonalite and granodiorite (Rajesh and Santosh, 2004). It has magnesian, calcic to calc-alkaline characteristics and Cordilleran-type affinities (Figs. 6a and 6b). The Paleoproterozoic rocks or charnockites from the Trivandrum Block and the Nagercoil Block fall in both ferroan and magnesian, Cordilleran and A-type granitoid fields (Rajesh and Santosh, 2004; Ravindra Kumar and Sreejith, 2016).

The Madurai Block underwent multiple magmatic events during Neoarchean (∼ 2500 Ma) and Neoproterozoic period (∼ 750 Ma). During the final stage of Gondwana amalgamation (Plavsa et al., 2012), the Madurai Block experienced high to ultrahigh-temperature metamorphism (∼ 1000 °C and ∼ 12-9 Kbar) along a clockwise P-T path typical of a collisional tectonic setting (Brown and Raith, 1996). The geochemical features of rocks show calcic to alkalic, magnesian to ferroan characteristics, and a wide range of silica content (Figs. 6c and 6d). Intermediate magmas that formed the protoliths to charnockite were generated by dehydration melting of hydrous basaltic lower crust. In contrast, felsic charnockite may be the product of differentiation and/or partial melting of more felsic sources. More ferroan, A-type chemistries were produced in extensional settings, in contrast to Cordilleran-type granitoids. The diversity in geochemistry indicates the operation of various tectonic processes in the history of the block (Rajesh and Santosh, 2004). However, it is unlikely that these processes happened simultaneously. For example, the concentration of ferroan granitoids in the vicinity of KKPTSZ may be related to an extensional event that did not develop in the western Madurai Block.

The Antananarivo Block in Madagascar was adjacent to the Madurai Block in the Gondwana assembly mainly comprises granulite-facies rocks, including charnockite, orthogneiss, and metasedimentary gneiss (Kröner et al., 2000). Based on U-Pb zircon crystallization age and Sm-Nd whole-rock isochron results, granitoid emplacement happened in the Antananarivo Block about ∼ 2500 Ma and was later interlayered with granite, syenite and gabbro at about 824-719 Ma (e.g., Tucker et al., 1999; Kröner et al., 2000; Collins, 2006). During the Neoproterozoic, the Antananarivo Block was structurally and thermally reworked at granulite grade, but the gneisses and granulites produced retained primary tonalite-granodiorite-granite compositions. The granulites of the Antananarivo Block exhibit similar geochemical features to the western Madurai Block, with calcic to alkalic-calc, ferroan to magnesian affinities (Figs. 6e and 6f), consistent with arc magmatism (Collins, 2006; Brandt et al., 2014; Ichiki et al., 2015).

The charnockites from the Coorg Block, Nilgiri Block, Biligiri Rangan Block, and Shevroy Block have similar geochemical characteristics produced by arc magmatism in the Archean and Paleoproterozoic. The Madurai, Trivandrum, and Nagercoil Blocks exhibit geochemical features in the same tectonic settings. High-temperature metamorphism converted the rocks in all these blocks into gneisses and granulites. Similar rocks have been found in the Antananarivo Block, where Neoarchean ages are identical to those found in parts of the Madurai Block (Plavsa et al., 2012). Consequently, the northern part of the western Madurai Block may be correlated with the Neoarchean Antananarivo Block of Madagascar (Brandt et al., 2014; Ichiki et al., 2015).

CONCLUSION

The metamorphic rocks of the western Madurai Block manifest various chemical compositions from tonalite to granite, calcic to alkalic, and magnesian to ferroan affinity. They fall in both Cordilleran and A-type granitoid classifications and can attributed to different tectonic settings at various stages of crustal evolution. The close resemblance between the geochemical characteristics of charnockite from Madurai and other crustal blocks within the SGT and adjacent continental blocks in Madagascar indicates a coeval origin. Based on evidence from lithology, geochronology, and geochemical features, the Antananarivo Block of Madagascar can be correlated with the Madurai Block.

ACKNOWLEDGMENTS

The first author thanks the Kerala State Council for Science, Technology and Environment (KSCSTE) for providing financial support as a Research fellowship 001/FSHP-MAIN/2015/KSCSTE and expresses sincere thanks to the Centre for Earth Sciences, Indian Institute of Science, Bangalore, India and J.K. Tomson, National Centre for Earth Science Studies, Thiruvananthapuram, India for providing the lab facilities. We especially thank Daniel J. Dunkley, Durgalakshmi, and Chiranjeeb Chatterjee for their critical comments, continuous support and valuable discussions, which helped to improve the manuscript. Our sincere thanks for the constructive comments and the careful editorial handling by K. Sajeev and M. Satish-Kumar.

SUPPLEMENTARY MATERIALS

Supplementary Tables S1 and S2 are available online from https://doi.org/10.2465/jmps.221212.

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