Charnockitic magmatism in southern India H M Rajesh1∗ and M Santosh2 1Department of Geographical Sciences and Planning, University of Queensland, St Lucia, 4072 Queensland, Australia. 2Department of Natural Environmental Science, Faculty of Science, Kochi University, Kochi 780-8520, Japan. ∗e-mail: [email protected] Large charnockite massifs cover a substantial portion of the southern Indian granulite terrain. The older (late Archaean to early Proterozoic) charnockites occur in the northern part and the younger (late Proterozoic) charnockites occur in the southern part of this high-grade terrain. Among these, the older Biligirirangan hill, Shevroy hill and Nilgiri hill massifs are intermediate charnockites, with Pallavaram massif consisting dominantly of felsic charnockites. The charnockite massifs from northern Kerala and Cardamom hill show spatial association of intermediate and felsic charnock- ites, with the youngest Nagercoil massif consisting of felsic charnockites. Their igneous parentage is evident from a combination of features including field relations, mineralogy, petrography, ther- mobarometry, as well as distinct chemical features. The southern Indian charnockite massifs show similarity with high-Ba–Sr granitoids, with the tonalitic intermediate charnockites showing simi- larity with high-Ba–Sr granitoids with low K2O/Na2O ratios, and the felsic charnockites showing similarity with high-Ba–Sr granitoids with high K2O/Na2O ratios. A two-stage model is suggested for the formation of these charnockites. During the first stage there was a period of basalt under- plating, with the ponding of alkaline mafic magmas. Partial melting of this mafic lower crust formed the charnockitic magmas. Here emplacement of basalt with low water content would lead to dehy- dration melting of the lower crust forming intermediate charnockites. Conversely, emplacement of hydrous basalt would result in melting at higher fH2O favoring production of more siliceous felsic charnockites. This model is correlated with two crustal thickening phases in southern India, one related to the accretion of the older crustal blocks on to the Archaean craton to the north and the other probably related to the collision between crustal fragments of East and West Gondwana in a supercontinent framework. 1. Introduction example, in Limpopo Belt, South Africa, Bohlen- der et al (1992) proposed in situ biotite dehydra- Charnockites characterized by orthopyroxene- tion to form orthopyroxene in the presence of a bearing granitic mineral assemblages are a common fluid phase with locally different activity of CO2 constituent of granulite-facies metamorphic ter- for the metamorphic charnockite and crystalliza- rains. However, the relative importance of igneous tion from calc-alkaline magmas derived by partial versus metamorphic processes involved in their ori- melting of lower crustal rocks for the spatially asso- gin is debated. They are either granitic rocks meta- ciated igneous charnockite. morphosed to the granulite facies (metamorphic Charnockitic rocks usually show spatial asso- charnockites; e.g., Newton et al 1980) or rocks ciation of rocks (charnockite–charnoenderbite– whose pyroxene crystallized directly from magma enderbite) differing in modal abundance of (igneous charnockites; e.g., Wendlandt 1981). For the dominant feldspar species. There is a Keywords. Charnockitic magmatism; southern India; intermediate charnockite; felsic charnockite; petrogenetic model. Proc. Indian Acad. Sci. (Earth Planet. Sci.), 113, No. 4, December 2004, pp. 565–585 © Printed in India. 565 566 H M Rajesh and M Santosh ∗ ∗ Figure 1. Fe /(Fe + MgO) vs. SiO2 and Na2O+K2O − CaO vs. SiO2 plots illustrating the representative trends of I-type granitoids, A-type granitoids, intermediate charnockites, and felsic charnockites. great deal of scientific literature covering the rocks interpreted by Dobmeier and Raith (2000) charnockite (opx-bearing granite)–charnoenderbite as enderbite are, in reality, charnockites or char- (opx-bearing granodiorite)–enderbite (opx-bearing noenderbites. Hence, Bhattacharya and Sen (2002) tonalite) (Holland 1900; Tilley 1936; Le Maitre proposed a common origin for charnockite and 2002) suite of rocks although no consensus exists enderbite at Chilka Lake. about the origin of at least some charnockite Charnockites, like granitoids, have a common occurrences including classic localities. A case compositional range and can be broadly divided of ongoing uncertainty is the origin of arrested into intermediate charnockites and felsic charnock- charnockite in the Chilka Lake area, India. Dob- ites, with most reported occurrences showing vari- meier and Raith (2000) postulated that these rocks ation from intermediate to felsic compositions formed as a result of localized synkinematic fluid (Rajesh 2004b). Intermediate charnockites are migration and were genetically linked to the host dominantly calc-alkalic (in terms of modified alkali- leptynite (garnet-biotite gneiss) in which they are lime index) and ferroan to magnesian (in terms enclosed. These authors also proposed that the of Fe-number), while felsic charnockites are dom- charnockites are completely unrelated to ender- inantly alkali-calcic and ferroan (figure 1). The bite layers in the same outcrops. In a comment, relatively iron-enriched nature of felsic charnock- Bhattacharya and Sen (2002) claimed that many ites imparts to them a predominantly tholeiitic Charnockitic magmatism in southern India 567 Figure 2. Shaded relief image of southern India showing the distribution of major charnockite massifs (cf. Rajesh 2004b, c). Not all charnockite massifs are considered in this study. The different crustal blocks are also shown. TB – Trivandrum Block; NB – Northern Block. affinity, in contrast to the predominantly calc- metamorphic charnockites showing relict composi- alkaline affinity of the intermediate charnockites tional banding and obliteration of foliation with the (Rajesh 2004b). adjacent gneiss, to patches and veins of charnock- The formation of charnockites may provide ite (incipient charnockites), representing in situ important constraints in trying to unravel the stages of granulite formation driven by the influx mechanisms involved in granulite formation, of CO2-rich fluids (carbonic metamorphism; New- because their granitic and often undepleted chem- ton et al 1980). The aim of this paper is to focus istry, which is more akin to the melt fraction on the characteristics of the magmatic charnock- rather than the restite, precludes their formation ites (charnockite hereafter, unless otherwise men- by removal of a hydrous partial melt. As dis- tioned) that constitute the major granulite masses cussed by Kilpatrick and Ellis (1992), the high in southern India. In the process we compile geo- temperature origin of charnockites compared to I- chemical data of these massifs to discuss and type granitoids, together with their more enriched develop an understanding of how geochemical char- incompatible element abundances, clearly excludes acteristics relate to their source rocks, petrogenetic the possibility that they represent the remelting of processes and tectonic environments. granulitic residues formed after the extraction of I- type granitoids, a model proposed for the origin of A-type granitoids by Collins et al (1982). Genera- 2. Geologic setting tion of charnockite magmas by high-temperature, water-deficient melting of mafic to intermediate, The southern granulite terrain (south of Dhar- high-K sources compositionally similar to alkaline war craton) includes several regional en echelon basalts or calc-alkaline basaltic andesites is one of Neoproterozoic shear zones which dissect the ter- the alternatives (e.g., Rajesh 2004a). rain into different Late Archaean and Proterozoic In the granulite-facies terrain of southern India, crustal blocks such as the Madras, Northern, Nil- charnockites include intrusions of gabbroic and giri, Madurai and Trivandrum blocks (Harris et al tonalitic-enderbitic composition occurring over 1994) (figure 2). The northern part of the southern large areas (massif igneous charnockites), massive granulite terrain consists primarily of charnockite 568 H M Rajesh and M Santosh massifs that form highland areas interspersed with of Palghat-Cauvery shear zone contains ample evi- lowlands consisting of felsic rocks generally in dence for Pan-African activity (metamorphism and amphibolite facies (the Madras, Northern, and magmatism). Compared to the older charnockite Nilgiri blocks). The nature of contacts between massifs from Madras-Northern-Nilgiri blocks, the highland and lowland rocks is highly controver- Madurai and Trivandrum blocks expose early- to sial, with faults suggested, but not proved, at late-Proterozoic charnockite massifs (Bartlett et al many of the boundaries of the massifs. With the 1995; Jayananda et al 1995; Miller et al 1996; exception of granitoids, carbonatites, and ultra- Mohan and Jayananda 1999). Recent studies from mafic rocks, Pan-African ages are absent from the Madurai block have suggested a polymetamorphic northern part of this granulite terrain. Granulite- and multistage P-T (700–1000◦C and 5–12 kbar; facies metamorphism affected the area at ca. 2.5 Ga Harris et al 1982; Mohan and Windley 1993; Raith and 2.8–3.0 Ga. The Nilgiri block was metamor- et al 1997; Satish Kumar et al 2002) evolution phosed under medium-high pressure (6–10 kbar; for the terrane. Nd-model ages from the Madurai Harris et al 1982; Janardhan et al 1982; Raith block range from 2.1–3.1 Ga (Harris et