Magmatic events

The evolutionary history of the EGB is punctuated with magmatic activities associated with tectonometamorphic events. Magmatic rocks of diverse chemistry are present all throughout the EGB. A large mafic-ultramafic complex is found to intrude the metapelitic granulites in Kondapalle in the southern EGB (Ongole Domain of Krishna Province). The complex consists of a gabbro-norite-pyroxenite-anorthosite suite of rocks which is presently metamorphosed to granulite grade (Leelanandam, 1990, 1997; Sengupta et al., 1999). Geochemical signatures suggest that the complex was formed at deep-crustal level (9-10 kbar) from high-Mg basalt in an arc-related setting (Leelanandam and Vijaya Kumar, 2007). Emplacement of this magma caused ultrahigh temperature (UHT) regional contact metamorphism of the surrounding crust (Sengupta et al., 1999). Emplacement age of this mafic complex is unknown, but must be older than ~1.72 Ga. The latter is the emplacement age of the enderbitic magma that intrudes the mafic complex at Kondapalle (Kovach et al., 2001). A similar mafic-ultramafic complex was emplaced at Chimakurthy (Ongole domain) in further south of EGB. This too caused contact metamorphism of the surrounding crust, but the depth of emplacement was much shallower (~6 kbar) (Dasgupta et al., 1997). From the northern part of EGB near Rayagada (Vishakhapatnam Domain), Shaw et al. (1997) determined the crystallization age of mafic magma (now mafic granulite) as ~1.45 Ga based on Sm-Nd data. Excepting this occurrence, the innumerable pockets of mafic granulite disseminated in the entire stretch of EGB remain largely undated, although such data may prove to be extremely important in understanding the geodynamic milieu for mafic magmatism. Voluminous amounts of enderbite/charnockite magma were emplaced in the entire stretch of EGB. Of particular mention is the linear stretch of western margin of EGB (Jeypore Province) where this forms a major mappable lithounit. Available petrological data show that this magma was emplaced broadly synchronous with the fabric forming deformation event (M2-D2) and metamorphosed to granulite grade (reviewed in Dasgupta and Sengupta, 2003 and Dobmeier and Raith, 2003). Although such magma is regarded as a member of calc-alkaline rock suite generated at Andean-type active continental margin setting (Dobmeier and Raith, 2003), its geochemical and geochronological attributes are not explored in detail. Such information would be essential in order to understand the paleotectonic setting under which such magmatism took place. Scattered geochronological data suggest the earliest age of enderbite emplacement in the southern part of EGB (Ongole Domain) is ~1.72-1.70 Ga (Kovach et al., 2001). A second generation of charnockite/enderbite magma intrudes the gneissic enderbite at ~1.60 Ga (Mezger and Cosca, 1999). Porphyritic granitoids, some of which contain orthopyroxene were emplaced in different parts of EGB (Eastern Ghats Province, in general) broadly in the time span of ~1000-950 Ma (Grew and Manton, 1986; Paul et al., 1990; Shaw et al., 1997; Aftalion et al., 1988; Kovach et al., 1998). This phase of granitoid magmatism was associated with Grenvillian-age metamorphism of the EGB (as discussed later).

Anorthosite complexes of variable size are found as scattered bodies in different parts of EGB (particularly in Eastern Ghats Province). These anorthosite bodies represent mantle pulses presumably concomitant with major tectonometamorphic activities (Leelanandam, 1990; Maji et al., 1997; Bhattacharya et al., 1998; Krause et al., 2001; Dobmeier, 2006). Available geochronological data suggest emplacement age of ~1000-900 Ma for these anorthosite bodies (references as above). Only one amongst these, the Chilka Lake Anorthosite, has some conflicting ages. Its previous emplacement age of ~792 Ma (Krause et al., 2001) has recently been re-estimated as ~983 Ma (Chatterjee et al., 2008). The Pangidi Anorthosite in the southern EGB has a much older emplacement age of ~1700 Ma (Dharma Rao et al., 2004). It is interesting to note that these emplacement ages in different provinces/domains of EGB have strong resemblances with timing of granulite-grade metamorphism (as discussed in the following section). It is argued that the anorthosite magma was emplaced in thinned crust during differentiation of high-Al basic magma. However, there is hardly any clue whether this anorthosite has any consanguinity with more widely distributed mafic granulites.

Series of alkaline rocks occur along the western boundary of the EGB with the surrounding Bastar, Singhbhum and Dharwar cratons (Upadhyay, 2008 and references therein). Notably, most of the alkaline plutons follow the strike of two major mega-lineaments i.e. Sileru Shear Zone and Eastern Ghats Boundary Shear Zone (Dobmeier and Raith, 2003) or Terrain Boundary Shear Zone (Biswal et al., 2007). Important members of these rock suites include syenite, nepheline syenite, hornblende syenite, monzosyenite and quartz syenite. Almost all of the alkaline complexes are metamorphosed to granulite-grade, while the sheared counterparts show amphibolite-grade of metamorphism. Such rocks are believed to have formed by fractionation of basanitic magma produced by partial melting of enriched subcontinental mantle source with variable degree of crustal contamination (Vijaya Kumar et al., 2007; Upadhyay, 2008). In a recent study, Upadhyay (2008) argued that the alkaline magma was emplaced in a rift-related tectonic setting at the cratonic margin, which was earlier conceived by others (Leelanandam, 1998; Leelanandam et al., 2006). All these rocks were emplaced in the time span of ~1500-1300 Ma during break-up of the supercontinent Columbia (Upadhyay, 2008).