Phase relations in mafic dykes and their host rocks from Kondapalle, Andhra Pradesh, India: Implications for the time-depth trajectory of the Palaeoproterozoic (late Archaean?) granulites from southern Eastern Ghats Belt

Abstract

Kondapalle area, situated in the southern part of the Eastern Ghats Belt (EGB), expose an assemblage of high grade rocks with protracted geological history of more than 1 billion years. Field and petrological studies in combination with the existing geochronological data indicate intermittent tectonic activities in this region punctuated by intrusions of three suites of mafic dykes during early Palaeoproterozoic (or late Archaen?) to late Mezoproterozoic time. The earliest suite of dykes (gabbronorite-Suite I) was emplaced after the Palaeoproterozoic/late Archaean (?) UHT metamorphism in a suite of metapelites at 8-10 kbar pressure. This event was followed successively by intrusions of a suite of felsic magma (now enderbitic rocks) at 1.72 Ga and their high grade metamorphism (culminated at ca. 900 °C and ca. 8 ± 1 kbar) at 1.67 Ga. Four sets of folding, F₁-F₄, accompanied this second granulite facies event. The Suite II noritic dykes were emplaced almost synchronous with the F₃ folding. Subsequent to the F₄ folding, a suite of dolerite dykes (Suite III) having typical ophitic and intergranular textures were emplaced. Finally, the entire rock assemblage of this region including the mafic dykes were affected by roughly N-S trending ductile shear zone and accompanying superposed metamorphism under granulite to amphibolite facies conditions during the period 1.53-1.39 Ga. Results from multiprong techniques including extant experimental data in the basaltic system, pseudosections for the mafic dykes and the topological relations of the contact metamorphic assemblages unequivocally demonstrate that the rocks of the studied area resided at the 8-10 kbar lithostatic pressures during the emplacement of all the three phases of mafic dykes. Integrating this information with the P-T conditions of the superposed metamorphic events and the existing geochronological data, a time-depth trajectory for the southern Eastern Ghats Belt (SEGB) has been constructed. This trajectory indicate that the Palaeoproterozoic/late Archaean (?) UHT granulites of this segment resided at or below the middle crust for more than 1 billion years before being exhumed to near surface condition. The constructed time-depth trajectory for the SEGB belt show striking resemblance with the east Antarctic craton and provide additional support in favour of a Palaeoproterozoic supercontinent 'Columbia'.