Metamorphic evolution and thermo-tectonic history of Metamafic Dykes: Insights into Continental Collision between Coorg Block and Western Dharwar Craton

The Mesoarchaean Coorg granulite block-Mercara Shear Zone (MRSZ) - Meso- to Neoarchaean Western Dharwar Craton (WDC) crustal section is ideally suited to study the Early Earth tectonics, and in particular, to establish collisional tectonics that led to their amalgamation. However, despite numerous petrological and geochronological studies, the nature of tectonic relationships among the three litho-tectonic domains is not well understood. Importantly enough, there is a dearth of metamorphic studies from the western part of the WDC, and in its absence, the amalgamation tectonics between the WDC and the Coorg block is poorly constrained. The south-western part of the Western Dharwar Craton (WDC), in contact with the Mesoarchaean Coorg Granulite Massif, consists of NNW-SSE trending mafic to ultramafic dyke swarm that is variably metamorphosed. Based on field observations and petrographic study, we have classified the metamafic dykes into two broad types: (a) Undeformed to foliated metagabbro, locally with coarse coronal Grt around Cpx, Pl, Ilm, and Hbl₁ (mineral abbreviations after Kretz, 1983) and differentially preserved igneous textures (Type-1) and (b) well-foliated and banded metamafites that lack magmatic textures and mineralogy, locally migmatitic with porphyroblastic Grt and Cpx and in others, garnetiferous amphibolite with porphyroblastic garnet (Type-2). Based on the degree of foliation development in these metamafites, we observe a south-westward increase in strain. Type-1 metamafites record a sequence of textural evolution, namely recrystallization of the magmatic Cpx±Opx+Pl assemblage→partial high-T hydration, producing Ti-Hbl (Ti=0.24-0.28)→growth of coarse coronal Grt [with a broad homogeneous magnesian core (X_Mg=0.25-0.23, X_Grs=0.20-0.22) and slightly ferroan rim (X_Mg=0.21-0.22, X_Grs=0.21-0.22), particularly in contact with Cpx] on the recrystallized matrix→a late Hbl-defined foliation (Ti=0.19-0.23). Type-2 metamafites show the development of a pervasive titaniferous Hbl-defined foliation (Ti=0.14-0.19), followed by the growth of compositionally homogeneous porphyroblastic Grt (X_Mg=0.21-0.22; Prp_14-15Grs_22-27) with or without Cpx (X_Mg=0.65-0.70, Al_total=0.03-0.08, and Na_total=0.01-0.03), and including localised crustal anataxis, producing tonalitic melt at the metamorphic peak. This was followed by the formation of late low Ti-Hbl (Ti=0.04-0.05). Using conventional thermobarometry, the peak P-T of type-1 metamafite has been estimated at ~ 8kb and 800°C.  In the type-2 metamafites, the peak and retrograde P-T is estimated at ~9kb and 800°C and ~7kb and 500°C respectively.  The effective bulk rock composition has been used to calculate the phase equilibria modelling of the type-2 metamafite, in which the intersections of compositional isopleths of X_Mg(Grt), X_Grs(Grt), X_An(Pl), Al(Cpx), Ti(Hbl) defines a peak P-T of 9.4 kb and 800°C which is similar to that calculated by conventional thermobarometry. The peak and retrograde P-T conditions together record the retrograde segment of a clockwise P-T path of evolution. We relate the textural sequence, results of thermobarometric computations and phase equilibria modelling, and strain patterns in the metamafic dykes to suggest a pervasive thermo-tectonic event that led to the prograde burial of the extended cratonised WDC beneath the Coorg Granulite Block to high-pressure upper amphibolite to granulite facies metamorphic conditions. We link this event with continental collisions between the WDC and Coorg Block at the dawn of the Proterozoic.