METAMORPHISM OF THE BUCHAN-TYPE (?) IN CHAMPANER SUPRACRUSTALS WITHIN AN ACCRETIONARY ZONE OF THE GREATER INDIAN PROTEROZOIC FOLD BELT: AN ENIGMA

The Greater Indian Proterozoic Fold Belt (GIPFB) comprises two crustal scale accretionary zones i.e., N/NNE trending Aravalli Delhi Fold Belt (ADFB) and E trending Central Indian tectonic zone (CITZ). The convergence of these two arms is in the form of an arcuate bend at the southernmost extremity of ADFB in west-central India. The Champaner supracrustals is a very significant lithodemic unit of Western India located in Eastern Gujarat and is positioned as this arc at the curvature part of the GIPFB. Despite being a smaller geological domain of c. 1400 km², Champaner records a unique evolutionary history in terms of its deformation patterns and metamorphic episodes. The metasedimentary rocks of the Champaner region are polydeformed (D_1-3), in which the last two phases are contemporaneous with widespread S-type calc-alkaline syn-post granitic intrusions (0.95-0.93 Ga). A distinctive prograde metamorphic sequence of phyllite, spotted phyllite, andalusite-cordierite schist, and migmatites/gneiss in metapelites have been demonstrated in the Champaner region from west to east, respectively. Further, the regional metamorphic rocks located at the eastern margin of these supracrustals in proximity to the granitic intrusion have been gradually recrystallized to hornfels. These metapelites exhibit the prograde mineral isograds sequence as biotite-cordierite-andalusite-garnet-sillimanite representing an increasing geothermal gradient induced by the prolonged felsic intrusions. P-T pseudo-sections built from the metapelitic compositions indicates a temperature pressure range of 450-650⁰C, and 1.5-4 kbar, suggesting an up-temperature P-T path and gently increasing pressure. Overall, the metamorphic episodes within Champaners shows concomitance with the “Buchan type-area” which includes an initial phase of low-grade regional metamorphism, followed by a widespread combined effect of regional-contact metamorphism, ending with a high-temperature contact metamorphic episode.