Neoarchean high-pressure margarite-phengitic muscovite-chlorite corona mantled corundum in quartz-free high-Mg, Al phlogopite-chlorite schists from the Bundelkhand craton, north central India

In quartz-free Fe, Na-poor and high-Mg, Al schists from the Bundelkhand craton in north central India, corundum porphyroblasts in finely interleaved phlogopite-chlorite aggregates with rare clinozoisite are mantled by fine-grained (< 3 mu m) intergrowths of outer collars (> 200 mu m wide) of phengitic-muscovite and chlorite (phlogopite + corundum + H2O -> phengitic-muscovite + chlorite), and < 100 mu m wide inner collars of margarite-muscovite (corundum + clinozoisite + phengitic muscovite -> margarite + muscovite + H2O). Wide-beam electron probe microanalyses indicate Mg in the bi-layered corona increases from corundum outwards, with a complementary decrease in Al and K. Si and Ca increase and then decrease to matrix values. The sharp chemical gradients across the highly structured bi-layered corona are inferred to suggest that the corona-forming reactions were promoted by local grain-boundary-controlled thermodynamic instability as opposed to element transport by advective diffusion. The P-T convergence of KMASH reactions and NCKMASH pseudosection phase relations computed using micro-domain compositions indicate the chlorite-phengitic muscovite outer collar formed at 18-20 kbar and ca. 630A degrees C. The NCKMASH margarite-muscovite inner collar yielded lower metamorphic P-T conditions of 11 +/- A 3 kbar, ca. 630A degrees C. U-Th-Pb chemical dating of metamorphic monazite and LA-ICPMS U-Pb isotope dating of re-equilibrated zircon yield ca. 2.78 Ga ages, which are interpreted to date corona formation and Neoarchean high-P metamorphism in the Bundelkhand craton, hitherto unknown in the Indian Precambrian. (220).