P–T path reconstruction in a multi‐layered garnet‐bearing schist: Evidence for 2.2–2.1 Ga near‐isothermal loading in the northern margin of the Western Dharwar Craton (India), and regional implications

Abstract

P–T paths were reconstructed using P–T pseudosection modelling from garnets in a finely laminated mylonitic hornblende‐biotite schist with four sub‐cm wide mineralogically distinct layers separated by poly‐crystalline quartz layers from the northern margin of the Western Dharwar Craton (WDC). The almandine‐grossular‐rich garnets, syn/post‐tectonic with respect to the mylonite foliation, exhibit considerable variations in the zoning patterns of divalent cations in the different layers. By contrast, the compositions of the matrix minerals dominated by biotite, chlorite, hornblende and plagioclase are tightly constrained within and across the layers. The varying abundances of major element oxides in the layers reflect variations in the abundances of the minerals, rather than the mineral compositions. P–T conditions estimated using mineral thermo‐barometry in the garnet‐muscovite‐biotite‐plagioclase assemblages are 520–575°C and 6–8 kbar; somewhat higher P–T values (615–665°C, 7–9 kbar) are obtained from hornblende‐garnet‐plagioclase‐quartz assemblages. The P–T paths obtained from the multi‐mineralic layers using P–T pseudosection modelling yield near‐isothermal loading (4–7 kbar) at 500–550°C in all layers. The near‐isothermal loading experienced by the multi‐layered rock suggests rapid crustal thickening involving overthrusting followed by nucleation of steeply dipping transpressional shear zones due to oblique convergence of the WDC with a crustal domain in the north. U‐Th‐total Pb chemical dating of metamorphic monazites indicates the crustal convergence occurred at 2.2–2.1 Ga. The event does not have known equivalents in the neighbouring crustal domains eastwards in India as well as in Madagascar in the west. This suggests the crustal domain may be an exotic block.