Paired metamorphism in peel back tectonics at the Archaean-Proterozoic boundary: New insights into Early Earth Plate tectonics from the Western Dharwar Craton, South India

Abstract

In this study, we deduce a spatially and temporally linked paired metamorphism from the Western Dharwar Craton (WDC), South India, for which we use representative rocks from the medium-grade (MG) (e.g. pelitic schist) and high-grade (HG) (e.g. mafic and metasedimentary granulites) domains in the craton. Using metamorphic reconstructions (P-T-t paths of evolution and thermobaric, T/P ratios at the metamorphic peak), garnet-biotite diffusion chronometry, monazite chemical dating techniques of the investigated rocks and published metamorphic and geochronological dataset, we demonstrate that the two domains, despite having a record of uniform burial to lower crustal depths, differ in their thermal (T-t) history. The medium-grade domain records relatively cooler, intermediate T/P type of metamorphism in the stability field of kyanite with TMax at ∼650-630°C, multiple metamorphic cycles, all along clockwise P-T paths of evolution, and representing recurring burial-exhumation and heating-cooling cycles and finally, relatively short-lived nature of residence at the medium- to lower crustal levels (Δt∼10-20 Myrs) and faster metamorphic cooling (dT/dt ∼10-20°C/Myr) and exhumation rates (dz/dt∼0.6-1.1 to 1-2 mm/yr). The high-grade domain, in contrast records relatively warmer, high T/P metamorphism (T/P ratios at ∼800-870°C/GPa) in the stability field of sillimanite with TMax at ∼800–870°C, recurring burial-exhumation stages as part of one prograde heating event along a monocyclic CW P-T path of evolution and finally, extremely long-lived lower-mid-crustal residence (Δt∼238 Myrs) and ultra-slow cooling (∼2°C/Myr) and exhumation (∼0.1 mm/yr) history. Both domains show broadly coeval metamorphism as part of the same orogenesis between c.2.53 and 2.41 Ga. We use these findings from the WDC to suggest the development of a paired metamorphism as part of a peel-back convergence style of plate tectonics at the Archaean-Proterozoic boundary in the planet Earth.