Mesoarchean (ultra)-high temperature and high-pressure metamorphism along a microblock suture: Evidence from Earth's oldest khondalites in southern India

Abstract

Early continent-building processes on Earth are challenging to investigate, particularly since juvenile felsic crust formed during the Early Archean (4.0–3.2 Ga) is rarely preserved. Thus, associated sedimentary records are of fundamental importance, although in many cases these have been metamorphosed and reworked to various degrees since deposition. Here we present new petrological and zircon and monazite U–Pb age data from one of the Earth's oldest ‘khondalite’ (granulite-facies aluminous metapelite) belt, which we define as the Mercara khondalite belt, and associated charnockite and mafic granulite from the Mercara suture, the collision zone welding the Coorg and Western Dharwar Blocks in southern India. Petrologic analyses and phase equilibria modelling of the khondalites and associated charnockite and mafic granulite reveal a clockwise pressure–temperature (P–T) path with a peak temperature of above ca. 900 °C, and pressures up to 12 kbar. Detrital zircon grains in the metasedimentary rocks have magmatic cores with oscillatory zoning and ages up to ca. 3.5 Ga, and metamorphic overgrowths with ages of 3.1–3.0 Ga. Monazite in the khondalites yield identical metamorphic ages in the range of 3.1 to 3.0 Ga. Some of these rocks are overprinted by a younger thermal event at ca. 2.8–2.6 Ga. We correlate the high P–T metamorphism with the subduction-collision history between the northern margin of the Mercara block and the Western Dharwar Craton during the Mesoarchean, which indicates that plate tectonics had been established on Earth by at least ca. 3.1 Ga, in agreement with many independent lines of evidence. The Mesoarchean Mercara khondalite belt signals emergence of continents on the early Earth with active drainage systems leading to the deposition of voluminous detritus. The ca. 3.1 to 3.0 Ga (ultra) high temperature and high pressure metamorphism also coincides with the timing of assembly of the Earth's oldest supercontinent Ur