Garnet as a reliable timekeeper in Archean polymetamorphic terranes

Reliably reconstructing the pressure–temperature–time (P–T–t) history of Archean polymetamorphic terranes is key to gaining insight into the tectonic processes operating on the early Earth. In this regard, garnet is arguably the most important mineral. Not only does it capture information about both the timing and the conditions of metamorphism, but can also preserve this record through multiple tectonic cycles. Here, we used in situ Lu–Hf dating of garnet from two greenstone belts in the Western Dharwar Craton (WDC) in southern India to investigate their tectono-metamorphic evolution. Garnet ages reveal a record of two distinct medium- to high-pressure tectono-metamorphic events during the Archean eon. Whereas garnet from the Holenarsipur Greenstone Belt grew during late Neoarchean metamorphism at 2.53 Ga, garnet from the Kalyadi Greenstone Belt records a 2.96 Ga regional metamorphic episode that has not previously been recognized. Coupled with results from in situ monazite U–Pb dating and thermodynamic modelling, our data suggest that the two neighboring greenstone belts, which were previously thought to share a common tectonothermal history, underwent contrasting P–T–t evolutions throughout the Mesoarchean and Neoarchean, challenging existing models for the tectonic evolution of the WDC. Obtaining reliable age constraints in polymetamorphic terranes via in situ Lu–Hf dating of garnet that can be directly linked to P–T information represents a key advance in deciphering the cryptic record of crustal metamorphism throughout Earth history.