Ultrahigh thermal gradient granulites in the Narryer Terrane, Yilgarn Craton, Western Australia, provide a window into the composition and formation of Archean lower crust

Abstract

Granulites from the Narryer Terrane in the northern Yilgarn Craton, Australia, record evidence for high to ultrahigh thermal gradients during the Meso–Neoarchean. U–Pb zircon ages reflect a complex history of high-grade, prolonged and poly-phase metamorphism, with evidence for several thermal pulses at ca. 2745–2725, ca. 2690–2660, and ca. 2650–2610 Ma. Forward phase equilibrium modeling on rocks with varying bulk compositions and mineral assemblages suggests that peak temperatures reached 880–920°C at pressures of 5.5–6 kbar at ca. 2690–2665 Ma, followed by near-isobaric cooling. These new P–T results also indicate that these rocks experienced some of the hottest thermal gradient regimes in the metamorphic record (≥150°C/kbar). Based on P–T models, U–Pb ages, and geochemical constraints, our data suggest that the geodynamic setting for the formation of this unusual thermal regime is ultimately tied to cratonization of the Yilgarn Craton. Previous models have inferred that ultrahigh thermal gradients and coeval large-scale anatexis in the Narryer Terrane were primarily generated by mantle-driven processes, despite most of the lithological, isotopic, and geochemical observations being at odds with the expected geological expression of large-scale mantle upwelling. We re-evaluate the mechanisms for high-grade metamorphism in the Narryer terrane and propose that long-lived high crustal temperatures between ca. 2690 Ma and 2610 Ma were instead facilitated by elevated radiogenic heat production in thickened, highly differentiated ancient crust. Mantle-derived magma input and new crustal addition may not be the only drivers for high- to ultrahigh-temperature metamorphism and stabilization of ancient crustal blocks.