Magmatic and anatectic history of a large Archean diapir: Insights from the migmatitic core of the Yalgoo Dome, Yilgarn Craton

Abstract

The genesis of domes in the Archean was fundamentally different than those on modern Earth as crustal reworking was far more voluminous and triggered by extensive reheating of the crust dictated by major pulses of mantle magmatic activity. In this study, we investigate how such mantle magmatism, reflected by extrusion of mantle-derived mafic–ultramafic greenstones sequences (2825–2760 Ma) in the Youanmi Terrane of the Yilgarn Craton, Australia caused a cascade of crustal processes between 2760 and 2740 Ma to form the 40 × 60 km-diameter Yalgoo Dome. Voluminous deposition of the greenstone sequences initiated at 2800–2760 Ma and followed by voluminous felsic magmatism lasting between 2760 and 2740 Ma. This process melted older basalts at depth and formed the Goonetarra Granodiorite. In turn, magmas, caused water-fluxed melting of older tonalite and amphibolite (2970–2900 Ma) that are preserved in the migmatitic core of the dome. Thermodynamic and geobarometry results on these rocks show that anatexis occurred between 3 and 9 kbar and ~700–800 °C, with 0.5–2 wt% H2O provided by the Goonetarra Granodiorite boosted melt production up to 15–30 vol%. The growth of this voluminous magmatic body accompanied by migmatization of country rocks, caused dramatic crustal weakening, allowing for sagduction of the thick overlying greenstones and the consequent diapiric rise of the buoyant, hot and voluminous granitoid bodies (partial convective overturn; Collins et al., 1998). Thus, the internal evolution and the doming process of the Yalgoo Dome resulted from the arrival of a major thermal (i.e. magma-induced) mantle anomaly into the crust. This event first gave rise to a gravitationally unstable crustal stratigraphy (greenstone over older felsic crust), followed by a wave of conductive heat traveling upward through the crust. This heat first produced voluminous and hydrous magmas as the Goonetarra Granodiorite. Then this melt advected heat and H2O to mid-crustal levels causing water-fluxed anatexis of older rocks and triggering partial convective overturn, including diapiric doming of the felsic middle crust, in a manner quite unlike the formation of Phanerozoic domes that develop due to orogenic-driven processes.