Burial and exhumation during Archean sagduction in the East Pilbara Granite-Greenstone Terrane

Abstract

Archean granitic domes and intervening volcano-sedimentary basins are commonly interpreted as the product of “sagduction”, a process involving the gravitational sinking of surficial greenstone cover sequences into narrow belts and the coeval exhumation of deeper granitic crust into broad domes. Alternatives to the sagduction model that can account for the regional dome and basin pattern include fold interferences and extensional metamorphic core complexes. In order to provide quantitative constraints on the pressure–temperature–time (P–T–t) evolution experienced by greenstone–granite pairs we investigate the Warrawoona greenstone belt and adjacent Mount Edgar granitoid dome the East Pilbara craton (Western Australia). We adopt a multidisciplinary approach that includes structural, metamorphic, geochronological and numerical investigation of the 3.5–3.2 Ga Mount Edgar high-grade metamorphic rocks. Garnet-bearing metasediments and metabasalts collected along the SW and SE rims of the Mount Edgar Dome show higher pressure but lower temperature of equilibration (9–11 kbar and 450–550 °C) than enclaves collected in the core of the dome (6–7 kbar and 650–750 °C). In situ oxygen isotope analysis and U–Pb dating of zircons from the enclave indicate a metasedimentary origin (∂18O∼+13‰) for the protoliths and a metamorphic age of 3311.9±4.9 Ma. In addition, monazites included in garnet from the SW dome margin yield an age of 3443.4±4.5 Ma. These monazites suggest the existence of an older metamorphic cycle and imply a polymetamorphic evolution of the unit. The P–T–t data support fast, gravity-driven tectonics, wherein sedimentary rocks were buried to lower crustal conditions, metamorphosed and exhumed back to the surface during a cycle lasting only a few million years. Forward thermo-mechanical modeling confirms the P–T–t evolutions deduced from thermobarometry and geochronology. Our model shows a large range of possible apparent geothermal gradients during sagduction, including low apparent geothermal gradients that are similar to those proposed for Archean and modern subduction.