Archean gravity-driven tectonics on hot and flooded continents: Controls on long-lived mineralised hydrothermal systems away from continental margins

摘要

We present the results of two-dimensional numerical modelling experiments on the thermal evolution of Archean greenstones as they sink into a less dense, hot and weak felsic crust. We compare this thermal evolution to that obtained via the analysis of isotopic data and fluid inclusion microthermometry data obtained in the Paleoarchean to Mesoarchean Warrawoona Synform (Eastern Pilbara Craton, Western Australia). Our numerical experiments reveal a two-stage evolution. In the first stage, cooling affects zones of downwelling as greenstone belts are advected downward, whereas adjacent domes become warmer as deep and hot material is advected upward.Weshow that this is consistent with stable isotopes data from the Warrawoona Synform, which reveal an early episode of seafloor-like alteration (90.160°C) strongly focused along steeply dipping shear zones. In a second long-lived stage, lateral heat exchanges between domes and basins dominate the system as domes cool down while downwelling zones become increasingly warmer. In the Warrawoona greenstone belt, stable isotopes in gold-bearing quartz veins post-dating the sagduction-related vertical fabrics reveal that rock.fluid interaction occurred at much higher temperatures (234.372°C) than seafloor-like alteration. We propose that emplacement of thick and dense continental flood basalts, on flooded hot and weak continental plates, led to conditions particularly favourable to hydrothermal processes and the formation of mineral deposits.Wefurther argue that sagduction was able to drive crustal-scale deformation in the interior of continents, away from plate margins. On largely flooded continents, sagduction-related shear zones acted as fluid pathways promoting gold mineralisation far away from active plate boundaries, continental rift zones or collisional mountain belts.