Fast exhumation of Earth’s earliest ultrahigh-pressure rocks in the West Gondwana orogen, Mali

Abstract

Abstract Did exhumation of ultrahigh-pressure (UHP) rocks proceed at comparable rates in the Neoproterozoic and in modern collisional orogens? We address this question with a multimineral geochronological study of UHP rocks from the Gourma fold-and-thrust belt in Mali. Integrated petrology and zircon U-Pb geochronology reveal peak metamorphic conditions of 820–740 °C and 3.3–3.4 GPa at 611.7 ± 3.6 Ma, providing evidence for subduction of the passive margin of the West African craton to ~125 km depth in the West Gondwana orogen. Rutile U-Pb cooling ages indicate further exhumation of the Gourma UHP unit to mid-crustal levels (~35 km) at 601.7 ± 3.2 Ma. These two ages provide a time lag between peak conditions and exhumation to 35 km of 10 ± 3.1 m.y., constraining an average vertical exhumation rate of 0.9 ± 0.3 cm/yr. Our data indicate a fast exhumation rate for the oldest known UHP rocks, comparable to that reported for modern collisional orogens. We argue that exhumation of the deeply subducted UHP rocks along the West Gondwana orogen contributed to significant changes in the Neoproterozoic atmosphere and biosphere.