Igneous, hydrothermal, or both? Exploring the origin of pegmatites and aplites at the Amis intrusion, Namibia

Abstract The origin of pegmatite- and aplite-bearing complexes is studied at the late-stage Amis center associated with the Early Cretaceous Brandberg pluton (Damaraland igneous province, Namibia). Highly fractionated Amis alkali granite and aplites, as well as aplites from the main pluton show elevated Rb, Nb, Y, and Zr but low Ba (<50 ppm) compared to the host granites. The Amis alkali granites and aplites have negative δ138/134Ba (−0.17‰ ± 0.01‰ to −0.64‰ ± 0.04‰), indicative of favored fractionation of lighter Ba isotopes into hydrous fluids in equilibrium with a parent silicate melt. Fluid-related Ba isotope fractionation signals cannot be detected in the host granites due to the high bulk-rock Ba concentrations (>500 ppm). Feldspar from the Amis alkali granite contains significant concentrations of structural OH (~90 ppm) and ~550 ppm H2O distributed in numerous fluid inclusions, the latter due to late or post-magmatic overprinting by fluids exsolving from the crystallizing granite magma (“autometasomatism”) as the system transitions from predominantly silicate melts to hydrous fluids. Highly water-enriched (plus CO2, F) exsolved melts or fluids during the near-solidus stage of the Brandberg reservoir precipitated at least part of the minerals in the intrusive rocks. The highly fractionated Amis graphic granite shows δ138/134Ba similar to that of upper continental crust (−0.02‰ ± 0.04‰) and low concentrations of structural OH in feldspar (~15 ppm), indicative of formation from the complementary water-poor melts. Magmatic volatile exsolution and scavenging of flux elements in such a magmatic fluid is a crucial step in the formation of pegmatites even in dry intraplate settings.