Pluton Formation and the Volcanic-Plutonic Connection in Large Igneous Provinces: Does the Brandberg Granite Intrusion Mark the Extinction of Etendeka Supervolcanoes?

Abstract

The northwestern Namibian section of the Cretaceous Paraná-Etendeka Large Igneous Province (LIP) displays granitic complexes and voluminous, coeval ignimbrite deposits, providing a rare opportunity to study silicic magmatism in an LIP-related setting at two different crustal levels: the upper crust and the surface. Here, using high-precision zircon geochronology, petrology and geochemistry, we focus on the ∼2000 km3 Brandberg intrusion and two of the most voluminous ignimbrites of the Northern Etendeka area, the ∼8600 km3 Sarusas and ∼ 4000 km3 Khoraseb quartz latites. Brandberg zircon crystals record incremental pluton growth over ∼1.4 Myr, beginning with the intrusion of the main granite body at 133.20 ± 0.19 to 132.62 ± 0.29 Ma, followed by the emplacement of quartz monzonites and peripheral dykes at 132.733 ± 0.042 to 132.14 ± 0.18 Ma, and finally the highly fractionated, fluid-enriched and partly pegmatitic peralkaline Amis granite from 131.910 ± 0.039 to 131.838 ± 0.043 Ma. Field observations suggest incremental emplacement of silicic magmas at shallow subvolcanic crust levels following a caldera collapse event, but there is no preserved evidence for chemically evolved volcanic deposits around Brandberg that could be related to a caldera-forming eruption or any other volcanic event during the pluton assembly. In the upper part of the Northern Etendeka volcanic stratigraphy, the two quartz latite ignimbrites Khoraseb (133.319 ± 0.073 Ma) and Sarusas (132.572 ± 0.034 Ma) are coeval to the Brandberg intrusion. These hot, unzoned and crystal-poor ignimbrites are comparable to the main units of the Brandberg pluton in terms of major and trace elements. Hence, the Brandberg pluton cannot be representative of a cumulate left-over of volcanic units such as the Khoraseb and Sarusas, but must have accumulated in the upper crust without much loss to the volcanic record (yielding a very low volcanic/plutonic ratio) during the late stages of the Paraná-Etendeka LIP evolution, as the crust was thermally mature. The estimated fluxes of silicic magmas into the Brandberg magma reservoir are relatively low (∼10-3 km3/yr), consistent with a waning magmatic source and leading to crustal conditions that were very favourable for magma entrapment in the crust. The large volumes of quartz latites erupted from magma reservoirs that are not exposed as plutonic remnants in the Northern Etendeka today; such crustal reservoirs were likely deeper than the Brandberg pluton, which allowed the accumulation of large amounts of hot, crystal-poor melt and avoided the collapse of large caldera structures.