Conditions of magma crystallization in the Henties Bay-Outjo dyke swarm, Namibia: implications for the feeder system of continental flood basalts

The Henties Bay-Outjo dyke swarm (HOD) in NW Namibia presents a well-exposed example of the magma feeder system for continental flood basalts. This study uses bulk-rock, mineral and melt inclusion compositions from dyke samples to define pressure-temperature crystallization conditions and thus contribute to understanding the magma plumbing and storage beneath the province. The thermobarometry calculations for near-equilibrium mineral-melt pairs (the latter proxied by whole-rock) indicate olivine crystallization at ~1170 to 1350°C and lower, overlapping, temperatures for clinopyroxene and plagioclase (~1070 to 1210°C) in keeping with the order of crystallization inferred from petrographic observation. The dykes yielding the highest temperatures (>1300 °C) are in a specific region of the HOD near the litho-tectonic boundary of the Neoproterozoic Kaoko and Damara Belts, where magma permeability of the crust may have been enhanced. Pressure estimates from clinopyroxene-melt pairs range from 0 to 10 kbar overall (13 dykes) indicating polybaric crystallization. The lowest pressures are recorded by clinopyroxene oikocrysts intergrown with plagioclase, which likely represent the dyke emplacement depths. Clinopyroxene phenocrysts and plagioclase free oikocryst cores yield a higher range of crystallization pressures at 4-6 kbar, corresponding to mid-crustal depths of 11-17 km. There is no spatial pattern in the pressure variations, suggesting a rather uniform level of magma stagnation and crystallization in all areas of the HOD. Partial crystallization at intermediate depths is consistent with the inference of entrained crystals and with geochemical evidence for crystal accumulation in some of the dykes. Comparison of model magma densities and the crustal density derived from seismic velocity profiles suggest the dyke magmas had positive buoyancy in the lower- and middle crust and near-neutral buoyancy in the upper crust. The depth of magma stagnation or pooling at 11-17 km depth may relate to this or to the brittle-ductile transition in the felsic Damara crust.