Picrites of the Jungfrau and Sargdeckel, central Namibia: Relative roles of mantle and crust in the Southern Etendeka large igneous province

Abstract

A suite of doleritic rocks from the Jungfrau and Sargdeckel hills of Karibib form part of the southern Etendeka Province of Central Namibia. The dolerites occur as sills or thick lava flows and are classified as picrites based on high MgO contents (15–26 wt.%) and the presence of skeletal olivine. Both Jungfrau and Sargdeckel picrites are characterized by low TiO2 contents (0.32–0.61 wt.%) and plot in the field of low-Ti magmas of the Etendeka flood basalts. The high MgO, Ni, and Cr whole-rock contents suggest that olivine and chromite accumulation occurred. Elevated Sr concentrations and positive Eu-anomalies in the Sargdeckel picrites indicate additional plagioclase accumulation.REE patterns of the picrites range from 5 to 30 times chondritic values, are slightly REE-enriched ([La/Yb]N ranges from 2.3 to 3.2), and have small Eu-anomalies (0.93–1.12). Multielement patterns (normalized to primitive mantle) show marked positive Sr and Ba anomalies relative to Th, U and Rb and negative Nb and Ti anomalies resulting in high LILE/HFSE ratios. 87Sr/86Sr and 143Nd/144Nd ratios in Jungfrau and Sargdeckel picrites exhibit an inverse covariation, with 87Sr/86Sr ranging from 0.70937 to 0.71322 and εNd from −7.2 to −8.7.All picrites exhibit a crustal signature characterized by the enrichment of LILE relative to HFSE, negative Nb-anomalies, radiogenic Sr and unradiogenic Nd isotopes. However, it is predominantly the picrites with the highest Mg#’s and compatible elements that show the more pronounced crustal signature and the lowest Nd- and the highest Sr-isotope ratios. Hence, it seems unlikely that the crustal signature seen in the picrites is the result of combined assimilation and fractional crystallization or direct interaction with continental crust. Altogether, the combined major and trace element, and isotope data suggest that the picrites are derived by plume-induced melting of the local metasomatized subcontinental lithospheric mantle, similar to the source of Horingbaai dolerites, that has been variable enriched with a crustal component that was likely introduced into the mantle during Neoproterozoic subduction.