Picrites of the Jungfrau and Sargdeckel Hills, Karibib, Namibia

Abstract:

A suite of doleritic rocks from the Jungfrau and Sargdeckel hills of Karibib form part of the Etendeka of central Namibia. These dolerites with rare vesicles are inferred to be a sill or a thick lava flow although field relationships are obscured. All samples are fresh and comprise holocrystalline medium-grained rocks with ophitic textures. The rocks are classified as picrites based on high MgO (15 to 26 wt. %) together with the presence of skeletal olivine. SEM-EDS data indicate the olivines exhibit a range in composition from Fo₇₄ to Fo₈₅ and, within any one rock the olivines are fairly homogenous and show limited variation. Orthopyroxene compositions are in the range Ca₄Mg₇₄Fe₂₂ to Ca₂Mg₈₂Fe₁₆. The clinopyroxenes are augites and range in composition from Ca₃₉Mg₄₇Fe₁₃ to Ca₄₀Mg₅₁Fe₁₃ but, as with olivine, the composition variation within individual rocks is limited. Plagioclase feldspars are calcic and show normal zoning from An₈₉ to An₃₀. The high-MgO basal rocks contain alkali feldspar (Or_9-70) as rims on plagioclase crystals and as small intergranular crystals. Geochemically the picrites belong to the low-Ti group and are relatively depleted in incompatible elements. The Mg-number varies from 0.74 to 0.81 in some cases due to olivine accumulation and in other cases, because of near-primary compositions. Hence, trends on geochemical plots can be attributed to the fractionation or accumulation of olivine plus chromite. A general decrease in Mg#, compatible trace elements, Fo-content of olivines are observed from bottom to top. Furthermore, incompatible trace elements like Ba and Nb show a general increase with lithostratigraphic height consistent with the increasing degree of differentiation. However, the basal high-MgO rocks appear to be the most contaminated having the highest indices of contamination (e.g., high Rb/Sr) and enhanced K concentration in the plagioclase rims suggesting that the hot high MgO rocks are more susceptible to crustal contamination.