Oxygen isotope ratios of pyroxene phenocrysts in the silicic volcanic rocks from the Cretaceous Etendeka-Paraná flood basalt province (Namibia, South America) are believed to reflect the δ18O values of the original magmas. We recognize a high δ18O value type (δ18O pyroxene ∼+10‰) found in the south of both regions, and a low δ18O value type (δ18O pyroxene ∼ +6.5‰) found in the north. Other differences between these two rhyolite types include higher concentrations of incompatible elements and lower initial 87Sr/86Sr ratios in the low δ18O value type. We suggest that the regional distribution of rhyolite types reflects differences in source composition, which can best be explained if the sources are lower crustal, Late Proterozoic mobile belt material (high δ18O) and Archean lower crust (low δ18O).

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