The geology and geochemistry of the Etendeka Formation quartz latites, Namibia

Abstract

Abstract provided by author:

Apart from some systematic differences between pitchstone and devitrified quartz latite, largely explained by alteration processes, individual quartz latite units exhibit remarkably uniform compositions with no significant vertical or lateral variation. It has been possible to use geochemistry as a primary criterion for the correlation of major quartz latite units over much of the southern Etendeka, and this has enabled the reconstruction of a stratigraphic model for the Etendeka Formation in this region

A comparison of the composition of quartz latites from the southern Etendeka with those from the more northerly, Sarusas region reveal that the latter show notable enrichments in Ba, Sr, Nb, Zr, P, Ti, Y and LREE. In the Karoo and Parana Igneous Provinces a laterally extensive geochemical discontinuity separates relatively incompatible trace and minor element "enriched" basalts in the north from compositionally more "normal" basalts in the south. This feature is also apparent in the Etendeka quartz latites

Petrogenetic modelling indicates that the quartz latites are minimum partial melts of mid- to lower-crustal material of basic to intermediate composition. Melting probably occurred at depths of 30 - 35 km (8 - 10 kb), induced by the underplating or ponding of basaltic magma within the lower crust. The major and trace element and isotopic (Sr and 0) composition of the quartz latites indicates that they underwent little mixing or interaction with basaltic magma, and further precludes their derivation from a basaltic precursor either by simple closed system fractional crystallisation or by differentiation and crustal contamination

Mineral composition data and geochemical modelling indicate that variation trends defined by different groups of quartz latites in the southern Etendeka are not the result of magmatic differentiation. However, small compositional variations within individual groups of. quartz latite are probably the result of limited degrees of fractional crystallisation. The compositional differences between different groups of quartz latite are probably due to slight variations in the composition of the source, its mineralogy and the degree of partial melting

Simple trace element modelling and geodynamic considerations indicate that the "enriched" Sarusas quartz latites are not the products of partial melting of "enriched" underplated basalt, as might be implied from their trace element and isotopic characteristics. If one accepts that the "enriched" basalts were derived from enriched sub-continental mantle, then it is possible that the enrichment process (es) may also have affected the source regions of the quartz latites at the base of the lower crust