The Geology And Geochemistry Of The Etendeka Formation Quartz Latites, Namibia.

The Etendeka Formation volcanics of north-western Namibia form part of the Karoo Igneous Province of southern Africa and consist of a series of basalts interbedded with quartz latites and minor volumes of latite. This thesis examines various aspects of the geology and geochemistry of the quartz latites, in particular the volcanological and petrogenetic origin of these rocks . This study has involved the geological mapping of ca. 5000 km2 of the southern Etendeka region and the documentation of the field and petrographic characteristics of the quartz latites. 183 whole rock quartz latite samples have been analysed for 32 elements by x-ray fluorescence spectrometry and 17 of these samples were selected for detailed mineral analyses by electron microprobe. The quartz latites make up a significant proportion of the succession (> 25 X) and constitute as much as 60 X of the outcrop in the southern Etendeka region. They occur as voluminous (80 800 km3 ), widespread (up to 4500 km2 ) sheet-like units, typically between 40 and 300 m thick. Individual units consist of basal, main and upper zones. The main zone usually constitutes over 70 X of the unit and typically consists of featureless devitrified quartz latite. In contrast the basal and upper zones of the flow are characterised by flow banding , pitchstone lenses and breccias which may contain rare pyroclastic textures. The quartz latites are sparsely porphyritic (< 10 X) with glassy or devitrified groundmass textures. The phenocrysts consist of plagioclase, pyroxene, titanomagnetite and rare ilmenite. Pyroxene and plagioclase geothermometry indicate high (1000 - 1100 °C) temperatures of crystallisation, which coupled with an absence of primary hydrous phases, indicate that the quartz latites were relatively hot, H20-undersaturated magmas. The quartz latites display features common to both rhyolite lavas and ignimbrites and are clearly the products of an unusual eruption style. The local preservation of pyroclastic textures and the broad areal extent of these units have led to the conclusion that the quartz latites were erupted as dense, high-temperature ash flows which underwent en masse lava-like flowage prior to their final emplacement (i.e. they are rheoignimbrites). A combination of high eruption temperatures and relatively low viscosity help to explain the often completely welded, homogeneous textures encountered at most outcrops. The apparent mobility of the quartz latite ash flows, coupled with their high temperatures and strongly welded nature require high rates of magma production (i) and/or descent from very high eruption columns. 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 LR.EE. 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 (ii) 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 .