The xenoliths of the Okenyenya volcanic breccia

Abstract

Abstract provided by author:

The Okenyenya igneous complex is one of the several Mesozoic anorogenic ring complexes which define a northeast trending linear feature in Damaraland, northwestern Namibia. The intrusive complex contains a wide variety of rocks ranging from tholeiitic to highly alkaline, which were emplaced over a minimum period of ~ 5 Ma, from approximately 123-129 Ma (Milner et al 1983). A volcanic diatreme of ultramafic lamprophyre, most closely resembling an alnöite, which pierces the Okenyenya igneous complex, hosts mafic and ultramafic xenoliths and megacrysts which are the focus of this study

The ultramafic xenolith suite consist of lherzolites ± spinel, wehrlites ± spinel ± amphibole, and clinopyroxenites. The mafic xenoliths comprise two-pyroxene granulites, amphibole clinopyroxene granulites, amphibolites, and minor clinopyroxenites and eclogites. The megacryst suite is dominated by Ti-rich amphibole, with lesser clinopyroxene, ilmenite, apatite and mica

The petrography, mineral chemistry, and geothermobarometry of the ultramafic xenoliths suggest that the spinel lherzolites have their origin in the upper mantle, and represent depleted residua remaining from partial melt extraction. These lherzolites are classified as typical Type I Cr-diopside peridotites. Geothermometry on a suite of spinel lherzolites gives estimated equilibration temperatures of 950 to 1050 °C. Equilibration pressure estimates are hampered by the absence of garnet in the lherzolite suite although phase stability fields suggest maximum equilibration pressures of approximately 19 kb. The amphibole bearing wehrlites have similar textural features to the lherzolites, although their mineral chemistry classifies them into the Type II Al-augite pyroxenite group. The wehrlite xenoliths are believed to represent metasomatically altered wallrocks adjacent to a Type II pyroxenite intrusions. The stability field of amphibole in the wehrlites suggests a range in possible equilibration conditions for the wehrlite assemblage, from pressures of 7 kb at 900 °C, to 18 kb at 1030 °C

The mafic granulites are interpreted to represent fragments of the lower crust. The presence of amphibole in these granulites is believed to result from a reaction between anhydrous phases and a hydrous mantle derived melt. Geothermometry on the two-pyroxene granulite suite indicates equilibration temperatures of 660 to 740 °C. The amphibolites occur as veins in the granulite, as igneous textured rocks, and as recrystallised banded rocks. These textural features in combination with the petrography and compositions suggest that the amphibolites represent the crystallisation products of igneous melts which have intruded the upper mantle and lower crust. The amphibole eclogites have basaltic compositions and are interpreted as the crystallisation products of alkaline basaltic magmas. Calculated equilibration temperatures for the eclogites range from 1120 °C at 20 kb to 1160 °C at 30 kb

The compositions and textural relations of the megacryst suite indicates that they represent crystallisation products of a melt at depth in the upper mantle. Trace element data on the amphibole megacrysts suggests that this melt may have been lamprophyric in composition, but the megacrysts themselves are not cognate with the host magma. Major element data on the clinopyroxene, ilmenite, apatite and mica megacrysts suggest that this group of megacrysts are also genetically unrelated to their host magmas but have crystallised together from a more evolved magma at shallower depths than the amphibole megacrysts

The wide variety of upper mantle and lower crustal xenoliths and megacrysts attests to a complex history for the subcontinental lithosphere beneath Okenyenya. A detailed schematic section of this area is presented, illustrating the occurrence of widespread magmatic activity, and resultant processes such as partial melting, metasomatism and metamorphism