GEOLOGY OF THE EARLY CRETACEOUS DOROS LAYERED MAFIC INTRUSION, NAMIBIA: COMPLEXITY ON A SMALL SCALE

Abstract

The Doros Complex of north-western Namibia is a relatively small (~17 km 2 in area; ≥500 m thick), lopolithic layered mafic intrusion (LMI) within the early Cretaceous Parana-Etendeka Large Igneous Province. It is hosted in Neoproterozoic granitoids and metasedimentary rocks of the Southern Kaoko Zone of the Damara Orogen. It is undeformed and unmetamorphosed and therefore represents a good candidate for the comprehensive characterisation and investigation of a layered intrusion. Detailed mapping and petrographic analysis in this study show that the intrusion consists of a stacked series of semi-concordant gabbroic layers, dipping shallowly in towards the centre. The fundamental mineralogy throughout the layered sequence is plagioclase + calcic clinopyroxene + oxy-exsolved Fe-Ti oxides ± olivine. Orthopyroxene is completely absent in the Doros Complex. The stratigraphy broadly comprises a partially-exposed, fine-grained gabbroic sill (An 46-21 ; clinopyroxene Mg# 80 to 59), overlain by a thick sequence of massive, olivine-cumulate melagabbros (Fo 75-70 ; An 73-23 ; clinopyroxene Mg# 80 to 76), followed by a massive, plagioclase-cumulate gabbro layer (Fo 61-54 ; An 75-51 ; Mg# 79 to 73), and topped by a series of interlayered variable, strongly foliated, plagioclase-, olivine- or magnetite-cumulate gabbro layers (Fo 63-60 ; An 72-24 ; Mg# 79 to 69). The foliated gabbros are also host to pegmatitic and monomineralic enclaves. The basal contact displays a glassy, micro-porphyritic chilled margin (An 2-0 ; Mg# 80 to 78). A syenitic (bostonite) phase (An 27-01 ; Mg# 66 to 3) – a late-stage differentiate – occurs as cross-cutting dykes. The intrusion is also cut by dolerite dykes, from a later event. Fold and foliation structures within the Damaran rocks likely facilitated magma emplacement. The layering in the intrusion is defined by variations in the modal proportions of the major minerals, and in the mineral and rock textures. The fine-scale rhythmic or cyclic layering characteristic of the larger LMIs is not evident. Despite the small size and simple mineralogy of the intrusion, this study reveals unexpectedly high degrees of complexity in the textures, mineralogy, compositions, styles of layering, internal layer variations and contact relations in the Doros Complex. We document systematic stratigraphic changes in modal abundances and mineral compositions through the intrusion that are consistent with the fractional crystallisation of a basaltic system, but also reveal compositional breaks and subtle reversals. Together with evidence of intrusive internal contacts and grain-size layering, these are consistent with emplacement as multiple magma pulses.