The ~132 Ma Paraná-Etendeka Large Igneous Province consists of an extensive succession of bimodal flood volcanic rocks and intrusions across Brazil, Paraguay, Uruguay, Argentina, Namibia and southern Angola. This magmatism has been attributed to the impact of the Tristan mantle plume and the associated opening of the South Atlantic Ocean, during the Early Cretaceous breakup of West Gondwana. In southern Africa, this is preserved as the Etendeka flood volcanic succession and dyke suites, and the Damaraland Intrusive Suite, a series of subvolcanic intrusions that occur in a north-east-trending band within the Neoproterozoic Damara Orogenic Belt.
The Doros Gabbroic Complex is a relatively small (~3.5 km x 7.5 km) layered mafic intrusion that forms part of the Damaraland Suite in north-western Namibia. It is hosted by deformed Damaran metaturbidites, Damaran granitoids and Karoo-age metasedimentary strata in the Southern Kaoko Zone of the Damara Belt. The intrusion is a shallow lopolith, with an estimated thickness of at least 500 m, which consists of a sequence of roughlyconcordant, sill-like gabbro layers, dipping in towards the centre, cross-cut by dolerite and bostonite dykes. It is undeformed and unmetamorphosed.
The fundamental mineralogy is essentially the same throughout the main body of the intrusion (plagioclase + calcic clinopyroxene + oxy-exsolved Fe-Ti oxides ± olivine). However, variations in the modal proportions of these minerals, and in the mineral and rock textures and compositions, define a series of layers. The stratigraphy broadly comprises: 1) a fine-grained gabbroic sill with chilled margin, present mostly as subcrop; 2) a sequence of massive, olivine-cumulate melagabbros (the Lower Zone), with a basal chilled margin, that form the outer ring of the complex; 3) a massive, plagioclase-cumulate olivine gabbro (the Intermediate Zone), comprising the inner ring of the complex; and 4) a sequence of variable, strongly foliated, plagioclase-, olivine- or magnetitecumulate gabbros (the Upper Foliated Series), in the core of the complex. A syenitic (bostonite) phase occurs as cross-cutting dykes or enclaves within the gabbros.
In this thesis, I present a detailed petrographic study, an extensive set of whole-rock and mineral major and trace element analyses, whole-rock Sr-, Nd- and Pb-isotopic analyses, melt inclusion analyses and aeromagnetic data
for the Doros intrusion. Interpretation of these data, combined with major element and trace element modelling, shows that the stratigraphic order of appearance of cumulus minerals and overall trends in rock compositions in the Doros intrusion are consistent with the fractional crystallisation of olivine, clinopyroxene, plagioclase, magnetite, K-feldspar and apatite, accompanied by variable degrees of accumulation, from a basaltic parental magma. From this combined evidence, I have put forward a compelling argument in favour of an origin for the Doros Complex by multiple, closely-spaced influxes of crystal-bearing magmas (magma mushes). This evidence includes intrusive layer relations, textural evidence for primocrysts, disequilibrium features, and stratigraphic reversals in mineral and whole-rock chemistry and magnetic properties. At least seven distinct major injections of magma have been identified in the stratigraphy, as well as several smaller pulses. Based on this interpretation, a comprehensive emplacement history for the Doros intrusion has been compiled. These findings represent a departure from the traditional model for the formation of such intrusions by the postemplacement differentiation of a single batch of crystal-free melt, and have fundamental implications for our understanding of layered mafic intrusions in general, which I elaborate on.
The Doros suite shows depleted ɛNd of +3.67 to +6.46, moderate initial 87Sr/86Sr = 0.703970 - 0.709525,
206Pb/204Pb = 18.17 - 18.47, 207Pb/204Pb = 15.55 - 15.59 and 208Pb/204Pb = 37.93 - 38.42, at 132 Ma. The clustering of isotopic data and trends in incompatible trace element ratios indicate that all the magmas in the complex were derived from the same mantle source. I demonstrate, with the assistance of quantitative isotopic modelling, that this source comprised a dominant depleted MORB-like mantle component (60 - 80%), with a significant Tristan plume-derived, asthenospheric component (20 - 40%) and a minor contribution likely from recycled oceanic crust (< 5%). I further demonstrate that chilled margin to the complex is the only rock type that shows significant evidence of crustal contamination, due to local assimilation of the Damaran host rock on emplacement.
In addition, this research shows that the Doros suite has strong geochemical affinities with the Tafelkop group “ferropicrite” lavas of the Etendeka Province. Common features include their near-Bulk Silicate Earth initial Sr ratios, low positive εNd, trends in 207Pb-space and relatively low heavy rare earth element concentrations.
Furthermore, trace element modelling of the Doros cumulates from their whole-rock chemistry suggests primary liquid compositions remarkably similar to the Tafelkop magma type. This provides crucial evidence in support of Doros as the eruptive site for the Tafelkop lavas, and thereby links the Doros magmatism to the earliest eruptive phase in the Paraná-Etendeka event. The distinctive chemistry of this magma group is attributed to the early, relatively deep, decompression melting of pyroxenitic material in the heterogeneous Tristan plume starting head, corresponding with the initial impact of the plume on the base of the lithosphere.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/14842 |
| Date | 01 July 2014 |
| Creators | Owen-Smith, Trishya M. |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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