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Titre : Geochemie und Geochronologie des Erongo-Komplexes, Namibia

Auteur : Wigand, Marcus Oliver

Université de soutenance : Universität Potsdam

Grade : Doktor der Naturwissenschaften - Dr. rer. nat. - 2003

Résumé partiel
The Erongo complex is the largest of the Cretaceous igneous complexes in the Damaraland, Southern Etendeka Province, Namibia. Erongo is made up of a series of mainly silicic volcanic and intrusive units, like many of the Damaraland complexes, but it is unique by virtue of its size and well-preserved igneous sequence silicic magmas as well as tholeiitic and alkaline basalts.The goals of this study were to determine the ages and petrogenetic relationships of the silicic and basic units in the Erongo complex, and to contribute to understanding the magmatic evolution of the Damaraland province. As part of this investigation, Os isotope compositions were determined on basic rocks from the complex, and this is the first study of Os isotopes in the Etendeka Province. The Erongo is built up on a sequence of tholeiitic basaltic lavas which are compositionally equivalent to the Southern Etendeka flood basalts (Tafelberg type) and probably represent erosional remnants of these (see below). The felsic volcanic units at Erongo include two types of rhyodacites and one rhyolite. The most voluminous of these is the Ombu rhyodacite, which has a thickness of up to 500 m and makes up most of the topographic expression of the complex. The Ombu rhyodacite rests directly on the basal tholeiites in the southern and eastern part of the complex, but to the north and west, a second rhyodacite occurs below it, the Erongorus rhyodacite. The stratigraphically youngest silicic unit in the complex is the Ekuta rhyolite, which is exposed as patchy erosional remnants in the higher reaches of the Erongo massif. Intrusive equivalents of the felsic volcanic units include the Ombu grandiorite, which is compositionally identical with the Ombu rhyodacite, and the Erongo granite, a biotite and tourmaline-bearing granite which is the intrusive equivalent of the Ekuta rhyolite. There is no known intrusive equivalent of the Erongorus rhyodacite. The silicic units were intruded by a group of basic alkaline stocks and dikes in the northern part of the complex. Late stage tholeiitic basic magmatism is documented by a ring dike and dolerite sills present at the N and NW margins of the Erongo. The Nd, Pb, and Sr isotopic ratios of the basal tholeiite basalts confirm the geochemical evidence that they are remnants of the Etendeka flood basalts. Unlike the late stage dike dolerite sills and ring dike, these basal tholeiites display strong variations in εNd(130 Ma) (–0.4 to –7.3) and 87Sr/86Sr(130 Ma) ratios (0.71477 to 0.71648) due to crustal contamination. The initial Sr, Nd and Pb isotopic ratios from alkali-basaltic stocks and dikes are close to those of the Tristan da Cunha hotspot (Tristan mantle plume) and there is no indication for involvement of either depleted mantle or enriched continental lithosphere component in their magma sources.

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