Geochemie und Geochronologie des Erongo-Komplexes, Namibia

Abstract

(TYPE=abstract)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 eNd(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. Compared with the basic rocks, the felsic volcanics and intrusives overlap in isotopic composition with the Damara basement rocks and are therefore likely to represent crustal melts of the mid to lower crust. Initial Nd and Sr isotopic ratios for the Ombu rhyodacite, Ekuta rhyolite, Ombu granodiorite, and Erongo granite are similar, and trace element modelling confirms that they can be derived from a common magma source by fractional crystallisation of plagioclase, alkali feldspar, quartz and biotite. The Erongorus rhyodacites, in contrast, were probably not derived from the Erongo. This is supported by: 1) the uneven distribution of the unit, 2) the lack of an intrusive equivalent, 3) the absence of locally-derived lithic fragments and basement-inherited zircons, and 4) the overlap in 87Sr/86Sr130 Ma (0.72048 to 0.72265), 143Nd/144Nd130 Ma ratios (0.512045 to 0.512069). Based on these argument and the age equivalence of Erongorus (U-Pb zircon 131.8±1.1 Ma; 1s error) with low-Ti Parana Etendeka rhyodacites, it is suggested that the Erongorus rhyodacites are erosional remnants of felsic Etendeka volcanics. Re-Os isotopic data for Cretaceous basic rocks in the Etendeka Province were determined in this study for the first time. The goals were to provide additional insights into nature of the mantle component involved in magma genesis, and to aid in identifying the influence of crustal contamination. High 187Os/188Os 130 Ma ratios of the basal tholeiites (0.1903 to 0.3705) are interpreted as a result from crustal contamination, in agreement with evidence from the Sr, Nd and Pb isotope systems. By contrast, ring dike dolerites and foidites-tephrites from late-stage alkali-basaltic stocks have different 187Os/188Os 130 Ma ratios (0.1164 to 0.1265 and 0.1319 to 0.1334, respectively). The negative values of gOs (130 Ma) for the ringdike dolerites (-0.2 and –8.1) are similar to those observed in alkaline rocks and mantle-derived xenoliths entrained in kimberlites from the Parana Province, Kaapvaal, Wyoming, and Siberian cratons. This suggests a derivation of the magmas from the subcontinental lithospheric mantle. On the other hand, the positive gOs (130 Ma) values of the alkaline stockes (+4.1 and +5.3) overlap with the so called enriched plume mantle component, which further supports the conclusion of a plume origin suggested by Sr, Nd and Pb isotope data. Results of geochronology using 40Ar/39Ar and high spatial resolution U-Pb zircon dating demonstrate that emplacement of the various igneous units at Erongo took place within a time span equivalent to or shorter than the geochronologically resolvable age differences (ca. 2 Ma). They also indicate that Erongo magmatism coincides with the peak of regional flood basalt activity in the Etendeka–Parana province. The Ombu rhyodacite was previously dated at 135.0±1.6 Ma (Pirajno et al., 2000), and for this study, the intrusive equivalent, Ombu granodiorite was chosen for dating. The granodiorite yielded concordant 40Ar/39Ar and U-Pb zircon ages of 132.6±1.1 Ma and 132.3±1.9 Ma (1s), respectively. The Ekuta rhyolite and compositionally equivalent Erongo granite yielded U-Pb zircon ages of 131.9±2.9 Ma and 130.3±1.4 Ma (1s), respectively. Two samples of the granite also gave overlapping 40Ar/39Ar biotite ages averaging 132.2±0.8 Ma. The final stage of magmatism at Erongo involved emplacement of basic alkaline stocks and dikes. Phlogopite and kaersutite from foidite stocks of this series yielded 40Ar/39Ar ages of 130.8±1.0 Ma and 132.0±1.0 Ma. Erongo is the last of the large silicic complexes in the Damaraland to be dated. In a regional context, the new age results indicate that silicic magmatism in the Damaraland complexes (Erongo, Brandberg, Paresis, Messum) was contemporary. It began simultaneously with the peak of flood basalt effusion at about 132 Ma throughout the Damaraland province and ceased within a very short time, by about 130 Ma. The silicic magmas are hybrid, with varying degrees of crustal and mantle-derived melts, and the age constraints suggest that crustal melting was caused by a short-lived thermal pulse related to the main flood basalt event. Low-volume basic magmatism in the Damaraland province continued sporadically thereafter to about 123 Ma, but mantle-derived heat input was insufficient to cause further crustal melting.