Mineralogie und Geochemie peralkalischer Granite des Brandberg Intrusivkomplexes/Namibia

Abstract

English abstract by author:

The Brandberg intrusive complex belongs to the Mesozoic Damaraland igneous province in NW Namibia. The magmatic activity in this province is closely related to rifting, formation of flood basalts and opening of the South Atlanic at 135-120 Ma. The Brandberg intrudes Pan-African granitoids and metasediments, Karoo clastic sediments and volcanics of the Etendeka Formation. The major part of the complex is formed by a homogenous biotite-hornblende granite (main granite). Other intrusions include leucogranitic dykes and a monzonite body exposed in the central part of the main granite. The monzonite, main granite and leucogranites have metaluminous composition. The Amis peralkaline complex is a mineralogically and chemically distinct intrusion at the SW margin of the main granite. Dykes and sills of a peralkaline arfvedsonite granite cut the main granite and adjacent country rocks. Although it is small in volume, the Amis complex is important because of its extreme enrichment in incompatible elements like Rb, Zr (max. 1. 7 wt. percent), Nb (2900 ppm), Y (2000 ppm), Th and U. High fluorine contents are also characteristic. Compared to the metaluminous rocks of the Brandberg complex the Amis rocks are depleted in Sr, Ba and Eu. The chemical composition of the Amis complex indicates a high degree of differentiation. The relationship between the Amis complex and the main granite was examined by major- and trace element modelling. Extreme fractional crystallisation (PI, Kfs, Qtz, ± Hbl, Bt) of a metaluminous parental magma similar to the main granite would account for the major elements, Rb, Sr and Ba, but cannot produce the extreme enrichment in elements like Zr, Nb, Y. The initial Nd- and Sr isotopic compositions of the main granite and for the Amis rocks are: 87Sr/86Sr = 0. 7123±0. 0003 vs. 0. 721±0. 004 and eNd = -3 vs. -1 (ages used: 125 Ma and 118 Ma, resp.)