Petrogenesis of the Gross Spitzkoppe topaz granite, central western Namibia: a geochemical and Nd-Sr-Pb isotope study

The early Cretaceous Gross Spitzkoppe granite is one of several igneous complexes that formed in the Damaraland of Namibia during an episode of continental rifting and magmatism associated with opening of the South Atlantic. This is a classic setting of rift-related, anorogenic magmatism showing the typical association of mantle-derived mafic and alkaline rocks along with silicic units. The Gross Spitzkoppe granite is an important example of the latter because its peraluminous composition and crustal isotopic signature makes it well suited to characterize the crustal contribution to magmatism. The Gross Spitzkoppe stock (GSS) is composed of three texturally distinct topaz-bearing biotite monzogranites (average 76 wt.% SiO2, similar to30 wt.% Rb/Sr and similar to0.5 wt.% F). Iron-rich biotite is the only mafic silicate. In proximity to the stock are numerous felsic dikes (rhyodacites to rhyolites). The elevated high-field-strength elements (HFSE) contents and high Ga/Al ratios of the granites and felsic dikes are typical of A-type granites. The granites and dikes show similar geochemical differentiation trends and the compositions of the more evolved dikes overlaps with the granites. This and similar initial isotope ratios suggest that the dikes and granites have a common origin. Major and trace element modeling confirms that the composition of the Gross Spitzkoppe granite can be produced by fractional crystallization from magma with the average felsic dike composition. Rb-Sr isotope data for nine whole-rock samples of the GSS yield an age for emplacement of the complex at 125 +/- 1 Ma. initial Sr-87/Sr-86 ratios vary from 0.710 to 0.716. Neodymium isotope compositions of the granites and dikes yield initial epsilonNd(125) values from -5.6 to -6.4. The Pb isotopic ratios (K-feldspar or age-corrected whole rock) fall in the range Pb-206/Pb-204 18.70-18.82, Pb-207/Pb-204 15.66-15.71, and Pb-208/Pb-204 38.57-39.07. These geochemical and isotopic characteristics are consistent with derivation of the Gross Spitzkoppe granitic magma from a granulite-facies metasedimentary source. Batch melting calculations suggest that low (10-20%) degree partial melting of a felsic granulitic source is capable of producing melt compositions similar to those of the felsic dikes, with further fractional crystallization being required to produce the granites. The source rocks are thought to be Damara Belt metasediments that were melt-depleted in the early Paleozoic by extraction of the S-type Damara granites. The most likely scenario for crustal melting is by basaltic underplating of the source region in the lower crust. The range in initial Nd and Sr values is attributed to minor mixing of basaltic magma with the crustal melts, although Sr was also affected by minor contamination from the radiogenic Damara granites near the level of emplacement. (C) 2004 Elsevier B.V All rights reserved.