ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract Fifty-six new oxygen isotope analyses of minerals separated from nine Mesozoic anorogenic complexes of Damaraland in northwest Namibia have been used to estimate the δ18O values of the original magmas (δmagma). These complexes range in composition from nepheline syenite to alkaline and peraluminous granites, often with a variety of rock types present at a single centre. The silica-undersaturated rock types show a relatively small spread of values of δmagma from 6.0 to 7.0‰ (mean 6.6‰), which is consistent with their derivation from the mantle with little or no subsequent crustal contamination. The silica-oversaturated rocks show a wide range of δmagma values from 4.9 to 12.0‰, with a mean value of 8.3‰. The high values of δmagma in the silica-oversaturated rocks can only be explained with considerable involvement of the continental crust in their petrogenesis, and those rocks with δmagma〉10‰ are interpreted as essentially crustal melts. It is generally accepted that the Damaraland complexes were generated as a result of rifting across the Tristan plume, with the plume providing both mantle-derived magma and with it the heat required for crustal melting. In addition to their mantle-like oxygen isotope ratios, the undersaturated rocks of the Damaraland complexes possess the trace element characteristics (e.g. low Zr/Nb ratios) of ocean island basalt, suggesting that their parental magmas were produced from the plume itself. In contrast, the oversaturated complexes generally have higher Zr/Nb ratios that are consistent with a larger crustal input. The highest values of δmagma in the Damaraland complexes are found in granitic rocks that intrude the central zone of the Pan-African Damara Orogen where presumably there is a substantial component of sedimentary origin in the lower to middle crust.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/s004100050130
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