The Rapakivi granites of South Greenland—stable isotope characteristics of their black and white facies and the nature of their protolith

Abstract

The rapakivi granites of South Greenland were emplaced into the Ketilidian orogenic belt at about 1.74 Ga during an episode of extensional tectonics. Petrographically, they can be subdivided into a roughly subequal black facies and white facies, distinguished by the colour (turbidity) of the alkali feldspars and by the associated mineralogy which is more hydrous (i.e., amphibole and biotite compared to olivine and pyroxene) in the white facies. The oxygen isotope composition of these rocks is notably homogeneous; from thirteen outcrops sampled over an area extending some 200 km×100 km all alkali feldspars have an average δ 18 O of 10.2±0.4‰. This lack of variation is consistent with the consensus petrogenetic model which invokes a very well mixed two-component protolith; the high abundance of 18 O (mean whole rock δ 18 O =9.4‰) argues that the major contribution (perhaps 90%), although originally mantle-derived, had undergone 18 O enrichment in a low temperature sedimentary cycle. There is no oxygen isotopic distinction between alkali feldspars in the black and white facies, which is argued to be a consequence of the relatively anhydrous nature of the melt. Hydrogen isotope compositions vary widely—by 30‰—but are coherent on a hand specimen scale, as demonstrated by approximately equal δD of biotite and alkali feldspar. It is not clear at which stage the D/ H heterogeneity was introduced.