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

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 faci es and white facies, distinguished by the colour (turbidity) of the all;ali feldspars and by the associated mineralogy which is more hydrous (i,e,, am phibole 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 X 100 km all a lkali feldspars have an average delta(18)O of 10.2 +/- 0.4 parts per thousa nd, This lack of variation is consistent with the consensus petrogenetic mo del which invokes a very well mixed two-component protolith; the high abund ance of O-18 (mean whole rock delta(18)O = 9.4 parts per thousand) argues t hat the major contribution (perhaps 90%), although originally mantle-derive d, had undergone O-18 enrichment in a low temperature sedimentary cycle. Th ere is no oxygen isotopic distinction between alkali feldspars in the black and white facies, which is argued to be a consequence of the relatively an hydrous nature of the melt. Hydrogen isotope compositions vary widely-by 30 parts per thousand-but are coherent on a hand specimen scale, as demonstra ted by approximately equal delta D of biotite and alkali feldspar. It is no t clear at which stage the D/H heterogeneity was introduced. (C) 1999 Elsev ier Science B.V. All rights reserved.