COEXISTING SILICATE AND CARBONATITIC MAGMAS IN THE QASSIARSUK COMPLEX, GARDAR RIFT, SOUTHWEST GREENLAND

Abstract

The Mesoproterozoic Qassiarsuk complex in the Gardar Rift in southwestern Greenland comprises lavas, pyroclastic rocks and subvolcanic intrusions of alkaline silicate rocks and carbonatites. The volcanic rocks are interlayered with sandstones and basalts belonging to the lower part of the Eriksfjord Formation (Mussartut member). The carbonatites at Qassiarsuk are mainly dolomite carbonatite, with subordinate calcite carbonatite. Reconstruction of the composition of heterogeneous, devitrified silicate glass found in lavas, pyroclastic rocks and as lapilli by raster analysis with the electron microprobe indicates that silicate magmas with K-poor, ultramafic and K-rich, basic to intermediate compositions were present during eruption. Samples of the latter today show extreme K2O/Na2O at (Na + K)/Al well below 1.0, reflecting selective loss of Na from a potassic to mildly ultrapotassic magmatic glass during postmagmatic alteration. The presence of coexisting lapilli of silicate rocks and carbonatite suggest that a magnesiocarbonatitic liquid was present together with both of the silicate magmas during eruption. These coexisting magmas were ultimately derived from a common source in the upper mantle, but they probably do not represent pairs of immiscible liquids separated from a single precursor in the shallow crust. It is most likely that the carbonatitic magma at Qassiarsuk arose by immiscible separation from a carbonated mafic or ultramafic silicate parental magma evolving by fractional crystallization at some depth in the magmatic plumbing system, the magnesiocarbonatitic liquid remaining immiscible with silicate magmas during ascent and eruption.