From source migmatites to to plutons: tracking the origin of ca. 435 Ma S-type granites in the East Greenland Caledonian orogen

Abstract

In the Caledonian orogen of East Greenland, superb exposure along steep fjord walls allows direct observation of the origin of ca. 435 Ma S-type granites by anatexis of paragneisses, and collection of the resulting melts into wider sheets and plutons. Field observations suggest that 'fertile' metasediments of the late Mesoproterozoic Krummedal supracrustal sequence were the source of the granites, and there is no evidence of participation of the 'less fertile' Archaean and Palaeoproterozoic orthogneisses in the region. Comparison of inherited zircons in the granites with detrital zircons in the metasediments by SHRIMP, as well as chemical analyses and Rb-Sr isotope data, support this contention. In addition to the ca. 435 Ma granites, ca. 930 Ma S-type granites are also present: The two age groups cannot always be differentiated on field criteria. The formation of late Caledonian plutons may have been triggered by decompression during gravitational collapse following crustal thickening by Caledonian collision, but structural relationships are too complex to allow a precise interpretation of the tectonic setting of most granites.