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

In the Caledonian orogen of East Greenland, superb exposure along steep fjo rd walls allows direct observation of the origin of ca. 435 Ma S-type grani tes by anatexis of paragneisses, and collection of the resulting melts into wider sheets and plutons. Field observations suggest that 'fertile' metase diments of the late Mesoproterozoic Krummedal supracrustal sequence were th e 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 th e metasediments by SHRIMP, as well as chemical analyses and Rb-Sr isotope d ata, support this contention. In addition to the ca. 435 Ma granites, ca. 9 30 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 fol lowing crustal thickening by Caledonian collision, but structural relations hips are too complex to allow a precise interpretation of the tectonic sett ing of most granites. (C) 2001 Elsevier Science B.V. All rights reserved.