Contemporaneous magnesian (I-type) and ferroan (A-type) magmatism in the Great Proterozoic Accretionary Orogen: Insights from the Julianehåb Igneous Complex, Greenland, and the Adlavik Domain, Labrador, Canada

The Julianehåb Igneous Complex (JIC) in the Ketilidian Orogen, Greenland, together with the Makkovik Province in Labrador, Canada, are part of the Great Proterozoic Accretionary Orogeny and represent an important period of Paleoproterozoic orogeny and crustal growth. The JIC is dominated by metaluminous, magnesian, hornblende-bearing, I-type granitoids, with abundant field evidence for co-mingling with mafic magmas. The rocks have subduction trace element signatures and elemental variations indicative of amphibole and plagioclase control on compositions during melting and melt evolution. Nd and Hf isotope compositions (whole rock and zircon) are relatively juvenile and positive in the south and become more variable and evolved to the NW. Increased involvement of North Atlantic Craton (NAC) Archean crust and enriched lithospheric mantle during magma generation and increasing crustal thickness approaching the inboard margin are indicated by more evolved Nd and Hf isotopic compositions coupled with increasing La/Yb in the NW part of the JIC. Mantle-like O isotope compositions in zircon indicate little involvement of supracrustal components. Contemporaneous granitoids in the Adlavik Domain of the Makkovik Province are chemically distinct, and are dominated by higher SiO 2 , ferroan compositions, HFSE enrichment and are classified as A-types. Nd isotope compositions of the JIC and Adlavik Domain overlap, indicating generation of contemporaneous A-type and I-type granites from similar sources during a magmatic flare-up and in the same overall tectonic setting. The JIC rocks formed in a continental arc setting, through fractionation of subduction-derived mafic melts and melting of juvenile mafic lower crustal sources with variable amounts of Archean NAC components to the north. The Adlavik Domain rocks are interpreted to have formed laterally to the JIC in an extending back-arc region, with less influence from subduction zone fluids and NAC crust. Upwelling of hot asthenospheric and lithospheric mantle during extension resulted in melting of crustal sources in the Adlavik Domain potentially comprising residual lower crustal sources and tonalitic to granodioritic products of earlier 1.85 Ga magmatism to produce the distinctive ferroan compositions. Combined, the JIC and Adlavik Domain are evidence for complex continental margin activity and crustal growth and reworking along the Paleoproterozoic orogenic margin to the Colombia/Nuna supercontinent and more data are needed to fully understand this long-lived, large and complex event. This study indicates that caution is needed when interpreting tectonic setting using trace element discrimination plots alone for examining regional scale and complex continental margins. • The Julianehåb Igneous Complex, S Greenland is a pulse of crustal growth at 1.8 Ga. • Dominantly magnesian I-type granites with no evidence for supracrustal components. • Granite compositions show increased North Atlantic Craton influence in the North. • Trace element evidence for increased crustal thickness to the north. • Contemporaneous magmas in Makkovik, Canada are more ferroan and A-type in nature.