FORMATION AND FRACTIONATION OF HIGH-Al THOLEIITIC MAGMAS IN THE AHVENISTO RAPAKIVI GRANITE - MASSIF-TYPE ANORTHOSITE COMPLEX, SOUTHEASTERN FINLAND

Abstract

The ~1640 Ma Ahvenisto AMCG complex in southeastern Finland is the most prominent occurrence of massif-type anorthosite and associated monzodioritic rocks in the Fennoscandian rapakivi suite. The petrology of the complex has been reasonably well studied, but the petrogenetic relationships among the different rock-types nevertheless remain ambiguous. In this study, we shed light on the source of AMCG complexes by providing updated geochronology, petrogenetic considerations, and an extensive geochemical framework for the Ahvenisto complex. Similar U–Pb mineral ages and Nd–Sr–Pb isotope compositions, as well as complementary major-element and REE data, suggest that the anorthositic and monzodioritic rocks of the Ahvenisto complex are comagmatic. Geochemical evidence implies a two-stage magmatic evolution, where isotopic differences between the slightly more juvenile olivine-bearing gabbroic rocks ( e Nd in the range +0.2 to +0.4, Sr i in the range 0.7034 to 0.7035) and leucogabbronorites and anorthosites ( e Nd between −0.9 and −0.5, Sr i between 0.7037 and 0.7041) reflect the influence of crustal contamination of the primary mantle-derived melt during the first stage. The bulk of the monzodioritic rocks ( e Nd in the range −1.1 to −0.2, Sr i in the range 0.7028 to 0.7040) represent second-stage residual liquids after extensive fractionation of plagioclase from a relatively evolved, high-Al tholeiitic magma with a pronounced crustal isotope signature. Although the Rb–Sr isotopic systematics of the rapakivi granites have been compromised, their geochemistry and Nd isotope compositions ( e Nd between −2.1 and −0.1) are consistent with a derivation mostly from a crustal source.