Petrogenesis of the Post-kinematic Magmatism of the Central Finland Granitoid Complex II; Sources and Magmatic Evolution

Abstract

The 188--187Ga post-kinematic granitoids of the Central Finland Granitoid Complex (CFGC) provide a key geochemical link to understanding granite formation in Paleoproterozoic orogenic and post-orogenic terrains. Thickness of the crust and intra-crustal differentiation processes played an important role in the formation of three granitoid types that shortly followed the peak of the Svecofennian orogeny. In the eastern CFGC, pyroxene-bearing plutons with C-type geochemical affinities predominate. These were formed from a mixture of low- to moderate-degree partial melts (30%) of mafic mantlederived (basaltic, 49% SiO2) source rocks and partial melts of pre-existing mafic granulite lower crust at depth. In the western CFGC, high-silica, iron-rich, fluorite-bearing plutons with A-type granite characteristics predominate. A higher thermal gradient, thinner upper and lower crust, and significantly more shallow Moho depth resulted in higher proportions of crustal melts (03--0 4v s 0 1--02 in the eastern CFGC) incorporated into the partial melts of a mafic mantle-derived source. A geochemical model focusing on the J and Honkajoki plutons of the post-kinematic suite is presented, constraining the nature of the source rock(s), the degree of partial melting, proportions of partial melts and fractionation processes involved.