Garnet granulite xenoliths from the Northern Baltic Shield - the underplated lower crust of a palaeoproterozoic large igneous province

Garnet granulite facies xenoliths hosted in Devonian lamprophyres from the Kola Peninsula are interpreted to represent the high-grade metamorphic equi valents of continental flood tholeiites, emplaced into the Baltic Shield Ar chaean lower crust in early Proterozoic time. Geochronological data and sim ilarities in major and trace element geochemistry suggest that the xenolith s formed during the same plume-related magmatic event that created a widesp read Palaeoproterozoic large igneous province (LIP) at 2.4-2.5 Ga. They are , thus, the first samples of the lower crust of a Palaeoproterozoic LIP to be studied in petrological detail. The suite included mafic granulites (gar + cpx + rutile +/- opx +/- phlog +/- amph), felsic granulites (plag + gar + opx + rutile +/- qtz +/- Kspar +/- phlog +/- amph) and pyroxenites (+/- p hlog +/- amph), but mafic garnet granulites predominate. Although some samp les are restites, there is no evidence for a predominance of magmatic cumul ates, as is common for Phanerozoic lower-crustal xenolith suites. Metasedim ents are also absent. Phlogopite and/or amphibole occur in xenoliths of all types and are interpreted to be metasomatic in origin. The K-rich metasoma tic event occurred at similar to 2.0 Ga, and led to substantial enrichment in Rb, K, LREE/HREE, Th/U, Th/Pb and, to a lesser extent, Nb and Ti. The fl uids responsible for this metasomatism were probably derived from a second plume that arrived beneath the region at this time. Evidence for partial me lting of mafic crust exists in the presence of migmatitic granulites. The t iming of migmatization overlaps that of metasomatism, and it is suggested t hat migmatization was facilitated by the metasomatism. The metamorphism, me tasomatism and migmatization recorded in the Kola granulite xenoliths may b e representative of the processes responsible for converting Archaean LIP-g enerated proto-continents into continental crust.