PETROLOGY AND GEOCHEMISTRY OF XENOLITHS FROM THE NORTHERN BALTIC SHIELD - EVIDENCE FOR PARTIAL MELTING AND METASOMATISM IN THE LOWER CRUST BENEATH AN ARCHEAN TERRANE

Lower crustal xenoliths entrained in a Paleozoic ultramafic lamprophyr e breccia pipe on Elovy island, Kola peninsula, Russia, represent some of the oldest lower crustal material yet investigated from Europe. Th e xenoliths vary from feldspar-poor, garnet-rich rocks which resemble eclogites, to feldspar-rich garnet granulites. Quartz-rich felsic gran ulites, as well as pyroxenites and amphibole-rich rocks are also prese nt. The mafic granulites/eclogites represent a suite of gabbros and no rites that is related by olivine fractionation. The igneous protoliths may have formed in a manner analogous to lower crustal rocks from mos t other European xenolith localities, i.e. by basaltic underplating, b ut magmatic cumulates are not in evidence. The Kola lower crust was su bjected to one or more metasomatic events which introduced up to 45% p hlogopite and/or amphibole into both eclogites/granulites and pyroxeni tes. The resulting rocks have strong enrichments in Rb, Ba, and K, ind icating that the lower crust is not uniformly depleted in LIL and heat -producing elements. Siliceous (65% SiO2) and mafic (< 50% SiO2) litho logies coexist in migmatitic xenoliths, which provide evidence for par tial melting processes and restite formation in mafic metaigneous lowe r crust. The relationship, if any, between partial melting and metasom atism is unclear.