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Petrology and geochemistry of xenoliths from the Northern Baltic shield: evidence for partial melting and metasomatism in the lower crust beneath an Archaean terrane |
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| Authors: | P D Kempton H Downes E V Sharkov V R Vetrin D A Ionov D A Carswell A Beard |
| Institution: | a NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, NG12 5GG, UK b Department of Geology, Birkbeck College, University of London, Malet St., London, WC1E 7HX, UK c IGEM, Staromonetry per 35, Moscow, Russia d Kola Science Centre, Apatity, Russia e School of Earth Sciences, Macquarie University, North Ryde, N.S.W. 2113, Australia f Department of Earth Sciences, Sheffield University, Sheffield, S3 7HF, UK |
| Abstract: | Lower crustal xenoliths entrained in a Paleozoic ultramafic lamprophyre breccia pipe on Elovy island, Kola peninsula, Russia, represent some of the oldest lower crustal material yet investigated from Europe. The xenoliths vary from feldspar-poor, garnetrich rocks which resemble eclogites, to feldspar-rich garnet granulites. Quartz-rich felsic granulites, as well as pyroxenites and amphibole-rich rocks are also present. The mafic granulites/eclogites represent a suite of gabbros and norites that is related by olivine fractionation. The igneous protoliths may have formed in a manner analogous to lower crustal rocks from most other European xenolith localities, i.e. by basaltic underplating, but magmatic cumulates are not in evidence. The Kola lower crust was subjected to one or more metasomatic events which introduced up to 45% phlogopite and/or amphibole into both eclogites/granulites and pyroxenites. The resulting rocks have strong enrichments in Rb, Ba, and K, indicating that the lower crust is not uniformly depleted in LIL and heat-producing elements. Siliceous (65% SiO2) and mafic (< 50% SiO2) lithologies coexist in migmatitic xenoliths, which provide evidence for partial melting processes and restite formation in mafic metaigneous lower crust. The relationship, if any, between partial melting and metasomatism is unclear. |
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