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Journal of Petrology Volume 42 Number 4 Pages 731-763 2001
© Oxford University Press 2001

Garnet Granulite Xenoliths from the Northern Baltic Shield—the Underplated Lower Crust of a Palaeoproterozoic Large Igneous Province?

P. D. KEMPTON1,*, H. DOWNES2, L. A. NEYMARK3, J. A. WARTHO4, R. E. ZARTMAN5,{dagger} and E. V. SHARKOV6

1NERC ISOTOPE GEOSCIENCES LABORATORY, KINGSLEY DUNHAM CENTRE, KEYWORTH NG12 5GG, UK
2BIRKBECK/UCL RESEARCH SCHOOL OF GEOLOGICAL AND GEOPHYSICAL SCIENCES, BIRKBECK COLLEGE, MALET STREET, LONDON WC1E 7HX, UK
3US GEOLOGICAL SURVEY, DFC, BOX 25046, MS 963, DENVER, CO, USA
4DEPARTMENT OF EARTH SCIENCES, OPEN UNIVERSITY, WALTON HALL, MILTON KEYNES MK7 6AA, UK
5DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF CAPE TOWN, RONDEBOSCH 7700, SOUTH AFRICA
6IGEM, STAROMONETNY PER 35, MOSCOW, 109017, RUSSIA

Garnet granulite facies xenoliths hosted in Devonian lamprophyres from the Kola Peninsula are interpreted to represent the high-grade metamorphic equivalents of continental flood tholeiites, emplaced into the Baltic Shield Archaean lower crust in early Proterozoic time. Geochronological data and similarities in major and trace element geochemistry suggest that the xenoliths formed during the same plume-related magmatic event that created a widespread 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 includes mafic granulites (gar + cpx + rutile ± plag ± opx ± phlog ± amph), felsic granulites (plag + gar + cpx + rutile ± qtz ± Kspar ± phlog ± amph) and pyroxenites (± phlog ± amph), but mafic garnet granulites predominate. Although some samples are restites, there is no evidence for a predominance of magmatic cumulates, as is common for Phanerozoic lower-crustal xenolith suites. Metasediments are also absent. Phlogopite and/or amphibole occur in xenoliths of all types and are interpreted to be metasomatic in origin. The K-rich metasomatic event occurred at ~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 fluids responsible for this metasomatism were probably derived from a second plume that arrived beneath the region at this time. Evidence for partial melting of mafic crust exists in the presence of migmatitic granulites. The timing of migmatization overlaps that of metasomatism, and it is suggested that migmatization was facilitated by the metasomatism. The metamorphism, metasomatism and migmatization recorded in the Kola granulite xenoliths may be representative of the processes responsible for converting Archaean LIP-generated proto-continents into continental crust.

KEY WORDS: granulite xenoliths; metasomatism; migmatization; Sr–Nd–Pb isotopes; geochronology


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