Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Ninth Annual V. M. Goldschmidt Conference 7044.pdf GARNET GRANULITE XENOLITHS FROM THE NORTHERN BALTIC SHIELD – THE LOWER CRUST OF A PALEO-PROTEROZOIC LIP. P.D. Kempton1, H. Downes2, L. Neymark3, E.V. Sharkov4, R. Zartman5 J. Wartho6 1NERC Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, NG12 5GG, U.K. ([email protected]), 2Birkbeck/UCL Research School of Geological and Geophysical Sciences, Birkbeck College, Malet Street, London WC1E 7HX, U.K. ([email protected]), 3U.S. Geological Survey, Denver, CO, U.S.A. ([email protected]), 4IGEM, Staromonetny per 35, Moscow, 109017, Russia ([email protected]) Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa ([email protected]), 5Department of Earth Sciences, The Open University, Milton Keynes MK7 6AA, U.K. ([email protected]). Introduction: It is generally believed that growth of continental crust has occurred through either or both of two mechanisms: volcanic arc accretion [1] and oceanic plateau accretion [2–4]—but have the relative contributions of these two mechanisms varied with time? Although the former is a model of long standing, the latter may be able to explain periods of rapid continental growth better than the volcanic arc accretion model. In order to improve our understanding of the processes involved in the formation and evolution of continental crust in Archean/Proterozoic time, we have undertaken a geochemical and isotopic study of a suite of Proterozoic lower crustal xenoliths from the northern Baltic Shield [5]. The Kola xenoliths were brought to the surface by a Devonian ultramafic lamprophyre diatreme on Elovy island (Kola Peninsula). The xenolith suite includes mafic granulite (gar + cpx + rutile ± plag ± opx ± phlog ± amph), felsic granulite (gar + cpx + plag + rutile ± qtz ± Kspar ± phlg ± amph) and pyroxenite (± phlog ± amph), but “eclogitic” plagioclase-poor 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. Instead, most of the protoliths of the granulites were mafic, LREE-enriched norites to gabbronorites. Major element, trace element and isotopic evidence indicates that the Kola xenoliths are the high-grade metamorphic equivalents of continental flood tholeiites, emplaced into the Baltic Shield lower crust in early Proterozoic time (~2.4–2.6 Ga). Similarities in major and trace element systematics suggest that they formed in response to the same plume-driven magmatic event that created the widespread Paleoproterozoic igneous province in the area at 2.4–2.5 Ga. Thus, the mafic protoliths may have formed in a manner analogous to Phanerozoic lower crustal rocks (i.e. basaltic underplating), but in this case the underplating was the result of an actively upwelling plume rather than passive upwelling asthenosphere beneath an extending region. The Kola xenoliths, therefore, represent the first lower crust of a Paleoproterozoic LIP to be studied in petrological detail. The granulites exhibit a wide range in isotopic values that can be related to primary lithology and the presence or absence of secondary hydrous phases. Present-day Sr-isotope values range from 0.70520.9564; Nd-isotopes range from 0.5112–0.5123. Pbisotopes exhibit an extreme range of values: 6/4Pb = 17.39–22.47, 7/4Pb = 15.42–16.03 and 8/4Pb = 37.46– 54.83. The unusual radiogenic Pb-isotope compositions of the xenoliths are the product of an ancient metasomatic event and not a function of initial protolith composition, such as might be the case for ancient metasediments. Indeed, amphibole and/or phlogopite occur commonly in xenoliths of all types, and are interpreted to be metasomatic in origin. Ar-Ar dating of a phlogopite separate from one strongly metasomatized xenolith indicates that introduction of K-rich fluids occurred sometime ca. 2 Ga; this event resulted in 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 alkalic volatile-rich melts generated from enriched lithospheric mantle above an incubating plume head that arrived beneath the region at ~2.0 Ga. These data indicate that the lower crust has not been uniformly depleted in U-Th-Pb since ancient times, but may have locally had extreme Th/Pb, Th/U and U/Pb ratios for ~2 b.y. Evidence for partial melting of mafic crust exists in the presence of migmatitic granulites. The timing of migmatisation overlaps that of metasomatism, and it is suggested that migmatisation was faciliated by the metasomatism. The metamorphism, metasomatism and migmatisation recorded in the Kola granulite xenoliths may be representative of the processes responsible for converting Archean LIP-generated proto-continents into continental crust. References: [1] Taylor, S. R., (1967). Tectonophysics 4, 17. [2] Stein, M. & Hofmann, A. W. (1994) Nature 372, 63–68. [3] Stein, M. & Goldstein, S. (1996). Nature 382, 773–778. [4] Abbott, D. & Mooney, W. (1995). Reviews in Geophysics 33, 231– 242. [5] Kempton, P.D., Downes, H., Neymark, L., Sharkov, E.V. and Zartman, R (J. Petrol., submitted). Ninth Annual V. M. Goldschmidt Conference SHORT TITLE HERE: A. B. Author and C. D. Author 7044.pdf