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Ninth Annual V. M. Goldschmidt Conference
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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
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