2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 14-12
Presentation Time: 11:10 AM

THE LARGEST OF THE CAMP INTRUSIONS: A GEOCHEMICAL AND ISOTOPIC STUDY OF THE FREETOWN LAYERED COMPLEX (SIERRA LEONE)


CALLEGARO, Sara1, MARZOLI, Andrea1, BERTRAND, Hervé2, BLICHERT-TOFT, J.3, REISBERG, Laurie4, JOURDAN, Fred5, CAVAZZINI, Giancarlo6, ZANETTI, Alberto7, PARISIO, Laura1 and BERT MANOZ, Romain Bouchet2, (1)Geoscienze, Università di Padova, via Gradenigo 6, Padova, 35100, Italy, (2)Laboratoire de Géologie de Lyon, Ecole Normale Superieure de Lyon, 46 allée d’Italie, Lyon, 5276, France, (3)Laboratoire de Géologie de Lyon, Ecole Normale Supérieure de Lyon and Université Claude 7 Bernard Lyon 1, CNRS, UMR 5276, 46 Allée d’Italie, Lyon, 69007, France, (4)Crpg, CNRS, Nancy, CNRS, France, (5)Western Australian Argon Isotope Facility, JdL Centre & Department of Applied Geology, Curtin University of Technology, GPO Box U1987, Perth, 6845, Australia, (6)Igg Cnr, Padova, via Gradenigo 6, Padova, 35100, Italy, (7)Igg Cnr, Pavia, via Ferrata 1, Pavia, 27100, Italy

The Freetown Layered Complex (FLC) is a massive (65 × 14 × 7 km) tholeiitic mafic intrusion injected into the Kasila Precambrian belt along the coastal Sierra Leone. It consists of cumulates ranging from olivine gabbros to anorthosites. The FLC is also cut by coast-parallel basaltic dykes, which extend from a Liberian dyke swarm, previously attributed to the Central Atlantic Magmatic Province (CAMP).

40Ar/39Ar dating of a plagioclase separate from one of the FLC gabbros gave an age of 202.3±2.4 Ma, which assigns this complex to the time of CAMP activity and the early stages of Pangea breakup and opening of the Central Atlantic Ocean.

Based on LA-ICPMS spot analyses of clinopyroxene and orthopyroxene crystals, the calculated trace element contents of the equilibrium melts indicate variously enriched and high-Ti compositions in agreement with those observed in the Liberia-FLC dyke swarm.

Except for three outliers, the intrusive rocks cluster in a tight range of 87Sr/86Sri (0.70337-0.70402) and 143Nd/144Ndi (0.51241-0.51252), at lower Nd for comparable Sr isotopic compositions relative to other high-Ti CAMP rocks from nearby Liberia, Guyana and northern Brazil in pre-drift reconstructions. Pb isotope systematics of the FLC show low 206Pb/204Pbi (17.520-17.850) and fall above the NHRL (207Pb/204Pbi = 15.549-15.594 and 208Pb/204Pbi = 37.730-37.936). A basaltic dyke and one gabbro plot towards the DMM field (with up to 0.70311, 0.51259, 0.28290, 17.702, 15.527, 37.651 for Sr, Nd, Hf and Pb isotopes, respectively). 176Hf/177Hfi (0.28279-0.28285), preliminary 188Os/187Osi (ca. 0.130), and previously published δ18O data for the FLC argue against significant crustal assimilation. These data rather suggest that the FLC derived from a mantle source dominated by an old (high Δ7/4Pb), EMI-flavored (low 206Pb/204Pb) component, while the subsequently emplaced dykes reflect substantial asthenospheric contribution.

Session No. 14

T117. Magmas and Their Sources I: A Tribute to the Distinguished Career of Fred Frey
Sunday, 19 October 2014: 8:00 AM-12:00 PM
205/206 (Vancouver Convention Centre-West)
Geological Society of America Abstracts with Programs. Vol. 46, No. 6, p.58
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