Geochemistry of lamprophyres from the Western Alps,Italy: implications for the origin of an enriched isotopic component in the Italian mantle |
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| Authors: | Jennifer Paige Owen |
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| Institution: | (1) Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, ON, Canada, K1S 5B6;(2) Department of Geoscience, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada, T2N 1N4 |
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| Abstract: | Cenozoic lamprophyres (minettes, spessartites, kersantite) from the Western Alps, northern Italy, represent small volume,
mafic melts with high Mg#s and high Ni and Cr contents. All the lamprophyres show light REE enrichment, high incompatible
element contents, and Ta, Ti and Nb troughs on chondrite-normalized diagrams. Age-corrected 87Sr/86Sr isotopic ratios (assuming t = 30 Ma) are highly variable and range from 0.70590 to 0.71884; 143Nd/144Nd ratios range from 0.51203 to 0.51242. Pb isotopic ratios are: 206Pb/204Pb = 18.669–18.895, 207Pb/204Pb = 15.605–15.689 and 208Pb/204Pb = 38.224–39.134. 87Sr/86Sr ratios show a negative correlation with 143Nd/144Nd, and a positive correlation with K, Ba, and Rb as well as with Ti, Th, Ta, Nb and Zr abundances. The primitive nature of
the lamprophyres, coupled with their enriched incompatible trace element and isotopic signatures, suggest derivation from
a metasomatized upper mantle source. Linear arrays in isotope space and elemental data plots suggest mixing between two distinct
end-members in the Italian mantle; an enriched end-member that is isotopically similar to pelagic sediments, and a significantly
less enriched end-member that approaches Bulk Earth values. New isotopic data indicate that the mantle source(s) of the lamprophyres
from the Western Alps contain a very high proportion of the enriched end-member. The geochemical signature of the enriched
end-member is attributed to fluids or melts derived from pelagic sediments subducted during the closure of the Tethyan Ocean
in the late Cretaceous to early Tertiary. |
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| Keywords: | Italy Subduction Metasomatism Geodynamics Lamprophyre |
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