La sizunite (cap Sizun, Finistere) et le probleme de l'origine des lamprophyres

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Abstract
Sizunite, a micaceous lamprophyre including small granitic xenoliths, is found in a NW-SE trending vein in granites of the Hercynian orogeny (western France). The xenoliths are of two types--numerous small globular forms containing biotite, and less abundant angular forms containing two micas. The sizunite is distinguished by its extreme richness in potassic elements and by the fact that microcline is the only feldspar present. Chemical analysis shows that this rock is the most potassic and phosphoric of the 382 lamprophyres known. Results of modal analyses and micrographic studies classify it in the alkalic syenite group. It is probably the product of selective assimilation of certain granitic elements by a deep-seated basic magma, related to a late orogenic phase. The magma contaminated itself while cutting across freshly folded and granitized, mesothermal sial.

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