A mega-porphyritic dolerite on Long Island, Seychelles Archipelago

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Abstract
SUMMARY A dolerite intrusion on Long Island, Seychelles contains exceptionally large plagioclase (labradorite) phenocrysts essentially unzoned, set in a doleritic groundmass. Many of the mega-phenocrysts have a dark redbrown interior. In the groundmass, zoned plagioclase is in a doleritic texture with a slightly titaniferous ferro-augite that has been almost completely replaced by an actinolite uralite. The mega-phenocrysts are considered to have crystallized slowly at depth and concentrated locally in a chamber of basaltic magma. The portion of magma containing these plagioclase crystals was later injected to higher levels as dykes. Although emplaced in a granitic (continental) environment, the intrusion has oceanic basalt affinities, and in chemical composition resembles high alumina basalts.

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