Melatroctolite-anorthositic gabbro complex, Cumberland, Rhode Island: petrology, origin, and regional setting.

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The mafic complex outlining a NE-trending lineament in N Rhode Island consists of a small melatroctolite body adjacent to a much larger (1 km 2 ) anorthositic gabbro body. The melatroctolite is composed of approx 49% olivine (Fo 63 ), 32% Ti-magnetite, 15% plagioclase (An 59 Ab 40 Or 1 ) and minor ilmenite and Al-spinel. The gabbroic rocks are composed of thin, tabular cumulus plagioclase of identical composition to the above and an interstitial volume filled by Ca-pyroxene, zoned overgrowth on the plagioclase, much less olivine (Fo 57-52 ), magnetite, ilmenite and apatite. Various lines of evidence indicate that the two rocks are genetically related; both formed simultaneously by gravitational settling of olivine, Ti-magnetite, and cumulophyric plagioclase + olivine aggregates, whereas discrete plagioclase crystals did not sink. According to the magnetite-ilmenite pairs in both rocks, the final crystallization between these two minerals took place at 950 + or - 100oC and fO 2 of 10 -12.5(1.0) .-L.di H.

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Geological Society of America Bulletin