Melatroctolite–anorthositic gabbro complex, Cumberland, Rhode Island: Petrology, origin, and regional setting

An igneous complex exposed in Cumberland, Rhode Island, is one of a series of mafic intrusions that outline a northeast-trending lineament extending from the Preston Gabbro in Connecticut to the Boston, Massachusetts, area. The age of these intrusions is not well constrained but appears to be between 620 and 370 m.y. B.P. The mafic complex in northern Rhode Island consists of a small body of melatroctolite adjacent to a much larger body (1 km2) of anorthositic gabbro. The contact between the two rock types is not exposed, but inclusions of anorthositic gabbro and large cumulophyric crystal aggregates identical to the gabbro occur in the melatroctolite. The latter rock is composed of ∼49% olivine (Fo63), 32% titaniferous magnetite, 15% plagioclase (An59 Ab40 Or1) and minor ilmenite and aluminum spinel. The plagioclase in the melatroctolite occurs as tabular cumulophyric crystal aggregates (0.2 cm × 0.5 cm) in a matrix of, and often including, finer-grained (<3 mm) olivine and titaniferous magnetite. The tabular plagioclase aggregates are oriented to give the rock a weak foliation. The gabbroic rocks are composed of a framework of thin, tabular (1.0 cm × 0.2 cm) cumulus plagioclase identical in composition to that in the melatroctolite. Interstitial volumes in the gabbro are filled by Ca-pyroxene, zoned overgrowths on the plagioclase, and smaller amounts of olivine (Fo57−52), magnetite, ilmenite, and apatite. The prominent igneous lamination and parallel but weak rhythmic layering in the gabbro are not parallel to the lamination in the melatroctolite, but the rocks have been subjected to one and possibly two episodes of deformation. Faulting and rotation accompanying these deformations may explain the fact that the two foliations are not parallel, or the relatively weak lamination in the melatroctolite may have formed during postcumulus flow. Compositionally, the gabbroic rocks range from a high alumina gabbro (19.5% A12O3) to anorthositic gabbro (25% A12O3). A porphyritic chilled-margin locality on the northern edge of the gabbro contains plagioclase phenocrysts (An61), as much as 8 cm across and oriented parallel to the contact, in a finer-grained (0.5 mm) groundmass of plagioclase and altered ferromagnesiaia minerals. The mineralogy, whole-rock chemistry, cumulus textures, and igneous laminations indicate that the melatroctolite and anorthositic gabbro are genetically related igneous rocks. This evidence, together with the presence of the gabbroic inclusions in the melatroctolite, indicates that the two rocks formed simultaneously by gravitational settling of olivine, titaniferous magnetite, and cumulophyric plagioclase + olivine aggregates, whereas discrete plagioclase crystals did not sink. The fine-grained ground-mass of the marginal gabbro sample is thought to represent the parent magma, on the basis of the composition (An61) and planar orientation of the plagioclase phenocrysts. The calculated density of the proposed parent magma is 2.66 g/cc (at 1250 °C and 1 atm), which is between that of plagioclase (2.63) and the plagioclase + olivine aggregates (2.68), as would be required. The crystal settling occurred slowly enough to permit unzoned adcumulus overgrowths to completely fill the interstices in the melatroctolite. According to magnetite-ilmenite pairs in the melatroctolite and gabbro, the final equilibration between these two minerals took place at 950 °C ± 100 °C and a fO2 of 10−12.5 ± 1.0.