Abstract

A sequence of five fractionated, subvolcanic, pyroxenite–gabbro sills of tholeiitic basalt initial composition was intruded into the Archean Favourable Lake metavolcanic–metasedimentary sequence. Individual sills are up to 2.7 km long, range in thickness from 17 to 240 m, and are confined to a relatively thin lacustrine metachert and fine clastic unit. The sills were emplaced sequentially upward into water-rich, unlithified country rocks, with succeeding sills being emplaced prior to final consolidation of underlying sills. Sills are either in contact or separated by thin country-rock septa, but later sills did not intrude earlier sills.Greenschist facies metamorphism and pre-metamorphism alteration have destroyed primary mineralogy and have modified chemical variations, but primary textures are well preserved by pseudomorphs. Each sill comprises a lower ultramafic zone of clinopyroxene–olivine and clinopyroxene–orthopyroxene cumulates and an upper mafic zone of clinopyroxene–plagioclase – iron–titanium oxide and plagioclase – iron–titanium oxide cumulates. The upper part of the mafic zone is largely cumulus plagioclase that either accumulated in situ or floated upwards. The lower part of the ultramafic zone is relatively constant in composition and crystallization appears to have been largely in a closed system with only local magma flowage. In the mafic zone, crystallization was apparently in a more open system with surges of magma related to volcanic processes being derived either from elsewhere in the sills or externally. The most fractionated component is granophyric gabbro that is locally concentrated into large intrusive lenses.

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