Petrology and geochemistry of gabbroic and ultrabasic rocks from eastern Rhum

Abstract

This work is concerned with the lower part of the Eastern Layered Series (LELS) of Rhum, and with the marginal relationships of the ultrabasic complex in eastern Rhum. The Lower Eastern Layered Series comprises approximately Units 1-5 of previous workers. Remapping has revealed considerable along-strike lithological variation in the units of the L.E.L.S. It is suggested, on the basis of field and geochemical evidence, that two layers formerly regarded as 'conformable intrusive sheets of fine grained olivine gabbro'. may be evolved allivalite layers rather than later intrusions. Xenolith suites in these layers and elsewhere, indicate a component derived from the roof or walls of the magma chamber. Cryptic variation is more extensive in the L.E.L.S. than in other parts of Rhum: olivine forsterite content varies from 85.6 to 70, and clinopyroxene MgX100/(Mg+Fe) varies from 88 to 74. Post-cumulus effects and sub-solidus re-equilibration have altered the initial compositions of the mineral phases. The migration of interstitial liquids has had a major effect on mineral chemistry. Replacement of plagioclase-rich rocks by peridotite is a significant process in parts of the sequence. This is ascribed to disequilibrium between migrating pore liquids and plagioclase. The data are consistent with a model of repeated replenishment by picritic magma, although the replenishing liquids may have been slightly less magnesian than those subsequently available during the formation of the upper ELS .Re-examination of the eastern margin of the ultrabasic complex suggests that the ultrabasic rocks formed more or less in situ, and that fragments of the roof to the intrusion occur in places. Locally under these roof fragments variolitic olivine-rich gabbros are developed, which may represent the chilled margin to the ultrabasic complex.