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TitleA re-evalutation of the origin and nature of layered peridotite, troctolite and gabbro in the Eastern Layered Series of the Rhum ultrabasic complex, Inner Hebrides
AuthorBédard, J H; Sparks, R S J; Renner, R; Hunter, R; Cheadle, M
SourceEarly Tertiary volcanism and the opening of the NE Atlantic; by Morton, A C (ed.); Parson, L M (ed.); Geological Society, Special Publication no. 39, 1988 p. 391,
Alt SeriesEarth Sciences Sector, Contribution Series 2005288
PublisherGeological Society of London
Mediapaper; on-line; digital
File formatpdf
AreaInner Hebrides; Isle of Rhum; Hallival; United Kingdom of Great Britain and Northern Ireland
Subjectsigneous and metamorphic petrology; igneous rocks; plutonic rocks; gabbros; peridotites; troctolites; lithology; layering; allivalites; host rocks; intrusions; stratiform intrusives; sills; ultramafic rocks; magma differentiation; crystal fractionation; cumulus processes; basalts; pyroxenization; textures; metasomatism; geological history; porosity; Eastern Layered Series; updoming; downwarping; plugs; cumulates; lamellae
Released1988 01 01
AbstractMapping of vertical and lateral lithologic variations in the Eastern Layered Series (ELS) on the northern flank of Hallival shows that both peridotite and allivalite (troctolite or gabbro) layers are laterally discontinuous and vary in thickness and lithology. This is particularly evident in some of the allivalites (e.g. Unit 10), where troctolite is replaced by gabbro along strike. Some troctolite layers terminate as isolated, fingered blocks in peridotite. Peridotite generally has sharp contacts against the allivalites, but reaction, dissolution and hybridization effects are developed locally. Peridotitic layers commonly transgress the allivalite layering at a shallow angle, but markedly discordant contacts are also observed. The smaller conformable peridotites cause updoming and downwarping of the host allivalite. Peridotite plugs that are clearly intrusive into allivalite are petrographically and geochemically very similar to the stratiform peridotites and a common origin is proposed. Our preferred interpretation is that most stratiform peridotites in the ELS represent sill-like intrusions of ultramafic magma into a partly solidified layered troctolite complex. However, the peridotites are cumulate rocks and significant fractions of residual basalt must be accounted for. Some of the residual basaltic melt appears to have percolated laterally (up-dip?) through stillporous troctolite and reacted to form gabbro (pyroxenization). The best evidence for pyroxenization of troctolite comes from the top of Unit 9 where the wavy (metre-scale) gabbro-troctolite contact cuts across pre-existing grain size, modal and rhythmic layering, but causes little disturbance to it. Gabbros formed by pyroxenization of troctolite mimic the textures, grain size and rhythmic layering of their troctolitic protoliths. Relict troctotite lamellae and fossil pyroxenization fronts are common. We propose that many of the ELS gabbros formed metasomatically as a result of interaction between the porous host troctolites and low-temperature basaltic melt. The ultimate origin of this basaltic melt is uncertain. It could be: (1) residua released from the peridotite sills; (2) residua from the chamber above that penentrated into high-porosity horizons in the cumulates; (3) related to the Askival Plateau or Atlantic Coire gabbros to the W and SW; (4) related to the basaltic replenishment events documented by Renner & Palacz (1987); or (5) represent residual melt extracted from compacting troctolitic cumulates and concentrated along high-porosity horizons.

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