| Summary: | The Siluro-Devonian Fogo Island Batholith is a high-level, bimodal, sill-like unit about 7 km thick, intruded by a slightly younger, heterogeneous mafic unit. Stratigraphic evidence suggests emplacement of the batholith by raising of the roof without strong deformation of the host rocks. Large composite gabbro-microgranite dykes below the sill, intruded along axial planar cleavage, probably served as feeders. The upper 3– 4 km of the sill comprise homogeneous to slightly zoned, coarse, hastingsite-biotite granite. Ignimbrite sheets of similar composition above the sill suggest that the granite was emplaced beneath cover rocks no more than 1 km thick. The lower 3 km of the sill consist of heterogeneous, locally layered, mafic rocks. Metre-scale, non-intrusive sheets of contrasting composition mark the contact between the granitic upper and mafic lower part of the sill. A late influx of mafic magma disrupted older mafic rocks and hybridised with granite, producing complex, heterogeneous "diorite". The local presence of A-type compositions in the granitic portion of the batholith may reflect diffusion during this process. A model for emplacement of the batholith assumes crustal anatexis above mantle-derived, underplated mafic magma, followed by passive emplacement of composite magma along tensional fractures related to movements on the Dog Bay Line, a dextral terrane boundary. Later movements on this feature controlled emplacement of further batches of magma, and finally tipped the batholith about 25° to the north, producing its present configuration. RÉSUMÉ Le Batholithe siluro-dévonien de l'île Fogo forme une unité à l'aspect d'un filon-couche bimodale de niveau élevé et d'environ sept kilomètres d'épaisseur, dans laquelle fait intrusion une unité mafique hétérogène légèrement plus récente. Les observations stratigraphiques laissent supposer une mise en place du batholithe par soulèvement du toit sans déformation marquée des roches hôtes. Des dykes composites de gabbro-rhyolite de fortes dimensions au-dessous du filon-couche, ...
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