Abstract

An extensive suite of minor intrusions, contemporaneous with the late-stage differentiated lavas of Piton des Neiges volcano, occurs within an old agglomeratic complex. An 8-m sill within this suite is strongly differentiated with a basaltic centre (Thorton Tuttle Index 27.6), residual veins of benmoreite (T. T. Index 75.2), and still more extreme veinlets of quartz trachyte. Although gravitative settling of olivine, augite and ore in situ is believed to be responsible for some of the observed variation, the over-all composition of the sill is considerably more basic than the mugearite which forms the chilled contacts. It is therefore concluded that considerable magmatic differentiation must have preceded the emplacement of the sill. This probably took place in a dyke-like magma body with a compositional gradient from mugearite at the top to olivine basalt in the lower parts.

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