The Older Series volcanics of Mauritius form a widely differentiated transitional basalt suite, in which two distinct stages of activity can be recognized. The first stage, shield-building, is composed principally of alternating picrite-basalt flows and agglomerate; this is followed by a second, evolved stage composed of feldsparphyric basalt, hawaiite, mugearite, and high-level trachytic intrusive rocks.

Three suites of nodules, each with specific associations, occur in the volcanic rocks; these are probably derived from layered subvolcanic cumulates. The nodular suites are: (a) dunite and wehrlite (restricted to the picrite basalt); (b) Bytownitic anorthosite (found only in the feldsparphyric basalt); and (c) mafic syenite (exclusive to the trachytic intrusive rocks). There is some evidence, from the extremely calcic nature of the plagioclase in the anorthosite nodules and feldsparphyric basalt and from the presence of kaersutite in evolved lava, that a hydrous period of crystallization developed - probably in the later stages of activity.

Variation diagrams indicate clearly the close control of phenocryst mineralogy on bulk-lava chemistry, compositions with more than 5 or 6 percent MgO lying along a pronounced olivine -+- clinopyroxene control line and more evolved lava along a trend developed from fractionation of olivine + clinopyroxene + plagioclase + titanomagnetite. Trachytic compositions show pronounced trace-element trends, probably controlled by anorthoclase fractionation, but it can be demonstrated that they are unlikely to represent successive differentiates from a common trachytic magma.

A distinct “silica gap” within the series, reflected in the absence of benmoreite, is tentatively ascribed to secondary boiling phenomena.

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