Mineralogy of the ejected plutonic blocks of the soufriere volcano St. Vincent: Olivine,pyroxene, amphibole and magnetite paragenesis |
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| Authors: | John F Lewis |
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| Institution: | 1. Department of Geology, Oxford University, UK
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| Abstract: | Ejected plutonic blocks from the Soufrière volcano, St. Vincent, consist of the mineral phases plagioclase (An96?An89, average An93), olivine (Fo79?Fo67, most frequent interval Fo74?72), salite containing 5–6 percent Al2O3, hastingsitic amphibole and magnetite containing 6% Al2O3, 4% MgO and 7% TiO2. Aluminous salitic pyroxenes are not confined to alkaline suites but also occur as high temperature phases in certain calc-alkaline suites. Similarly low silica amphiboles with comparatively high Fe+3+Ti contents are unusual in common sub-alkaline rocks. Magnetite in the St. Vincent blocks is an example of a single homogeneous iron oxide phase precipitated in equilibrium with the other four phases in the blocks. Conditions of crystallisation are just as important as silica activity in determining the compositions of phases separating from basaltic magmas at relatively shallow depths (P tot< 8 kb). It is argued mainly on crystal chemical grounds that the mineral assemblage in the St. Vincent blocks can crystalline from a sub-alkaline magma under conditions of relatively high temperature, high water vapor pressure, and oxygen fugacity. |
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