The petrology of lavas and associated plutonic inclusions of Ascension Island

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Authors:Harris, Chris
Author Affiliations:Primary:
Oxford Univ., Dep. Geol. and Mineral., Oxford OX1 3PR, United Kingdom
Volume Title:Journal of Petrology
Source:Journal of Petrology, 24(4), p.424-470. Publisher: Clarendon Press, Oxford, United Kingdom. ISSN: 0022-3530
Publication Date:1983
Note:In English. 2 p. refs.; illus. incl. 96 anal., 12 tables, sketch maps
Summary:These relatively young lavas and pyroclastic rocks contain a suite of plutonic inclusions ranging in composition from olivine gabbro to peralkaline granite, whereas the lavas range from alkali basalt through hawaiite, trachybasalt, trachyandesite and trachyte to comendite. Basalt sensu stricto is relatively rare and there is a scarcity of analyses with 57 < SiO2 < 63; no probable mantle fragments have been found. One suite of gabbros sometimes contains cumulus and intercumulus orthopyroxene, absent from the lavas and other plutonic inclusions; these gabbros also have differing Pb-isotopic compositions, suggesting derivation from a different mantle source. Major- and trace-element variations within the lavas suggest that crystal fractionation at low P is the prime differentiation process and the presence of cumulate blocks implies that crystal settling occurred. Partial melting is seen in some plutonic inclusions but trace-element evidence suggests that the acid lavas and plutonic inclusions did not evolve via partial melting of plutonic intermediate material. Representative probe analyses are reported for feldspars, clinopyroxenes, olivines, sodic pyroxenes, amphiboles, ilmenites and magnetites. Mixing models for the major elements suggest that feldspar was the principal fractionating phase, along with olivine, clinopyroxene and ilmenite and/or magnetite in proportions in general agreement with the abundance of phenocrysts in the lavas and the modal mineralogies of the plutonic inclusions. Coexisting magnetite and ilmenite in the gabbroic inclusions and the assemblage aegirine + arfvedsonite + or - aenigmatite + or - fayalite in the granites suggest that both crystallized under moderately reducing conditions, slightly to the HM side of the FMQ reaction curve. [R.A.H.]
Subjects:Basaltic composition; Cumulates; Differentiation; Fractional crystallization; Gabbroic composition; Granitic composition; Igneous rocks; Inclusions; Lava; Magmas; Major elements; Mantle; Melting; Minor elements; Petrology; Phase equilibria; Trachytes; Volcanic rocks; Xenoliths; Ascension Island; Atlantic Ocean Islands
Abstract Numbers:84M/2060
Record ID:1986006241
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom, Reference includes data from PASCAL, Institute de l'Information Scientifique et Technique
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