Petrology and Mineral Chemistry of Plutonic Igneous Xenoliths from the Carbonatite Volcano, Oldoinyo Lengai, Tanzania |
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| Authors: | DAWSON J B; SMITH J V; STEELE I M |
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| Institution: | 1DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF EDINBURGH WEST MAINS ROAD, EDINBURGH EH9 3JW, UK
2DEPARTMENT OF THE GEOPHYSICAL SCIENCES, UNIVERSITY OF CHICAGO CHICAGO, IL 60637, USA |
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| Abstract: | Magmatic plutonic rocks in the Oldoinyo Lengai pyroclasticsare jacupirangite, pyroxenite, ijolite, nepheline syenite andwollastonitite. Mainly cumulates, they are combinations of nepheline,clinopyroxene, Ti-andradite, spinel (sensu lato), apatite, perovskite,titanite, wollastonite, sulphides, mica, glass and K-rich feldspar,most of which are strongly zoned. Low analytical sums for glasses,vesiculation of intergranular glasses, and the generally explosivenature of the volcanicity point to significant concentrationof dissolved volatiles in the parent magma; the absence of hydrousphases suggests that the dominant volatile is CO2. Cumulatetextures, widely variable modes, veining and variation in specimenconsolidation and metasomatism all indicate derivation froma structurally complicated and multiply injected sub-volcaniccomplex. Complex zoning of phases and mineral disequilibrium is attributedto convective percolation of fluids through permeable cumulates,possibly complicated by magma replenishment during crystallizationof individual magma batches. Olivine, mica and pyroxene megacrystsin some ijolites indicate polybaric crystallization; mixingof potassic and sodic magmas may be the cause of these megacryst-bearingijolites, but the main parent is highly evolved, of carbonatedijolitic (nephelinitic) composition and with Nd and Sr isotopecharacteristics slightly more depleted than Bulk Earth. KEY WORDS: xenoliths; ijolite; jacupirangite; nepheline syenite
*Corresponding author. Telephone 031 650 4837. Fax: 031 668 3184. e-mail: jbdawson{at}glg.ed.ac.uk |
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