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Journal of Petrology | Volume 39 | Number 10 | Pages 1721-1764 | 1998
© Oxford University Press 1998

Magmagenesis at Soufriere Volcano, St Vincent, Lesser Antilles Arc

Emily Heath1, Ray Macdonald1,*, Harvey Belkin2, Chris Hawkesworth3 and Haraldur Sigurdsson4

1 Environmental Sciences Division, Iebs, Lancaster University Lancaster La1 4YQ, UK
2 Us Geological Survey Reston, Va 22092, USA
3 Department of Earth Sciences, the Open University Milton Keynes Mk7 6AA, UK
4 Graduate School of Oceanography, University of Rhode Island Narragansett, Ri 02882, USA

Received May 10, 1997; Revised typescript accepted April 16, 1998


  Abstract

Soufriere volcano of St Vincent (<0.6 Ma) is composed of basalts and basaltic andesites, the most mafic of which (mg-number 75) may be representative of the parental magmas of the calc-alkaline suites of the Lesser Antilles arc. Parental, possibly primary, magmas at Soufriere had MgO ~12.5 wt % and were probably nepheline-normative. They last equilibrated with mantle at ~17 kbar pressure, at temperatures of around 1130°C and f(O2) exceeding FMQ (fayalite–magnetite–quartz) + 1. They fractionated, along several liquid lines of descent, through to basaltic andesites and rarer andesites over a range of crustal pressures (5–10 kbar) and temperatures (1000–1100°C), separating initially olivine + Cr-spinel + clinopyroxene + plagioclase ± titanomagnetite and then clinopyroxene + plagioclase + titanomagnetite + orthopyroxene assemblages. The total amount of crystallization was some 76 wt %. Amphibole was apparently not a fractionating phase. Sr and Nd isotopic and trace element systematics show no evidence for significant crustal assimilation. There is conflicting evidence as to the pre-eruptive water contents of Soufriere magmas; compositions of clinopyroxene phenocrysts and melt inclusions suggest H2O >3 wt %, whereas various projections onto phase diagrams are more consistent with relatively anhydrous magmas. Primary magmas at Soufriere were generated by around 15% melting of mid-ocean ridge basalt type mantle sources which had been modified by addition of fluids released from the slab containing contributions from subducted sediments and mafic crust.

KEY WORDS: high-MgO arc magmas; geochemistry; magmagenesis; Lesser Antilles; Soufriere St Vincent

* Corresponding author. Telephone: 01524 593934. Fax: 01524 593985.


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