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Soufriere of St. Vincent is the most active subaerial volcano in the Lesser Antilles arc, and is composed of basalts and basaltic andesites. Eruptive style has tended to alternate between predominantly effusive and explosive, although magma compositions show no systematic variations with time. New Ar-Ar and 14C dates help to constrain the geological evolution of the (- 0.6 Ma) volcano. Parental, possibly primary, magmas at Soufriere had MgO contents exceeding 12 wt.% (mg# 75) and were probably nepheline-normative. They may be representative of the parental magmas of the calc-alkaline suites of the Lesser Antilles arc. The source mantle probably resembled that of N-MORB, prior to metasomatic enrichment by hydrous slab-derived fluids, containing contributions from subducted sediments and oceanic crust. Parental magmas last equilibrated with the mantle at - 17 kbar pressure, with temperatures greater than 1130·C and f02 exceeding FMQ +1. Near-primary basaltic lavas were only erupted during the earliest (Pre-Somma) phase of volcanism. Basaltic andesites (and occasional andesites) were produced by fractionation of 01 + spinel + cpx + plag ± opx over a range of crustal pressures (5-10kbar), at temperatures mainly in the range 1000 - 1l00·C. The total amount of crystallization was some 76 wt. %, and amphibole was apparently not a fractionating phase. There is conflicting evidence as to the pre-eruptive water contents of Soufriere magmas; phenocryst compositions suggest H20 > 3 wt. %, whereas various projections into phase diagrams are more consistent with relatively anhydrous magmas. Magma mixing did not apparently play a significant role in the evolution of most Soufriere magmas, and fractional crystallization was not accompanied by crustal assimilation, judging from trace element and Sr-Nd isotope systematics. New U-Th mineral isochrons suggest that magmas resided for tens of thousands of years within the crust, which requires rather stable thermodynamic conditions in the magma chamber(s) beneath Soufriere.

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