Title:
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Genesis and evolution of calc-alkaline magmas at Soufriere volcano, St Vincent, Lesser Antilles arc.
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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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