The geochemistry and petrogenesis of the volcanic rocks of Carriacou,Grenadine Islands,West Indies |
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| Authors: | G Caldwell T E Smith C H Uang T A Jackson |
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| Institution: | (1) Department of Geology, University of Windsor, N9B 3P4 Windsor, Ontario, Canada;(2) Department of Geology, University of West Indies, Mona, Kingston 7, Jamaica |
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| Abstract: | Carriacou is small volcanic island located near the southern end of the Lesser Antillean chain. Field relationships, petrography and geochemistry of the Tertiary lavas, outcropping in the southern half of the island, are used to identify the rocks present and to determine their petrogenesis and assess their significance within the island arc.Six main volcanic units are present. From oldest to youngest, these are the clinopyroxene-phyric basalt (CPB) sequence, the amphibole-phyric andesite (APA) sequence, the clinopyroxene-megaphyric basalt (CMB) sequence, the olivine-microphyric basalt (OMB) sequence, the clinopyroxene phyric andesite (CPA) sequence, and the amphibole-megaphyric andesite (AMA) sequence. Volcaniclastic deposits are associated with the APA, CMB, and AMA sequences. The APA sequence is calcalkaline, whereas the other five sequences are tholeiitic.Sr isotope and rare earth element (REE) data suggest that these volcanic rocks were derived from partial melts of garnet-peridotite generated deep within the mantle. The OMB lavas have the highest temperature assemblages of intratelluric minerals and the least evolved chemical characteristics, and are considered to be closest in composition to a parental melt. Phenocryst assemblages and chemical variation suggest that the andesite sequences have been derived from the mafic melts by low pressure fractional crystallization of approximately 20% clinopyroxene and 20% olivine, plus smaller amounts of plagioclase and amphibole. The high concentrations of incompatible and compatible elements and the high87Sr/86Sr ratios may indicate that subduction is slower in the southern part of the arc, and fluids released during slab dehydration rich in Incompatible trace elements, in Radiogenic strontium, and in Silica (IRS), have modified the parental melts. |
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