HR: 0800h
AN: V31B-0494    [Abstracts]
TI: Stratigraphy and Petrology of the Grande Soufriere Hills Volcano, Dominica, Lesser Antilles
AU: * Daly, G
EM: gedaly@prodigy.net
AF: California State University, Department of Geological Sciences, San Bernardino, CA 92407, United States
AU: Smith, A L
EM: alsmith@csusb.edu
AF: California State University, Department of Geological Sciences, San Bernardino, CA 92407, United States
AU: Garcia, R
EM: rebeccga@nmsu.edu
AF: New Mexico State University, Department of Geological Sciences, Las Cruces, NM 88003, United States
AU: Killingsworth, N
EM: nlkillingswo@ualr.edu
AF: University of Arkansas Little Rock, Department of Earth Sciences, Little Rock, AR 72204, United States
AB: The Grande Soufriere Hills volcanic center is located on the south east coast of the island of Dominica in the Lesser Antilles. Although the volcano is deeply dissected, a distinct circular crater that opens to the east can be observed. Within the crater is a lava dome and unconsolidated pyroclastic deposits mantle the southeast flanks of the volcano. These pyroclastic deposits are almost entirely matrix-supported block and ash flows and surges suggesting that Pelean-style eruptions have dominated its most recent activity. Within this sequence is a relatively thin (30-50 cm) clast-supported deposit that has been interpreted as a possible blast deposit. Two age dates from these younger deposits suggest that much of this activity occurred between l0,000 and 12,000 years ago. On the southeastern coast at Pointe Mulātre and extending approximately 4 km north and at a maximum 2 km west, is a megabreccia of large (up to 3 m) flow-banded andesite clasts set in a semi-lithified medium grained ash matrix. At Pointe Mulātre this megabreccia is overlain by unconsolidated block and ash flow deposits. To the north of the megabreccia, exposures in the sea cliffs reveal a consolidated sequence of well-bedded alternating coarse and fine deposits suggesting deltaic foreset beds; which in turn appears to be overlain by a yellow- colored relatively coarse flow deposit with an irregular upper surface. The uppermost deposits in the sea cliffs are a sequence of unconsolidated block and ash flow deposits and interbedded fluviatile conglomerates equivalent to the younger flow deposits logged inland. Volcanic rocks from the Grande Soufriere Hills are all porphyritic andesites often containing hypabyssal inclusions. Dominant phenocrysts are plagioclase often with inclusion-rich cores and well developed zoning. Mafic phenocrysts include hornblende, augite and hypersthene. Geochemically these andesites range from 58- 63% SiO2 and show trends of decreasing values for Al2O3, FeO, MgO, CaO, TiO2, Sr, V, and Sc and increasing values for Na2O, K2O, Ba, Rb, and Zr with increasing silica. Samples from the megabreccia can be chemically distinguished from the younger rocks of this center. Petrologic models suggest that the younger rocks from the Grand Soufriere Hills can be produced by fractional crystallization of basaltic magma such as those erupted from other centers (such as Morne Anglais to the west). Minor variations within this suite of andesites can be related to upper crustal fractionation of phenocryst phases.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3640 Igneous petrology
DE: 3690 Field relationships (1090, 8486)
DE: 8404 Volcanoclastic deposits
DE: 8428 Explosive volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting