Volcanic stratigraphy and geochemistry of the Soufrière Volcanic Centre, Saint Lucia with implications for volcanic hazards
Introduction
Eruptions of silicic magma have produced some of the largest and most explosive volcanic events in Earth's history. Although relatively infrequent compared to more mafic eruptions, large silicic eruptions have the potential to produce severe socio-economic impacts on both regional and global scales. Most of the recent work on understanding evolution and hazards of large silicic magma bodies has focussed on areas where this magmatism is most common: continental arcs such as the Taupo Volcanic Zone (e.g. Wilson et al., 2009) or long-lived intra-continental settings such as Long Valley and Yellowstone (e.g. Christiansen, 2001, Reid, 2008). Less attention has been given to silicic magmas in island arc settings, although recent work on the Izu–Bonin, Scotia, Aegean and Tonga–Kermadec arcs (e.g. Tamura and Tatsumi, 2002, Leat et al., 2003, Mortazavi and Sparks, 2004, Smith et al., 2006, Bachmann et al., 2012) has shown that silicic magmatism can be a significant component of island arc systems.
The island of Saint Lucia in the Lesser Antilles provides an opportunity to study the eruptive history and potential hazards of silicic systems in an archetypal, well-established island arc setting. Saint Lucia and its neighbour Dominica have erupted dominantly medium-K calc-alkaline andesite and dacite, in contrast to islands to the north and south in the arc, which have produced dominantly more mafic magmas (Macdonald et al., 2000). The largest eruptions of such magmas in Saint Lucia are represented by a series of extensive pyroclastic deposits considered by previous workers to have erupted between 40 and 20 ka ago (Wright et al., 1984, Wohletz et al., 1986). This silicic pyroclastic activity is not typical for the Lesser Antilles arc, the only comparable deposits being the Pleistocene–Holocene Roseau tuff, Grand Savanne ignimbrite, Layou Tuff and Grand Bay ignimbrite and associated domes on Dominica (Sparks et al., 1980a, Sparks et al., 1980b, Lindsay et al., 2003, Lindsay et al., 2005a, Tinnin et al., 2006). These deposits in Saint Lucia and Dominica, which represent the largest known explosive eruptions in the Lesser Antilles arc, can be considered a flare up of silicic activity in an otherwise dominantly basaltic-andesite to andesitic portion of the arc. Interestingly, several of these silicic deposits are associated temporally and geographically with major sector collapse and debris avalanche events, yet the relationship between the collapses and the eruptions is unclear (e.g. Lindsay et al., 2005a, Lindsay et al., 2005b).
The pyroclastic deposits on Saint Lucia, together with associated lava flows, domes, block-and-ash-flow deposits and explosion craters, form the Soufrière Volcanic Centre (SVC) in the southwest of the island (Fig. 1). The SVC is located within a major collapse feature called the Qualibou depression, and is associated with vigorous geothermal activity at the Sulphur Springs geothermal field (Fig. 1). There has been no historical magmatic activity in Saint Lucia, however there was a series of phreatic eruptions in the SVC in 1766 (Lindsay, 2005). Because of its vigorous geothermal activity, recent intense seismic activity (in 2000–2001) and potential for violent explosive eruptions, the SVC is the focus of volcanic monitoring and hazard assessment in Saint Lucia by researchers at the Seismic Research Centre, University of the West Indies, Trinidad (Lindsay et al., 2002, Lindsay et al., 2005b, Joseph et al., 2013).
Previous workers developed contrasting theories for the source and significance of the pyroclastic units in southern Saint Lucia. Wright et al. (1984) proposed that they were derived from andesitic stratovolcanoes in the central highlands, while Wohletz et al. (1986) concluded that they were sourced from within the Qualibou depression during caldera collapse. This distinction has important implications for the assessment of volcanic hazards related to the Qualibou depression, and has motivated the renewed study of the volcanism reported here. One major difficulty in understanding the volcanic history of this region has been the lack of reliable correlation criteria from the various widespread pyroclastic units. Recently, Schmitt et al. (2010) used combined (U–Th)/He and U–Th zircon geochronology to establish a new chronostratigraphic framework for the SVC. In our study, we refine the stratigraphic relations within the SVC in light of this and other new data, and by applying new petrographic and geochemical analysis from a suite of lava flows, lava domes, and pyroclastic flow deposits that include the stratigraphically oldest and youngest units of the SVC. The aim is to reconstruct the volcanologic history of the SVC, in particular that of the silicic pyroclastic units associated with the Qualibou depression, and to discuss the implications for volcanic hazards.
Section snippets
Regional setting
Currently about one million people in the Caribbean region are threatened by the direct effects of volcanic eruptions. The ongoing Soufrière Hills eruption on Montserrat has had a major impact on that island's population since it began in 1995 (Sparks and Young, 2002), and is a constant reminder of the potential for volcanic activity in other islands. The island of Saint Lucia (620 km2 area) has a population of 163,267 (2001 census), all of whom would be affected should there be renewed volcanic
Geology of Saint Lucia
Saint Lucia consists almost entirely of volcanic rocks (Fig. 1), which can be categorised into three main groups based on age and distribution (Lindsay, 2005). From older to younger these are: (1) eroded basalt and andesite centres, (2) dissected andesite centres, and (3) the Soufrière Volcanic Centre. The oldest radiometric ages obtained from basalts in Group 1 are 15–18 Ma (Briden et al., 1979, Le-Guen de Kerneizon et al., 1983), although shallow marine volcaniclastic deposits in the northern
Stratigraphy and dating of the Soufrière Volcanic Centre
The dense vegetation, extreme weathering of outcrops, and high degree of leaching of organic material that could be used for dating have made it difficult to constrain the volcanic stratigraphy of the SVC. Early studies of the units associated with the SVC (Tomblin, 1964, Wright et al., 1984, Wohletz et al., 1986) lead to conflicting interpretations. More recent dating studies (e.g. Samper et al., 2008, Schmitt et al., 2010) have revised older chronostratigraphy for several volcanic features
Sampling and analytical methods
For this study we sampled all units associated with the SVC, and analysed a total of 58 pumice and lava samples for whole-rock geochemistry, which represent pyroclastic deposits (36 samples) as well as dacitic domes and associated lava flows (13 samples). The dissected andesite centres of the central highlands are the source of the numerous andesitic volcaniclastic units in and around the SVC. These centres are densely forested and for the most part inaccessible; our coverage is limited to a
Petrography and mineral chemistry
There is significant variation in textures and modal abundances among the various pumiceous pyroclastic units in the SVC, particularly between the Belfond pyroclastic flow deposit and units that were previously referred to collectively as the Choiseul tuff (e.g. Wright et al., 1984). Schmitt et al. (2010) showed that the quartz-poor pumice deposits collectively mapped as the ca. 40 ka old Choiseul tuff include products of at least three other eruptions at ca. 264 ka (Bellevue), 104 ka (Anse John)
Geochronology
In order to close key gaps in the chronology of the Soufrière Volcanic Complex we carried out age determinations on (1) pumice from Micoud pyroclastic flow deposit to date deposits previously mapped as Choiseul tuff on the east coast of Saint Lucia; (2) pumice from the Choiseul type-location to test the previously determined 14C age of 40 ka; and (3) lava from near the summit of the pre-Qualibou-collapse Morne Tabac, in order to further constrain the age of the collapse (Fig. 1).
The new
Overview
Mineral assemblage and compositional data were combined with age data to identify key silicic units and their correlations within the SVC (Table 2) as a basis for grouping samples in the geochemical descriptions given here. This was done according to the following scheme. Firstly, units for which a unique age is known are identified in all plots. This includes the Belfond, La Pointe, Anse John, Bellevue and Micoud pyroclastic deposits and the Terre Blanche and Belfond domes. Secondly,
Re-definition of pyroclastic units and relationship to domes
The geochronological, mineralogical and geochemical constraints discussed here permit a revised definition of the SVC pyroclastic flow deposits. The Belfond unit is confirmed as a large, valley-filling pumiceous pyroclastic flow deposit; with deposit thicknesses ranging from 1 to over 100 m (it occurs in the Piaye river valley where thick inaccessible cliffs visible from the Londonderry village appear to be > 100 m high). It is distributed to the north, northeast, south and southeast of the
Conclusions
The geological history of the Soufrière Volcanic Complex (SVC) in Saint Lucia, has been reconstructed in light of new and recently published (U–Th)/He, U–Th and U–Pb zircon chronostratigraphic data, aided by mineralogical and geochemical correlations. The main conclusions of this work are:
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Lava and pumice from domes and pyroclastic flows of the SVC are monotonous medium-K, calc-alkaline andesites and dacites with 61.6 to 67.7 wt.% SiO2, and with similar trace element abundances.
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Combined U–Th
Acknowledgements
Rudolf Naumann and Daniel Goode are thanked for their assistance in XRF analyses at the GFZ-Potsdam, Ian Smith for LA-ICP-MS analytical work at the Research School of Earth Science, Australian National University, and Roman Kislitsyn for the assistance with the (U–Th)/He dating. We thank Daniel Goode, Richard Robertson, and Roman Kislitsyn for their assistance with field work. This work was supported by the National Geographic Society grant 8517-08. The ion microprobe facility at UCLA is partly
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