Volcanic stratigraphy and geochemistry of the Soufrière Volcanic Centre, Saint Lucia with implications for volcanic hazards

• We revise the stratigraphy of the Soufrière Volcanic Complex in Saint Lucia.
• Combined (U–Th)/He and U–Th zircon geochronology provides new ages.
• Pyroclastic deposits range in age from 640 to 20 ka.
• Future activity is likely to occur from within the Qualibou depression.

The Soufrière Volcanic Complex (SVC), Saint Lucia, represents one of the largest silicic centres in the Lesser Antilles arc. It comprises extensive pumiceous pyroclastic flow deposits, lava flows as well as Peléan-style domes and dome collapse block-and-ash-flow deposits. These deposits occur within and around the Qualibou Depression, a ~ 10-km diameter wide sector collapse structure. To date, vent locations for SVC pyroclastic deposits and their relationship to the sector collapse have been unclear because of limited stratigraphic correlation and few radiometric ages. In this study we reconstruct the geologic history of the SVC in light of new and recently published (U–Th)/He, U–Th and U–Pb zircon chronostratigraphic data, aided by mineralogical and geochemical correlation. Compositionally, SVC deposits are monotonous medium-K, calc-alkaline rocks with 61.6 to 67.7 wt.% SiO2 and display similar trace element abundances. Combined U–Th and (U–Th)/He zircon dating together with 14C ages and mineral fingerprinting reveals significant explosive eruptions at 640, 515, 265, 104, 60 and 40 ka (producing deposits previously grouped together as the “Choiseul” unit) and at 20 ka (Belfond unit). The mineralogically and geochemically distinct Belfond unit is a large, valley-filling pumiceous pyroclastic flow deposit distributed to the north, northeast, south and southeast of the Qualibou Depression that was probably deposited during a single plinian eruption. The unit previously referred to as ‘Choiseul tuff’ is much less well defined. The typical Choiseul unit comprises a series of yellowish-white, crystal-poor, non-welded pumiceous pyroclastic deposits cropping out to the north and southeast of the Qualibou depression; however its age is poorly constrained. A number of other units previously mapped as Choiseul can be distinguished based on age, and in some cases mineral and whole rock chemistry. Pyroclastic deposits at Micoud (640 ± 19 ka), Bellevue (264 ± 8 ka), Anse John (104 ± 4 ka) and La Pointe (59.8 ± 2.1 ka), Anse Noir and Piaye were all previously grouped with or associated with the Choiseul tuff (all uncertainties 1σ). We suggest that these units represent individual periods of activity spanning a range of ages, whereas Choiseul pumice at the type locality has yielded a (U–Th)/He zircon age of 515 ± 19 ka. Their overall geochemical and mineralogical similarities with the Choiseul at the type locality suggest that they might have all originated from the same centre. Morne Tabac (532 ± 21 ka) is a dome truncated by the depression escarpment, whereas Morne Bonin (273 ± 15 ka), Gros Piton and Petit Piton (71 ± 3 ka and 109 ± 4 ka, resp.), Belfond (13.6 ± 0.4 ka) and Terre Blanche (15.3 ± 0.4 ka) are domes within the Qualibou Depression. Belfond and Terre Blanche have whole rock geochemistry and mineral assemblages similar to the Belfond pyroclastic flow deposit, thus possibly representing late-erupted degassed portions of the magma that produced the Belfond pyroclastics. The geochemical characteristics and similar zircon age distributions of the silicic lava domes and pyroclastics of the SVC suggest that these share a common magma source beneath the Qualibou depression. The distribution of the pyroclastic flows and the wide range in their eruption ages makes it unlikely that these were erupted during caldera-forming activity, and we instead invoke a series of smaller-volume explosive eruptions from the area of the current depression, the earliest of which occurred from a large proto-Qualibou edifice that subsequently underwent sector collapse. Activity from this proto-Qualibou centre may have ceased sometime between 38 and 59 ka ago, it therefore seems unlikely given our present understanding that there will be another eruption from the southern central highland region. However, the young dome-forming activity in the Qualibou depression may have occurred in or close to the Holocene, and there have been dome collapse events and explosion craters formed since then. A new dome eruption or renewed activity at a dome within the depression, growing in the style of the ongoing Soufrière Hills lava dome on the nearby island of Montserrat, is possible; as is a future plinian eruption from this area. Such an eruption would not only have a devastating impact on Saint Lucia, but would also have significant regional and global impacts.