Late quaternary explosive silicic volcanism on St Lucia, West Indies

Abstract

AbstractMany explosive eruptions of dacitic magmas have occurred on St Lucia during the late Quaternary. These have produced widespread aprons and fans of pumice flow and ash flow deposits radiating around the central highlands, with co-eruptive air-fall and surge layers interbedded with palaeosols and epiclastic deposits. Vents in the highlands have not been located because of the dense tropical jungle but we suspect they are now plugged by lava domes surrounded by aprons of block and ash flow deposits. Young magmatically related dacitic lava domes have been extruded in the Qualibou depression. The pumice succession can be divided into older quartz-poor deposits forming the Choiseul Pumice and younger crystal-rich deposits with abundant large quartz which are called the Belfond Pumice. The Choiseul Pumice groups together scattered remnants of the products of many eruptions of different low-silica dacitic magma types. The Belfond Pumice is the product of several eruptions of a high-silica magma type and 14C ages have dated these between 20900 to 34200 years B.P.The pumice flow deposits occur as small-volume valley fills. A granulometric study of Belfond pumice flow deposits shows them to be strongly depleted in finer ash and vitric components. It is suggested that the narrow, winding and vegetated valleys on the island locally induced turbulence and the flows moved with large, highly fluidized and inflated heads, resulting in substantial loss of fine vitric ash. One ash flow deposit which is extremely rich in crystals and carbonized vegetation is highly depleted in fines and shows enhanced vitric losses. This flow may have been a much more violent ash hurricane or blast which surmounted topography ingesting large amounts of lush vegetation. Ignition of this released the large quantities of gas needed to elutriate most of the fines.A model is suggested for the recent volcanic activity on St Lucia in which separate batches of silicic magma, each having a distinctive petrological and chemical character, rose into high level chambers over a large area. Eruptions of volatile-rich magma led to highly explosive pumice-forming activity from vents in the central highlands. Degassed and more crystal-rich magma was extruded later from the same vents or in the attenuated flank of the Qualibou depression to from lava dome complexes.