The volcanic history and petrology of the Soufrière region, St. Lucia

The Soufrière region, which includes the most recently active volcanic centre on the island of St. Lucia in the West Indies, is the site of a relatively recent volcanic caldera. This structure, with a diameter of approximately 4 miles, has an age of definitely more than 50,000 years and probably of several times this age. Apart from continuous solfataric activity, there have been no historic eruptions, and the morphology of the caldera has been considerably modified by erosion since the time of its formation. The history of volcanic events in the Soufrière region may be divided into four principal phases: the first involved the effusion of basalt lava from vents both within and to the east of the site of the ultimate caldera. During the second phase, following what was probably a long time-interval, a group of andesitic strato-volcanoes developed in an area slightly to the northeast of the ultimate caldera. These emitted andesite flows and, apparently, a very large volume (estimated a 6 cubic miles) of pyroclastic material, probably mainly as glowing avalanches. The third phase opened with the emission of andesite pumice fall and pumice flow from a vent which probably lay within the ultimate caldera. After an erosion interval, activity was renewed on a more violent scale, in a series of ultravulcanian explosions, and these were followed by a second period of voluminous, andesite pumice flows which immediately preceded caldera subsidence. The fourth and final phase has occupied the relatively long period since collapse, during which fifteen volcanic domes and seven craters have developed within the caldera: initially of andesitic composition, around the margin of the caldera, and subsequently of dacite lava, towards the centre of the collapsed area. The effusion of these lavas was accompanied by pyroclastic eruptions of pumice fall (with an estimated volume of 1.23 cu. miles) and pumice flows (with an estimated volume of 0.27 cu. mile). The youngest large pumice flow, dated by the radiocarbon method, occurred 39,050 years ago.
The mineralogy of the St. Lucian lavas compares closely with that of typical, calcium-rich, island-arc suites. All rocks contain calcic plagioclase phenocrysts, the cores of which generally consist of bytownite while the rims, especially in the andesites and dacites, include oscillatory zones in which the calcium content commonly decreases to a minimum of about An40. The mafic minerals in the basalts include augitic clinopyroxene, with or without orthopyroxene and olivine, whilst in the andesites, orthopyroxene (hypersthene or ferrohypersthene) predominates. The dacites are characterized by amphibole (common hornblende, oxyhornblende, or cummingtonite) and biotite, with subordinate orthopyroxene and occasional olivine phenocrysts, accompanied by 5–15% of large, rounded or bipyramidal phenocrysts of quartz. The youngest dacites of the Soufrière region are unusual in that they contain phenocrysts of calcium-poor, cummingtonitic amphibole.
Twenty new analyses of whole rocks from St. Lucia are presented, and these extend over a silica range from 50% in the earliest to 66% in the most recent products. The suite is relatively rich in alumina (15–19%) and poor in alkalis (Na2O up to 3.5% K2O up to 1.5%), although by West Indian standards the potash content is high. The rocks as a whole are members of the calc-alkaline, "Pacific" series, characteristic of mobile structural belts. The silica variation diagram for St. Lucia illustrates the compositional heterogeneity of the basalts, and the gap (from 53–59%) in silica content, which also corresponds to the gap in time between the eruption of the basalts (phase 1) and the andesites (phase 2). The andesites, on the other hand, appear to be closely related to the dacites, forming a single, continuous series. A comparison of the Fe : Mg : Na+K ratios reveals the close chemical similarity of the andesites and dacites of St. Lucia to those of Crater Lake, Oregon, and to the "hypersthenic" series of Hakone volcano, Japan. The Ca : Na : K ratios show that the rocks of St. Lucia contain a higher proportion of potash relative to soda and lime than most other suites of the orogenic regions.
The distribution of the minor elements in ten rocks from St. Lucia conforms closely to that found in other West Indian and orogenic volcanic suites, with the notable exception of rubidium, which is much more abundant in the acid rocks of St. Lucia than in those of other suites from the Lesser Antilles. A co-variance plot of seven minor elements in the analysed samples from St. Lucia provides an arrangement of specimens along the abscissa which is closely parallel to the order according to silica content, and this in turn corresponds to the chronologic sequence of eruption.
The basalts of St. Lucia are believed to have crystallized from essentially unmodified, primary magma. The occurrence in the Soufrière region of two basalt varieties, namely porphyritic and aphyric, is explicable either as the result of the accumulation at depth of plagioclase felspar crystals, or alternatively may be attributed to the independent development two primary magmas, of tholeiitic and alumina-rich types. There is no support in the Soufrière region for differentiation of basalt, or assimilation by primary basalt magma, on a sufficiently extensive scale to produce the observed volume and sequence of the more acid lavas. It is, therefore, proposed that the andesites and dacites of St. Lucia were derived largely by partial melting of local crustal material.

Advisor: Wager, L
Brown, G