The volcanic evolution of Montserrat using 40Ar/39Ar geochronology

Abstract

Abstract40Ar/39Ar dating has facilitated a substantial reinterpretation of the volcanic evolution of Montserrat. Three volcanic centres with non-overlapping volcanic activity are identified: Silver Hills (c. 2600 to c. 1200 ka); Centre Hills (at least c. 950 to c. 550 ka); South Soufrière Hills-Soufrière Hills (at least c. 170 ka to present). The geochronological data show that old xenocrysts are common in the porphyritic andesite, implying that reliable ages are best obtained by dating the groundmass. Soufrière Hills evolved from early eruptions dominated by two-pyroxene andesite to eruptions of hypersthene-hornblende andesite at c. llOka. Between the two varieties of andesite there was an interlude of mafic volcanism at c. 130ka to form South Soufrière Hills. There is evidence of tectonic uplift of early products of the complex along with older submarine volcanic rocks. Consideration of stratigraphy and age data indicates that only a proportion of the dome-forming eruptions are recorded as domes in the geological record. Older products are removed from the subaerial edifice by sector-collapse events. The time-averaged eruption rate of the South Soufrière Hills-Soufrière Hills centre is estimated at 0.005 m3 s-1 (c. 0.15 km3 ka-1) (dense rock equivalent). The ongoing eruption is very similar in style to previous activity at Soufrière Hills, and future activity is likely to pose similar hazards. Soufrière Hills have been characterized by alternations of periods of enhanced activity and periods of dormancy, both lasting of the order of 104 years. During periods of elevated activity several major dome-forming eruptions are separated by quiescent interludes lasting less than c. 103 years. The ongoing eruption may mark the onset of a fourth period of enhanced volcanic activity at Soufrière Hills.