STRATIGRAPHY AND ERUPTIVE HISTORY OF THE LATE DEVONIAN MOUNT PLEASANTCALDERA COMPLEX, CANADIAN APPALACHIANS

Stratigraphic, petrographic and geochemical evidence indicate that the volcano-sedimentary rocks of the Late Devonian Piskahegan Group, loca ted in the northern Appalachians of southwestern New Brunswick, repres ent the eroded remnants of a large epicontinental caldera complex. Thi s complex - the Mount Pleasant Caldera - is one of few recognizable pr e-Cenozoic calderas and is divisible into Exocaldera, Intracaldera and Late Caldera-Fill sequences. The Intracaldera Sequence comprises four formations that crop out in a triangular-shaped area and includes: th ick ash flow tuffs, thick sedimentary breccias that dip inward, and st ocks of intermediate to felsic composition that intrude the volcanic p ile or are localized along caldera-margin faults. The Exocaldera Seque nce contains ash how tuffs, mafic lavas, alluvial redbeds and porphyri tic felsic lavas that comprise five formations. The Late Caldera-Fill Sequence contains rocks that are similar to those of the outflow facie s and comprises two formations and two minor intrusive units. Geochemi cal and mineralogical data support the stratigraphic subdivision and i ndicate that the basaltic rocks are mantle-derived and have intraplate chemical affinities. The andesites were probably derived from basalti c magma by fractional crystallization and assimilation of crustal mate rial. The various felsic units are related by episodes of fractional c rystallization in a high-level, zoned magma chamber. Fractionation was repeatedly interrupted by eruption of material from the roof zone suc h that seven stages of caldera development have been identified The ge nesis of the caldera is related to a period of Lithospheric thinning t hat followed the Acadian Orogeny in the northern Appalachians.