Stratigraphic, petrographic and geochemical evidence indicate
that the volcano-sedimentary rocks of the Late Devonian Piskahegan
Group, located in the northern Appalachians of southwestern New
Brunswick, represent the eroded remnants of a large epicontinental
caldera complex. This complex – the Mount Pleasant Caldera
– is one of few recognizable pre-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: thick ash flow
tuffs, thick sedimentary breccias that dip inward, and stocks of
intermediate to felsic composition that intrude the volcanic pile or
are localized along caldera-margin faults. The Exocaldera Sequence
contains ash flow tuffs, mafic lavas, alluvial redbeds and
porphyritic felsic lavas that comprise five formations. The Late
Caldera-Fill Sequence contains rocks that are similar to those of the
outflow facies and comprises two formations and two minor intrusive
units. Geochemical and mineralogical data support the stratigraphic
subdivision and indicate that the basaltic rocks are mantle-derived
and have intraplate chemical affinities. The andesites were probably
derived from basaltic magma by fractional crystallization and
assimilation of crustal material. The various felsic units are
related by episodes of fractional crystallization in a high-level,
zoned magma chamber. Fractionation was repeatedly interrupted by
eruption of material from the roof zone such that seven stages of
caldera development have been identified. The genesis of the caldera
is related to a period of lithospheric thinning that followed the
Acadian Orogeny in the northern Appalachians.
Mount Pleasant Mansion, in Fairmount Park, Philadelphia, Pa.
