Stratigraphy and eruptive history of the Late Devonian Mount Pleasant Caldera Complex, Canadian Appalachians

Abstract

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. Figure 1. Tectonic lithofacies map of southwestern New Brunswick (after Fyffe, McLeod & Ruitenberg, 1992) with location of the Mount Pleasant Caldera complex.

Re-Os geochronological constraints on the mineralizing events within the Mount Pleasant Caldera: implications for the timing of sub-volcanic magmatism

“…Based on previous published geochronological data, the age of mineralization at Mount Pleasant could only be broadly limited to a range between the Late Devonian and Early Carboniferous (i.e., Sinclair et al 1988;McGregor and McCutcheon 1988;Anderson 1992;McCutcheon et al 1997;Tucker et al 1998). The results from our Re-Os dating provides direct information on the timing of the tungstenmolybdenum mineralizing event at Mount Pleasant and highlights the need to modify previous interpretations of the magmatic-hydrothermal evolution of the Mount Pleasant Caldera Complex.…”

“…It underlies and is intercalated with the Little Mount Pleasant Formation (Fig. 3b) described below (Harris 1964;van de Poll 1967;Sinclair et al 1988Sinclair et al , 2006McCutcheon 1990;McCutcheon et al 1997).…”

“…1). The deposit is located near the southwestern margin of the Late Devonian Mount Pleasant Caldera Complex (McCutcheon et al 1997), a subcircular feature that underlies an area approximately 13 km by 34 km (Fig. 2).…”

A Review of Proterozoic to Early Paleozoic Lithotectonic Terranes in New Brunswick, Canada and their Tectonic Evolution during Penobscot, Taconic, Salinic and Acadian Orogenesis

“…Thinner, essentially flat-lying strata overlie the New Brunswick Platform to the north of the Belleisle-Beaver Harbour Fault. The lower part of the platformal section comprises Upper Devonian subaerial, mainly felsic volcanic flows and tuffs of the Harvey and Piskahegan groups (Fyffe and Barr 1986;Payette and Martin 1986a, b;McGregor and McCutcheon 1988;McCutcheon et al 1997). The Upper Devonian volcanic rocks are unconformably overlain on the platform by Mississippian coarse-to fine-grained redbeds and local alkaline basalt and peralkaline felsic volcanic rocks of the Mabou Group (St. Peter and Johnson 2009;Gray et al 2010).…”

“…Bradley (1982) interpreted early rapid subsidence in local subbasins followed by widespread, slow subsidence in the Maritimes Basin to be related to transcurrent rifting and later thermal relaxation in a pull-apart basin. McCutcheon and Robertson (1987) and McCutcheon et al (1997) attributed subsidence to extension and crustal thinning of over-thickened continental crust resulting from isostatic upwelling of hot asthenosphere following the Acadian orogeny. It is likely that both processes were involved in development of the successor basin (Murphy and Keppie 2005).…”