Mount Pleasant Caldera

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The Mount Pleasant Caldera is a large and wide eroded Late Devonian caldera complex, located in the northern Appalachian Mountains of southwestern New Brunswick, Canada. It is one of few noticeable pre-Cenozoic calderas, and its formation is associated to a period of crustal thinning that followed the Acadian orogeny in the northern Appalachian Mountains.[2] It sits relatively near to the coastline.[3]

Geology

The large elliptical feature is dated back to the late Devonian Period, and is partially covered in the north by overlying Middle Mississippian and Pennsylvanian-period strata.[2] The volcano is north-south trending in its elliptical shape, with minimum dimensions of 13 by 34 kilometres (8 mi × 21 mi) as outlined by regional gravitational and magnetic studies. The northern half of the volcano has since been covered by depositional rock strata.[4] The caldera is bounded to the east and west by fused Ordovician to Silurian turbiditic metasedimentary rocks of the local Digdeguash and Flume Ridge geological formations.[4]

Late Silurian to Devonian granitic rocks of the Saint George Batholith bound part of the southern margin of the caldera. Rocks within the summit itself date back to the Upper Devonian, and show multiple fill sequences late in its history.[4]

The magma produced by Pleasant is rich in silica, as indicated by a large amounts of ignimbrite, tuff, rhyolite, and other igneous rocks rich in the mineral. Silica-rich magma does have a high viscosity, and therefore does not flow easily like basalt. As a result, gases tend to become trapped at high pressure within the magma. When the magma approaches the surface of the Earth the rapid off-loading of overlying material causes the trapped gases to decompress rapidly triggering explosive destruction of the magma and spreading volcanic ash over wide areas.[5]

Intrusion-related gold veins have recently raised great interest among economic geologists. In southwestern New Brunswick, which is part of the Canadian Alleghenian orogeny, several gold deposits have been recorded in the past. The positive early results have created great interest for gold-finding efforts, and Mount Meager has proven to be an ideal candidate.[6] This is also one of the reasons we know so much about the volcano's minerology.

Granitic intrusions within the caldera complex include the McDougall Brook Microgranite and the somewhat younger Mount Pleasant Granite. Yielding gold quartz breccias and veins cut the McDougall Brook Microgranite and its volcanic wall-rock, while molybdenum-bismuth-tungsten and later polymetallic mineralization are hereditarily related with the multiphase Mount Pleasant Granite.[7]

The numerous felsic sections are associated by episodes of fractional crystallization in a high-level, zoned magma chamber. Fractionation was continually interrupted by eruption of material from the roof zone such that seven phases of caldera growth have been recognized.[7]

Mount Pleasant lies along the southwestern margin of the caldera complex. Two mineralized zones, termed the Fire Tower Zone and the North Zone, occur within volcanic plugs about 1 kilometre (1 mi) apart. The volcanic necks are defined by magmatic-hydrothermal breccias.