The Mount Pleasant Caldera is a large eroded Late Devonian volcanic 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. It sits relatively near to the coastline.
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. 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. 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. 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. 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. 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 the Mount Meager massif has proven to be an ideal candidate. Granitic intrusions within the caldera complex include the McDougall Brook Microgranite and the somewhat younger Mount Pleasant Granite. Gold quartz breccias and veins cut the McDougall Brook Microgranite and its volcanic wall-rock, while molybdenum-bismuth-tungsten and later polymetallic mineralization are related to the multiphase Mount Pleasant Granite. The numerous felsic sections are associated with 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. 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.
Eruptive history The eruptive history of the Mount Pleasant Caldera can be divided into three stages of activity: the exocaldera sequence, the intracaldera sequence, and the late caldera-fill sequence. These can further be subdivided into strata based on their depth below the surrounding rock. The Intercaldera Sequence comprises formations that crop out from overlying flows in triangularly shaped area, and includes thick volcanic ash (tuff), thick breccia layers, and intermediate to felsic igneous rocks that tend to intrude the above layers and are typically located along caldera margin faults. The Exocaldera Sequence contains ash flow tuffs, mafic lavas, alluvial redbeds, and porphyritic felsic lavas that are distributed across five different layers. The late Caldera-Fill Sequence contains rocks that are similar to those of the outflows of the other, older layers, and comprises two formations and two relatively minor intrusive lava flows. The volcanic flows are generally mafic. The stratigraphic subdivision is supported by geochemical and mineralogical analyses, which indicate that the basaltic rocks are mantle-derived and have, unusually, relatively intraplate (or hotspot) type chemical affinities. The multiple andesite flows were probably derived from basaltic magma by the crystallization of the magma material. The relatively rare, more felsic flow units, are thought to have originated from high-end crystallization inside the magma chamber. The various stages of fractionation are continually interrupted by eruptions, and have allowed scientists to recognize seven stages of caldera development. The genesis of the caldera is related to a period of lithospheric thinning that followed the Acadian Orogeny in the northern Appalachians. The relative position of the Exocaldera and Intracaldera sequences is based on several observations:
The upper part of the Rothea formation (Exocaldera) contains about 1% biotite. The only intracaldera rocks with this much biotite is volcanic strata within sedimentary breccia of and a tuff unit near the Scoullar Mountain formation (Intracaldera). Andesitic strata occurs only in two flow units: the South Oromocto Andesite of the Exocaldera sequence, and the Scoullar Mountain formation of the Intracaldera sequence. The exocaldera Carrow formation contains clasts from the intracaldera Seelys formation. The exocaldera Bailey Rock Rhyolite intrudes and overlies the Carrow formation, but is intruded by the intracaldera McDougall Brook Granite formation.
… excerpt ends here. Continue reading the full article.
