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.
Lower to Middle Cambrian volcanic rocks occur within the Avalon Zone of southern New Brunswick at Beaver Harbour and in the Long Reach area. The Beaver Harbour rocks are intensely altered, but the major- and trace-element geochemistry indicates that they could be highly evolved (basaltic andesites) within-plate basalts. The mafic flows from the Long Reach area form two chemically and petrologically distinct groups: (1) basalts with feldspar phenocrysts that represent evolved continental tholeiites with some oceanic characteristics; and (2) a group of aphyric basalts showing extremely primitive continental tholeiite compositions, also with oceanic affinities and resembling some rift-related Jurassic basalts on the eastern seaboard. Felsic pyroclastic rocks in the Long Reach area make the suite bimodal. This distribution of rock types supports conclusions from the mafic rocks that the area experienced tension throughout the Early to Middle Cambrian.
Juxtaposition of the Mississippian Ristigouche and Cannes-de-Roches Basins, which are subbasins of the composite late Paleozoic Maritimes Basin, occurred through dextral movement along the northwest-striking Percé Fault system in the eastern Gaspé Peninsula of Quebec. The northwest-striking faults are truncated by small north-northwest-striking dextral strike-slip faults, which probably developed as regional stress gradually rotated clockwise from north-northwest-south-southeast to northeast-southwest. This study provides the first evidence in eastern Quebec for significant post-Acadian block displacement other than normal faulting and indicates that Alleghanian deformation extended much farther north than previously thought. Identification of these structures formed during the Alleghanian orogeny but more than 1000 km away from areas of peak Alleghanian metamorphism in the southeastern United States underlines the far-reaching effects of continental collisions. It also casts doubt on the age attribution of brittle strike-slip faults elsewhere in the Gaspé Peninsula, away from Mississippian exposures. Such brittle faults were previously associated with late stages of the Acadian orogeny but could in fact be considerably younger.
Two spatially and temporally related subvolcanic intrusive suites, the McDougall Brook Granitic Suite (MBG) and the Mount Pleasant Granitic Suite (MPG), occur within the Late Devonian Mount Pleasant Caldera, southwestern New Brunswick. Auriferous greisenized quartz breccias and veins are associated with the MBG, whereas Sn-W-Mo-Bipolymetallic (Cu-Zn-Pb-In) mineralization is genetically related to the slightly younger MPG. The low-silica (< 70 wt. %) MBG and the high-silica (> 74 wt. %) MPG are calc-alkaline and metaluminous to weakly peraluminous (A/CNK = 0.91 -1.28), and exhibit some crustal A-type granite affi nities. However, compared to the MPG, the MBG has lower amounts of incompatible trace elements, a more pronounced Nb anomaly, enriched REE patterns with smaller negative Eu anomalies, and lower negative Ti, Sr, and Ba anomalies. Trace-element modeling, major-element chemistry, and petrologic evidence indicate that the MBG and MPG may have formed through pulse injection of magmas produced by fractional crystallization from felsic magma chambers at depth, involving an assemblage of clinopyroxene, amphibole, plagioclase, K-feldspar, ± biotite, ± zircon, ± apatite. Supracrustal contamination is evident, resulting in abnormally high alkalies (e.g., K) and high initial ( 87 Sr/ 86 Sr) i ratio. The parental magmas may be mainly derived from juvenile materials within the crust. Extremely high F (≤ 0.99%), Li (≤ 610 ppm), Rb (≤ 1210 ppm), Cs (≤ 28 ppm), U (≤ 43 ppm), Th (≤ 50 ppm), Nb (≤ 107 ppm), Ta (≤ 26 ppm), and heavy REE (e.g., Yb ≤ 36 ppm) in the MPG rocks, with very low K/Rb ratio (≤ 56) and Nb/Ta ratio (≤ 7.6), suggest fl uid fractionation played an important role in late-stage magmatic differentiation.Although gold background contents in both the MBG and MPG are typically low, ranging from < 2 to 8 ppb, W, Sn, and Mo contents in the MBG are much lower than those in the MPG. Gold mineralization appears to be related to the early MBG, as it behaves as a compatible element during magmatic evolution. The MBG is broadly comparable to intrusion-related gold systems elsewhere. However, the MPG has characteristics of evolved topaz-bearing granites commonly associated with Sn-W-Mo-Bi-polymetallic (Cu-Zn-Pb-In) mineralization; the youngest phase of the MPG appears have high potential for generating Sn deposits. Both intrusive suites formed within an epicontinental caldera complex and generally have within-plate geochemical character. RÉSUMÉDeux suites intrusives subvolcaniques spatialement et temporellement apparentées, la suite granitique du ruisseau McDougall (MBG) et la suite granitique du mont Pleasant (MPG), sont présentes à l'intérieur de la caldeira du Dévonien tardif du mont Pleasant, dans le Sud-Ouest du Nouveau-Brunswick. Des fi lons et des brèches de quartz greisenifi és aurifères sont associées à la MBG, tandis qu'une minéralisation de Sn-W-Mo-Bi-polymétallique (Cu-ZnPb-In) est génétiquement apparentée à la MPG légèrement plus récente. La MBG à faible teneur en silice (< 70 % pds) et la MBG à f...
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