Pierre Doucet
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M. Sc. Thesis
The Petrology and Geochemistry of the Middle River Area, Cape Breton Island, Nova Scotia
(PDF - 152 Mb)
The Middle River area of the southern Cape Breton Highlands is underlain by a sequence of interlayered metasedimentary and metavolcanic rocks. The Middle River unit, in the south, consists of two distinct east-west belts. Low- to medium-grade schists and phyllites with thin metabasite sheets form the "underlying" southern belt while high-grade paragneisses form the "overlying" central belt. The Egypt Highland unit, composed of foliated granitic rocks grading into a less deformed granite, makes up a third belt in the northern part of the area.
Four phases of deformation are recognized. The first is associated with a poorly preserved enclosed schistosity found only in the central belt. The second is represented by the principal fabric of the rocks of the Middle River and Egypt Highland units and by east- and west-plunging folds possibly related to the "stacking" of the belts along the shear zones. The third and fourth phases are represented by north-trending small- and large-scale folds respectively. A southward and upward movement of the complex along a northeast-southwest mylonite zone and parallel north-south faults produced the observed structural configuration of the Middle River complex.
The grade of metamorphism increases rapidly northward in the southern belt from the biotite zone to the staurolite-kyanite zone. Peak metamorphic conditions reach the kyanite zone in the central belt during D1 but were subsequently overprinted by garnet zone conditions during D2. Calculated temperatures from calcite-dolomite and garnet-biotite geothermometers cluster between 500'C and 600'C and reflect these overprinting conditions. 40Ar/39Ar ages of 377+9 Ma for biotite and 386+9 Ma for coexisting hornblende from a metabasite sheet in the central belt apparently ascribe the cooling of the sequence during uplift following D2 to the Middle Devonian Acadian orogeny.
The study area is enclosed by Mississippian sedimentary rocks to the west, a granodiorite of uncertain age to the east, a Devonian monzogranite to the northeast, and deformed volcanic and volcaniclastic rocks to the southeast.
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Pages: 361
Supervisor: Rebecca Jamieson