麻豆传媒

The Brookville terrane of southern New Brunswick consists of the Green Head Group, Brookville Gneiss, Dipper Harbour volcanic unit, and associated plutonic units. The Mesoproterozoic Green Head Group is mainly a low-grade platformal sequence of carbonate and pelitic rocks that is in faulted contact along a ductile shear zone with the low-pressure/high-temperature Brookville Gneiss, composed of cordierite-bearing paragneiss, amphibolite, tonalitic to granodioritic orthogneiss, minor marble and quartzite. The paragneiss has a maximum depositional age of ca. 641 Ma, the orthogneiss has an igneous crystallization age of ca. 605 Ma, and peak regional amphibolite facies metamorphism occurred at a ca. 564 Ma. Hence, the Brookville Gneiss is younger than the Green Head Group and does not represent basement; however, the original relationship between these units is unclear. The Green Head Group was moderately to intensely folded prior to the Late Neoproterozoic regional deformation and amphibolite facies metamorphism associated with the prolonged juxtaposition of the Brookville Gneiss with adjacent parts of the Ashburn Formation of the Green Head Group. Based on contrasts in age and metamorphic conditions, the ca. 610 Ma high-pressure/low-temperature Hammondvale metamorphic unit is not a high-grade metamorphic equivalent of the Green Head Group and is excluded from the Brookville terrane.

The Late Neoproterozoic Dipper Harbour volcanic unit consists of rhyolitic and andesitic tuffs with minor siltstone and marble, preserved in a Carboniferous thrust in the southwestern part of the terrane.

Twenty-six granitoid plutons in the Brookville terrane are broadly grouped on the basis of composition into: 1) diorite to granodiorite; 2) monzogranite to granodiorite; and 3) syenogranite to monzogranite suites. They have I-type, calc-alkaline characteristics and have yielded crystallization and cooling ages from ca. 548 to 500 Ma. They are exposed to more shallow crustal levels in the southwest, where they are associated with the Dipper Harbour volcanic unit, compared to the northeast, where they intruded the Brookville Gneiss and Green Head Group. Although the isotopic ages obtained from the Brookville terrane span the Neoproterozoic-Cambrian boundary, the tectonothermal history of the terrane is not compatible with the transition from Late Neoproterozoic magmatic arc to stable Cambrian platform that is recorded in the adjacent Caledonia terrane (Avalon terrane sensu stricto). Thus, the Brookville terrane is interpreted to have been a distinct tectonostratigraphic assemblage in the Neoproterozoic through early Paleozoic. It is correlated with the Bras d'Or terrane of Cape Breton Island and parts of the Hermitage Flexure of southern Newfoundland.

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