Abstract

Flemish Cap, a large isolated submarine knoll 600 km east of Newfoundland, consists of a central core of Hadrynian rocks and an onlapping sequence of undisturbed to disturbed Mesozoic–Cenozoic sediments. The central core of the cap, sampled with an electric rock-core drill, comprises pink, fine- to medium-grained granodiorite, dacite, and volcanic siltstone. The granodiorite samples, collected in six out of eight cores, are remarkably similar lithologically, despite a separation of up to 65 km, and probably represent a single pluton. The aphanitic dacite and laminated cherty volcanic siltstone have been metamorphosed to subgreenschist (prehnite) facies but show no evidence of contact metamorphism.U–Pb analyses of coarse and fine zircon fractions from a granodiorite core yielded upper intercepts of 751 and 833 Ma, respectively. Although not precise, these ages probably represent the age of intrusion sometime in the 750–830 Ma range and are older than those reported for similar Hadrynian granitic rocks of eastern Newfoundland. AK–Ar age of 657 ± 29 Ma on hornblende from the same core and a K–Ar age of 615 ± 20 Ma on biotite from a granodiorite core elsewhere in the body probably represent incomplete degassing during the superimposed subgreenschist metamorphism of uncertain age.Thus the Flemish Cap granodiorite and associated rocks are part of the Avalon Zone and may represent a much older part of the Avalon Terrane than parts toward the west on Avalon Peninsula and Burin Peninsula.

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