Abstract

Permian plutonism is widespread in the U.S. Appalachians, but not in the Canadian sector of the Appalachians. The German Bank pluton, sampled in two boreholes offshore southwestern Nova Scotia, gave a U-Pb age of 300 ± 1 Ma, determined by isotope dilution–thermal ionization mass spectrometry (ID-TIMS) on thermally annealed and chemically etched zircon grains. Electron-microprobe dating of monazite yielded an age of 308 ± 13 Ma. Late Paleozoic plutonism in the Canadian Appalachians was dominated by northwestward subduction of the Rheic Ocean, whereas the Rheic Ocean in the U.S. Appalachians closed by southeasterly subduction. A slab tear therefore developed at the southwestern margin of the Rheic Ocean slab beneath the Canadian Appalachians. The high Sr and Ba metaluminous magnetite granite of the German Bank pluton resulted from melting of underplated lower crust and/or lithospheric mantle by a rising asthenospheric diapir related to this slab tear. Geochemically, the German Bank pluton is similar to most 300–265 Ma plutons of the southern Appalachians, except in its strongly negative Nd isotopic composition that results from the underthrusting of a ca. 2 Ga lower crustal block of Saharan craton.

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