SILURIAN METAMORPHISM OF PROTEROZOIC ANORTHOSITE AND RELATED ROCKS: U-PB, TRACE-ELEMENT AND HF ISOTOPIC EVIDENCE FROM ZIRCON, RED RIVER ANORTHOSITE SUITE, NORTHERN CAPE BRETON ISLAND, NOVA SCOTIA, CANADA

For thirty years, the Blair River Inlier (BRI) in northern Cape Breton Island, Nova Scotia, has, with some debate, been considered the southeasternmost surface extent of native Laurentia in the northern Appalachian orogen. Mesoproterozoic gneiss and massif-type anorthosite, both overprinted by Silurian metamorphism with local plutonism, are key to the BRI constraining the tectonic configuration of this segment of the orogen. New data and analytical capabilities have prompted a re-examination of the BRI using laser-ablation methods for U-Pb, trace elements, and Hf in zircon.

Two granitoids from the southern BRI yield concordant U-Pb zircon spot ages of ca. 430 Ma, with no sign of Proterozoic inheritance and initial epsilon Hf (at 430 Ma) of -1.7 to +2.5. In contrast, 300 zircon spot analyses from four different lithologies within the Red River anorthosite suite yield discordant U-Pb arrays indicating Stenian or Tonian ages; lower intercepts are consistent with Silurian Pb-loss but no analyses plot at Silurian ages. The best-constrained anorthosite sample has an upper intercept of ca. 950 Ma, initial epsilon Hf values cluster tightly at 0 to +1.7 (at 950 Ma), and CL images and trace-element data imply U-Pb discordance due to Pb-loss by hydrothermal alteration, not due to core-overgrowth mixing.

The anorthosite suite is more isotopically evolved than the Silurian granitoids. Zircon in Silurian mafic rocks may also show this juvenile signature and could effectively discriminate them from Proterozoic mafic components of the anorthosite suite, regardless of the complexity of the U-Pb systematics. Other anorthosites in the Appalachian orogen (e.g., Lower Coverdale, Montpelier, Roseland) have Stenian or Tonian crystallization ages but variable isotopic characteristics and different petrology, whole-rock geochemistry and/or tectonostratigraphic relationships. Some anorthosite-bearing Mesoproterozoic pre-Appalachian inliers may have originated in the Amazonian segment of Rodinia, but the mechanism of their tectonic incorporation into the Appalachian orogen remains unclear.