STENIAN-TONIAN ANORTHOSITE AND RELATED ROCKS IN MARITIME CANADA: U-PB, TRACE-ELEMENT AND HF ISOTOPIC EVIDENCE FOR PROVENANCE

The Blair River Inlier (BRI) in northern Cape Breton Island, Nova Scotia, is interpreted as the southeasternmost surface extent of native Laurentia in the northern Appalachian orogen. It consists of Mesoproterozoic gneiss and massif-type anorthosite, both overprinted by Silurian metamorphism and, locally, plutonism. New analytical capabilities have prompted a re-examination of the BRI using laser-ablation methods for U-Pb, trace elements, and Hf in zircon. Four samples of the Red River anorthosite suite (RRAS) in the BRI yield discordant U-Pb arrays indicating Stenian or Tonian 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, 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 Lower Coverdale alkalic anorthosite in southeastern New Brunswick is known only from legacy drill cores located in an area included in Ganderia. No host rocks were encountered in the drill core. LA-ICP-MS dates confirm previous dates by ID-TIMS, and show that gabbro, mela-anorthosite and ferronorite in the plutonic suite have similar Tonian ages. Best constrained are ages of 981±5 Ma (LA-ICP-MS concordia age) and 975.5±2.3 Ma (ID-TIMS upper intercept), interpreted to be best estimates of the igneous crystallization age of the plutonic suite. Trace-element and Hf-isotopic characteristics of the zircon grains from all the rock types are closely similar; REE have steep positive slopes, large positive Ce and small negative Eu anomalies and other trace-element characteristics suggest origin in a continental arc setting. The anorthosite and ferronorite have more evolved (higher Pb, lower Nd and Hf) isotopic signatures compared to typical Laurentian Grenville regions. The data are consistent with BRI being Laurentian Grenville, but the Lower Coverdale rocks may have originated in the Amazonian segment of Rodinia.