TONIAN ALKALI ANORTHOSITE, FERRONORITE, AND GABBRO IN THE LOWER COVERDALE PLUTONIC SUITE, SOUTHERN NEW BRUNSWICK, CANADA: AN AMAZONIAN PART OF GANDERIAN BASEMENT?

The Lower Coverdale plutonic suite lies below 100-200 m of Carboniferous sedimentary rocks and is known only from 11 drill holes. Rock types in the drill core are entirely plutonic and include anorthosite, mela-anorthosite, ferronorite, and gabbro. New U-Pb analyses by LA-ICPMS have confirmed previous dates by ID-TIMS, and show that the gabbro, as well as the mela-anorthosite and ferronorite, have similar Tonian ages. Best constrained are ages of 981±5 Ma (LA-ICPMS 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. Although data from the 5 samples show some scatter, they clearly demonstrate the close timing of Tonian zircon crystallization in the entire suite; Th/U ratios are consistent with an igneous origin for the zircon. 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. Maximum temperature estimates from Ti-in-zircon thermometry give similar magmatic equilibrium temperatures (ca. 750 degrees C) for the three lithologies. The Lower Coverdale anorthosite has characteristics of alkali anorthosite massifs, including andesine plagioclase (An_30-35) with Ba-bearing K-feldspar exsolution lamellae. Original orthopyroxene in mela-anorthosite has been altered to amphibole and chlorite. Ferronorite consists of plagioclase and orthopyroxene with varying amounts of apatite and ilmenite, grading into Fe-Ti-P rocks. Anorthosite and ferronorite are typically interlayered with sharp contacts, and both are cut by abundant gabbroic dykes with tholeiite composition and possible arc setting. The anorthosite and ferronorite have more evolved (higher Pb, lower Nd and Hf) isotopic signatures compared to Laurentian Grenville regions like the Adirondacks but are similar to other Mesoproterozoic inliers in the Appalachians. The suite differs in age and geological setting from the Montpelier anorthosite and associated rocks in the Goochland terrane, and may have been derived from the Amazonian part of the Grenville orogen. Its current position, apparently within Ganderia, remains enigmatic.