New U-Pb (zircon) age and geochemistry of the Wedgeport pluton, Meguma terrane, Nova Scotia

Abstract

A new U-Pb (zircon) age of 357 ±1 Ma for the Wedgeport pluton in the Meguma terrane of southwestern Nova Scotia confirms that it is younger than other granitoid plutons of the Meguma terrane but not as young as the ca. 316 Ma age based on previous geochronological work. The pluton consists of mainly homogeneous biotite monzogranite characterized by abundant accessory phases including garnet, zircon, epidote, titanite, and ilmenite. Major minerals are quartz, plagioclase (albite-oligoclase), and perthitic microcline. The pluton has consistently high SiO2 content (average ca. 75%) but shows some internal variation consistent with fractional crystallization of feldspar and biotite, together with accessory phases such as zircon. Tectonic setting during emplacement of the pluton was likely within plate, but classification is ambiguous because the pluton has characteristics of all three granite types. Peraluminous composition and presence of ilmenite and aluminous minerals such as Fe-rich biotite and garnet suggest S-type affinity, but other characteristics such as high-level emplacement, accessory fluorite, and some chemical characteristics (high Ga/Al ratio, elevated Nb and Y) indicate A-type affinity, whereas abundant titanite and overall chemical character is compatible with felsic I-type composition. The granite has specialized characteristics such as elevated Rb relative to Sr and Ba, and high U and Th compared to all of the average granite types. High ε Nd values (+2 to+3) indicate a relatively primitive source compared to other plutons of the Meguma terrane. The Wedgeport pluton appears to be unique in the Megurna terrane in both age and composition, although similar plutons may occur in inboard terranes the Appalachian orogeń.