Late silurian bimodal volcanism of southwestern New Brunswick, Canada: products of continental extension

Silurian-Devonian bimodal volcanism--a common feature of the northern Appalachians of the northeastern United States and eastern Canada forms--major belts of igneous rocks including the Coastal volcanic belt of New England and New Brunswick. The Coastal volcanic belt is one of the largest bimodal volcanic provinces in the world. Controversy remains regarding the tectonic setting of this volcanism; suggestions range from volcanic arc, to extensional continental rift, transpressional basin, backarc, or plume-modified subduction zone. The mapped Passamaquoddy Bay volcanic sequence of the Coastal volcanic belt of southwestern New Brunswick has a minimum thickness of 4 km and comprises 63 lithologic units that are divided into four distinct cycles of mafic-felsic volcanism. A preliminary U-Pb zircon date indicates an age of 423 Ma, similar to the age of the Cranberry Island bimodal volcanic series of the Coastal volcanic belt in Maine. Our analysis of 115 volcanic rocks for major and trace element concentrations indicates that the sequence is subalkalic and bimodal (basalt-rhyolite) with within-plate tectonic affinities. Most mafic rocks range in composition from basalt to basaltic andesite. The Mg# [defined as 100 x MgO/(MgO + FeO), where FeO = Fe[O.sub.T] X 0.9] of the basalts ranges from 30 to 70, and there is a trend toward more primitive compositions upward in the section. The light rare earth element abundances of the basalts are enriched to levels of 30-130 times chondritic values. The flows are enriched in incompatible elements with respect to primitive mantle and show distinct Nb, Hf, and Sr anomalies. We interpret the basalts to be mantle melts modified by crustal contamination and mantle metasomatism from a previous subduction event. Most of the chemical variation can be explained by replenishment and fractionation of small magma chambers. The rhyolitic rocks are crustal melts, modified by crystal fractionation. Within each of the rhyolitic cycles there is an upward trend toward more evolved compositions as a result of an evolving magma chamber and magma-chamber zonation. We suggest that the rocks were erupted in an extensional environment and that there was a significant zone of continental extension during the Late Silurian in the northern Appalachians. However, controversy remains regarding the precise nature of the tectonic setting and the duration and extent of this volcanism. Keywords: bimodal volcanism, Coastal volcanic belt, continental extension, northern Appalachian volcanism, petrogenesis.