GEMOC ARC National Key Centre

Dadd, K.A.1, van Wagoner, N.A.2 and Leybourne, M.3
1. GEMOC Macquarie
2. Acadia University, Nova Scotia, Canada
3. University of Texas, Dallas, USA

Siluro-Devonian bimodal volcanism is a common feature of the northern Appalachians of the northeastern United States and eastern Canada, forming major volcanic belts including the Coastal Volcanic Belt, (CVB) of New England and New Brunswick. The CVB is one of the largest bimodal volcanic provinces in the world. This paper reports on the detailed lithogeochemistry of the thickest and most continuous part of the CVB; the Passamaquoddy Bay volcanic sequence (PBV) of southwestern New Brunswick, and discusses implications for tectonic setting and comparisons to modern analogues. Controversy remains regarding tectonic setting of this volcanism, with suggestions ranging from volcanic arc, to extension-related volcanism at a continental rift, transpressional basin, backarc, or plume modified subduction zone. The mapped PBV sequence has a minimum thickness of 4 km, that includes 58 lithologic units, comprising 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 CVB in Maine. We analysed 115 volcanic rocks for major and trace elements. The sequence is subalkalic with within plate tectonic affinities. Most mafic rocks range in composition from basalt to basaltic andesite. The Mg#  ranges from 28 to 65, and there is a trend toward more primitive compositions upward in the section. Most of the chemical variation can be explained by replenishment and fractionation of small magma chambers. Chemical variations within each of the rhyolitic cycles indicates an upward trend toward more evolved compositions as a result of an evolving magma chamber and magma chamber zonation. Based on the nature, thickness, cyclicity and chemistry of volcanism, we explore tectonic models and modern analogues for this impressive Silurian volcanic sequence.

In, Abstracts with Programs, Vol. 32, No. 7