Tectonic significance of Late Ordovician silicic magmatism, Avalon terrane, northern Antigonish Highlands, Nova Scotia

Avalonia was a microcontinent during most of the Ordovician, separating the Iapetus Ocean to the north from the Rheic Ocean to the south. In the northern Antigonish Highlands, Nova Scotia, volcanic rocks (Dunn Point and McGillivray Brook formations) underlie Early Silurian Early Devonian strata (Arisaig Group) and were thought to represent extension that heralded the development of the basin into which Arisaig Group strata were deposited. However, recent U-Pb (zircon, thermal ionization mass spectrometry) data from rhyolite in the Dunn Point Formation (DPF) yielded an age of 460.0 +/- 3.4 Ma, and, here, we report a concordant age of 454.5 +/- 0.7 Ma for an ignimbrite in the overlying McGillivray Brook Formation (MBF). These data confirm a ca. 10 million year gap between volcanism and onset of Arisaig Group deposition, which occurred after accretion of Avalonia to Baltica. The DPF and MBF both resemble A-type SiO2-rich magmas. The MBF has very high concentrations of Zr (745-1965 ppm), Y (65-213 ppm), Nb (57 to 185 ppm), and high Ga/Al. Several MBF samples exhibit strong LREE depletion, consistent with fractionation of LREE-bearing accessory phases. epsilon Nd-t values for MBF (t = 455 Ma) range from +1.5 to +3.9, and overlap with DPF rhyolites that range from +2.9 to +3.7. Depleted mantle model ages for MBF and DPF samples unaffected by accessory phase fractionation are between 0.9 and 1.2 Ga and are similar to T-DM values in older (Neoproterozoic, Cambrian) crustally derived felsic rocks, suggesting derivation from the lower crustal basement beneath the Antigonish Highlands. DPF and MBF rocks were probably erupted in a local extensional environment within an ensialic arc, perhaps analogous to the modern Taupo Volcanic Zone in northern New Zealand.