Tectonic significance of Late Ordovician silicic magmatism, Avalon terrane, northern Antigonish Highlands, Nova Scotia ¹ This article is one of a series of papers published in CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.² Contribution to International Geological Correlation Programme (IGCP) Project 497.

Abstract

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. 3Ndt 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. Résumé : Avalonia était un microcontinent durant la plus grande partie de l'Ordovicien, séparant l'Océan Iapetus au nord de l'Océan Rhéique au sud. Dans la partie nord des terres hautes d'Antigonish, Nouvelle-Écosse, des roches volcaniques (formations de Dunn Point et de McGillivray Brook) reposent sous des strates datant du Silurien précoce -Dévonien précoce (Groupe d'Arisaig) et elles devaient représenter l'extension qui annonçait le développement du bassin dans lequel les strates du Groupe d'Arisaig ont été déposées. Toutefois, des données U-Pb récentes (zircon, spectrométrie de masse à ionisation thermique (TIMS)) d'une rhyolite de la Formation de Dunn Point ont indiqué un âge de 460,0 ± 3,4 Ma; nous rapportons ici un âge concordant de 454,5 ± 0,7 Ma pour une ignimbrite de la Formation de McGillivray Brook sus-jacente. Ces données confirment une lacune stratigraphique d'environ 10 millions d'années entre le volcanisme et le début de la déposition du Groupe d'Arisaig, qui a eu lieu après l'accrétion d'Avalonia à Baltica. Les formations de Dunn Point et de McGillivray Brook ressemblent toutes deux à des magmas de type A, riche en SiO 2 . La Formation de McGillivray Brook a des concentrations très élevées en Zr (745-1965 ppm), en Y (65-213 ppm), en Nb (57 à 185 ppm) et un rapport...

Seismic Evidence for Proterozoic Collisional Episodes along Two Geosutures within the Southern Granulite Province of India

“…90 Ma (Tables DR1 and DR2). Cambrian-Ordovician ages are common in Avalonian sedimentary and metasedimentary rocks but are also consistent with plutonic rocks in the Avalon and Gander terranes [34,67,69,70]. The oldest concordant inherited zircon from the anorthosite dyke has a 206 Pb/ 238 U age of 657 ± 15 Ma (1σ) which is more indicative of the Avalon terrane than the Gander terrane [59].…”

“…The oldest age group consists of two subgroups where four of the zircons are between 440 ± 11 Ma (1σ) and 446 ± 11 Ma (1σ), and the remaining two are 464 ± 11 Ma (1σ) and 468 ± 11 Ma (1σ). This group is considered to be inherited due to their relatively minor abundance but also because middle to late Ordovician rocks are known in Cape Breton and throughout Avalonia [33,34]. We interpret the crystallization age to be indicative of the weighted-mean 206 Pb/ 238 U age (423:8 ± 2:5 Ma, 2σ) of the zircons that comprise the middle group ( Figure 5(b)).…”

A re-appraisal of the petrogenesis and tectonic setting of the Ordovician Fishguard Volcanic Group, SW Wales

“…There are several other known Ordovician Avalonian terranes that have comparable magmatism and originated in similar tectonic settings to the Welsh marginal basin rocks. Similar terranes from the southern flank of the Iapetus include the Antigonish Highlands Avalonian terrane in Nova Scotia, Canada, and the Avoca volcanic rocks, Ireland; both formed owing to rifting within an overall arc environment (McConnell, Stillman & Hertogen, 1991; Murphy, Hamilton & LeBlanc, 2011). These comparable origins, therefore, reinforce the suggested model of formation for the Fishguard Volcanic Group.…”

“…Avalonia (the microcontinent of which Wales was a part) moved northward from 55°S to 30°S in the period from the Arenig (478–467 Ma) to the Ashgill (451–444 Ma) (Fitton, Thirlwall & Hughes, 1982). During this period a number of island arc chains and marginal basins formed above the southeastward-dipping subduction zone on the north side of Avalonia along a NE-trending plate margin (Fitton & Hughes 1970; Phillips, Stillman & Murphy, 1976; Fitton, Thirlwall & Hughes, 1982; Murphy & Nance, 2008; Murphy, Hamilton & LeBlanc, 2011). Ordovician sequences of both basic and/or silicic lavas are present not only in the Fishguard region but throughout Wales, in areas such as Snowdonia and Ramsey Island (Fig.…”

The Cape Porcupine Complex, northern mainland Nova Scotia – no longer a geological orphan

“…These felsic rocks have compositions similar to A-type granites and were interpreted to represent on-going extension following on from that represented by the West Barneys River plutonic suite at ca. 469 Ma (Escarraga et al 2012;Murphy et al 2012). The syenitic suite and metarhyolite in the Cape Porcupine Complex appear to extend this period of A-type extensional and bimodal magmatism in this part of Avalonia back to at least 478 Ma.…”