40Ar/39Ar age of the Jurassic North Mountain Basalt, southwestern Nova Scotia

Abstract

Two whole-rock samples of the Jurassic North Mountain Basalt have been dated using the 40 Ar/ 39 Ar method. A single sample of fresh, medium-grained, holocrystalline basalt from the lower fl ow unit of this thick (i.e., 400 m) sequence gave coincident plateau and isochron correlation ages of 201 ± 2.5 Ma, in agreement with a zircon U-Pb age of 202 ± 1 Ma for this same fl ow unit. This 40 Ar/ 39 Ar age contrasts with earlier conventional K-Ar whole-rock ages of ca. 192 Ma for the North Mountain Basalt, which are similar to other K-Ar and Ar/Ar ages for correlative basalts of eastern North America. The second dated sample is a zeolite-bearing basalt from the middle fl ow unit of the North Mountain Basalt. This sample gave a discordant age spectrum with excess argon, but the isochron correlation age of 206 Ma, albeit with a large error (i.e., 56 Ma), is similar to that for the fresh sample. The data indicate that reliable whole-rock 40 Ar/ 39 Ar ages for the basalts are attainable, but that fresh samples must be used. In light of this, it is suggested that the younger 192 Ma ages may refl ect a widespread thermal event related to zeolite formation. RÉSUMÉ

Remnants of Early Mesozoic basalt of the Central Atlantic Magmatic Province in Cape Breton Island, Nova Scotia, Canada

“…The petrographic features noted above for basalt samples from the Ashfield Formation compare very well with those described for the more extensive North Mountain Formation (Kontak et al 2002;Kontak 2002Kontak , 2008. In particular the plagioclase-and pyroxene-phyric character, the modal percent of phases, and the presence of well-preserved glass in the matrix are distinctive.…”

“…7a) shows that the first part of the spectrum has a much older age and also lower Ca/K ratio than the rest of spectrum, which suggests that the gas was not liberated from the same phase. The latter part of the spectrum, which provides the more robust age data, has a higher Ca/K ratio in keeping with gas being D r a f t liberated from plagioclase and is similar to age spectra from samples of the North Mountain Basalt (e.g., Kontak and Archibald 2003;Jourdan et al 2009).…”

“…As shown in Figure 13b, the slightly higher TiO 2 and the higher (La/Yb) N ratio are characteristic of the Orange Mountain Group, the stratigraphically lowest group of Mesozoic basalts in the eastern North America CAMP basins in the USA (e.g., Kent and Olsen 2000;Cirilli et al 2009;Merle et al 2014;Marzoli et al 2011). Whole-rock and plagioclase 40 Ar/ 39 Ar ages obtained from the North Mountain Basalt range from 197.0 ± 1.0 to 201.5 ± 1.1 Ma (Kontak and Archibald 2003;Jourdan et al 2009), and U-Pb zircon ages between 201.3 ± 0.1 and 201.7 ± 1.4 Ma (Hodych and Dunning 1992;Schoene et al 2006Schoene et al , 2010Blackburn et al 2013). Many of the flows, sills and dykes related to CAMP in eastern North America have similar ages (Fig.…”

Structural, stratigraphic and sedimentological characterisation of a wide rift system: The Triassic rift system of the Central Atlantic Domain

“…Marzoli et al, 1999;Nomade et al, 2007). Dykes and lava flows were dated at 199.8 ± 1.1 ( 40 Ar/ 39 Ar; Jourdan et al, 2009) in the Hartford Basin (USA), at ages ranging from 197.6 ± 1.1 to 199.5 ± 1.8 (Beutel et al, 2005;Hames et al, 2000) in South Carolina (USA), at 202±1 Ma (U/Pb; Hodych & Dunning, 1992), 202± 4 Ma ( 40 Ar/ 39 Ar; Dunn et al, 1998), 201± 2.5 Ma ( 40 Ar/ 39 Ar; Kontak & Archibald, 2003), 199.6 ± 0.6 ( 40 Ar/ 39 Ar; Jourdan et al, 2009), 201.38 ± 0.0.02 Ma (U-Pb; Schoene et al, 2010) in the Fundy Basin, at 199.9±0.5 Ma ( 40 Ar/ 39 Ar; Knight et al, 2004) and 199.1±1 Ma ( 40 Ar/ 39 Ar ; Verati et al, 2007) in Morocco,and in Portugal at 198.1± 0.4 Ma ( 40 Ar/ 39 Ar ; Verati et al, 2007).…”

A Review of Proterozoic to Early Paleozoic Lithotectonic Terranes in New Brunswick, Canada and their Tectonic Evolution during Penobscot, Taconic, Salinic and Acadian Orogenesis

“…3) (Pringle et al 1974;Greenough and Papezik 1987;Wade and Jansa 1994;McHone 2011). Correlative basalt on the south shore of the Bay of Fundy in Nova Scotia yielded 40Ar/39Ar dates of 199 ± 1 Ma and 201 ± 1 Ma, and a U-Pb zircon date of 201.3 ± 0.3 Ma indicative of a Late Triassic (Rhaetian) age (Kontak and Archibald 2003;Schoene et al 2006;Cirilli et al 2009;Jourdan et al 2009). The development of the Fundy rift basin is related to the initial opening stages of the Atlantic Ocean (Nadon and Middleton 1984;Cirilli et al 2009).…”