Palaeozoic within-plate volcanic rocks in Nova Scotia (Canada) reinterpreted: isotopic constraints on magmatic source and palaeocontinental reconstructions

Abstract

Palaeozoic volcanism in the Avalon Terrane of northern Nova Scotia occurred during three time intervals: Cambrian-early Ordovician, late Ordovician-early Silurian and middle-late Devonian. In the Meguma Terrane of southern Nova Scotia, Palaeozoic volcanism is limited to the middle Ordovician. Geochemical data show that most of these volcanic rocks are bimodal, within-plate suites. Initial ε Nd signatures range from +5.4 to -1.9 in the rhyolites and +6.8 to +2.7 in the basalts, a difference attributable to the absence or presence, respectively, of a significant crustal component. The data and regional tectonic settings of the Avalon and Meguma terranes suggest that the volcanism was generated in three different within-plate settings: (1) Cambrian-early Ordovician volcanism related to thermal decay of late Proterozoic arc magmatism during transtensional deformation; (2) middle Ordovician-early Silurian volcanism during sinistral telescoping between Laurentia and Gondwana where extensional bends in the Appalachians produced rifting; and (3) Devonian volcanism resulting from lithospheric delamination during dextral transpression and telescoping. In each setting, active faults served as conduits for the magmas. Nd isotopic data indicate that the source of the Palaeozoic felsic volcanic rocks is isotopically indistinguishable beneath southern and northern Nova Scotia and did not substantially change with time. This crustal source appears to have separated from the mantle during the Proterozoic, a conclusion consistent with the hypothesis that the Palaeozoic rocks in Nova Scotia were deposited upon a late Proterozoic oceanic-cratonic volcanic arc terrane. The Nd data, when combined with published faunal, palaeomagnetic and U-Pb isotopic data, suggest that the Avalon Terrane was peripheral to Gondwana off northwestern South America during Neoproterozoic and early Palaeozoic times. 426 J. D. K E P P I E A N D OT H E R S

Tectonomagmatic origin of some volcanic and sub-volcanic rocks from the Lower Benue rift, Nigeria

“…This alkaline nature is also corroborated by their light REE enrichment (Fig. 8-c) as reflected in their generally high fractionation indices [(La/Yb) N ], 7.06 to 17.65 for the basaltic rocks, and 23.59 to 135.35 for the trachytic rocks, as opposed to low values common in subalkaline (tholeiitic) series (see Keppie et al, 1997;Sprung et al 2007). Also, the strong enrichment in the incompatible elements is an indication of their alkaline affinity (Fitton and Dunlop, 1985).…”

“…Both enrichment in Th and depletion in Nb indicate crustal contamination (Murphy et al, 1985;Rollinson, 1993;Keppie et al, 1997). The La/Nb, Th/La and Ba/La ratios are also considered useful for identifying crustal contamination of magmas (Thompson et al, 1984).…”

Neoproterozoic?Early Paleozoic evolution of peri-Gondwanan terranes: implications for Laurentia-Gondwana connections

“…These igneous rocks have elemental Sm/Nd ratios of ca. 0.19, which is typical of intracrustal melts (Allgre and Ben Othman 1980) and show similar initial Nd values that range between -2.5 and +5.0 (Thorogood 1990; Barr and Hegner 1992;Whalen et al 1994;Kerr et al 1995;Murphy et al 1996a;Keppie et al 1997;Murphy et al 2000;Samson et al 2000). Extrapolated to the depleted mantle curve, Nd growth lines consistently yield overlapping model ages (T DM ) of 0.8-1.1 Ga in Atlantic Canada and 1.0-1.3 Ga in southern Britain (Thorogood 1990;Murphy et al 2000).…”

Stratigraphy, provenance and tectonic setting of the Lumsden Dam and Bluestone Quarry formations (Lower Ordovician), Halifax Group, Nova Scotia, Canada

“…These data were interpreted by Greenough et al (1999) to suggest that the Meguma succession rests upon a basement with Avalonian affinities. It has been proposed that the West Avalonia and the Meguma terrane once formed a part of the same microcontinent and that the Meguma succession was deposited on Avalonian crust (e.g., Keppie et al 1997;Landing 2004;Murphy et al 2004a;Linnemann et al 2012). Others interpret the contact between the two terranes as a thrust (e.g., Eberz et al 1991;Keppie and Dallmeyer 1987;Waldron et al 1989;Greenough et al 1999) along which the Meguma terrane was emplaced over Avalonia.…”