⁴⁰Ar/³⁹Ar dating of metamorphic and igneous rocks of the Liscomb Complex, Meguma Terrane, southern Nova Scotia, Canada

Abstract

Recent geological mapping in the Meguma Terrane of southern Nova Scotia has revealed the presence of previously unrecognized high-grade gneisses and gabbroic rocks that form, along with previously recognized granitic rocks, the Liscomb Complex. Field relationships indicate emplacement of the suites, in the sequence gneisses, gabbros, and granites, after the ca. 400 Ma Acadian deformation of the Meguma Group metasedimentary rocks. 40Ar/39Ar dating of amphibole and mica phases from these suites indicates (I) an amphibole inverse isochron age of 375 f 3 Ma for the gneisses, and amphibole plateau ages of 371 f 4 and 372 + 5 Ma for the gabbros; (2) muscovite plateau ages of 367 f 3 Ma for the gneisses, and 369 f 3 to 373 f 4 Ma for the granites; and (3) variably discordant biotite age spectra with integrated ages sometimes greater than the amphibole and muscovite plateau ages (371 -383 Ma (n = 5) gneisses; 373-385 Ma (n = 3) gabbros; 367 and 368 Ma in granites). The spectral discordance in the biotites is attributed to irradiation-induced recoil in the presence of submicroscopic chlorite, whereas the high integrated ages reflect excess argon.Collectively, the age data indicate rapid cooling of the Liscomb Complex through the argon retention temperatures of amphibole (approx. 500°C) and micas (approx. 350-300°C), consistent with previous 40Ar/39Ar dating of amphibole and micas in this area. This cooling history is attributed to rapid uplift and exhumation of the area postemplacement of the units because of its close proximity (i.e., along its northern boundary) to the Cobequid-Chedabucto Fault System. Les travaux rtcents de cartographie geologique dans le terrane de Meguma, du sud de la ~ouvelle-~cosse, ont rtvtlt l'existence des gneiss de degrt tlevt inconnus auparavant, et aussi la prtsence des roches gabbroi'ques formant avec les roches granitiques dtj8 connues, le Complexe de Liscomb. Les relations sur le terrain indiquent que la mise en place des suites s'est faite selon la skquence suivante : gneiss, gabbros et granites, et qu'elle est posttrieure a la dtformation acadienne des roches mCtastdimentaires du Groupe de Meguma datBe de 400 Ma. La datation 40Ar/39Ar sur amphibole et micas de ces suites indique (1) un lge isochrone inverst de 375 f 3 Ma sur l'amphibole des gneiss et des lges plateaux de 371 f 4 et 372 f 5 Ma sur l'amphibole des gabbros; (2) des lges plateaux sur muscovite de 367 + 3 Ma pour les gneiss et de 369 f 3 8 373 f 4 Ma pour les granites; et (3) un spectre d'lges discordants irrkguliers sur biotite d'avec les lges inttgrts qui, parfois, sont plus grands que les lges plateaux sur amphibole et muscovite (371 -383 Ma (n = 5) pour les gneiss; 373-385 Ma (n = 3) pour les gabbros; 367 et 368 Ma pour les granites). La discordance spectrale observk pour les biotites est attribute 2 un recul cr& par une irradiation en prtsence de chlorite submicroscopique, alors que les Lges inttgrts reflktent un surplus d'argon.Les domtes chronologiques, prises globalement, suggbrent un refroidissement rapide du Complex...

Geophysical and structural signatures of syntectonic batholith construction: the South Mountain Batholith, Meguma Terrane, Nova Scotia

“…The reflective layer is interpreted to be the structural basement to the Meguma Terrane (Keen et al . 1991), and could correspond to sillimanite‐grade gneisses and schists of the Liscomb Complex (Clarke Chatterjee & Giles 1993; Kontak & Reynolds 1994) that are exposed in a tectonic window south of the Cobequid–Chedabucto fault zone.…”

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

“…These turbidites, which constitute the Meguma Group are overlain by 2.3-4.5 km of middle Ordovician to early Devonian subaerial to shallow marine, bimodal within-plate volcanic rocks, and sediments containing a Rhenish-Bohemian fauna (Boucot, 1960;Keppie & Dostal, 1980). The Liscomb Complex, exposed in a gneiss dome, was intruded into the Meguma Group during the late Devonian (c. 372 Ma) and has been inferred to represent remobilized basement of the Meguma Terrane (Kontak et al 1990;Clarke, Chatterjee & Giles, 1993). Granulite facies metasedimentary and meta-igneous xenoliths in late Devonian lamprophyre dykes in the southern Meguma Terrane may also represent samples of the basement (Eberz et al 1991).…”

Field relations and petrology of the Trafalgar Plutonic Suite and comparisons with other Devonian granitoid plutons in the Meguma terrane, Nova Scotia, Canada

“…A range of 40 Ar/ 39 Ar cooling ages has been reported for amphibole, biotite, and muscovite from several of the plutons in the Trafalgar Plutonic Suite (Kontak and Reynolds 1994). These ages indicate the time of cooling of the plutons through the closure temperatures for these minerals (~600°C, ~ 300°C and ~400°C, respectively; Winter 2001), and are summarized on Figure 3.…”