U–Pb and Rb–Sr geochronology of the Wedgeport granitoid pluton, southwestern Nova Scotia

Abstract

Zircons from biotite monzogranite of the Wedgeport Pluton, intrusive into deformed metasediments of the Cambrian(?) Goldenville Formation in the southwestern Meguma Tenane of Nova Scotia, yield concordant U-Pb ages of 316 + 5 Ma. This is interpreted as the time of intrusion and crystallization. Within the error limits, the 323 f 12 Ma Rb-Sr whole-rock isochron age is identical and gives an initial 87Sr/86Sr ratio of 0.7137 f 0.0056. Rb-Sr analyses of mineral separates of biotite, potassium feldspar, and quartz -plagioclase from several samples yield subparallel, internal isochrons with an average age of 257 f 8 Ma. Initial ratios of the internal isochrons range from 0.716 to 0.759. A slow-cooling model for these latter data is discarded because the mineral data fall on straight lines. Instead, a reheating event related to plutonism ca. 257 Ma ago, which was sufficient to cause local grain-to-grain migration and reequilibration of strontium and rubidium but not large-scale redistribution, is invoked. This reheating is also inferred to be responsible for the hydrothermal alteration and Sn-U mineralization concentrated along the northwestern margin of the pluton. A dextral northeast-southeast shear zone cutting the pluton is also inferred to be ca. 257 Ma old. It may be related to the last stages of westward obduction of the Meguma Terrane.These results provide a clear example of Permo-Carboniferous plutonism in the southwestern Meguma Terrane and suggest a similar interpretation may apply to other anomalously young ages recorded in this area. In light of these results, the Permo-Carboniferous age of the large East Kemptville tin deposit and its location in a dextral shear zone suggest that the association of younger plutonism and shear zones may be a significant factor for economic mineralization.Les zircons d'un monzogranite ii biotite du pluton de Wedgeport, lequel recoupe les m6tas6dim~nts d6formts de la Formation de Goldenville du Cambrien(?), dans la partie sud-ouest du tenane de Meguma, en Nouvelle-Ecosse, fournissent des Lges U -Pb concordants de 316 f 5 Ma. Cette date est considtke comme indiquant le moment de la mise en place de l'intrusion et de la cristallisation. En considkrant les limites d'erreur, on peut affirmer que 1' Age isochrone de 323 f 12 Ma obtenu par la m6thode Rb-Sr sur roche totale est identique, et le rapport initial B7Sr/86Sr est de 0,7137 f 0,0056. Les analyses Rb-Sr de concentks de biotite, feldspath potassique et quartz -plagioclase provenant de plusieurs tchantillons fournissent des courbes isochrones internes subpalalleles avec un Lge moyen de 257 f 8 Ma. Les rapports initiaux de ces isochrones internes varient de 0,716 0,759. Le modtle d'un refroissement lent pour expliquer ces derniers ksultats ne peut &re applique car les d6terminations B partir de ces mintraux tombent sur des lignes droites. I1 faut plut6t faire appel ? i un tv6nement de kchauffement associt au plutonisme apparu il y a environ 257 Ma et suffisant pour produire une migration grain-a-grain et une d6quilibration du...

Early Mesozoic Alkaline Mafic Dykes, Southwestern Nova Scotia, Canada, and Their Bearing on Triassic-Jurassic Magmatism

“…In the Plymouth area of southwestern Nova Scotia (Fig. 2), the mid-Carboniferous Wedgeport Pluton (Cormier et al 1988) and the surrounding Cambrian-Ordovician Goldenville Formation are cut by mafic dykes of lamprophyre and olivine diabase (Cant et al 1978, Chatterjee & Keppie 1981. The dykes are poorly exposed in outcrop, but were sampled in a series of diamond drill-holes put down by Shell Canada Resources.…”

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

“…We also note that, in contrast to the widespread but episodic resetting of 40Ar/39Ar and Rb-Sr mineral ages (Cormier et al 1988;Keppie 1987, 1988;Reynolds et al 1987;Kontak and Cormier 1991) in the western part of the Meguma Terrane at 340-250 Ma (Fig. 6 ) , there is no such disturbance detected in the eastern Meguma Terrane.…”

New U-Pb (zircon) age and geochemistry of the Wedgeport pluton, Meguma terrane, Nova Scotia

“…Its specialized characteristics were recognized in the Nova Scotia "granite boom" of the late 1970s and early 1980s, when the poorly exposed pluton was extensively explored and drilled for Sn, W, and U (Cant et al 1978;Wolfson 1983). Subsequently, the pluton was reported to be signifi cantly younger than the other plutons of the Meguma terrane, with a late Carboniferous U-Pb (zircon) and Rb-Sr whole rock age of 316 ± 5 and 323 ± 12 Ma, respectively (Cormier et al 1988). This age is similar to some 40 Ar/ 39 Ar ages from mica in plutonic and metasedimentary units and shear zones in the area (e.g., Reynolds et al 1981Reynolds et al , 1987Keppie 1987, 1988;Muecke et al 1988;Keppie and Dallmeyer 1995;Culshaw and Reynolds 1997).…”

“…The pluton also yielded younger 40 Ar/ 39 Ar biotite and Rb-Sr mineral ages of ca. 257 Ma (Reynolds et al 1981;Cormier et al 1988), which, together with the inferred crystallization age, were used to suggest that Permian-Carboniferous plutonism was a signifi cant factor for economic mineralization in the Meguma Terrane (Cormier et al 1988). It was suggested more recently that emplacement of the Wedgeport pluton may have been linked to extension related to delamination (Keppie and Dallmeyer 1995) or transtension along shear zones as a result of relative motion of the Gondwanan and Laurentian plates (Pe-Piper and Jansa 1999).…”