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 (1) an amphibole inverse isochron age of 375 ± 3 Ma for the gneisses, and amphibole plateau ages of 371 ± 4 and 372 ± 5 Ma for the gabbros; (2) muscovite plateau ages of 367 ± 3 Ma for the gneisses, and 369 ± 3 to 373 ± 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 sub-microscopic 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.

You do not currently have access to this article.