40Ar/39Ar hornblende ages from southwestern Maine: evidence for Late Paleozoic metamorphism

Abstract

40Ar/39Ar analyses of hornblendes from a 120 km long NE-trending transect within the high-grade rocks of the Casco Bay Group provide information regarding the timing of thermal events and subsequent cooling history of this region. In the north, plateau ages of 368-372 Ma record the time of cooling through argon closure temperatures (500°C) following Acadian metamorphism. In the central portion of the transect, hornblendes display younger ages (350-323 Ma) and release spectra which show evidence of a Late Paleozoic thermal overprint. To the south, significantly lower plateau ages are recorded (270-290 Ma). These lower ages date either the time of a Late Paleozoic thermal event or the time of cooling following a prolonged period of burial at deep crustal levels. The former interpretation is favored on the basis of the disturbed release spectra in the central portion of the transect and the fact that ages over the 60 km long southern portion of the transect are relatively constant at 280+10 Ma. If a Late Paleozoic thermal event occurred in this region it was evidently not accompanied by large amounts of deformation as structural evidence for such an event is lacking. The exact nature of the Late Paleozoic resetting of hornblendes in this portion of the Casco Bay Group is not clear; however, it may be related to the events which lead to granite emplacement 275 Ma ago in the Massabesic Gneiss Complex and the 272-282 Ma ages for monazites from the nearby Sebago Batholith. Des analyses 40Ar/39Ar, effectuees sur des hornblendes prelevees sur une traverse NE de 120 km de longueur au sein des roches metamorphiques de haut degre du Groupe de Casco Bay, ont foumi des donnees sur Page des episodes thermiques et l'histoire du refroidissement qui leur succeda dans ce Secteur. Au nord, des ages-plateaux de 368 a 372 Ma datent le refroidissement via les temperatures de fermeture de P argon (500°C) suivant le metamorphisme acadien. Dans la portion centrale dela traverse, les hornblendes exhibentdes ages pi us jeunes (350 a 325 Ma) etdes spectres de diffusion qui portent la trace d 'une surimpression thermique tardi-paleozoi'que. Vers le sud, on enregistre des ages-plateaux bien plus jeunes (270 a 290 Ma). Ces demiers datent soit un episode thermique tardi-paleozoi'que, soit un refroidissement apres un enfouissement prolonge profondement dans la croute. On favorise la premiere interpretation a cause des spectres de diffusion deranges que montre la portion centrale de la traverse et parce que les ages obtenus au long des 60 km de la portion sud de la traverse sont relativement constants a 280±10 Ma. Si cette region a subi un episode thermique tardi-paleozoi'que, ce ne fut assurement pas de pair avec une deformation importante puisque la structure ne foumit aucun indice d 'un tel evenement. La nature exacte de la remise a zero tardi-paleozoi'que des hornblendes dans cette portion du Groupe de Casco B ay n 'estpas apparente. On pourrait cependant larelier aux evenements a l'origine de l'emplacement de granite dans le Complexe de Gneis...

Dextral transpression at the Casco Bay restraining bend, Norumbega fault zone, coastal Maine

“…4B) suggest a phase of localized uplift related to this transpression. Evidence for structural thickening and associated uplift can be found in the younger hornblende ages for the northwest side of the fault (West et al, 1989), which suggests burial by oblique thrusting. Additional support for localized uplift in the Casco Bay area is provided by the pattern of current indicators in the late Paleozoic (310-280 Ma) basins to the northeast.…”

“…The Saco-Harpswell sequence southeast of the Flying Point fault shows older hornblende ages of 355-324 Ma. The younger high-temperature hornblende ages in the Falmouth-Brunswick sequence northwest of the Flying Point fault were interpreted (West et al, 1989) to reflect a late Paleozoic thermal event, but the relationship to regional deformation at that time was unclear. The rocks northwest of the Flying Point fault show evidence for strong internal shearing as a phase of oblique dextral thrusting and might be linked to this suggested thermal event.…”

Paleozoic orogens in New England, USA

“…A program of 40 Ar-39 Ar dating in south-central Maine by West et al (1988), has shown that hornblende gives plateau ages of 368 to 372 Ma in the northeast, decreasing in age across 40 km toward the southwest to 323 Ma. They interpreted this pattern to reflect slow cooling in response to differential unroofing following Acadian metamorphism, in which more deeply buried rocks to the south cooled later.…”

Characterizing the Southwestern Extent of the Norumbega Fault System, a Mid-Paleozoic Crustal-Scale Strike-Slip Fault System in the New England Appalachians

“…The start of dextral deformation and the initiation of the regionally extensive NFS is suggested to be syn-to post-metamorphism, based on microstructural studies that indicate amphibolite facies mineral growth during dextral deformation (West and Lux, 1993;Price et al, 2010). Dextral shear along the fault system has been interpreted to have initiated in the Middle Devonian by 380 Ma based on 40 Ar/ 39 Ar hornblende data (West et al, 1988;Ludman and West, 1999) and U-(Th)-Pb zircon and monazite ages (Gerbi and West, 2007). A wider zone of dextral shear developed during the Late Devonian to Early Carboniferous (West and Hubbard, 1997).…”

“…Outcrops of the NFS in ME were examined at three primary locations (see below). Each outcrop location has been defined as the Norumbega fault system by others (West et al, 1988(West et al, , 1995(West et al, , 1999Price et al, 2010), and were examined in this study to document key characteristics of the NFS.…”