Structural and ⁴⁰Ar/³⁹Ar mineral age constraints for the tectonothermal evolution of the Green Head Group and Brookville Gneiss, southern New Brunswick, Canada: Implications for the configuration of the Avalon composite terrane

Abstract

In the late Precambrian Avalon composite terrane of the Canadian Appalachians, the local juxtaposition of Avalonian successions against gneiss complex-platformal metasedimentary rock associations of uncertain relationship to the Avalonian overstep sequence has raised importad i u d o n s about the configuration of the composite terrane. Typical of this relationship is the juxtapostion of Avalonia:. arc-related packages (Caledonia assemblage) with the migmatitic Brookville Gneiss and metacarbonate-quartzite Green Head Group (Brookville assemblage) along the Caledonia Fault in southern New Brunswick. Polyphase deformation of the predominantly greenschist facies Green Head Group accompanied development of a regional ductile shear zone that separates the group from the amphibolite facies Brookville Gneiss. Heterogeneous ductile flow in carbonate rocks and the development of a regional foliation was followed by NW-directed shortening and the local development of a penetrative axial planar fabric that intensifies towards the shear zone. Associated structural elements suggest regional dextral transpression, consistent with the metamorphic contrast across the shear zone. Steeply plunging east-west folds may record younger, sinistral movement on associated NE-SW faults. Deformation coincident with metamorphic culmination in the Brookville Gneiss produced a gneissic foliation that was later deformed to produce widespread minor folds of sheath-like geometry. These folds are best developed proximal to the shear zone where they locally document dextral shear, and probably include several generations that overlap early phases of deformation of the Green Head Group. Kinematic indicators within the gneiss are predominantly dextral. '6Ar?oAr versus "Ar/JOAr isotope-correlation ages recorded by metamorphic hornblende suggest that regional cooling of the Brookville Gneiss through ca. 500°C occurred a t ca. 540 Ma. providing a minimum age for initial deformation and concomitant metamorphic culmination in the gneiss. aAr/J9Ar plateau ages for metamorphic muscovite suggest cooling through ca. 375°C at ca. 500-520 IMa, providing a minimum age for progressive deformation in both lithotectonic sequences. Low temperature age discordance in the muscovite spectra suggest partial rejuvenation in the mid-and late Palaeozoic.Protracted Cambrian tectonothermal activity in the Brookville assemblage contrasts with the Avalonian tectonostratigraphic record of the Caledonia assemblage in which late Precambrian arc-related packages are overstepped by Cambrian-Ordovician shallow marine strata. Significant Cambrian separation between the two assemblages is therefore suggested, despite Precambrian similarities in their tectonothermal evolution. Separation as a consequence of terrane dispersal is suggested. and may imply a significant rearrangement of the Avalon composite terrane at this time. Final juxtaposition of the two assemblages pre-dates their shared late Palaeozoic rejuvenation, and may correspond to an earlier, mid-Palaeozoic thermal overprint...

A comparative analysis of pre-Silurian crustal building blocks of the northern and the southern Appalachian orogen

“…In the context of comparing peri-Gondwanan realm crustal blocks, the most significant observation concerning Ganderia is that arc activity continued into the Paleozoic (for example, Barr and others, 2003;Rogers and others, 2006). Late Neoproterozoic-earliest Paleozoic tectonism of these arc rocks has been documented throughout Ganderia (for example, Nance and Dallmeyer, 1994;White and Barr, 1996) although the cause of tectonism is ambiguous. Ganderian arc rocks are succeeded by the hallmark of the domain, a Middle Cambrian-Early Ordovician passive margin sequence of quartzose and feldspathic clastic rocks (for example, van Staal and others, 1996).…”

Constraints on the age of Archaeozoon acadiense and evidence for hydrothermally transported zircon in the Ashburn Formation (Green Head Group), Saint John, New Brunswick, Canada

“…In the classic work of Williams (1978Williams ( , 1979, the areas now known from southeast to northwest as Caledonia, Brookville, and Kingston belts (Fig. 1) were included in the Avalon Zone, and its characteristic units were considered as an integral part of the Avalonian stratigraphic sequence (e.g., Nance 1987;Murphy and Nance 1989;Nance and Dallmeyer 1994). However, differences in stratigraphic, magmatic, metamorphic, and tectonic history compared to Avalonia (e.g., Barr and Raeside 1989;Raeside and Barr 1990; Barr and White 1996; White and Barr D r a f t 1996) led to the subsequent inclusion of both the Brookville and Kingston terranes in Ganderia (e.g., Hibbard et al 2006;van Staal et al 202a, b and references therein).…”