STRUCTURAL AND AR-40 AR-39 MINERAL AGE CONSTRAINTS FOR THE TECTONOTHERMAL EVOLUTION OF THE GREEN-HEAD GROUP AND BROOKVILLE GNEISS, SOUTHERNNEW-BRUNSWICK, CANADA - IMPLICATIONS FOR THE CONFIGURATION OF THE AVALON COMPOSITE TERRANE/

In the late Precambrian Avalon composite terrane of the Canadian Appal achians, the local juxtaposition of Avalonian successions against gnei ss complex-platformal metasedimentary rock associations of uncertain r elationship to the Avalonian overstep sequence has raised important qu estions about the configuration of the composite terrane. Typical of t his relationship is the juxtapostion of Avalonian are-related packages (Caledonia assemblage) with the migmatitic Brookville Gneiss and meta carbonate-quartzite Green Head Group (Brookville assemblage) along the Caledonia Fault in southern New Brunswick. Polyphase deformation of t he predominantly greenschist facies Green Head Group accompanied devel opment of a regional ductile shear zone that separates the group from the amphibolite facies Brookville Gneiss. Heterogeneous ductile flow i n carbonate rocks and the development of a regional foliation was foll owed by NW-direted shortening and the local development of a penetrati ve axial planar fabric that intensifies towards the shear zone. Associ ated structural elements suggest regional dextral transpression, consi stent with the metamorphic contrast across the shear zone. Steeply plu nging east-west folds may record younger, sinistral movement on associ ated 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. T hese folds are best developed proximal to the shear zone when they loc ally document dextral shear, and probably include several generations that overlap early phases of deformation of the Green Head Group. Kine matic indicators within the gneiss are predominantly dextral. Ar-36/Ar -40 versus Ar-39/Ar-40 isotope-correlation ages recorded by metamorphi c hornblende suggest that regional cooling of the Brookville Gneiss th rough ca. 500 degrees C occurred at ca. 540 Ma, providing a minimum ag e for initial deformation and concomitant metamorphic culmination in t he gneiss. 40Ar/Ar-39 plateau ages for metamorphic muscovite suggest c ooling through ca. 375 degrees C at ca. 500-520 Ma, providing a minimu m 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 tect onothermal activity in the Brookville assemblage contrasts with the Av alonian tectonostratigraphic record of the Caledonia assemblage in whi ch late Precambrian are-related packages are overstepped by Cambrian-O rdovician shallow marine strata. Significant Cambrian separation betwe en the two assemblages is therefore suggested, despite Precambrian sim ilarities in their tectonothermal evolution. Separation as a consequen ce of terrane dispersal is suggested, and may imply a significant rear rangement of the Avalon composite terrane at this time. Final juxtapos ition of the two assemblages pre-dates their shared late Palaeozoic re juvenation, and may correspond to an earlier, mid-Palaeozoic thermal o verprint correlated with tectonothermal activity accompanying accretio n of the Avalon and outboard Meguma terranes to more inboard tectonic elements of the northern Appalachians.