Late Precambrian-Early Palaeozoic evolution of part of the Avalon terrane in southern New Brunswick, Canada

Abstract

Abstract An important Proterozoic to early Ordovician record of the Avalon terrane is exposed between the Wheaton Brook Fault and the Bay of Fundy in southern New Brunswick, Canada. Ductilely mobilized tonalitic ?basement of possibly Helikian age (Brookville gneiss) a platform carbonate-quartzite cover (Green Head Group) containing ?mid-Riphaean stromatolites. ?Unconformably overlying carbonate slump breccias and turbidites (Martinon Formation) are interpreted to record the collapse of this platform and may be broadly contemporary with minor mafic magmatism and a mid-Hadrynian ( c. 800 Ma) tectonothermal event apparently recorded in the Brookville gneiss. However, the association may equally reflect the onset of late Hadrynian ( c. 625–550 Ma) subduction recorded in calc-alkaline granitoid plutonism (Golden Grove suite) and associated arc-related volcanism (Coldbrook Group). The broadly coeval? emplacement of bimodal sheeted dykes (Kingston complex) appears to have occurred under conditions of sinistral transtension suggesting subduction was oblique. Mafic dykes are mainly continental tholeiites and the complex is interpreted to represent the floor of a volcanic arc rift. Major ductile shear zones of dextral sense, which border the dyke complex, may record late Precambrian rift closure but were active in the mid-Palaeozoic. Post-tectonic ‘Eocambrian’ volcanogenic redbeds and continental rift volcanics pass disconformably into Cambro-Ordovician sandstones and shales of the Saint John Group which contain shallow-marine Acado-Baltic fauna of the Iapetus cycle and represent an overstep sequence to Avalon’s Precambrian evolution. Termination of subduction and the tectonic transition to Cambro-Ordovician platformal conditions lacks evidence of a major collisional event. Instead, they coincide with the break-up of a late Precambrian supercontinent that was responsible for the inception of the Iapetus cycle and are likely to reflect the major plate reorganizations that would accompany such an event.