PERI-GONDWANAN TERRANES OF SOUTHERN NEW BRUNSWICK, CANADA: NEW INSIGHTS AND ENIGMAS

Southern New Brunswick consists of fault-bounded belts of rocks which show both similarities and contrasts in Late Proterozoic - Early Paleozoic evolution. The number of distinct terranes represented by these belts, and their affinity to the classical Avalon and Gander zones of the northern Appalachian orogen, remain controversial, in spite of a large body of accumulated field, petrochemical, and geochronological data. Myriad Carboniferous and younger faults in the area obscure the original nature of boundaries between rock packages, and it is likely that many of the original components are missing. Remnants of high-pressure metamorphic belts provide evidence for the presence of former terrane boundaries. P-T estimates for the Hammondvale metamorphic suite (HMS) on the northwestern margin of Caledonia terrane indicate peak metamorphic conditions at 9.5-12 kbar and 580-420°C. The HMS is interpreted to represent a remnant of an accretionary complex formed in association with ca. 620 Ma subduction in Caledonia terrane, and its presence confirms that the now-adjacent Brookville terrane was not part of the Caledonia (Avalon) terrane at that time. The Pocologan metamorphic suite at the boundary between the Brookville and Kingston terranes had peak P-T conditions of 9.5 kbars and 550°C, probably at ca. 420 Ma. A complex later history is indicated by ⁴⁰Ar/³⁹Ar cooling ages as young as ca. 320 Ma. Modelling of magnetic and gravity data, constrained by magnetic susceptibility and density measurements from surface samples, indicates that present fault boundaries are sub-vertical or dip slightly to the southeast, although more shallow structures indicative of northwesterly thrusting are evident in some areas. The models require geophysically distinct bodies at depth (ca. 6 km or more) that are interpreted to correspond to Avalon, Brookville, and Ganderia basements. Tracing terranes of southern New Brunswick into Maine is complicated by abundant Silurian and Devonian igneous units, and by apparent offset to the southeast to Grand Manan Island, where rock types and ages show elements of Avalon, Brookville, New River, and/or St. Croix terranes, in fault-bounded packages. Correlation with the mainland of New Brunswick or Nova Scotia is not yet resolved.