UNRAVELLING TERRANES IN SOUTHERN NEW BRUNSWICK, CANADA: THE SAGA CONTINUES

Mapping, U-Pb dating, and petrological studies continue to provide enhanced information about the geology of southern New Brunswick but have yet to resolve the original relationships among the various fault-bounded belts of rocks that comprise the area. The Caledonia terrane consists of mainly Neoproterozoic ca. 620 Ma and 560-550 Ma volcanic, sedimentary, and plutonic rocks overlain by fossiliferous Cambrian - Ordovician sedimentary rocks considered typical of the Avalon terrane. Northwest of the Caledonia terrane, the Brookville and Kingston terranes display evidence for arc magmatism of Cambrian and Silurian age, respectively. Between the Caledonia-Brookville and Brookville-Kingston terranes are remnants of high-P/low-T metamorphic rocks related to collisional tectonics. The next terrane to the northwest, the New River terrane, appears composite, with some rocks of typical Avalonian age and character but also younger units that may be more closely linked to Ganderian terranes of the “central mobile belt” of the northern Appalachian orogen. Current studies in the New River terrane are focusing on defining the distribution and relationships among ca. 630-620 Ma plutonic units, voluminous ca. 555 Ma volcanic and plutonic units, recently dated ca. 540 Ma volcanic units, and younger Cambrian and Ordovician volcanic and sedimentary units. Tracing the terranes of southern New Brunswick to the southwest across the international border into Maine is complicated by the abundance of Silurian and younger cover sequences and gabbroic to granitic plutons.

Offshore on Grand Manan Island, new U-Pb (zircon) ages suggest that most of the pre-Mesozoic rocks, like those on the mainland, are of late Neoproterozoic to Cambrian age, between ca. 618 Ma and 535 Ma. However, the rocks differ in detail from any one of the mainland terranes, with similarities to components of the Caledonia terrane (ca. 620-611 Ma tuffaceous and granitic rocks), Brookville terrane (isolated occurrences of marble and quartzite and ca. 540-535 Ma plutonic rocks), and New River terrane (quartzite-pebble conglomerate and ca. 540 Ma volcanic and sedimentary rocks). No rocks with affinities to the Meguma terrane of Nova Scotia have been recognized, and aeromagnetic data suggest the presence of major faults both north and east of Grand Manan Island.