THE COMPOSITE NEW RIVER BELT, SOUTHWESTERN NEW BRUNSWICK

The composite New River belt forms a linear, >100 km long northeast-trending belt of Neoproterozoic and Early Paleozoic rocks situated immediately west of the Belleisle Fault in southern New Brunswick. Although the composite nature of the belt was not originally recognized, recent mapping and radiometric studies indicate that Neoproterozoic rocks in the belt can be separated into two major fault-bounded blocks, each containing a distinct Lower Paleozoic cover.

In the central part of the belt, the Robin Hood Lake Fault zone defines the boundary between distinct lithological sequences with contrasting styles of deformation. Southwest of the fault in the Pocologan River area, ductilely deformed and mylonitic, ca. 555 Ma Neoproterozoic granitoid rocks are overlain by Early Paleozoic volcanic and sedimentary rocks that appear to have greater affinity to Ganderian rocks in the St. Croix belt to the west, than to those in the Avalon Terrane in southeastern New Brunswick. In contrast, rocks to the northeast of the fault in the Long Reach area comprise brittlely deformed ca. 620 Ma Neoproterozoic granitoid rocks and relatively undeformed ca. 555 Ma bimodal volcanic rocks overlain by fossiliferous Cambrian strata. Avalonian fauna within the latter, and in a similar sequence exposed in a small fault wedge immediately west of the Belleisle Fault at Beaver Harbour, are directly correlative to those in the Avalon Terrane in the Saint John area.

The composite New River belt is situated between Early Silurian rocks of the Kingston belt and Late Ordovician to Early Devonian rocks of the Mascarene belt. The recent interpretation of the Kingston and Mascarene belts as remnants of a Silurian arc and back-arc sequence, respectively, and the location of the New River belt with respect to these rocks, has led to a model of northwestward subduction beneath a composite Gander Terrane to explain the current spatial relationships. This is in contrast to tectonic interpretations based on evidence from Maine, which invoke an east-dipping subduction zone beneath a composite Avalon Terrane.