Tectonic implications of 40Ar/ 39Ar hornblende ages from late Proterozoic-Cambrian plutons in the Avalon Composite Terrane, Nova Scotia, Canada

Abstract

The Avalon Composite Terrane in Nova Scotia mainly represents a late Proterozoic, rifted, cratonic, magmatic arc complex. The geo-chemistry of the volcanic rocks indicates that most of southeastern Cape Breton Island is underlain by a volcanic arc, whereas the Antigonish and Cobequid Highlands expose intra-arc rift sequences. In Cape Breton Island, late Proterozoic rocks are both volcanic and plutonic south of the Macintosh Brook fault (transecting the Boisdale Hills), whereas they are mainly plutonic rocks north of the fault. These igneous units are unconformably overlain by subaerial to shallow-marine Cambrian-Ordovician sedimentary sequences containing an Atlantic fauna that are interbedded with Early to Middle Cambrian, within-plate, continental-rift volcanic rocks. 4O Ar/ 39 Ar plateau ages recorded by hornblende from plutons spatially associated with these volcanic rocks correspond to three groupings: (1) ca. 600-635 Ma in Cape Breton Island south of the Macintosh Brook fault; (2) ca. 525-555 Ma in Cape Breton Island north of the Macintosh Brook fault (Boisdale Hills, North Mountain and Creignish Hills); (3) ca. 600-625 Ma in the Antigonish and Cobequid Highlands. Based upon the low metamorphic grade of the late Proterozoic and early Paleozoic rocks, and the low pressures recorded in contact aureoles and plutons, it is inferred that these plutons were emplaced at relatively shallow crustal levels, and therefore the 40 Ar/ 39 Ar plateau ages probably closely date their emplacement. Interpretation of the age data suggests that plutonism in Nova Scotia associated with the rifted magmatic arc lasted from ca. 635-600 Ma. Igneous activity continued between ca. 555-525 Ma and may have been related to the Cambrian, within-plate, rift volcanism.