NEOACADIAN PLUTONISM IN THE NEWFOUNDLAND APPALACHIANS: A POORLY CONSTRAINED BUT ECONOMICALLY IMPORTANT MAGMATIC EVENT

The record of Appalachian-cycle plutonism in Newfoundland extends from the Cambrian to the late Devonian, and most Ordovician and Silurian plutons are now dated accurately, for they constrain deformational and/or accretionary events. However, there is far less incentive to obtain precise crystallization ages on post-tectonic suites of Middle Devonian and younger age. Thus, our understanding of the temporal and spatial distribution of such magmatism is inadequate. As currently understood, "NeoAcadian" plutonism in Newfoundland is largely confined to an arcuate belt within the eastern Gander Zone and adjacent Avalon Zone. It appears to be spatially associated with this boundary, although it postdates juxtaposition of the two zones. Compared to extensive Silurian and early Devonian magmatism in the Dunnage and Gander zones, this late plutonism is spatially displaced towards the southeast, i.e., outboard with respect to Laurentia. The compositional spectrum of late plutonism is bimodal but felsic-dominated. Granites are typically silica-rich, strongly potassic, and metaluminous to variably peralkaline in character, showing strong regional enrichments in U, Th and F. The Nd isotopic signatures of late granites provide a striking illustration of the fundamental (crustal-scale) importance of the Gander-Avalon boundary, which here demarcates discrete lower crustal blocks.

This magmatism is of more than academic interest, for exploration activity in these suites has grown with rising commodity prices. Late, evolved granites are associated with potentially economic deposits of molybdenum, tungsten and fluorite, and several plutons are now under active exploration for uranium. Hopefully, this industry interest will help to develop a better geological framework, and will lead ultimately to an improved understanding of this important magmatic episode.