APPALACHIAN-CYCLE MAGMATISM IN NEWFOUNDLAND: EVOLUTIONARY CHANGES IN SPACE, TIME AND COMPOSITION

Newfoundland geology has influenced models for the evolution of the Appalachian- Caledonian Orogenic Belt for many years, and it continues to do so. Our extensive record of magmatism in both ensimatic and ensialic settings demonstrates that the familiar tectonic zones established by Hank Williams each contain distinct sequences of plutonic rocks.

Cambro-Ordovician plutonism is largely restricted to the Dunnage Zone. In the west, there are both trondhjemites (intraoceanic arcs ?), and calc-alkaline suites (mature or continental arcs ?). In contrast, Ordovician suites in the east record anatexis of the structurally underlying Gander Zone metasediments following Dunnage/Gander juxtaposition. Plutonism expanded during the Silurian, notably to the east, and affected all zones except the Avalon Zone. Its onset in the Humber and Gander Zones is very significant, most likely indicating the closure of the main Iapetus basin. Dunnage Zone Silurian plutonic suites are bimodal, and have an alkali-calcic, "A-type" geochemistry, in sharp contrast to Ordovician suites. This compositional shift may record a transition from compressional to extensional tectonic environments. Compositional commonality across the Dunnage Zone does not support wide separation of the western and eastern parts during most of the Silurian. The Humber and Gander Zones, on opposite sides of the orogen, are both dominated by Silurian peraluminous, "I-type" granite suites (commonly K-feldspar megacrystic), with subordinate "S-type" muscovite-biotite granites. In the east, these form a continuum, and were derived from diverse supracrustal, infracrustal and subcrustal sources. Silurian suites in the east could represent distal arc-related magmas, linked to subduction of oceanic crust originally located between the Gander and Avalon zones, but more primitive arc-type rocks are absent. They could equally represent a late-orogenic, post-closure assemblage, within which spatial compositional variations reflect the nature of lower crustal blocks. In the Devonian, the locus of plutonism shifted eastward, and these post-orogenic granites are most abundant in the Gander - Avalon boundary boundary region. Devonian granites are typically more compositionally evolved than the Silurian suites. Their contrasting isotopic compositions indicate that the Gander-Avalon boundary is a crustal-scale structure.