Geology and geochemistry of Late Precambrian volcanic and intrusive rocks of southwestern Avalon Zone in Newfoundland

Abstract

The Hermitage Peninsula, at the southwestern extremity of the Avalon Zone in Newfoundland, is underlain by a Late Precambrian conformable succession of volcanic and sedimentary rocks, named the Connaigre Bay Group, which is subdivided into four formations: a lowest formation of corundum-normative and garnet-bearing acidic volcanic rocks (Tickle Point Formation), a sedimentary sequence (Great Island Formation), a formation of sub-alkaline mafic volcanic rocks (Doughball Point Formation) and, at the top, a sequence of red sandstones, conglomerates and shales (Down's Point Formation). Igneous rocks which intrude the Connaigre Bay Group include a hornblende-rich gabroic-granitic intrusion (Hermitage Complex) and a separate granitic intrusion (Straddling Granite). These plutonic rocks are chemically similar to the volcanic rocks, which along with field and petrographic evidence suggests that they are genetically related. The assemblage is classified as a bimodal calc-alkaline suite dominated by amphibole fractionation. The Late Precambrian Hermitage—Connaigre Bay Assemblage, and the presumed correlative Simmons Brook Batholith and the Long Harbour Group about 10 km to the east, are intruded by Upper Devonian—Lower Carboniferous homogeneous granitic plutons, the Pass Island, Harbour Breton and Belleoram stocks. They are readily distinguished from the older calc-alkaline suite both petrographically and chemically, especially by their higher alkalies and associated trace elements. These relatively undeformed rocks of the Avalon Zone are juxtaposed against deformed granitic rocks of the Gander Zone by the Hermitage Bay Fault. The fault is characterized by a 50–100-m-wide zone of brecciation with the main movement having a reverse southeastward component. This movement is post-Ordovician and possibly Devonian, but the Hermitage Bay Fault does not represent the original structure which marked the boundary between the two zones. The Hermitage—Connaigre Bay Assemblage marks the westernmost extremity and perhaps best-exposed area of the vast late Precambrian volcanic-plutonic bimodal sequence recognized throughout the North Atlantic region, from Florida to Newfoundland and in West and North Africa and western Europe. As such it has an important bearing on petrological and tectonic models of this whole ‘Avalon terrain’ in Late Precambrian times.