The stratigraphy and geochemistry of late Devonian to early Carboniferous volcanic rocks of the northern Chignecto peninsula, Cobequid Highlands, Nova Scotia

Abstract

The volcanic succession of the Fountain Lake Group of the northern Chignecto peninsula consists of basalt and andesite flows, rhyolite shallow intrusions and minor flows, and rhyolitic pyroclastic rocks. Fault scarp breccia, fluvial conglomerate and sandstone, and lacustrine siltstone and minor limestone are interbedded with the volcanic rocks. The succession is cut by mafic dykes, some of which predate and others postdating brittle north-vergent thrusting. Detailed geochemical and petrographic study shows that the volcanic rocks geochemically resemble the plutonic rocks of the Cape Chignecto pluton that are thrust over the volcanic succession. Diabase dykes may record the contamination of gabbroic magma by residual base-of-crust plagioclase-pyroxene granulite from which felsic magma had been extracted. Fractionation of this mafic magma resulted in enrichment in incompatible elements in some dykes. Andesites evolved from basaltic parents either by fractionation or by melting of hydrous gabbros. Much of the volcanic succession shows low-temperature alteration including silicification.