Petrological and geochemical characteristics of the Bolton Igneous Group, Southern Québec Appalachians

Abstract

This study presents new geochemical data for the rocks of the Bolton Igneous Group in the Appalachians. These rocks are pre-Taconian basaltic pillow lavas with comagmatic diabases and gabbros forming a north-south trending chain of small massifs on the west side of Memphremagog Lake, in the Dunnage Zone. Their REE patterns are flat to enriched in light REE with La/Yb = 1.5 to 4.2, whereas the MORB-normalized incompatible element plots are enriched in LILE with Hf/La = 0.2 to 0.6. Their compositions resemble transitional mid-ocean ridge basalts. This study also subdivides mafic volcanic rocks of the Dunnage Zone of the Appalachians in southern Québec into three groups according to their TiO2 contents. The first group, mainly found in the Thetford Mines Ophiolitic Complex, has low TiO2 (<0.5 wt percent) and displays boninitic affinities. The second group of medium TiO2 content (0.5-2.0 wt percent) is represented by tholeiitic mafic volcanic rocks of the Bolton Igneous Group. The third, High TiO2 group (>2.0 percent) occurs in the Orford Ophiolitic Complex. These data suggest that rocks of the Bolton Igneous Group were generated in an ocean ridge environment of the Iapetus Ocean and occur as tectonic slices in an accretionary prism, represented in the Québec Appalachians by the St-Daniel Mélange. A similar setting has been invoked for the Stowe Formation volcanics in Vermont.