Abstract

There are two phases of post-Taconic, pre-Acadian magmatism in the eastern Gaspé Peninsula of Québec, (A) the Middle to Upper Ordovician Pabos Suite and (B) widespread Silurian-Devonian volcanics and hypabyssal intrusions. The Pabos Suite forms two large intrusions composed of LREE-depleted, N-MORB-like gabbros and ferrogabbros. Evolution within this suite was controlled by fractional crystallization, and the presence of the intrusions implies a tensional event between the Taconic and Acadian orogenies. The Silurian-Devonian volcanic and intrusive rocks are chemically variable (tholeiitic to alkaline, basaltic to rhyolitic) and do not belong to a single fractionation series. Quartz xeno-cryst-bearing rocks are enriched in Si, K, Zr, and LREE's, providing evidence of disequilibrium contamination by a felsic melt similar to locally occurring rhyolites. The lavas and dikes that do not carry felsic xenocrysts also show patterns of enrichment/depletion of trace elements, which suggest that felsic contamination/hybridization was an important factor in their development, but show little Si enrichment. This is interpreted to be due to maintenance of chemical equilibrium during contamination. This would have forced the composition of the magma to remain on the plagioclase + clinopyroxene + olivine cotectic and would thus have inhibited Si enrichment. The xenocryst-bearing rocks superficially resemble a calc-alkaline series, but most of the pre-Acadian Silurian-Devonian volcanics of the Gaspé Peninsula do not appear to have been directly associated with subduction. The preferred model is that of local tensional environments in an orogenic foreland.

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