The Colliers Peninsula, a typical area in the western part of the Harbour Main Group in eastern Newfoundland, is underlain by three steeply-dipping sequences of ash-flow tuffs 250–500 m thick, separated by clastic sediments, conformably overlain by subaerial mafic flows and cut by small bodies of high-level intrusive rocks rich in plagioclase phenocrysts. All rocks are of low metamorphic grade.The two lower pyroclastic sequences have a bulk composition of rhyolite and alkali rhyolite; they contain phenocrysts of albite, quartz, and biotite. Na/K ratios are highly variable; the albite crystals may be pseudomorphous after anorthociase. The upper sequence, rhyodacitic in bulk composition, has lower Si and higher Na; it lacks quartz and biotite phenocrysts but contains small amounts of calcic augite and altered olivine. The mafic flows (50% SiO2) show high Al and alkalies and low Ca and Ti. The porphyritic intrusives have 60–65% SiO2, but their bulk chemistry is otherwise similar to that of the ash-flow tuffs.The present composition of the rocks reflects complex chemical adjustments, both contemporaneous with and subsequent to the magmatic event. However, mineralogical and chemical data and trends considered to be a part of the original magmatic heritage are consistent with suggestions that the western (probably lower) part of the Harbour Main volcanic pile was formed in a continental, extensional tectonic environment.

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