Petrology and tectonic implications of Silurian(?) metavolcanic rocks in the Clyburn Brook area and on Ingonish Island, northeastern Cape Breton Island, Nova Scotia
Abstract
Metavolcanic rocks in the Clyburn Brook area of northeastern Cape Breton Island range in composition from mafic to felsic, include flows and tuffs, and are interlayered with less abundant semi-pelitic metasedimentaiy rocks. In contrast, metavolcanic rocks on nearby Ingonish Island are mainly rhyolitic flows and tuffs, and lack interlayered metasedimentary rocks. Because correlation between these two areas cannot be confirmed without more radiometric dating, continued use of separate names (Clyburn Brook formation and Ingonish Island rhyolite) is advocated. Rocks in the Clyburn Brook formation contain greenschist facies mineral assemblages and are mainly well foliated. Although also metamorphosed, the Ingonish Island rhyolite is mainly unfoliated, and the metamorphism may be a result of contact metamorphism. A contact metamorphic overprint is also present in the Clyburn Brook formation adjacent to the ca. 402 Ma Cameron Brook Granodiorite, and felsic dykes perhaps related to the intrusion occur in both the Clyburn Brook formation and the Ingonish Island rhyolite. These observations suggest that both units are older than ca. 402 Ma, consistent with a previously published imprecise Rb-Sr date of 412 ±15 Ma for the Ingonish Island rhyolite. Petrochemical characteristics of mafic rocks in the Clyburn Brook formation indicate tholeiitic affinity and origin in an arc-related setting, possibly a back-arc. Although they show chemical differences, felsic rocks in both the Clyburn Brook formation and on Ingonish Island have compositions consistent with an extensional setting, possibly within a continental arc or back-arc such as the modern-day Taupo volcanic zone in New Zealand. Overall, the Clyburn Brook formation shows lithological and chemical similarity to the Money Point Group, a Silurian (ca. 430 Ma) volcanic arc/back-arc unit in the Aspy terrane of Cape Breton Island, and is inferred to be of similar age.
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