The predominantly volcanic Harbour Main Group of Proterozoic (Hadrynian) age forms a northerly-trending belt in the central part of the Avalon Peninsula, Newfoundland. The rocks include flows, pyroclastics, and several belts of well-preserved ignimbrites.Twenty-two recent chemical analyses of volcanic rocks, sixteen of them not previously published, show that the Harbour Main volcanic suite is distinctly sodic and weakly alkalic, ranging from basic rocks of the hawaiite–mugearite type to sodic rhyolites with relatively high alkali content; the alkali-lime index of the suite is 52.The Harbour Main rocks are shown to be chemically similar to the Late Precambrian Uriconian volcanic suite in Central Britain, with which it has been previously tentatively correlated on other grounds. It is suggested that the volcanic rocks of the Avalon Peninsula and the British Midlands were formed in different parts of a possibly continuous tectonic belt in a period of post-orogenic block-faulting of the Basin and Range type, prior to the opening of the present Atlantic Ocean.

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