Origin and U–Pb geochronology of amphibolite-facies metamorphic rocks, Miramichi Highlands, New Brunswick

Abstract

The Miramichi Highlands of New Brunswick are underlain by subgreenschist- to greenschist-facies sedimentary and volcanic rocks of the Cambro-Ordovician Tetagouche Group and by amphibolite-facies paragneisses, amphibolites, and felsic orthogneisses of the Trousers Lake and Sisson Brook suites. New field, geochemical, and geochronologic data for the amphibolites and felsic orthogneisses suggest that they are high-grade metamorphic equivalents of the Tetagouche volcanic rocks and their associated intrusions.Amphibolites in the Miramichi Highlands occur as striped and unstriped varieties that possess chemical characteristics indicative of an igneous origin. However, the two types are compositionally distinct: the striped amphibolites resemble volcanic-arc tholeiites, whereas the unstriped amphibolites are like within-plate tholeiites. The geochemically inferred tectonic origin of these amphibolites is compatible with a recently proposed intracontinental back-arc tectonic setting for the Tetagouche Group.Felsic orthogneisses (Fox Ridge augen granite and Trousers Lake felsic orthogneiss) exhibit concordant contacts with the unstriped amphibolites. U–Pb zircon ages for the Fox Ridge augen granite [Formula: see text] and Trousers Lake felsic orthogneisses [Formula: see text] indicate a Late Ordovician intrusive event. Thus, there is no evidence for Precambrian granite and orthogneiss in the Miramichi Highlands, as had been previously inferred from a correlation with purported Precambrian rocks in the Gander Zone of Newfoundland. The age of the unstriped amphibolites is interpreted as being the same as that of the felsic orthogneisses because these two rock types always exhibit close relationships in the field. The age of the striped amphibolites is less certain, although a correlation with Ordovician basalts of the Tetagouche Group is consistent with their field relationships and tectonic setting.