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Pre-Arenig metamorphism in the Appalachians

Summary Our understanding of the distribution of pre-Arenig metamorphism in the Appalachians is rudimentary because of polymetamorphism, complex structure and the need for further combined field, structural, isotopic and petrological studies. Available data indicate that in the northern Appalachians pre-Arenig metamorphism is preserved in distinct terranes often collectively referred to as Avalonia. In Maritime Canada 800–700 Ma dates on amphibolite-facies gneisses are interpreted as metamorphic ages. Amphibolite- to greenschist-facies metamorphism is older than 600 Ma granitic rocks. Several areas in eastern New England may record late Proterozoic to early Ordovician amphibolite- to greenschist- facies metamorphism. In the central Appalachians metamorphism older than about 500 Ma is granulite facies in southeastern Pennsylvania and amphibolite to greenschist facies in northern Virginia. Except in the Grenville basement, metamorphism in the southern Appalachians is generally inferred to be Arenig or younger.