Western Grenville Province holds key to midcontinental Granite‐Rhyolite Province enigma

Abstract

The Mesoproterozoic Granite‐Rhyolite Provinces in North America consist of A‐type granitic and rhyolitic rocks, and their formation is commonly ascribed to continental `extension' or `rifting'. The tectonic environment of the extension is, however, poorly understood. New Sm–Nd isotopic data from the Central Gneiss Belt, Grenville Province, Ontario, suggest that rocks compositionally and temporally similar to those in the Granite‐Rhyolite Provinces formed in and behind an active continental arc undergoing intra‐arc and back‐arc extension. Basaltic underplating accompanied extension, providing heat and juvenile material. Source rocks vary from juvenile within the arc, to more mature with increasing distance behind the arc. Recent investigations from SW Baltica show that the Rjukan Group, a proposed Granite‐Rhyolite Province correlative, also formed inboard of an active continental arc. A tectonically and temporally similar, and probably recurrent, evolution along the vast Laurentia–Baltica margin points to active continental‐margin processes as first‐order controls on back‐arc granite‐rhyolite magmatism.