Plutonic Evolution of the Cutler Area, Ontario

Abstract

In the Cutler area, on the northern shore of Lake Huron, Huronian sedimentary rocks and Nipissing diabase sills have been isoclinally folded and metamorphosed to staurolite grade during an erogenic episode dated at about 1.7 b.y. Emplacement of the Cutler batholith culminated this episode. In an aureole around, and probably above, gently dipping contacts of the batholith, muscovite was eliminated from metasedimentary rocks while staurolite remained stable. Rock chemistry and consideration of experimental data on mineral stabilities suggest that muscovite elimination occurred through an alkali exchange reaction, probably propagated by an influx of Na-bearing solutions associated with the granite. Mineralogical and structural studies indicate that the granite is magmatic, probably of anatectic origin. It is shown that a steep geothermal gradient ( dp/dt is large) could result in the stabilization of staurolite at temperatures of the granite minimum. Structural studies indicate that the granite was emplaced along the axial culmination of a doubly plunging antichnorium during a period of passive flow folding. The regional tectonite fabric of the metasediments is preserved even in terranes of 75 percent or more granitic rock and the fabric of the granitic rocks is symmetrically related to that of the metasediments. Contrast in viscosity (and therefore strain rate) between the granitic and metamorphic rocks during folding resulted in segregation of anatectic magma from the parent metamorphic rocks, with the granite being concentrated in higher portions of the growing fold.