U-Pb evidence for polymetamorphic history of Huronian rocks within the Grenville front tectonic zone east of Sudbury, Ontario, Canada

This paper presents U-Pb geochronological data for zircon, titanite, monazi te, apatite, allanite, and rutile from Street Township east of Sudbury that help unravel two main facets of the geology of the Grenville Front Tectoni c Zone (GFTZ). They document the presence of extensive Huronian Supergroup magmatic (ca. 2480-2460 Ma) and sedimentary rocks inside the GFTZ, and high light the significance of bimodal magmatism in the genesis of the Huronian Supergroup. The data also provide further insights into the complex orogeni c evolution of the GFTZ, showing in particular that amphibolite facies meta morphism, migmatization and deformation during Grenvillian compression at 9 95-980 Ma overprinted, but did not totally erase, the memory of earlier met amorphic events at ca. 1720 and 1470 Ma. From the point of view of U-Pb sys tematics, the results emphasize the nearly ubiquitous state of isotopic dis equilibrium in these rocks. Most of the minerals investigated comprise two or more generations and are generally in a state of partial resetting becau se of the superimposed effects of younger events. The preservation of pre-G renvillian ages in titanite and allanite demonstrates that processes relate d to final Grenvillian collision and exhumation occurred at great speed. On ly apatite and to some degree rutile yield ages that are clearly post-tecto nic, thereby reflecting the final stages of exhumation and cooling of the r egion. (C) 2000 Elsevier Science B.V. All rights reserved.