The significance of titanite and apatite U-Pb ages: Constraints for the post-magmatic thermal-hydrothermal evolution of a batholithic complex, Berens River area, northwestern Superior Province, Canada

Titanite, apatite, and in one case rutile, from a suite of twenty-seven samples representing the major phases of an extensive and well dated Archean plutonic domain of the northwestern Superior Province, have been dated by U-Pb to determine the temporal progress of thermal and hydrothermal events accompanying and following major plutonism and to assess the isotopic behaviour of these geochronometers. Most samples contain: (1) dark-brown titanite with 350-20 ppm U, showing shapes and textural relationships indicative of a primary magmatic origin and yielding ages corresponding either to the time of magmatic crystallization or to that of secondary metamorphic or magmatic events known in the area; (2) pale titanite with 30-2 ppm U, showing textural evidence for a secondary origin by breakdown of biotite, hornblende, oxide, or older titanite and yielding ages that in general post-date the major plutonic and metamorphic events in the area; (3) apatite containing 30-5 ppm U (occasionally up to 150 ppm U), showing igneous shapes and textural relationships, but yielding post-magmatic ages similar to those of coexisting pale titanite; (4) rutile tin one sample) postdating the ages of coexisting titanite and apatite. The common coexistence of two noncoeval titanite generations demonstrates that the often invoked mechanism of Pb diffusion during slow cooling in this case has no general validity. The distribution of primary dark titanite ages, however, could be explained by late closure to diffusion or, alternatively, as the result of resetting by the thermal or hydrothermal effects caused by younger plutonic activity. The ages of the secondary pale titanites probably reflect crystallization during discrete retrogressive events promoted by moderate heating or enhanced fluid activity. The correspondence between pale titanite and apatite ages suggests that the same processes also caused the isotopic resetting of apatite. On a regional scale the data pattern permits the identification of domains characterized by differences in the length and intensity of the thermal and hydrothermal processes. The most protracted activity is recorded in northern domains of the Berens River area, whereas the gold-mineralized Red Lake area in the south underwent a very rapid decline of the thermal/hydrothermal activity. These data also highlight important differences between the Red Lake and other gold camps of the Superior Province such as Hemlo and Val d'Or where hydrothermal activity, locally associated with mineralization, occurred as much as 30-80 m.y. after the major orogenic developments. Copyright (C) 1998 Elsevier Science Ltd.