THERMAL RECORDS OF LOWER CRUSTAL XENOLITHS AND GRANULITE EXPOSURES: A PERSPECTIVE FROM GRANULITES OF THE SNOWBIRD TECTONIC ZONE, NORTHERN SASKATCHEWAN

Lower crustal xenolith studies play a critical role in constraining lithospheric evolution, in part because of the scarcity of deep crustal exposures and the possibility of bias in this exposed sample. However, it is essential to use the in situ context afforded by well-preserved deep crustal exposures to better interpret the xenolith record. The East Athabasca area of the Snowbird tectonic zone in northern Saskatchewan contains an enormous tract (> 3000 km2) of high grade rocks (10 to > 20 kbar, > 750 °C) that include eclogites, mafic granulites, and heterogeneous gneisses. This region, composed of several disparate deep crustal domains, is an exceptional location to consider the constraints on thermal histories provided by the xenolith vs. granulite exposure records.

New U-Pb thermochronologic data place precise constraints on the timing of high-grade metamorphic events and subsequent exhumation of granulites in the East Athabasca area. Two episodes of high-grade metamorphism recorded in mafic granulites at ca. 2.54 Ga and ca. 1.9 Ga imply deep crustal residence within a stable lithosphere for ca. 640 m.y. Lithospheric reactivation at ca. 1.9 Ga is recorded by 1896 Ma amphibolite anatexis (> 750 °C) of a major mafic dike swarm intruded into one of the domains. Subsequent cooling to < 450 °C by ca. 1.76 Ga is recorded by titanite, apatite and rutile. Contrasts in cooling records between domains may indicate variability in isobaric cooling and exhumation. This perspective on deep crustal thermal evolution from the unique East Athabasca exposure can shed insight into the xenolith record and provide a better understanding of lithospheric growth, stabilization, and reactivation.