Multiple burial–exhumation episodes revealed by accessory phases in high-pressure granulite-facies rocks (Rae craton, Nunavut, Canada)

Abstract

The Big Lake shear zone (BLsz) is a 60\xa0km long deep crustal structure adjacent to the northern segment of the Snowbird Tectonic zone (STZ: Nunavut, Canada) in a region characterized by intensely sheared high-pressure granulite-facies rocks. The units exposed here provide an exceptional record of the lower crust that preserves crucial evidence bearing on interpretation of superimposed Neoarchean and Paleoproterozoic tectono-metamorphic events. Deformation along the BLsz postdates 2650\xa0Ma, the age of a mylonitized monzogranite, and predates 2190\xa0Ma, the age of cross-cutting MacQuoid mafic dykes. Metamorphic assemblages and P–T modeling suggest crustal thickening in the Neoarchean with peak conditions of 800\xa0°C and 14–15 kbar at ca. 2530\xa0Ma. Mylonitic fabrics developed at ca. 2505\xa0Ma (T\u2009~\u2009800\xa0°C, P\u2009<\u200913 kbar). P–T data on the metamorphosed Paleoproterozoic dykes suggest ca. 1900\xa0Ma recrystallization and partial reactivation of the BLsz at 770\xa0°C–13\xa0kbar. In this work, we demonstrate that U–Pb geochronology and diffusion modeling in Neoarchean titanite can be used to resolve time–temperature paths for lower crustal rocks. Preservation of Nb and Ta zoning with sharp boundaries in titanite and diffusion models of geochemical tracers (e.g., Pb), are indicative of a scenario involving fast cooling (>\u200910\xa0°C/Ma) and partial exhumation post-ca. 2505\xa0Ma followed by re-burial at 1900\xa0Ma, rather than slower isobaric cooling at depth. These results indicate that the long-term residence models proposed by several authors for domains in the central STZ are not applicable to the BLsz region, and are more compatible with a ca. 1.9\xa0Ga collisional setting.