Crustal evolution and age of thermotectonic reworking in the western hinterland of the Trans-Hudson Orogen, northern Saskatchewan

Abstract

The Hearne province in northern Saskatchewan forms the western hinterland of the Trans-Hudson Orogen. It is a fault-bounded terrane, lying structurally and geographically between the Rae province to its west and the Wathaman batholith and orogenic internides to its east. The Hearne province is comprised primarily of high-grade metamorphic Archean rocks structurally overlain by Archean or Proterozoic metasedimentary rocks and intruded by Paleoproterozoic granites, whereas the Rae terrane on its west boundary is comprised primarily of still higher-grade Archean rocks. The calc-alkaline Wathaman batholith was intruded at ca. 1.855 Ga, and the orogenic internides to the east of it include mostly high-grade mafic and intermediate volcanic rocks crystallized between 1.92 and 1.86 Ga. Terminal collision and orogenesis in the internides have been constrained to be later than 1.82 Ga and earlier than 1.78 Ga. Mineral U/Pb data presented here unequivocally demonstrate that high-grade metamorphism and large-volume anatexis across the southern Hearne province were approximately synchronous with terminal collision and high-grade metamorphism across the Trans-Hudson Orogen. Zircon data for most samples form complex arrays in concordia space, but demonstrate a high-grade metamorphic overprint at 1810–1800 Ma. Zircons typically have inherited components of Archean age, suggesting the presence of extremely old material in the Hearne and a potential Archean connection with the Rae province. Sm–Nd data indicating ca. 3500 Ma model ages for similar parageneses in both terranes further support the proposed Archean connection between Hearne and Rae province and the great antiquity of Hearne crust. Sm/Nd data also elucidate the similarity of the ca. 1.80 Ga anatectic granites of the Hearne to the 1.82 Ga Junction granite, which intruded the Virgin River shear zone and constrains the timing of final motion on that structure. Monazite 207Pb/ 206Pb ages are not older than 1812 Ma or younger than 1769 Ma, and rutile 207Pb/ 206Pb ages and U/Pb ages are tightly constrained at about 1750 Ma. These data, in combination with results of standard thermobarometric analysis, permit determination of post-orogenic cooling and denudation rates. High cooling (7°C/Ma) and denudation (240 m/Ma) rates suggest fault-accomodated uplift during post-orogenic collapse.