Abstract

An augened, strongly flasered, and multiply folded monzonitic gneiss occurs in a structural dome in the Black Bear Island Lake region of northern Saskatchewan, within the ca. 1850 Ma Paleoproterozoic southern Rottenstone Domain of the Trans-Hudson Orogen (THO). A sample of this rock has yielded thermal ionization mass spectrometry (TIMS) and sensitive high-resolution ion microprobe (SHRIMP) zircon data consistent with a formation age of at least 2500 Ma. Zircons also show somewhat younger, ca. 2380 Ma overgrowths, indicating a complex history. The Nd model age (TDM) of 2726 Ma also confirms the late Archean age of the rock, as does the isotopic composition of common Pb from a K-feldspar sample. U–Pb analyses of titanites yield ages of ca. 1800 Ma, indicating recrystallization during terminal closure of the THO. It is unlikely that the Archean rocks are a part of the Archean Sask craton in the Glennie Domain, for Lithoprobe seismic sections indicate that the Sask craton dips westward beneath the La Ronge and Rottenstone domains. It is more likely that the rocks are part of a klippen of Hearne Province crust emplaced during closure of the THO, a large pendant in the ca. 1850 Ma Wathaman batholith, or a crustal fragment exotic to the orogen. Further study should shed light on the tectonic and paleogeographic history of the THO.

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