Paleomagnetism and geobarometry of the Big Creek Batholith suggests that the Yukon–Tanana Terrane has been a parautochthon since Early Jurassic

Abstract

The ∼190±5 Ma Big Creek hornblende-syenite and monzogabbro batholith has an outcrop area of ∼400 km 2 within the Yukon–Tanana Terrane (YTT) in the west-central part of the Yukon Territory of Canada. Paleomagnetic analysis of 245 specimens from 21 sites isolates a characteristic remanent magnetization residing principally in magnetite and subordinately in hematite. The unit mean direction for the batholith from 16 normal polarity and two reverse polarity sites is D=305.6°, I=72.1° ( α 95=3.9°). Al-in-hornblende geothermobarometry at six locations yields emplacement depths of ∼16±2 km. Three arguments suggest that the characteristic remanent magnetization was acquired on exhumation at ∼180 Ma. More significantly, the pole position indicates no northward translation but counterclockwise rotation, implying that the YTT formed close to its present position on the Jurassic margin of the North American craton. This implies, in turn, that the allochthonous terranes of the Intermontane and Coast belts of the western Cordillera impacted onto, and overrode, the YTT upon accretion during Late Cretaceous to Middle Eocene time.