U-Pb geochronologic constraints on Paleoproterozoic tectonism in the Monashee complex, Canadian Cordillera: Elucidating an overprinted geologic history

Abstract

U-Pb isotopic data and field relationships are used to outline the Paleoproterozoic geologic history of the Monashee complex, the largest of three Paleoproterozoic exposures in the Canadian Cordillera and one of the westernmost exposures of crystalline basement in North America. The effects of Paleoproterozoic tectonism are best preserved in deep structural levels of the complex, where overprinting related to high-grade Cordilleran (early Tertiary) metamorphism and deformation was incomplete. Determination of precise crystallization ages is hindered by the U-Pb age discordance in the zircon, monazite, and titanite; the discordance is attributed to inherited Pb in some of the grains and to Pb loss, overgrowth, and recrystallization that occurred during one or more thermal overprints. Igneous crystallization ages are interpreted from the upper intercepts of linear arrays that are defined by three or more analyses. Varying degrees of confidence are attributed to the ages based on the probability of inheritance. Large bodies of augen orthogneiss and granodioritic orthogneiss yield precise igneous crystallization ages of 2077 ± 2 and 1862 ± 1 Ma, respectively. Crystallization ages of about 2.27 and 2.10 Ga are interpreted with less certainty from dioritic orthogneiss and granitic orthogneiss, respectively. Deformation associated with a migmatitic gneissosity occurred after intrusion of the 2077 ± 2 Ma augen gneiss, the youngest dated rock that contains the fabric, and before intrusion of a 1848 ± 3 Ma granite, the oldest confidently dated rock that postdates the fabric. Postdeformational pegmatite dikes are dated as 1845 ± 3 and 1836 ± 2 Ma. Metamorphism is interpreted as occurring during monazite growth at 2060 ± 1 Ma in pelitic schist and during titanite growth at ca. 1.85 Ga in amphibolitic gneiss. The gneisses are basement to an unconformably overlying cover sequence, the lower part of which was deposited prior to intrusion of a 1852 ± 4 Ma pegmatite. There is sufficient age resolution on the intrusive rocks to conclude that Monashee basement has affinities with other Paleoproterozoic exposures in the Cordillera and the western Canadian shield. The knowledge that a Paleoproterozoic geologic history can be elucidated through the Cordilleran overprint suggests that older histories may be preserved in the other Paleoproterozoic exposures in the Cordillera.