U–Pb geochronologic constraints on the cover sequence of the Monashee complex, Canadian Cordillera: Paleoproterozoic deposition on basement

Abstract

The cover sequence in the Monashee complex is a platformal metasedimentary succession that occupies a nearly unique position in the Canadian Cordillera due to its unconformable contact with exposed crystalline basement. Zircon U–Pb data and field observations show that the lower part of the sequence contains Paleoproterozoic rocks, the oldest known metasedimentary rocks in the Cordilleran miogeocline, and the upper part of the sequence is Mesoproterozoic or younger. Maximum age constraints on the lower part are provided by 1.99 Ga detrital zircons from the basal unit and a 1862 ± 1 Ma orthogneiss upon which it was presumably deposited. Minimum age constraints are provided by rocks that intruded into the lower part: 1852 ± 4 Ma pegmatite, 1762 ± 6 Ma leucogranite, and 724 ± 5 Ma syenitic gneiss. The upper part of the sequence must be considerably younger than the lower part because it contains a detrital zircon dated at ~1.21 Ga. Other detrital zircons, dated at Neoarchean (2.95–2.86 Ga) and Paleoproterozoic (1.85–1.81, 1.75 Ga), suggest a source in the western Canadian Shield. These ages constrain the thickness of Mesoproterozoic and Neoproterozoic metasedimentary rocks in the cover sequence to be < 2 km. Combining these ages with previously interpreted Paleozoic deposition ages for the middle and upper parts of the sequence constrains the thickness to be <0.2 km, considerably less than that of coeval rocks above the Monashee complex in the hanging wall of the Monashee décollement. Such a contrast suggests that deposition above and below the décollement occurred in different parts of the Cordilleran miogeocline.