Abstract

U-Pb ages of Permian detrital zircons within eastern Quesnellia are similar to ages of western and southwestern Laurentian basement provinces, suggesting sedimentologic ties between the Quesnellian arc terrane and the Laurentian continental margin. The Permian to Lower Triassic Mount Roberts Formation forms the easternmost exposures of the Harper Ranch subterrane of Quesnellia. The formation consists of interbedded lithic graywacke, argillite, limestone, and volcanic rocks deposited within an ensimatic arc environment. Petrology of Mount Roberts Formation sandstones indicate derivation from lithologically diverse sources that consisted predominantly of volcanic rocks but also included significant amounts of sedimentary, metamorphic, and plutonic rocks. U-Pb ages of 27 individual detrital zircons from a single Permian to lower Triassic sandstone of the Mount Roberts Formation indicate that detritus was derived from source terranes with ages of approximately 0.37–0.40 Ga, 1.0–1.3 Ga, 1.50 Ga, 1.61–1.66 Ga, 1.8–2.1 Ga, 2.36 Ga, 2.48 Ga, and 2.5–2.7 Ga.

Devonian zircons are interpreted to represent first-cycle detritus derived from an igneous source terrane that may make up local basement to the Mount Roberts Formation. Most Precambrian zircons are well rounded by sedimentary abrasion and thus were probably derived from older sedimentary rocks. The ultimate sources of 1.0–1.66 Ga detrital zircons are unknown, but it is unlikely that they were derived from the presently adjacent craton. Zircons 1.0–1.66 Ga may represent detritus ultimately derived from southwestern Laurentia. Zircons 1.8–2.7 Ga are similar in age to geologic provinces within western Laurentia, suggesting presently adjacent provinces within the Laurentian craton as ultimate sources of some Precambrian detritus within the Mount Roberts Formation. Although Laurentia may have been the ultimate source of Precambrian detritus within the Mount Roberts Formation, sedimentologic and petrologic data and regional geologic considerations strongly suggest that most of this detritus was recycled from craton-derived sedimentary and metasedimentary rocks that composed part of Quesnellia.

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