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Provenance of Jurassic sedimentary rocks of Quesnellia : implications for paleogeography Petersen, Nathan Todd

Abstract

Jurassic sedimentary rocks of Quesnellia record the history of the Quesnellian magmatic arc and reflect increasing continental influence throughout the Jurassic history of the terrane. Three provenance methods, including standard petrographic point counts, geochemistry and Sm-Nd isotopes and detrital zircon geochronology, were found to be complementary in reflecting source area characteristics. Each method provided valuable information about the source areas of the Jurassic sedimentary rocks of Quesnellia. Lower Jurassic proximal and proximal basin sedimentary rocks show characteristics of derivation from an arc source area. Sandstones are immature, grain compositions are dominated by plagioclase grains and volcanic lithics, and quartz is rare. The rocks are characterized by their geochemically and isotopically primitive nature. Detrital zircon populations, based on a limited number of analyses, are homogeneous Late Triassic or Early Jurassic groups, reflecting local derivation from Quesnellian arc sources. Middle Jurassic proximal and proximal basin sedimentary rocks show a trend toward more evolved characteristics. This trend is reflected in a change to more stable grain populations, with enrichment of plagioclase over volcanic lithics, subtle increases in quartz content, and noticeable amounts of chert and sedimentary lithics. The change to more evolved characteristics is emphasized by the overall decrease in eNd values of the sedimentary rocks and is reflected by the presence of Proterozoic detrital zircon grains. This change is probably due to a combination of two factors: (1) the erosion and weathering of the Late Triassic-Early Jurassic arc source of Quesnellia, linking the terrane to the continent, and/or (2) the onset of continental contamination of the Middle Jurassic phase of the arc. Distal environments are characterized by more efficient mixing of sediment from the local arc sources and continental material. This latter source is relatively prominent, showing up in the evolved geochemistry and Sm-Nd isotopes of the sedimentary rocks. This indicates that Quesnellia was tied to the continent throughout the Jurassic but the ties are not evident when the sedimentary environment dictates a dilutional effect from proximal arc sources. Western Quesnellia is distinctly different from eastern Quesnellia in terms of basement characteristics, ammonite faunas, facies association and detrital zircon populations. These differences point to the possibility that the two parts of the terrane underwent unique histories and support models calling for northward displacement of Quesnellia as a whole or northward displacement of just the western part of Quesnellia.

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