Provenance and Tectonic Implications of LA-ICP-MS Zircon Depth-Profiling, Nanaimo Basin, British Columbia, Canada
Date
2019-03-26
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Abstract
Detrital zircon studies from sedimentary rocks of the Nanaimo Basin provide an important independent test of the Late Cretaceous paleogeographic position for the basin, and the associated Insular Superterrane, Coast Plutonic Complex (CPC), and North Cascades of the North American Cordillera. Detrital zircon populations not derived from the CPC provide the best constraints on paleolatitudes for the Nanaimo Basin in the Late Cretaceous. Non-CPC sources for the basin are restricted to the Lemhi sub-basin (Idaho and Montana), or the Mojave-Sonoran Region (MSR) of the southwestern US and northern Mexico. We distinguish between these potential non-CPC sediment source areas by combining sandstone petrographic observations with detrital zircon depth-profiling of thin zircon rims observed on >30% of zircons from Maastrichtian Nanaimo Group rocks. These data allow us to interpret tectono-metamorphic events that led to rim development in the sediment source region of the Nanaimo Basin, and provide additional provenance constraints for the non-CPC source region. Our results show that zircon rims are ubiquitous on Proterozoic grains but are also present on Mesozoic populations. Rim growth indicates metamorphism and magmatism in the non-CPC source area from ~100 – 69 Ma and 110 – 64 Ma, respectively. Core ages (modes ~1698 Ma and ~1388 Ma) are similar for grains with metamorphic or magmatic rims, suggesting they both derive from a single source region that underwent metamorphism and partial melting in the Late Cretaceous. The timing of rim growth closely matches the timing of tectono-metamorphic events in the MSR, and our data suggest that metasedimentary rocks are the source of non-CPC grains in the Nanaimo Basin. Our interpretation of a metasedimentary source differs from previous claims that Nanaimo Basin sediments derived from crystalline basement rocks and recycled sedimentary successions in the MSR. We interpret that drainages never connected the Nanaimo Basin directly to rocks of the MSR and propose instead that metasedimentary rocks were the source of non-CPC grains in the basin. The arrival of non-CPC detritus in the basin ~72 Ma is interpreted to represent the onset of northward translation of the Baja-BC block from paleolatitudes near modern-day southwestern California to its present position.
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Keywords
Tectonics, Provenance, Baja-BC, U-Pb Geochronology, Zircon Depth-Profiling
Citation
Boivin, M.-P. (2019). Provenance and Tectonic Implications of LA-ICP-MS Zircon Depth-Profiling, Nanaimo Basin, British Columbia, Canada (Master's thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.