Date

2017

Department or Program

Geosciences

Primary Wellesley Thesis Advisor

David P. Hawkins

Abstract

U-Pb geochronology of detrital zircon grains in clastic sedimentary rocks has become an important tool for constraining the provenance of ancient sediment and reconstructing the paleogeography (landscapes, prevailing wind directions) of ancient continents (e.g., Soreghan and Soreghan, 2013). Such detrital zircon studies typically rely only on U-Pb dates of at least 100 grains per sample. In contrast, this study employs an emerging petrochronological approach that combines zircon crystal growth histories, U-Pb dates, and trace element geochemistry on hundreds of grains to fingerprint sources of ancient sediment with greater fidelity. Such an approach is used to evaluate the provenance of Cambrian detrital zircon in the Late Paleozoic Brushy Canyon Formation of the Delaware Basin in west Texas. Previous work by the Hawkins research group suggests that these ca. 500 Ma detrital zircon grains were derived from a local source in the ancient Florida uplift, preserved today in the Florida Mountains of southwestern New Mexico. This study evaluates their hypothesis by further characterizing the petrochronology of zircon grains in rocks that were exposed in, and eroded from the Florida uplift during the Late Paleozoic. Two rock units exposed in the Florida Mountains of southwestern New Mexico, the Cambrian Florida Mountain granite, and the Ordovician Bliss sandstone, yield igneous zircon grains (n=309) with 206Pb/238U dates of 500-530 Ma, and distinctive trace element compositions consistent with within plate crustal magmatism. Circa 500 Ma detrital zircon grains (n=359) from sandstones in the Brushy Canyon Formation of the Delaware Basin overlap in both age and trace element composition with zircon from the hypothesized source rocks in the Florida Mountains (as well as related granite sources in Oklahoma). These results are consistent with the proposed hypothesis that the Cambrian plutons and related rocks were an important local/regional source for sediment in the Delaware Basin. Additionally, this study demonstrates the efficacy of zircon petrochronology as an approach for fingerprinting the provenance of sediment on ancient continents.

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