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Petrology and Correlation of Newly Recognized Cenozoic Dacite and Tuff Units in NW Knoll Basin, NE Nevada

Session: 37th Annual Undergraduate Research Exhibition Sponsored by Sigma Gamma Epsilon (Posters)

Presenting Author:

Stefanie Cannan

Authors:

Cannan, Stefanie D.¹, Robinson, Sophia², Jenkins, Travis³, Clark, Ashlyn⁴, Deibert, Jack E.⁵, Camilleri, Phyllis A.⁶

(1) Department of Earth and Environmental Sciences, Austin Peay State University, Clarksville, TN, USA, (2) Department of Earth and Environmental Science, Austin Peay State University, Clarksville, TN, USA, (3) Department of Earth and Environmental Science, Austin Peay State University, Clarksville, TN, USA, (4) Department of Earth and Environmental Science, Austin Peay State University, Clarksville, TN, USA, (5) Earth and Environmental Sciences, Austin Peay State University, Clarksville, TN, USA, (6) Austin Peay State University Department of Earth and Environmental Sciences, Clarksville, TN, USA,

Abstract:

Geologic mapping in the NW part of Knoll Basin in northeastern Nevada revealed the presence of two newly recognized, distinct Cenozoic volcanic units below the 16–14 Ma Jarbidge Rhyolite. These new units include felsic lava, the Boies dacite and an underlying ash-flow tuff, Jakes tuff. Based on their stratigraphic position, we infer that the age of these rocks is between ca. 45–16 Ma. The goals of this project were to analyze three samples of each unit along the northwestern margin of the basin to assess possible lateral petrological and geochemical changes, and to compare them to known volcanic rocks in the region for possible correlation. Samples were analyzed for constituents using a petrographic microscope and for major and trace element chemistry using XRF. The Boies dacite contains phenocrysts of plagioclase, sanidine, biotite, hornblende, and minor quartz in a microcrystalline groundmass. Chemical analyses indicate that the two northernmost Boies dacite samples are dacite, and the southernmost is a rhyolite. The major and trace element composition of the Boies dacite was compared to rocks in the nearby 43–39 Ma NE Nevada volcanic field in the Pequop Mountains and East Humboldt Range area. Our data indicate that the two northernmost Boies dacite samples are most similar to hornblende-biotite dacite in the East Humboldt Range. Thin-section analysis of the southernmost Boies dacite reveals the presence of secondary quartz-filled voids, suggesting its rhyolitic chemistry is a product of this added silica. The Jakes tuff samples are dominantly composed of unsorted glass shards and pumice fragments up to 2 cm in diameter. Additionally, the tuff contains a minor amount of plagioclase, sanidine, quartz, biotite, and hornblende phenocrysts, as well as randomly-distributed lithic fragments of local Triassic shale up to 2 cm in diameter. These characteristics suggest a pyroclastic flow origin of the tuff. Our study suggests that the NE Nevada volcanic field may extend farther north than previously recognized. Future research will include radiometric dating of samples to assist in correlating these units.