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Provenance Determined by Ree, Th, and Sc Analyses of Metasedimentary Rocks, Boyden Cave Roof Pendant, Central Sierra Nevada, California
Gary H. Girty (1), Andrew D. Hanson (1), Charles Knaack (2), Diane Johnson (2)
REE, Th, and Sc analyses of metasedimentary rocks collected from the western sequence of the Boyden Cave roof pendant, when compared with similar analyses of uppermost Precambrian to Cambrian miogeoclinal mudstones from the Nopah Range, support the idea that rocks from the former area are a part of a large tectonically displaced fragment of the Cordilleran miogeocline, as proposed by R.A. Schweickert and M.M. Lahren. For example, REE distribution diagrams for samples from the western sequence of the Boyden Cave pendant show parallel light-REE enrichment trends and negative Eu anomalies that range from 0.61 to 0.86, and average 0.69. Likewise, REE distribution patterns for samples from the uppermost Precambrian to Cambrian Cordilleran miogeoclinal section in the Nopah Range also show p rallel light-REE enrichment trends, and Eu anomalies that range from 0.65 to 0.75 and average 0.68. On a La-Th-Se diagram, samples from the Boyden Cave pendant cluster with specimens from the Nopah Range. Finally, sediments from both areas have broadly similar Th/Sc and Th/U ratios. These results suggest that samples from the western sequence of the Boyden Cave pendant are composed of detritus like that shed from differentiated western North American upper continental crust during the early development of the Cordilleran miogeocline. Such an interpretation is consistent with: (1) the quartz-rich to slightly feldspathic character of sandstones in the Boyden Cave pendant, which implies a continental to transitional continental-block provenance; (2) a Precambrian U-Pb age for detrital zirco extracted from a single sandstone bed; and (3) the complete absence of volcanic material in the western sequence of the Boyden Cave pendant. Our results support the regional correlations and tectonic model proposed by R.A. Schweickert and M.M. Lahren, and further suggest that although underutilized, REE, Th, and Sc analyses are useful in evaluating stratigraphic, sedimentologic, and tectonic problems in complexly metamorphosed and deformed metaclastic terranes.
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