9-3 Whole-Rock Geochemical Constraints on Jurassic Subduction in the Western Sierran Foothills: Insights from the Colfax and Mariposa Formations

Session: Sedimentary Systems and Provenance in the Western Cordillera (Posters)

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Abstract:

The development of the Western Sierra Nevada Metamorphic Province (WSNMP) during Jurassic time is key to understanding recent tectonics in the North American Cordillera, but questions remain about the number and polarity of subduction zones. We focus on characterizing sediment sources of Jurassic strata to evaluate the magmatic arc(s) associated with subduction.  The Middle-Late Jurassic Colfax Formation (CXF) and the Late Jurassic Mariposa Formation (MF) of the WSNMP include turbidites formed in a forearc position along the continental margin and underwent only low-grade post-depositional metamorphism. All CXF and MF slates sampled for whole-rock geochemistry presented here are associated with meta-sandstones analyzed for detrital zircon and petrography. Major element results show that the MF sources (mean ICV = 0.87 ± 0.11) were more weathered than the CXF sources (mean ICV = 1.21 ± 0.06). Trace-element results indicate intermediate to mafic source compositions for both formations, with no evidence of ultramafic material. CXF and MF samples consistently plot between reference values for continental arcs and the North American Shale Composite (NASC) on Ta/Yb vs Th/Yb and La-Th-Sc plots, but closer to NASC. Samples from both formations normalized to chondrite, MORB, and NASC on multi-element plots most closely resemble NASC, suggesting that their intermediate to mafic provenance signal derived from the continent rather than MORB. εNd results show a clear continental signal for both the MF and CXF, with mean εNd values of  -7.1 ± 6.6 and -5.7 ± 2.7, respectively.

CXF data cluster tightly compared to MF data, suggesting more variable sources in MF. This could result from: (1) the smaller geographic area represented by CXF samples compared to MF samples; (2) more proximal CXF source rocks that swamped detrital signals; and/or (3) more mixing of CXF sources. MDAs calculated from detrital zircon ages in associated sandstones indicate that both formations were deposited over >10 million years, so we do not attribute the less variable geochemical character of CXF to more rapid deposition. 

Taken together, our results indicate that both CXF and MF received abundant sediment derived from the North American continent, with lesser input from an active continental magmatic arc and minimal evidence for sources in MORB or accreted island arc terranes.

Geological Society of America Abstracts with Programs. Vol. 58, No. 3, 2026

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