Abstract
Rocks of the Triassic-Jurassic belt of the northern Sierra Nevada are exposed near the town of Auburn in the western Sierra Nevada foothills. These rocks, well exposed along the American River, consist of metamorphosed sedimentary, mafic, and ultramafic rocks, interpreted to be an accretionary melange, and later intruded by felsic plutons and deformed in the Bear Mountain Fault Zone. During the 1960s and 1970s this area was intensively mapped and studied during construction of the never-completed Auburn Dam. Later studies presented structural, petrographic, and geochronological work; however, detailed geochemistry was never performed. Here we present new geologic mapping and geochemistry of exposures on the northern bank of the American River. Geologic mapping focused on determining the protolith of deformed rocks and the nature of the contacts between units. Rock identification was done with thin-section petrography. To aid in identification of rocks, and to determine tectonic setting, whole-rock samples were collected and sent to a commercial lab for trace-element geochemistry by ICP-MS. Mapping shows high-angle contacts between units that may have originated as thrust faults separating slices of ocean-plate stratigraphy. Fine-grained mafic rock, previously thought to be sheared basalts, contain undeformed domains of coarse-grained rocks and have similar trace-element chemistry to gabbros, suggesting they are highly-strained gabbros. These rocks, and surrounding less-deformed gabbros, have high Th/Nb ratios indicating formation in a subduction-zone setting. Basalts, identified by interbedded cherts, have enriched incompatible element chemistries, but lack high Th/Nb ratios. The two different types of trace-element chemistries may indicate that mafic rocks at the Auburn Dam site may represent two different tectonic settings.