GEOCHEMISTRY OF BASALTS AND TRACHYANDESITES IN NORTHERN OWENS VALLEY, INYO COUNTY, CALIFORNIA

As the Mendocino Triple Junction migrated north past the latitude of northern Owens Valley (OV) at approximately 5 Ma, the plate boundary progressed from a subduction zone to a transform fault. One hypothesis is that the composition of rising basalt magmas (e.g. Ce, Y, Zr, and Ba) altered as the plate boundary changed. A second hypothesis is that delamination of the lithospheric root of the Sierra Nevada batholith around 4 Ma also altered the composition of basalt magmas (i.e., higher K₂O).

Basalts in northern OV have ⁴⁰Ar/³⁹Ar ages of 3 Ma and 11 Ma, which precede and follow these tectonic events. Basalt flows in the White Mountains (WM) to the east are dated to 11.5 Ma. Relations between the OV and WM flows and magma are unclear. We make three hypotheses. If the Sierra delamination raised K₂O in magma, then the 3 Ma basalts will have higher K₂O content. If the Ce, Y, Zr, and Ba concentrations change, then the 3 Ma basalts reflect plate boundary and magma pathway changes. Also, if the OV and WM basalts are geochemically similar, then they may represent the same flow or magmatic source.

To test these hypotheses, one 3 Ma and three 11 Ma basalts in northern OV were sampled and field observations were made about the geologic nature of the outcrops. Samples were analyzed by x-ray fluorescence. Our findings do not support the hypothesis that the OV and WM basalts represent the same flow or magmatic source. The samples are geochemically dissimilar: 11 Ma MZP-OV-2 is a trachyandesite (59.50 wt% SiO₂; 3.51 wt% Na₂O; 3.35 wt% K₂O) and 11 Ma MZP-OV-3 is a mantle-sourced basalt (0.32 Zr/Ba; 3.44 Ce/Y; 1.64 wt% K₂O). MZP-OV-4 is an 11 Ma lithosphere-sourced basalt (0.19 Zr/Ba; 4.36 Ce/Y; 2.72 wt% K₂O) geochemically similar to the WM basalts; however, field observations suggest MZP-OV-4 is a small vent and not part of a larger flow. Our findings also do not support the hypothesis that the 3 Ma basalts in northern OV are mantle-sourced. The 3 Ma “basalt”, MZP-OV-1, is a basaltic trachyandesite (51.42 wt% SiO₂; 2.43 wt% Na₂O; 3.68 wt% K₂O) showing lithosphere interaction (0.15 Zr/Ba; 2.35 Ce/Y). The 3 Ma basaltic trachyandesite and 11 Ma trachyandesite had similar K₂O concentrations, thus we were unable to discern the influence of Sierra delamination on magma composition in this area. This relationship may instead be linked to crustal assimilation.