Tertiary extension‐related volcanism, Old Woman Mountains area eastern Mojave Desert, California

Abstract

Tertiary volcanism in the Old Woman Mountains area, eastern California overlapped temporally and spatially with lithospheric extension during the early Miocene. Field relations and restoration of movement along major faults indicate that the primary locus of magmatism probably lay along or flanked the axis of the Piute Mountains. The maximum age of volcanism is unknown but is probably about 20–21 Ma (after regional extension began). Most of the volcanism terminated after emplacement of the 18.5 Ma Peach Springs Tuff. Contrary to conventional models which suggest that extension‐related volcanism is basaltic or bimodal, the volcanic rocks of the Old Woman Mountains area represent a calc‐alkaline continuum dominated by mafic andesite; the most mafic rocks are mildly alkalic. Compositional variability within the suite is the result of mixing a mantle component with silicic crust, accompanied initially by minor olivine fractionation. Petrographic and field observations indicate that both magma mixing and assimilation of granitic crust have operated to produce the observed chemical variation. Trace elements and isotopes preclude binary mixing and suggest that mixing involved several crustal end members. This is consistent with the heterogeneous nature of both the exposed basement and a lower crustal xenolith suite sampled by a Tertiary dike. The mildly alkalic nature of the mafic lavas, the enrichment in large ion lithophiles and light rare earth elements, and the relatively radiogenic character of the most primitive uncontaminated basalt suggest a source similar to enriched mantle lithosphere. Alternatively, these geochemical and isotopic signatures could be attributed to contamination of asthenospheric basalt with mafic lower crust or low‐degree partial melts of this crust.