JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. B8, PAGES 13,629-13,643, JULY 30, 1991 Tertiary Extension-Related Volcanism,Old Woman Mountains Area Eastem Mojave Desert, California JONATHAN S. MILLER 1 AND CALVIN F. MILLER Geology Department, Vanderbilt University, Nashville, Tennessee 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 PeachSprings Tuff. Contraryto conventional models which suggest that extension-related volcanism is basaltic or bimodal, the volcanic rocks of the Old Woman Mountains area representa calc-alkaline continuum dominated by mafic andesitc; 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 graniticcrust have operated to produce the observed chemical variation. Trace elements and isotopespreclude 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 sampledby 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 crustor low-degree partial melts of this crust. INTRODUCTION The relationshipbetween magmatismand extensionin the southern Great Basin and northern Basin and Range has received considerable attention over the past two decades [e.g. Lipman et al., 1972; Christiansen and Lipman, 1972; Hildreth, 1981; Glazner and Bartley, 1984; Gans, 1987; Wernicke et al., 1987; Glazner and Ussler, 1989; Taylor et al., 1989; Coleman and Walker, 1991]. These studies have been primarily concerned with three major issues. The largest and perhaps most controversial issue concerns the large-scale space-time patterns of volcanism and extension across the Basin and Range [e.g., Lipman et al., 1972; Christiansenand Lipman, 1972; Glazner and Bartley, 1984; Wernicke et al., 1987], which we will not discuss in this study. The second issue concerns whether magmatism induces crustal extension("active rifting") or is causedby extension ("passive rifting"; terminology after Sengor and Burke [1978]). Gans et al. [1989] propose that basalt intrusion thermally weakensthe lower and middle crust causing brittle failure of the uppercrust. In this model, basaltic magmatism provides a thermal driving mechanism for continental extension, hence the term active rifting. In the passive rifting model [Taylor et al., 1989], tectonic thinning of the lithosphere may inducemagmatism by decompression melting. Magmatism is related to extension but is not the driving mechanism. The third issue concerns the genetic link, if any, between extension and the compositional characteristicsof related i Now at Department of Geology, University of North Carolina, Chapel Hill. Copyright1991 by the AmericanGeophysical Union. Papernumber 91JB00607. 0148-0227/91/91JB-00607 $05.00 magmatism. Recent models [e.g., Gans et al., 1989; Glazner and Ussler, 1989] have challenged the notion that extension- related magmatism is always basaltic or bimodal [e.g., Christiansen and Lipman, 1972; Eichelberger, 1978; Hildreth, 1981] by showing that basaltic volcanism in the Basin and Range generallypostdates large-magnitude extension. In these more recent models, the thermal structure and density (as opposed to stress state) of the lithosphere have a strong influence on the composition of volcanism, and volcanism which is associated with significant extension is likely to show effects of crustal interaction; hence volcanism may be more intermediate in composition rather than bimodal. Although studies of the structural geology of the southeastern Mojave Desert are numerous [e.g., Howard et al., 1987, Howard and John, 1987, Hileman et al., 1990], there are few studies of the Tertiary volcanic rocks associatedwith extension. Given the dearth of data on the southeastern Mojave Desert, we have examined the Old Woman Mountains volcanism focusing on the issues raised above. Ongoing structural studies, this work, and studies of other volcanic centers (e.g., Mopah Range [Hazlett, 1990]) may eventually allow a synthesis of the broader space-time-compositional featuresof this large-scale extensionalterrane. Location The Old Woman Mountainsare a 40-km-long,north trending range located approximately60 km west of the Colorado River in southeastern California (Figure 1). The Tertiary volcanic rocks (including intercalated volcaniclastic rocks) are found in small ranges adjacent to and north of the core of the range (Figure 2). These ranges include the Little Piute Mountains,the Piute Mountains, and the Ship Mountains. We refer collectively to these localities as the Old Woman Mountains area. The Stepladder Mountains(Figure 1) were investigated in reconnaissance during the course of this study, and some preliminary geochemical and isotopic data from this range are included in this report. 13,629