Chapter 9: Recycling of continental crust in Miocene volcanic rocksfrom the Mojave block, southern California

Abstract

Tertiary volcanism in the Mojave Desert region of southern California followed a long early Tertiary hiatus in volcanism and sedimentation. Volcanism began simultaneously across the region, producing a broad belt of volcanoes that stretched at a high angle to the continental margin from the western tip of the Mojave Desert eastward into Arizona, a distance of more than 500 km. This band of volcanoes was part of a northward-moving wave of volcanism that passed through the southwestern United States in the Tertiary, inland of the migrating Mendocino triple junction. Volcanism coincided (at the temporal resolution of present data) with crustal extension and with the change from subduction to transform-fault tectonics.Although the suite includes rocks ranging from alkali basalt to high-silica rhyolite, petrologic data indicate that volcanism was caused by large-scale injection of the crust by high-alumina basalt magma. Basalt magmas were probably trapped in the crust owing to low crustal density. Intermediate rocks are hybrids produced by mixing of silicic crustal material into mafic magma. Evidence for mixing includes (1) ubiquitous disequilibrium phenocryst textures (e.g., quartz xenocrysts in mafic rocks; reversezoned, spongy plagioclase), and (2) positive correlations of 87 Sr/ 86 Sr and S 18 0 with Si(>2. Major-element, trace-element, and isotopic trends from rocks with <65 weight percent SiC>2 are consistent with a simple mixing model involving addition of rhyolite and minor fractionation of olivine. Above 65 weight percent SiC>2, addition of rhyolite alone can explain observed chemical trends.The tectonic significance of Mojave volcanism remains obscure. The hybrid nature of intermediate rocks and lack of a clear connection to subduction rule out the possibility of using the rocks to make inferences about paleosubduction geometries. The basalt magmas that triggered the volcanic outburst may represent the release, during lithospheric extension, of partial melts of sub-Mojave lithosphere, which was significantly metasomatized by the long period of early Tertiary nonvolcanic subduction.

Cenozoic evolution of the Mojave block of southern California

“…At ca. 24-22 Ma, volcanic rocks were erupted along an east-trending belt that stretched from the westernmost Mojave Desert region inland to the Whipple Mountains and beyond (Glazner and Bartley, 1984;Glazner, 1990). The onset of magmatism was accompanied by the onset of extensional faulting, as both swept northwestward out of Arizona (Glazner and Supplee, 1982;Glazner and Bartley, 1984).…”

“…Composite volcanoes have been identified locally (Glazner, 1988;Sabin et al, 1994). Some areas are dominated by intermediate-composition and silicic rocks, whereas others are bimodal accumulations of basalts and basaltic andesites with silicic tuffs (Glazner, 1990;Miller and Miller, 1991). East of Barstow, mafic flows and silicic tuffs typically overlie thick sequences of andesite and dacite lavas, but geochronologic data indicate that these lithologically correlative sequences are not strictly time correlative (Gardner, 1940;Glazner, 1988;Glazner et al, 2000).…”

“…Volcanic rocks of the Mojave block are broadly calcalkalic, but the mafic end of the spectrum is typically high in titanium and mildly alkalic, unlike typical subduction-related suites such as the Cascades (Glazner, 1990). In a given area, the most mafic rocks are typically basaltic andesites.…”

“…In the central Mojave block, 24-20 Ma volcanic strata are widespread in the ranges southeast of Barstow (Glazner, 1990;Walker et al, 1995) and extend west in scattered outcrops to the very western edge of the Mojave block (e.g., Armstrong and Higgins, 1973;Matthews, 1976;Dokka and Baksi, 1989). Volcanic rocks north of the latitude of Barstow are younger, predominantly 20-14 Ma, with scattered outcrops of late Miocene and younger basalts (Burke et al, 1982;Schermer et al, 1996;Sabin et al, 1994;Smith et al, this volume).…”

The origin of Mount St. Helens andesites

“…These crystals exhibit various textures, including clear phenocrysts and phenocrysts with "sieve" textures. Although "sieve" implies that the growing crystal engulfed melt during growth (e.g., Glazner, 1990), we use this term to describe cores that appear to have undergone resorption or partial dissolution; some, in fact, may have formed by inclusion of melt during crystallization. We note that plagioclase textures which Kawamoto (1992) rims); this range exceeds that (5-10 mole%) typical of olivine phenocrysts in the mafic lavas.…”