The early Miocene Quechan volcanics, SE California, U.S.A.: slab-window or continental-margin arc volcanics?
Department of Geological Sciences,
San Diego State University,
Advisor Dr. Gary Girty
Located between Indian Pass and Picacho State Recreation Area, SE California are widespread exposures of an ~4 to 1.2 km thick early Miocene volcanic and epiclastic unit. The 23.4 ± 0.4 Ma Quechan volcanics form the lowest and thickest part of the volcanic and epiclastic unit, and formed contemporaneous with the conversion of the North American plate boundary from a convergent to a transform setting. Although some authors have suggested that this conversion was associated with the formation and growth of a slab window, the influence of volcanic activity produced by such a process has remained controversial. Here we evaluate whether or not slab-window magmatism played any role in the genesis of the Quechan volcanics.
The Quechan volcanics consists of a series of lava flows, flow breccias, and lesser amounts of laharic breccia that is over 3 km thick. Based on stratigraphic position, flow thickness, and phenocryst mineralogy, we have informally subdivided the Quechan into lower, middle, and upper members. On the Zr/TiO2 versus SiO2 classification diagram, samples from the lower and upper members plot mostly in the alkali and sub-alkaline basalt fields. In contrast, samples from the middle member plot mostly in the andesite field with three specimens falling in the dacite/rhyodacite fields. On an AFM diagram, samples from the lower and middle members exhibit the Fe-depletion trend characteristic of calc-alkaline magmas. However, specimens from the upper member cluster near the high iron end of the trend defined by the lower and middle members.
Major and trace element data suggest that the lower member was derived from basaltic to basaltic-andesitic magma that was undergoing olivine, pyroxene, plagioclase and probably magnetite fractionation. If the middle member was derived from the same magma, then it was either (1) mixed with other magma(s) and/or (2) underwent rapid decompression as it rose quickly with only minor heat loss as it traversed the crust. Trace element plots of Zr/TiO2 versus Sc, Ni, Cr and V and sieve textures in plagioclase are most consistent with magma mixing.
LREE enriched and HREE depleted chondrite-normalized patterns for the analyzed samples from the lower, middle, and upper members suggest that magma feeding the Quechan volcanics was derived from small degrees of partial melting within the garnet stability field. Reconstructed Paleogene crustal thicknesses reported in the literature suggest a crustal thickness between ~30 and 40 km. Hence, the melts supplying the Quechan volcanics are interpreted to have been derived from sub-crustal lithospheric North American mantle at depths probably greater than ~60 km.
A small Eu anomaly is present in chondrite-normalized REE patterns from the middle member. We interpret the anomaly to be result of plagioclase fractionation prior to eruption of the mixed middle member.
MORB-normalized trace-element patterns of the Quechan are similar to those derived from the central volcanic zone of South America. They are characterized by large positive peaks for Th, Ba, and Rb, negative Ta, Nb, and Ti troughs, a positive peak for Ce, relatively flat to slightly depleted patterns between P and Sm, and relatively steep depleted trends between Sm and Yb. We interpret the enrichments in such patterns to be the result of magma passing through and interacting with continental crust on its way to the surface. In contrast, we suggest that the small negative Ta, Nb, and Ti anomalies in the patterns derived from the middle member may be the result of mixing and assimilation of a Ti-poor possibly dacitic crustal melt with sub-alkaline basaltic to basaltic andesite magma prior to eruption of the middle member. Mixing models of Zr versus Nb and Rb/Sr versus Ba/Rb generally support such a scenario
The above characteristics are most consistent with evolution of the Quechan volcanics within a continental-margin volcanic arc setting, and thus support published interpretations that slab-window volcanic activity, if present at all, was restricted to the immediate coastal region of the evolving early Miocene North American plate margin.