NEW AGE AND GEOCHEMICAL PERSPECTIVE ON LATE JURASSIC DIKE SWARMS OF THE SW MOJAVE

The Independence Dike (ID) swarm records extensional magmatism across 600 km of the California Cordillera in response to sinistral shear during changes in Late Jurassic plate movement. In the southwestern Mojave Desert, IDs have greater geochemical variation, and are more alkalic than in northern parts of the swarm, potentially a result of shallower crustal levels (Carl & Glazner, 2002; Glazner et al., 2008). To test this hypothesis, and provide greater understanding of swarm in general, we have examined northwest-striking dikes at Stoddard Ridge and in the Sidewinder, Ord, and Fry Mountain ranges of the southern Mojave. Field and petrographic relations are combined with bulk geochemistry by XRF (n= 73 dikes), and U-Pb age (LA-ICP-MS) and O isotope (Laser fluorination) analysis of zircon.

In the study area, the dikes are trachy-basalts, basaltic-andesites, a handful of dacites, and quartz + K-feldspar phyric to aphyric rhyolites. Spider plots show that felsic dikes show varying degrees of Sr and Ti depletion consistent with variable input of melts of mafic (gabbroic?) sources. Other indices (Total alkalis, Rb/Sr, Zr, Ti/Zr, Nb/Y, Cu/Zn) distinguish among IDs from range to range. Felsic dikes from the Fry Mts show the most alkalic character and high Zr (500-1500 ppm) and Rb/Sr (>100 ) consistent with small degrees of partial melting of crust. Similarities to IDs of the Black Mt. area (Stone et al., 2013), indicates a regional felsic pulse timed to eruption of the Late Jurassic tuffs at in the Sidewinder volcanics. These younger, felsic volcanics appear genetically related to the tuffs, although they are chemically distinct from coeval plutons.

U-Pb zircon ages of seven dikes in the study area range from 150 to 158 Ma. The 158 Ma rhyolite from the East Ord Mts is in close association with 153 Ma basaltic andesites, and 151 Ma dacites that contain 158 Ma xenocrysts. Values of δ¹⁸O from 150–152 IDs are 5.34–5.39‰ (n=3), but δ¹⁸O is higher in the 157 Ma dike (6.37‰). Collectively, the age, geochemical and isotopic data indicate a more protracted period of emplacement than previously recognized and decreasing crustal input with time. In addition, the dikes are older than in the Owens Valley and suggest a south-to-north sweep of ID magmatism.