GEOLOGY, PETROLOGY, AND SIGNIFICANCE OF RAMOS CRATER, DEATH VALLEY, CA

A volcanic crater and two associated flows, informally known as Ramos Crater (35⁰59.3’N, 116⁰24.8’W) are located in the center of and near the south end of Greenwater Valley, one valley east of Death Valley, CA. The crater is ca. 0.5 km in diameter, 50 m high, and consists of two lava flows with little to no cinder. The flow units are 3-5 m thick and extend for ≤1.6 km to the southwest across the valley bottom, gradually climbing the west side of the valley. Both the crater and flows are undeformed and postdate crustal extension and southeastward tilting of Greenwater Valley and the Greenwater Range.

Magmas from Ramos Crater are relatively alkaline (transitional), conspicuously porphyritic, and vary in composition from trachyandesite to trachydacite (61-65 percent SiO₂, 7-8 percent Na₂O+K₂O). Phenocryst assemblage is variable and includes plagioclase, olivine (altered), clinopyroxene, hornblende, and biotite. Plagioclase phenocrysts are twinned and characterized by sieve textures; subhedral to euhedral hornblende crystals are rimmed by Fe-Ti oxides. The pilotaxitic groundmass consists of plagioclase, Fe-Ti oxides, and other phases similar to the phenocrysts; voids filled with secondary carbonate and zeolite are common.

Geochemical composition of the magmas at Ramos Crater is quite evolved (Mg-number 44-50, low values of compatible trace elements). Rare earth elements are relatively high in abundance (total REE 160-176 ppm) and moderately enriched in LREE relative to HREE (La/Yb_CN 13.5-15.9). The crater and flows yield ε_Nd values between -5.8 and -6.9, T-DM model ages between 975 and 1,055 Ma, and ⁸⁷Sr/⁸⁶Sr values between 0.70812 and 0.70842. Our stratigraphic, geochemical, and isotopic data suggest that magmas from Ramos Crater are post extension, may represent a relatively evolved endmember of extension-related Neogene-Recent volcanism across Death Valley (cf. Calzia et al., 2016, GSA Cordilleran Section Meeting), and suggest the presence of an Fe-enriched (peridotitic/pyroxenitic) lithospheric source component underneath Mojavia.