EVOLUTION OF THE GREENWATER RHYOLITE AND ADJACENT BASALT FIELD, GREENWATER RANGE, DEATH VALLEY, CALIFORNIA: NEW U-PB ZIRCON DATES AND MG-ISOTOPES

The northern Greenwater Range on the eastern boundary of Death Valley National Park contains a 9 km diameter crater filled with rhyolite domes, flows and pyroclastic material. A basalt field with >20 cinder cones borders the rhyolite to the north and east. Twelve new U-Pb zircon dates for a continuous section of rhyolite ash-flow tuff, surge, air-fall and flows provide a comprehensive temporal history for the eruption of the rhyolite. Eruptions began at 6.5 Ma with a 2 m thick surge deposit, containing megabreccia blocks of basalt, that was followed by three rhyolitic episodes: 1) ash-flow tuff at 5.8 Ma, 2) domes, flows, and pyroclastic material at 5.6 – 5.4 Ma, and 3) ~100 m succession of surge deposits and flows (5.3 and 5.0 Ma). The rhyolite is capped by 4.0 Ma Funeral Formation basalt. The southern crater margin is composed of 8.5 Ma rhyolite and basalt (Shoshone Volcanics), while the northern margin is Artist Drive Formation sediments with a 7.6 Ma ash layer toward the top. Because the 6.5 Ma surge deposit laps against an already-formed crater wall, we suggest that the crater formed initially between 6.5 and 7.6 Ma. In several samples, zircons show Cretaceous inheritance (75-84 Ma), which, along with geochemical data, suggests the rhyolite formed by partial melting of Cretaceous basement.

Funeral Formation basalts in the Greenwater Range are evolved alkali basalts with MgO between 4-7 wt. %. They fall into 3 groups based on Nd isotopes: Group 1 with ε_Nd -13 – -10, Group 2 with ε_Nd -8 – -9, and Group 3 with ε_Nd 0 – -6. Initial Sr is similar among the three groups at 0.7045 – 0.7079, although the variability is greater in Group 3. We suggest that basalts formed by partial melting of peridotite or pyroxenite in the lithospheric mantle with variable crustal assimilation among the eruptive groups. Mg-isotopes (δ²⁶Mg) vary from -0.22 to -0.36 and hover about the mantle value of -0.26. Plots of δ²⁶Mg vs (Gd/Yb)n indicate Group 3 basalts have a spinel-bearing source and trend lower δ²⁶Mg values indicating a metasomatized mantle component. By contrast, Groups 2 and 3 basalts are from a mixed spinel- and garnet-bearing source with δ²⁶Mg values at mantle value, suggesting no metasomatized mantle component.