GEOCHEMICAL INVESTIGATION OF NEOGENE BASALTS OF THE HORSE THIEF HILLS REGION, NW OF DEATH VALLEY: IMPLICATIONS FOR INITIATION OF BASIN AND RANGE EXTENSION

Geologic maps of the Last Chance Range (LCR; NW Death Valley), White Mountains (WM; WNW from Death Valley), Horse Thief Hills (HTH; contiguous upland between WM and LCR) and Fish Lake Valley (FLV; N of Death Valley; E of WM and HTH) show a number of isolated outcrops of Tertiary (Neogene) olivine basalt. Basalt atop Chocolate Mountain (NW HTH) is correlated with the 10.8 Ma (K/Ar) basalt found overlying a 10.9 Ma (K/Ar) rhyolite to the NW in the WM. Basalt outcrops atop ridges in the HTH have K/Ar ages ranging from 5.3-6.3 Ma.

Geochemical studies of western Basin and Range basalts show that basalts at this latitude that are younger than 5 Ma have a Zr/Ba ratio less >0.2, which is explained as a different magmatic source and path than basalts older than 5 Ma at this latitude. Our previous geochemical data show that the basalts atop the LCR differ in magmatic source from the 10.8 Ma basalt flow in the SE WM, Deep Springs Valley and Chocolate Mountain. To determine if basalts in the WM and HTH-FLV have the same source as the Chocolate Mountain basalt, the LCR basalts or some other source location, samples were analyzed for major and minor trace element composition by X-Ray Fluorescence Spectrometer (XRF).

Previously mapped outcrops and analyzed samples using XRF are shown to be olivine barring basalts in various states of weathering. Trace element plots (e.g., Zr/Ba vs Ce/Y) show that basalt from the WM, Deep Springs Valley and isolated ridges atop the HTH are similar, from the same source and likely the same flow. A HTH basalt previous dated at 5.3 Ma by whole-rock K/Ar has a Zr/Ba ratio below 0.2, which is inconsistent with younger basalt flows. Based on the geomorphic position and geochemistry, we are skeptical of the K/Ar age and suggest that this basalt is part of the presumably 10.8 Ma flows. Basalt overlying rhyolite and cropping out on the east side of FLV is also geochemically similar to the 10.8 Ma basalt.

We interpret the geochemical data, outcrop thickness and geomorphic data to indicate that a basalt flow traveled from ESE about 10.8 M from the WM, across Deep Springs Valley and the HTH and as far as eastern FLV. This basalt flow predates the formation of both Deep Springs Valley and FLV. Future work will reaffirm the ages of the basalts, which were originally dated by whole-rock K/Ar in the 1960s.