Abstract

Current detailed (1:16,000) geologic mapping along the southern extension of the Antelope Range is focusing on stratigraphy, ⁴⁰Ar/³⁹Ar dating and structural and remote-sensing analysis (see also Butler et al., 2001, this volume). East-dipping Cretaceous (Iron Springs Formation) and Tertiary (Claron Formation) sedimentary rocks are bounded to the west by the NE-SW striking Antelope Range Fault. However, the most extensive rocks in the quadrangle are Tertiary volcanic units. These include regional ash-flow tuffs (Needles Range Group, Isom Formation, Quichapa Group, and Racer Canyon Tuff) from calderas outside the area to the NW and W, more local ash-flow tuffs (Rocks of Paradise and Rencher Formations) derived from Iron Axis laccoliths to the south, and local volcanic rocks from vents within the map area. In the map area the volcanic rocks generally get younger from west to east (Needles Range, Isom, Quichapa, Rocks of Paradise, Rencher, and Racer Canyon). In the east the 19 Ma Racer Canyon Tuff is subhorizontal. Erosion of the east-tilted fault-block resulted in semi-consolidated, late-Miocene (~8–9 Ma) alluvial deposits located in the southwest. The youngest igneous rocks are sub-horizontal to gently east-dipping basalts also restricted to the southwest.

Historically, K/Ar mineral dates provide the main age constraints on the Miocene volcanic stratigraphy and Iron Axis magmatism of this region. We present the results of 40Ar/39Ar incremental release dates on plagioclase separated from six volcanic samples collected in the Pinto Quadrangle. The age of the Harmony Hills tuff (Quichapa Group), a key unit deformed by all of the intrusions, is only poorly constrained by six prior K/Ar dates ranging from 24.4 to 20.3 Ma. We obtained a well-defined plateau age (8 steps constituting 91% of the total 39Ar) of 22.03±0.15 Ma. This date is indistinguishable from a 21.93±0.07 Ma date (40Ar/39Ar, biotite) reported for the immediately overlying ash-flow tuff member of the Rocks of Paradise Formation (Hacker et al., 1997, GSAA). We also obtained a well-defined plateau age of 21.83±0.17 Ma (4 steps, 55% total 39Ar) on the Rencher Formation (and concordant isochron age of 21.46±0.40 Ma) which directly overlies the Rocks of Paradise. These data tightly constrain the age of several key volcanic units in the area as well as their sources to the south and west (Stoddard Mountain, Pinto Peak, and Bull Valley intrusions). Unfortunately, four of our samples gave U-shaped, discordant, age spectra suggesting either the presence of excess argon or xenocrystic contamination. The least contaminated sample (an east-tilted capping basalt at the southwestern end of the map area; Gum Hill) yielded a minimum age increment of 5.7 Ma which we interpret as a maximum age for the rock. If correct, this interpretation suggests that some of the Miocene capping basalts throughout the area may be younger than indicated by prior K-Ar ages (i.e, ~7.7 Ma, Bull Valley Mountains, Best and others, 1980, AJS).

(this work is supported in part by a USGS EDMAP grant and a Kent State University research grant)