Neenach Volcanics
Neenach Volcanics
early Miocene to late Oligocene
A volcanic sequence of andesitic to rhyolitic lava flows interbedded with lesser pyroclastic deposits and agglomerate cropping out in the western Mojave Desert immediately northeast of the San Andreas fault, in southern map area.
Compositions are calc-alkaline and include andesite, dacite, and rhyolite lavas and lesser pumice lapilli tuff (Matthews, 1976). Andesite lavas are variably porphyritic with plagioclase ± augite ± olivine ± hypersthene ± pyroxene phenocrysts; dacite lavas are volumetrically minor and typically porphyritic with plagioclase and lesser biotite and quartz phenocrysts; rhyolite lavas are aphyric to moderately porphyritic with plagioclase, biotite and quartz phenocrysts (Matthews, 1976). Pyroclastic deposits range in composition from andesitic to rhyolitic and include vitric lapilli tuff and pumice lapilli tuff, some of which contain minor accidental granitic clasts (Matthews, 1976). Also includes perlite and heterolithologic boulder agglomerate (Matthews, 1976).
The southwestern margin of the outcrop area is truncated by the San Andreas fault; this unit has been correlated with the Pinnacles Volcanic formation that occurs 315 km northwest on the opposite side of the fault (Matthews, 1976).
Matthews (1973) reported K-Ar dates of about 18 to 23 Ma for the Neenach Volcanics based on preliminary analyses conducted by Turner, similar to dates obtained by Turner (1968) at the Pinnacles Volcanics. Sims (1993) published recalibrated ages for these original analyses of about 19 to about 24 Ma. However, Weigand and Swisher (1991) reported Ar-Ar whole rock ages of only 11.95 to 14.28 Ma for the dacite and andesite units, respectively (also see notes in Stanley and others (2000)). Recent, unpublished Ar-Ar dating of the dacite and andesite units produced similar ages ranging from about 10.9 to 15.0 Ma for the andesite and dacite units, but an age of about 23 Ma for the lower rhyolite unit (Matt Heizler, New Mexico Tech Geochronology Lab, written commun., 2023). However, subsequent U-Pb dating yielded ages of about 27.0 to 27.6 Ma for the dacite and lower rhyolite units, respectively (Josh Schwartz, CSUN Laser Lab, written commun., 2023). Also, Anttila and others (2025) report a U-Pb zircon age of ~24.3 Ma on the flow-banded rhyolite. These new U-Pb dates and evidence of alteration in the dacite and andesite units suggest that Ar loss occurred during alteration, producing incorrect young ages.