AGE AND ORIGIN OF THE MALAPAI HILL BASALT, JOSHUA TREE NATIONAL PARK, CALIFORNIA

Malapai Hill (MH) is a small basaltic outcrop in California's Joshua Tree National Park (JTNP), located in the southern Mojave Desert near the left-lateral Blue Cut Fault. While research has investigated the timing and source of mafic magmatism at other volcanic fields in the Mojave, little is known about the basalts in JTNP. As such, this work sought to elucidate the age and origin of the JTNP magmas to better understand their relation to the volcanic fields and subcontinental mantle sources of Mojave volcanism.

The Malapai Hill Basalt intruded into the 151±1Ma White Tank monzogranite (Barth et al., 2008). Columnar jointing patterns suggest that MH was emplaced as a hypabyssal intrusion during two or more pulses of magmatism, and was cooled in part because of interaction with shallow groundwater. The basalt contains spinel-lherzolite nodules and xenoliths of the host monzogranite. ⁴⁰Ar/³⁹Ar laser incremental heating analysis of groundmass gives a plateau age of 15.93±0.08Ma, making Malapai Hill distinctly older than most large Mojave volcanic centers. Whole rock geochemistry shows MH samples range from basalt to trachybasalt (SiO₂=42.5–47.3%, NaO+K₂O=4.0–5.3%). Oxygen isotope ratios of olivine from basalt groundmass are 5.03–5.62‰ (n=2), with the higher value exceeding both normal mantle values and those of olivines in Dish Hill nodules (Mattey et al., 1994). Higher values are consistent with a locally enriched source and possible crustal contamination.

These results suggest MH represents asthenospheric magma that upwelled as a result of lithospheric extension and subsequent thinning, potentially due to evolution of the San Andreas Fault system. Additionally, MH may (1) be representative of a primitive source for later melts, including the "M" component magma identified by Glazner et al. (1991) at Amboy and Pisgah Craters; or (2) represent the composition of mafic crust later partially melted to generate the "A" and "P" component magmas also identified by Glazner et al. (1991) at Amboy and Pisgah. Of these, (1) has implications for the source of mafic volcanism and/or the timescales of mafic crust assimilation in the Miocene mantle, while both (1) and (2) imply MH and the xenoliths it contains offer a relatively primitive sample of Miocene-aged subcontinental mantle melt.