INSIGHTS INTO SOURCE VARIATION FOR SOUTHERN CALIFORNIA CORDILLERAN GRANITES VS. GRANODIORITES THROUGH ZIRCON GEOCHRONOLOGY, THERMOMETRY, AND TRACE ELEMENT CHEMISTRY: A CASE STUDY IN JOSHUA TREE NATIONAL PARK

Joshua Tree National Park (JTNP) in the Mojave desert, CA, comprises a segment of the Mesozoic Cordilleran arc intruded into a continental margin at its full thickness, represented by Western Mojave province gneisses which yield age peaks at 1.8-1.76 and 1.73 Ga, followed by a 1.71-1.69 Ga metamorphic event, and a subsequent magmatic event from 1.68-1.62 Ga. A-type magmatism occurred throughout the region at ~1.4 Ga. Laramide orogenesis produced a tilted section of this arc segment, which is now exposed at 20 km depth at its westernmost extent and at subvolcanic levels in the east. Data are for Cretaceous samples from a sheeted zone containing intrusions ranging from garnet bearing granites to diorites, exposed at ~15 km depth.

Zircons in sampled magmatic rocks contain abundant cores of pre-magmatic origin. Newly developed SHRIMP-RG techniques allow measurement of high precision U/Pb ages, Ti concentrations, and REE concentrations on single zircon domain, providing : 1) an age spectrum similar to those used for detrital studies, 2) crystallization temperature information, and 3) concentrations of REEs and other elements that can be used to calculate REE compositions of magmas during crystallization.

When grouped by peraluminosity, several chemical differences in both magmatic and pre-magmatic zircons are observed. Peraluminous samples display a group of high temperature zircons (>850^o) and a group of cool, high Yb/Gd grains; neither group is observed in the metaluminous population. Magmatic zircons from the garnet bearing Oasis granite record only sub-725^o temperatures.

Age histograms of pre-magmatic zircons in small volume peraluminous samples yield age peaks from 1.8-1.67 and 1.47-1.4 Ga. Metaluminous samples and the medium volume Palms granite lack any 1.4 Ga peak, indicating a variation in source components. All pre-magmatic samples share a common trend in log U/Th vs. Yb/Gd space, but while metaluminous samples display no significant scatter, peraluminous samples scatter broadly towards low Yb/Gd values (garnet influenced). This scatter is mostly observed in ~1.7 aged pre-magmatic zircons, suggesting that in addition to metamorphic sources, correlative deep crustal igneous bodies associated with this deformational event may be tapped during formation of the JTNP sheeted complex.