VARIABLE RECYCLING OF ANOMALOUS LOW- δ18O PROTEROZOIC CRUST IN THE ORD MOUNTAINS, MOJAVE DESERT, CA

Mesozoic plutonic and volcanic rocks in the Mojave Desert provide a scattered record of arc magmatism in western North America. Within the Ord Mountains, Proterozoic granitic and amphibolite gneisses that are intruded by a Jurassic, coarse-hornblende gabbro complex, that is itself cut by a mingling complex of tholeiitic basalt, basaltic-andesite, and rhyolite. Ca. 148 Ma Independence Dikes (IDs) cut all other units providing a minimum age of magmatism. Jurassic rhyolite and dacite tuffs of the Sidewinder volcanic series record voluminous silicic magmatism in in the region as well. Oxygen isotope (δ¹⁸O) analyses of zircon, which is highly resistant to alteration or original values, reveal three Proterozoic gneiss locations distributed over 4 km with δ¹⁸O values of 3.3–4.6‰, well below normal mantle δ¹⁸O values of 5.3±0.3‰. This discovery requires that gneiss protoliths were hydrothermally altered before Proterozoic zircon crystallization. Thus, the Ord Mts. expose a newly recognized example of low- δ¹⁸O crust in the Mojave Terrane. In contrast, δ¹⁸O of zircon from the IDs in the range and adjacent areas have restricted, mantle-like δ¹⁸O values (5.3–5.4‰; n = 4). Whole rock geochemistry shows that the IDs have crustal affinity similar to other IDs in the Mojave (Glazner et al., 2008, GSA Spec. Pap. 438), being mostly rhyolitic: SiO₂ = 67–78 wt.%; Na₂O+K₂O = 8–12 wt.%, with high Ba, Rb, and unusually high Zr concentrations (over 700 ppm in some dikes) consistent with the IDs’ alkaline nature. Given the alkaline nature of the IDs, and restricted mantle δ¹⁸O values, we suggest they originated as hot, dry melts of metasomatized lower crust and had minimal interaction with the Ord crustal rocks during their emplacement. Four tuffs of the Sidewinder series have δ¹⁸O zircon values of 4.7 to 5.8‰. This range of values suggests that source calderas tapped the low-δ¹⁸O Ord Mts. crustal reservoir, however, and alternate cause of low- δ¹⁸O magmas in caldera-formed volcanics is melting of hydrothermally-altered volcanic rocks at shallow levels in the crust. Ongoing work seeks to further refine the timing of the formation of the Ord Mts. low-δ¹⁸O rocks and the extent to which they are recycled into other Mesozoic magmas in the region.