EVIDENCE FROM XENOLITHS FOR A DYNAMIC LOWER CRUST, EASTERN MOJAVE DESERT, CALIFORNIA

Garnet-rich xenoliths in a Tertiary dike in the eastern Mojave Desert, California, preserve information about the nature and history of the lower crust. These xenoliths record pressures of similar to 10-12 kbar and temperatures of similar to 750-800 degrees C. Approximately 25% have mafic compositions and bear hornblende+plagioclase+clinopyroxene+quartz in addition to garnet. The remainder, all of which contain quartz, include quartzose, quartzofeldspathic, and aluminous (kyanite+/-sillimanite-bearing) varieties. Most xenoliths have identifiable protoliths-mafic from intermediate or mafic igneous rocks, quartzose from quartz-rich sedimentary rocks, aluminous from Al-rich graywackes or pelites, and quartzofeldspathic from feldspathic sediments and/or intermediate to felsic igneous rocks. However, many have unusual chemical compositions characterized by high FeO(t), FeO(t)/MgO, Al2O3, and Al2O3/CaO, which correspond to high garnet abundance. The mineralogy and major- and trace-element compositions are consistent with the interpretation that the xenoliths are the garnet-rich residues of high-pressure crustal melting, from which granitic melt was extracted. High Sr-87/Sr-86 and low Nd-143/Nd-144, together with highly discordant zircons from a single sample with Pb/Pb ages of similar to 1.7 Ga, demonstrate that the crustal material represented by the xenoliths is at least as old as Early Proterozoic. This supracrustal-bearing lithologic assemblage may have been emplaced in the lower crust during either Proterozoic or Mesozoic orogenesis, but Sr and Nd model ages >4 Ga require late Phanerozoic modification of parent/daughter ratios, presumably during the anatectic event. Pressures of equilibration indicate that peak metamorphism and melting occurred before the Mojave crust had thinned to its current thickness of <30 km. The compositions of the xenoliths suggest that the lower crust here is grossly similar to estimated world-wide lower-crustal compositions in terms of silica and mafic content; however, it is considerably more peraluminous, has a lower mg-number, and is distinctive in some trace element concentrations, reflecting its strong metasedimentary and restitic heritage.