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% h ave mafic compositions and bear hornblende+plagioclase+clinopyroxene+q uartz in addition to garnet. The remainder, all of which contain quart z, include quartzose, quartzofeldspathic, and aluminous (kyanite+/-sil limanite-bearing) varieties. Most xenoliths have identifiable protolit hs-mafic from intermediate or mafic igneous rocks, quartzose from quar tz-rich sedimentary rocks, aluminous from Al-rich graywackes or pelite s, and quartzofeldspathic from feldspathic sediments and/or intermedia te to felsic igneous rocks. However, many have unusual chemical compos itions 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 crust al melting, from which granitic melt was extracted. High Sr-87/Sr-86 a nd low Nd-143/Nd-144, together with highly discordant zircons from a s ingle sample with Pb/Pb ages of similar to 1.7 Ga, demonstrate that th e 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 Me sozoic orogenesis, but Sr and Nd model ages >4 Ga require late Phanero zoic modification of parent/daughter ratios, presumably during the ana tectic event. Pressures of equilibration indicate that peak metamorphi sm and melting occurred before the Mojave crust had thinned to its cur rent thickness of <30 km. The compositions of the xenoliths suggest th at the lower crust here is grossly similar to estimated world-wide low er-crustal compositions in terms of silica and mafic content; however, it is considerably more peraluminous, has a lower mg-number, and is d istinctive in some trace element concentrations, reflecting its strong metasedimentary and restitic heritage.