Evidence from Xenoliths for a Dynamic Lower Crust, Eastern Mojave Desert, California

Garnet-rich xenoliths in a Tertiary dike in the eastern MojaveDesert, California, preserve information about the nature andhistory of the lower crust. These xenoliths record pressuresof 10–12 kbar and temperatures of 750–800C. Approximately25% 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 (kyanitesillimanite-bearing) varieties. Mostxenoliths have identifiable protoliths—mafic from intermediateor mafic igneous rocks, quartzose from quartz-rich sedimentaryrocks, aluminous from Al-rich graywackes or pelites, and quartzofeldspathicfrom feldspathic sediments and/or intermediate to felsic igneousrocks. However, many have unusual chemical compositions characterizedby high FeO(t), FeO(t)/MgO, Al₂O₃, and Al₂O₃/CaO, which correspondto high garnet abundance. The mineralogy and major-and trace-elementcompositions are consistent with the interpretation that thexenoliths are the garnet-rich residues of high-pressure crustalmelting, from which granitic melt was extracted. High ⁸⁷Sr/⁸⁶Srand low ¹⁴³Nd/¹⁴⁴Nd, together with highly discordant zirconsfrom a single sample with Pb/Pb ages of 1.7 Ga, demonstratethat the crustal material represented by the xenoliths is atleast as old as Early Proterozoic. This supracrustal-bearinglithologic assemblage may have been emplaced in the lower crustduring either Proterozoic or Mesozoic orogenesis, but Sr andNd model ages> 4 Ga require late Phanerozoic modificationof parent/daughter ratios, presumably during the anatectic event.Pressures of equilibration indicate that peak metamorphism andmelting occurred before the Mojave crust had thinned to itscurrent thickness of <30 km. The compositions of the xenolithssuggest that the lower crust here is grossly similar to estimatedworld-wide lower-crustal compositions in terms of silica andmafic content; however, it is considerably more peraluminous,has a lower mg-number, and is distinctive in some trace elementconcentrations, reflecting its strong metasedimentary and restiticheritage. ^* Author to whom correspondence should be addressed. Present address: Rensselaer Polytechnic Institute, Department of Earth and Environmental Sciences, Troy, New York 12180, USA. Fax: 518–276–8627; email: hanchj{at}rpi.edu.