EVIDENCE FROM OUTCROPS AND SINGLE ZIRCONS FOR CONSTRUCTION AND EVOLUTION OF THE MOJAVE CRUSTAL PROVINCE

An east to west transect across the Mojave crustal province yields a comparatively complete record of assembly, post-orogenic sedimentation, and rifting along the southwestern Laurentian continental edge. The oldest exposed rocks are a heterogeneous assemblage of amphibolite, meta-gabbro, and gneisses derived from immature sediments. Evidence from protolith compositions and detrital zircons indicates that this assemblage records crustal growth and recycling of Archean (3.2-2.5 Ga) and Paleoproterozoic (2.1-1.8 Ga) lithosphere, forming the distinctive average Nd and Pb isotopic character of the Mojave province. The basement of this older assemblage has not been identified, but the composition of exposed crust, the association with older mafic igneous rocks, and the detrital zircon signature are consistent with formation along an extended continental margin. The older assemblage was intruded in apparently discrete magmatic episodes at approximately 1.79-1.73 and 1.69-1.64 Ga. Peak metamorphic conditions in the province reached middle amphibolite to granulite facies; metamorphism occurred locally at 1.79 Ga, and there is evidence of widespread metamorphism at 1.74, 1.71-1.69 Ga and 1.67-1.65 Ga. Older plutons are commonly tonalite, trondhjemite and granodiorite, whereas after 1.69 Ga porphyritic high K granitic rocks are more common, indicating that substantial crustal reworking began across the province at about 1.71 Ga. Proterozoic basement rocks are overlain locally by siliciclastic-carbonate sequences of Mesoproterozoic and Neoproterozoic age, and a more widespread late Neoproterozoic to Cambrian siliciclastic sequence records the transition from stable Mojave crust to the rifted Cordilleran margin. Following carbonate-dominant sedimentation through Pennsylvanian time, tectonic rearrangement of the Cordilleran margin led to Mesozoic plutonism in the central and western parts of the province. Mesozoic plutons contain magmatic zircons with a wide range of Th/U, many of which carry relict premagmatic cores, providing a record of recycling from a heterogeneous but apparently crudely stratified Mojave province metaigneous lower crust.