PAIRED U-PB AND LU-HF ISOTOPE ANALYSIS OF DETRITAL AND PLUTONIC ZIRCON TRACK BASIN FORMATION AND SUBSEQUENT CRATONIZATION IN THE PALEOPROTEROZOIC MOJAVE PROVINCE OF SOUTHWESTERN LAURENTIA

The crystalline basement of cratons commonly consists of intimately associated plutonic and supracrustal rocks believed to form along convergent margins. As these are among the most varied geologic environments on Earth, identifying the specific tectonic setting in which supracrustal and plutonic rocks form is challenging. It is also fundamental to understanding the processes by which continental lithosphere is formed and cratonized. Paleoproterozoic crust of southwestern Laurentia is regarded as a classic example of lithospheric growth though accretionary orogenesis. The Mojave crustal province has, however, remained enigmatic in models of lithospheric formation due to its distinctive isotope systematics. Single-crystal U-Pb and Lu-Hf isotope analysis of detrital zircon reveals that metasedimentary schists and gneisses across the Mojave province all share the same provenance. This detrital zircon signature is characterized by a prominent age peak at 2.0-1.8 Ga, and a 2.7-2.4 Ga population. Both populations of detrital zircon yield εHf(t) values that vary more than 10 epsilon units. Maximum depositional ages determined from U-Pb geochronologic results, and minimum depositional ages constrained by intrusive relationships with dated plutonic rocks become younger across strike in the orogen from west to east, ranging from 1.79-1.74 Ga over ~600 km. Arc related plutons that intrude these metasediments mimic this age progression, and show a systematic variation in initial Hf isotope composition. The oldest plutons yield εHf as low as -4 at 1.79 Ga, followed by progressively more radiogenic values culminating in a suite of arc plutons with εHf values as high as +10 from 1.74-1.71 Ga. Detrital zircon grains preserved in metasediments show the same progression in Hf isotope composition from 1.8-1.75 Ga, indicating that they were derived from extrusive counterparts of plutonic rocks. Initial Hf isotope compositions of syn-to-post orogenic crustally derived granites show a departure towards more evolved Hf isotope compositions after 1.71 Ga. This time corresponds to the onset of the Yavapai orogeny across the province. Temporal, spatial, and isotopic trends within the Mojave province are interpreted as the opening of a back-arc basin during slab rollback, followed by closure during the Yavapai orogeny.