OXYGEN ISOTOPES CONSTRAIN THE ORIGIN OF PROTEROZOIC CRUSTAL PROVINCES IN THE SOUTHWESTERN USA

Accretionary orogens are important sites for crustal growth and mineralization, as well as key records of Earth’s evolving tectonic styles. Within accretionary orogens, older crustal components provide crucial insights into these processes. The ~1300-km-wide system of Proterozoic orogenic belts in the southwestern USA is widely regarded as a classic example of a dominantly juvenile accretionary orogen with older components in the Mojave Province and in central to western Colorado (the Gunnison block). Here, we combine oxygen stable isotopes, U-Pb geochronology, and Nd-Pb-Hf geochemistry to evaluate the origin of these older components and how the different provinces acquired their isotopic signatures. Oxygen isotopes from granitic rocks yield distinct δ¹⁸O_magma compositions for each province. The isotopically juvenile Yavapai Province yielded a mean δ¹⁸O_magma of 7.1±0.8‰ that is consistent with a largely juvenile, mantle-derived source. Heavier δ¹⁸O_magma compositions from the Gunnison block (8.4±1.3‰) and Mojave Province (8.9±1.2‰) suggest greater relative contributions from supracrustal sources compared to depleted mantle. Individual samples form a general trend whereby samples with more evolved whole-rock Sm-Nd and zircon Lu-Hf are characterized by heavier δ18O. This is interpreted to reflect a mixing relationship between a depleted mantle-like source and older sources which contain supracrustal components. These results, along with new detrital zircon data, support models in which the Gunnison block incorporated, or formed on, similar material to the Mojave Province. The recognition of these distinct terranes in the Southwest, which also have with distinctive P-T-t paths, support models in which Proterozoic tectonism involved multiple arc-related domains that developed on pre-1.8 Ga crustal and 1.8 Ga oceanic material. These blocks are interpreted to have been progressively assembled in an outboard setting and subsequently accreted to Laurentia. Subsequently, the Proterozoic crust was modified by multiple phases of tectono-metamorphism during, and after its accretion to Laurentia. Ultimately, the provenance and history of the distinct crustal blocks provides new insight into the genesis of the 1.4 Ga Mountain Pass REE deposit in the Mojave Provinc and the Cenozoic Colorado Mineral Belt in the Gunnison block.