In situ oxygen isotope analysis of monazite as a monitor of fluid infiltration during contact metamorphism; Birch Creek Pluton aureole, White Mountains, eastern California

Monazite from the hydrothermal aureole of the Cretaceous two-mica Birch Creek Pluton in the White Mountains of eastern California records the infiltration of magmatic fluids into the metasedimentary Early Cambrian Deep Spring Formation. Monazite in the Birch Creek Pluton displays concentric, euhedral magmatic zoning, delta (super 18) O = 8.7+ or -0.2 ppm, and Th-Pb magmatic ages of 78.0+ or -0.7 Ma. The middle Deep Spring Formation approximately 0.5 km east of the contact underwent moderate- to low-temperature alteration by F-rich magmatic fluids; monazite displays patchy zoning but has similar delta (super 18) O values (8.7+ or -0.4 ppm) and Th-Pb ages (78.3+ or -1.6 Ma) to monazite in the Birch Creek Pluton. In contrast, monazite from the upper Deep Spring Formation approximately 0.6 km west of the contact and outside the mapped hydrothermal zone shows concentric zoning, delta (super 18) O = 5.2+ or -0.3 ppm, and partially reset detrital ages from 583 to 1069 Ma. Deep Spring Formation monazite within the hydrothermal alteration zone dissolved and reprecipitated during magmatic fluid infiltration, whereas monazite outside the zone was unaffected. In contrast, Deep Spring zircon within the hydrothermal alteration zone preserved its magmatic zoning and Cambrian- Precambrian U-Pb ages. Zircon can reliably date events prior to hydrothermal activity, whereas monazite, being more susceptible to alteration by fluids, is useful for mapping the extent and timing of fluid infiltration events.