Low temperature replacement of monazite in the Ireteba granite, Southern Nevada: geochronological implications

The Ireteba pluton is a relatively homogeneous, ∼64 Ma (zircon ion probe age) two-mica granite that was intruded by two 16 Ma Miocene plutons at depths ranging from 5 to 13 km. Deeper levels of the Ireteba and Miocene plutons were ductilely deformed at 15–16 Ma. At shallow levels remote from the Miocene plutons, the Ireteba granite appears to have experienced little Miocene heating and deformation. tes from different portions of the pluton reflect the different histories experienced by the host rock. Irregularly shaped (patchy) zones with high huttonite component (ThSiO4) are widespread in monazite at deep levels adjacent to Miocene plutons but less common in shallow-level rock; monazite grains with extensive replacement generally have irregular, embayed surfaces. In undeformed rocks distant from the Miocene plutons, monazites are less modified and more nearly euhedral, though fine networks of replacement veins are common and irregular rims are evident in some grains. Secondary monazite from these samples is poorer in huttonite. Ion probe Th–Pb dating yields 60–65 Ma ages for magmatic and some replacement zones in monazite from the shallow samples, and veins yield apparent ages as young as mid-Tertiary. Monazites from deep samples yield a few 55–65 Ma ages for remnant magmatic zones and abundant Miocene ages for replacement zones (∼14–18 Ma). These data demonstrate extensive Miocene replacement of magmatic monazite, especially at deep levels near Miocene plutons, and they suggest an early replacement episode as well. Both events were probably related to influxes of fluid; the first may have been associated with initial solidification of the Ireteba pluton and the second with the Miocene plutons and/or extensional deformation. Ambient temperatures at the time of replacement indicate that secondary monazite growth occurred at T as low as 400°C or less.