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

The Ireteba pluton is a relatively homogeneous, similar to 64 Ma (zircon io n probe age) two-mica granite that was intruded by two 16 Ma Miocene pluton s at depths ranging from 5 to 13 km. Deeper levels of the Ireteba and Mioce ne plutons were ductilely deformed at 15-16 Ma. At shallow levels remote fr om the Miocene plutons, the Ireteba granite appears to have experienced lit tle Miocene heating and deformation. Monazites from different portions of the pluton reflect the different histo ries experienced by the host rock. Irregularly shaped (patchy) zones with h igh 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 surfac es. In undeformed rocks distant from the Miocene plutons, monazites are les s modified and more nearly euhedral, though fine networks of replacement ve ins are common and irregular rims are evident in some grains. Secondary mon azite from these samples is poorer in huttonite. Ion probe Th-Pb dating yie lds 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 magmati c zones and abundant Miocene ages for replacement zones (similar to 14-18 M a). These data demonstrate extensive Miocene replacement of magmatic monazi te, especially at deep levels near Miocene plutons, and they suggest an ear ly replacement episode as well. Both events were probably related to influx es of fluid; the first may have been associated with initial solidification of the Ireteba pluton and the second with the Miocene plutons and/or exten sional deformation. Ambient temperatures at the time of replacement indicat e that secondary monazite growth occurred at T as low as 400 degreesC or le ss. (C) 2000 Elsevier Science B.V. All rights reserved.