USING MONAZITE TO MAP THE SPATIAL AND TEMPORAL DISTRIBUTION OF FLUIDS DURING CONTACT METAMORPHISM: BIRCH CREEK PLUTON

Monazite occurs in the central and border suites of the two-mica Birch Creek pluton in the White Mts., eastern CA and in its 1-3 km diameter hydrothermal aureole containing Precambrian metasediments. We used the Cameca ims 1270 ion microprobe at UCLA to measure in-situ Th-Pb ages, and oxygen isotope ratios uncorrected for mass bias and expressed in per mil relative to the SMOW standard. Monazites in the border and central suites display concentric euhedral zoning, mean d18O=1.7 ± 0.2‰ and Th-Pb ages of ~80 Ma and ~76 Ma respectively. Monazites in the Deep Springs quartzite ~ 1 km from the contact show concentric euhedral zoning and d18O=-1.1 ± 0.3‰, but monazites ~0.3 km from the contact display patchy chaotic zoning, d18O=2.4 ± 0.5‰ and Th-Pb ages of ~78 Ma, suggesting that fluids associated with intrusion of the border suite infiltrated quartzites close to the contact and exchanged oxygen with monazite while causing it to recrystallize and reset. In contrast, zircons in country rocks experienced no new growth or recrystallization, with most yielding 207Pb/206Pb ages of ~1.0 Ga to 2.7 Ga, but a few that experienced slight Pb loss lie along a discordia with a poorly-defined lower intercept of 80 Ma. Monazite in hydrothermal aureoles can be used to map the extent and date the timing of fluid infiltration during contact metamorphism.