DATING OF GARNET GROWTH IN UPPER AMPHIBOLITE FACIES SCHIST BY THORIUM-LEAD ION MICROPROBE DATING OF MONAZITE INCLUSIONS

Th-Pb ages from 8–20 µm monazite inclusions in three ~1 cm garnets are interpreted to record occlusion ages. The garnets are from the structurally lowermost horizon of the schist of Stevens Spring, Basin Creek area, Grouse Creek Mtns, Utah and grew from a continuous reaction involving the breakdown of staurolite. Element maps of Mg, Ca, Mn and Fe reveal primary growth zoning, with Ca and Mn decreasing toward the rims, Fe increasing, and Mg being generally flat. Narrow zones (<300 µm) of retrogradation along rims related to diffusive reequilibration are characterized primarily by a decrease in Mg. Gibbs method modeling of growth zoning and garnet-biotite geothermometry indicate that the garnets grew during increasing temperatures from ~600 °C to ~635 °C and through a positive change in pressure of ~0.9 kb, related to thrusting in the hinterland of the Sevier orogen. The ages determined for 44 garnet-hosted monazite inclusions range from 46–70 Ma. With few exceptions, ages decrease from core to rim. There are four possible interpretations: (1) Ages record monazite growth, (2) ages record different amounts of partial Pb loss while in the matrix prior to occlusion, (3) ages record the time of occlusion of monazite by garnet, and (4) ages record differential Pb loss after occlusion in garnet. Experimental diffusion studies of Pb in monazite are conflicting. Option 1 is remotely possible if monazite was being continuously produced in the matrix during garnet growth and if Pb diffusivities are as low as recent RBS depth profiling measurements suggest. We plan Th-Pb measurements of matrix monazites to test this possibility. If Pb diffusivities are as high as reported by Smith and Gilleti (1997), option 2 is discounted because grains would be expected to maintain isotopic equilibrium with their surroundings prior to occlusion; using Arrhenius constants for Pb diffusion in monazite reported in Smith and Gilleti (1997), it was calculated that a cylindrical grain of 6 µm radius retains only 0.23% of its Pb at 600 °C over 10 m.y. Option 4 is discounted by previous studies that estimated rates of Pb diffusion in garnet to be very low at the conditions of interest, consistent with negligible diffusion of radiogenic Pb outward from occluded monazite grains. Currently we favor option 3 and interpret the ages as dating garnet growth from core to rim.