A METAMORPHIC HISTORY FROM MICRON-SCALE PB-207/PB-206 CHRONOMETRY OF ARCHEAN MONAZITE

Ion microprobe measurements of Pb isotope ratios in monazites have been obtained, in situ, from thin sections using the Cambridge ISOLAB 120. Molecular interferences are sufficiently resolved at an RP of 6500 to allow Pb-207/Pb-206 dating of monazite with precisions as low as 4-5 Ma (2 sigma). The results presented here provide important information on the chronological history of the Late Archean metamorphism of the Wind River Range, Wyoming (USA). Matrix monazites and monazite inclusions in garnets from a metapelite from the northern Wind River Range have been analysed by SIMS. In a previous study peak metamorphic conditions (T=800 degrees C; P=8+/-1 kb *) were estimated using inclusion assemblages in garnets from this same sample. Isolated monazite inclusions in garnet yield Pb-207/Pb-206 age estimates of 2781+/-6 to 2809+/-10 Ma. Those along fractures yield lower ages (2603-2687 Ma) which are similar to TIMS and SIMS ages of matrix monazites. A single large(500 mu m) monazite grain locally preserves growth zoning, but has a recrystallised core and a resorbed (recrystallised?) rim. Age estimates for these three regions are 2788+/-9 Ma, 2663+/-4 and 2523+/-6 Ma, respectively. Thus the inclusion assemblages of Sharp and Essene * may record peak metamorphic conditions at ca. 2.8 Ga, and indicate a phase of metamorphism that predates by over 100 Ma the emplacement of the Bridger Batholith, the major lithologic component of the northern Wind River Range. The analysed monazite grains appear to preserve ca. 300 Ma history, even within a single grain. Monazite inclusions in garnet that are fully armoured may provide estimates for the time of garnet growth, even in high grade terranes where most chronometers are reset. The age pattern preserved by the large monazite grain cannot be simply related to diffusion controlled closure. Instead, a chronology is preserved which can be related to the petrographic setting of individual grains through in situ analysis.