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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 2°7pb/Z°6pb dating of monazite with precisions as low as 4-5 Ma (2~r). 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°C; P = 8 + 1 kb *) were estimated using inclusion assemblages in garnets from this same sample. Isolated monazite inclusions in garnet yield 2°7pb/Z°6pb 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 /xm) 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 Bridget 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.

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Lead isotopic data on leached K-feldspars, plagioclases, and muscovite provide constraints on the premetamorphic evolution of the Grenville Orogen in Ontario and New York. A boundary between reworked Archean crust to the northwest and new mantle-derived crust generated during mid-Proterozoic times to the southeast is identified within the northern portion of the Gneiss Belt of Ontario. The identification of juvenile mid-Proterozoic crust at the southeastern margin of the Orogen is consistent with previous investigations based on neodymium model ages. However, near the Grenville Front lead isotopes indicate the presence of reworked Archean crust rather than primary early Proterozoic crust postulated on the basis of neodymium model ages. This discrepancy between the lead and neodymium "isotopic terranes" underscores the necessity of using a multidi~iplina~ approach to identify possible terrane boundaries. Only through a combination of the lead with neodymium isotopes is it possible to distinguish between crust that formed during a single major erogenic cycle and crust that represents a mixture of old with new mantle-derived material. The combined neodymium and lead data imply that the Grenville Orogen in Ontario consists essentially of reworked Archean crust and juvenile mantle derived material generated around 1.5 Ga. The Grenville Orogen involved Archean crust that was depleted in thorium and uranium several hundred Ma before high grade metamorphism at ca. 1.1 Ga. Such a depletion may have a significant impact on the geotherm achieved during orogeny and may explain the lower metamorphic thermal gradients observed for the Gneiss Belt.

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