Investigations of monazite and garnet chronology and applications to the Archean gneiss terrane of the Wind River Range, Wyoming.

A technique for evaluating the contributions of monazite inclusions to the budgets of important trace elements (U, Pb, Sm, Nd) in garnet has been developed in this thesis. This technique combines the use of uranium mapping by the induced fission track method with stepwise dissolution of bulk garnets and isotopic analysis by thermal ionization mass spectrometry. Knowledge of the relative importance of inclusions in the budgets of these elements is important for accurate interpretation of U-Pb and Sm-Nd ages of metamorphic garnets. Lead isotope measurements on zoned monazites and monazite inclusions in garnet from the northern Wind River Range have been obtained by Secondary Ionization Mass Spectrometry. Small monazite inclusions (ca. 10-30 $\mu$m) in garnet that are isolated from fractures preserve older $\sp{207}$Pb/$\sp{206}$Pb ages than matrix monazites. A large zoned monazite preserves the oldest ages along a rim where lamellar zoning in major elements is also preserved. These results indicate that these monazites have had their U-Pb systematics reset by a process other than volume diffusion. Conventional U-Pb, $\sp{40}$Ar-$\sp{39}$Ar and SIMS Pb-Pb data indicate a ca. 200 Ma metamorphic history for the late Archean metamorphism of the Wind River Range. The cooling and denudation rates indicated by these data suggest two metamorphic events rather than a single P-T path involving early isothermal decompression. Electron microprobe analyses of monazites from metapelites, and S-type granites indicate significant variation in extent of huttonite-cheralite-monazite solid solution between grains in a single thin section. Preservation of this chemical variability in monazites from granulite facies metapelites may indicate low diffusivities for these elements. Pb isotopic compositions from leached feldspars are used to constrain the premetamorphic history of the Grenville Province. Evidence for juvenile mid-Proterozoic crust in the Central Metasedimentary Belt, Adirondack Highlands and Adirondack Lowlands. Evidence for an Archean Pb component, and a pre-Grenville reduction in $\sp{238}$U/$\sp{204}$pb is indicated for the northern Central Gneiss Belt.PhDGeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/103567/1/9332045.pdfDescription of 9332045.pdf : Restricted to UM users only