MONAZITE U-TH-PB GEOCHRONOLOGY OF THE CENTRAL METASEDIMENTARY BELT BOUNDARY ZONE (CMBBZ), ONTARIO CANADA

The Central Metasedimentary Belt boundary zone (CMBbz) is a crustal-scale Grenville shear zone exposed in the classic Grenville transect in New York and southern Ontario. The CMBbz juxtaposes the Central Gneiss Belt and the Central Metasedimentary Belt. Previous geochronological work on the timing of deformation and metamorphism in the CMBbz is ambiguous, and the questions that motivate our study are: How many episodes of shear zone activity did the CMBbz experience, and what is the tectonic significance of each episode? We present electron microprobe data from monazite (the U-Th-Pb method) in order to directly date deformation and metamorphism recorded in five garnet-biotite gneiss samples collected from three localities of the CMBbz (West Guilford, Fishtail Lake, and Killaloe). All three localities yield youngest monazite dates ca. 1045-1040 Ma; most of the monazite domains that yield these dates are high Yttrium (Y) rims that probably grew during late retrogressive metamorphism associated with crustal thinning and extensional shear zone activity at the base of an orogenic lid. In comparison to this common retrograde history, the prograde history of the three CMBbz localities is not clearly shared. The West Guilford samples have monazite grain cores that show older high Y domains and younger low Y domains; these cores yield a prograde Ottawan (1100-1075 Ma) history. The Killaloe samples yield a well-defined prograde Shawinigan history (i.e. 1220-1160 Ma) in addition to some evidence for a second prograde Ottawan event. In other words, the answers to our research questions are: three events; a prograde Shawinigan event associated with crustal thickening, a prograde Ottawan event associated with another round of crustal thickening, and a late Ottawan retrogressive event associated with crustal thinning at the base of an orogenic lid. We also present data from other CMBbz samples that contain monazite domains characterized by extremely low Th. Young dates ca. 950 Ma from these domains may record late resetting by fluid-related dissolution and reprecipitation.