NEW CONSTRAINTS ON THE TIMING OF DEFORMATION IN THE MABERLY SHEAR ZONE IN THE GRENVILLE PROVINCE OF SOUTHERN ONTARIO

We present new U-Th/Pb monazite dates for the development of the Maberly Shear Zone (also known as the Sharbot Lake Shear Zone) in the Grenville Province of Southern Ontario, Canada. This shear zone is of regional tectonic significance as a major boundary within the Central Metasedimentary Belt; it juxtaposes the Sharbot Lake Terrane to the west against the Frontenac Terrane to the east. Our sample is a garnet sillimanite schist with well-developed foliation and lineation parallel to the regional fabric in the Maberly Shear Zone. We report dates based on in-situ electron microprobe analysis of monazite grains, and compositional domains within these grains, conducted at the UMass Geosciences Electron Microprobe/Scanning Electron Microscope Facility. Dated monazite grains occur both within garnets and in the deformed matrix of the schist, and range in size from tens to hundreds of microns in scale; grains are relatively round and equant, although a few show unusual shapes. We dated domains in ten monazite grains, many of which show multiple domains that we defined based on domain geometries and variations in Y and Th content. We present twenty-six dates that range from 1222 ± 2.6 to 1145 ± 6.5 Ma (all errors reported here at 1 sigma). High-Y cores of monazite grains yield dates whose mean is 1206 ± 3.5 Ma, and lower-Y inner regions of grains yield mean dates of 1190 ± 4.4 Ma. Inner mantles yield a mean date of 1182 ± 3.4 Ma; outer mantles yield a mean date of 1172 ± 4.0 Ma. Some grains show geometric rims, not distinguished by unusually high Y content as is common in similar rocks; and mean rim dates are 1163 ± 4.2 Ma. All of these dates are in excellent agreement with predictions based on field mapping in the region and dates yielded by U-Pb analysis of zircon from neighboring granitoids (Corfu and Easton, 1997; Davidson and van Breemen, 2000). We interpret these dates as a record of shearing and high-grade metamorphism of metasediments in the Maberly Shear zone starting before and continuing through the later phases of the Shawinigan Orogeny.