MONAZITES IN THE NW ADIRONDACK LOWLANDS RECORD EVIDENCE OF THE OTTAWAN ORGOENY

Electron microprobe U-Th-Pb dating of monazite (Mnz) from two ductile deformation zones in the Adirondack Lowlands was conducted in order to evaluate the possibility of multiple recrystallization/neomineralization events. The dated rocks included three garnet-sillimanite gneisses of the Hailesboro Ductile Defomation Zone (HDDZ) separating the Lower Marble from the Popple Hill Gneiss (metadacite), and two garnet-sillimanite gneisses and calc-silicate rocks from the Hyde School Marginal Mylonite (HSMM) which separates the 1172 Ma Hyde School Gneiss (alaskite to diorite) from the Lower Marble. Twenty-five analyses on one Mnz grain from a HDDZ sample yield distinct age populations of 1250 ± 30, 1171 ± 16, and 1060 ± 28 Ma (weighted means reported with 95% confidence). These ages are interpreted as reflecting pre-Elzevirian inheritance, the Elzevirian orogeny, and the Ottawan orogeny, respectively. These ages occur also in other Mnz grains, along with intermediate dates interpreted as the result of age mixing. The pre-Elzevirian inherited ages are consistent within error with a published Rb-Sr WR age of 1296 ± 21 Ma for the Popple Hill Gneiss, whereas the Elzevirian ages are consistent with published Rb-Sr WR ages and U-Pb ages of zircon, sphene, and apatite that have been recognized previously throughout the Lowlands. In contrast to HDDZ Mnz grains, those in the HSMM record neither the pre-Elzevirian nor Elzevirian ages, unlike the Hyde School Gneiss per se in which both ages have been previously documented. Instead, HSMM Mnz grains show an apparent mixing array in the ~1120-1080 Ma range, a distinct ~1060 Ma Ottawan age signature, and subordinate ~900-950 Ma ages on grain rims. The absence of Mnz of ~1250 or ~1170 Ma age in the HSMM suggests that post-1170 Ma mylonitization overprinted such grains. The ~900-950 Ma Mnz ages in the HSMM are suggestive of late-stage recrystallization during reactivation of this ductile deformation zone. This time frame is also consistent with known regional extension, during incipient break-up of supercontinent Rodinia. Most importantly, this study reveals the first definitive indication that the Ottawan orogeny affected the NW Adirondack Lowlands, thus permitting the possibility that the Highlands and Lowlands domains were juxtaposed prior to or during Rodinia assembly.