EVIDENCE FOR OTTAWAN OVERPRINTING IN THE ADIRONDACK LOWLANDS, WITH IMPLICATIONS FOR LOWLANDS-HIGHLANDS JUXTAPOSITION HISTORY

The Adirondack Highlands experienced high-grade thermotectonism during the ~1090-1040 Ma Ottawan orogeny associated with the assembly of supercontinent Rodinia. Yet, it is widely believed that the adjacent Lowlands escaped this orogeny, based upon the apparent lack of appropriate mineral growth ages and, consequently, that the Lowlands and Highlands were separated laterally during Ottawan time. In this study, however, we report occurrences of polygenetic monazite in the Lowlands that preserve Ottawan growth ages and of coexisting hornblendes and biotites that record evidence of post-Ottawan cooling. Monazite was studied in mylonitized gneisses sampled from the Hailesboro Ductile Defomation Zone (HDDZ) which separates a carbonate unit (Lower Marble) and the Popple Hill gneiss and from the Hyde School Marginal Mylonite (HSMM) which separates the 1172 Ma Hyde School gneiss and the Lower Marble. One monazite grain from an HDDZ sample yielded distinct age populations of 1250 ± 30, 1171 ± 16, and 1060 ± 28 Ma (weighted means reported with 95% confidence), which are interpreted as reflecting the Elzevirian, Shawinigan, and Ottawan orogenies, respectively. In contrast, monazite grains from the HSMM samples record neither Elzevirian nor Shawinigan ages, preserving instead an array of ~1120-1080 Ma mixed ages, a distinct ~1060 Ma Ottawan orogenic signature, and subordinate ~900-950 Ma ages on grain rims. Significantly, both the HDDZ and HSMM yield conclusive evidence that the Ottawan orogeny affected the Lowlands, in addition to the Highlands, thus strongly suggesting that these domains were juxtaposed prior to or during Rodinia assembly. Published ⁴⁰Ar/³⁹Ar incremental-heating measurements in metamorphic hornblende and biotite constrain a ~180 m.y. history of uplift, exhumation, and slow cooling (1.6 ± 0.2 °C/m.y.) of the Lowlands following the ~1080-1070 Ma Ottawan peak. The youngest monazite growth ages and mica cooling ages are suggestive of latest HSMM reactivation and final isotopic closure at ~900-950 Ma. This time frame also coincides with the latest extensional normal faulting known along the Carthage-Colton shear zone that separates the Lowlands and Highlands. Regionally, this episode of extension may herald the Neoproterozoic breakup of Rodinia, which was well underway in the western Adirondacks by ~730 Ma.