40AR/39AR THERMOCHRONOLOGY OF HORNBLENDE AND BIOTITE, ADIRONDACK LOWLANDS (NEW YORK), WITH IMPLICATIONS FOR EVOLUTION OF A MAJOR SHEAR ZONE

⁴⁰Ar/³⁹Ar incremental-heating measurements of Grenville metamorphic rocks from the Adirondack Lowlands, New York, constrain a ca. 1100-900 Ma history of uplift, exhumation and cooling following ca. 1180-1130 Ma tectonomagmatism. Seven hornblende-biotite pairs yield a time-integrated, post-orogenic cooling rate of 1.5 ± 0.2 °C/m.y. between 1100 and 900 Ma (assuming 500 and 300 °C closure temperatures). Also, hornblende and biotite cooling ages exhibit parallel younging trends of 2.0-2.3 m.y./km from northwest to southeast across the Lowlands. Thus, the northwest Lowlands terrain was cooling slowly through ca. 500 °C at ca. 1100 Ma under tectonically quiescent conditions while the now-adjacent Adirondack Highlands terrain was being overprinted by ca. 1090-1045 Ma granulite-facies metamorphism (Ottawan orogeny) known to have peaked at ca. 1070-1080 Ma. This relationship suggests that the regional trends record final juxtaposition of the cooling Lowlands against a warmer Highlands some time between ca. 1100 Ma (oldest hornblende) and ca. 900 Ma (youngest biotite), and that regional cooling was thereby progressively delayed from northwest to southeast. The Lowlands ⁴⁰Ar/³⁹Ar data coupled with published ⁴⁰Ar/³⁹Ar data from the Highlands bear upon recently proposed models for evolution of the Carthage-Colton shear zone (CCSZ) that now separates these terrains. In particular, the regional younging trend observed in Lowlands hornblendes continues uninterrupted into the western Highlands, with no evidence for a 100 m.y. offset across the CCSZ as previously suggested by others; the parallel biotite trend flattens out at 900-920 Ma across the CCSZ. The apparent lack of offset in the hornblende ages may preclude a scenario of strictly post-950 Ma extension along the CCSZ, as proposed in one published model. Instead, the regional age trends may be more consistent with a published scenario of much earlier orogenic collapse along the CCSZ beginning at ca. 1050 Ma (i.e., late Ottawan), which largely juxtaposed the Lowlands (hanging wall) and Highlands (footwall) into their current structural positions. Following this model, slow cooling across the Lowlands progressed through gently northwest-dipping isotherms that remained inclined from 1100 Ma until final thermal equilibration with the Highlands at 900-920 Ma.