40Ar/39Ar Release Spectra of Biotite and Hornblende from the Cortlandt and Rosetown Plutons, New York, and Their Regional Implications

Biotite and hornblende from the western portion of the Cortlandt complex record undisturbed $$^{40}Ar/^{39}Ar$$ incremental-release spectra with average total-gas dates of 420 m.y. (hornblende) and 390 m.y. (biotite). Biotite from the younger phase of the Rosetown pluton also records undisturbed release spectra with an average total-gas date of 420 m.y. Coexisting hornblendes display more complex spectra (a result of excess argon contamination); however, they have similar high-temperature "plateaus" at 450 m.y. Both biotite and hornblende from the older portion of the Rosetown have disturbed release spectra, possibly a result of excess argon contamination (hornblende) and partial postcrystallization gas loss (biotite). The disturbed spectra preclude a definitive estimate of absolute age but the data do suggest a minimum date of ~485-500 m.y. Because field criteria (Ratcliffe 1968, 1971) clearly indicate that both the Cortlandt and the younger Rosetown postdate a regionally pervasive progressive metamorphism, it is suggested that their differences in geochronology reflect regional variations in times of postmetamorphic cooling and that the 450-m.y. date defined by the Rosetown hornblendes closely approximates the time of intrusion of both plutons. This conclusion requires that the regional metamorphic event was Taconic in age. The undisturbed incremental release spectra of the Cortlandt samples indicate no substantial argon loss which could be attributed to Acadian reheating. This substantiates the conclusion that regional metamorphic zonation is Taconic rather than Acadian, and suggests that this area was little affected by Devonian or later reheating. The difference in probable age of the older part of the Rosetown (485-500 m.y.) and the cross-cutting younger phase (450 m.y.) bracket a period of reactivation of the Ramapo fault system and suggests that fault activity was concurrent with Middle Ordovician Taconic metamorphism.