P-T-TIME PATHS IN EAST-CENTRAL MAINE: INSIGHTS FROM THERMOCHRONOLOGY AND GEOBAROMETRY OF DEVONIAN PLUTONS AND THEIR METAMORPHIC HOST ROCKS

⁴⁰Ar/³⁹Ar step-heating experiments conducted on hornblende (Hbl), biotite (Bt), K-feldspar (Kfs), and muscovite (Ms) from Devonian plutons east and west of the Central Maine Boundary fault (CMBF) support cooling from the Acadian orogeny. These data combined with published zircon U/Pb and whole-rock Rb/Sr ages for the plutons and new Al-in-Hbl geobarometer data constrain estimates for the P-T-t paths for both the igneous and the lower greenschist facies metamorphic host rocks.

The Al-in-Hbl geobarometer yields ~3.5-3.8 kbar average estimates for the pressure of emplacement of plutons, consistent with depths of ~10-11 km. The middle Silurian–early Devonian formations in the area were regionally metamorphosed to lower greenschist facies by ~410 Ma and stayed below the closure temperature (T_C) of Ar in Ms (~350°C) as evidenced by ⁴⁰Ar/³⁹Ar ages of multiple populations of Ms, and supported by assuming a geothermal gradient of 20-30 ºC/km, which implies that the temperature of the country rocks never exceeded 220-330 ºC and a loading rate of no greater than 0.5-2 mm/yr.

Crystallization temperatures of ~750°C from the dated granitoids along with Hbl, Ms, and Bt cooling ages (T_C ~500°C, 350°C, 300°C respectively ), constrain estimates of the cooling rate of the plutons, toward the ambient temperature of the country rocks. Cooling ages of Hbl are younger than the crystallization ages by 12-15 Ma implying a cooling rate of ~17-21°C/Ma. West of the CMBF, Hbl, Ms and Bt cooling ages overlap at ~390 Ma, whereas east of the fault, Bt cooling ages of ~360 Ma are ~5 Ma younger than Hbl cooling ages. The relatively rapid cooling of the western plutons could be due to their smaller sizes. Cooling from ~300°C to the T_C of Ar in Kfs is slower in plutons west of the fault, at a rate of 1.3 °C/Ma compared with 2.7°C/Ma for plutons to the east. This comparison suggests tectonically induced cooling of the plutons east of the CMBF, which supports east-side-up movement along the fault. The net cooling rate through the Triassic of ~1.1ºC/Ma is consistent with an exhumation rate of 0.03 mm/yr. These results along with the ~410-385 Ma ages of Acadian cleavages indicate that peak loading in east-central Maine ended by the Late Devonian, consistent with regional trends established in western Maine.