Thermal and tectonic history of east central Maine: Insights from 40Ar/39Ar geochronology

The thermal history of east-central Maine was established using 40Ar/39Ar geochronology, which places the ages of metamorphism and cleavage development in a regional tectonic context. The cooling histories of four Devonian plutons obtained using 40Ar/39Ar cooling ages of amphiboles, muscovite, biotite, and K-feldspar form the core of this work. Comparison of the cooling ages of amphibole, through ∼500+/-50ºC, with the cooling ages of biotite, through ∼300+/-50ºC, suggests that the plutons had similar rapid cooling rates of ∼40-60ºC/m.y. Following this rapid cooling, simple regional slow cooling (∼1.3ºC/m.y.) monitored by the cooling ages of K-feldspar began at ∼350 Ma. However, the ages of K-feldspar span a range of up to 50 m.y. The younger of these feldspars show variable and patchy recrystallization in cathodoluminescence images and relatively low Al-Si ordering states with X-ray diffraction analysis. The samples with younger apparent ages are thus explained by recrystallization below the closure temperature of Ar diffusion in K-feldspar. The ages of the regional Acadian foliation (S1) and a later foliation (S2) in the local Chester shear zone were constrained using 40Ar/39Ar geochronology and the ages of the four crosscutting Devonian plutons. The age of S1 cleavage was constrained to be younger than ∼411 Ma and older than crosscutting plutons at ∼407 Ma. The age of the S2 cleavage in the Chester zone was constrained to be younger than ∼380 Ma and older than crosscutting plutons at ∼377 Ma. Thus, the metasediments of central Maine were metamorphosed to lower greenschist facies conditions by Early Devonian, and they were later deformed in the Middle Devonian. The time of S2 cleavage suggests that deformation in the Chester shear zone occurred during the regional accretion of the Meguma terrane to coastal Maine along the Norumbega fault.