CRYPTIC “BARROVIAN” METAMORPHISM IN A REGIONAL LOW-PRESSURE METAMORPHIC TERRANE: AN EXAMPLE FROM SOUTH-CENTRAL MAINE

The eastern metamorphic lobe of south-central Maine is a classic example of a regional low-pressure metamorphic terrane characterized by the pervasive presence of andalusite in pelitic rocks. Previous workers have suggested that these rocks experienced a complex polyphase metamorphic history, but the conditions and timing of metamorphic events early in that history are poorly understood. We present petrography, phase equilibria modelling, and in-situ U-Pb monazite petrochronology that suggest these rocks record an early phase of “Barrovian-style” metamorphism (M1) that was subsequently overprinted by the low-pressure parageneses (M2) that dominate the terrane. Phase equilibria modelling of garnet-staurolite-andalusite and garnet-staurolite-cordierite schists suggests a polyphase history: garnet cores in these rocks grew at c. 0.6-0.8 GPa, 550-600°C (M1), and garnet rims grew at c. 0.3-0.4 GPa, 650-700°C (M2). In contrast, a garnet-chloritoid phyllite only records garnet growth during M1 conditions of c. 0.6-0.7 GPa, 500-550°C, with no record of the low-pressure M2 event despite being within 1 km of andalusite-staurolite-bearing rocks.

In-situ U-Pb petrochronology on compositionally zoned monazite (mnz) in a garnet-cordierite-staurolite schist elucidates the timing of M1 and M2. Euhedral mnz cores grew at c. 420 Ma and typically have high Th/U, low Y, and a negative Eu anomaly. These trace element characteristics are consistent with mnz growth during prograde allanite breakdown and/or staurolite growth, likely during M1. Mnz rims grew at c. 390 Ma with low Th/U, slightly elevated Y, and minimal Eu anomaly, which we interpret to record the timing of the low-pressure M2 event. We interpret the higher pressure M1 event to be the result of crustal thickening or terrane accretion at c. 420 Ma, and M2 to result from low-pressure heating at c. 390 Ma, which obscures obvious traces of M1 in the rock record. The timing of M2 is coincident with crystallization ages of several plutons in the region, which may have provided heat and fluids to drive spatially extensive low pressure metamorphism, though the spatial extent of M2 recrystallization may be more limited than previously thought.