100 MILLION YEARS OF METAMORPHISM IN WESTERN MAINE REVEALED IN POLYMETAMORPHIC TITANITE

Here we present titanite U-Pb dates from two banded calc silicate gneisses (SSP18-1A and 1B) from the Sangerville formation, Lewiston, Maine. Microtextures suggest multiple phases of metamorphism. The peak assemblage is Cpx + Kfs + Pl + Ttn, consistent with lower granulite-facies conditions. Little to no calcite (<3 vol %) is observed and Ep + Tr after clinopyroxene suggests a late influx of H₂O-rich fluids (X_H2O > 0.90). Compositional maps of SSP18-1B reveal early clinopyroxene was intensely fractured and rehealed by one or more phases of higher X_Di clinopyroxene. Fractured clinopyroxene is also infilled by K-feldspar and rimmed by epidote. Albite infills fractures in earlier cyclically zoned higher X_An plagioclase. In contrast, SSP18-1A is more texturally equilibrated and contains only rare Ep + Tr. SSP18-1A titanite exhibits complex overprinting zoning in Al, Fe³⁺, and F, whereas titanite in SSP18-1B exhibits typical core to rim zoning.

Titanite was dated by LA-ICP-MS at the University of Maine using an ESI NWR193^UC excimer laser ablation system coupled to an Agilent 8900 ICP-MS. Single spot U-Pb ages range from 280 to 400 Ma with 12-20 Ma propagated 2SE. In SSP18-1B, four titanite domains are identified: A. 400 ± 8 Ma (dark BSE cores), B. 372 ± 4 Ma (bright BSE cores), C. 342 ± 6 Ma (bright BSE cores, no Eu anomaly), and D. 302 ± 3 Ma (dark BSE rims, low LREE). Low LREE in Group D rims are consistent with the formation texturally late allanite and epidote. BSE dark rims observed in SSP18-1A are low in total REE relative to titanite cores and mantles and give a pooled U-Pb age of 306 ± 3 Ma. Age, texture, and REE pattern are decoupled in interior zones, which suggests incomplete resetting of ^207/206Pb and poor equilibration of REEs during overprinting. Zr-in-titanite temperatures for Group D rims in the quartz-bearing SSP18-1B give a weighted mean T of 764 ± 2 °C at 4.5 ± 0.1 GPa, consistent with field and microtextural evidence for muscovite dehydration and melting reactions. The oldest ages observed here are consistent with the estimated position of the Acadian deformation front in Maine at 400 Ma, whereas our youngest ages correspond to the peak of the Alleghenian Orogeny. These data also show that titanite reliably preserves U-Pb ages and T over many subsequent high-T metamorphic events.