TOO HOT FOR ZIRCONS? U-PB ZIRCON GEOCHRONOLOGY OF THE ULTRAHIGH-TEMPERATURE (UHT) REGION OF THE CENTRAL MAINE TERRANE, CONNECTICUT, USA

Determining the timing of UHT metamorphism can be difficult as UHT rocks are often overprinted and undergo long cooling histories. While zircon is one of the most robust minerals for geochronology, it too can be affected by extreme temperatures. This study focuses on 18 UHT and non-UHT samples from the southern end of the Central Maine terrane in Connecticut. In-situ U-Pb spot analyses on polished zircon grains were done via SIMS using the CAMECA ims 1270 at UCLA. Most samples are from metapelitic and felsic gneisses of the Brimfield Schist and the Upper Member of the Bigelow Brook Formation. Peak UHT conditions were at least 1000 °C and 1 GPa (Ague et al. 2013; Axler and Ague 2015). Rocks outside the UHT region were also sampled for zircons including the Southbridge formation and the western, non-UHT part of the Brimfield schist. The rocks are thought to have initially been metamorphosed during the Acadian orogeny (423–385 Ma in southern New England; Robinson et al. 1998). The non-UHT rocks preserve mostly (~65% of analyses) pre-Acadian, likely detrital ages ranging from ~430 Ma to ~1800 Ma. The oldest age (1823 ± 24 Ma; 2σ) is from the western Brimfield schist cut off from the UHT zone by a westward dipping thrust fault. In contrast, only about 14% of the UHT zircon analyses yielded detrital ages and these were only found in half of the UHT rocks. The UHT samples do not even preserve the Acadian episode of metamorphism very well; most analyses (nearly 70 out of 110) are Neo-Acadian (355-360 Ma). Furthermore, three samples were found to contain only Neo-Acadian zircons. We infer that at UHT conditions, rocks with lower Zr (and zircon) contents would not retain detrital zircons as easily as rocks with higher Zr contents due to Zr incorporation into rutile and partial melt (Kelsey et. al 2008). Partial melting and migmatization occurred throughout the region; a felsic migmatite vein yields 358.9 ± 5.8 Ma (2σ), consistent with the 355–360 Ma time frame. We conclude that zircons were largely eliminated from low-Zr rocks at UHT conditions and then reprecipitated at 355–360 Ma only after cooling below the zircon saturation temperature. Comparing the populations of zircons in rocks of varying bulk composition, specifically Zr content, can help determine the minimum age of UHT metamorphism.