UNUSUAL ORIENTED MULTIPHASE NEEDLES IN GARNET FROM THE ULTRAHIGH-TEMPERATURE GRANULITES OF NORTHEASTERN CONNECTICUT, USA

Garnets in mantle rocks, ultrahigh-pressure (UHP) rocks, and high-temperature to ultrahigh-temperature (UHT) granulites may contain multiphase solid inclusions which are generally thought to be the remnants of fluid/melt trapped in growing garnet. Multiphase inclusions are typically “boxy” or preserve a negative crystal shape. This study investigates unusual oriented needle-shaped multiphase inclusions found in the cores of garnets in felsic granulites of the Upper Member of the Bigelow Brook Formation, CT. The rocks are part of the Acadian/Neoacadian orogen and contain garnet+sillimanite+K-feldspar+plagioclase+quartz+cordierite+biotite±spinel. The sillimanite is commonly pseudomorphous after kyanite. Common phases in the inclusions include quartz, feldspars, micas, chlorite, rutile, ilmenite, graphite, and a quenched felsic melt phase. The multiphase inclusions and associated needle-shaped precipitates of rutile and ilmenite are oriented parallel to <111> of garnet. Compared to most multiphase inclusions, these oriented multiphase inclusions are much more elongate and tubular. The closest analogs of which we are aware are found in UHP garnets of the Erzgebirge (Perchuk, 2008). The multiphase needles are surrounded by radiating cracks; we infer that the inclusions were originally more equant, but expanded when the internal pressure of the inclusion exceeded the combined external pressure and tensile strength of the garnet (e.g., Stöckhert et al, 2001; 2009) along intersections of {110} partings that define <111>. Chemical analyses of phases were done using the field emission gun electron microprobe at Yale University. Zr-in-rutile thermometry (Tomkins et al., 2007) yields an average temperature of 1008 ± 11 ºC (2σ; n=10) for the rutile needles within garnet cores at a representative P of 1 GPa. Therefore, the newly discovered UHT zone of the Central Maine terrane of northeastern CT (Ague et al., 2013; Geology) has been extended to the east to include the Upper Member of the Bigelow Brook Formation. Consequently, the only confirmed UHT locality in the United States is larger than initially recognized, and is at least 25 km long and 5-10 km wide. The oriented, elongate multiphase inclusions and oriented rutile needles may be useful indicators of extreme T and/or P rocks in other field areas.