ECLOGITE-FACIES METAMORPHIC ROCKS OF THE WORCESTER MOUNTAINS, STOWE, VERMONT, USA

Recent studies have identified evidence for Taconic high-pressure (HP) (Chu et al., 2016; Honsberger, 2023) and ultra-high-pressure (UHP) (Gonzalez et al., 2020) blueschist/eclogite-facies metamorphism in the northern Appalachian Mountains. The Worcester Mountain Complex (consisting of mafic and pelitic schist of the Stowe Formation) occurs ~25 km south and at the same structural level as the Tillotson Peak Complex where coesite was discovered (Gonzalez et al., 2020). The Worcester Mountain Complex is interpreted to represent fragments of Iapetus oceanic lithosphere that underwent subduction zone metamorphism during the Ordovician Taconic Orogeny and subsequent regional metamorphism during the Devonian Acadian Orogeny (Lanphere and Albee, 1974). It is a key locality to assess the extent of (ultra)-high-pressure, eclogite-facies metamorphic conditions in the northern Vermont Appalachian Mountains.

We present whole-rock geochemical data, mineral compositions, trace element (Zr-in-rutile; Ti-in-quartz) data, as well as pseudosection calculations for samples from three localities within the Worcester Mountain Complex. Pelitic samples contain phengitic white mica (3.1–3.2 Si-per-formula unit), garnet, kyanite, and quartz. Mafic samples contain calcic amphibole, garnet, white mica, and quartz. Accessory minerals include rutile, ilmenite, apatite, zircon, and monazite. Chlorite, chloritoid, and biotite are retrograde minerals that generally occur as pseudomorphs after garnet, kyanite, and white mica. The extent of retrogression varies at the outcrop scale. Garnet is compositionally zoned, with pyrope components as low as 4% in garnet cores to as high as 27% in garnet rims. Titanium concentrations of quartz inclusions in garnet vary little from core (6–18 ppm) to rim (10–19 ppm), whereas zirconium concentrations of rutile inclusions are generally higher in garnet rims (250–375 ppm) as compared to garnet cores (160–250 ppm). Our combined results indicate that garnet grew at eclogite facies conditions (12–20 kbar; 600–685°C). These pressures are considerably higher than previous estimates (6–8 kbar; Laird et al., 2007) and further demonstrate that Taconian rocks in the northern Appalachian Mountains preserve evidence for high-pressure metamorphism.