NEWLY RECOGNIZED PETROLOGIC AND GEOCHEMICAL EVIDENCE FOR SILURO-DEVONIAN FELSIC VOLCANISM ALONG THE MARGIN OF THE CONNECTICUT VALLEY- GASPÉ BASIN

Abstract

A Na-rhyolite tuff member, originally mapped as part of the Silurian-aged Shaw Mountain Formation (SMF), occurs in the Bull Run watershed (Roxbury Quadrangle, central Vermont) and suggests a new phase of volcanism. The tuff is found within the Dog River Fault Zone (DRFZ) along the western margin of the Connecticut Valley–Gaspé Basin (CVGB). The tuff occurs in a 0.5 km-wide shear zone as isolated lenses with thicknesses ranging from 1 cm to >10 m, embedded within sheared rocks of the Cram Hill and Northfield Formations. The tuff lenses differ rheologically from the strongly foliated surrounding rocks, aiding preservation of their original textures in the lens centers despite metamorphism and deformation during the Acadian Orogeny and shear in the DRFZ. Where strain is extensive, sericitic phyllosilicates occupy anastomosing microfractures and define a cleavage. Within the lenses, euhedral, twinned phenocrysts of sodic plagioclase, up to one millimeter in length, are set in a fine-grained matrix containing feldspar, quartz and muscovite. ICP-AES analyses of two samples reveal that the tuff has a composition of SiO₂ 71.8-79.0 wt%, Al₂O₃ 12.40-15.20 wt%, Na₂O 1.08-4.77 wt%, and K₂O 3.74-11.25 wt%, classifying the rock as a rhyolite. REE concentrations are similar from sample to sample and have pronounced negative europium anomalies. Zircon crystals extracted from the rhyolite tuff are uniformly euhedral, occurring as bipyramidal prisms. ICP-MS laser ablation of zircons provided U-Pb isotope ratios with a weighted mean average age of 418.2 ± 2 Ma (earliest Devonian) based on 50 points measured. The presence of euhedral plagioclase in the cores of the tuff lenses, the euhedral character of the zircons, and uniform age measurements suggest initial crystallization in a magma chamber preceding explosive volcanism near the CVGB. While tuffs were initially mapped as a part of the SMF along with its quartz pebble conglomerate, clean quartz sandstone, and calcareous arenite members, these volcanic rocks have not been observed in contact with other units in the SMF. These data provide new evidence for felsic volcanism in this area as the CVGB was opening and suggests that the tuffs may not belong to the SMF.