THERMOBAROMETRIC COMPARISON ACROSS THE RICHARDSON MEMORIAL CONTACT, BETHEL VERMONT

The Richardson Memorial Contact (RMC) in central Vermont marks the contact between Pre-Silurian rocks to the west with the Silurian-Devonian Connecticut Valley Synclinorium (CVS) to the east. There is speculation as to whether the RMC is a fault or sedimentary unconformity. Application of the quartz-in-garnet (QuiG) barometer to metamorphic rocks from both sides of the RMC in Bethel, Vermont provides an additional method for studying this structural feature. QuiG uses shifts of the characteristic quartz 464 cm^-1 Raman band to determine internal pressures on quartz inclusions. The internal pressures are used in equations of state to estimate their pressure of entrapment thus providing an estimate of the conditions at which garnet grew.

Samples collected from the Moretown Formation west of the RMC contain mica-rich layers alternating with coarse quartz-feldspar layers. Mica-rich layers contain a garnet zone assemblage of garnet and biotite porphyroblasts in a matrix of muscovite, chlorite, epidote, quartz, ilmenite, and plagioclase. Garnet and biotite show little zoning in Fe and Mg, and garnet shows depletion of Mn from core to rim. Apatite and zircon are minor phases, and epidote replaces allanite in the matrix. Garnet separates from this layer maintain Raman shifts of 2.31cm^-1, indicating a pressure of 9.4 kbar at the peak temperature of 510°C from garnet-biotite thermometry. A metapelite, with plentiful garnet porphyroblasts, on the east side of the contact provides a pressure estimate of 9.5 kbars at a similar garnet-biotite peak temperature estimate. The similarity of these P-T estimates suggests rocks on both sides of the RMC were metamorphosed at similar depths during the Acadian orogeny. It can be inferred that the RMC in this locality is a sedimentary unconformity, as differing shifts would be expected for a post-metamorphic fault. Previously determined QuiG pressure calculations across the CVS indicate a pressure range of 7.5-12 Kbars, placing the rocks west of the RMC within the peak pressure range of Acadian CVS rocks. QuiG barometry reveals that, in general, garnet nucleated and grew at conditions close to peak metamorphic conditions after considerable overstepping of the garnet isograd.