REGIONAL TRENDS AND VARIATION OF RAMAN SHIFTS OF QUARTZ INCLUSIONS IN GARNET THROUGHOUT THE ACADIAN TERRANES IN VERMONT AND MASSACHUSETTS

Several studies have utilized quartz in garnet Raman barometry (QuiG) within the Connecticut Valley Synclinorium (CVS) to show that garnet nucleates close to peak P-T conditions, thus serving as reliable monitor of the peak burial attained during the Acadian Orogeny. Raman spectrometry measurements of shifted peaks in quartz inclusions were made along traverses of samples, at multiple locations in the CVS in Vermont and Massachusetts to infer terrane scale trends and to determine how well quartz inclusion Raman shifts correlate with changing metamorphic grade and tectonic features.

Generally, samples show little to no systematic core-rim variation in Raman shifts within individual garnet porphyroblasts indicating garnet nucleated and grew at nearly isobaric and isothermal conditions. West of the Strafford Dome in central VT, Raman shifts between 2.4-3.0 cm^-1 were observed across both the staurolite-kyanite and garnet zones. East of the dome, a discontinuity is observed as Raman shifts fall abruptly from 3.0 to 1.5 cm^-1 across the St-Ky zone and low-grade garnet zone. This observable break in Raman shifts coincides with the location of rocks with mylonite fabric indicating a structural break between these two zones. Within the Goshen Dome a traverse of Raman shifts was collected across the recognized structural boundary separating the Devonian Goshen Fm metasediments from the underlying Ordovician metamorphic rocks. Raman shifts typically vary from 2.8-3.0 cm^-1 within the Goshen Fm., and decrease as the boundary is crossed from a maximum of 2.9 within the Ord. Hawley Fm. to 1.5-2.0 cm^-1 within the western Ord. Moretown Fm. This is broadly consistent with structural observations that there is considerable tectonic movement along a decollement where the higher-grade Goshen Fm. sequence was thrust above the lower grade Ordovician rocks. The overall range of Raman shifts between the Goshen and Strafford traverses are largely similar indicating similar depths of burial for St-Ky and upper Grt zone rocks. Preliminarily collection of Raman shifts from northern VT, in the vicinity of St. Johnsbury, reveal a range of maximum Raman shifts between 1.2-2.0 cm^-1, significantly lower than either traverse. This Raman shift decrease follows the trend of south-north decreasing metamorphic grade along strike.