Investigating Records of Prograde Metamorphism in Quartz with TitaniQ Thermobarometry: Initial Results from the Northfield Mountains, Vermont

We present results of microstructural, petrological, and chemical analyses on pelitic schists of the Green Mountain Cover Sequence in the Northfield Mountains of central Vermont. Sampling was conducted along a structural transect from Royalton to Rochester, Vermont, crossing the Richardson Memorial Contact and encompassing the transition from strictly Acadian to combined Taconic and Acadian deformation and metamorphism. The transect covers a region exhibiting a series of interbedded phyllites, quartzites, and calcic schists that form the eastern limb of the Green Mountain Anticlinorium. Petrology and microstructures record two major prograde events resulting from nappe emplacement events. An increase in peak metamorphism from chlorite grade in the west to garnet grade in the east provides a well-defined thermal and barometric transect along which to investigate the application of the Ti-in-quartz (TitaniQ) thermobarometer to constrain P-T-t-D histories. Cathodoluminescence (CL) imaging of quartz provides a qualitative depiction of [Ti] and intensity based upon the positive relationship at λ = 415 nm. Based on our results to date, two major trends in CL patterns have been recognized: a distinct difference between matrix and inclusion quartz in garnet-bearing samples, and a progressive transition in zoning patterns across the entire transect. In chlorite grade rocks to the west, matrix quartz is more uniform under CL. In higher grade rocks to the east, matrix quartz typically has dark cores with bright rims, while inclusions within garnet typically have either patchy dark cores with bright rims or uniformly bright cores with brighter rims. Furthermore, quartz grains from the lower grade samples frequently exhibit banding in CL, potentially related to planar defects that have been removed at higher P-T conditions. Zones of banding are restricted to the Q domains of the dominant (S2) schistosity. Banding appears to be frequently aligned with the sweeping direction of undulatory extinction within the quartz grains, although several grains exhibit either multiple orientations or a single orientation of banding oblique to the sweeping of extinction. Initial X-ray mapping of garnets from Acadian prograde metamorphism show decreasing Xsps, and increasing Xalm, Xprp, Xgrs. This variation of chemistry indicates pressure increase and minor temperature decrease during garnet growth after the development of the S3 crenulation cleavage, indicative of crustal thickening. These observations indicate that quartz in different microstructural contexts record different stages of the P-T-t-D history of the Northfield Mountains and highlights the need to integrate microstructural analyses and CL intensity patterns to guide the placement of [Ti] spot analyses and interpret the results of TitaniQ thermobarometry.