ABSTRACT Much of the early prograde history in metamorphic rocks is lost due to overprinting at near-peak conditions or through retrograde modification during exhumation. Fortunately, inclusions encapsulated in rigid porphyroblasts may preserve a record of early burial conditions. Quartz inclusions in garnet porphyroblasts from the Strafford Dome, eastern Vermont, have homogeneous Ti concentrations ([Ti]) that differ from matrix quartz, which retains a history of Si-liberating metamorphic reactions and fluid influx. We applied growth-composition models to evaluate potential processes associated with Ti partitioning in quartz before encapsulation in garnet, including a model for constant-volume growth of quartz due to mineral dissolution-transfer processes and growth as a result of Si-liberating diagenetic and metamorphic reactions. Because these processes typically occur at low temperatures, quartz with exceedingly low [Ti] (<<1 ppm) would be formed and cannot account for the homogeneous Ti distribution at concentrations between 2.5 and 5 ppm observed in the sample. This suggests that chemical reequilibration through dynamic recrystallization must have taken place prior to encapsulation in garnet. Analysis of fluid and graphite inclusions with Raman spectroscopy in different microstructural settings allowed the characterization of fluid composition and temperature of microstructure development early in the prograde history. The findings from this study exemplify the utility of garnet hosts to shield inclusion minerals from chemical modification and recrystallization during later events. As such, they provide a window into the early stages of orogenesis and provide insights concerning the mechanisms controlling equilibration of quartz.

Abstract The Magoffin Member of the Four Corners Formation (Breathitt Group) outcrops in eastern Kentucky as a predominantly coarsening-upward succession of rhythmically interstratified sandstone, siltstone, and mudstone with minor occurrences of limestone at its base. Primary sedimentary structures, trace fossils, vertical successions of facies, and sediment body geometries suggest that these rhythmically bedded sediments were deposited in a delta-front/distributary-mouth-bar setting. The overall thickness of the Magoffin Member is highly variable. Within the study area, it ranges in thickness from less than 25 m to greater than 40 m. Evidence indicates that in areas where the Magoffin Member is thickest, it tends to be sandier and contain rhythmite intervals that are thicker and more complete than in areas where it is thin. The member displays several orders of centimeter- to decimeter-scale cycles that are consistent with semi-diurnal, diurnal, semi-monthly, and monthly tidal periodicities. Halfsynodic (semi-monthly neap-spring-neap) and anomalistic (monthly) lunar periodicities are manifested by the systematic thickening and thinning of the shorter duration cycles. The rhythmite interval ranges in thickness from less than 5 m up to 20 m and records up to four months of deposition. Accumulation rates for the rhythmites typically ranged from 20–100 cm per neap-spring-neap event, but reached rates of over 30 cm per day in areas where the Magoffin Member is thickest and the most proximal deltaic facies are preserved. The nature of tidal bundling suggests that these rhythmites accumulated in a mixed, dominantly semi-diurnal tidal system where both lunar phases and lunar declination influenced tidal cyclicity.