Deformed polymictic conglomerates of the Cambro-Ordovician Tyson Formation are exposed as a number of slivers separated by high-angle thrusts on the east limb of a large anticlinorium at Dry Hill, central Vermont. The clasts of the conglomerate are highly strained with aspect ratios (ratios of the major to minor axis) ranging from 2 to more than 40. Aspect ratios are found to vary as a function of mineral composition and to correlate inversely with average grain size (300-150µm; Evans et al., 1980). Laser fluorination techniques were used to study the variation of d18O values of quartz and muscovite from quartzite and pegmatite quartz clasts as a function of deformation of these conglomerates. In addition, quartz and muscovite were analyzed from the Precambrian Mount Holly Complex, the source of these clasts in the conglomerate. Analyses of this type provide important information on the mechanism and extent of mass transfer involved during extreme plastic deformation at greenschist facies grades.
d18O values of quartz separated from quartzites and pegmatitic quartz within the quartzite source rocks, have similar and relatively homogenous d18O values of 14.0 ±0.9” (2s; n=13). Quartz-muscovite fractionations in these source rocks correspond to temperatures of 510°C (±44°C), that is temperatures which are somewhat low for the primary metamorphism of the source rocks, but consistent with partial re-equilibration in a later, lower-grade event. Quartz from "moderately" deformed quartzite clasts (aspect ratios up to 10) and undeformed pegmatitic quartz within the same conglomerate also have similar d18O values whose range and mean value are identical to those of the source area (d18O = 14.1 ±0.8” (2s), n=15). In contrast, d18O values of quartz from quartzite clasts in highly strained conglomerates, whose aspect ratios exceed about 25, are significantly lower with average values of 12.5 ±0.6” (2s; n=8). Quartz-muscovite fractionations within these samples provide average temperatures of 430°C (±30°C), which are in good agreement with the regional metamorphism in this area (370 to 420°C). Small-scale traverses within moderately and highly deformed quartzite clasts, which focused on a comparison of quartz d18O values between cores, rims and material deposited in pressure shadows of the clasts, exhibit random variations of only ±0.3”, that is, a variation which barely exceeds the analytical error of 0.2”. Variations of similar magnitude are found on the same scale in source area quartzites and are likely related to variations in d18O values of detrital quartz grains within the original sediment.
These observations allow for an interpretation where the plastic deformation in the clasts occurred via recrystallization by grain-boundary migration. This process involved little or no communication between the quartzite clasts and the surrounding matrix (quartz, microcline, albite, sericite, chlorite, biotite and epidote), nor with significant volumes of fluid. Only in quartzite clasts with aspect ratios exceeding 25, that is in clasts of reduced grains size and volume/surface ratio, do the d18O values show a marked decrease. This decrease may be related to diffusive oxygen exchange between the clast and the matrix and/or surrounding fluid, a process that will be tested further through oxygen isotope analyses of matrix quartz.
Evans, B., Rowan, M., & Brace, W.F., J. Struc. Geol. 2, 411-424 (1980).