OXYGEN ISOTOPE THERMOMETRY OF QUARTZITES, SOUTHERN ADIRONDACK HIGHLANDS

Quartz-garnet oxygen isotope thermometry in quartz-rich metasediments from the southern Adirondack Highlands (Grenville Province, New York) yields metamorphic temperatures 700-800°C, consistent with granulite-facies mineral assemblages. Samples from the Irving Pond quartzite (Canada Lake antiform) record D(Qtz-Grt)=2.58±0.58 (n=16), corresponding to peak metamorphic conditions of 733±37°C. This agrees well with the 710-740°C estimate from garnet-biotite exchange thermometry (Storm and Spear, NEIGC 2002). Similar temperature estimates are obtained from Swede Pond (682±47°C, n=3) and King’s Station (ca. 700°C). The Whitehall shear zone records higher temperatures (798±25°C, n=3). All of these temperatures are higher than regional temperature estimates (Bohlen et al., J. Pet. 1985). Only nearly monomineralic rocks are predicted to preserve peak stable isotope fractionations during closed-system slow cooling after metamorphism. Absence of a second exchangeable oxygen reservoir (e.g. feldspar) should limit the amount of retrograde oxygen exchange. In garnet-bearing quartzites, quartz-garnet fractionations should not change during cooling because quartz would have no other minerals with which to exchange (and slow oxygen diffusion in garnet). Additional feldspar should cause systematic retrograde resetting from exchange between quartz and feldspar. To test this hypothesis, rocks of variable quartz:feldspar ratio were analyzed. Peak metamorphic temperatures are preserved in both quartz-rich and feldspar-rich rocks, and no correlation between fractionation and modal mineralogy is observed. This suggests that slow oxygen diffusion in both quartz and feldspar has been caused by low water fugacity during metamorphism.