EVIDENCE FOR A LOWER-PRESSURE TRAJECTORY IN THE POST-OTTAWAN COOLING PATH IN WESTERN ADIRONDACK GNEISSES

Petrologic studies of low-variance pelitic assemblages along the Moose River in the western Adirondack Highlands reveal a nearly isobaric cooling trajectory from granulite facies metamorphism through fluid inclusion formation in quartz. Net transfer reactions and exchange equilibria in grt + opx gneiss indicate maximum preserved P-T conditions of 850+/-20º C, 6.6+/-0.6 kbar. Grt + sil + qtz + crd assemblages that cooled through 675+/-50º C at 5.0+/-0.5 kbar preserve post-Ottawan, retrograde metamorphic conditions. Fluid inclusions in quartz constrain the retrograde path to ~225-350º C at between 1-4 kbar. This cooling trajectory approximately parallels the retrograde path deduced from similar studies in the northeastern Adirondack Highlands, but at pressures lower by 1-3 kbar.

These results cast doubt on proposals that maximum temperatures of metamorphism varied systematically across the Highlands. However, these results are consistent with the suggestion of DeWaard (1965) that the central and eastern Adirondacks achieved relatively higher metamorphic pressures, reflected in garnet formation in two-pyroxene amphibolites by the pressure-dependent net transfer reaction opx + pl=grt + cpx + qtz. Further support for relatively lower pressure conditions west of DeWaard’s garnet-out isograd comes from ~3-5.5 kbar conditions described for anatexis in pelitic gneiss around Port Leyden, on the western edge of the exposed Highlands. Florence et al. (1995) originally interpreted anatexis at Port Leyden as coeval with AMCG-related magmatism, but subsequent geochronology has shown that it is likely that melting here occurred during the Ottawan Orogeny.

All petrologic studies considered here are to the north of the Piseco Lake Shear Zone and presumably represent syn-to-post deformational cooling histories within a coherent, mid-to-deep crustal block. Possible explanations for a west-to-northeast increasing pressure gradient include greater crustal thickening in the northeastern Adirondacks during Ottawan tectonics and differential uplift during post-Ottawan exhumation.