P-T-X-t constraints on ductile deformation zones within the Adirondack Lowlands

Thermobarometric, geochemical, and Rb-Sr whole-rock geochronologic results constrain ductile deformation and synchronous chemical changes in mylonitic rocks from two ductile deformation zones (50-200 m thick), the Hailesboro Ductile Deformation Zone and the Hyde School Marginal Mylonite, to the time (ca. 1150 Ma) of regional metamorphism and granitoid emplacement in the Adirondack Lowlands portion of the Proterozoic Grenville Province. Both zones occur at or near marble-gneiss contacts, and while tectonic fabrics indicate some differences in history, each zone is characterized by garnet {dollar}pm{dollar} sillimanite-bearing rocks that developed from gneiss protoliths during deformation partitioning and stress-induced neomineralizations (ca. 620-670{dollar}spcirc{dollar}C, 5.5-8.0 kb).; Textural features in all mylonites indicate that high temperature replacement reactions were synchronous with deformation, but heterogeneously distributed within the rocks. In both zones, garnet, sillimanite, and potassium feldspar are the principle products of syntectonic reactions involving biotite and plagioclase. Chemical results indicate that the protolith of the Hailesboro Ductile Deformation Zone is the Popple Hill Gneiss and that the Hyde School Marginal Mylonite is a marginal facies of the Hyde School Gneiss. Both zones have been modified by syntectonic metasomatic additions and depletions, primarily involving Na, Ca., Fe, Sr, K, and CO{dollar}sb2{dollar}. These zones are proposed as regional, structural discontinuities along which contrasting lithologies were transposed at the time of peak metamorphic conditions in the Lowlands. As such, they are analogues of the synorogenic shear zones that occur both between and within the major tectonic belts that comprise the Grenville Province of Ontario and should be considered significant features that developed during Himalayan-like tectonics of the Grenville Orogen.; This work represents the first comprehensive analysis of the chemistry and age of regional ductile deformation zones in the Lowlands. Although sedimentary protoliths have frequently been proposed for the garnet {dollar}pm{dollar} sillimanite-bearing rocks in both zones, this study indicates that the presence of these phases is the result of localized deformation and not original bulk rock chemistry. Geochronologic results demonstrate that Rb-Sr dating of shear zones can yield the age of synchronous deformation and metamorphism if textural and chemical evidence indicate that retrograde changes were unimportant. The geothermobarometric results show that reconstructions of metamorphic conditions in the Lowlands need to recognize that temperatures within 300 m are {dollar}sim{dollar}100{dollar}spcirc{dollar}C higher than regional temperatures. Retention of high-grade equilibrium Fe/Mg ratios in coexisting biotite and garnet appears to be more likely if the phases are not zoned and the rock contains sillimanite. The addition of sillimanite as a constraint may be a useful in other high-grade terranes where garnet-biotite temperature determinations have been erratic.