VARIATIONS IN BIOTITE 40AR/39AR AND RB-SR AGES ACROSS THE ADIRONDACK LOWLANDS, NY

The behavior of the K-Ar and Rb-Sr isotopic systems in biotite from diverse metamorphic lithologies of the slowly-cooled Adirondack Lowlands has been examined with variety of approaches. Our previous reports have constrained the ~1100-900 Ma uplift, exhumation, and cooling in the Lowlands, showing a regional pattern based upon incremental-heating of hornblende and biotite separates. These ages demonstrate trends of decreasing age from the St. Lawrence River to the Carthage-Colton shear zone, NW to SE, which is most readily explained as reflecting regional tilting by ~9° subsequent to ~900 Ma and define a ~1.5 °C/m.y. cooling rate. Variations in biotite ⁴⁰Ar/³⁹Ar age around the central trend are large, with excess Ar suspected as the cause in general. Rb-Sr analyses of small biotite samples were undertaken to examine for excess Ar; these demonstrate that most biotites anomalously older than the trend contain excess Ar. The variations in biotite age do not appear to be related to composition or crystal chemistry. Analyses of small sample aliquots and single grain fragments for Ar were undertaken to examine small-scale variations; these show significant intrasample variations. These age variations are seen for all samples, being most pronounced (> 200 m.y.) where biotite contains appreciable excess Ar with smaller ones (10's of m.y.) for samples without obvious excess. This includes even biotites that give a concordant release spectrum with a plateau. Obviously, the small-scale age variations are homogenized, albeit to varying degrees, with incremental Ar release during vacuum heating. The variations are too large to reflect age gradients that might be produced during the slow cooling of the Lowlands, nor do they appear to reflect presence of other mineral impurity phases. These results underscore the desirability of combining Rb-Sr measurements and microsampling techniques in regional thermochronology studies even for relatively “simple” terranes.