2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 4
Presentation Time: 8:45 AM


MILLER, M.R.1, DAHL, P.S.1, FOLAND, K.A.2 and RAYMOND, C.C.3, (1)Dept. of Geology, Kent State Univ, Kent, OH 44242, (2)Geological Sciences, Ohio State Univ, Columbus, OH 43210, (3)Dept. of Chemistry, SUNY Oswego, Oswego, NY 13126, pdahl@kent.edu

Argon retention properties of hornblende have been studied using diverse metamorphic lithologies in the Adirondack Lowlands and Highlands — using mass spectrometry, microprobe, and X-ray diffraction. Incremental-heating of hornblendes and biotites constrain ~1100-900 Ma uplift, exhumation, and cooling in the Lowlands following the ~1270-1170 Ma Elzevirian orogeny. Ages of hornblendes and biotites define parallel younging trends of ~3 m.y./km across the ~45 km between the St. Lawrence River and the Carthage-Colton shear zone that separates the Lowlands from the Adirondack Highlands domain. These trends, especially in hornblende, are attributable to post-900 Ma tilting of the Lowlands domain by ~9° to the NW. Hornblende 40Ar/39Ar ages range from ~1125 to ~1000 Ma from NW to SE across the Lowlands, and age-distance regression indicates that ~67% of the age variance is attributable to the regional pattern (i.e., tilting) alone. The remaining ~33% apparently embodies other factors potentially related to Ar retention per se, such as microtexture and crystal chemistry. Indeed, the apparent age spread for a given locality is ~40 m.y., which, coupled with a known cooling rate of ~1.5 K/m.y., translates into a nominal ~60 K range in hornblende Ar closure temperature. The regional age-distance trend for hornblende resolves into three subparallel compositional trends: magnesio-(hastingsitic) (oldest), pargasitic-tschermakitic, and edenitic (youngest) — listed in order of decreasing Ar retention and increasing ionic porosity. Multiple linear regressions of 40Ar/39Ar age (dependent variable) versus distance and various combinations of ~35 crystal-chemical parameters (independent variables) have been performed. The most robust of these, statistically, relates age to a combination of distance, Mg#, Na content (A-site), Si content (T-site), and the beta angle. Another highly significant regression, in which ages are first normalized by sample location, also involves several Highlands hornblendes; this regression of normalized age incorporates the compositional terms listed above, plus bulk ionic porosity and several parameters specifically related to A-site geometry. These regressions both account for ~86-90% of the 40Ar/39Ar age variance observed among Lowlands (and Highlands) hornblendes, suggesting that Ar retention reflects crystal chemistry.