AN INVESTIGATION OF ARGON RETENTION PROPERTIES OF K-BEARING AMPHIBOLES, SOUTHERN GRENVILLE PROVINCE, USA AND CANADA

Argon retention properties have been studied in K-bearing amphiboles from diverse metamorphic lithologies in three Grenville province terrains, using mass spectrometric, electron microprobe, and X-ray diffraction techniques. These amphiboles exhibit broad ranges in both Mg# (41-61) and A-site occupancy (0.6-1.0), thereby classifying as edenitic hornblendes to hastingsites. Unit cell parameters range as follows:  a  = 9.84-9.93 Å,  b  = 18.05-18.17 Å,  c  = 5.30-5.34 Å, and  b  = 105.0-105.2°. Published ⁴⁰Ar/³⁹Ar cooling ages range from: 1098 to 1012 Ma (Adirondack Lowlands [AL],  n  =  12), 948 to 907 Ma (same outcrop; Adirondack Highlands [AH],  n  = 2), and 1047 to 1007 Ma (Morin Terrain [MT], Québec;  n  = 3). Within the AL, cooling ages decrease systematically from northwest to southeast, a trend that is largely explained by post-closure tilting of the terrain. However, minor local scatter (~5-20 m.y.) along this age-distance trend is potentially attributable to compositional and/or microstructural effects on Ar closure behavior, following published studies documenting such behavior elsewhere. For example, in Zimbabwe and Norway compositional effects on Ar retention are mirrored in calculations of ionic porosity (Z), yet Z values for all but one of the AL amphiboles lie within a tight 37.7-37.9% range indicating that composition does not account for localized age variance here. X-ray analysis indicates that two of the AL amphiboles are twinned. These same amphiboles are also the youngest in their respective localities, suggesting that twin planes provided microstructural avenues for enhanced Ar loss. Another AL amphibole, yielding the highest Z (38.3%), is also the most altered (to biotite) and exhibits the youngest cooling age. Possibly, the relatively open structure favored the alteration, which in turn promoted the enhanced Ar loss, thereby explaining the inverse age-Z relationship. Likewise, the older (less altered) and younger (more altered) AH amphiboles yield Z values of 37.6 and 38.0%, respectively, suggesting that Z proxies for both alteration progress and relative age. Inverse age-Z behavior is also suggested among the MT amphiboles, with an F-rich Mg-hastingsitic hornblende yielding both the oldest cooling age (1047 Ma) and the lowest Z (37.0%) within a calculated Z range of 37.0-37.3%.