IMPLICATIONS OF CHEMICALLY ZONED TSCHERMAKITES IN AMPHIBOLITES OF THE BRONSON HILL TERRANE, NORTHERN CONNECTICUT

Preliminary results of detailed electron microprobe analysis of lineated amphiboles are used to constrain the metamorphic evolution of a portion of the Bronson Hill Terrane, CT. These rocks are classically understood to be part of the Acadian metamorphic high, but recent ⁴⁰Ar/³⁹Ar data show that these rocks reached amphibolite facies metamorphic conditions in the Alleghanian (Wintsch et al., 2003). We report results of microchemical analyses of strongly lineated amphiboles to characterize this Alleghanian metamorphic evolution. The amphibolites analyzed are composed primarily of amphibole + plagioclase + epidote + titanite and show strong north-northeast striking foliation and northwest plunging lineation. Amphibole and plagioclase spot analyses were oriented parallel to the lineation-defining c-axis of the amphiboles. Amphibole compositions lie in the tschermakite field with 6.1 to 6.3 Si cations p.f.u. Plagioclase composition varies from Ab₆₆ to Ab₇₃. Amphiboles show zoning with Si, Mg and Na decreasing from core to rim, and Al, Ti, and K increasing from core to rim. Holland and Blundy (1994) edenite-richterite thermometry is used to calculate the temperature of crystallization of these grains. Calculated temperatures range between ~685 and ~730°C from core to rim with a maximum variation of about 30°C in a single grain. This type of temperature zoning pattern is suggestive of prograde growth and is consistent with the measured increase in Ti concentration toward the rims, which is generally indicative of an increase in temperature. Based on one-dimensional thermal modeling (Wintsch et al., 2003) we conclude that these grains were crystallizing during decompression and exhumation along a clockwise P-T-t path after the peak pressure but before the peak temperature during the Alleghanian orogeny ~275 million years ago.