CARBONIFEROUS OVERPRINTING RELATIONSHIPS IN THE AMMONOOSUC AMPHIBOLITES OF THE MONROE THRUST SHEET, NORTH HARTLAND QUADRANGLE, VT-NH

A petrologic and geochronologic transect of 3 km across amphibolites of the Ammonoosuc Volcanics near West Lebanon, NH shows Carboniferous cooling through amphibole closure with variable preservation of Devonian ages. Amphiboles in the west are 100-1000 μm long, have aspect ratios of ~ 5:1, and are tschermakitic (~6.3 Si per formula unit (p.f.u)). These amphibole grains show decreasing Na and Si towards amphibole beards suggestive of overgrowth during exhumation. An₅ rims on coexisting plagioclase cores (~An₂₀) confirm overgrowth at lower grade. Rare garnet shows increasing MgO and decreasing CaO towards rims suggestive of a clockwise path, and garnet-amphibole-plagioclase thermobarometry (TWQ, Berman, 1991) using rim compositions yields 600° C and 6 kb. ⁴⁰Ar/³⁹Ar step-heating experiments on amphibole from two of these western samples have simple age spectra and reproducible correlation ages of 338 ± 8 and 330 ± 2 Ma.

In contrast, amphiboles in the east, closer to the mapped biotite zone, are as coarse or coarser (up to 5 mm) than those in the west but similar in composition and zoning pattern. Some samples also preserve prograde zoning from actinolitic cores (7.7 Si p.f.u) to tschermakitic rims (6.3 Si p.f.u). Evidence for retrograde deformation includes tschermakitic amphibole grains truncated by chlorite and biotite matrix folia, boudined amphiboles with chlorite-filled necks, and fine grained amphibole fabrics of higher Si content (Si. 6.4-6.7 p.f.u). A U-shaped amphibole spectrum from this area has a minimum age of 372 Ma. A second sample with a retrograde chlorite matrix foliation yields an age spectrum that climbs from ~345 Ma to ~370 Ma, similar to the minimum age of the U-shaped age spectrum.

From these observations we suggest that the western samples underwent a high-grade metamorphism followed by cooling through amphibole closure at ~335 Ma with no preservation of the older metamorphism by the argon isotopic system. The eastern samples also saw high-grade metamorphism, but amphiboles locally preserve prograde zoning, are more clearly affected by a later greenschist facies overprint, and have complex ⁴⁰Ar/³⁹Ar age spectra. We suspect these disturbed age spectra reflect cooling through amphibole closure during the Devonian and partial resetting of the argon isotopic system in the Carboniferous.