INTEGRATION OF AMPHIBOLE COMPOSITION DATA AND MAFIC SCHIST MODAL SPACE WITH RECENT GEOLOGIC MAPS AND CROSS SECTIONS FROM THE PRE-SILURIAN LITHOTECTONIC BELTS OF CENTRAL AND NORTHERN VERMONT

1:24,000 scale maps and cross sections of central and northern Vermont by the Vermont Geological Survey and collaborators provide a detailed framework for understanding the geologic implications of petrologic, geochemical, and geochronological data on mafic schists. In turn, these data place constraints on the maps, cross sections, and tectonic models.

We show amphibole and plagioclase compositions and 40Ar/39Ar data superposed on geologic maps and cross sections from the Pre-Silurian rocks of northern Vermont; we focus on the mafic complexes from the Worcester Mts and Belvidere Mt/Tillotson Pk farther to the west. The amphibole compositional data are in the form of NaM4(PL), AlVI+Fe3+2Ti+Cr(TK), and NaA+K (ED) substitutions and are divided into pressure facies series for cores and rims; the plagioclase data show mol% anorthite. Modal space calculations are from Am. Mineral., v. 90, p. 843-856. The published 40Ar/39Ar age data indicate that the highest grade metamorphism was Taconian.

In the Worcester Mts, the cores of amphibole from amphibolite in the structurally highest lithotectonic package (Worcester Complex(WC)) shows amphibole growth with increasing PL and TK consistent with epidote-amphibolite facies metamorphism and pressures <7kb. This complex tectonically overlies the Stowe lithotectonic package (LP), a package of albitic schists, phyllite and mafic volcanic rocks, that has amphibole cores with distinctly greater PL/(PL+TK). In modal space plagioclase decreases more to account for the increasing PL in amphibole and is consistent with metamorphism at generally greater pressure (>7kb) than in the Worcester Complex. The Stowe LP structurally overlies the Gold Brook LP comprised of green and black phyllites and mafic rocks, where amphiboles are greenschist facies and locally have elevated PL/(PL+TK).

Amphibole cores from the Belvidere Mt Complex have PL/(PL+TK) ratios similar to those from the Stowe LP. This ratio is significantly greater for amphibole cores from the Tillotson Complex where glaucophane occurs and pressures of 9-14 kb are reported. Generally, amphibole rims in the study area have low PL and TK and indicate, in conjunction with the 40Ar/39Ar data, greenschist facies retrograde metamorphism in the Acadian.