Pressure, temperature, and time indicators in mafic schist; their application to reconstructing the polymetamorphic history of Vermont

Abstract

Mineral chemistry and overgrowth relationships in mafic schist from Vermont are shown to be sensitive indicators of pressure, temperature, and relative time and to be good chronicles of the Paleozoic history of this polymetamorphic terrane. Within the common assemblage, amphibole + chlorite + epidote + plagioclase +quartz+ Ti-phase ± carbonate ± K-mica ± Fe^(3+)-oxide, electron microprobe analyses show that increasing metamorphic grade (as defined by intercalated pelitic schist) is recorded by an increase in the edenite, glaucophane, and tschermakite contents of amphibole, in the anorthite content of plagioclase, and in the substitution of (Al^(VI),F^(2+), Ti), Al^(IV) for (Fe^(2+), Mg, Mn), Si in biotite, chlorite, and muscovite. With increasing pressure the glaucophane component of amphibole increases. For medium-pressure metamorphism the albite-oligoclase gap is in the garnet zone, where amphibole has between 1.2 and 1.8 formula proportion Al^(IV) and (Al^(VI) + Fe^(3+) + Ti + Cr). This gap is below the garnet isograd in low-pressure mafic schist where Al^(IV) and (Al^(VI) + Fe^(3+) + Ti + Cr) in the amphibole are both less than 0.6. Mineral growth periods observed are characterized by metamorphic grade and facies series and are assigned to two Ordovician (Taconic) and two Devonian (Acadian) events. Silurian-Devonian mafic schist in northeastern Vermont records two periods of low-pressure, Devonian metamorphism. These events are expressed by medium-pressure and low-pressure metamorphism in the Cambrian and Ordovician rocks to the west and south. In the pre-Silurian rocks high-pressure and medium-pressure metamorphism is assigned to the Ordovician. The observed high-pressure metamorphism is confined to a 110 by 40 km area along the Green Mountain anticlinorium axis in north-central Vermont.