Metamorphism of lower Paleozoic rocks in the vicinity of the Taconic Range in west-central Vermont

Rocks from the Taconic Range and adjacent areas, near Castleton in W.-central Vermont, have been studied for their mineral assemblages. Most of the rocks are argillaceous or arenaceous, of Early Cambrian to Middle Ordovician age, and are from the Taconic allochthone. A number of samples, including carbonates, are from the Lower Ordovician to Middle Ordovician rocks of the surrounding autochthonous marble belt. Regional metamorphism was later than the emplacement of the Taconic allochthone; the metamorphic grades in the taconic sequence cut across structures and conform to the trend in the surrounding autochthone. The grade decreases from E. to W., and the rocks range correspondingly from phyllites to slates and shales. Study of the mineralogy of the samples made use both of the X-ray diffractometer and the petrographic microscope. Two chlorite samples, 1 from a slate and 1 from a phyllite and a chloritoid sample from a phyllite, in addition to 4 rock samples, were chemically analyzed. The most significant mineral assemblages found are: 1) muscovite-chlorite-quartz, 2) muscovite-paragonite-chlorite-chloritoid-quartz, 3) muscovite-chlorite-chloritoid-epidote-hematite-magnetite-quartz, 4) muscovite-stilpnomelane-chlorite-albite-microcline-quartz, 5) muscovite-biotite-quartz-graphite, 6) calcite-dolomite-chlorite-muscovite, and 7) calcite-zoisite-chlorite. The prevalence of assemblage (1), with its simple mineralogy, indicates that chlorite may be variable in the (Fe,Mg)/Al ratio in addition to the Fe/(Fe+Mg) ratio. This is borne out by the chemical analyses; the analyzed chlorites are among the highest reported in their Al/(Fe,Mg) ratios. The basal spacings of coexistent muscovite and paragonite in assemblage (2), found in the phyllites, are 9.97(5) Å and 9.62(3) Å, respectively, indicating very limited solid solution between these phases at this metamorphic grade. No paragonite has been found in the slates; it is suggested that paragonite may possess a lower limit of stability, with the more hydrous assemblage, kaolinite plus albite, taking the place of paragonite plus quartz. Rocks relatively rich in Al contain chloritoid (2,3), and the chloritoid-chlorite association is common even in slates. The chlorite is always more Mg rich although the analyzed chloritoid also shows significant solid solution of Mg for Fe. The only sodic phase found with chloritoid is paragonite. Rocks relatively poor in Al do not have chloritoid but contain stilpnomelane (4), in addi