Phase Equilibria in Mafic Schist and the Polymetamorphic History of Vermont

Citation

Laird, Jo (1977) Phase Equilibria in Mafic Schist and the Polymetamorphic History of Vermont. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/nd16-8e08. https://resolver.caltech.edu/CaltechTHESIS:10312023-155456278

Abstract

Phase relationships in metamorphosed mafic rocks from Vermont and adjacent Quebec are presented and used to develop an understanding of the petrogenesis of this common rock type and to reconstruct the Paleozoic history of the region. Samples have been collected from Cambrian to Devonian mafic and pelitic schists. The metamorphic grade defined by the phase assemblages in the pelitic layers ranges from biotite to sillimanite. Electron microprobe analyses of the minerals in the mafic rocks are correlated with the physical conditions of metamorphism, and overgrowth relationships in amphibole grains record relative time. From these data and field and isotopic studies, mineral growth periods are characterized by metamorphic grade and facies series and assigned to two Ordovician and two Devonian events. The continuous and discontinuous reactions which describe the mineral chemistry and abundance variations in basaltic compositions are inferred from graphical and least-squares analyses.

In mafic rocks the common assemblage observed is amphibole-chlorite-epidote-plagioclase-quartz-carbonate-K mica-Ti phase-Fe³⁺ oxide. Medium pressure progressive metamorphism of this assemblage causes the following continuous changes. 1) Ca-amphibole is enriched in the tschermakite and edenite end members. 2) Biotite and chlorite become richer in Al. 3) The discontinuous change from albite to oligoclase is followed by a continuous increase in the anorthite content of plagioclase. 4) Calcite becomes more ankeritic. Hornblende occurs with albite in the low garnet zone and oligoclase in the medium to high garnet zone. Accompanying these compositional variations are the growth of amphibole, the consumption of chlorite, epidote, and plagioclase, and the liberation of H₂O. In contrast, during the medium- to low-pressure metamorphism of mafic rocks, plagioclase grows and the transition from actinolite to hornblende is not complete in rocks where the albite to oligoclase jump has already occurred. Epidote disappears at a lower grade than for medium-pressure metamorphism.

Higher pressure metamorphism results in an increase of the glaucophane/riebeckite substitution in the amphibole and of the phengite substitution in the K-rich white mica. Ultimately, the transition from medium-pressure to high-pressure metamorphism yields the observed assemblage Ca-rich amphibole - glaucophane which results from the cross-over reaction: actinolite + chlorite + albite + glaucophane + epidote.

The cross-over reaction: hornblende+chlorite+epidote+plagioclase₁ +quartz = cummingtonite+plagioclase₂ , where the product plagioclase₂ is more abundant and anorthitic than the reactant plagioclase₁ , occurs in Ca-poor mafic rocks in the medium- to low-pressure garnet zone. In medium-pressure Fe-rich basaltic rocks, the assemblage actinolite-chlorite-stilpnomelane occurs in the biotite-albite zone. It is replaced by hornblende-chlorite in the garnet- albite zone and by hornblende-chlorite-garnet in the garnet-oligoclase zone.

The Silurian and Devonian mafic rocks in northeastern Vermont record medium- to low-pressure and low-pressure mineral growth which is assigned to two events during the Devonian Acadian orogeny. It is proposed that the lower pressure event was the later. These two events are expressed by medium-pressure and medium- to low-pressure mineral growth in the Cambrian and Ordovician rocks to the west.

Two periods of Ordovician metamorphism are recorded in the Lower Paleozoic units and are assigned to the Taconic orogeny. High-pressure, medium- to high-pressure, and medium-pressure metamorphism are observed. The higher than "normal" Barrovian facies series metamorphism is confined to northern Vermont and southern Quebec. Locally, glaucophane-bearing rocks and ornphacite assemblages record high-pressure facies series metamorphism.

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