ABSTRACT
The prograde evolution of mineral assemblages in metapelites of a Barrovian sequence from the tri-state area (Massachusetts, Connecticut and New York) of the southern Taconic Range shows the displacement of the assemblages Ga+Ch+Ct+Bi, Ga+Ch+Ct+St, and Ga+Ch+Bi+St, displaying the reaction sequence [St] -> [Bi] -> [Ct].
Detailed petrologic observations on Ga+Ch+Ct+Bi assemblages show: (1) garnet high in Mn and Fe but low in Mg can be stable with chlorite at low grades before the stability of the Ct+Bi join; (2) such garnet reacts partially to form Ct+Bi at intermediate grades; and (3) at higher grades after the Ct+Bi join has been broken garnet resumes growth with a higher Mg/Mn ratio and coexists with chlorite. Garnet zoning and systematic mineral compositions suggest that (1) in the KFMASH system Ga+Ch is stable at higher T than Ct+Bi via the discontinuous Fe-Mg reaction Ct+Bi => Ga+Ch (2) Ct+Bi is stable at higher T than Ga+Ch via the same reaction in the MnKFASH system, and the Fe-Mn discontinuous reaction is Ga+Ch => Ct+Bi; and (3) in the MnKFMASH system, Ga+Ch may be stable at higher or at lower T than Ct+Bi depending on the effective bulk Mn-content of the rock.
A petrogenetic grid and associated T-X diagrams for the above mineral assemblages are modeled thermodynamically in the MnKFMASH system using the Gibbs method. The [As, Cd] invariant point is calculated to be at about P = 6.5±1.0 kbar and T = 540±50ºC. The topology of the univariant curves around the invariant point [As, Cd] in the KFMASH system is different from published ones. The stable part of the univariant curve [St] is shown to radiate upwards in the same direction as those of [Bi] and [Ct]. In the KFMASH system, the predicted [St] reaction is Ct+Bi => Ga+Ch, with Ga+Ch stable at high T. In the MnKFASH system, however, the predicted reaction is Ga+Ch => Ct+Bi. These results are contrary to previous work, but consistent with the petrologic observations of this study.
This thesis emphasizes the following important effects of the "extra" component, Mn, in the KFMASH system: (1) Mn allows the reaction sequence [Bi] => [Ct] to occur in lower P/T terranes; (2) Mn allows the mineral assemblages Ga+Ch+Ct+Bi and Ga+Ch+Ct+St assemblages to be stable at lower T; (3) Mn allows the assemblage Ga+Ch+Bi+St to be stable at higher T; and (4) most importantly, Mn causes the [St] reaction directions to be reversed.

Wang, P., 1989. Progressive metamorphism of pelitic schists from the Tri-State, MA, CT and NY, area: a field and theoretical analysis of Garnet + Chlorite + Chloritoid + Biotite assemblages.  Unpublished PhD dissertation, State University of New York at Albany. 263pp., +xiii
University at Albany Science Library call number:  SCIENCE MIC Film QE  40 Z899 1989 W34
Copies of this PhD dissertation can be ordered from Proquest UMI

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