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
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