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Chemographic analysis of assemblages involving pyrophyllite, chloritoid, chlorite, kaolinite, kyanite, quartz: application to metapelites in the Witwatersrand goldfields, South Africa
Author(s) -
ZHOU T.,
DONG C.,
PHILLIPS G. N.
Publication year - 1994
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1994.tb00049.x
Subject(s) - kyanite , chlorite , pyrophyllite , geology , kaolinite , quartz , geochemistry , mineral , andalusite , metamorphic rock , mineralogy , materials science , metallurgy , paleontology
The Witwatersrand goldfields contain abundant assemblages that include pyrophyllite, chloritoid, chlorite, kaolinite and/or kyanite, with quartz. A chemographic analysis of the system Fe(Mg)‐Al‐Si‐O‐H involving these minerals yields 22 potential phase diagrams. Using orientation criteria and thermodynamic calculations as further constraints, this list has been reduced to three possible diagrams. New thermodynamic data favour one of these in particular. This chemographic analysis demonstrates that formation of chloritoid is not restricted to the breakdown reaction of kaolinite plus chlorite in the F(M)ASH system, as stated by previous studies, but could be from pyrophyllite + chlorite → chloritoid + quartz + H 2 O. The metamorphic temperature variation between Witwatersrand goldfields exceeded 65d̀ C, based on chlorite and chloritoid compositions. The lower and upper pressure limits are constrained by the andalusite to kyanite, and the sudoite/chlorite to carpholite boundaries, i.e. 1.5–2.8, and 7 kbar, respectively. The widespread pyrophyllite, chlorite and Fe‐chloritoid in all the Witwatersrand goldfields, and the local occurrence of sudoite indicate a consistent low‐pressure environment in which Mg‐chloritoid would not be stable.