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Plagioclase relations in pelites, central Menderes Massif, Turkey. II. Perturbation of garnet‐plagioclase geobarometers
Author(s) -
ASHWORTH J. R.,
EVIRGEN M. M.
Publication year - 1985
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.1525-1314.1985.tb00318.x
Subject(s) - plagioclase , geology , kyanite , massif , geochemistry , metamorphism , albite , mineralogy , quartz , paleontology
Activity‐composition relations in oligoclase near the peristerite gap are investigated in pelites from the Central Menderes Massif. The pressure of metamorphism is estimated independently, from garnet‐rutile‐ilmenite‐kyanite‐quartz, as being in the range 4–7 kbar. In the temperature range, 450–600°C approximately, both the Newton‐Haselton calibration of the garnet‐plagioclase‐kyanite‐quartz geobarometer and a related simple treatment of garnet‐plagioclase‐muscovite‐biotite give a wide range of apparent pressures, correlated with plagioclase composition and ranging up to 11–12 kbar where the plagioclase is most sodic. This effect is attributed to failure of the activity model for plagioclase used in the Newton‐Haselton treatment. It is inferred that, in the present area, γ plag An decreases with increasing X plag An in the range An 15 ‐An 25 . The data can be interpreted in terms of high γ plag An in the high‐albite structure at these temperatures, modified to lower values by ‘e’ordering in the more calcic oligoclases. The ordering appears to be independent of the peristerite gap, and the data do not support the interpretation of the gap as a solvus. Garnet‐plagioclase assemblages are unreliable as geobarometers where the plagioclase is more sodic than approximately An 20 and T < 700°C, and should instead be used to investigate the γ ‐X behaviour of the plagioclase where independent geobarometry can be used as a constraint.