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Controls of mineral reactions in high‐grade garnet‐wollastonite‐scapolite‐bearing calcsilicate rocks: an example from Anakapalle, Eastern Ghats, India
Author(s) -
SENGUPTA P.,
SANYAL S.,
DASGUPTA S.,
FUKUOKA M.,
EHL J.
Publication year - 1997
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.1525-1314.1997.tb00635.x
Subject(s) - geology , granulite , metamorphism , metamorphic rock , wollastonite , mineral , geochemistry , mineralogy , geomorphology , chemistry , raw material , organic chemistry , facies , structural basin
A suite of garnet‐wollastonite‐scapolite‐bearing calcsilicate granulites from the Eastern Ghats has been investigated to document the controls of mineral reactions during the metamorphic evolution of the deep continental crust. The rocks studied show heterogeneity in modal mineralogy and phase compositions in millimetre‐sized domains. Textural relations, and the compositional plots of the phases, established that the clinopyroxene exerts a strong influence on the formation and composition of garnet in the complex natural system. P‐T estimates using the vapour‐independent equilibria involving garnet define a near isobaric cooling path from c. 850C at c. 5.5–5.2 kbar. The deduced trajectory tallies well with the terminal segment of the overall retrograde P‐T path construed from the associated rocks using well‐calibrated thermobarometers. The ubiquitous occurrence of wollastonite and scapolite in the main calcsilicate body suggests low a CO2 during peak metamorphic condition. Fluid compositions constrained from mineral‐fluid equilibria of the garnet‐bearing assemblages show domainal variations as a function of the compositions of the solid phases, e.g. garnet and clinopyroxene. A quantitative log/ CO2 ‐log/ O2 diagram has been constructed to depict the stability of the different calcsilicate assemblages as functions of the compositions and the behaviour of these fugitive species. The results of the mineral‐fluid equilibria and the quantitative fluid/rock ratio calculations, in conjunction with the topological constraints, imply vapour‐deficient meta‐morphism in the rocks studied. It is argued that f O2 during peak metamorphism was monitored by the ambient f O2 . Subsequently, during retrogression, different domains evolved independently, whereas the fluid composition was controlled by the mineral‐fluid equilibria.