Formation of patchy charnockite from garnet bearing quartzofeldspathic gneiss: evidences of fluid induced metasomatism under granulite facies conditions

Fluid-induced metasomatic changes and origin of patchy charnockite via CO₂ influxed dehydration of biotite and amphibole is frequently encountered in different high-grade terrains. The current study reports fluid-mediated transition of garnet-bearing quartzofeldspathic gneiss (GG) to orthopyroxene-bearing quartzofeldspathic gneisses (charnockitic gneisses; CG) from parts of Eastern Ghats mobile belt. Field study reveal that irregular patches of charnockitic gneisses have developed within a foliated garnet-bearing quartzofeldspathic gneiss. The foliation in the garnet-bearing quartzofeldspathic gneiss is continuous and pervasive in the charnockitic gneiss while the contact between the two lithotypes is transitional which advocates for a fluid-induced transformation. Petrography suggest that the peak metamorphic mineralogy in the garnet-bearing quartzofeldspathic gneiss comprises garnet+ plagioclase+ alkali-feldspar (perthite) + quartz+ ilmenite. Garnet (Alm₇₆Py₁₈Gr₅Sps₁ to Alm₆₈Py₂₆Gr₅Sps₁) grains in the garnet-bearing quartzofeldspathic gneisses are dominantly anhedral to subhedral, often occur as aggregates and define the melanosomes. Small, polygonal garnet grains often show recrystallized boundaries, indicating presence of deformation. In the charnockitic gneisses, large orthopyroxene (X_Mg ~0.54 to 0.58; Al: 0.26 to 0.28 a.p.f.u.) grains have inclusions of polygonal garnet grains (Alm₆₈Py₂₈Gr₄Sps₁)along with ilmenite and quartz. Plagioclase in both the rocktypes are dominantly albitic and plagioclase in garnet-bearing quartzofeldspathic gneiss is more sodic than that of charnockitic gneiss (An₃₁Ab₆₉ in GG and An_38-40Ab_62-60 in CG). In both the rocktypes biotite develops as secondary minerals replacing both garnet and orthopyroxene.

Reaction modelling suggest the following reactions that possibly led to the development of orthopyroxene:

1.9 Garnet + 8.3 Plagioclase _GG + 5.5 Quartz + 3.1 Mg²⁺+ 1 Ca²⁺ = 4.7 Orthopyroxene + 8.3 Plagioclase _CG

The reaction predicts mobilization of Mg and Ca being essential in forming the charnockitic patches.

Conventional geothermobarometry constrain the temperature-pressure conditions of orthopyroxene formation between 800 to 850°C and 7-8 kbar. Pseudosection constructed in NCKFMASHT (Na₂O-CaO-K₂O-FeO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂) system and the intersection of compositional isopleths of garnet from each rocktypes further yield a similar physical conditions corresponding to the observed mineral assemblages. High metamorphic temperatures also corroborates well with the high Al₂O₃ (4.30-5.53 wt.%) contents of the orthopyroxene. Mass balance calculations using the whole rock compositions predict extensive mobilization of Mg which corroborates the inference from the modelled reaction. T-X_Mg and P- X_Mg diagram indicate that the stability of the orthopyroxene is strongly influenced by X_Mg rather the physical conditions. T-X_H2O diagram further suggest that low H₂O (possibly high CO₂ or brine-rich) fluids favour the stabilization of orthopyroxene in the observed assemblage and at ~800°C and ~7 kbar, increasing H₂O content even up to 2.5 wt.% does not hinder the stability of the orthopyroxene suggesting that temperature and the Mg-influx were the probable parameter that triggered the transition. Low H₂O and high CO₂ fluid presumably facilitated the process.