Petrology of gedrite‐bearing rocks in mid‐crustal ductile shear zones from the Eastern Ghats Belt, India

Abstract

A suite of metapelitic, basic and quartzofeldspathic rocks intruded by enderbitic gneiss from the southernmost tip of the Eastern Ghats Belt, India, and metamorphosed at c. 750–800 °C, 6 kbar, were subjected to repeated ductile shear deformation, hydration, cooling and accompanying alkali metasomatism along narrow shear zones. Gedrite‐bearing assemblages developed in the shear zones traversing metapelitic rocks. Interpretation of the reaction textures in an appropriate P–T grid in the system FMASH, an isothermal–isobaric μH2O–μNa2O grid in the system NFMASH, and geothermobarometric data suggest a complex evolutionary history for the gedrite‐bearing parageneses. Initially, gedrite‐bearing assemblages were produced due to increase in μNa2O at nearly constant but high μH2O accompanying cooling. Gedrite was partially destabilized to orthopyroxene+albite due to progressively increasing μNa2O. During further cooling and at increased μH2O a second generation of gedrite appeared in the rocks.

Sapphirine and corundum bearing ultrahigh temperature rocks from the Palghat-Cauvery Shear System, southern India

“…As the Napier samples are from the vicinity of shear zones, Harley (1985) concluded that the gedrite is a product during later shearing. Dasgupta et al (1999) reported gedrite with up to 2.6 wt.% Na 2 O in sheared rocks from the Eastern Ghats Belt (India). Since the Paramati rocks are also located within the northern domain of the Palghat Cauvery Shear System, we do not rule out a similar possibility of gedrite formation during a retrograde event during shearing.…”

Syn-kinematic emplacement of the Pangong metamorphic and magmatic complex along the Karakorum Fault (N Ladakh)

“…5). The presence of orthoamphiboles suggests an early retrogression due to shearing in granulite grade conditions (Dasgupta et al, 1999). Clinopyroxene to hornblende transformation indicates a thermal drop to 750-775°C at a pressure of 4-8 kbar (Spear, 1981).…”

Sodicgedrite in ultrahigh-temperature Mg-Al-rich rocks from the Palghat-Cauvery Shear Zone system, southern India

“…As the rocks are collected from the vicinity of shear zones, Harley (1985) discussed that the gedrite is a product of later shearing. Dasgupta et al (1999) also reported gedrite with up to 2.6 wt% Na 2 O in sheared rocks from the Eastern Ghats Belt, India. Although a metasomatic origin through the activity of fluids along shear zones is a possible mechanism to explain the occurrence of sodicgedrite in our samples, halogen analyses of the gedrites demon-* Number of oxygens.…”

“…The mineral has also been reported in several Mg Al rich rocks from ultrahigh temperature (UHT) metamorphic terranes (e.g., Harley, 1985;Ouzegane et al, 1996;Dasgupta et al, 1999). Such gedrite in UHT rocks generally contain high Na 2 O (Na A > 0.5) as sodicgedrite (Leake et al, 1997).…”

“…Additional experimental study of Na solubility in gedrite and thermodynamic calculations of sodicgedrite bearing assemblages will be necessary to further address this interesting topic. facies rocks from Eastern Ghats (Dasgupta et al, 1999), Karur , and In Ouzzal (Ouzegane et al, 1996) …”