A multi‐stage pressure—temperature record in the Chilka Lake granulites: the epitome of the metamorphic evolution of Eastern Ghats, India?

Abstract

Metapelitic and charnockitic granulites exposed around Chilka Lake in the northern sector of the Eastern Ghats, India, preserve a multi-stage P-T record. A high-T decompression from above 10 kbar to 8 kbar around 1100" C has been determined from Mg-rich metapelites (XMg > 0.60) with quartz-cordieriteorthopyroxene-sillimanite and cordierite-orthopyroxene-sapphirine-spinel assemblages. Between this and a second decompression to 6.0 kbar, isobaric cooling from 830 to 670" C at 8 kbar is evident. These changes are registered by the rim Compositions of orthopyroxene and garnet in charnockites and metapelites with an orthopyroxene-quartz-garnet-plagioclase-cordierite assemblage, and are further supported by the garnet + quartz .$ orthopyroxene + cordierite and biotite-producing reactions in sapphirine-bearing metapelites. Another indication of isobaric cooling from 800 to 650" C at 6.0 kbar is evident from rim compositions of orthopyroxene and garnet in patchy charnockites. Two sets of P-T values are obtained from metapelites with a quartz-plagioclase-garnet-sillimanite-cordierite assemblage: garnet and plagioclase cores yield 6.2 kbar, 700" C and the rims 5 kbar, 650" C, suggesting a third decompression.The earliest deformation (Fl) structures are preserved in the larger charnockite bodies and the metapelites which retain the high P-T record. The effects of post-crystalline F2 deformation are observed in garnet megacrysts formed during or prior to F1 in some metapelites. Fold styles indicate a compressional regime during F1 and an extensional regime during F2. These lines of evidence and two phases of cooling at different pressures point to a discontinuity after the first cooling, and imply reworking. Two segments of the present P-T path replicate parts of the P-T paths suggested for four other granulite terranes in the Eastern Ghats, and the sense of all the paths is the same. This, plus the signature of three phases of deformation identified in the Eastern Ghats, suggests that the Chilka Lake granulites could epitomize the metamorphic evolution of the Eastern Ghats. Recent work on P-T-t paths indicates cooling from moderately high temperatures and decompression at ROCK TYPES A N D F I E L D RELATIONS medium temperatures in several sectors of the Eastern There are five major rock types in the Chilka area: Ghats. The paucity of evidence for the prograde path does metapelites (with intercalated quartzites and rarely 287 288 s. K . S E N Er A L '. ' : I Bf'3d 290 9 290

New constraints on the metamorphic evolution of the Eastern Ghats Belt, India, based on relict composite inclusions in garnet from ultrahigh‐temperature sapphirine granulites

“…A decompressive P-T path has also been described from some areas in the EGB (Lal et al, 1987;Sen et al, 1995;Mohan et al, 1997;Sarkar et al, 2003). However, opinion is sharply divided as to whether the P-T path of the granulites of the EGB is clockwise or anticlockwise (Sengupta et al, 1990;Sen et al, 1995;Bhattacharya, 1996;Mohan et al, 1997;Mukhopadhyay and Bhattacharya, 1997). Recent work suggests a clockwise P-T path and near ITD from high temperatures in rocks from Paderu terrane during the Neoproterozoic Era (Bhattacharya and Kar, 2002).…”

“…North of the Godavari Rift (Figure 1), an early UHT ($10008C) metamorphic event (M 1 ) at crustal depths of 30-35 km (8.4 kbars) is reported from high Mg-Al granulites in G. Madugula (Mohan et al, 1997), and several nearby areas including Paderu (Lal et al, 1987;Pal and Bose, 1997;Bhattacharya and Kar, 2002), Anantagiri (Sengupta et al, 1990), Anakapalle (Dasgupta et al, 1994;Sanyal and Fukuoka, 1995;Rickers et al, 2001a;), Araku and Vizianagram (Kamineni and Rao, 1988). Contrasting post-peak IBC and isothermal decompression (ITD) have been recorded from several areas in the Eastern Ghats granulite belt (Lal et al, 1987;Sengupta et al, 1990;Dasgupta et al, 1991;Dasgupta, 1993;Dasgupta et al, 1994;Sen et al, 1995;Bhattacharya, 1996;Shaw and Arima, 1996;Mohan et al, 1997). A decompressive P-T path has also been described from some areas in the EGB (Lal et al, 1987;Sen et al, 1995;Mohan et al, 1997;Sarkar et al, 2003).…”

“…Contrasting post-peak IBC and isothermal decompression (ITD) have been recorded from several areas in the Eastern Ghats granulite belt (Lal et al, 1987;Sengupta et al, 1990;Dasgupta et al, 1991;Dasgupta, 1993;Dasgupta et al, 1994;Sen et al, 1995;Bhattacharya, 1996;Shaw and Arima, 1996;Mohan et al, 1997). A decompressive P-T path has also been described from some areas in the EGB (Lal et al, 1987;Sen et al, 1995;Mohan et al, 1997;Sarkar et al, 2003). However, opinion is sharply divided as to whether the P-T path of the granulites of the EGB is clockwise or anticlockwise (Sengupta et al, 1990;Sen et al, 1995;Bhattacharya, 1996;Mohan et al, 1997;Mukhopadhyay and Bhattacharya, 1997).…”

Discussion of the origin of ‘arrested’ charnockitization in the Chilka Lake area, Eastern Ghats Belt, India

“…We have analysed several orthopyroxene grains from the patchy charnockites of this area (Sen, Bhattacharya & Acharyya, 1995). The MgO contents of these orthopyroxenes vary from 9.8 to 14.2 % approximately.…”

Relative Chronology in High-Grade Crystalline Terrain of the Eastern Ghats, India: New Insights

“…Isoclinal and rootless F 1 folds with a NE-SW trending steep axial-plane foliation, S 1 and common structural repetitions suggest a regional NW-SE directed compression and shortening during the development of first generation folds [4]. Beside the polyphase deformation history, the Eastern Ghats Belt is also characterized by polycyclic metamorphic record and dehydration melting in different crustal protoliths [5][6][7][8][9]. However, in view of different crustal residence ages from different parts of the regional granulite terrain, as also distinct isotopic records of granulite facies metamorphism across this regional granulite terrain, the tectonic-metamorphic evolution should be discussed separately for the different crustal domains & provinces identified [10,11].…”