Reaction textures and metamorphic evolution of sapphirine–spinel-bearing and associated granulites from Diguva Sonaba, Eastern Ghats Mobile Belt, India

Abstract

The Diguva Sonaba area (Vishakhapatnam district, Andhra Pradesh, South India) represents part of the granulite-facies terrain of the Eastern Ghats Mobile Belt. The Precambrian metamorphic rocks of the area predominantly consist of mafic granulite (±garnet), khondalite, leptynite (±garnet, biotite), charnockite, enderbite, calc-granulite, migmatic gneisses and sapphirine–spinel-bearing granulite. The latter rock type occurs as lenticular bodies in khondalite, leptynite and calc-granulite. Textural relations, such as corroded inclusions of biotite within garnet and orthopyroxene, resorbed hornblende within pyroxenes, and coarse-grained laths of sillimanite, presumably pseudomorphs after kyanite, provide evidence of either an earlier episode of upper-amphibolite-facies metamorphism or they represent relics of the prograde path that led to granulite-facies metamorphism. In the sapphirine–spinel-bearing granulite, osumilite was stable in addition to sapphirine, spinel and quartz during the thermal peak of granulite-facies metamorphism but the assemblage was later replaced by Crd–Opx–Qtz–Kfs-symplectite and a variety of reaction coronas during retrograde overprint. Variable amounts of biotite or biotite+quartz symplectite replaced orthopyroxene, cordierite and Opx–Crd–Kfs–Qtz-symplectite at an even later retrograde stage. Peak metamorphic conditions of c. 1000°C and c. 12 kbar were computed by isopleths of XMg in garnet and XAl in orthopyroxene. The sequence of reactions as deduced from the corona and symplectite assemblages, together with petrogenetic grid and pseudosection modelling, records a clockwise P–T evolution. The P–T path is characteristically T-convex suggesting an isothermal decompression path and reflects rapid uplift followed by cooling of a tectonically thickened crust.

Significance of post‐peak metamorphic reaction microstructures in the ultrahigh temperature Eastern Ghats Province, India

“…Late cordierite is a common feature of many (62%) of the studies listed in Table . In evaluating photomicrographs and accompanying descriptions in the published studies, the late cordierite is commonly not developed to great extent, typically being confined to narrow coronas or symplectites on higher pressure post‐peak M1 assemblages such as orthopyroxene–sillimanite, garnet–sillimanite and sapphirine–orthopyroxene, amongst others (Table ; Bose et al., , ; Das, Bose, et al., ; Dasgupta et al., ; Dharma Rao et al., ; Grew, ; Grew et al., ; Kamineni & Rao, ; Mohan et al., ; Pal & Bose, ; Prakash et al., ; Sarkar et al., ; Sengupta et al., ; Shaw & Arima, ). Late cordierite was interpreted in numerous studies to reflect the down‐pressure segment (i.e.…”

“…The M1 event in the EGP was interpreted to involve UHT metamorphism followed by near‐isobaric cooling (e.g. Bhowmik, ; Bose & Das, ; Bose, Das, Dasgupta, Miura, & Fukuoka, ; Bose, Fukuoka, Sengupta, & Dasgupta, ; Das, Bhattacharya, Raith, Bhadra, and Banerjee, ; Das, Bose, Ohnishi, & Dasgupta, ; Dasgupta & Ehl, ; Dasgupta & Sengupta, , ; Dasgupta, Sengupta, Fukuoka, & Bhattacharya, ; Dasgupta et al., ; Dharma Rao et al., ; Grew, ; Kamineni & Rao, , 1988b; Korhonen, Brown, et al., ; Korhonen et al., ; Lal, Ackermand, & Upadhyay, ; Mohan, Tripathi, & Motoyoshi, ; Mukhopadhyay & Bhattacharya, ; Nasipuri, Bhattacharya, & Das, ; Pal & Bose, ; Prakash et al., ; Sarkar, Dasgupta, & Fukuoka, ; Sengupta, Karmakar, Dasgupta, & Fukuoka, ; Sengupta, Raith, & Dasgupta, ; Sengupta et al., ; Shaw & Arima, , ). Most researchers consider M1 to be characterized by an overall counter‐clockwise P–T path (see Table and preceding references).…”

“…The M2 event was classified as temporally distinct from M1, occurring later and at granulite facies conditions—but not necessarily UHT—and reworked the M1 granulites (e.g. Bose & Das, ; Bose et al., ; Dasgupta & Sengupta, ; Dasgupta et al., , , ; Dharma Rao et al., ; Dobmeier & Raith, ; Grew, ; Grew, Rao, Raju, & Yates, ; Lal et al., ; Mohan et al., ; Mukhopadhyay & Bhattacharya, ; Pal & Bose, ; Prakash et al., ; Rickers, Raith, et al., ; Sanyal & Fukuoka, ; Sarkar et al., ; Sengupta et al., , ; Simmat & Raith, ). The P–T path for M2 was stated as being characterized by a down‐pressure evolution (Table ) followed by a period of near‐isobaric cooling (Bhowmik, ; Das et al., ; Dasgupta, ; Dasgupta et al., , ; Mukhopadhyay & Basak, ; Sanyal & Fukuoka, ; Sengupta et al., , ).…”

Neoproterozoic evolution and Cambrian reworking of ultrahigh temperature granulites in the Eastern Ghats Province, India

“…Multiple localities within the EGP preserve evidence for UHT conditions (Das et al., ; Korhonen, Brown, et al., ; Korhonen, Clark, et al., ; Korhonen et al., , ; Prakash et al., ), suggesting that UHT metamorphism was regional in extent. The P–T evolution of rocks within the EGP has been constrained using a variety of methods at different localities.…”

Ultra-High Temperature metamorphism of Southern Granulite Terrain, India: A Review

“…It is worthwhile to mention here that sapphirine-bearing granulites is a strong evidence for UHT metamorphism. For interpreting pressuretemperature-time (P-T-t) path the sapphirine-bearing granulites serveas an irreplaceable tool because this refractory mineral holds the signatures of metamorphic history experienced throughout their evolution (Prakash et al 2013(Prakash et al , 2015(Prakash et al , 2019. Petrological investigations on the sapphirinebearing granulites in Palni Hills of southern block of SGT from different localities located south of the PCSZ have provided signatures of UHT metamorphism (Tewari et al, 2018).The clockwise-iso-thermal decompression paths (CW-ITD)have been characteristically noticed.…”