Contrasting Styles of Lower Crustal Metamorphism from a Granulite Suite of Rocks from Angul, Eastern Ghats Belt, India: Implications for the India-Antarctica Correlation

The present work is focussed on a suite of high-grade rocks including mafic granulite, aluminous granulite, khondalite, charnockite, and augen gneiss along with medium-grade rocks like olivine-bearing metanorite, gabbro, and porphyritic granite of the Angul domain at the northern margin of the Proterozoic Eastern Ghats Province (EGP). Based on the petrological and geothermobarometric data, two distinct metamorphic events have been identified. The imprints of the earlier event (M-A1) are preserved in the mafic granulite, aluminous granulite, khondalite, augen gneiss, and fine-grained charnockite, but those are best preserved in mafic granulite and aluminous granulite. In mafic granulite, orthopyroxene + clinopyroxene + plagioclase +/- garnet+ ilmenite +/- quartz assemblage was stabilised at the peak M-A1 conditions, whereas the peak M-A1 assemblage is represented by Fe3+-garnet + hematite + magnetite + cordierite + K-feldspar + plagioclase + sillimanite + quartz + melt in aluminous granulite. Phase equilibria modelling and thermobarometric data suggest the P-T conditions of >850 degrees C, 7 to 8 kbar for this event. The retrograde metamorphism (M-A1R) involved minor decompression (down to similar to 5 kbar) and subsequent cooling to form biotite- and hornblende-bearing mineral assemblages in aluminous granulite and mafic granulite, respectively. Texturally constrained monazite (U-Th-total Pb) and zircon (U-Pb) data from the former rock suggest ca. 1200 Ma age of the M-A1 metamorphism, which was associated with granitic and charnockitic magmatism as constrained from oscillatory-zoned zircon domains in the augen gneiss and fine-grained charnockite. The rock ensemble was affected by a younger metamorphic event (M-A2), which is texturally characterised by partial replacement of hornblende (developed during M-A1R) to orthopyroxene +/- clinopyroxene + plagioclase +/- ilmenite + melt assemblage in mafic granulite. Moreover, biotite of aluminous granulite has undergone dehydration melting to produce garnet + cordierite-bearing assemblage. Garnet in the above assemblage did not form as porphyroblastic phase and overgrew the M-A1 garnet. The M-A2 event followed a counterclockwise P-T trajectory, causing heating (up to 800 degrees C) with associated loading (from 4.0 to 5.8 kbar) along the prograde path. Monazite U-Th-total Pb data from aluminous granulite and khondalite suggest M-A2 ages of 987 +/- 12 and 975 +/- 16 Ma, respectively. Fine-grained charnockite and augen gneiss also recorded the imprints of M-A2 event by developing thin to thick sector-zoned overgrowth yielding group ages of 979 +/- 12 and 982 +/- 29 Ma, respectively. Zircon overgrowth in mafic granulite formed at 962 +/- 13 Ma. The M-A2 event coincides with the crystallisation of coarse-grained charnockite at 983 +/- 22 Ma and porphyritic granite at 960 +/- 10 Ma. Geochronological data, thus, indicate that the Angul domain underwent the M-A2 metamorphism and associated magmatism at ca. 990 to 960 Ma. The apparent absence of M-A1 event (similar to 1200 Ma) in the greater part of the EGP and its dominance in the Angul domain suggest that the latter was possibly an exotic block at ca. 1200 Ma and joined with the rest of the EGP only after ca. 960 Ma. We further suggest that the metamorphic history of the Angul domain is strikingly different from the rest of the EGP, but matches well with that of the Prydz Bay region of the East Antarctica.