Formation of aluminous clinopyroxene-ilmenite-spinel symplectic assemblage in a regionally metamorphosed calc-silicate granulite from the Chotanagpur Granite Gneiss Complex, East Indian shield

Clinopyroxene-ilmenite-spinel (+plagioclase) symplectites are rare in metamorphic rocks. The present study examines this extremely rare symplectic assemblage in association with high aluminous clinopyroxene, from a calc-silicate granulite in the north-eastern part of the Chotanagpur Granite Gneiss Complex (CGGC), East Indian shield. The studied rock comprises plagioclase+ titanite + garnet + clinopyroxene + amphibole +ilmenite + spinel ± calcite + zircon ± magnetite, non-uniformly distributed between the leucocratic, melanocratic and garnet-rich bands. Textural relations indicate the formation of peak metamorphic minerals: garnet, clinopyroxene (Cpx1), plagioclase (Pl1), amphibole (Am1), titanite ± ilmenite, via the devolatilisation reaction Aminc + Plinc + ilmenite ± calcite = garnet + Cpx1 + titanite + vapour, which together defines a granoblastic fabric in all the three bands. The absence of quartz-alkali feldspar segregations implies that the rock did not melt during this prograde reaction. The symplectic intergrowth of clinopyroxene (Cpx2) + ilmenite+ spinel+ plagioclase (Pl2) formed subsequently, by the breakdown of these peak metamorphic minerals via the dehydration reaction: garnet + titanite + Am1 = Cpx2 + ilmenite +spinel + Pl2 + vapour. Irrespective of the textural type, clinopyroxene is highly aluminous (Al2O3 up to ∼11.5 wt%) and plagioclase is highly calcic (XAn = 0.98–0.99). Phase equilibria modelling (pseudosection) in the Na2O-CaO-FeO-MgO-Al2O3-SiO2-TiO2-H2O-CO2-O2 system demonstrates that (i) the granoblastic assemblage was formed at the culmination of metamorphism, at ∼790–890 °C and ∼ 8.4–9.5 kbar, followed by (ii) a near isothermal, steeply decompressive retrograde pressure-temperature path up to ∼780–860 °C and ∼5–5.8 kbar, which led to the formation of the symplectic assemblage (Cpx2-ilmenite-spinel-Pl2). UPb isotopic dating from metamorphically grown zircons constrains the timing of peak metamorphism and symplectite formation between ∼1060–970 Ma. This study corroborates with the published experimental data and natural rock record, which suggest a decrease in pressure to be the driving force for the transformation of garnet+ titanite+ amphibole to clinopyroxene+ ilmenite± spinel in natural rocks. Presence of detrital zircon grains suggests a clastic sedimentary protolith for the studied rock. The variable 207Pb/ 206Pb ages (∼2470 Ma, 2000–2190 Ma and ∼1790 Ma) from detrital cores of zircons indicate multiple provenance. Additionally