Origin and evolution of FeAl-granulite in the thermal aureole of the Chilka Lake anorthosite, Eastern Ghats Province, India

The Chilka Lake anorthosite (792 ± 2 Ma) in the northeastern part of the Eastern Ghats Province provides an excellent example of deep-crustal diapiric emplacement and ultra-high temperature (UHT) contact metamorphism. Within a narrow contact zone, supracrustal granulates were heated to temperatures in excess of 1000°C, causing renewed partial anatexis and melt segregation under dynamic conditions, with hybridization of the marginal leuconorite. The thermal history of the innermost aureole is well manifested by reaction textures in a FeAl-granulite occurring at the immediate contact SE of Balugaon. The tightly folded rock shows a finely banded structure. Typically, spinel+ilmenite layers are separated from quartz+garnet1 ± perthite layers by successive coronitic layers of sillimanite and garnet2. The sillimanite fabric indicates synkinematic development of the corona. The reaction texture, mineral and bulk chemical characteristics, and geothermobarometric modelling suggest the following thermal evolution: (1) near-isobaric heating of a banded metapelitic granulite (khondalite: grt1+sil+qtz+kfsp) to c. 1100°C (7–8 kbar). Coeval with partial melting and extraction of the felsic melt, the divariant reaction (FeMg)grt+2sil → 3(FeMg)spl+6qtz is crossed at c. 1050°C, leading to the formation of a restitic FeAl-granulite made up of alternating spl+ilm and qtz+grt1 layers. The segregated felsic melt causes hybridization of the adjacent leuconorite. (2) Subsequent near-isobaric cooling of the FeAl-granulite (spl+qtz+ilm+grt1) from c. 1100°C to the stable geotherm. Reversal of the divariant reaction [3(FeMg)spl+6qtz → (FeMg)grt+2sil] produces the multi-layered coronitic texture. During this stage, coronal garnet forms also in the closely associated ferrodiorite via the reaction opx+pl → grt+qtz. The P-T data indicate that the Chilka Lake anorthosite intruded thickened crust at a depth of c. 28 km.