A HFSE- and REE-enriched ferrodiorite suite from the Bolangir Anorthosite Complex, Eastern Ghats Belt, India

Abstract

The massif-type anorthosite complex at Bolangir in the northern part of the Eastern Ghats belt occurs in a milieu of predominantly supracrustal granulite-grade rocks. The massif is separated from the host gneisses by coarse-grained garnetiferous granitoid gneisses which are interpreted as coeval crustallyderived melts. Melanocratic ferrodiorite rocks occur at the immediate contact with the anorthosite massif which they intrude in cross-cutting dikes and sheets. The emplacement age of the anorthosite diapir and the associated igneous suites is deemed to be pre-D₂. Recrystallization of the igneous assemblages of the ferrodiorite suite (750–800°C, 7–8kbar,$a_{H_2 O}< 0.25, logf_{O_2 }< - 18 bar$) during a period of near-isobaric cooling from the igneous crystallization stage to the regionalP-T regime led to extensive development of coronitic garnet at the interface of plagioclase phenocrysts with the mafic matrix assemblage (opx + fay + cpx + ilm ± amph, bio). Abundant accessory phases are zircon, apatite and thorite. The mafic phases have extremely ferrous compositions (X_Fe gar: 0.93-0.87, fay: 0.90-0.87, opx: 0.80-0.60, cpx: 0.70-0.47, amph: 0.81-0.71) reflecting the low Mg-number (16-8) of the rocks. Compared to worldwide occurrences of similar rocks, the Bolangir ferrodiorites (SiO₂ 36–58 wt.%, FeO*: 39-10 wt.%) are characterized by exceptionally high concentrations of HFSE and REE (TiO₂: 4.8-1.0 wt.%, P₂O₅: 1.7-0.5 wt.%, Zr: 5900-1300 ppm, Y: 240-80 ppm, La: 540-100 ppm, Ce: 1100-200 ppm, Yb: 22-10 ppm, Th: 195-65 ppm). Well defined linear variation trends for major and trace elements reflect progressive plagioclase accumulation towards the felsic members of the suite. The ferrodiorites are interpreted to represent residual liquids of anorthosite crystallization which after segregation and extraction from the ascending diapir became enriched in HFSE and REE through selective assimilation of accessory phases (zircon, monazite, apatite) from crustal felsic melts. Ferromonzodioritic rock presumably formed through hybridization between the ferrodiorite and overlying felsic melts.