Petrology, geochemistry, and petrogenesis of calcic-ferroan-metaluminous garnetiferous magmatic charnockites from eastern Chhotanagpur Gneissic Complex, Eastern Indian Craton

We report calcic-ferroan-metaluminous garnetiferous magmatic charnockites that are extremely rare in nature and hence interesting to study. The garnetiferous porphyritic granite pluton of the Tilaboni area of Chhotanagpur Gneissic Complex of Eastern Indian shield contains older enclaves of enderbite-charnoenderbite-charnockite (charnockitic suite). Garnetiferous metagabbro are spatially associated with charnockitic rocks. Plagioclase, K-feldspar, quartz, ortho-, and clinopyroxene, garnet, biotite ± amphibole, ilmenite ± magnetite are major mafic phases. Biotite is sub-alkaline to alkaline. Plagioclase compositions vary from andesine to oligoclase. Garnet is rich in almandine (70.28–74.04 mol%) and grossular (17.77–21.41 mol%) but contains low pyrope (2.83–7.67 mol%) and spessartine (4.09–4.59 mol%). Amphibole formed through the hydration of hypersthene, clinopyroxene, and garnet.

Garnet-clinopyroxene and orthopyroxene-clinopyroxene geothermometry and garnet-orthopyroxene-plagioclase-quartz geobarometry give granulite-facies (750-850°C; 7.5-8.0 kb) of metamorphism of the charnockitic rocks. Amphibole-plagioclase thermobarometry yields temperature and pressure (733−795 °C; 5−6 kbar) that suggest amphibolization of the mafic minerals at a relatively shallower level. Pseudosection modeling shows that the garnets and orthopyroxene finally equilibrated at around 560°C temperature and 5.8 kb.

Primary ilmenite and high Fe/(Fe+Mg) ratios of amphibole-biotite indicate these charnockites metamorphosed under reduced conditions (ΔNNO −2).

These charnockites are dominantly calcic and ferroan to slightly magnesian (Fe-number: 0.74–0.97); dominantly metaluminous to weakly peraluminous (A/CNK: 0.84–1.08); high- and medium-K calc-alkaline and shoshonite series.

These exhibit moderate variations of Al2O3 (12.44–18.19 wt.%), K2O (1.16–5.7 wt.%), and CaO (1.01–5.72 wt.%) contents. Na2O (3.71–3.89 wt.%) show a slight variation in concentration. Abundances of Fe2O3(total) (2.45–7.88 wt.%) and TiO2 (0.21–1.11 wt.%) are generally moderate, whereas the concentration of MgO (0.08–1.99 wt.%) remains low.

These rocks show enrichments of the Rb, Ba, Th, K, Zr, and Hf but depletion in Nb, Ta, and Ti relative to the primitive-mantle composition. They also show strong depletions in Sr and P, whereas enrichment in Pb. LaN/SmN (2.68–12.95) and GdN/YbN ratios (1.57–2.89) of these rocks are high. Five of the six samples show negative Eu-anomalies (0.29–0.91), one sample shows pronounced positive Eu-anomaly (3.09).

These rocks exhibit similar multicationic trace-element and REE patterns and a nearly collinear array of sample plots in Harker diagrams. Further, these samples follow a calcic to alkali-calcic trend in SiO2 vs. MALI diagram. These factors are the result of magmatic differentiation. Decreases in CaO and Fe2O3t with increasing SiO2 but increasing agpaitic index with increasing silica alkalis are due to fractional crystallization from a common parental magma. Decreasing modal plagioclase following the calc-alkaline trend also supports magma differentiation. High Nb/U (av. 22.48) and Ce/Pb (av. 12.64) ratios but low Th/U (average 7.76) ratios suggest mantle source of the magma parental to these charnockites.

Their ferroan and reduced characters resulted from intense fractionation of early-formed allanite, magnetite, etc. Geochemical modeling shows the calcic charnockites evolved by fractionation of garnet and clinopyroxene from basaltic magma derived from a depleted mantle.