Ree Geochemistry of Pink Granulites from North Arcot District, Tamil Nadu

Abstract

Petrography, mineral chemistry, major elements as well as Rb, Sr, Ba and REE geochemistry of pink granulites and the associated charnockites have been studied. Pink granulites are prograde rocks metamorphosed under granulite facies condition. There is no genetic relation between potassic pink granulites and tonalitic charnockites and their geochemical evolutionary trends are distinctly different. K/Rb ratios are in the same range, although abundance level of K and Rb in pink granulites is high. Charnockites have higher Sr and lower Rb/Sr. Pink granulites have variable Sr level, high Rb/Sr and variable Ba/Sr indicating heterogeneity in the parent rocks. REE abundance level in pink granulites shows wide variation; non-uniform but negative Eu anomaly, strong fractionation with respect to LREE, positive slope after Gd and lower ∑LREE/∑HREE ratio (5 to 26). Charnockites have coherent REE patterns, with strongly fractionated LREE and slight positive Eu anomaly and higher ∑LREE/∑HREE (30-70) ratios. Field relation, mineral paragenesis, whole rock chemistry, Sr-isotope ratios and REE geochemistry strongly support sedimentary parentage for the pink granulites. The highly fractionated REE patterns of the associated charnockites reflect not only igneous inheritance but also the effect of granulite grade metamorphism.