The geochemistry of prograde and retrograde charnockite-gneiss reactions in southern India

Exposures in many quarries in southern India exhibit field evidence for incipient charnockitization of tonalitic and granitic gneiss (prograde relationship), or retrogression of charnockite to produce tonalitic gneiss (retrograde relationship). Few systematic geochemical relationships exist between adjacent gneisscharnockite sample pairs during either prograde or retrograde reactions. Most elements and element ratios exhibit inconsistent variations; however, prograde chamockites appear enriched in Ta, Pb, volatiles (chiefly CO₂), and in transition metals relative to Mg, and depleted in REE and Y compared to adjacent gneiss protoliths. Retrograde gneisses have higher Rb, Pb, Th, Hf, Zn relative to Co, Nb relative to Ta, Hf relative to Zr, and volatiles (chiefly H₂O) compared to parental charnockites. Of those elements (U, Th, Rb, Cs, Pb) significantly depleted in high-pressure charnockites exposed south of the prograde transition zone, only Pb is significantly replenished during retrogression. Evidence suggests that prograde fluids are relatively rich in CO₂ and retrograde fluids in H₂O and that the typical non-systematic geochemical variations during prograde and retrograde reactions reflect local effects at the wave front.