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

Philip Allen*, Kent C. Condie, B. L. Narayana

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

44 Citations (Scopus)

Abstract

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.

Original languageEnglish
Pages (from-to)323-336
Number of pages14
JournalGeochimica et Cosmochimica Acta
Volume49
Issue number2
DOIs
Publication statusPublished - 1985
Externally publishedYes

Fingerprint

Dive into the research topics of 'The geochemistry of prograde and retrograde charnockite-gneiss reactions in southern India'. Together they form a unique fingerprint.

Cite this