Geochemistry of high-grade supracrustal rocks from the Kerala Khondalite Belt and adjacent massif charnockites, South India

Abstract

The Kerala Khondalite Belt (KKB) comprises a suite of supracrustal rocks of probable Proterozoic age that was metamorphosed to the granulite facies. The KKB consists of an interlayered sequence of: (1) garnet-biotite ± orthopyroxene ± graphite gneisses; (2) khondalites (graphite-garnet-biotite-sillimanite ± cordierite gneisses); and (3) cordierite gneisses (garnet-biotite-cordierite ± orthopyroxene), as well as less abundant mafic granulites, calc-silicates and quartzites. Petrologic and geochemical data suggest that these rocks were originally arkosic and argillaceous sediments (possibly accompanied by felsic volcanic rocks), along with minor intercalations of basalts, carbonates and quartz-rich sandstones. The REE patterns of the metasediments are typically LREE enriched with large negative Eu anomalies, indicating a continental provenance. The major- and trace-element compositions of some of the arkosic rocks of the KKB resemble those of the massif charnockites that bound the KKB to the north which suggests that the massif charnockites may have been the source of some of the KKB sediments. Modern-day analogues of the pelite-arkose lithologic association observed in the KKB occur in intracratonic rifts and rifted continental margins. This association is clearly distinct from the graywacke-mafic volcanic association found in the southern Indian greenstone belts and many other lower-grade Archean sedimentary sequences. The KKB supracrustals show both lithologic and chemical similarities to many Proterozoic supracrustal successions.