Composition of the middle and late Archaean upper continental crust as sampled from the Kaldurga Conglomerate, Dharwar craton, India

Abstract

Abstract The greywacke matrix of the polymict Kaldurga Conglomerate (KCM) of the Bababudan schist belt has preserved the composition of the upper continental crust exposed during the middle and late Archaean. The boulder-to pebble-size clasts are of tonalitic and trondhjemitic gneisses (TTG), cross-bedded quartzites, amphibolites, banded iron formations (BIF), vein quartz and carbonates. The recrystallized matrix, mainly amphibolitic and quartzofeldspathic, also includes heavy minerals such as kyanite derived from older schist belts. Geochemical parameters such as K 2 O/Na 2 O, MgO/Fe 2 O 3 , MgO/K 2 O, Cr/Zr, Zr/Ti, La/Lu, Eu/Eu ∗ and LREE/HREE indicate that the conglomerate and its matrix were derived from a multicomponent source area. A mixing model of provenance consisting of about 50% TTG, 35% ultramafic rocks, 10% mafic rocks and 5% sediments, is consistent with the average composition of the KCM. The exposure ratio of 90:10 of TTG and mafic-ultramafic rocks older than 2.6 Ga suggest that during the middle and late Archaean, the exposed crust had a relatively high proportion of mafic-ultramafic components. Similar characteristics of multicomponent mixing are indicated by a large variation in REE abundances and Eu anomalies. It is inferred that the Kaldurga Conglomerate represents a transitional stage in unidirectional crustal evolution from dominantly mafic to felsic composition.