Geochemistry of late Archaean metagreywackes from the western Dharwar Craton, South India; implications for provenance and nature of the late Archaean crust
Hegde, V.S.; Chavadi, V.C.
Gondwana Research 15.2
2009
ISSN/ISBN: 1342-937X DOI: 10.1016/j.gr.2008.09.006
Accession: 037061769
Late Archaean metagraywackes of the Ranibennur Formation, Dharwar Supergroup, in the Dharwar-Shimoga schist belt of the western Dharwar Craton (WDC) are texturally and mineralogically immature of the quartz-intermediate type. The SiO2 content in them ranges from 60.58 to 65.26 wt.%. Chemical Index of weathering (CIW) values varies between 50 and 65.4, indicating a low degree of chemical alteration of the provenance rocks. A high degree of correlation between K2 O and Al2 O3 (r = -0.73) and low Rb/Sr ratios also suggest a low degree of alteration of provenance rocks. Abundances of transition group elements (Cr = 118-221; N = 89-154; V = 89-192 and Sc = 11-16 ppm) as well Zr (132-191 ppm) suggest a mixed mafic-felsic provenance for the metagraywackes. Low HREE and Y content, and low Tb/Yb ratios (0.23-0.41) suggest the presence of tonalite as an important component in the provenance areas. Values of Eu/Eu*(0.78) and Th/Sc (0.55) suggest that the granodioritic upper crust had evolved prior to serving as the provenance. Mixing calculations suggest 50-55 vol.% tonalite, 20-25 vol.% granite, 18-20 vol.% basalt and approximately 5 vol.% komatiite composition for the provenance. Geochemical characteristics of the Ranibennur metagraywackes suggest that sedimentary basin formed in the vicinity of a magmatic arc in a continental island arc setting, and the detritus were shed from the arc rock.