ABSTRACTThe quartz-sillimanite-garnet rocks of the Precambrian khondalite assemblage of Orissa consist dominantly of SiO2 + Al2O3 + Fe2O3 + FeO (average c. 95%) with Fe2O3 > FeO. An average analysis (H2O and CO2 free) also shows MgO, CaO and Na2O having 1·0, 0·5 and 0·4%, respectively. Compared with average crustal abundances, enrichment is shown in SiO2, Al2O3 and Fetot and depletion shown in MgO, CaO, Na2O and P2O5 with that for CaO being more than an order of magnitude and that for Na2O being a factor of >7. On an SiO2–Al2O3–Fetot plot a clearly defined field, elongate nearly parallel to the SiO2–Al2O3 sideline, is similar to that for deeply weathered soil profiles in Brazil. This correspondence also extends to enrichment, stability or depletion, compared to crustal averages, for Ce, Co, Cu, Ga, La, Ni, Nb, Th, U, Y, Zn and Zr, but not for Ba, Cr and Rb. In addition comparison of the proportions and ratios of alkalis and alkaline earths in average khondalite and in a weathering profile over a granodiorite, considered to be typical of the weathering of continents, shows remarkable similarities.The major and trace element data are consistent with the khondalites being granulite facies—upper amphibolite facies metamorphic equivalents of a deeply weathered soil profile. Associated quartzites and calc-silicate granulites are interpreted as having been silcretes and calcretes, respectively. This interpretation implies (1) the previous existence of a large stable cratonic mass on which the soil profile formed, (2) climatic conditions suitable for the development of such a profile, (3) topography, drainage systems and groundwater movement in Precambrian times similar to those of present day peneplane regions, (4) the presence of free oxygen in the atmosphere, (5) rapid covering (e.g. by products of volcanism) to preclude mass wasting, (6) a Precambrian stratigraphy in the crystalline rocks of the Eastern Ghats region similar to that of South India, and (7) orogenesis that involved tectonic repetition of lithological units and a mechanism for taking products of surface weathering down to granulite facies P–T conditions and subsequently elevating them.
Wood Ash in Canavalia ensiformis L. Cultivation on Highly Weathered Soil in Brazil