V. Rajamani

Geochemistry of the loessic sediments on Delhi ridge, eastern Thar desert, Rajasthan: implications for exogenic processes

Abstract Quartzite ridges in the Delhi region include in their local depressions thick piles of unconsolidated, unstratified, yellowish brown, homogenous loamy sediments. These Delhi ridge sediments (DRS) have textural, mineralogical and geochemical characteristics similar to those of loess sediments. Geochemically, DRS, just as other loess sediments, are similar to average exposed upper continental crust of Taylor and McLennan [Taylor, S.R., McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell, London, 311 pp.]. The calculated chemical index of alteration [Nesbitt, H.W., Young, G.M., 1984. Prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations. Geochim. Cosmochim. Acta 54, 1523–1534; Nesbitt, H.W., Young, G. M., 1989. Formation and diagenesis of weathering profiles. J. Geol. 97, 129–147.] and other chemical parameters suggest a provenance of chemically less weathered rocks. During the process of wind transport and deposition, not only the dilution effect of silica on the REE abundance was counterbalanced but also the REE pattern was kept similar to the UCC and PAAS by the appropriate concentration and mixing of the REE bearing phases. The immediate source to these sediments is likely to be mechanically ground sediments of glacial origin. The last phase of the upliftment of the Himalayas, accompanied by Pleistocene glaciation could have provided a vast glacio–fluvial outwash plain. The aeolian processes associated with prevailing W to SW winds in the hot arid condition seem to have been selectively transporting fertile silt materials leaving behind infertile desert sands.

Weathering of gneissic rocks in the upper reaches of Cauvery river, south India: implications to neotectonics of the region

In the upper reaches of the Cauvery catchment area in southern India, the Archean Peninsular Gneisses exhibit a greater degree of weathering in the field relative to the massive charnockites and granites. Although the gneisses have been physically broken down along foliation planes, fractures and compositional discontinuities, secondary mineral (such as chlorite, smectite, kaolinite and Fe-oxyhydroxide) formation is insignificant. Consequently, there is little chemical change even up to the stage of saprolite development. Only in the regolith there is some loss of Ca, Na, Sr, Ba, Mg and SiO2. Other elements such as Fe, Mn, Al, Cr, Ni and REE all show enrichment relative to TiO2 in areas which have developed profiles by in situ weathering. The chondrite normalized REE patterns, however, show little change excepting a slight flattening of the patterns because of greater HREE mobility during weathering. Clay minerals also have flattened REE patterns because of relative HREE enrichment. The depletion and the enrichment of different elements appear to be related to mild leaching of primary mafic minerals by meteoric water rather than by mineral breakdown. This and the extent of variation in Chemical Index of Alteration values suggest that that the gneisses have suffered only incipient chemical weathering for the extent of physical weathering undergone by them. It is likely that this contrasting weathering extents of these >2500 Ma rocks is due to their exposure to surface geological processes only recently because of ongoing periodic uplift and physical erosion of this region.

Major Element, REE, and Other Trace Element Behavior in Amphibolite Weathering under Semiarid Conditions in Southern India

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Geochemistry and origin of ferruginous nodules in weathered granodioritic gneisses, Mysore Plateau, Southern India

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Petrography and geochemical characteristics of the sediments of the small River Hemavati, Southern India: Implications for provenance and weathering processes

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Nd and Sr isotope characteristics of Quaternary Indo-Gangetic plain sediments: Source distinctiveness in different geographic regions and its geological significance

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