Regolith is largely a product of weathering where bedrock may be exposed to water or other compounds that percolate through an outcrop. The chemical compounds can alter the minerals present in the rock over time, breaking down some material into smaller components and separating it from the bedrock layer (Birkeland, 1999). Cleavages, cracks, and other weak planes are suitable sites for weathering. In the present case, the cracks have developed in the granitic parent rock with the advancement of weathering and the same has been filled with the water during precipitation (rainfall). The fragmentation of the rock created saprolite with ferruginous and clay coatings during the chemical weathering. Alteration of the minerals and their complete fragmentation/isolation created sandy Bw-horizon in the regolith profiles. The red and yellow colors of the Bw-horizon may be due to its formation in oxidising conditions and the presence of iron minerals. As per the USDA Soil Classification (1999), the studied regolith can be classified as Inceptisol that is a poorly weathered mountainous soil of the humid to sub-humid zone where clay production is low.
The petrographic study of the regolith reveals that the oblique cracks in the feldspars might have developed during dry phases and were filled with clay minerals and iron oxides during wet seasons. The precipitation of submicron-sized hematite within orthoclase pores at the replacement front results in the red coloration (e. g. Plumper and Putnis, 2009). The occurrence of sericite, illite, and kaolinite associated with the feldspars may indicate dissolution and reprecipitation processes in the regolith development (e. g. Plumper and Putnis, 2009). The single-channel voids suggest faunal activities and the bifurcated channel voids may be rhizoliths formed by the plant roots (e. g. Singh et al., 2007).
The up-section increase in alumina in the regolith profiles looks directly related to clay enrichments as a result of weathering and pedogenesis. The TiO2, supposed to be a stable oxide, shows some variation in the regolith profiles may be effect of leaching or dissolution. The up-section increase of K2O and the up-section decrease of Na2O may be related to the higher stability of K2O as compared to Na2O in weathering environments (e. g. Pettijohn, 1984). The appreciable concentration of K2O may increase the nutrient value of the regolith and crops may be grown on it.
In the regolith profiles, the decrease of SiO2 concentration from bottom to top may be due to an alkaline environment (pH > 7) which also favors the enrichment of CaO (Fig. 8). This may also be due to the dilution effect and alteration of feldspar into clay minerals (e. g. Singh 2009a, b). The depletion of Na2O shows the alteration of sodic plagioclase (particularly albite) more than that of alkali feldspars. The enrichment of K2O may be due to its incorporation in the illite during progressive weathering. The depletion of CaO and MgO as compared to UCC (after Rudnick and Gao, 2003) can be explained by their relatively more mobile nature during weathering (Nesbitt and Young, 1982).
The chemical index of alteration values of all the regolith samples are higher than the upper continental crust (UCC = 50) suggested by McLennan (1993). The CIA values to a maximum of 61 in the lower-most horizon, 69 in the lower regolith, and 78 in the top-most part of the regolith reveal that the chemical weathering increased progressively from the lower-most horizon to the top in the regolith. In the ternary A-CN-K diagram, saprolite is plotting just above the plagioclase-K-feldspar tie line and regolith is clustering away from the A-CN line, close to the A-K line, and close to the illite zone. The proximity of some of the regolith samples to the plagioclase-K feldspar tie line indicates the existence of a sub-humid environment in the early stage of soil formation whereas the appearances of illite and kaolinite indicate chemical alteration due to more rainfall. Also, the PIA (plagioclase index of alteration) values from 67 in the lower part and 92 in the upper-most part of the regolith suggest that the plagioclase alteration is intense in the upper-most horizon. The depletion of Na2O and enrichment of Al2O3 led to higher values of PIA during a progressive stage of weathering (e. g. Fedo et al., 1995). A positive correlation has been observed between CIA and PIA because of the similar trend of the weathering where the advancement of weathering led to the leaching of K2O in the regolith.
The behavior of Sr is more or less similar to Na and/or Ca-rich feldspar, which shows its association and strong correlation with plagioclase feldspars. The Sc shows its enrichment due to the development of secondary ferromagnesian minerals during progressive weathering. The V, Ni, Zn, and Co show a significant correlation with Fe2O3 due to their association with iron minerals or their sequestration with iron oxyhydroxide or Fe-coated minerals due to the dissolution of Fe-hosting minerals such as biotite. The significantly lower values of Sr and Rb as compared to the upper continental crust (e. g. Rudnick and Gao, 2003) are because of feldspar disintegration and development of secondary clay minerals during the weathering processes. The depletion of Ga from the lower horizon to the upper horizon of the regolith indicates oxidizing environment particularly in the upper-most part (e. g. Sharma and Rajamani, 2000; Vyshnavi and Islam, 2015). Although Zr is resistant, its depletion in some profiles (particularly in the saprolite horizons) is due to the extensive leaching of the host minerals. Ti/ Zr values around 40% of the host rock suggest in-situ soil formation (Maynard, 1992). The Ti/Zr values around 40% in the regolith suggest their in-situ development in the present case.
The REEs are very useful in determining crustal compositions due to insignificant changes in their composition by secondary processes and diagenesis (Bhatia and Crook, 1986; Armstrong-Altrin et al., 2004). The higher LREE / HREE ratios (ranging from 2.41 to 12.53 in all the studied regolith profiles and negative Eu anomalies are signatures of the felsic rocks (granite) and their weathering products (Cullers and Graf, 1984). The progressive enrichment of LREE in the studied profiles of the regolith along with the negative Eu anomaly is because of the granitic parent rock and the in-situ weathering processes. The lower value of Eu / Eu* in the regolith (0.73) suggests the dissolution of plagioclase feldspars. The mobilization of LREE resulted due to their absorption by the clay minerals (e. g. Verma et al., 2022). Cerium in the oxidizing environment occurs as Ce 4+, highly insoluble as CeO2 whereas the other REE maintain their Ce3+ ionic states and are leached by circulating water (Marsh 1991). The positive Ce anomaly in all the four profiles suggests that Ce is fixed as Ce 4+ due to oxidation after dissolving initially as Ce 3+. The Ce anomaly reflects oxidation of Ce and coprecipitation by Fe- and Mn-oxides and organic matter (Mao et al., 2017). Furthermore, the positive Ce anomaly in the regolith profiles indicates that Ce was rapidly precipitated during weathering and retained at the upper regolith horizons. Mao et al. (2017) found slight REE enrichment in an upper layer and no obvious light REE/heavy REE (LREE/HREE) fractionation (LaN/YbN of 0.9) in one granitic profile from Mongolia, while significant enrichment in LREEs in the upper portion (LaN/YbN > 1.8) in the second profile and China. Furthermore, the Eu, Ce, and Gd anomalies of the two profiles are different. Slightly negative Eu anomaly indicates slow dissolution of primary minerals and little secondary products (Mao et al. 2017). The negative Eu anomaly in the present case suggests slow dissolution of primary minerals and little secondary products.
Weathering systems of granitic rocks display a relative depletion of SiO2, Na2O, CaO, Rb, and Sr and relative enrichments of sesquioxides (Fritz, 1988). However, we have recorded only depletion of SiO2 and Na2O and enrichment of CaO, Rb, and Sr from the lower-most horizons to the top-most horizons of the regolith profiles in the present study. This difference in mobility of the elements may be because of the local hydromorphic conditions. SiO2 and Na2O show systematic increases with precipitation, runoff, and temperature in the watersheds underlain by granitic rocks under normal climates and erratic behavior under extreme climates (White and Blum, 1995). In the area under study, the present climate is sub-humid, but we cannot rule out extreme climates in the past that must have been responsible for the depletion of SiO2 and Na2O in the regolith profiles. However, there is more likelihood of moderate rainfall in the past that resulted in moderate weathering in the regolith that is depleted in SiO2 and Na2O. It is a known fact that SiO2 leaches during summers and is removed during the next rainy season making regoliths silica deficient in seasonal climatic conditions.