The Paleoproterozoic Aravalli Supregroup of rocks, hosted in Aravalli Craton of NW shield, is deposited in shallow and deep water conditions. The major lithologies are phyllites and quartzites with significant components of greywacks and dolomite. Geochemical indices in particular, CIA (Chemical Index of Alteration) values (avg. phyllites: 51.6 - 81.5, avg. quartzites: 57.4 - 95.5) calculated from the data of clastic rocks of ASG suggest minimum to highly intense weathering in the source region. Other indices including PIA, CIW and ICV along with plot patterns on the A-CN-K diagram also nearly endorse CIA based interpretation. These rocks possess relatively high Th/U ratios compared to that found in fresh igneous rocks or their high grade metamorphic equivalents. This high Th/U ratio is neither a source inheritance nor a result of oxidation state rather a manifestation of Th hosting mineral accumulation through sorting. Viewed in the context of present stratigraphic succession, the weathering history adduced from geochemistry does not seem compatible but matches well with earlier classification scheme wherein the evolution of Aravalli Supergroup was considered episodic.
Sedimentary rocks are of great interest since quite long time as they have been effectively used in unfolding the history of diverse geological settings. Clastic material can be act upon through different practices and observations. The chemical and mineralogical compositions of clastic sedimentary rocks are directed by many factors including, transportation mechanism, composition of the source rocks, environmental parameters, duration and intensity of weathering, depositional environment and post depositional makeup (e.g., diagenesis and metamorphism) [
Aravalli craton of NW Indian shield preserves thickest and extensively developed Proterozoic sedimentary sequence [
The Aravalli Mountain range is the main edifice of the Aravalli craton of northwest Indian shield. It extends over 700 km in length, with a general NE-SW trend. It is composed of a Neoarchaean cratonic nucleus [
The rocks of Paleoproterozoic Aravalli Supergroup cover a wide region in the eastern and south-eastern parts of the Aravalli Mountain Range. The entire region can be divided from north to south into three sectors; 1) the Bhilwara sector; 2) Udaipur sector; 3) Lunavada sector. Out of these three sectors, the Udaipur sector is known as the type area of Aravalli Supergroup where they show evidence of complete development of stratigraphic succession and structural evolution. The rocks of the Udaipur sector characteristically show two contrasting sedimentary facies associations which are distributed along two roughly N-E trending belts (
Supergroup is characterized by the presence of alumina-rich and iron-poor palaeosols [
The Lower Aravalli Group is constituted by the rocks of Delwara and Jhamarkotra formations. Basal Delwara Formation is made up of an intercalated sequence of metabasaltic rocks and well-sorted feldspathic quartzites whereas Jhamarkotra Formation comprises dolomites, carbonaceous phyllites with intercalated quartzites. The Middle Aravalli Group commences with deep water turbidite sequences of the Udaipur Formation comprising metagreywacke and phyllite, indicating deepening of the carbonate platform and active tectonism. Udaipur Formation is overlain by thick rock sequences namely dolomite (intermittent phyllite/quartzite), quartzite (intermittent phyllite), and phyllite-dolo- mite-quartzite, grouped respectively as Mochia (Zawar), Bowa and Tidi formations in continuous succession. Debari quartzites and Kabita dolomites constitute Upper Aravalli Group whereas Jharol Formation comprising mica schists and ultramafics represent western deep facies sequence of the Aravalli basin [
Seventy four samples for major and forty two samples for trace elements of quartzites and phyllites representing various formations of Aravalli Supergroup were analysed and the results are presented in
Compositionally, the average contents of major oxides classify the quartzites of Aravalli Supergroup as quartzarenite (SiO2: 91.53%, Al2O3: 4.49%, Fe2O3: 0.90%, CaO: 0.70%, Na2O: 1.06% and K2O: 0.73%) except few samples (Condie, 1993).; The Delwara Quartzites contain lowest silica (avg. 83.73%) and highest alumina (avg. 9.53%) contents whereas the Debari Quartzites posses highest silica (avg. 96.18%) and lowest alumina (avg. 2.58%) abundances. The average Na2O concentration of the quartzites of Udaipur belt (shallow facies) ranges between 0.01 to 3.26%. It could be due to the inconsistent presence of Na rich plagioclase in the different formations. Deep water Jharol Quartzites have low Na2O content (<1%) compatible to low plagioclase in their petrographic mode. Upper continental crust (UCC) normalized spidergrams of quartzites show depletion in all major oxides except SiO2. The magnitude of depletion is maximum in Tidi Quartzites and minimum in Delwara Quartzites (
Among the various formations of Aravalli Supergroup, SiO2 content is maximum in Zawar Phyllites (avg. 71.47%) and minimum in Delwara Phyllites (avg. 55.99%) whereas the Al2O3 abundances are maximum in Jharol Phyllite (avg. 21.66%) and minimum in Delwara Phyllite (avg. 11.58%). However, the Delwara Phyllites have higher contents of Fe2O3 (avg. 14.42%) and CaO (avg. 6.35%). The SiO2 concentrations of the Udaipur and Jharol Phyllites are almost same (avg. 65.79% & 64.47% respectively) but they differ in their Al2O3 abundances as the former is depleted (avg.18.24%) relative to later (avg. 21.66%) (
The general correlation matrix for clastic rocks indicates that Al2O3 is in general positively correlated with chief major oxides (MgO, TiO2, K2O and Na2O)
but shows negative variation with SiO2 in both types of Aravalli clastics. Furthermore, Al2O3 is also in good positive correlation with Th and U. This indicates that the chemical composition of the Aravalli clastics is nearly primary. Minor inconsistencies in some samples of phyllites and quartzites may be due to post depositional effects (
Palaeoweathering in source area is one of the most important processes affecting the composition of clastic sedimentary rocks. To constrain the intensity of chemical weathering in the source area, the geoscientists have employed several indices e.g., Chemical Index of Alteration (CIA) [
It is considered one of the most useful indices to ascertain and quantify the intensity of chemical weathering. The index is calculated as follows:
CIA = [ Al 2 O 3 / Al 2 O 3 + CaO * + Na 2 O + K 2 O ] × 100
The values of oxides are in molecular proportions and CaO* represents CaO in silicate minerals. This weathering index is not only related to the degree of
Lower Aravalli Quartzite | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
SiO2 | Al2O3 | TiO2 | Fe2O3 | MgO | CaO | Na2O | K2O | Th | U | |
SiO2 | 1 | |||||||||
Al2O3 | −0.70 | 1 | ||||||||
TiO2 | −0.36 | 0.29 | 1 | |||||||
Fe2O3 | −0.70 | 0.09 | 0.57 | 1 | ||||||
MgO | −0.68 | −0.05 | 0.17 | 0.88 | 1 | |||||
CaO | −0.71 | 0.00 | 0.10 | 0.83 | 0.98 | 1 | ||||
Na2O | −0.62 | 0.96 | 0.03 | −0.09 | −0.11 | −0.04 | 1 | |||
K2O | −0.46 | 0.53 | 0.93 | 0.48 | 0.05 | 0.02 | 0.28 | 1 | ||
Th | −0.84 | 0.89 | 0.82 | 0.89 | 0.43 | −0.33 | 0.25 | 0.89 | 1 | |
U | −0.23 | 0.44 | 0.39 | 0.46 | 0.35 | −0.85 | 0.22 | 0.45 | 0.57 | 1 |
Middle Aravalli Quartzite | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
SiO2 | Al2O3 | TiO2 | Fe2O3 | MgO | CaO | Na2O | K2O | Th | U | |
SiO2 | 1 | |||||||||
Al2O3 | −0.98 | 1 | ||||||||
TiO2 | −0.81 | 0.72 | 1 | |||||||
Fe2O3 | −0.84 | 0.74 | 0.98 | 1 | ||||||
MgO | −0.79 | 0.67 | 0.87 | 0.94 | 1 | |||||
CaO | −0.65 | 0.60 | 0.39 | 0.52 | 0.74 | 1 | ||||
Na2O | −0.97 | 0.98 | 0.66 | 0.71 | 0.68 | 0.68 | 1 | |||
K2O | −0.90 | 0.93 | 0.72 | 0.69 | 0.51 | 0.30 | 0.87 | 1 | ||
Th | −0.74 | 0.75 | 0.64 | 0.90 | 0.64 | 0.67 | 0.64 | 0.79 | 1 | |
U | −0.38 | 0.38 | 0.48 | 0.62 | 0.29 | 0.33 | 0.26 | 0.50 | 0.88 | 1 |
Upper Aravalli Quartzite | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
SiO2 | Al2O3 | TiO2 | Fe2O3 | MgO | CaO | Na2O | K2O | Th | U | |
SiO2 | 1 | |||||||||
Al2O3 | −0.95 | 1 | ||||||||
TiO2 | −0.91 | 0.79 | 1 | |||||||
Fe2O3 | −0.91 | 0.95 | 0.88 | 1 | ||||||
MgO | −0.12 | −0.02 | −0.10 | −0.28 | 1 | |||||
CaO | −0.75 | 0.51 | 0.89 | 0.57 | 0.22 | 1 | ||||
Na2O | −0.64 | 0.45 | 0.90 | 0.65 | −0.26 | 0.88 | 1 | |||
K2O | −0.65 | 0.86 | 0.40 | 0.77 | −0.19 | −0.01 | 0.01 | 1 | ||
Th | −0.89 | 0.93 | 0.87 | 1.00 | −0.33 | 0.55 | 0.65 | 0.76 | 1 | |
U | −1.00 | 0.94 | 0.94 | 0.92 | 0.07 | 0.78 | 0.69 | 0.62 | 0.91 | 1 |
Lower Aravalli Phyllite | ||||||||||
SiO2 | Al2O3 | TiO2 | Fe2O3 | MgO | CaO | Na2O | K2O | Th | U | |
SiO2 | 1 | |||||||||
Al2O3 | −0.18 | 1 | ||||||||
TiO2 | −0.68 | 0.10 | 1 | |||||||
Fe2O3 | −0.75 | −0.33 | 0.58 | 1 | ||||||
MgO | −0.84 | −0.13 | 0.47 | 0.76 | 1 | |||||
CaO | −0.64 | −0.43 | 0.38 | 0.56 | 0.60 | 1 | ||||
Na2O | −0.59 | −0.04 | 0.52 | 0.42 | 0.35 | 0.59 | 1 | |||
K2O | 0.25 | 0.71 | −0.24 | −0.69 | −0.44 | −0.54 | −0.39 | 1 | ||
Th | 0.46 | 0.01 | −0.65 | −0.45 | −0.59 | −0.55 | −0.42 | 0.62 | 1 | |
U | 0.57 | −0.01 | −0.39 | −0.68 | −0.65 | −0.41 | −0.36 | 0.31 | 0.40 | 1 |
Middle Aravalli Phyllite | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
SiO2 | Al2O3 | TiO2 | Fe2O3 | MgO | CaO | Na2O | K2O | Th | U | |
SiO2 | 1 | |||||||||
Al2O3 | −0.40 | 1 | ||||||||
TiO2 | −0.51 | 0.58 | 1 | |||||||
Fe2O3 | −0.73 | −0.24 | 0.12 | 1 | ||||||
MgO | −0.57 | −0.39 | −0.17 | 0.76 | 1 | |||||
CaO | −0.16 | −0.26 | 0.09 | 0.28 | 0.09 | 1 | ||||
Na2O | 0.05 | −0.19 | 0.21 | −0.02 | −0.04 | 0.18 | 1 | |||
K2O | −0.06 | 0.63 | 0.27 | −0.43 | −0.52 | −0.11 | −0.53 | 1 | ||
Th | 0.05 | 0.27 | −0.30 | 0.00 | −0.61 | −0.33 | −0.44 | 0.31 | 1 | |
U | −0.06 | 0.35 | −0.20 | 0.00 | −0.57 | −0.46 | −0.22 | 0.26 | 0.85 | 1 |
Upper Aravalli Phyllite | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|
SiO2 | Al2O3 | TiO2 | Fe2O3 | MgO | CaO | Na2O | K2O | Th | U | |
SiO2 | 1 | |||||||||
Al2O3 | −0.83 | 1 | ||||||||
TiO2 | −0.45 | 0.04 | 1 | |||||||
Fe2O3 | −0.19 | −0.37 | 0.51 | 1 | ||||||
MgO | −0.82 | 0.42 | 0.46 | 0.61 | 1 | |||||
CaO | 0.45 | −0.06 | −0.95 | −0.55 | −0.40 | 1 | ||||
Na2O | −0.63 | 0.22 | 0.52 | 0.58 | 0.91 | −0.34 | 1 | |||
K2O | −0.96 | 0.72 | 0.68 | 0.26 | 0.77 | −0.68 | 0.63 | 1 | ||
Th | −0.53 | 0.84 | 0.94 | −0.92 | −0.80 | −0.92 | −0.53 | 0.82 | 1 | |
U | −0.48 | 0.81 | 0.93 | −0.90 | −0.83 | −0.94 | −0.57 | 0.80 | 1 | 1 |
weathering but also controlled by source composition and grain size. The source rocks with felsic composition have greater CIA values than mafic ones. Selective removal of cations (e.g. Ca2+, Na+, K+) over stable residual constituents (Al3+, Ti4+) during weathering in a warm and humid climate results in high CIA values [
Average CIA values of the phyllites of Aravalli Supergroup range from 51.6 to 81.5 (DWP―Delwara Phyllites: 51.6, JKP―Jhamarkotra Phyllites: 64.9, UPP― Udaipur Phyllites: 66.2, ZWP―Zawar Phyllites: 71.5, BWP―Bowa Phyllites: 81.5, TDP―Tidi Phyllites: 71.4, JHP―Jharol Phyllites: 76.3). It is evident from CIA values of phyllites of different formation that chemical weathering of Aravalli Supergroup increased with stratigraphic younging except the Bowa Phyllites which possess highest CIA values, although these phyllites are not at top in stratigraphic hierarchy (
Harnois (1988) [
CIW = [ Al 2 O 3 / ( Al 2 O 3 + CaO + Na 2 O ) ] × 100
In this index Al2O3 is used as immobile element that remains in the system as proposed in earlier indices whereas CaO and Na2O are the mobile components as they are readily leached during weathering. This index is alike to the CIA except to the elimination of K2O. Maynard [
The average CIW values of phyllites of Aravalli Supergroup ranging from 56.9 to 93.3 (DWP: 56.9, JMP: 82.9, UPP: 87.7, ZWP: 92, BWP: 81.7, TDP: 88, JHP: 93.3) advocate for least to intense chemical weathering. Similarly, average CIW values of quartzites of Aravalli Supergroup ranging from 67 to 99.1 (DWQ: 75.5, JMQ: 67.0, UPQ: 76.0, ZWQ: 83.5, BWQ: 85.5, TDQ: 96.6, DBQ: 99.1, JHQ: 82.4) nearly duplicate the phyllite trend.
Plagioclase Index of Alteration (PIA) has been used as an alternative to CIW. It is used to monitor the weathering of the plagioclase [
PIA = [ ( Al 2 O 3 − K 2 O ) / ( Al 2 O 3 + CaO * + Na 2 O − K 2 O ) ] × 100
All the oxides of this formula are in molecular proportions and CaO* is the CaO content incorporated in silicate minerals only. Like CIA and CIW, PIA around 50 suggests derivation of detritus from fresh rocks and values closer to 100 indicate the complete conversion of plagioclase into clay minerals [
Index of compositional variability (ICV) ratio is used to access the original detrital mineralogy [
ICV = ( Fe 2 O 3 + Na 2 O + CaO + MgO + MnO + K 2 O + TiO 2 ) / Al 2 O 3
ICV is used to measure the abundance of alumina in relation to the other major oxides. The clay minerals owing to their higher Al content possess lower ICV whereas non clay silicate minerals contain higher proportion of silica and lower amount of alumina as compared to the clay minerals. The ICV decreases in the order of pyroxene and amphibole (~10 - 100), biotite (~8), alkali feldspar (~0.8 - 1), plagioclase (~0.6), muscovite and illite (~0.3), montmorillonite (~0.15 - 0.03), and kaolinite (~0.03 - 0.05) [
The Th/U ratio of clastic rocks is also used to interpret weathering conditions [
The average Th/U ratio in the phyllites of Aravalli Supergroup ranges from 5.32 to 8.52 (DWP: 5.6, JMP: 6.2, UPP: 7.1, ZWP: 8.5, BWP: 5.3, TDP: 6.9, JHP: 8.2). The average Th/U ratio of quartzites of Aravalli Supergroup ranges from 1.7 to 11.4 (DWQ: 11.3, JMQ: 1.7, UPQ: 4.9, ZWQ: 4.7, BWQ: 3.2, TDQ: 2.3, DBQ: 2.8, and JHQ: 2.6). The average Th/U ratios in the clastic rocks of Aravalli Supergroup are higher than the Th/U ratio of upper crustal rocks (3.5 - 4.0) and can be interpreted either due to chemical weathering under oxidizing conditions in the depositional basin or source characteristics or accumulation of Th bearing minerals through sorting [
Most authors favour the use of Al2O3-(CaO*+Na2O)-K2O (A-CN-K) ternary plot in evaluating the chemical weathering trends than simple comparison of numerical values [
This diagram portrays the molar proportions of Al2O3 (A apex), CaO* + Na2O (CN apex) and K2O (K apex), where CaO* represents CaO incorporated into silicate minerals [
The A-CN-K plot is also useful for evaluating fresh rock composition and examining their weathering trends since unweathered primary igneous rocks have CIA values close to 50 [
In the A-CN-K triangular diagram, the data of the Aravalli Supergroup appears to be plotted in scattered form. Such patterns are interpreted to indicate derivation of detritus from different sources. However, close scrutiny of the diagram indicates that data points of individual formations show parallelism with A-CN or A-K axes.
Most of the samples of phyllites and quartzites of Aravalli Supergroup lie above the feldspar join in A-CN-K plot (
Sediments/sedimentary rocks are the final products of a chain of dynamic phenomena including weathering, ablation, transport, deposition and diagenesis. Therefore, chemical composition of the sedimentary rock is necessarily different from that of its parent rock because of the loss of the most soluble elements into water and the particle hydrodynamic sorting during transport by rivers to sea. After deposition, sediment may experience several mineral transformations during its burial diagenesis history. These transformations depend on the local physico-chemical conditions that prevail at a given time. Some of the primary minerals may be dissolved (feldspars) or re-crystallized (clays) while new ones precipitate in pores (quartz, carbonates, chlorites). Consequently, the composition of the sediments is modified according to the type and intensity of the different possible diagenetic reactions. Nonetheless, the dismantling of a given weathered rock (same parent rocks forming the drainage basin, same climatic conditions) may produce different types of sediments according to the energy and length of transport. Thus, the chemical composition of arkoses, greywackes and shales gives different CIA values in spite of their common source [
The overall synthesis of geochemical data of Aravalli clastic rocks discussed, suggests low to extreme degree of chemical weathering in their source area. Close scrutiny of weathering indices reveal a pattern of change in the weathering intensity from base to top. The Delwara phyllites as well quartzites possess minimum CIA values (51.6, 57.4 respectively) whereas Bowa and Jharol clastics have maximum CIA values (BWP: 81.5, BWQ: 78.9, JHP: 81.6, JHQ: 76.3) (
the inference adduced from CIA. However, A-CN-K diagrams fail to provide any emphatic relationship. Since Nesbitt’s pioneer work defining Chemical Index of Alteration (CIA) [
Sample/ Element | Delwara | Jhamarkotra | Udaipur | Zawar | Bowa | Tidi | Debari | Jharol | |||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Quartzite | Phyllite | Quartzite | Phyllite | Quartzite | Phyllite | Quartzite | Phyllite | Quartzite | Phyllite | Quartzite | Phyllite | Quartzite | Quartzite | Phyllite | |
SiO2 | 83.73 | 55.99 | 94.78 | 65.02 | 84.10 | 65.79 | 91.30 | 71.47 | 96.24 | 57.76 | 98.64 | 66.88 | 96.18 | 96.87 | 64.47 |
Al2O3 | 9.53 | 11.58 | 1.76 | 15.27 | 8.37 | 18.24 | 5.70 | 16.45 | 2.64 | 14.35 | 1.30 | 17.50 | 2.58 | 2.06 | 21.66 |
TiO2 | 0.10 | 1.37 | 0.02 | 0.80 | 0.16 | 0.71 | 0.17 | 0.60 | 0.05 | 0.92 | 0.00 | 0.96 | 0.05 | 0.06 | 0.96 |
Fe2O3 | 1.29 | 14.42 | 0.69 | 7.59 | 1.71 | 6.50 | 0.84 | 5.63 | 0.33 | 14.20 | 0.02 | 8.39 | 0.61 | 0.53 | 6.59 |
MnO | 0.02 | 0.20 | 0.03 | 0.09 | 0.02 | 0.11 | 0.02 | 0.04 | 0.02 | 0.08 | 0.03 | 0.01 | 0.03 | 0.02 | 0.04 |
MgO | 0.15 | 5.59 | 0.89 | 4.09 | 0.67 | 2.68 | 0.16 | 1.15 | 0.07 | 10.04 | 0.01 | 1.39 | 0.47 | 0.05 | 1.12 |
CaO | 0.55 | 6.35 | 1.72 | 2.21 | 0.59 | 0.62 | 0.20 | 0.23 | 0.10 | 0.62 | 0.04 | 0.20 | 0.04 | 0.05 | 0.22 |
Na2O | 3.26 | 2.52 | 0.02 | 1.22 | 2.59 | 1.30 | 0.46 | 0.72 | 0.26 | 1.94 | 0.01 | 1.23 | 0.01 | 0.37 | 0.74 |
K2O | 1.39 | 1.83 | 0.11 | 3.54 | 1.75 | 4.55 | 1.16 | 3.60 | 0.32 | 0.04 | 0.01 | 3.36 | 0.08 | 0.03 | 4.12 |
P2O5 | 0.01 | 0.16 | 0.02 | 0.18 | 0.05 | 0.23 | 0.03 | 0.11 | 0.01 | 0.09 | 0.01 | 0.09 | 0.02 | 0.01 | 0.09 |
CIA | 57.40 | 51.58 | 63.69 | 64.89 | 66.99 | 66.23 | 67.64 | 71.47 | 78.91 | 81.47 | 95.52 | 71.35 | 95.26 | 81.57 | 76.26 |
PIA | 73.46 | 51.38 | 65.79 | 78.83 | 70.85 | 84.87 | 78.45 | 89.42 | 84.75 | 81.58 | 96.57 | 84.40 | 99.02 | 82.06 | 90.95 |
CIW | 75.54 | 56.86 | 67.24 | 82.89 | 76.01 | 87.69 | 83.50 | 91.98 | 85.55 | 81.66 | 96.61 | 87.98 | 99.06 | 82.37 | 93.33 |
ICV | 1.18 | 4.30 | 1.89 | 2.09 | 1.04 | 1.57 | 0.80 | 1.14 | 0.53 | 3.61 | 0.09 | 1.36 | 0.94 | 0.64 | 1.05 |
Th | 15.01 | 17.77 | 1.60 | 19.40 | 5.4 | 20.30 | 6.52 | 23.82 | 2.25 | 6.86 | 0.32 | 34.51 | 1.33 | 1.2 | 28.44 |
U | 1.32 | 3.20 | 0.81 | 4.12 | 1.13 | 2.89 | 1.40 | 2.76 | 0.67 | 1.29 | 0.11 | 5.03 | 0.45 | 0.36 | 3.61 |
Th/U | 11.35 | 5.56 | 1.69 | 6.24 | 4.94 | 7.09 | 4.67 | 8.52 | 3.18 | 5.32 | 2.26 | 6.87 | 2.77 | 2.64 | 8.24 |
based on mineralogical or chemical data to evaluate the intensity of alteration. Nevertheless, the CIA remains, by far, the most widely used one.
The major factors which control the intensity of weathering are heavy rainfall, vegetation cover, relief, high surface temperature and high atmospheric PCO2. A low to moderate degree of weathering as indicated by the composition of lower Aravalli do not suggest more CO2 enriched atmosphere and usually high surface temperature in absence of vegetation. Various geochemical parameters such as CIA, CIW, PIA, Th/U ratio, ICV values as described above suggest moderate climatic conditions during the deposition for the sediments of Lower and Middle Aravalli and the Upper Aravalli formation reflect the extreme weathering in the source region. The higher values of ICV (>1) for Aravalli sediments indicate that they are compositionally immature and possibly derive from a tectonically active environments where sediment recycling is not active. High Th/U ratios in the Aravalli clastics are due to the low mobility of Thorium (which got accumulated through hydraulic sorting) under all environmental conditions, mainly due to the high stability of the insoluble oxide ThO2 and the strongly resistant nature of its carrier minerals such as monazite.
The stratigraphic division of the lithosequences of Aravalli fold belt has been in debate since the elevation of their status from group to supergroup [
Heron (1953) [
A critical examination of chemical weathering trend inferred from weathering indices of clastic rocks of various formations of Aravalli Supergroup in present study reveals that the rocks of Delwara Formation are almost unweathered, Jhamarkotra and Udaipur largely moderate, Tidi and Jharol are intense and Bowa and Debari are highly intense. In the present stratigraphic succession, this variation in chemical weathering is difficult to explain as it requires protuberance in source area followed by peneplanation stage and then moderate uplift. If the samples are re-arranged according to the succession given by Roy (1988) [
peneplanation of the source region. It is argued and observed that there were consistent attempts within the Indian shield to change its evolution style from typical Archean greenstone to Proterozoic Wilson Cycle [
The geochemical indices used to constrain weathering intensity in the source region prescribe three episodes of chemical weathering. Each cycle began with physical weathering and culminated in intense chemical weathering. The conglomerate/quarzite horizons occurring at various levels in the stratigraphic hierarchy show maximum chemical weathering and thus mark the end of a weathering cycle. This interpretation advocates that stratigraphic classification proposed by Roy (1988) [
The authors are thankful to the Chairmen, Department of Geology, AMU, Aligarh, for providing requisite research facilities. We are also indebted to Dr. A.V. Mudholkar, Chief Scientist, NIO, Goa, Dr. Indra S. Sen, IIT, Kanpur and Dr. Srinivas Sharma, Principal Scientist, NGRI, Hyderabad for the sample analysis. Dr. Tavheed Khan, PDF, NGRI, Hyderabad, is thanked for fruitful help during the course of this study. University Grant Commission, New Delhi is acknowledged for financial support in the form of Basic Scientific Research (BSR) fellowship to PKS.
Singh, P.K. and Khan, M.S. (2017) Geochemistry of Palaeoproterozoic Rocks of Aravalli Supergroup: Implications for Weathering History and Depositional Sequence. International Journal of Geosciences, 8, 1278-1299. https://doi.org/10.4236/ijg.2017.810074