Download CHAPTER 6 WEATHERING HISTORY AND PALEOCLIMATE

CHAPTER 6
WEATHERING HISTORY AND PALEOCLIMATE
6.1. Introduction
Weathering acts to break down source rock minerals partitioning weathering
components into
1. Gravel and sand sized residual minerals and lithic grains.
2. New minerals, principally clays and in modern environments, oxides.
3. And materials in solution, with continued weathering, mineral breakdown
becomes more complete.
The term chemical weathering is restricted to the interactions between the
waters derived directly from precipitation (generally rain water containing inorganic
and organic acids derived from decay of litter in the soil zone) and rocks and their
weathering residues (Garrels, 1967; Nesbitt and Young, 1984, 1989; Drever, 1988).
The definition excludes reactions between permanent or perched ground waters and
rocks (or residues). Interactions between ground waters, rocks and the materials of the
weathering profile are considered to the diagenetic.
The degree of chemical weathering is mainly a function of climate and erosion
rate, the latter of which is controlled by relief and vegetation cover of the source area.
Studies on the relationship between climate and the degree of weathering have shown
that higher rainfall corresponds with increased loss of labile minerals and higher
Chemical Index of Weathering (CIA) values in the resulting sediments (Basu, 1981;
James et al., 1981; Suttner et al., 1981; Dutta and Suttner, 1986; Girty, 1991; White
and Blum, 1995).
Consistent rainfall will continuously flush a weathering profile with
unsaturated fluids for hydrolysis and removal of the products of ion exchange and
volumetrically more parent rock material will be subject to decomposition over a
given unit time. With increased rainfall comes increased vegetation and this has added
effects of providing organic acids to promote acid mineral decay and of binding soils
against erosion, allowing longer and more complete weathering.
Under modern arid climatic conditions, sediments tend to have compositions
nearly same to those of their source rocks and even more soluble and easily weathered
rock and mineral types tend to be well represented in the weathering products, mainly
as resistates (Hessler, 2001). In addition, erosion and transport under arid climatic
condition tends to be dominated by flashy streams, characterized by sudden high
114
volume of rainfall leading to flash floods and debris flows that transport a large
amount of immature sediments with a minimal amount of working and abrasion.
In the absence of vegetation, the erosive power of rainfall depends on two
factors viz. cohesion of particles and friction of particles acting on the slope (Birot,
1968). These factors, which are related to a complex set of sub factors like grain size,
sorting, steepness of slope, composition and specific gravity of grains and thickness of
sedimentary cover, determine how effectively rainfall will erode the sedimentary
veneer. It is generally observed, that the erosional effect of rainfall on barren slopes
outpaces rock decomposition (Birot, 1968). In a temperate environment with
continuous ground vegetation, rainfall has only a minor erosional effect. However,
where vegetative cover is highly discontinuous as in a desert environment, rainfall
will erode sediments before much chemical weathering can occur (Birot, 1968;
Hessler, 2001).
Increase in either PCO2 or surface temperature will enhance the rate of
weathering. Higher temperatures will significantly enhance the rate of mineral
decomposition and the potential for more resistant minerals such as quartz and
potassium feldspar to undergo hydrolysis. Vant’ Hoff’s equation gives that the rate of
chemical reaction will increase two or three times with every 100C rise in temperature
(Retallack, 1990) and in general, weathering rates will similarly increase with higher
temperatures.
The low degrees of chemical weathering may thus reflect high erosion
rates, leaving not enough time for intense chemical weathering, coupled with short
transport distances and rapid sedimentation in a tectonically active basin setting and
the lack of vegetation.
Chemical weathering strongly affects the major elements geochemistry and
mineralogy of sediments (Nesbitt and Young, 1982; Johnsson et al., 1988; McLennan,
1993). Thermodynamic stability of minerals, experimental kinetic data and
compositional data from weathering solutions and profiles are in accordance, provide
a framework for interpretation of compositional changes accompanying weathering.
The framework can be used to interpret weathering trends produced in the past, to
decipher the climatic regimes influencing the composition of sediments (Nesbitt et al.,
1980; Nesbitt and Young, 1982, 1984).
115
To investigate and quantify the degree of chemical weathering to which the
rocks have been subjected, several weathering indices have been proposed. A useful
weathering index is that which satisfies the following criteria viz.
1. Yields a unique value or range of values characteristics of different
weathered materials.
2. Includes only those elements which have consistent geochemical behavior
during weathering.
3. Be independent of the degree of oxidation of the weathering material.
4. Involve chemical elements commonly reported in soil analyses.
5. And be as simple as possible and easy to use.
Kramer (1968), stated that in general, the ultimate weathering equation
may be considered on exchange of Na, Ca, Mg and K for H in the mineral structure
with (or without) a loss of Si. Although silica does tend to be mobile during
weathering, its movement within a profile is often irregular, and the total proportion
lost is usually small. In contrast, alkali and alkaline earth metals display marked
depletions (Nesbitt et al., 1980; Button and Tyler, 1979; Nesbitt and Young, 1982;
Harriss and Adams, 1966; Craig and Loughnan, 1964; Kronberg and Nesbitt, 1981;
Schan and Henderson, 1983; Grandstaff et al., 1986; McLennan et al., 1995; Gu,
1994, 1996). Consideration of this characteristic, together with the probable
mechanism of weathering as advanced by Kramer (1968), make Na, Mg. K, and Ca
more suitable elements than Si as a base for developing a weathering index.
Each of these elements has a different susceptibility to weathering. In order to
include in the index, some estimate of this susceptibility is required. Thus, the
strength of the element to oxygen bonds as given by Nicholls (1963) has been used for
the purpose of developing the indices by various workers. These strengths are
measures of the energy needed to break the bonds and hence of the relative likelihood
of the elements being involved in a weathering reaction. The assumption is made that
the bonds are ionic.
Subsequently, attempts have been made to generate an equation to quantity the
degree of chemical weathering based on major element analysis involved many
variables in complex calculations (Harnois, 1988). For example, the Ruxton ratio
(Ruxton, 1968), Vogt’s Residual Index (V) (Vogt, 1927; Roaldset, 1972), Weathering
Index of Parker (WIP) (Parker, 1970) and Modified Weathering Potential Index
(MWPI; Reiche, 1943; Vogel, 1975).
116
The interpretation of weathering from weathering indices requires caution,
because of the interplay of post depositional metasomatic effects on major element
content especially, for deformed and metamorphosed rocks. The extent of
metasomatism on profiles is dependent on the hydrology of the basin and porosity and
permeability of the weathering profiles. Porosity and permeability of profiles is
generally higher (Grant, 1963). Rapid decrease in porosity and permeability during
burial should preclude such metasomatism. When permeability remains comparable to
or higher than the surrounding sediment, the buried profile may act as a basal aquifer
with large volumes of fluids migrating through it. Metasomatism then may be
extensive. However, several workers (Nesbitt and Young, 1982; Fedo et al, 1995)
have proposed methods to investigate effects of post-depositional metasomatic
modifications on major element compositions.
Though the sediments under study have suffered metamorphism from
greenschist to upper amphibolite facies (Heron, 1953), yet the relations of SiO2 and Zr
with Na2O and K2O attest that in general, the major element geochemistry has not
been greatly affected by these processes. The preservation of pristine mineralogy in
the thin section of the samples particularly plagioclase, K-feldspar and some pyroxene
grains endorses the fact that metamorphism has not greatly affected the major element
geochemistry.
6.2. Weathering of some common rock forming minerals and their
products
Plagioclase is the most abundant mineral of the exposed unweathered
continental crust (Nesbitt and Young, 1984) and among the more rapidly weathered
silicates (Garrels and Mackenzie, 1967; Grant, 1963). Experiment by Busenberg and
Clemency (1976) and petrographic and XRD studies (Wahlstrom, 1948; Lovering,
1959; Meunier and Velde, 1976; Markovics, 1977; Nesbitt, 1977; Nesbitt et al.,
1980), indicate that during weathering, large amounts of kandites (Kaolinite group of
minerals) are ultimately produced at the expense of plagioclase.
Potassium feldspars apparently weather more slowly than plagioclase
(Busenberg and Clemency, 1976; Nesbitt and Young, 1984), leading to the formation
of illite (Busenberg and Clemency, 1976; Weaver, 1967; Weaver and Pollard, 1973).
117
Weathering of phyllosilicates is also important, but the amphiboles, pyroxenes
and olivines are volumetrically small, their importance in weathering of exposed crust
and their contributions to sediments being proportionately less. Biotite and chlorites
commonly weather to vermiculite. But kaolinite and smectite are also reported
(Tsawlassov, 1971; Clauer, 1981).
Muscovite may remain unaltered during the incipient stage of weathering
(Meunier and Velde, 1976).
When it is in contact with plagioclase or biotite,
however, it may be replaced by kaolinite. Degradation of bulk of muscovite probably
occurs during the more advanced stages of weathering. Smectites are abundant
secondary products of mafic minerals and basic rocks (Patterson, 1971).
6.3. Chemical Index of Alteration (CIA)
Nesbitt and Young (1982) introduced the relatively simple chemical index of
alteration (CIA), which monitors the progressive alteration of plagioclase and
potassium feldspars to clay minerals. The wide applicability of this index stems from
the fact that feldspars are the dominant minerals of the upper crust (Nesbitt and
Young, 1984). The index is calculated using molecular proportions in the equation
CIA= [Al2O3/Al2O3+CaO*+Na2O+K2O)] x100
In the equation CaO* is the amount of CaO incorporated in the silicate fraction of
rock (corrected for calcium residing in carbonates and phosphates). For
unconsolidated sediments, it is sometimes also necessary to make a correction for
Na2O for seawater salt (McLennan et al., 1990). The CIA values of sediments are
widely used as an indicator of the intensity of weathering of the provinces (Nesbitt
and Young, 1982). The high CIA values reflect the removal of labile cations (Ca2+,
Na+, K+) relative to the static residual constituents (Al3+) during weathering (Nesbitt
and Young, 1982). Conversely, low CIA values indicate the near absence of chemical
alteration and consequently might reflect cool and or arid conditions. On such a scale,
unweathered igneous rocks have values of 50 or below, residual clays have values of
near 100, and typical shales average about 70 to 75. CIA values for unaltered
plagioclase and K-feldspars are approximately equal to 50 and values of 100 indicate
complete conversion of feldspar to aluminous clay minerals like kaolinite and gibbsite
(intense chemical weathering, Fedo et al., 1995).
118
In the analyzed samples, the CO2 data is not available and thus correction for
Ca in carbonates to obtain CaO* cannot be done. But overall LOI content is low and
suggests that carbonates are not controlling factor for the elevated CaO level, as is
present in few samples. The petrographic inspection of thin sections confirms the
above observation as there are no carbonate minerals or carbonate cement in these
samples. Therefore, the CaO content of the analyses corresponds to the CaO content
in the silicate fraction. The elevated values of CaO, as compared to their counterparts
in few of the samples are probably due to the contributions from amphibole and
feldspars that occurs in significant proportions in these samples. The two samples of
calc gneiss have been excluded from CIA calculations for their obvious high CaO
values.
The CIA values for phyllites ranges from 56.25 to 72.64 (average=65.26).
Those for pelitic schist range from 43.35 to 79.81 (average=59.46). Among the low
values of CIA in pelitic schist are those of KPSH1 (43.35), KPSH12 (47.81) and
KPSH14 (51.14). For greywackes the CIA values ranges from 40.98 to 63.11
(average=56.47). The values of CIA of pelitic gneiss range from 44.44 to 67.77
(average=55.46). The quartzites CIA values range from 36.54 to 91.16
(average=59.53). The KQRT18 sample of Kumbalgarh quartzite records highest CIA
value (CIA=91.16) primarily due to the low contents of CaO, K2O and Na2O. Overall
the quartzites have low CIA values probably due to the low Al2O3 content in their
samples. As these samples have high SiO2, content thereby, decreasing all other
mineral phases in the samples, due to quartz dilution. The low CIA values of few
samples (<50) present in nearly all the rock types of Kumbalgarh clastics is probably
due to the low values of their Al2O3 or Na2O contents (Table 5.1). Furthermore, these
low CIA values (<50) may be attributed to be a primary character of these rocks.
Altrin et al., (2004) in Kudankulam Formation sandstone have observed such low
values of CIA in course of their studies and have correlated it to mafic rock
signatures.
The average CIA values of sediments of Kumbalgarh Group varying from
55.48 to 65.15, lie between unweathered clastic rocks (CIA<50) and average standard
shales (CIA=70-75; McLennan et al., 1993) suggesting less to moderate weathering
condition in the source area of these sediments.
To further assess intensity of weathering suffered by source rocks, the data of
Kumbalgarh clastic rocks is plotted in the Al/Na ratio versus CIA diagram of Servaraj
119
and Arthur (2006) (Fig.6.1). For comparison CIA value of PAAS (75) is also plotted.
In this diagram, all samples (except the calc gneiss) plot in the CIA range of 40 to 80,
suggesting low to intermediate intensity of weathering.
Figure-6.1: Al/Na ratio versus CIA plot for the Kumbalgarh clastic rocks. (After
Servaraj and Arthur, 2006)
6.4. Al2O3-(CaO*+Na2O)-K2O or A-CN-K ternary diagram
Most authors favor the use of Al2O3-(CaO*+Na2O)-K2O (A-CN-K) ternary
plot in evaluating the chemical weathering trends than simple comparison of
numerical value (Nesbitt and Young 1984, 1989; Rodddaz et al., 2006; Raza et al.,
2010, 2011) as geological systems are not simple and a single calculated value may
not adequately reflect their complexities.
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 (Nesbitt and Young, 1984, 1989). Plagioclase and K-feldspar plot at 50%
Al2O3 on the left and right boundaries, respectively to form the ‘feldspar join’. Biotite
plots with K-feldspar, augite and amphiboles plot near the CN apex and calcite plots
at the Ca apex. Illite and smectites plot on the diagram at 70% and 85% Al2O3. The
clay mineral groups, kaolin, chlorites and gibbsite plot at the A apex (100% Al2O3).
The initial weathering trends of igneous rocks are sub parallel to CN-A join as Ca and
Na from plagioclase are leached from the profile in preference to Al2O3, thus the
120
residues are enriched in Al2O3. As weathering progresses, clay minerals are produced
at the expense of feldspars and bulk composition of soil/sediments samples evolve up
the diagram towards A apex, along the weathering trend. The most intensely
weathered samples will therefore, plot highest on the diagram, reflecting the
preponderance of aluminous clay minerals. The weathering trend intersects then A-K
boundary once all plagioclase is weathered and then is redirected towards the A apex
because K is extracted from the residues in preference to Al.
Several workers (Fedo et al., 1995; Bhat and Ghosh, 2001; Roddaz et al.,
2006) have proposed various approaches in order to ascertain the weathering trend in
the A-CN-K plot, for a given set of data. Bhat and Ghosh (2001), advocate the use of
a best fit line through the data points as the probable weathering trend, while Roddaz
et al. (2006) define the weathering trend as line parallel to the A-CN join through
samples with low K2O content. Fedo et al. (1995) and Jayant and Rajamani (2003) on
the other hand follow the general trend line of Upper Continental Crust (UCC).
The A-CN-K plot is useful for evaluating fresh rock composition and
examining their weathering trends since unweathered primary igneous rocks have
CIA values close to 50 (Fedo et al., 1995) i.e. close to the feldspar join on A-CN-K
diagram, backward projection of the weathered samples should lead to a point on the
feldspar join which approximates plagioclase/K-feldspar ratios of their source rock.
And also the amount of K enrichment and the paleoweathering index prior to such
enrichment can be ascertained from A-CN-K plot (Fedo et al., 1995). Because, this
type of K-enrichment involves addition of K2O to aluminous clays, which follows a
path towards the K2O apex of the triangle. K metasomation of sediments can take two
different paths representing either conversion of aluminous clay minerals (kaolinite as
matrix) to illite and/or conversion of plagioclase to K-feldspar. Both these processes
result in the samples being enriched in K2O and therefore offset from the weathering
trend. Conversions of secondary aluminum clay minerals to illite results in a CIA
value lower than the premetasomatised one while in the second case, where Kmetasomatism involves replacement of plagioclase by K-feldspar, the CIA does not
change because the process involves mole by mole substitution of K for Ca or Na.
An estimate of the K-metasomatism can be made by drawing a line from K2O
apex through individual data point, the intersection point of this line with the
weathering path gives the pre-metasomatised CIA value, which can be directly read
121
from the scale on the left. The difference between the pre-metasomatised and the
current CIA values allows quantitative estimation of K-enrichment in a rock.
The phyllites of Kumbalgarh Group plot in A-CN-K diagram (Fig. 6.2) above
the feldspar join from 75% to 55% Al2O3 almost parallel to the A-CN side. The pelitic
schists of the group plot in nearly same region as phyllites (Fig. 6.2). The greywackes
plot in A-CN-K diagram above the feldspar join from 65% to 50% Al2O3 region very
close and/or to the A-CN side (Fig. 6.2). The pelitic gneisses also plot above the
feldspar join, from nearly 70% to 50% Al2O3 (Fig. 6.2). The weathering trend of these
sediments is obtained by taking a best fit line through their data points, as advocated
by Bhat and Ghosh (2001). In case of Kumbalgarh quartzites, the data points are
scattered but all points plot close to the A-CN side close to or above the feldspar join
and the best fit line for these points is nearly parallel to A-CN join (Fig. 6.2). The
weathering trends of greywackes and quartzites are nearly parallel to that of Average
Upper Continental Crust (AUCC). Some samples of pelitic gneiss, greywackes and
pelitic schists plotting below the feldspar join but close to the A-CN side of the
diagram, attest the fact that, due to high content of CaO they posses low CIA values.
Overall the samples of pelitic gneisses, greywackes, phyllites, quartzites and pelitic
schists plot along the linear trend of tonalite and granodiorite as given by Nesbitt and
Young (1984).
The few samples of Kumbalgarh clastic rocks plotting below the feldspar join
in the A-CN-K diagram, due to the low CIA values can be said to be sourced from a
high Mg parent. As Sharma and Rajamani (2000), during the course of their studies of
amphibolites of southern India observed, the CIA values for such rocks to be in the
range of 26 to 53 and thus, the present low CIA values in few samples indicate a
possibility of some ultramafic or Mg rich rock in the source area.
122
Phyllite
Pelitic schist
123
Greywackes
Gneiss
124
Quartzite
Figure-6.2: A-CN-K (in molecular proportion) ternary plot (Nesbitt and Young, 1984)
for the Kumbalgarh clastic rocks, where CaO* is the silicate fraction of rocks.CIA=
[Al2O3/(Al2O3+CaO*+Na2O+K2O)] x 100. Mineral abbreviations: ka=kaolinite,
gi=gibbsite, chl=chlorite, sm=smectite, K-sp=potassium feldspar, pl=plagioclase
AUCC=Average Upper Continental Crust
6.5. Al2O3 – (CaO*+Na2O+K2O)-(FeO+MgO) or A-CNK-FM ternary
diagram
A second diagram was introduced by Nesbitt and Young (1989), to illustrate
the relationship between leucocratic and melanocratic constituents in weathering
profiles. CaO*, Na2O and K2O (CNK) are plotted at the lower apex, Al2O3 (A apex) at
the top and FeO (total iron as FeO) and MgO are summed to form the third variable.
All variables are taken in molecular proportions. The diagram is referred to as the ACNK-FM diagram. It is similar to the ACF diagram used in metamorphic petrology.
Plagioclase plus K-feldspar (Fel) plot on the left hand boundary at 50% Al2O3. Illite
plots on the left boundary at approximately 75% (and greater) Al2O3, and kaolin and
gibbsite plot at the A apex. Biotite plots three quarters of the way along the line
between feldspars and FM apex. Chlorite plots on the right hand boundary as a solid
solution ranging from approximately 15% to 25% Al2O3. Smectite plots within the
triangle somewhat above 50% Al2O3.Weathering trends cannot be calculated from this
125
diagram because the requisite kinetic data are not available and therefore, trends are
derived from weathering profile alone.
Most of the data points of the Kumbalgarh clastic rocks lie above the line
joining Fel point on the A-CNK boundary to the FM apex. Though, plots of quartzites
show no preferred trend, majority of the data points plot close to the line joining Fel
and FM apex (Fig. 6.3). In contrast the phyllites, greywackes, pelitic gneiss and pelitic
schists occupy nearly the same region in the A-CNK-FM diagram (Fig. 6.3). A best fit
line through these data points falls close to the trend line of granodiorite (Nesbitt and
Young, 1989).
Thus, it can be concluded from the above plot that the Kumbalgarh clastic
sediments are derived from a mixed source. And probably the source of pelitic schists,
pelitic gneiss, greywackes and phyllites are closely related.
Quartzite
126
Phyllite
Greywackes
Gneiss
127
Pelitic schist
Figure-6.3: A-CNK-FM (in molecular proportion) ternary plot (Nesbitt and Young,
1989) for the Kumbalgarh clastic rocks, where CaO* is the silicate fraction of rocks
6.6. Chemical Index of Weathering (CIW)
Of the two major elements indices proposed to monitor chemical weathering
CIA (Nesbitt and Young, 1982) and CIW (Harnois, 1988), only CIW index avoids
problems related to the remobilization of K during diagenesis or metamorphism.
Condie (1992) considers CIW (Chemical index of weathering) as a best measure of
intensity of chemical weathering because CIA calculations involve K2O, which is a
mobile oxide. This limits its application to sediments in which potassium has been
actually leached.
CIW = [Al2O3/Al2O3+(CaO+Na2O)]x100 where Al2O3, CaO, Na2O are in
molecular proportions. In this index Al2O3 is used as the immobile component. CaO
and Na2O are the mobile components, because they are readily leached during
weathering. This index does not incorporate potassium because it may be leached or it
may accumulate in the residue during weathering.
The CIW index increases with the degree of depletion of sediment in Na and
Ca relative to Al. The difference between CIW index values for source rock and
sediment reflects the amounts of chemical weathering experienced by the weathered
material. The CIW index has been applied to Precambrian paleosols and soil profiles
developed on basaltic and granitic rocks. For both, the index values show a continued
increase as the degree of weathering increases.
It can be applied to silicate rocks of felsic to mafic composition and is superior
to other weathering indices, in that it involves a restricted number of components
128
which have simple well known and consistent geochemical behavior during
weathering.
The CIW index for phyllites of Kumbalgarh Group ranges from 60.60 to 91.60
(average=78.36) while that of pelitic schist and greywackes varies from 43.52 to
94.97 (average=69.51) and 40.99 to 64.76 (average=58.57) respectively. For pelitic
gneisses the index ranges from 48.30 to 83.45 (average=63.90). Among the quartzites
the average value of CIW index ranges from 42.68 to 98.83 (average CIW=67.88).
Like CIA, the CIW index is also low in some samples. The CIW index values are
higher than CIA values of the analyzed samples, due to the exclusion of K2O from the
index. On the basis of CIW, the Kumbalgarh clastic rocks maybe interpreted to show
low to moderate weathering.
6.7. Plagioclase Index of Alteration (PIA) and Al2O3-CaO-Na2O or
ACN ternary diagram
The chemical weathering can also be indicated by PIA i.e. Plagioclase Index
of Alteration. (Fedo et al., 1995), especially when plagioclase weathering needs to be
monitored. The values are calculated using the formula PIA= (Al2O3-K2O)/(Al2O3K2O +CaO* + Na2O-K2O)x100, where all variable are in molar proportions and CaO*
is the CaO content of silicate fraction only. PIA values around 50 are those of
unweathered plagioclase and values close to 100 indicate complete conversion into
secondary aluminous clay minerals such as kaolinite, illite, and gibbsite (Fedo et al.,
1995). The PIA values of Kumbalgarh clastic rocks are plotted in (Al2O3-K2O)-CaONa2O (A-C-N) diagram (Fig. 6.4).
The PIA values for phyllites are highest and ranges from 57.30 to 88.64
(average=73.46) followed by pelitic schists (43.30 to 92.55, average=65.33), pelitic
gneisses (41.56 to 78.47, average=57.79) and greywackes (40.97 to 63.73,
average=56.90). In general the few samples with low CIA also show low values of
PIA as compared to their counterparts. The quartzites PIA values range from 31.11 to
98.15 (average PIA=62.43).
In the ACN diagram the phyllites data points plot close to the A apex along the
A-C join, with A content close to 60% to 95%, thereby indicating near alteration of
their feldspars (Figure. 6.4). The pelitic schist occupy nearly same compositional
space in the A-C-N diagram only the A content varies between 50% and 95% (Figure.
129
6.4). Thereby, indicating moderate to high alteration of feldspars in pelitic schist. The
pelitic gneiss, greywackes and quartzite data points show scattering in the A-CN
diagram (Figure. 6.4), without any preferred trend unlike the pelitic schists and
phyllites which show a roughly linear trend parallel to AC join. Thus no inference can
be drawn based on these lithologies from this diagram.
Phyllite
Pelitic schist
130
Gneiss
Greywackes
131
Quartzite
Figure-6.4: A-C-N (in molecular proportion) ternary plot (Fedo et al., 1995) for
Kumbalgarh clastics. PIA = [(Al2O3-K2O)/(Al3O3+CaO+NaO-K2O)] x 100
6.8. Index of Compositional Variability (ICV) and K/Al
Oxide ratios can be used to characterize differences between samples. Clay
minerals and non-clay silicate minerals are characterized by very different proportions
of alumina. In view of this several workers (Cox et al., 1995; Cox and Lowe, 1995),
have proposed the use of Index of Compositional Variability (ICV) and K2O/Al2O3
ratio.
The
index
of
compositional
variability
[ICV=(Fe2O3+K2O+Na2O+CaO+MgO+MnO+TiO)/Al2O3] measures the abundances
of alumina relative to the other major cations in a rock or mineral. Silica is excluded
to eliminate problems of quartz dilution. Non clay silicates contain a lower proportion
of Al2O3 than clay minerals, relative to other constituents therefore, they have a higher
ICV. In addition there is a compositional gradient within the non clay silicates. The
ICV tends to be highest in minerals high in the weathering sequence of Goldich
(1938), such as the pyroxene and amphiboles (≈10-100) and decreases in more stable
minerals such as the alkali feldspars (≈0.8-1). The ICV decreases further in the
montmorillonite group clay minerals (≈0.15-0.3) and is lowest in the kaolinite group
minerals (≈0.03-0.05). Since minerals show a relationship between resistance to
weathering and ICV, it may be applied to sediments as a measure of compositional
maturity. Compositionally immature sediments that contain a high proportion of non
clay silicate minerals, or that are rich in clay minerals such as montmorillonite and
132
sericite will have high values of this index. On the other hand compositionally mature
sediments, poor in non clay silicates or dominated by minerals such as those of the
kandite family will have low values. Compositionally immature sediments tend to be
found in tectonically active setting and are characteristically first-cycle deposit (Van
de Kamp and Leake, 1985). Compositionally mature sediments characterize
tectonically quiescent or cratonic environments (Weaver, 1989), where sediment
recycling is active, but may also be produced by intense chemical weathering of first
cycle material (Barshad, 1966).
The ICV index for phyllites ranging from 0.19 to 0.32 (average=0.25) is
similar to that of pelitic schist (0.10 to 0.67; average=0.29) and pelitic gneiss (0.11 to
0.68; average=0.28) while that of greywackes ranges from 0.05 to 0.70
(average=0.26). In the quartzites average ICV value is 0.12. The lower values of ICV
(<1) in case of Kumbalgarh clastic rocks indicate that they are compositionally mature
sediments, poor in non clay silicates or dominated by minerals such as those of the
kandite family that have low values and possibly derived from a tectonically quiescent
or cratonic environments (Weaver, 1989), where sediment recycling is active, but may
also be produced by intense chemical weathering of first cycle material (Barshad,
1966).
The K2O/Al2O3 ratio may suggest how much alkali feldspar vs. plagioclase
and clay minerals were present in the original rocks (Cox et al., 1995). The
K2O/Al2O3 ratio decreases from
alkali feldspar (≈0.4-1), illite (≈0.3) to other clay
minerals (≈0) (Cox et al., 1995). Sediments with K2O/Al2O3 ratio greater than 0.5
suggest a significant quantity of alkali feldspar relative to other minerals in the
original source, while sediments with ratio less than 0.4 suggest minimal alkali
feldspar in the source (Cox et al., 1995). All the Kumbalgarh clastic rocks have
K2O/Al2O3 ratio around 0.2 (< 0.4), thus indicating minimal alkali feldspar in the
source rock of these sediments and in turn presence of plagioclase oriented rocks.
6.9. Th/U ratio
The Th/U ratio can also be used to highlight weathering condition (McLennan
et al., 1993) as the ratio of sedimentary rocks is expected to increase with increasing
weathering (Taylor and McLennan, 1985; McLennan and Taylor, 1991; McLennan et
al., 1990, 1995). As in most cases, weathering and sedimentary recycling under
oxidizing conditions typically result in oxidation of U4+ to U6+, the latter being more
133
soluble, is dissolved and lost during sedimentation. For most upper crustal rocks, the
Th/U is typically about 3.5 to 4.0. Many types of sediment from active margin
tectonic settings with major components of young undifferentiated crust have Th/U
significantly below 3.5 accompanied by low Th and U contents, and this is interpreted
as dominantly reflecting a low ratio in the source rocks (McLennan, 1989; McLennan
and Taylor, 1991). Low Th/U ratios are rather common in mantle-derived volcanic
rocks and reflect the geochemically depleted nature of such reservoirs (Newman et al.,
1984).
In cases where low Th/U exists due to sedimentary process, it is likely that it
resulted from U enrichment and accordingly this will be accompanied by high U
content reflecting the greater mobility of this element. Workers like Wedephol (1991),
have also shown that the whole rock Th/U ratio is a sensitive indicator of
metamorphism in all types of rocks. The Th/U ratios may reach 8-10 in upper
amphibolites facies and as high as 25 in the granulite facies.
The Th/U ratio for quartzite ranges from 1.36 to 25.67 (average=4.24). The
Th/U ratios of Kumbalgarh phyllites ranges from 1.46 to 9.90 (average=5.25)
followed by those of pelitic gneiss and pelitic schists ranging from 1.07 to 7.73
(average=4.32) and 0.42 to 5.12 (average=3.16) respectively. While in case of calc
gneiss and greywackes it ranges from 0.39 to 1.30 (average=0.70) and 0.27 to 3.09
(average=1.40) respectively. The Th/U average ratios of quartzites, pelitic gneiss and
phyllite are comparable with the Th/U ratio of upper crustal rocks (about 3.5 to
4.0).While the low ratios of calc gneiss and greywackes can be interpreted as
dominantly reflecting a low ratio in the source rocks (McLennan, 1989; McLennan
and Taylor, 1991). The Th and U content show high positive correlation (Table 5.6) in
the Kumbalgarh clastic rocks (expect in P2 and phyllites) which is suggestive of the
fact that the U content of these rocks is not due to weathering processes, but inherent
from the source. And their low Th/U ratios are also primary. Barring two samples of
quartzites, the Th/U ratios of Kumbalgarh rocks <10 is well in accordance with their
metamorphic grade, from lower greenschist to amphibolite facies. To further confirm
the interpretation Th/U is plotted against Th abundances (Fig. 6.5). In this diagram
also, phyllites and most of the quartzite samples fall in the upper crust field (Th/U>4)
while greywackes lie below this value in the depleted mantle source field along with
most of the pelitic schists. While the pelitic gneisses data points occur in both the
fields.
134
Figure-6.5: Plot of Th/U vs. Th (After McLennan et al., 1993)
6.10. Interpretation
Overall synthesis of geochemical data of Kumbalgarh clastic sediments as
discussed above suggest low to moderate degree of weathering in their source area.
The major factors which control the intensity of weathering are heavy rainfall,
vegetation cover, relief, high surface temperature and high atmospheric PCO2. The low
to moderate degree of chemical weathering as shown by Kumbalgarh clastic
sediments, probably reflect relatively high erosion rate giving less time for intense
chemical weathering. Other factors may be short distance and rapid rate of
sedimentation in a tectonically active basin and lack of vegetation during the
Proterozoic. A moderate degree of weathering as indicated by the composition of
these rocks do not suggests more CO2 enriched atmosphere and unusually high
surface temperature in absence of vegetation. Various geochemical parameters such as
CIA, CIW, PIA, Th/U ratio, ICV values and K2O/Al2O3 as described above suggest
moderate climatic conditions during deposition of Kumbalgarh sedimentary fill and
indicate that extreme weathering conditions were probably negligible in the source
area. The lower values of ICV (<1) in case of Kumbalgarh clastic rocks indicate that
they are compositionally mature sediments and possibly derived from a tectonically
quiescent or cratonic environments where sediment recycling is active. The detritus of
Kumbalgarh sediments is considered to have been derived from not much more than
tens of kilometers upholding the observation for less transport.
135