A-CN-K Ternary Case Study

A-CN-K (Al2O3- (CaO*+Na2O)-K2O) diagram
A–CN–K ternary plot is useful tool to evaluate weathering trends in the source rock and to estimate K-metasomatism [Nesbitt and Young (1982, 1984); Fedo et al., 1995]. The ideal weathering lines are parallel to A-CN boundary because the removal rates of Na and Ca from plagioclase are greater than the removal rate of K from k-feldspars.
On the A–CN–K ternary plot (Fig.6.16) the samples are plotted more or less close to A - K boundary indicating that removal of CaO, Na2O and K2O under moderate degree of chemical weathering.
6.2.1. Weathering and weathering indices of metasedimentary rocks
Weathering processes decompose and breakdown the mineral grains from the parent rocks and it enhanced the mobility
Weathering indices are useful to measure the degree of depletion of mobile elements relative to immobile elements during weathering (Harnois,
The correction also made for carbonate and apatite content [CaO* = (mole CaO − mole P2O5 × 10/3)].
The higher values of CIA indicates that the removal of mobile elements such as Ca, Na, K relative to Al during weathering, whereas low values of CIA indicates near absence of chemical weathering (Nesbitt and Young ,1982 , Fedo et al,.1995) .
Fedo et al. (1995) classified the CIA values into low (50%–60 %), intermediate (60%–80%) and extreme (80%-100%) high chemical weathering. Thus, the CIA values of the samples (Table.6.3) ranges from 57.99 % to 68.39 % (average 63.73%). This value indicates that, the sediments shows intermediate degree of chemical weathering in the source area.
Harnois (1988) developed chemical index weathering to measure and evaluate the extent of conversion of feldspar to clay minerals. Moreover, this index removes the remobilization and redistribution of problem of K during diagenesis and metasomatism processes.
The chemical index weathering defined as:
CIW = [Al2O3/ (Al2O3+CaO*+Na2O