petrographically by the formation of K-feldspar replacing plagioclase. CIA values in this case do not change because the process involves mole for mole substitution of K for Ca or Na. There is no
petrographic evidence for post-depositional burial diagenetic
changes that could have affected the bulk composition after deposition. Moreover, the illite present in the sandstones studied is of
detrital origin from weathering horizons developed on silicic rocks,
and is identified as a mica polymorph (Fig. 5). According toNesbitt
and Young (1989), the depletion of Na and Ca is related to progressive chemical weathering of the granite that destroys plagioclase
rapidly, leading to little Ca- or Na-bearing phases remaining in
the weathering residue. The dominant minerals that remain are
kaolinite and potassic phases such as illite and/or K-feldspar.
Although we can not completely neglect the effect of post-depositional processes in altering the mineralogy and chemistry, the textural and chemical immaturity of the sandstones studied indicate
that their bulk chemistry, including the K-enrichment, was inherited from the source area.