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Description
The study site, located near the top of an ~20° – 30° E-SE dipping slope underlain by ~125 Ma granite, consists of a large mushroom-shaped corestone, ~4 m in width and extending ~3 m above the land surface. Such geometry implies that as much as 3 m of regolith was removed during erosional exhumation of the corestone. Twelve samples collected from the granitic corestone and 12 from the adjacent saprock were analyzed for their major element chemistry. In addition, the clay mineralogy of the < 2 μm fraction separated from representative saprock samples was evaluated by standard XRD procedures. Following conversion to molar A, CN, and K concentrations, non-central principal component analysis revealed that principal component 1 explained 94.6% of the variance of saprock data about a calculated compositional linear trend. In A-CN-K ternary space, the perturbation vector derived from eigen vectors associated with principial component 1 directed the compositional linear trend away from the CN apex and toward the A-K join, and thus suggested that formation of the saprock involved a loss of Ca and Na mass. Given that these elements are largely held within the crystal lattice of plagioclase, the above data suggest leaching of plagioclase during transformation of granitic basement to saprock. MgO passed statistical tests for immobility, and was used as a reference framework in mass balance calculations. Results supported the above interpretations, indicating a 37% loss in Ca and a 32% loss in Na mass. Such losses also are supported by thin section work which shows that plagioclase has been extensively converted to clay, and XRD work which shows that the < 2 μm fraction is dominated by kaolinite and minor vermiculite. In addition, a 14% loss in Mn mass likely reflects leaching from biotite. In contrast to the above losses in elemental mass, gains of 18% Al, 38% Fe, and 36% Ti mass are statistically significant. Thin section work revealed that the saprock crack system is lined with a yellowish-brown clay mineral that is likely kaolinite (Al2Si2O5(OH)4). The Si:Al ratio in kaolinite is 2:1. Hence, depending on the volume of kaolinite lining the crack system, an increase in Si mass also should be evident. Relative to the overall change in bulk mass, an 18% increase in Al mass translates into an increase of ~2.5 grams of Al2O3 per 100 grams of granodiorite. Thus, if 2.5 grams of Al2O3 were added, then ~5 grams of SiO2 are required to balance that needed in kaolinite. Unfortunately, the volume of kaolinite lining the crack system is small, and the gain of 10% (+12%/-11%) Si mass, though statistically insignificant, translates into an addition of ~7.9 grams of SiO2. Though more work is required, the above data are consistent with the idea that within the erosional removed 3 m thick section of regolith, particles of kaolinite, along with ions of Fe and Ti were eluviated, and then transferred downward where they were illuviated, exchanged, or adsorbed onto the walls of the saprock crack system.
