CHARACTERIZING THE EFFECTS OF ILLUVIATION ON THE PETROLOGY AND CHEMISTRY OF TONALITIC SAPROCK: IMPLICATIONS FOR INTERPRETING COMPOSITIONAL LINEAR TRENDS

In order to characterize the petrological and chemical effects of illuviation, we collected 17 samples of saprock adjacent to, and 14 samples of corestone from, an ~6.7 m high tonalitic corestone (tor). Based on thin section observations, all saprock samples are characterized by a network of transgranular and intergranular cracks filled or lined with illuviated clay. In contrast, the silicate framework has been weakly to mildly weathered, and as result, biotite has been partially transformed into mixed-layer biotite/vermiculite, plagioclase has been weakly weathered to a dusting of smectite, and hornblende has been weakly weathered to Fe- and/or Mn-oxyhydroxide. The weathering of biotite translates into an ~5-6% loss of K mass while the weathering of plagioclase resulted in no statistically significant loss of Ca or Na mass.

The above effects of eluviation contrast markedly with the statistically significant additions of Si, Al, Fe, Mn, Ti, Sc, Cr, Cu, Rb, Y, and Yb mass produced by illuviation. Such additions translate into an overall statistically significant 12-15% increase in bulk mass. The above increases in elemental and bulk mass are a reflection of eluvial processes operating in the now eroded overlying ≥6.7 m section of regolith. Within that overlying section, kaolinite; minute particles of biotite, hornblende, and ilmenite; and ions derived from leaching of these mineral were suspended into downward percolating fluids. The dominance of kaolinite, along with the apparent severity of leaching implied by the downward transfer of significant elemental mass, suggest a climatic regime that is unlike that currently existing in the Peninsular Ranges, or during the Quaternary, ~35,000 years ago, the exhumation age of a nearby corestone. Though poorly constrained, after removal of ~300 km of displacement along the San Andreas fault, the section of regolith studied during this investigation, may reflect Paleocene weathering within a subtropical setting.

On centered p(A)-p(CN)-p(K) ternary diagrams, illuviation resulted in a compositional linear trend anchored by the geometric mean of the corestone samples and the projected composition of kaolinite at the p(A) apex. Notably, this trend is unlike that documented for biotite-controlled and plagioclase-controlled weathering.