ABSTRACT
The purpose of this thesis was to investigate the in-situ weathering patterns of protomylonitic tonalite in a mild Mediterranean climate at three variably weathered locations within Santa Margarita Ecological Reserve, SW California. Samples of corestone, saprock, and/or rind were collected and analyzed from each of the three sites. The Chemical Index of Alteration was used to determine the intensity of weathering between the corestone, saprock and/or rind, and the C# program Statistical Analysis of Compositional Data was utilized to determine the magnitude of elements and volatile matter entering and exiting the weathered material along with associated 95% confidence intervals. Resulting data suggest site I, located near the base of the regolith, is weakly weathered, while sites II and III located in the upper half of the regolith are increasingly weathered. This conclusion is supported by field, petrological, and chemical data. Hence, it's likely that fluids migrating through the saprock were increasingly reactive in the upper regolith compared to the base of the regolith. The leaching of K and increases of volatile mass is attributed to the weathering of biotite, a process that occurred at all three sites. In contrast, at the most intensely weathered site losses in Ca, Na, and Ba mass are also evident and attributable to the partial dissolution of plagioclase. In short, data presented in this thesis suggests that in K-feldspar poor systems, weathering first involves biotite, and once the majority of potassium mass in biotite is removed, then the weathering of plagioclase proceeds or increases in intensity. The weathering of K-feldspar was not observed at any of the three sampled locations suggesting that the widely recognized weathering model of granitoids should be altered for systems that are K-feldspar poor or free.