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Description
Though tectonics and climate are known to affect the composition of siliciclastic sediments in varying degrees, suprisingly little is known about the effects of in situ weathering on sediment composition prior to mass wasting and erosion. Though a wide variety of rock types are exposed at the Earth's surface, granodiorite is commonly viewed as the most voluminous. Hence, the purpose of my thesis was to explore the question, does in situ weathering affect the mineralogical and chemical composition of granodiorite? In order to address the above question, I selected a well-developed weathering profile developed on the Early Cretaceous La Posta pluton for detailed study. The studied site receives three times as much precipitation in a winter month than in the driest summer month, and precipitation in the driest month is less than 3 cm. Hence, it lies within a Mediterranean hot summer climate. At the research site, the weathering profile is divisible into R, C, and A horiz.ons. The R horiz.on represents unweathered hornblende biotite granodiorite. In contrast, material making up the 2.5 to 3 meter thick C horiz.on is best characterized as highly friable and partially unconsolidated. Roots penetrate the upper 1 meter of C. The 50.8 centimeter thick A horiz.on is a loamy sand that supports the development of plants. Point count and petrographic studies revealed that the proportion of plagioclase and K-feldspar decrease from the R to A horiz.on. In addition, on A-CN-K and A-CNK-FM ternary diagrams, chemically analyzed samples indicate a loss of molecular CaO, Na20, and K20 during development of the C and A horizons. Similarly, when CIA (Chemical Index of Alteration) values are plotted against concentrations of major and trace elements, CaO, Sr, and Na20 systematically decrease from the R to A horizon. The observed increase of K20 and Ba is likely due to chelation by overlying plants, whose roots permeate the A horizon. Statistical analysis of chemical data suggest that -6 grams/I 00 grams of rock were lost during the development of the C and A horiz.ons. The calculated mass loss as well as chemical and mineralogical changes noted in the preceding paragraphs are attributed to the partial dissolution of plagioclase and K-feldspar during the development of the C and A horizons. Thus, data presented in this thesis illustrate well that significant elemental loss occurs even in immaturely developed weathering profiles. This conclusion is significant because it implies that the mineralogical and chemical compositions of siliciclastic sediments may reflect more the conditions of weathering than the conditions of tectonic setting or climate. This statement is particularly true for QP- K values and the elements CaO, Na20 , K20 , Rb, Sr, Ba, and Ms. However, before such ideas can gain wide acceptance, more work documenting the effects of weathering in other climatic zones and on different rock materials is needed.
