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Description
Twenty-five samples of plutoniclastic sand-sized grus were collected, consolidated with epoxy, thin-sectioned, and chemically stained for easy identification of K-spar and plagioclase. All samples represent grus produced by the in situ mechanical and chemical breakdown of the underlying 95 Ma La Posta pluton in arid, semi-arid, and Mediterranean (hot summer) climatic zones. The La Posta pluton is a large concentrically zoned pluton which forms a part of the moderately to deeply dissected Peninsular Ranges batholith which in turn represents a large Cretaceous-aged continental margin magmatic arc system. Each plutoniclastic sand sample was point-counted utilizing the Basu and Gazzi-Dickinson point-count methods. The results from the former method yield a plutoniclastic sand composition that is markedly dependent upon grain size and dramatic changes in temperature and precipitation. For example, finer grain sizes are invariably poor in rock fragments and are enriched in feldspar and quartz. In contrast, coarser grain sizes contain greater amounts of rock fragments. The results of the Basu point-count method when combined with the literature indicate that a plutoniclastic sand produced in a generally dry climate will have far less quartz than a plutoniclastic sand produced in a generally humid climate for all grain sizes. In contrast to the Basu point-count method, the Gazzi-Dickinson method yields a plutoniclastic sand composition that is affectively independent of grain size and small changes in temperature and precipitation. In addition, the resulting sand composition reflects more directly the composition of the source rock and its tectonic setting than does the Basu method. Thus, the results of this study suggest that in order to extract maximum information about provenance characteristics sand/sandstone petrologists should use both point-count methods.
