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Description
The effect of climate upon the composition of sand is poorly understood. In his 1976 study of sand weathering from undeformed plutonic rocks in humid, semi-humid, and semi-arid climates, A.J. Basu established grain-size dependent compositional curves for each climatic zone. Sands weathering in more humid environments plotted closer to the quartz pole of a QFL diagram, and the abundance of lithic fragments in all climates decreased with decreasing grain-size. Basu claimed that these compositional differences were produced by climatic effects, and he promulgated an empirical model which he claimed explains the effect of climate upon sand composition. Basu asserts that climatic effects will produce similar compositional differences in detritus weathering from any source rocks, but provided little data to support this all-encompassing statement. In 1986, L.J. Suttner and P.K. Dutta established a climate-discrimination model which uses the ratio of resistate to labile materials to determine climatic zones. Suttner and Dutta established their climatic fields using flora and fauna, and validated their model using sandstones ranging in age from Permian to Triassic. This model has never been tested using Holocene detritus. W.R. Dickinson indicated that rocks weathering in known tectonic settings should yield sands with QFL and QmFLt values unique to that setting. In 1983, Dickinson and associates established seven tectonic-dependent compositional fields for the standard QFL and QmFLt ternary diagrams. This study presents the results of a petrologic analysis of sand-sized detritus weathering from mylonitic plutonic rocks in the Peninsular Ranges of southern California. Grus derived from the Harper Creek gneiss was sampled in a Mediterranean cool summer climate, and grus derived from the Borrego Springs shear zone was sampled from an arid climate. Results presented here provide the first documented description of detritus derived from mylonitic plutonic rocks. There is a compositional difference between the two climatic zones; sands in a Mediterranean cool summer climate plot closer to the quartz pole of a QFL diagram than sands in an arid climate. However, quantitative analysis of the grain-size dependent modal trends of each constituent of the collected detritus indicates that these differences are controlled by source rock composition rather than climate. This observation indicates that Basu's climatically-controlled compositional model is not valid when applied to mylonitic detritus weathering in Mediterranean cool summer and arid climatic zones. Ratios of resistate to labile fragments from grus utilized during this investigation were inconsistent with the climatic zones in which they were produced. These results suggest that the model by L.J. Suttner and P.K. Dutta is not valid when applied to Holocene detritus. Finally, data presented here generally support the provenance-discrimination model established by W.R. Dickinson.
