CORRELATIONS BETWEEN VOLCANIC LITHIC FRAGMENTS AND COMPOSITION OF SOURCE VOLCANIC ROCK

Volcanic lithic fragments are commonly used in sandstone petrographic studies to represent the composition of the parent volcanic rock. Felsitic lithic fragments are inferred to have a rhyolitic source, microlitic lithic fragments an andesite source, and lathwork lithic fragments a basaltic source. This project seeks 1) to test this hypothesis and 2) to propose a statistically valid predictive model for volcanic lithic fragment populations and corresponding parent volcanic rock compositions.

In this study, we borrowed thin-sectioned volcanic rocks with known whole rock geochemistries from various collections. We point counted these samples using the Gazzi-Dickinson method for sandstones, with a ~ 1 mm area under the crosshairs serving as a sand-sized grain. Finding previous classifications insufficient, we redefined the categories of volcanic lithic fragments by examining matrix textures and identifying the relative ratios of glass and non-phenocryst phase crystals (microlites) in order to constrain the boundaries between the lithic fragment types.

Results show generally poor correlations between lithic fragment type and parent volcanic rock composition. Statistical analysis could not be performed because of the wide range of lithic types at a specific composition of volcanic parent rock and the wide range of compositions at a specific lithic population. However, broad associations are recognizable. For example, volcanic rocks with a high ratio of microlitic lithic fragments range in composition from 50 - 68 % SiO2, but the majority are found between 56 - 65 % SiO2. Thus, a volcaniclastic sandstone with high microlitic lithic fragment content could be considered ‘andesitic’ in source.