Quantitative Analysis of Volcanic Lithic Fragments

Textural subcategories of volcanic lithic fragments in sand and sandstone have been correlated qualitatively with volcanic compositional types, but these relationships have not been tested quantitatively. Lathwork grains were assumed to be derived mainly from mafic lavas, microlitic from intermediate lavas, and felsitic (sometimes divided into granular and seriate) from silicic lavas. Vitric volcanic lithic fragments were not initially assigned a composition, but they commonly have been associated with silicic compositions. More precise definitions of volcanic lithic types facilitate the quantification of modal proportions of these types, which can be used to test correlations between lithic type and SiO2 content of volcanic rocks. In order to establish which types of volcanic lithic fragments are produced by volcanic rocks of specific compositions, 94 thin sections from diverse volcanic rocks with 45.57 to 76.93 weight % SiO2 were counted using the Gazzi-Dickinson point-counting method. Relationships between volcanic lithic-fragment type and SiO2 content are complex. The only definitive conclusion is that samples containing dominantly vitric and granular volcanic lithic fragments can be confidently assigned to high-SiO2 sources. More tentative conclusions are that mafic volcanic rocks have lithic types anywhere on the seriate-microlitic-lathwork spectrum, and that intermediate volcanic rocks dominantly consist of microlitic and lathwork textures.