Volcaniclastic sand provenance in magmatic arc setting: an example from Lipari Island beaches (Aeolian Archipelago, Tyrrhenian sea)

In this study, 24 modern volcaniclastic beach sand samples, from Lipari, one of the volcanic Aeolian Islands of southern Italy, were petrographically characterized, focusing on the role of processes controlling the composition of the volcaniclastic detritus, including bedrock sources. Several distinctive types of bedrock, in the drainage basins, that are the likely prominent sources for sand at each sampled beach were recognized, and divided into two categories of provenance lithotypes: lavas and pyroclastic rocks ranging in composition from basaltic andesitic, to andesitic, to rhyolitic. Using the Gazzi-Dickinson point counting method, lithic volcaniclastic petrofacies were defined. On QmFLt ternary plot, medium- and fine-sized sand compositions from Lipari beaches overlap although finer sands are richer in individual monocrystalline feldspar grains (P+K). The proportions of various types of volcanic individual grains and lithic groups components in the medium and fine sand identified more distinct relationships between sources. Volcanic lithic fragments from Lipari beach sand, in the coastal stretch from Acquacalda to the north and the south western side of the island (Valle Muria), consist of colorless and black glassy volcanic fragments with lathwork, felsitic, vitric and microlitic textures. Moreover, high amounts of detrital unstable minerals such as pyroxene, olivine and Fe-oxides, illustrate how the analyzed sands preserve the source rock(s) provenance signals in a weathering-limited erosion regime such that of the Aeolian volcanic arc. The overall composition and texture of sand is influenced by the bedrock type that constitutes the source rocks of the drainage basins. In some cases, sand composition may be determined by fragment types [i.e., lavas vs. pyroclastic rocks] that represent only a subordinate portion of the outcropping source area because of their higher Sand Generation Indices. Finally, our results show that these lithotypes have different propensity to create terrigenous detritus, in both in terms of grain size and composition. The Sand Generation Index (SGI) of lavas is 0.9 to 12.3 times higher than of pyroclastic rocks, even if the lavas constitute a minor outcrop area of the island. In this regard, we suggest a modified SGI, detailing the Sand (S) in terms of grain size classes: in a weathering-limited erosion regime, under a Mediterranean climate, in the case of felsic to intermediate lavas the SGI is characterized by a medium (m) to fine (f) sand generation (mfSGI) whereas in the case of pyroclastic rocks the grain size of sand is in the range of very coarse (vc) to coarse (c) sand (vccSGI). Clastic contribution from pyroclastic rock outcrops such as pumice, are not found in the studied sand samples, probably because they produce gravel-size detritus or very fine sand and/or finer detritus such as silt and clay. In the future, studies of the conversion from “pumice source rock to pumiceous detritus” could more thoroughly document and evaluate the propensity to create gravel- or silt-sized detritus, namely if texture and composition of this source rock could be characterized by a Gravel Generation Index (GGI) or a silt generation index (SiGI).