Ignimbrite correlation using whole-rock geochemistry: an example from the Sulcis (SW Sardinia, Italy)

Abstract

AbstractIgnimbrites are useful chronological markers in the geological record at local and regional scales. They also provide information on the dynamics of the eruption that produced them, making their study of great importance in terms of volcanic hazard assessment. However, their study is usually hampered by their lateral variation and discontinuity. When stratigraphic and lithologic criteria are not sufficient for correlation purposes, the use of multiple complementary correlation tools may be necessary to correctly determine their areal extension, volume and facies variations. Whole-rock geochemistry is considered one of the less reliable correlation techniques due to the pyroclastic nature of these deposits and their emplacement dynamics. These may introduce vertical and horizontal geochemical heterogeneity in the final deposit. In addition, the occurrence of zoned ignimbrites due to magma supply of changing composition is common. In this work we show that, if appropriately used, whole-rock geochemistry can be a valid and highly useful tool for ignimbrite correlation. We provide an example from the study of an ignimbrite sequence containing 18 units (sensu lato) in the Sulcis region (SW Sardinia, Italy). A protocol has been developed for unit recognition based on successive simple binary diagrams where the whole-rock composition of a problem sample can be plotted. Immobile trace elements have been preferentially used to minimize effects of element mobilization associated with alteration and weathering. The diagrams provided here are designed for the Sulcis, but the methodology followed to develop them may be applied to other study areas.