Lithofacies characteristics of diatreme deposits: Examples from a basaltic volcanic field of SW Sardinia (Italy)

摘要

A deeply eroded diatreme field, consisting in several, decametric-sized, vertical, mainly clastic volcanic bodies of basaltic composition is described for the first time in the Variscan basement of SW Sardinia. The recognition and description of four different lithofacies in these diatremes allowed discussion of the role of the different processes which control magma eruption and conduit infilling, and making general inferences about diatremes. The studied diatremes have a cross-sectional shape from elliptical to sub-triangular, and are slightly elongated nearly parallel to the main foliation of the intruded meta-sedimentary rocks. Foliation of host rocks is locally reoriented or folded close to the contact with the diatremes, suggesting that magma possibly rose to the surface through fissures oriented nearly parallel to host rock foliation. Textural features of the volcanic bodies show many analogies with kimberlitic diatremes, despite the difference in petrography and composition. Juvenile lapilli are mainly made by ghosts of mafic phenocrysts (olivine and clinopyroxene) set in a groundmass formed by plagioclase microlites immersed in a cryptocrystalline, chlorite-rich matrix. The four lithofacies were described mainly based on the shape and physical features of the dasts and textural anisotropy: a globular, juvenile-rich, lapilli tuff facies (GJLt); an angular, juvenile-rich, lapilli tuff facies (AJLt); a lithic-rich, lapilli tuff facies LiRLt), and a coherent, lava-like facies (COH). All the clastic lithofacies are generally well sorted and typically lack a fine-grained matrix. Juvenile fragments are lapilli sized and from equant to oblate in axial ratio, and from rounded-globular to very angular in shape. Conversely, lithic dasts are largely variable in shape and size, and are mainly represented by basement-derived dasts. The absence of bedding, the scarcity of the coherent facies and the dominance of dast supported, structureless, volcanidastic fades suggest that the outcropping portion of these volcanic bodies represents the lower diatreme zone. The presence of diffuse welding and the globular shapes of some juvenile fragments, together with their vesicularity, suggest that magma fragmentation was mainly driven by magmatic gas exsolution occurring at a deeper level respect to classical, basaltic explosive activity. Textural features, facies assodation and fades architecture of the studied deposits are suggestive of an important affinity with kimberlitic and other ultramafic diatremes.