Trachy-phonolite lava pebbles used in the ancient settlement of Oplontis (Torre Annunziata, Naples): petrochimical data supporting the origin from an old effusive activity of the Somma-Vesuvius volcano

In the southern and south-eastern sectors of Somma-Vesuvius volcano (Campania region, southern Italy), archaeological sites covered by the deposits of A.D. 79 Vesuvius eruption, and dating back to Protohistoric and Roman periods are characterized by the presence of numerous lava pebbles used in the human settlements. In this work we present the new case study of the lava samples found with other materials used in building foundations of the Roman site of Oplontis (the “Villa of Poppea” or “Villa A” or Oplontis A; and the ancient commercial structure of Oplontis B). This new data set of lava pebbles of different lithologies was also compared with that of the archaeological site of Longola, in order to unravel the source provenance from distinct evolutionary phase of the Somma-Vesuvius volcano. According to major and trace elements composition, lava pebbles are all compatible with the Somma-Vesuvius magmas erupted before 8 ka BP and are represented, according to modal mineralogy and whole-rock geochemistry, by two groups belonging to the slightly silica undersaturated series: several basic-intermediate lithotypes (mainly phonolitic tephrites to basaltic trachyandesites) and trachy-phonolites. Despite the obvious abundance of volcanic rock pebbles, as the settlements were built on the Somma-Vesuvius volcano, the trachyphonolite lava pebbles do not match with any lithostratigraphic units mapped in the up-to-now available literature data. Nevertheless, the trachy-phonolite lava pebbles show (i) a comagmatic relationships with the products erupted by Somma-Vesuvius before 8 ka BP as pyroclastic products (i.e. pumices) and (ii) a good petrological affinity with lava trachyte ejecta included within the Plinian eruptions preceding the A.D. 79 event. The basic-intermediate and the trachy-phonolite lava pebbles from both the sites of Oplontis and Longola, have to be interpreted as an old effusive activity of the Somma-Vesuvius. In order to form lava pebbles, reworking could have taken place both on the original lava flows/autoclastic breccias or the lithic lava fragments of pyroclastic/breccia deposits produced by the explosive events preceding the A.D. 79 eruption.