The long-lasting, multistage activity of the Astroni volcano (Campi Flegrei, Italy): new evidence from petrological and chemostratigraphic data

The Astroni volcano is atypical within the Campi Flegrei district, where most volcanic centres are monogenetic. It consists of a 1.5 km wide tuff ring formed during repeated eruptions between ca. 4.6 and 4.0 ka. The products emplaced during its activity offer a rare opportunity to investigate the temporal evolution of a relatively long-lived volcanic system from various perspectives. These include pre- to syn-eruptive magma chamber processes, as well as eruptive and emplacement dynamics. Building on a recent revision of the Astroni stratigraphic framework, which identified eight distinctive eruptive units and five laterally traceable unconformity surfaces marking phases of volcanic quiescence, the preliminary results of a detailed petrological and chemostratigraphic study of the Astroni deposits are presented here. Most of the stratigraphic sequence (Units I-VI) is made of pyroclastic density current and minor fallout deposits dominated by highly vesicular juvenile clasts, with low crystal content and relatively uniform feldspar-rich paragenesis. The topmost levels include a small lava dome/plug with associated welded scoria deposits emplaced inside the crater (Unit VII), and a succession of fine-grained stratified ash deposits (Unit VIII). The two topmost units are stratigraphically separated by the deposits of the nearby Fossa Lupara crater (about 1 km NW of the Astroni edifice). Juvenile compositions of the Astroni units are consistently trachytic/phonolitic, showing relatively narrow ranges for most major oxides (e.g. 58.2-60.2 wt% SiO2, 0.48-0.58 wt% TiO2, 3.01-3.82 wt% CaO, 0.72-1.31 wt% MgO). Trace elements are relatively more variable, especially Sr (428-734 ppm), Zr (371-462 ppm) and Ba (424-1169 ppm). The Fossa Lupara juvenile clasts are slightly, though systematically less evolved, with e.g. 56.5-59.2 wt% SiO2, 0.53-0.65 wt% TiO2, 3.34-4.70 wt% CaO, 1.06-1.7 wt% MgO, 723-943 ppm Sr, 311-388 ppm Zr and 1026-1571 ppm Ba. An overall trend of decreasing degree of evolution is observed throughout the Astroni sequence and within most eruptive units, based on trace element contents and ratios. In more detail, Units I and II are homogeneous and strongly evolved in composition, whereas Units III to VII show subtle but consistent trends of decreasing degree of evolution with increasing stratigraphic height. These findings, slightly differing from those of previous literature, indicate a plumbing system involving either 1) multiple independent reservoirs feeding individual eruptive units or 2) a single reservoir subject to periodic recharges by less evolved magmas, possibly concomitant to phases of volcanic quiescence.