Volcanological constraints on the post-emplacement zeolitisation of ignimbrites and geoarchaeological implications for Etruscan tomb construction (6th-3rd century B.C) in the Tufo Rosso a Scorie Nere, Vico Caldera, Central Italy

摘要

We examine the role of physical volcanological processes including eruption style (magmatic versus phreatomagmatic) as well as transport and depositional processes (pyroclastic fall versus pyroclastic flow) in promoting the ideal hydrologic conditions necessary for large scale, homogeneous, post-emplacement zeolitisation of ignimbrites, associated with caldera forming eruptions. We consider the Tufo Rosso a Scorie Nere (TRSN) of Vico Caldera (151 ka), in central Italy. The TRSN exhibits pervasive, homogenous alteration of high alkali tephriphonolitic and phonolitic glass to zeolite minerals (chabazite and phillipsite) in all regions of the study area and at all stratigraphic levels with the exception of the basal 1 m. Based on detailed lithological studies, we propose that a large geothermal field around the vent area was destroyed during the closing stages of the Sutri eruption. Subsequent incorporation and entrapment of superheated geothermal fluids into the ensuing pyroclastic flow during eruption column collapse greatly influenced the emplacement temperature and provided the necessary water required for post-emplacement zeolitisation of the TRSN. We suggest that the absence of zeolitisation at the base of the ignimbrite is directly related to transport conditions reflecting cooler regions in the lower portions of the deposit where the flow came into contact with the underlying substrate.rnWe also consider the geoarchaeological implications of enhanced strength and cohesiveness provided by the zeolite rock framework on Etruscan tomb location and burial architecture in the Vico region. The TRSN contains literally hundreds of hypogeum-style Etruscan tombs at a number of sites across the study area. This study focuses on two sites in particular, the Norchia Necropoli and the San Guiliano Necropoli. Considering the enhanced mechanical properties of zeolitised ignimbrites we infer that physically the TRSN would still have been a relatively soft rock, suitable for the carving of tombs. However, we infer the increased strength and cohesiveness provided by the zeolite framework enhanced the conservation potential of these tombs, preserving them for over two thousand years.