Petrography and Mineral Chemistry of Monte Epomeo Green Tuff, Ischia Island, South Italy: Constraints for Identification of the Y-7 Tephrostratigraphic Marker in Distal Sequences of the Central Mediterranean

Massimo D'Antonio,Richard J Brown,Biagio Giaccio,Paola Petrosino,Carlo Pelullo,Ilenia Arienzo

doi:10.3390/min11090955

Abstract

The 56 ka Monte Epomeo Green Tuff (MEGT) resulted from the largest volume explosive eruption from Ischia island (south Italy). Its tephra is one of the main stratigraphic markers of the central Mediterranean area. Despite its importance, a detailed characterisation of the petrography and mineral chemistry of MEGT is lacking. To fill this gap, we present detailed petrographic description and electron microprobe mineral chemistry data on samples collected on-land from the MEGT. Juvenile clasts include pumice, scoria, and obsidian fragments with porphyritic/glomeroporphyritic, vitrophyric, and fragmental textures. The porphyritic index is 13–40 vol.%, and phenocryst phases include alkali-feldspar, plagioclase, clinopyroxene, ferrian phlogopite, and titano-magnetite, in order of decreasing abundance; accessory phases include sphene, hydroxy-fluor-apatite, and rare edenite. Plagioclase varies from predominant andesine to subordinate oligoclase, whereas alkali-feldspar is more variable from sanidine to anorthoclase; quasi-pure sanidine commonly occurs as either rim or recrystallisation overgrowth of large phenocrysts due to hydrothermal alteration. Secondary minerals include veins and patches of carbonate minerals, Fe-Mn oxyhydroxides, clay minerals, and zeolites. Clinopyroxene is ferroan diopside (En45–29Fs7–27) and never reaches Na-rich compositions. This feature allows the discrimination of MEGT from aegirine-bearing, distal tephra layers erroneously attributed to MEGT, with implications for the areal distribution of Ischia explosive deposits.

Highlights

The Monte Epomeo Green Tuff (MEGT) is the most important volcanic unit of Ischia island (Gulf of Naples, south Italy; Figure 1A)
We present detailed petrographic description and electron microprobe mineral chemistry data on samples collected on-land from the MEGT
Clinopyroxene is ferroan diopside (En45–29Fs7–27) and never reaches Na-rich compositions. This feature allows the discrimination of MEGT from aegirine-bearing, distal tephra layers erroneously attributed to MEGT, with implications for the areal distribution of Ischia explosive deposits

Summary

Introduction

The Monte Epomeo Green Tuff (MEGT) is the most important volcanic unit of Ischia island (Gulf of Naples, south Italy; Figure 1A). It is the product of a complex, paroxysmal explosive eruption that occurred ≈56 ka [1,2] on the island and that resulted in caldera collapse and the wide dispersal of ash across the central Mediterranean. The typical green colour of the MEGT deposits allows their quick recognition in the field This results from the alteration of the tuff in a shallow marine environment ([12] and references therein), following the emplacement of large volumes of ignimbrite within the caldera during the eruption. They are potassic trachyte and phonolite in composition [11]

Materials and Methods

Results

Clinopyroxene

Carbonate