Mineralogy and geochemistry of the older (> 40 ka) ignimbrites on the Campanian Plain, southern Italy

摘要

The Campanian Plain in southern Italy has been volcanically active for at least the last 300 ka. The Campanian Ignimbrite (CI) erupted at 39.3 ka, has a volume of >= 310 km(3) and a great areal extent. However, significant, but scattered deposits of older ignimbrites underlie the CI and document a long history of volcanism. We examined the mineralogy and geochemistry of 11 older ignimbrite strata by optical petrography, electron microprobe, scanning electron microscope, X-ray diffraction, and various whole-rock geochemical techniques. We have analyzed strata at Durazzano (116.1 ka), Moschiano (184.7 ka), Seiano Valley (245.9 and 289.6 ka), and Taurano Acqua Feconia (157.4, 183.8, 205.6, and 210.4 ka) that have been previously dated on unaltered sanidine. The older ignimbrites are highly altered with loss on ignition (LOI) that ranges from 17 to 8 wt%. Whole-rock compositions reflect variable element mobility during weathering; e.g., CaO is enriched and Na2O depleted relative to hydration. X-ray diffraction identified major chabazite, kaolinite, and illite alteration products in some samples. Rhabdophane-(Nd), usually intergrown with chabazite and Mn-carbonate, indicates that some LREE were also mobilized during weathering. The phenocryst mineralogy is typical for Campanian Plain (CP) magmas and consists of plagioclase (An(88) Ab(11) Or(1) to An(32) Ab(63) Or(5)), potassium feldspar (Or(40) Ab(57) An(3) to Or(79) Ab(18) An(3)), biotite (TiO2 =similar to 4-7 wt%, BaO = up to 2 wt%, F = up to 2 wt%), diopside (Ca47Mg47Fe6 to Ca48Mg23Fe23), and titaniferous magnetite. Relatively immobile trace elements Zr, Hf, Th, Ta, V, and Nb were used to investigate element abundance and ratio compared to the Campanian Ignimbrite and other CP magmas. Zr/Hf of the older ignimbrites is similar to that of the CI, but Ta is depleted relative to Th and V is enriched compared to Cl. Th/Fa and Nb/V distributions for most of the older ignimbrites are similar to those in the Neapolitan Yellow Tuff with the exception of the sample MS-1 from Moschiano that is more evolved and similar to Campanian ignimbrite. All older ignimbrite Zr/Hf (w/w) and many Nb/Ta (w/w) ratios are superchondritic that suggests that the older ignimbrites represent fractional crystallization products of parental magmas generated from enriched mantle without significant addition of continental crust. (C) 2016 Elsevier B.V. All rights reserved.