Original paper
The Orvieto-Bagnoregio Ignimbrite: pyroxene crystal-chemistry and bulk phase composition of pyroclastic deposits, a tool to identify syn- and post-depositional processes
Gentili, Silvia; Comodi, Paola; Nazzareni, Sabrina; Zucchini, Azzurra
European Journal of Mineralogy Volume 26 Number 6 (2014), p. 743 - 756
published: Dec 19, 2014
Abstract
This study aims to characterize the crystallization conditions of the ignimbritic flux, the spatial and temporal evolution as well as the post-depositional processes of the Orvieto-Bagnoregio pyroclastic deposits from Bolsena Complex (Roman Magmatic Province, Italy) from a mineralogical and a geochemical point of view. Single-crystal X-ray diffraction together with electron-microprobe analysis (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) data were used to determine the crystal-chemistry of ten clinopyroxene samples from three outcrops of the Bolsena Complex: Orvieto, Bagnoregio and Porano. In addition, quantitative X-ray powder diffraction analyses of amorphous and crystalline phases by the Rietveld method were performed both on the bulk rock and on the juvenile products (mainly pumices and scoriae). The chemical composition and lattice parameters of the Orvieto and Bagnoregio clinopyroxenes indicate diopsides with a common mineralogical and petrogenetic affinity, but slightly different crystallization conditions from the samples of the Porano deposit. Cell volume and M1 polyhedral volume indicate a very low pressure of crystallization in comparison to those calculated for other eruptive centres of the Roman Magmatic Province (Sabatini, Vico and Alban Hills). Notwithstanding a common very high REE content in clinopyroxenes, the decrease of the REE content from the bottom to the top of the stratigraphic sequence of Orvieto is interpreted as an indication of the chemical zoning of the magmatic chamber. The bulk mineralogical composition is characterized by the presence of clinopyroxene, biotite, anorthite, leucite, analcime and zeolite, in particular phillipsite and chabazite. The Orvieto sequence is affected by strong differences in the amorphous content ranging from 1.3 to 82.0 wt%, as well as in the sanidine/zeolite ratio: sanidine content changes from 15.5 to 55.9 wt% and chabazite from 3.4 to 68.4 wt%. The large amount of zeolites along the pyroclastic sequence testifies that the Orvieto-Bagnoregio Ignimbrite was emplaced by hydromagmatic eruption at quite high temperatures with diffuse zeolitization that indicates strong variations in the chemical-physical composition of the pyroclastic flows which in turn have influenced the post-depositional processes and the cohesion of the rock.
Keywords
pyroclastic deposit • x-ray diffraction • syn- and post-depositional processes • bolsena district • clinopyroxene • ultra-potassic alkaline rocks