Petrological constraints on the high-Mg basalts from Capo Marargiu (Sardinia, Italy): Evidence of cryptic amphibole fractionation in polybaric environments

Abstract

This study deals with the textural and compositional characteristics of the calc-alkaline stratigraphic sequencefrom Capo MarargiuVolcanicDistrict (CMVD;Sardiniaisland,Italy). The areaisdominatedbybasalticto interme-diate hypabyssal (dikes and sills) and volcanic rocks (lavaflows and pyroclastic deposits) emplaced during theOligo-Miocene orogenic magmatism of Sardinia. Interestingly, a basaltic andesitic dome hosts dark-grey,crystal-rich enclaves containing up ~50% of millimetre- to centimetre-sized clinopyroxene and amphibolecrystals. This mineral assemblage is in equilibrium with a high-Mg basalt recognised as the parental magma ofthe entire stratigraphic succession at CMVD. Analogously, centimetre-sized clots of medium- and coarse-grained amphibole + plagioclase crystals are entrapped in andesitic dikes that ultimately intrude the stratigraphicsequence. Amphibole-plagioclase cosaturation occurs at equilibrium with a differentiated basaltic andesite. Majorand trace element modelling indicates that the evolutionary path of magma is controlled by a two-step processdriven by early olivine + clinopyroxene and late amphibole + plagioclase fractionation. In this context, enclavesrepresent parts of a cumulate horizon segregated at the early stage of differentiation of the precursory high-Mg ba-salt. This is denoted by i) resorption effects and sharp transitions between Mg-rich and Mg-poor clinopyroxenes,indicative of pervasive dissolution phenomena followed by crystal re-equilibration and overgrowth, and ii) reac-tion minerals found in amphibole coronas formed at the interface with more differentiated melts infiltrating with-in the cumulate horizon, and carrying the crystal-rich material with them upon eruption.....