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This paper analyses the xenoliths found in the phreatomagmatic products of the most recent Via del Laghi maar field at Colli Albani volcano in order to investigate the nature of the substratum and its interaction with the magmatic system. The pre-volcanic sedimentary xenoliths are from the same rock formations that form the pre-orogenic Mesozoic–Cenozoic carbonatic pelagic succession cropping out in the nearby Apennine mountain belt, and the Pliocene post-orogenic marine sedimentary succession. Along with the sedimentary xenoliths, a large variety of thermo-metamorphic, metasomatic and magmatic xenoliths are present. Skarn xenoliths have been studied to estimate peak metamorphic P–T conditions and assess the role of fluids during prograde and retrograde metamorphism. The presence of hydrous minerals such as phlogopite and amphibole and the carbonatic nature of the substratum suggest a binary H2O–CO2 mixture as the dominant fluid. Microscale wollastonite filling fractures indicates a temporal link between fracturing and fluid infiltration. The occurrence of retrograde hydrous textures superimposed on anhydrous associations indicates at least a two step process: metamorphism with a T peak at dry conditions (CO2 saturated), followed by fracturing of the substratum in the late stage, allowing abundant aqueous fluids to penetrate the thermally modified rocks. A significative amount of rare earth minerals crystallized in the late stage.

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