Petrology of the most recent ultrapotassic magmas from the Roman Province (Central Italy)

We report on the newly discovered lava flow that erupted in the Colli Albani Volcanic District, which is the most recent and, geochemically the most peculiar effusive event recognised in the entire ultrapotassic Roman Province (Central Italy). This lava flow is associated with the Monte Due Torn scoria cone, located approximately 5 km south of the Albano hydromagmatic centre (69-36 ka). The Monte Due Torn scoria cone displays well-preserved morphological characteristics and the 40 +/- 7 ka age determined for the associated lava flow indicates that its activity was nearly contemporaneous to the most recent, explosive activity that occurred at the Albano centre from 41 to 36 ka. By comparing chemical and petrological features of the Monte Due Torn lava flow, Albano products, and older products (>69 ka), we show that the youngest Colli Albani eruptions were fed by two new batches of parental magmas that originated in a phlogopite-bearing metasomatised mantle, each one feeding one of the two youngest eruptive cycles (at 69 ka and 41-36 ka). The trace element signature, e.g., very low Pb content, of primitive (MgO>3 wt.%) magmas feeding the initiation of the hydromagmatic activity at Albano (69 ka) and the subsequent effusive activity at Monte Due Torri (40 ka) indicates that a magma chamber located in the deep anhydrite-bearing dolomite formation was tapped. However, the polygenic activity, the changes in magma composition, and the variable thermometamorphic clasts occurring in the hydromagmatic deposits (recording variable substrata) suggest, particularly for the Albano eruptive centre, a more complex plumbing system consisting of at least two more magma chambers at a shallower depth, i.e., in the Mesozoic limestone and Pliocene pelite formations. The large amount of stratigraphic, volcanological, and geochemical data collected for the Colli Albani Volcanic District, one of the main districts in the ultrapotassic Roman Province, enable us to contribute insights into the still open debate regarding the temporal variation of the metasomatised mantle source of the Italian potassic magmas. Based on our data, i.e., variation of radiogenic and trace elements over time, we suggest that the observed variation in the mantle source of the ultrapotassic magmas can be related to progressive consumption of the phlogopite component in the metasomatised source rather than the transition from lithosphere- to asthenosphere-derived magmatism and/or the transition from orogenic to anorogenic magmatism. (C) 2011 Elsevier B.V. All rights reserved.