Geochemical and isotopic (Sr, Pb) evidence of crust-mantle interaction in acidic melts - The Tolfa-Cerveteri-Manziana volcanic complex (central Italy): a case history

The Tolfa-Cerveteri-Manziana volcanic complex (TVC, 3.5 Ma), located about 100 km north of Rome, represents the southernmost outcrop of the Tuscan magmatic province. The overlapping of Tuscan acidic and Roman alkaline volcanic rocks in the area makes it suitable for the study of the interaction between crustal and mantle-derived melts. Major, trace element (Rb, Sr, Ba, Y, Nb, Zr, Pb, U, and Th), rare earth element, and Sr and Pb isotopic data on lavas (quartz-latites, rhyolites, and high-silica rhyolites), magmatic enclaves, and metasediments from the basement, allow the following conclusions to be drawn. 1. (a) The TVC volcanic rocks are derived from the anatexis of a metasedimentary source having a composition similar to that of the metasediments present in the basement beneath the M. Amiata geothermal region at depths ranging from 1.4 to 3.2 km. 2. (b) The anatectic magma, during its differentiation by fractional crystallization, interacted with two mafic magmas with differing Sr and Pb isotopic compositions and variable degrees of enrichment in incompatible elements. The most differentiated rocks (high-silica rhyolites) are the most highly contaminated. 3. (c) The inferred mafic contaminants appear to be derived from a heterogeneous metasomatized mantle with isotopic and geochemical characteristics similar to those of the mantle which presently underlies the Roman and Campanian regions. © 1991.