Petrography and chemistry of the buried Pliocene Castel di Pietra pluton (Southern Tuscany, Italy)

Deep boreholes drilled in search of ore minerals near Castel di Pietra (Sou thern Tuscany), have revealed the occurrence of a shallow plutonic body of Pliocene age belonging to the acidic igneous association of the Tuscan Magm atic Province (TMP). The pluton intruded the quartz-phyllites of the Tuscan sequence producing a similar to 200 m thick thermal aureole. The maximum d epth of emplacement is constrained at about 8-9 km by mineral equilibria in the hornfels. On the basis of petrographical and chemical data, three faci es have been distinguished in the Castel di Pietra pluton: Darker Orthopyro xene-bearing Facies (DOF), a Porphyritic Facies (PF) and a Leucocratic Faci es (LF). Within the granitoids of the TMP, this pluton is unique in that it shows the largest compositional range, and the DOF is the most basic litho logy yet recognised. The character of the internal contacts suggests that t he intrusion of PF took place when the main mass (i.e. DOF) was not yet com pletely solidified, indicating that the pluton is a multiple intrusion prod uced by at least two separated pulses of magma. DOF and PF are also isotopi cally distinct, with DOF having a significantly lower initial Sr-87/Sr-86 r atio. Collected data suggest that geochemical and petrographical variation of the pluton can be ascribed dominantly to mixing processes combining evol ved magmas with a more basic one, having relatively high (crustal source) a nd low (mantle source) radiogenic Sr content, respectively. The occurrence of orthopyroxene in the DOF points to a mantle-derived basic liquid with an oversaturated character, more compatible with basic magmas of TMP than wit h those belonging to the Roman Magmatic Province.