Deep boreholes drilled in search of ore minerals near Castel di Pietra (Southern Tuscany), have revealed the occurrence of a shallow plutonic body of Pliocene age belonging to the acidic igneous association of the Tuscan Magmatic Province (TMP). The pluton intruded the quartz-phyllites of the Tuscan sequence producing a ~200 m thick thermal aureole. The maximum depth of emplacement is constrained at about 8-9 km by mineral equilibria in the hornfels. On the basis of petrographical and chemical data, three facies have been distinguished in the Castel di Pietra pluton: Darker Orthopyroxene-bearing Facies (DOF), a Porphyritic Facies (PF) and a Leucocratic Facies (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 lithology yet recognised. The character of the internal contacts suggests that the intrusion of PF took place when the main mass (i.e. DOF) was not yet completely solidified, indicating that the pluton is a multiple intrusion produced by at least two separated pulses of magma. DOF and PF are also isotopically distinct, with DOF having a significantly lower initial 87Sr/86Sr ratio. Collected data suggest that geochemical and petrographical variation of the pluton can be ascribed dominantly to mixing processes combining evolved magmas with a more basic one, having relatively high (crustal source) and 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 with those belonging to the Roman Magmatic Province
Petrography and chemistry of the buried Pliocene Castel di Pietra pluton (Southern Tuscany, Italy)
INNOCENTI, FABRIZIO;TAMPONI, MARCO;
2000-01-01
Abstract
Deep boreholes drilled in search of ore minerals near Castel di Pietra (Southern Tuscany), have revealed the occurrence of a shallow plutonic body of Pliocene age belonging to the acidic igneous association of the Tuscan Magmatic Province (TMP). The pluton intruded the quartz-phyllites of the Tuscan sequence producing a ~200 m thick thermal aureole. The maximum depth of emplacement is constrained at about 8-9 km by mineral equilibria in the hornfels. On the basis of petrographical and chemical data, three facies have been distinguished in the Castel di Pietra pluton: Darker Orthopyroxene-bearing Facies (DOF), a Porphyritic Facies (PF) and a Leucocratic Facies (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 lithology yet recognised. The character of the internal contacts suggests that the intrusion of PF took place when the main mass (i.e. DOF) was not yet completely solidified, indicating that the pluton is a multiple intrusion produced by at least two separated pulses of magma. DOF and PF are also isotopically distinct, with DOF having a significantly lower initial 87Sr/86Sr ratio. Collected data suggest that geochemical and petrographical variation of the pluton can be ascribed dominantly to mixing processes combining evolved magmas with a more basic one, having relatively high (crustal source) and 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 with those belonging to the Roman Magmatic ProvinceI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
