Tuscan Magmatic Province

The Tuscany Magmatic Province (Fig.1) comprises several mafic to silicic intrusive and extrusive centres scattered through southern Tuscany and the Tuscan archipelago. The silicic rocks of the Tolfa-Manziana-Cerite area, north-west of Rome, and a mafic ultrapotassic dyke from Sisco (Corsica) are also traditionally included into the Tuscany Province.
Magmatic rocks form stocks, dykes, necks, lava flows and domes, and the large volcanoes of Monte Amiata, Monti Cimini and Capraia island. Ages range from about 14 Ma for the Sisco dyke to about 0.2 Ma for Monte Amiata, and show a tendency to decrease from west to east.


Fig.1: Location of intrusive and extrusive rocks of the Tuscany Magmatic Province. Numbers in parentheses indicate ages (in Ma).

The igneous rocks of the Tuscany Province consist of an association of mafic to silicic rocks exhibiting contrasting compositions and genesis. Silicic rocks SiO2 > 65 wt %) consist of a large number of intrusive and extrusive bodies having a peraluminous character (Alumina Saturation Index, ASI > 1), and moderate variations of major and trace elements at a given silica level. The mafic rocks (MgO > 3 wt %) range from calc-alkaline and shoshonitic to potassic and ultrapotassic. The Tuscan ultrapotassic rocks are slightly undersaturated to oversaturated in silica, in contrast with the ultrapotassic rocks from the Roman Province, which are strongly undersaturated in silica.

The Tuscany Magmatic Province mafic to silicic intrusive and extrusive centres are:

San Vincenzo (4.5 Ma): Rhyolite lava flow and dome.
Roccastrada (2.5 Ma): Rhyolite lava flow and dome.
Tolfa-Manziana-Cerite (3.5 Ma): Multicentre complex made of trachydacite to rhyolite lava flows, domes and pyroclastic flows.
Monti Cimini (1.3-0.9 Ma): Volcanic complex formed of trachydacite to latite lava flows, domes and ignimbrites, with a few late erupted olivine-latite and shoshonite lavas.
Monte Amiata (0.3-0.2 Ma): Central cone of prevailing trachydacite lava flows and domes, with a few late stage olivine-latite and shoshonite lavas.
Elba island (8.5-6.8 Ma): Monzogranites and minor granodiorites, sienogranites, alkali feldspar granites, aplites and pegmatites forming stocks, laccoliths, dykes and sills. Late (5.8 Ma) calc-alkaline mafic dike.
Vercelli seamount (7.2 Ma): Small intrusive body from which syenogranitic rocks have been dredged.
Montecristo island (7.1 Ma): Monzogranite stock cut by aplite and pegmatite veins and porphyritic dikes.
Giglio island (5 Ma): Monzogranite stock intruded by leucocratic monzogranite and by aplite-pegmatite dikes.
Campiglia-Gavorrano (5.9-4.3 Ma): Leucocratic monzogranite, alkali-feldspar granite, monzogranite and tourmaline-bearing leucogranite forming large intrusion mostly hidden beneath surface. Altered mafic dykes with an apparently ultrapotassic composition also occur.
Sisco (14 Ma): Minette dyke showing a high-silica lamproitic composition.
Capraia island (7.6-4.6 Ma): Stratovolcano formed by high-K calc-alkaline andesites and dacites, and by late shoshonitic basalts.
Montecatini Val di Cecina (4.1 Ma): Minette neck with high-silica lamproitic composition, permeated by leucocratic veins.
Orciatico (4.1 Ma): Mafic hypabyssal body with high-silica lamproitic composition.
Radicofani (1.3 Ma Ma): Mafic neck and remnants of lava flow with shoshonitic to ultrapotassic composition.
Torre Alfina (0.9-0.8 Ma): Mafic necks and lava flow with high-silica lamproitic composition.

The thickness of the crust in Tuscany is moderate (about 20 to 30 km), and reaches a minimum beneath the Tyrrhenian border of southern Tuscany. A vertical zone of high S-wave velocity (up to 4.6 km/s) has been detected below a depth of about 70 km beneath the north-central Apennine area.
This has been interpreted as representing a relict lithospheric slab from the Adriatic plate (east of the Apennine chain), which is passively sinking into the upper mantle.
An important geophysical feature of the area is denoted by a layer in the uppermost mantle that exhibits crustal-like seismic wave velocities (VP = 6.8 km/s). This layer may represent either the remnants of the Ligure-Piemontese slab, which was subducting beneath this area until Oligocene time, or partially molten mantle material.
Heat flow is high in the Tuscany area (around 100 to 200 mW/m2 in some areas), as testified by the occurrence of well-known geothermal fields at Larderello and Amiata.


Silicic Magmatism:
A crustal anatectic origin is widely accepted for Tuscany silicic magmatism. This is strongly supported by a wealth of petrological and geochemical data, including the peraluminous nature of most rocks and their crustal-like geochemical and isotopic signatures. However, only a few silicic rocks actually represent pure anatectic melts. These include the Roccastrada rhyolites, some of the San Vincenzo lavas and some leucocratic granitoid bodies occurring, for example, at Elba and Giglio. Several studies have shown that the compositions of unmodified crustal anatectic magmas in Tuscany can be modelled by assuming large degrees (some 40-50%) of partial melting of metasediments. Garnet micaschists and gneiss, such as those found by drilling in Tuscany, have been successfully used as source rocks to model trace element and isotopic compositions of silicic magmas at Roccastrada and other localities in Tuscany.

Petrological data and geochemical modelling suggest that melting of metasedimentary rocks occurred in fluid absent conditions at pressure of at least 0.4-0.6 GPa, leaving a garnet- and cordierite-bearing residue. This indicates that metasediments occur at great depths in the Tuscany Province. The majority of the silicic rocks exhibit ample textural and geochemical evidence suggesting a more complex genesis than simple crustal anatexis. The occurrence of microgranular mafic enclaves and mafic xenocrysts in several granitoids and lavas is the most obvious indication of interaction (i.e. mixing or mingling) between felsic and mafic magmas.
The nature of the mantle end-member involved in the mixing processes is difficult to define, since several enclaves show clear evidence of being equilibrated with host rocks. However, Poli et al. (2002) noticed that the mafic enclaves from some plutons (e.g. Elba and Giglio) have patterns of incompatible elements that are similar to those of calc-alkaline rocks from Capraia island. This has led them to suggest that the mafic end-member of Tuscany plutonism was represented by a calc-alkaline melt.

Mafic Magmatism:
The mafic rocks in Tuscany have highly variable compositions in terms of major elements, incompatible element abundances and isotopic signatures. There is little doubt that the Tuscany mafic magmas have been subject to fractional crystallisation, mixing and crustal assimilation (e.g. Conticelli 1998). However, their high MgO, Ni and Cr concentrations, whose values are close to those of primary mantle melts, exclude that the mafic magmas with different enrichments in potassium and incompatible elements can be derived from each other by any common evolution process. Therefore, it has been concluded that the variable petrological and geochemical compositions of mafic rocks in Tuscany basically result from anomalous and heterogeneous mantle sources (Peccerillo et al. 1987).

Petrogenetic History

Petrological and geochemical data suggest a complex series of events in petrogenesis of the Tuscany Province. These can be summarised as follows:

1. Mantle contamination by addition of variable amounts of upper crustal material (e.g. metapelites) to peridotite, possibly represented by both harzburgite and lherzolite.
2. Melting of heterogeneously contaminated harzburgite-lherzolite mantle source to generate calc-alkaline to ultrapotassic magmas with variable enrichments in incompatible elements and radiogenic isotope signatures, but exhibiting very similar shapes of incompatible element patterns.
3. Mixing between different types of mantle-derived melts to form a variety of magmas with continuous compositional variations.
4. Moderate evolutionary modification of mafic melts during emplacement, with some fractional crystallisation, crustal assimilation (e.g. at Torre Alfina) and unmixing of felsic residual melts at Montecatini Val di Cecina.
5. Crustal melting, probably triggered by the emplacement of mafic melts, with generation of peraluminous, highly silicic magmas which were emplaced either as unmodified melts or mixed with different types of mantle-derived magmas giving less strongly silicic hybrids products.
6. Fractional crystallisation of hybrid silicic melts to produce high-silica aplites and pegmatites.


• Peccerillo. A. Plio-Quaternary Volcanism in Italy. (2005)