Gavorrano Granite

The Gavorrano Granite (Fig 1) is a small granite body (about 3 km² in map view) of early Pliocene age (4.4 Ma) with a NNW-SSE elongated shape. It is bounded by the Caldana-Monticello Fault and Gavorrano Fault on the eastern and western sides, respectively. In the field two main intrusive facies have been distinguished : a dominant monzogranite facies and a leucogranite facies.

Fig.1: Geological sketch map of Gavorrano Antiform. LU, Ligurian units; TN, Tuscan Nappe; TMC, Tuscan Metamorphic Complex; PF, Palaie Fault; CMF, Caldana-Monticello Fault; GF, Gavorrano Fault; RF, Rigoloccio Fault. After G. Musumeci (2005).

Fig.2: Geological cross section of the Gavorrano Antiform. After G. Musumeci (2005).

The monzogranite has a generally porphyritic texture with scattered K-feldspar megacrysts up to 8 cm in length. Phenocrysts of zoned plagioclase, biotite, and quartz are embedded in a medium-grained matrix mainly composed of quartz and alkali feldspars with pinitized cordierite grains. The most common accessory phases are apatite, zircon, monazite, magnetite and pyrite.

The leucogranite is a tourmaline-bearing alkali feldspar granite with aplitic texture. Its mineral assemblage is dominated by K-feldspar associated with minor plagioclase, quartz, Fe²⁺-rich tourmaline (schorl) and muscovite.
The leucogranite occurs as small bodies and dikes in the eastern and northern parts of the intrusion. Dikes, centimeter to meter size in thickness, strike mainly ENE and NNW and are often mineralized, containing pyrite and sulphides. Field relationships suggest that the alkali feldspar leucogranite intruded the monzogranite. Two lines of evidence support this: (1) leucogranite dikes are evident intruding the monzogranite and (2) angular fragments of monzogranite occur within the leucogranite.

In the Gavorrano Granite, host rocks contact metamorphism is testified by the occurrence of pelitic and calcsilicate hornfelses mainly cropping out at the northern and southern sides of the intrusion (Fig.1). Andalusite and cordierite-bearing spotted schists occur in the phyllites and metasandstone of the TMC (Tuscan Metamorphic Comple). These extend over a distance of some tens of meters from the contact with the intrusion. In the TN (Tuscan Nappe), the thermal aureole is up to 100 m wide and is characterized by marble and calc-silicate hornfelses derived from the carbonate formations at the base of the sedimentary sequence. These rocks crop out at the southeastern side of the intrusion, near the village of Ravi, and occur as tectonized clasts and/or slices (meter to decameters in size) along the Gavorrano fault on the western side of intrusion (Fig.1). The cordierite + andalusite + K-feldspar mineral assemblage of the pelitic hornfels indicates that the contact metamorphism developed under LP/HTconditions (P < 0.15 GPa, T = 550-600°C ).

Bibliography

• Musumeci, G., Mazzarini, F., Corti, G., Barsella, M., & Montanari, D. (2005). Magma emplacement in a thrust ramp anticline: The Gavorrano Granite (northern Apennines, Italy). Tectonics, 24(6).

Photo
Tourmaline (brown-green), quartz (colorless) and alkali feldspar (colorless) crystals. Gavorrano leucogranite (Italy). PPL image, 2x (Field of view = 7mm)	Tourmaline (brown-green), quartz (colorless) and alkali feldspar (colorless) crystals. Gavorrano leucogranite (Italy). PPL image, 2x (Field of view = 7mm)	Tourmaline, quartz (I order white-gray) and alkali feldspar (I order gray) crystals. Gavorrano leucogranite (Italy). XPL image, 2x (Field of view = 7mm)
Alkali feldspar (I order gray) crystals. Gavorrano leucogranite (Italy). XPL image, 2x (Field of view = 7mm)	Tourmaline (brown-green) crystal. Gavorrano leucogranite (Italy). PPL image, 10x (Field of view = 2mm)	Tourmaline (brown-blue), quartz (colorless) and alkali feldspar (colorless) crystals. Gavorrano leucogranite (Italy). PPL image, 21x (Field of view = 7mm)
Tourmaline (brown-blue), quartz (colorless) and alkali feldspar (colorless) crystals. Gavorrano leucogranite (Italy). PPL image, 21x (Field of view = 7mm)	Tourmaline, quartz (I order white-gray) and alkali feldspar (I order gray) crystals. Gavorrano leucogranite (Italy). XPL image, 2x (Field of view = 7mm)	Tourmaline (brown-green), quartz (colorless) and alkali feldspar (colorless) crystals. Gavorrano leucogranite (Italy). PPL image, 2x (Field of view = 7mm)
Tourmaline (brown-green), quartz (colorless) and alkali feldspar (colorless) crystals. Gavorrano leucogranite (Italy). PPL image, 2x (Field of view = 7mm)	Tourmaline (brown-green), quartz (colorless) and alkali feldspar (colorless) crystals. Gavorrano leucogranite (Italy). PPL image, 2x (Field of view = 7mm)	Tourmaline (brown-green), quartz (colorless) and alkali feldspar (colorless) crystals. Gavorrano leucogranite (Italy). PPL image, 2x (Field of view = 7mm)
Graphic intergrowths of quartz in orthoclase. Gavorrano leucogranite (Italy). XPL image, 2x (Field of view = 7mm)	Graphic intergrowths of quartz in orthoclase. Gavorrano leucogranite (Italy). XPL image, 2x (Field of view = 7mm)	Graphic intergrowths of quartz in orthoclase. Gavorrano leucogranite (Italy). XPL image, 2x (Field of view = 7mm)
Graphic intergrowths of quartz in orthoclase. Gavorrano leucogranite (Italy). XPL image, 2x (Field of view = 7mm)	Tourmaline (brown-green), quartz (colorless) and alkali feldspar (colorless) crystals. Gavorrano leucogranite (Italy). PPL image, 2x (Field of view = 7mm)	Tourmaline, quartz (I order white-gray) and alkali feldspar (I order gray) crystals. Gavorrano leucogranite (Italy). XPL image, 2x (Field of view = 7mm)
Tourmaline (brown-green), quartz (colorless) and alkali feldspar (colorless) crystals. Gavorrano leucogranite (Italy). PPL image, 2x (Field of view = 7mm)	Tourmaline, quartz (I order white-gray) and alkali feldspar (I order gray) crystals. Gavorrano leucogranite (Italy). XPL image, 2x (Field of view = 7mm)	Tourmaline (brown-green) and quartz (colorless) crystals. Gavorrano leucogranite (Italy). PPL image, 2x (Field of view = 7mm)
Tourmaline and quartz (I order white-gray) crystals. Gavorrano leucogranite (Italy). PPL image, 10x (Field of view = 2mm)	Graphic intergrowths of quartz in orthoclase. Gavorrano leucogranite (Italy). XPL image, 10x (Field of view = 2mm)

contact metamorphic aureole
Vesuvianite (high relief, colorless) and calcite crystals. calcsilicate marble. PPL image, 2x (Field of view = 7mm)	Vesuvianite (dark gray) and calcite crystals. calcsilicate marble. XPL image, 2x (Field of view = 7mm)	Vesuvianite (dark gray) and calcite crystals. calcsilicate marble. XPL image, 2x (Field of view = 7mm)
Vesuvianite (dark gray) and calcite crystals. calcsilicate marble. XPL image, 2x (Field of view = 7mm)	Vesuvianite (high relief, colorless) and calcite crystals. calcsilicate marble. PPL image, 2x (Field of view = 7mm)	Vesuvianite (dark gray) and calcite crystals. calcsilicate marble. XPL image, 2x (Field of view = 7mm)
Vesuvianite (dark gray) and calcite crystals. calcsilicate marble. XPL image, 2x (Field of view = 7mm)	Vesuvianite (dark gray) and calcite crystals. calcsilicate marble. XPL image, 2x (Field of view = 7mm)	Vesuvianite (dark gray) and calcite crystals. calcsilicate marble. XPL image, 2x (Field of view = 7mm)