# The Alteration of Giglio Island Granite: Relevance to the Conservation of Monumental Architecture
## 2 Geological Setting  
Giglio Island belongs to the Tuscan Archipelago and is located in the stretch of sea facing the Argentario promontory. The 90% of its extension is made of a monzogranite intrusion that rises to the surface and is linked to the extensive tectonic phase following the collision between the Adria and Corsica-Sardinia plates that formed the Apennine chain. In this tectonic domain, a series of intrusive, volcanic, and/or sub-volcanic magmatism episodes, known in literature as the Tuscan Magmatic Province, took place [14; 15]. In the area occupied by the Tuscan Archipelago, starting about 7 Ma ago, four grandiodrite and quartz-monzonite intrusive bodies have been emplaced, two in correspondence with Elba Island (Monte Capanne and the Porto Azzurro-Rio Marina area), one in Monte-cristo, and another in Giglio. This activity developed from the end of the Miocene to the whole Pliocene (from 7 to around 2 Ma ago) [16; 17; 18]. This magmatism is generally interpreted as deriving from a large-scale melting process of clayey sediments inside the Earth's crust (anatexis phenomenon). The intrusion of the monzogranite mass took place inside sedimentary rocks, whose remnants from contact metamorphism can be observed at Punta del Fenaio (north of the island) in two small outcrops of strongly foliated phyllites. Some authors report other small outcrops of intensely metamorphosed schists injected with agiltic and quartz veins in the locality of \"Vena\". The only part of the island not made up of plutonites is that of the Franco promontory, south-west of Campese village, which also diversifies morphologically. In this area, metamorphic and sedimentary Mesozoic rocks crop out, referable to two distinct structural units, the upper unit, consisting of grey argillites and metagabbros, and the lower unit consisting of crystalline limestone, cavernous limestone, quartz conglomerate, and quartzites of the Verrucano formation. The intrusions of these plutons caused the lifting of the sedimentary cover and the formation of a horst in correspondence with the Argentario itself. Subsequently, the evolution of the extensional tectonics caused a sinking of the area between Argentario and Giglio, transforming the latter into an island. Monzogranites are acidic intrusive rocks characterised by a mineralogical composition in which quartz (20%), K-feldspar (20%), and plagioclase (40%) predominate [19]. Other minerals are represented by biotite (10%) and cordierite and as the accessory minerals apatite, zircon, titanate, iron oxides, and sulphides. The minerals' grain sizes range from medium to fine (one to four millimetres), sometimes with centimetre-sized phenocrysts. The rock has a classic whitish grey colour that changes to rusty brown upon weathering. Two distinct magmatic intrusions [18], that of the Giglio monzogranite intrusion (GMI) occupying the most part of the island and that of the limited autcrop of _Le Scole_ islets, south-east of Giglio Porto, known as Scole monzogranite intrusion (SMI), are recognised. Within the GMI intrusion, two facies can be further distinguished, called \"facies of Pietrabona\" (PBF) and \"facies of Arenella\" (ARF), characterised by some petrographic and structural differences. The Pietrabona facies is characterised by very pronounced lineation given by the preferential orientation of flat or elongated micro-granular enclaves, while the Arenella facies is characterised by a more massive texture (only localised orientation of the megacrystals), abundance of K-feldspar megacrysts, and a lower amount of biotite. A further macroscopic characteristic of ARF is the presence of auto-intrusions of micro-granular enclaves with different contents of megacrysts. The monzogranite of the Le Scole facies (SMI) has a more acidic chemical composition than GMI and is practically free of xenoliths. The monzogranites undergo an alteration due to two phenomena often connected to each other. The first is essentially linked to fracturing, which creates practically isolated blocks.