Metamorfismo varisico dell’Unità tettonica del Monte Grighini,Sardegna centrale

Monte Grighini Complex (central Sardinia) consists, from the bottom to the the top of Monte Grighini Unit, Castello Medusa Unit and Gerrei Unit. Monte Grighini Unit is caracterized by medium grade metamorphism evidenced by mineral such garnet, staurolite, andalusite and sillimanite, and by the presence of a syn-kinematic late-Variscan intrusive complex related to a shear zone marked by ultramylonites and cataclasites. Geological map in scale 1:25000 of the Monte Grighini Complex has been done by combining geo-petrographical data with previous geological and structural data (Musumeci et al., under revision). A lito-stratigraphic column has also been done. That column shows relations between the two formations of the Monte Grighini Unit (from bottom to the top): Truzzulla Formation that consist of metavolcanics, metarkoses and metasandstones, and Toccori Formation formed by quartzites, micaschists, paragneiss, phyllites, graphitic phyllites and marbles. Monte Grighini Unit has been interested by four deformation phases. The phase D1 is characterized by the blastesis of biotite and potassic white mica; this phase is almost transposed by the D2 deformational phase. The phase D2 is the main foliation both at the outcrop and micro-scale and it is characterized by folds, foliation and blastesis of garnet, staurolite, biotite, potassic white mica, andalusite, sillimanite and cordierite. The phase D3 produces F3 macroscale folds. The D4 deformation phase is related to the development of the shear zone resulting in ultramylonites and cataclasites. Zircon U/Pb analyses in metavolcanics and metasandstones samples yield 447 ± 4 Ma (Katian – Upper Ordovician) and 419 ± 5 – 450 Ma (Middle Ordovician) respectively. This indicates that the Ordovician calc-alkaline volcanism went on until Upper Ordovician. Micro-structural, minero-petrographic and chemical data has been done in micaschists and phyllites of Toccori Formation and metamorphic zoneography has changed. Three zones have been set: garnet zone, andalusite and staurolite zone and sillimanite zone. Biotite zone were removed and andalusite zone and staurolite zone were merged. Mineralogical, structural and minero-chemical analyses shown that garnet of Toccori has compositional zoning in fact, almandine and pirope increasing and grossular and spessartin decrease, from the core to the rim (mineral association: Grt + Wmca + Bt + Qtz ± St ± And). A rare heuedral garnet without compositional zoning was observed in hornfels closed to the intrusive complex. Pseudosections of garnet and staurolite micaschists, garnet and sillimanite hornfels of Toccori Formation and leucogranite were calculated. P-T path of micaschists were done through garnet zoning yielding P = 6.5 - 9.5 kbar and T = 450 - 540°C for garnet core and P = 3-5.8 kbar e T = 510-580°C for garnet rim. The P-T path obtained starts with the baric peak (P = 6.5 - 9.5 kbar) and continues its path to the thermal peak (T = 510 – 580°C). A retrograde path is showing by chlorite and ilmenite growing. Thermodynamic modeling in garnet and sillimanite hornfels gave P = 3.5 – 4.5 kbar and T = 510 – 575 °C on the basis of garnet composition; these pressure and temperature are compatible to the condition of that obtained modeling garnet rim in zoned garnet. The emplacement of granitic intrusion conditions has been obtained by the intersection of the solidus curve and silica content in magmatic potassic white mica getting pressures between 4.2 and 5.5 kbar which corresponds to 16-21 km depth. These pressures of emplacement are consistent with those of the thermal peak of micaschists. A model of Monte Grighini Complex has been done. In the first stage the crustal thickening occurs with development of polyphasic foliations (phases D1 and earlier-D2) under green-schists and amphibolitic facies. During the second stage deformational phase D2 occurs with forming minerals such as biotite, garnet, staurolite, etc in the Monte Grighini Unit. During this stage an incipient shear zone and an early stage granite emplacement occur. In the third stage granites finish its emplacement with a strong developing of shear zone resulting in mylonites and ultramylonites. In the fourth stage the brittle deformation finish. Detachment structure marked by cataclasites occurs