Il basamento metamorfico della regione delle Baronie (NW Sardegna) offre una sezione strutturale continua e ben esposta di rocce appartenenti alla catena varisica sarda, mostrando un aumento del grado metamorfico verso nord. La Posada Asinara Shear Zone (PASZ; CAROSI et al., 2020) è una zona di taglio regionale a cinematica transpressiva destra che segna il limite tra il Low-to Medium Grade Metamorphic Complex e l’High Grade Metamorphic Complex (L-MGMC and HGMC; CARMIGNANI et al., 1994b), entrambi costituiti da rocce metapelitiche e metagranitiche caratterizzate da un metamorfismo barroviano in condizioni di facies anfibolitica. Inoltre, l’attività della PASZ è correlata all’esumazione di rocce appartenenti alla crosta profonda. In questo lavoro di tesi è stato condotto uno studio microstrutturale nell’ottica di una maggiore comprensione dell’evoluzione tettono-metamorfica di questa porzione di catena. I campioni da analizzare sono stati selezionati lungo due transetti ortogonali alla PASZ, dove sono presenti perlopiù micascisti e gneiss milonitici metamorfosati in facies anfibolitica. La distribuzione dei minerali indice e le relazioni tra blastesi e deformazione, combinate con nuove stime P-T, hanno permesso di ottenere nuovi P-T paths. In particolare, due campioni di micascisti milonitici a staurolite, plagioclasio e granato e un campione di micascisto milonitico a cianite, granato e staurolite, sono stati studiati con l’ausilio della termobarometria convenzionale e mediante il calcolo di pseudosezioni. In accordo con gli studi petrologici e strutturali precedenti (FRANCESCHELLI et al., 1989; CAROSI & PALMERI, 2002; CAROSI et al., 2020), è possibile correlare la crescita delle mineral assemblages dai primi stadi collisionali della catena (fase deformativa D1) fino all’inizio della transpressione indotta dalla PASZ (fase deformativa D2). La crescita post-D3 (desunta dai rapporti microstrutturali) di andalusite, cordierite, mica bianca e biotite ha rilevato uno stadio di riscaldamento a bassa pressione, suggerendo una evoluzione metamorfica polifasica in questo settore di catena. Tenendo conto delle variazioni composizionali osservate, la fase deformativa D1 è stata vincolata nel range 540-580 °C e 3-7,5 kbar (nucleo del granato), fino a c. 650 °C e c. 10 kbar (Pmax ottenuta dal contenuto di Si⁴⁺ nella mica bianca), la D2 in decompressione nel range 600-640 °C e 7,5-4,5 kbar, il riscaldamento a bassa pressione post-D3 a 630 °C e 2,5 kbar.
The metamorphic basement of the Baronie Region (NW Sardinia, Italy) offers a well-exposed and continue structural section of the rocks pertaining to the Sardinian Variscan Belt, showing a northward increase in the metamorphic field gradient. The Posada Asinara Shear Zone (PASZ; CAROSI et al., 2020) is a regional scale dextral transpressional shear zone that marks the boundary between the Low- to Medium-Grade and the High-Grade Metamorphic Complexes (L-MGMC and HGMC; CARMIGNANI et al., 1994b), both consisting of metapelitic and metagranitic rocks that underwent a Barrovian metamorphism in amphibolitic conditions. Moreover, the activity of the PASZ is linked to the exhumation of deep-seated crustal rocks. In order to further understand the tectono-metamorphic evolution of this portion of the belt, a microstructural study has been carried out in the present work. The samples have been selected along two transects orthogonal to the PASZ, where mostly amphibolite-facies mylonitic micaschists and gneisses are present. The distribution of index-minerals and the blastesis-deformation relationships, combined with new P-T estimates, have led to attain new P-T-D paths. Particularly, two specimens of garnet-plagioclase-staurolite-bearing mylonitic micaschists and one specimen of staurolite-garnet-kyanite-bearing mylonitic micaschist, have been investigated in detail for the reconstruction of P-T estimates, with the aid of conventional thermobarometry and with the calculation of P-T pseudosections. According to previous petrological and structural investigations of the area (FRANCESCHELLI et al., 1989; CAROSI & PALMERI, 2002; CAROSI et al., 2020), it is possible to relate the growth of the mineral assemblages from the late collisional stage (D1 phase) to the beginning of transpressional shearing along the PASZ (D2 phase). A post-D3 growth (based on the microstructures) of andalusite, cordierite, white mica and biotite, pointed out a low-pressure heating stage, suggesting a polyphasic metamorphic evolution in this part of the belt. Depending on the whole rock composition examined, the development of the D1 phase was constrained around 540-580 °C and 3-7,5 kbar (garnet core) up to c. 650 °C and c. 10 kbar (Pmax based on Si⁴⁺ content in white mica), the D2 phase around 600-640 °C and 7,5-4,5 kbar in decompression, the post-D3 low pressure heating stage around 630 °C and 2,5 kbar.
EVOLUZIONE TETTONO-METAMORFICA DI UNA PORZIONE DEL BASAMENTO VARISICO SARDO (REGIONE DELLE BARONIE): UN APPROCCIO ALLA MICROSCALA
PAGANO, VALENTINA
2020/2021
Abstract
The metamorphic basement of the Baronie Region (NW Sardinia, Italy) offers a well-exposed and continue structural section of the rocks pertaining to the Sardinian Variscan Belt, showing a northward increase in the metamorphic field gradient. The Posada Asinara Shear Zone (PASZ; CAROSI et al., 2020) is a regional scale dextral transpressional shear zone that marks the boundary between the Low- to Medium-Grade and the High-Grade Metamorphic Complexes (L-MGMC and HGMC; CARMIGNANI et al., 1994b), both consisting of metapelitic and metagranitic rocks that underwent a Barrovian metamorphism in amphibolitic conditions. Moreover, the activity of the PASZ is linked to the exhumation of deep-seated crustal rocks. In order to further understand the tectono-metamorphic evolution of this portion of the belt, a microstructural study has been carried out in the present work. The samples have been selected along two transects orthogonal to the PASZ, where mostly amphibolite-facies mylonitic micaschists and gneisses are present. The distribution of index-minerals and the blastesis-deformation relationships, combined with new P-T estimates, have led to attain new P-T-D paths. Particularly, two specimens of garnet-plagioclase-staurolite-bearing mylonitic micaschists and one specimen of staurolite-garnet-kyanite-bearing mylonitic micaschist, have been investigated in detail for the reconstruction of P-T estimates, with the aid of conventional thermobarometry and with the calculation of P-T pseudosections. According to previous petrological and structural investigations of the area (FRANCESCHELLI et al., 1989; CAROSI & PALMERI, 2002; CAROSI et al., 2020), it is possible to relate the growth of the mineral assemblages from the late collisional stage (D1 phase) to the beginning of transpressional shearing along the PASZ (D2 phase). A post-D3 growth (based on the microstructures) of andalusite, cordierite, white mica and biotite, pointed out a low-pressure heating stage, suggesting a polyphasic metamorphic evolution in this part of the belt. Depending on the whole rock composition examined, the development of the D1 phase was constrained around 540-580 °C and 3-7,5 kbar (garnet core) up to c. 650 °C and c. 10 kbar (Pmax based on Si⁴⁺ content in white mica), the D2 phase around 600-640 °C and 7,5-4,5 kbar in decompression, the post-D3 low pressure heating stage around 630 °C and 2,5 kbar.| File | Dimensione | Formato | |
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https://hdl.handle.net/20.500.14240/69961