A detailed structural and petrological study has been focused on the Sila Piccola Massif (Northern Calabria), inorder to obtain new constraints about the tectono-metamorphic history of the “Castagna Unit” (Dubois & Glangeaud,1965). This unit represents a pervasively mylonitized continental crust complex, comprised of amphibolite togreenschist facies metamorphic rocks intruded by late Variscan granitoids. Structural investigations highlighted as thepervasive mylonitic fabric (Sm) has obliterated older metamorphic ones, locally preserved as relics in the low straindomains of the metapelite horizons. The relics of these pre-mylonitic fabrics (D1 stage) are represented by agrt1+pl1+bt1+wm1+qtz assemblage in which garnet exhibits a quasi-absent crystal zoning, reflecting a long residencetime at deep crust conditions. Field investigations integrated with microstructural data highlight as the mylonitic process(D2 stage) can be subdivided into two different metamorphic events: i) the first one, indicating an early retrograde path(M2a) developed under amphibolite facies conditions (P = 5.7-6.8 Kbar; T = 595 °C), is characterized by the synkinematicgrowth of grt2+pl2+bt2+sil+qtz observable in the pressure shadows of syn-D1 garnet porphyroclasts; ii) thesecond one, consistent with a late-retrograde metamorphic overprint (M2b) under greenschist facies conditions (P = 2-3Kbar; T = 400-500 °C), is highlighted by the syn-shearing crystal growth of chl1+wm2(Si a.p.f.u. 3.15-3.25) +pl3+qtzobserved along C-planes. A further mylonitic event (D3; P = 4-8 Kbar; T = 400-500 °C) appears having developed athigher pressure conditions than the previous one, as testified by the syn-shearing white micas with high phengitecontent (Si a.p.f.u. 3.25-3.40). The bimodal composition of the white mica arranged along C-planes, points to theoccurrence in the same terrains of two different mylonitic processes, probably ascribable to two orogenic cyclesdeveloped under different baric regimes. This hypothesis is geologically consistent with the re-activation of a Variscanshear zone during an Alpine shearing event (D3) developed in a HP-LT regime. Such a hypothesis is also supported byquartz LPO analyses indicating the presence of two contrasting senses of shear developed under greenschist faciesconditions.In this scenario, the “Castagna Unit” can be interpreted as an extensional late-to-post Variscan shear zone, locallyre-activated during the building of the Alpine edifice. Finally, an asymmetrical folding involving the mylonitic fieldfoliation, suggests a shallower deformative event (D4) linkable to the stacking stages of the Alpine-Appennine tectonicactivity in the central Mediterranean area.

Alpine re-activation of a late post-Variscan shear-zone: microstructural and petrological investigations (Sila, northern Calabria)

CIRRINCIONE, ROSOLINO;ORTOLANO, GAETANO;PEZZINO, Antonino;FIANNACCA, PATRIZIA;PUNTURO, Rosalda;
2015

Abstract

A detailed structural and petrological study has been focused on the Sila Piccola Massif (Northern Calabria), inorder to obtain new constraints about the tectono-metamorphic history of the “Castagna Unit” (Dubois & Glangeaud,1965). This unit represents a pervasively mylonitized continental crust complex, comprised of amphibolite togreenschist facies metamorphic rocks intruded by late Variscan granitoids. Structural investigations highlighted as thepervasive mylonitic fabric (Sm) has obliterated older metamorphic ones, locally preserved as relics in the low straindomains of the metapelite horizons. The relics of these pre-mylonitic fabrics (D1 stage) are represented by agrt1+pl1+bt1+wm1+qtz assemblage in which garnet exhibits a quasi-absent crystal zoning, reflecting a long residencetime at deep crust conditions. Field investigations integrated with microstructural data highlight as the mylonitic process(D2 stage) can be subdivided into two different metamorphic events: i) the first one, indicating an early retrograde path(M2a) developed under amphibolite facies conditions (P = 5.7-6.8 Kbar; T = 595 °C), is characterized by the synkinematicgrowth of grt2+pl2+bt2+sil+qtz observable in the pressure shadows of syn-D1 garnet porphyroclasts; ii) thesecond one, consistent with a late-retrograde metamorphic overprint (M2b) under greenschist facies conditions (P = 2-3Kbar; T = 400-500 °C), is highlighted by the syn-shearing crystal growth of chl1+wm2(Si a.p.f.u. 3.15-3.25) +pl3+qtzobserved along C-planes. A further mylonitic event (D3; P = 4-8 Kbar; T = 400-500 °C) appears having developed athigher pressure conditions than the previous one, as testified by the syn-shearing white micas with high phengitecontent (Si a.p.f.u. 3.25-3.40). The bimodal composition of the white mica arranged along C-planes, points to theoccurrence in the same terrains of two different mylonitic processes, probably ascribable to two orogenic cyclesdeveloped under different baric regimes. This hypothesis is geologically consistent with the re-activation of a Variscanshear zone during an Alpine shearing event (D3) developed in a HP-LT regime. Such a hypothesis is also supported byquartz LPO analyses indicating the presence of two contrasting senses of shear developed under greenschist faciesconditions.In this scenario, the “Castagna Unit” can be interpreted as an extensional late-to-post Variscan shear zone, locallyre-activated during the building of the Alpine edifice. Finally, an asymmetrical folding involving the mylonitic fieldfoliation, suggests a shallower deformative event (D4) linkable to the stacking stages of the Alpine-Appennine tectonicactivity in the central Mediterranean area.
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/20.500.11769/57553
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