Strain localization within shear zones may partially erase the rock fabric and the metamorphic assemblage(s) that had developed before the mylonitic event. In poly-deformed basements, the loss of information on pre-kinematic phases of mylonites hinders large-scale correlations based on tectono-metamorphic data. In this study, devoted to a relict unit of Variscan basement reworked within the nappe stack of the Northern Apennines (Italy), we investigate the possibility to reconstruct a complete pressure (P) – temperature (T) – deformation (D) path of mylonitic micaschist and amphibolite by integrating microstructural analysis, mineral chemistry and thermodynamic modelling. The micaschist is characterized by a mylonitic fabric with fine-grained K-white mica and chlorite enveloping mica-fishes, quartz, and garnet pseudomorphs. Potassic white mica shows Mg-rich cores and Mg-poor rims. The amphibolite contains green amphibole+ plagioclase+garnet+quartz+ilmenite defining S1 with a superposed mylonitic fabric localized in decimetre- to centimetre-scale shear zones. Garnet is surrounded by an amphibole+plagioclase corona. Phase diagram calculations provide P-T constraints that are linked to the reconstructed metamorphic-deformational stages. For the first time an early high-pressure stage at > 11 kbar and 510 °C was constrained, followed by a temperature peak at 550-590 °C and 9-10 kbar and a retrograde stage (< 475 °C, < 7 kbar), during which ductile shear zones developed. The inferred clockwise P-T-D path was most likely related to crustal thickening by continent-continent collision during the Variscan orogeny. A comparison of this P-T-D path with those of other Variscan basement occurrences in the Northern Apennines revealed significant differences. Conversely, a correlation between the tectono-metamorphic evolution of the Variscan basement at Cerreto pass, NE Sardinia and Ligurian Alps was established.

High-pressure tectono-metamorphic evolution of mylonites from the Variscan basement of the Northern Apennines, Italy / Deborah Lo, Pò; Roberto, Braga; Hans joachim, Massonne; Molli, Giancarlo; Montanini, Alessandra; Bargossi, Giuseppe Maria. - In: JOURNAL OF METAMORPHIC GEOLOGY. - ISSN 1525-1314. - 36:(2018), pp. 23-39. [10.1111/jmg.12281]

High-pressure tectono-metamorphic evolution of mylonites from the Variscan basement of the Northern Apennines, Italy

MOLLI, GIANCARLO;MONTANINI, Alessandra;BARGOSSI, Giuseppe Maria
2018-01-01

Abstract

Strain localization within shear zones may partially erase the rock fabric and the metamorphic assemblage(s) that had developed before the mylonitic event. In poly-deformed basements, the loss of information on pre-kinematic phases of mylonites hinders large-scale correlations based on tectono-metamorphic data. In this study, devoted to a relict unit of Variscan basement reworked within the nappe stack of the Northern Apennines (Italy), we investigate the possibility to reconstruct a complete pressure (P) – temperature (T) – deformation (D) path of mylonitic micaschist and amphibolite by integrating microstructural analysis, mineral chemistry and thermodynamic modelling. The micaschist is characterized by a mylonitic fabric with fine-grained K-white mica and chlorite enveloping mica-fishes, quartz, and garnet pseudomorphs. Potassic white mica shows Mg-rich cores and Mg-poor rims. The amphibolite contains green amphibole+ plagioclase+garnet+quartz+ilmenite defining S1 with a superposed mylonitic fabric localized in decimetre- to centimetre-scale shear zones. Garnet is surrounded by an amphibole+plagioclase corona. Phase diagram calculations provide P-T constraints that are linked to the reconstructed metamorphic-deformational stages. For the first time an early high-pressure stage at > 11 kbar and 510 °C was constrained, followed by a temperature peak at 550-590 °C and 9-10 kbar and a retrograde stage (< 475 °C, < 7 kbar), during which ductile shear zones developed. The inferred clockwise P-T-D path was most likely related to crustal thickening by continent-continent collision during the Variscan orogeny. A comparison of this P-T-D path with those of other Variscan basement occurrences in the Northern Apennines revealed significant differences. Conversely, a correlation between the tectono-metamorphic evolution of the Variscan basement at Cerreto pass, NE Sardinia and Ligurian Alps was established.
High-pressure tectono-metamorphic evolution of mylonites from the Variscan basement of the Northern Apennines, Italy / Deborah Lo, Pò; Roberto, Braga; Hans joachim, Massonne; Molli, Giancarlo; Montanini, Alessandra; Bargossi, Giuseppe Maria. - In: JOURNAL OF METAMORPHIC GEOLOGY. - ISSN 1525-1314. - 36:(2018), pp. 23-39. [10.1111/jmg.12281]
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11381/2832290
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