The Early Paleozoic evolution of the northern margin of Gondwana is characterized by several episodes of bimodal magmatism intruded or outpoured within thick sedimentary basins. These processes are well recorded in th...The Early Paleozoic evolution of the northern margin of Gondwana is characterized by several episodes of bimodal magmatism intruded or outpoured within thick sedimentary basins. These processes are well recorded in the Variscan blocks incorporated in the Ligurian Alps because they experienced low temperature Alpine metamorphism. During the Paleozoic, these blocks, together with the other Alpine basements, were placed between the Corsica-Sardinia and the Bohemian Massif along the northern margin of Gondwana. In this framework, they host several a variegated lithostratigraphy forming two main complexes(Complexs I and II) that can be distinguished by both the protoliths and their crosscutting relationships, which indicate that the acidic and mafic intrusives of Complex II cut an already folded sequence made of sediments, basalts and granitoids of Complex I. Both complexes were involved in the Variscan orogenic phases as highlighted by the pervasive eclogite-amphibolite facies schistosity(foliation II). However, rare relicts of a metamorphic foliation at amphibolite facies conditions(foliation I)is locally preserved only in the rocks of Complex I. It is debatable if this schistosity was produced during the early folding event e occurred between the emplacement of Complex I and II e rather than during an early stage of the Variscan metamorphic cycle.New SHRIMP and LA ICP-MS Ue Pb zircon dating integrated with literature data, provide emplacement ages of the several volcanic or intrusive bodies of both complexes. The igneous activity of Complex I is dated between 507 ± 15 Ma and 494 ± 5 Ma, while Complex II between 467 ± 12 Ma and 445.5 ± 12 Ma.The folding event recorded only by the Complex I should therefore have occurred between 494 ± 5 Ma and 467 ± 12 Ma. The Variscan eclogite-amphibolite facies metamorphism is instead constrained between ~420 Ma and ~300 Ma. These ages and the geochemical signature of these rocks allow constraining the Early Paleozoic tectono-magmatic evolution of the Ligurian blocks, from a middleeupper Cambrian rifting stage, through the formation of an Early Ordovician volcanic arc during the Rheic Ocean subduction, until a Late Ordovician extension related to the arc collapse and subsequent rifting of the PaleoThetys. Furthermore, the ~420-350 Ma ages from zircon rims testify to thermal perturbations that may be associated with the Silurian rifting-related magmatism, followed by the subduction-collisional phases of the Variscan orogeny.展开更多
The investigated mantle bodies from the External Ligurians(Groppo di Gorro and Mt.Rocchetta)show evidences of a complex evolution determined by an early high temperature metasomatism,due to percolating melts of asthen...The investigated mantle bodies from the External Ligurians(Groppo di Gorro and Mt.Rocchetta)show evidences of a complex evolution determined by an early high temperature metasomatism,due to percolating melts of asthenospheric origin,and a later metasomatism at relatively high temperature by hydrothermal fluids,with formation of rodingites.At Groppo di Gorro,the serpentinization and chloritization processes obliterated totally the pyroxenite protolith,whereas at Mt.Rocchetta relics of peridotite and pyroxenite protoliths were preserved from serpentinization.The rodingite parageneses consist of diopside+vesuvianite+garnet+calcite+chlorite at Groppo di Gorro and garnet+diopside+serpentine±vesuvianite±prehnite±chlorite±pumpellyite at Mt.Rocchetta.Fluid inclusion measurements show that rodingitization occurred at relatively high temperatures(264-334℃ at 500 bar and 300-380℃ at 1 kbar).Garnet,the first phase of rodingite to form,consists of abundant hydrogarnet component at Groppo di Gorro,whereas it is mainly composed of grossular and andradite at Mt.Rocchetta.The last stage of rodingitization is characterized by the vesuvianite formation.Hydrogarnet nucleation requires high Ca and low silica fluids,whereas the formation of vesuvianite does not need CO2-poor fluids.The formation of calcite at Groppo di Gorro points to mildly oxidizing conditions compatible with hydrothermal fluids;the presence of andradite associated with serpentine and magnetite at Mt.Rocchetta suggests Fe^3+-bearing fluids with fO2 slightly higher than iron-magnetite buffer.We propose that the formation of the studied rodingite could be related to different pulses of hydrothermal fluids mainly occurring in an oceancontinent transitional setting and,locally,in an accretionary prism associated with intra-oceanic subduction.展开更多
基金supported by Italian 1:50,000 Geological Mapping (CARGdR egione Liguria Project, University of Pavia grants)
文摘The Early Paleozoic evolution of the northern margin of Gondwana is characterized by several episodes of bimodal magmatism intruded or outpoured within thick sedimentary basins. These processes are well recorded in the Variscan blocks incorporated in the Ligurian Alps because they experienced low temperature Alpine metamorphism. During the Paleozoic, these blocks, together with the other Alpine basements, were placed between the Corsica-Sardinia and the Bohemian Massif along the northern margin of Gondwana. In this framework, they host several a variegated lithostratigraphy forming two main complexes(Complexs I and II) that can be distinguished by both the protoliths and their crosscutting relationships, which indicate that the acidic and mafic intrusives of Complex II cut an already folded sequence made of sediments, basalts and granitoids of Complex I. Both complexes were involved in the Variscan orogenic phases as highlighted by the pervasive eclogite-amphibolite facies schistosity(foliation II). However, rare relicts of a metamorphic foliation at amphibolite facies conditions(foliation I)is locally preserved only in the rocks of Complex I. It is debatable if this schistosity was produced during the early folding event e occurred between the emplacement of Complex I and II e rather than during an early stage of the Variscan metamorphic cycle.New SHRIMP and LA ICP-MS Ue Pb zircon dating integrated with literature data, provide emplacement ages of the several volcanic or intrusive bodies of both complexes. The igneous activity of Complex I is dated between 507 ± 15 Ma and 494 ± 5 Ma, while Complex II between 467 ± 12 Ma and 445.5 ± 12 Ma.The folding event recorded only by the Complex I should therefore have occurred between 494 ± 5 Ma and 467 ± 12 Ma. The Variscan eclogite-amphibolite facies metamorphism is instead constrained between ~420 Ma and ~300 Ma. These ages and the geochemical signature of these rocks allow constraining the Early Paleozoic tectono-magmatic evolution of the Ligurian blocks, from a middleeupper Cambrian rifting stage, through the formation of an Early Ordovician volcanic arc during the Rheic Ocean subduction, until a Late Ordovician extension related to the arc collapse and subsequent rifting of the PaleoThetys. Furthermore, the ~420-350 Ma ages from zircon rims testify to thermal perturbations that may be associated with the Silurian rifting-related magmatism, followed by the subduction-collisional phases of the Variscan orogeny.
基金supported by the University of Parma,Fondi Ricerca Scientifica Locale di Ateneo(Universitádi Parma)and by MIUR-PRIN prot.2015C5LN35funded by the‘Departments of Excellence’program of the Italian Ministry for Education,University and Research(Ministero Istruzione UniversitáRicerca,Italy,2018-8562022)。
文摘The investigated mantle bodies from the External Ligurians(Groppo di Gorro and Mt.Rocchetta)show evidences of a complex evolution determined by an early high temperature metasomatism,due to percolating melts of asthenospheric origin,and a later metasomatism at relatively high temperature by hydrothermal fluids,with formation of rodingites.At Groppo di Gorro,the serpentinization and chloritization processes obliterated totally the pyroxenite protolith,whereas at Mt.Rocchetta relics of peridotite and pyroxenite protoliths were preserved from serpentinization.The rodingite parageneses consist of diopside+vesuvianite+garnet+calcite+chlorite at Groppo di Gorro and garnet+diopside+serpentine±vesuvianite±prehnite±chlorite±pumpellyite at Mt.Rocchetta.Fluid inclusion measurements show that rodingitization occurred at relatively high temperatures(264-334℃ at 500 bar and 300-380℃ at 1 kbar).Garnet,the first phase of rodingite to form,consists of abundant hydrogarnet component at Groppo di Gorro,whereas it is mainly composed of grossular and andradite at Mt.Rocchetta.The last stage of rodingitization is characterized by the vesuvianite formation.Hydrogarnet nucleation requires high Ca and low silica fluids,whereas the formation of vesuvianite does not need CO2-poor fluids.The formation of calcite at Groppo di Gorro points to mildly oxidizing conditions compatible with hydrothermal fluids;the presence of andradite associated with serpentine and magnetite at Mt.Rocchetta suggests Fe^3+-bearing fluids with fO2 slightly higher than iron-magnetite buffer.We propose that the formation of the studied rodingite could be related to different pulses of hydrothermal fluids mainly occurring in an oceancontinent transitional setting and,locally,in an accretionary prism associated with intra-oceanic subduction.