Mineralogical and petrological studies of Triassic Verrucano metasediments of the Northern Apennines are reported. The widespread occurrence of Al‐silicates allows the delineation of four metamorphic zones with increasing metamorphic grade: (1) kaolinite zone (well Perugia 2, Umbria); (2) kaolinite‐pyro‐phyllite zone (Monte Argentario and part of the Verrucano of the Monticiano‐Roccastrada area and Monti Leoni); (3) pyrophyllite zone (Monti Pisani, Iano, Monti Leoni, the Monticiano‐Roccastrada area and some wells in the Larderello region); (4) kyanite zone (Massa area and some wells in the Larderello area).
The four metamorphic zones correspond to temperatures ranging from 300°C to about 450°C. On the basis of the Si content of muscovite and geological arguments, pressures of between 3 and 5 kbar are estimated. The metamorphic zones are located more or less parallel to the bent north‐west‐south‐east trending structural zonation of the Northern Apennines, with the concave side towards the Tyhrrenian Sea.
During the Alpine orogeny, the Verrucano metasediments underwent three folding phases each of which has produced an axial plane schistosity (S1, S2, S3). During the first folding phase the Verrucano sediments were buried increasingly deeply within the crust from east to west. The climax of Alpine metamorphism was attained prior to the second folding phase with crystallization of porphyroblasts of kyanite and chloritoid in a central area located between Massa and Larderello. The inferred paleo‐temperature distribution pattern resembles an asymmetric thermal high defined by the kyanite zone, and surrounded by the pyrophyllite zone. A similar pattern is still present in the Tuscan crust, as indicated by a series of geothermal anomalies passing through the Northern Apennines.
The metamorphic evolution of a granulitized eclogite from Punta de li Tulchi NE Sardinia, Italy,\ud
reconstructed utilizing a combined microstructural (symplectitic, coronitic and kelyphytic features) and\ud
thermodynamic approach, involved a complex metamorphic history with equilibrium attained only at a\ud
domainal scale. Microstructural analysis and mineral zoning allow recognition of reactants and products\ud
involved in successive balanced mineral reactions. The P–T conditions at which each microstructure was\ud
formed are further constrained by calculating isochemical phase diagrams (pseudosections) for the\ud
composition of effectively reacting domains. A pre-symplectite stage developed during prograde\ud
metamorphism under conditions ranging from 660-680°C, 1.6-1.8 GPa to 660-700°C at 1.7-2.1 GPa.\ud
Pseudosections calculated for subsequent clinopyroxene + plagioclase and orthopyroxene +\ud
plagioclase symplectitic coronae using the composition of effectively reacting microdomains temperature in excess of 800°C and pressures of 1.0-1.3 GPa. Modelling the development of later plagioclase + amphibole coronae around garnet during decompression yields conditions of 730-830°C and 0.8-1.1 GPa. H2O (wt%) isomodes indicate that the granulitized eclogites were H2O-undersaturated\ud
at peak-P conditions and during most of the subsequent heating and decompression. This allowed the\ud
preservation of prograde garnet zoning in spite of the strong granulite facies overprint. The P–T\ud
evolution of Punta de li Tulchi granulitized eclogite is very similar in shape to that registered by other\ud
NE Sardinia retrogressed eclogites thus suggesting a common tectonic scenario for their evolution
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.