Tectono-metamorphic evolution of the siliciclastic units in the Middle Tuscan Range (inner Northern Apennines): Mg–carpholite bearing quartz veins related to syn-metamorphic syn-orogenic foliation

This paper accounts for a new finding of High Pressure–Low Temperature (HP-LT) metamorphism documented in continental-derived rocks exposed in the central part of the Middle Tuscan Range, representing the easternmost location, in the Tyrrhenian Region, where blueschist facies metamorphism has been documented. It describes the development mechanism of Mg–carpholite bearing quartz veins in the framework of the deformational events described for the inner Northern Apennines. The HP-LT peak metamorphism can be estimated in P ≥ 1.1 GPa and T = 370°–420 °C as supported by the mineralogical assemblage consisting of quartz–muscovite–chloritoid–pyrophylite–hematite ± chlorite ± paragonite ± Ti-oxides, documented in the aluminous metasediments (pelites and quartzose sandstones) of the Triassic Verrucano succession. This paragenesis defines the S1 schistosity, an axial planar tectonic foliation associated to map- and mesoscale isoclinals folds (F1); S1 schistosity is a composite foliation resulted from a non-coaxial progressive deformation associated to the DE1 collisional event (Late Oligocene–Early Miocene). Mg–carpholite bearing quartz veins developed as tension gashes in en-échelon arrays within brittle/ductile shear zones. The development of quartz veins was guaranteed by the high volumes of Si-rich fluids produced by the dynamic recrystallization process on the siliciclastic rocks.

► New finding of exhumed HP-LT metamorphic rocks in the inner Northern Apennines (southern Tuscany).
► Relationships between Mg–carpholite bearing quartz veins and deformational events characterizing the evolution of the inner Northern Apennines belt.
► HP-LT paragenesis for siliciclastic rocks.
► New finding of composite sin-collisional dynamic tectonic foliation (S1) in southern Tuscany.