Heterogeneous brittle-ductile deformation at shallow crustal levels under high thermal conditions: The case of a synkinematic contact aureole in the inner northern Apennines, southeastern Elba Island, Italy

We present an example of interaction between magmatism and tectonics at shallow crustal levels. In the Late
Miocene the metamorphic units of the eastern Elba Island (northern Apennines) were intruded at very shallow
crustal levels by a large pluton (> 60 km2) with the development of an hectometre-sized contact aureole defined
by growth of low-pressure/high-temperature mineral assemblages (Pmax < 0.2 GPa, Tmax ~ 650 °C). Structural
data show that the contact aureole is associated with a km-sized antiform of the foliation and by several metreto
decametre-thick high-strain domains consisting of strongly foliated rocks containing synkinematic HT/LP
mineral assemblages and ductile shear zones of variable thickness. These shear zones are characterized by a
mylonitic foliation variably overprinted by cataclasis. Quartz microfabrics indicate that the dynamic crystallization
processes progressively changed from grain boundary migration, associated with the thermal peak of
contact metamorphism, to subgrain rotation and bulging recrystallization, the latter mostly associated with the
cataclastic overprint. These transitions of recrystallization mechanisms in quartz are related to a progressive
decrease of temperature during deformation. Deformation accompanied the development and cooling of the
contact aureole, which recorded the switch from high temperature ductile to low temperature brittle conditions.
The geometry of the studied deformation structures is consistent with the constraints of the regional tectonic
evolution and its local interaction with the localized and transient thermal anomaly related to the coeval emplacement
of igneous rocks.