Exhumation dynamics of high-pressure metamorphic rocks from the Voltri Unit, Western Alps: constraints from phengite Rb–Sr geochronology

The Voltri Unit and adjacent Tertiary Piedmont Basin, Western Alps, preserve complementary bedrock and sedimentary archives of Alpine subduction and exhumation. Combined PT modeling and phengite Rb–Sr geochronology of bedrock and clast samples shows that Fe–Ti metagabbros and metasediments preserve a record of protracted high-pressure metamorphism, between ~ 50 and ~ 40 Ma. Bedrock and clast Fe–Ti metagabbros yield similar eclogite-facies peak conditions, between 23 and 25 kbar, 510–530 °C; phengite, zoned in celadonite content, with cores > 3.5 Si c.p.f.u. and rims < 3.3 Si c.p.f.u., constrains the timing of exhumation between 24 and 20 kbar to 45–49 Ma. A single impure quartzite bedrock sample records peak-P conditions of 18–19 kbar, 450–470 °C; partial phengite equilibration in this sample occurred continuously between 19 and 12 kbar, between ~ 45 and ~ 40 Ma. Exhumation-related recrystallization of high-pressure phengite to low-Si phengite in two metasedimentary samples occurred between ~ 33 and ~ 30 Ma, contemporaneous with the onset of deposition in the Tertiary Piedmont Basin and consistent with previous ⁴⁰Ar–³⁹Ar constraints on the timing of greenschist metamorphism. Combined with existing ages from the ophiolite, these data show that peak subduction-related high-pressure conditions, between ~ 18 and ~ 25 kbar, were attained at different times across the Voltri Unit, between ~ 50 and ~ 40 Ma, implying that the Voltri Unit comprises an assembly of discrete lithotectonic units that were juxtaposed prior to erosion and deposition in the Tertiary Piedmont molasse basin. The PTt data reported here support a model in which individual sheets of high-pressure material were detached from the downgoing plate, partially exhumed from peak pressures to blueschist facies conditions, while subduction continued, and were stored for > 10 Myr until subduction ceased on arrival of the European continent into the orogenic wedge. As shear tractions on the plate interface are considered to exceed available buoyancy forces for exhumation, we suggest that syn-subduction exhumation occurred along the wedge-plate interface.