Structure and PT estimates across late-collisional plutons: constraints on the exhumation of western Alpine continental HP units

Abstract

The ∼30 Ma Biella and Traversella late-collisional intermediate to granitic plutons intruded the innermost part of the Sesia-Lanzo Zone (SLZ) in the western Alps. The SLZ represents an early-Alpine continental high-pressure (HP) metamorphic complex and forms part of the subducted Adria margin along the Periadriatic lineament, the internal boundary of the axial Alpine belt. Intrusions postdate ductile deformation of the country rocks, but predate most of the late-orogenic brittle deformation. These plutons cut across regional ductile foliations that are dominated by eclogite-facies mineral assemblages generated during the Alpine subduction cycle. Structural mapping across the contact aureoles suggests that the late-stage ductile folds coeval with exhumation-related greenschist-facies assemblages are transected by the igneous stocks. Thermobarometry of the igneous and contact metamorphic rocks reveals that both plutons crystallized at a depth between ∼3.5 (shallower part) and ∼7 (deeper part) km at a temperature of ∼700–750°C. Comparative thermobarometry and multi-scale structural analyses across the aureole zones show that shallow crustal decompression-exhumation of the HP SLZ complexes under greenschist-facies conditions predated the late-orogenic intrusions. The pressure and temperature (PT) conditions and ages of the eclogitic metamorphic peak (∼65 Ma) and the Periadriatic intrusives (∼30 Ma) were taken into account to estimate an average pre-intrusive exhumation rate of ∼2 km/million years. The lower post-intrusive exhumation rate of these two plutons (∼0.2 km/million years) with respect to that of the Bergell pluton (∼0.6 km/million years) suggests that, in the internal central Alps, a larger volume of Oligocene intrusive rocks has been eroded than in the internal western Alps, where most of the plutons may still be buried.