Pressure gradient and original orientation of a lower-crustal intrusion in the Ivrea-Verbano Zone, northern Italy

The southern Ivrea-Verbano Zone of the western Italian Alps contains a huge complex of mafic to intermediate plutonic rocks that intruded the lower continental crust during the Permian. Recent geologic mapping of the complex has characterized the effects of Alpine deformation and has demonstrated that the complex contains an arcuate internal structure. Building on this geologic foundation, we report a thermobarometric study of the plutonic and associated metamorphic rocks that was performed to better constrain the depth and orientation of the complex at the time of intrusion. The results demonstrate a continuous increase in equilibration pressure from 5 +/- 1 kb along the eastern intrusive contact of the complex to 8 +/- 1 kb near the western limit of the complex. After correcting for the effects of Alpine faulting, the observed pressure gradient ranges from 0.32 to 0.38 kb/km. Given the large uncertainties inherent in the geobarometric calculations relative to the narrow pressure range recorded in the complex, we conclude that the observed gradient is indistinguishable from a normal, vertical pressure gradient in the lower crust. It appears that, following intrusion and equilibration at a depth of 15 to 25 km, the complex was uplifted and rotated approximately 90 degrees. The data also demonstrate that the internal arcuate structure formed in the complex before the observed pressure gradient was established. This result reinforces models for the growth of the complex by synmagmatic deformation and large-scale necking during crustal extension and excludes the possibility that the arcuate structure was produced during the more recent Alpine uplift.