Heat sources for Variscan high temperature-low pressure metamorphism: constraints from a petrochronological investigation of the Trois Seigneurs Massif, French Pyrenees

The thermal budget of metamorphic terranes with evidence for kilometric-scale partial melting in the shallow crust (< 15 km depth) cannot be solely explained by conductive relaxation of thickened crust. Such high temperature-low pressure (HT-LP) metamorphism demands a prodigious heat supply to overcome the cooling effect of heat loss from the Earth’s surface. In this study, we present results from a systematic monazite and zircon petrochronological investigation of a classic HT-LP terrane: the Variscan-aged Trois Seigneurs Massif, French Pyrenees.

The massif is composed of a progressive metamorphic sequence from chlorite-bearing phyllites to sillimanite-bearing migmatites, culminating in an S-type granitoid body that occupies over one-third of the massif’s area. Phase equilibrium modelling refines established pressure-temperature (PT) conditions of melting and granite formation to 4-6 kbar and >685 °C. Monazite from five metapelitic samples spanning the structural thickness of the massif records an extended period of metamorphism from 330-290 Ma, with only the low-grade andalusite schists recording a significant population of U/Th-Pb dates older than 310 Ma. Higher-grade schists and migmatites preserve dates from 310-295 Ma, constraining the duration of peak metamorphism, which overlaps zircon U-Pb dates obtained from the S-type granitoid (305.1 ± 1.9 Ma). Peak metamorphic conditions and granitoid emplacement dates at the Trois Seigneurs massif overlap with other published dates for HT-LP metamorphism and granitoid emplacement across the entire Variscan Pyrenees. Combining these PT estimates with those derived from proximal Variscan Pyrenean massifs defines a composite ‘dogleg’ geotherm with elevated dT/dz through the shallow crust (>50 °C/km, <12 km) but near-isothermal conditions through the mid-crust (12-25 km).

A simple thermal model is used to show that this ‘dogleg’ thermal structure can be attained in <15 Myr by advection of magmatic heat between the lower and shallow crust. For such a mechanism to operate on orogenic length scales, however, requires a critical combination of: i) a fertile lower crust buffering the deep crust at the wet solidus, ii) attenuated mantle lithosphere during the waning stages of orogenesis, and ii) significant focusing of melt through the crustal column. We speculate that melt-driven HT-LP metamorphism should be present in other orogenic belts where these conditions are met.