Heat Sources for Crustal Anatexis in the Upper-to-Middle Crust: Pressure-Temperature-Time Constraints from the Trois Seigneurs Massif, French Pyrenees

Abstract:

Melting of continental crust is required for its chemical differentiation and yet is energetically intensive, requiring large amounts of exogenic or endogenic heat. The prevailing paradigm is that granulite facies metamorphism, partial melting and attendant differentiation occur in the lower structural levels of thickened, radiogenic crust (e.g. Tibet, McKenzie & Priestley, 2008). Terranes that preserve evidence for partial melting under shallow crustal conditions thus pose a challenge to this paradigm and demand alternative heat sources and transport mechanisms. In this study, we report thermobarometric, petrochronological and thermal modeling results from an ongoing field-based investigation of a classic low-pressure high-temperature (LP-HT) metamorphic terrane: the Variscan age Trois Seigneurs Massif, France. Construction of a series of pseudosections for four Cambro-Ordovician (Wickham, 1987) pelites spanning 4 km structural thickness tightly constrains the metamorphic field gradient. Low-grade albite schists with prominent sedimentary layering (CCTS1927) and quartzites (CCTS1926B) preserve assemblages that are calculated to be stable <4kbar and 400-580 °C. Higher-grade garnet-cordierite-sillimanite schists (CCTS1915A) record conditions of 3.8-4.4 kbar and 590-690 °C. A migmatitic gneiss (CCTS1912) records the onset of muscovite dehydration melting at 3.25-6 kbar and >695 °C. Average P-T analysis is ongoing. Regression of these P-T data yields a field gradient of 48-75 °C/km, implying elevated heat flow at the time of peak conditions (e.g. Wickham & Oxburgh 1987). Preliminary zircon U-Pb dates from biotite granite, contiguous with migmatitic gneisses, constrains the age of crustal anatexis to 306 ± 3.25 Ma. Results from further monazite and zircon U-Pb petrochronology will be used to test whether the peak temperatures recorded by the massif occurred contemporaneously. Combined, these P-T-t data will be used to inform a thermal model for the development of the Trois Seigneurs, constructed to discriminate between different exogenic heat sources such as elevated mantle heat flow and advective emplacement of plutons.