COEVAL CONTRACTION, EXTENSION, AND MELT CRYSTALLIZATION IN THE MONTAGNE NOIRE DOUBLE DOME, VARISCAN FRENCH MASSIF CENTRAL

Exhumation of the high-grade core of the Variscan Montagne Noire (French Massif Central) has been attributed to diapirism and compressional, strike-slip, and extensional tectonics. This discrepancy of interpretations stems from the complex structural relations between the core of the E-W elongate dome (the Axial Zone), which consists of augen gneiss and migmatite, and the overlying units made of nappes of Cambrian to Devonian strata that were thrust southward; flysch deposits indicate that thrusting ended ~320 Ma. The eastern termination of the dome is interpreted as a top-to-the-E extensional detachment zone that developed around 300 Ma and exhumed the dome as a metamorphic core complex. The first-order structure of the well-exposed eastern part of the Axial Zone is a double dome (Espinouse and Caroux subdomes) separated by a synform (Rosis syncline), and all rocks contain an E-W subhorizontal lineation parallel to fold axes. The Caroux dome consists dominantly of stretched augen gneiss, and the Espinouse dome is made of migmatitic augen gneiss grading locally into diatexite and leucogranite (the Vialais granite). Abundant sillimanite-quartz nodules in the diatexite are stretched E-W, parallel to the regional lineation; leucogranite sheets filled tension gashes in response to E-W stretching of the dome. Dating the Vialais granite provides a link between the generation and crystallization of melt and the structural development of the Montagne Noire dome. Previous dating of the Vialais granite at 327 Ma (Matte et al., 1998) directed the interpretation of migmatization and E-W stretching to the period of shortening and nappe emplacement. New U-Pb zircon and monazite geochronological data, however, determine the age of Vialais crystallization around 300 Ma. Therefore, E-W stretching in the Axial Zone, formation of the double dome, leucogranite crystallization, and east-directed detachment shearing occurred together around 300 Ma. Our preferred model of dome formation reconciles the contractional structures at depth and extensional structures in the upper crust by invoking convergent flow of low-viscosity crust, forming the double dome and other contractional structures, in response to upper crustal thinning and graben development, as indicated by Permo-Carboniferous sedimentary and volcanic deposits.