Pre- to syn-extension melt-assisted nucleation and growth of extensional gneiss domes: The western French Massif Central (Variscan belt)

Evidence for melt-assisted development of extensional gneiss domes is provided using the combined analysis of geological, geophysical, and geochronological data on the Millevaches dome and the adjacent Argentat fault (AF), in the western Massif Central, France. The Millevaches dome consists of mica-schists associated with monzogranites (344–342 Ma), migmatites (337–334 Ma) and leucogranites (329–318 Ma). These melts were emplaced before, at the onset of, and during the slip of the AF (∼335–315 Ma), respectively. The AF is a 5-km-wide, crustal-scale, westward dipping shear zone along which S3 foliation strikes NNW and L3 lineation trends WNW. Kinematic indicators show uniform top-to-the WNW sense of shear coeval with retrograde low-pressure metamorphism, indicative of a normal-sense movement along the AF. The Millevaches dome is interpreted to be an extensional gneiss dome cored by migmatites and monzogranite derived from a mid-crustal molten layer inherited from collision and modified by addition of syn-extension leucogranites. The dome formed by crustal-scale normal shearing along the AF and coeval exhumation of deep-seated rocks in the footwall. Pre-extension melts have allowed dome nucleation at the onset of extension by localizing normal shearing within the molten layer. Syn-extension melts, given their quantity and emplacement mode within the dome, have enhanced the persistence of dome growth and its ultimate geometry.