Upper crustal record of migmatites exhumation: the South Armorican Domain

Abstract

The South Armorican Massif hosts a high-grade metamorphic domain mainly composed of medium to high-grade micaschists, migmatitic gneisses and anatectic granites [1]. At the end of the Carboniferous, these deep crustal units were exhumed rapidly during the extension associated with the collapse of the Hercynian chain [2]. To the North, this domain is limited by the listhospheric-scale South Armorican Shear Zone (SASZ). Giant quartz veins are associated with the SASZ and recorded important synmetamorphic fluid circulation [3]. Together with very low δ18O values for some euhedral quartz, down to -2‰, low-salinity fluid inclusions argue for a contribution from meteoric fluids [3]. Corresponding δ18Ofluid values estimated around -11‰ are probably related to the high palaeoelevation of meteoric precipitation. Scarce, but significant, CO2 fluid inclusions in euhedral quartz indicate also a metamorphic contribution. Metamorphic fluids were probably sourced from the exhumed metamorphic basement in the southern part of the Massif. Also, because of the synchronicity between the metamorphic event (exhumation) and the meteoric infiltration, it is proposed that the heat advected towards the surface by the exhumation of high-grade metamorphic rocks provided the driving force for meteoric fluid circulation on a regional scale. The meteoric infiltration is recorded regionally by the mylonites which actually define the SASZ and by the syn-kinematic granites which emplaced along the SASZ. Low δ18O values have been measured on some feldspar and zircon grains in the formers [4] while oxygen isotope disequilibirum was recorded by Qz-Fds pairs in the latters [5]. The muscovite Ar-Ar and monazite U-Th-Pb chronometers from these lithologies were highly disturbed [4,6]. In the Questembert granite, a classical example of a syn-kinematic granite, pervasive infiltration of oxydative meteoric water was facilitated by the penetrative character of the deformation (C/S planes are observed throughout the massif) and was probably responsible for the leaching of millions of tons of uranium while the granite was still at depth