EXHUMATION OF HIGH-PRESSURE METAMORPHIC ROCKS IN GNEISS DOMES

Gneiss/migmatite domes commonly contain rocks that record higher-pressures than those recorded by the majority of the dome rocks. Common examples are kyanite-bearing rocks included within cordierite gneiss, and eclogite within quartzofeldspathic gneiss and migmatite characterized by amphibolite-facies conditions. Even in cases in which the high-pressure (HP) metamorphism occurred during an earlier/different orogeny, dome formation is an important mechanism by which high-pressure rocks are exhumed. 2D numerical models for the formation of gneiss domes under extension predict that channels of partially molten crust converge in the deep crust beneath a zone of localized extension, driving ascent of hot, low-viscosity crust and creating a “double dome”. The Montagne Noire gneiss dome of the southern Massif Central (France) has a double-dome structure comprised of two subdomes separated by a high-strain zone. Eclogite occurs in the proposed zone of maximum exhumation between the two subdomes. In this zone, rare metabasaltic eclogite occurs as lenses in migmatite and quartzofeldspathic gneiss. Even more rare are fresh eclogites that have not been extensively converted to amphibole-plag symplectite. Our data are from a relatively fresh eclogite that contains cpx with 36% jadeite and garnet (core) composition alm43 sps2 prp32 grs24; a previous study of retrogressed eclogite reported a max jadeite content of 28% in cpx and garnet composition of prp22 grs26 (Demange 1985). Garnet is zoned; from core to rim, Mg increases and Ca decreases, indicating an increase in P and/or T during garnet growth. P-T conditions calculated from grt-cpx thermometry and a phase diagram for metabasalt are P > 1.4 GPa at T > 650 C, indicating that dome-forming processes may have sampled rocks from at least 40 km depth in the zone of maximum exhumation. New and recent published geochronology data for MN eclogite confirm that eclogite metamorphism was related to the same orogenic events that formed the migmatite dome.