RAPID VARISCAN EXHUMATION AND THE ROLE OF MAGMA IN-CORE COMPLEX-FORMATION - SOUTHERN BRITTANY METAMORPHIC BELT, FRANCE

The high-grade migmatitic core to the southern Brittany metamorphic be lt has mineralogical and textural features that suggest high-temperatu re decompression. The chronology of this decompression and subsequent cooling history have been constrained with Ar-40/Ar-39 ages determined for multigrain concentrates of hornblende and muscovite prepared from amphibolite and late-orogenic granite sheets within the migmatitic co re, and from amphibolite of the structurally overlying unit. Three hor nblende concentrates yield plateau isotope correlation ages of c. 303- 298 Ma. Two muscovite concentrates record well-defined plateau ages of c. 306-305 Ma. These ages are geologically significant and date the l ast cooling through temperatures required for intracrystalline retenti on of radiogenic argon. The concordancy of the hornblende and muscovit e ages suggest rapid post-metamorphic cooling. Extant geochronology an d the new Ar-40/Ar-39 data suggest a minimum time-integrated average c ooling rate between c. 725 degrees C and c. 125 degrees C of c. 14+/-4 degrees C Ma(-1), although below 600 degrees C the data permit an inf initely fast rate of cooling. Mineral assemblages and reaction texture s in diatexite migmatites suggest c. 4 kbar decompression at 800-750 d egrees C. This must have pre-dated the rapid cooling. Emplacement of t wo-mica granites into the metamorphic belt occurred between 345 and 30 0 Ma. The youngest plutons were emplaced synkinematically along shallo w-dipping normal faults interpreted to be reactivated Eo-Variscan thru sts. A penetrative, west-plunging stretching lineation developed in th ese granites suggests that extension was orogen-parallel. Extension wa s probably related to regional uplift and gravitational collapse of th ermally weakened crust during constrictional (escape) tectonics in thi s narrow part of the Variscan orogen. This followed slab breakoff duri ng the terminal stages of convergence between Gondwana and Laurasia; d etachment may have been consequent upon a change in kinematics leading to dextral displacement within the orogen. Dextral ductile strike-sli p displacement was concentrated in granites emplaced synkinematically along the South Armorican Shear Zone. Rapid cooling is interpreted to have resulted from tectonic unroofing with emplacement of granite alon g decollement surfaces. The high-grade migmatitic core of the southern Brittany metamorphic belt represents a type of metamorphic core compl ex formed during orogen-parallel extensional unroofing and regional-sc ale ductile flow.