TIME SCALE OF GNEISS DOME FORMATION AND DUCTILE FLOW: THE MONT-LOUIS AND AX-LES-THERMES PLUTONS AND RELATED GNEISS DOMES IN THE FRENCH PYRENEES

The Variscan segment of the Pyrenees is characterized by an important plutonism and by gneiss domes. A new geochronological and structural study was performed both on the orthogneissic/granitic cores of the Aston and Mont-Louis domes (eastern Pyrenees) and their related calc-alkaline plutons which intruded the upper crust. The aim of this study is to characterize the mechanism and timing of gneiss dome formation.

The Aston gneiss dome shows four main events. (i) D1 deformation appears only as relics in the orthogneisses located above the sillimanite isograd; it shows a NS non coaxial stretch associated to top to-the-south motions, attributed to a NS convergence. (ii) D2-a deformation appears in the orthogneisses and their country-rocks located below the sillimanite isograd, where the D1 structures are transposed, and in the peraluminous granites whatever their structural level; this deformation shows an EW to N120 stretch associated to a top-to-the-east flat shearing attributed to lateral flow in the hot middle crust in a transpressive regime. (iii) D2-b deformation is characterized by EW-trending megafolds with horizontal axes in the middle crust; during this event, calc-alkaline plutons emplaced in this upper crust.

The Mont-Louis dome is largely overlapped by the Mont-Louis pluton. Structural study shows that emplacement of the various facies of the pluton is coeval with the formation of the dome.

In situ ion probe U-Pb dating on zircons demonstrates that the orthogneisses belonging to the dome cores correspond to former Ordovician (469 ± 3 Ma) granitic laccoliths subsequently deformed during the Variscan orogeny. In situ LA-ICPMS U-Pb dating on zircons from the Variscan peraluminous granites of the Ax-les-Thermes pluton located in the core of the Aston gneiss dome and affected by the D2-a deformation gave an age of emplacement at 306.2 ± 2.3 Ma. LA-ICPMS U-Pb dating on zircons from three facies of the Variscan calc-alkaline Mont-Louis pluton gave an age of emplacement at 301.0 ± 2.1 Ma, 302.4 ± 2.9 Ma and 303.3 ± 1.1 Ma.

These new data show that the Variscan formations of the Pyrenees recorded the evolution in transpressive regime from a stage of ductile flow of the middle crust which occuring around 306 Ma to a stage of large scale folding (buckling of the upper crust and dome formation) which occurred between 303 and 301 Ma.