40Ar/ 39Ar mineral age constraints for the tectonothermal evolution of a Variscan suture in southwest Iberia

Abstract

Dallmeyer, R.D., Fonseca, P.E., Quesada, C. and Ribeiro, A., 1993. 40Ar/ 39Ar mineral age constraints for the tectonothermal evolution of a Variscan suture in southwest Iberia. Tectonophysics, 222: 177–194. Mafic units interpreted to represent an internally disrupted ophiolite succession are exposed along a Variscan suture situated between the South Portuguese and Ossa-Morena zones in southern Iberia. Structural characteristics of various units adjacent to the suture zone suggest maintainence of an oblique (transpressional), sinistral tectonic regime during late Paleozoic plate convergence. This involved initial, limited, north-directed subduction beneath the Ossa-Morena zone, and was followed by oblique collision of the South Portuguese Terrane. Metamorphic evolution of rocks adjacent to the suture occurred during maintainence of a moderately high geothermal regime. Five samples of amphibolite were collected within the hanging wall of the suture. These included samples from the ophiolitic terrane and from the Ossa-Morena zone. Two samples were also collected within the post-kinematic Beja Gabbro which intruded the other two tectonic units following their structural juxtaposition and penetrative ductile deformation. Hornblende concentrates from these seven samples record well-defined 40Ar/ 39Ar plateau ages which range between c. 342 and 335 Ma. These results imply that the regional tectonostratigraphic units were maintained at moderate crustal levels until the Visean, and that their juxtaposition and internal ductile deformation occurred in the late Devonian. Subsequent cooling was probably effected by transpressional uplift during oblique collision following closure of intervening oceanic elements.