Mesozoic emplacement and subsequent deformation of the Cuaro sills and dykes (Uruguay) from anisotropy of magnetis susceptibiliy
Résumé
The Cuaró doleritic sills and dykes correspond to a part of the plumbing system of the southern Paraná large igneous province. They emplaced at about 130 Ma once the southern Atlantic Ocean was opened. The dykes trend from N70 to N120 on aerial photographs and are mostly vertical within Permian and Jurassic sediments. They are 5 to 20 m thick. The sills are almost horizontal and about few tens of meters thick. A preliminary Anisotropy of Magnetic Susceptibility (AMS) study on 452 oriented specimens from 13 sites (8 sills and 5 dykes) shows that the bulk magnetic susceptibility ranges between 0.379x10-4 and 1142.45x10-4 SI with a degree of anisotropy varying between 0.1 to 25%. The shape parameters within the dykes present both oblate and prolate shapes, whereas they are dominantly oblate within most of the sills. Consistent with geological observations, the minimum axes of the AMS tensor (K3), estimated from bootstrap statistics on each sampling site in the sills, are sub-vertical and almost perpendicular to the intrusive contacts where they were observed. All the sills record a magnetic fabric indicating vertical flattening (K1~K2>K3; K1 and K2: maximum and intermediate AMS tensor axes estimated from bootstrap statistics, respectively). Only few of them record an additional magnetic fabric which possibly corresponds to a late deformation event. In contrast, the dykes often show superimposed fabrics in which K1>K2>K3. Some are consistent with magmatic flow directions within the dykes and indicate that the flows were oblique to the vertical and not strictly horizontal. Others hardly correspond to magnetic fabrics induced by magmatic flows and suggest that the dykes were deformed after their emplacement and subsequent cooling and/or that some late fluid circulations have induced magnetic mineral recrystallizations. The origins of these secondary magnetic fabrics are discussed in the light of both geological observations and rock magnetic analyses.