Field relations, petrography and 40Ar/39Ar cooling ages of hornblende in a part of the eclogite-bearing domain, Sveconorwegian Orogen

High-pressure metamorphism and deformation of the Ullared Deformation Zone (UDZ) in the Eastern segment of the Sveconorwegian Orogen, southwest Sweden is evidenced by ~ 970 Ma retro-eclogite found in association with upper amphibolite and high-pressure granulite facies rocks. Petrographic, structural and geochronological studies of the UDZ were carried out to better understand the regional metamorphism and deformation history with focus on a ~N-S trending part of the zone influenced by a late stage of deformation during Sveconorwegian orogeny, namely the Svarten Zone. The deformational history of the investigated area can be discussed in two scenarios. In the first one, an E-W extension component synchronized with the onset of migmatization was responsible for the development of an old upright and symmetric ESE-WNW generation of folds. The folds were later influenced by an intense phase of shear, which formed the isoclinal folds and the parallel mineral stretching lineation. The old ESE-WNW foliation was deflected by a later phase of NNW-SSE shear, following the isoclinal folds development. The second possible scenario is that a NNW-SSE shear phase has taken place synchronized with E-W extension, following the regional migmatization, resulted in E-plunging mineral stretching lineation and isoclinal folds development, which consequently deflected ESE-WNW foliations. Regardless of which scenario is correct, the sheared limb of a young, open and upright generation of folds indicate that a late phase of shear has taken place in association with ~N-S structure of the Svarten Zone. Mineral assemblages indicate the occurrence of retrograde metamorphism resulted from uplift of the Sveconorwegian Orogen, reflected in the amphibolite mineral assemblages overprinting of the eclogite and the granulite facies. 40Ar/39Ar dating of hornblende yielded cooling ages that range from 920 Ma to 927 Ma. The cooling rates of 5.5-6.4 °C/m.y. and 4.5-5.2 °C/m.y. were obtained from the onset of partial melting and migmatization down to the hornblende cooling age of 920 Ma to 927 Ma, respectively, upon the chosen closure temperatures of 500°C and 550°C. The cooling rates are associated with uplift of the Sveconorwegian Orogen and cooling after peak metamorphism.