Original paper

Age constraints for the tectonometamorphic evolution of the Pulo do Lobo Zone in SW Spain

Giese, Uwe; Hoymann, Karl-Heinz; Glodny, Johannes; Kramm, Ulrich; Dallmeyer, R. David

Kurzfassung

Kombinierte geologische, strukturelle und radiometrische Untersuchungen erlauben eine detaillierte Rekonstruktion der tektonometamorphen Entwicklung der Pulo do Lobo-Zone in SW-Spanien. Strukturen der ersten variszischen Deformationsphase sind nahezu vollständig überprägt worden und nur als Relikte in Mikrolithons der D2-Phase erhalten geblieben. Da oberdevonische Flyschsedimente nicht von der D1-Phase erfaßt worden sind, muß die tektonische Akkretion bereits im Oberdevon abgeschlossen gewesen sein. Während der Unterkarbons wurde die Pulo do Lobo-Zone zunehmend zwischen Südportugiesischer Zone und Ossa-Morena-Zone eingeengt und deformiert. Während dieser transpressiven Phase erfolgten die zweite und dritte variszische Deformationsphase, die synkinematische Intrusion basischer und saurer Schmelzen und eine temperaturbetonte Metamorphose. Auf Grund der engen Assoziation zu einer weitverbreiteten Kontaktmetamorphose wird angenommen, daß die Pulo do Lobo-Zone weiträumig von synkinematischen Intrusionen unterlagert wird. Rb/Sr- und Ar/Ar-Datierungen der synkinematischen Intrusionen ergeben Abkühlalter zwischen 337 Ma und 328 Ma. Danach erfolgte die variszische Transpressionsphase im späten Visé. Mit Beginn des Oberkarbons war die duktile variszische Deformation abgeschlossen und sinistrale Blattverschiebungen erfolgten final auf den bereits angelegten Störungen.

Abstract

Combined geological, structural, petrological and radiometric investigations provide constraints for the polyphase Variscan tectonothermal evolution of the Pulo de Lobo Zone in SW Spain. First phase deformation structures are intensively overprinted and only preserved in microlithons of the second Variscan phase. As Late Devonian overstep sequences are undeformed by the first Variscan deformation, this phase can be dated around the Frasnian/Famennian boundary. It argues for an early accretion of the Pulo do Lobo Zone to the Ossa-Morena Zone which was terminated in the late Devonian. During the Early Carboniferous the Pulo do Lobo Zone was successively deformed and sandwiched inbetween the Ossa-Morena Zone and the South Portuguese Zone. Oblique convergence resulted in a prominent phase of transpression. This stage comprises the second and third Variscan deformation phase, both with SSW directed thrusting and left-lateral displacement. Synkinematic intrusions started to ascend contemporaneously. Their final emplacement occured between the second and third deformation phase, cooling of the intrusions outlasted the third Variscan phase. The regional metamorphism of the Pulo do Lobo Zone reaches in the basal units upper greenschist to lower amphibolite facies conditions. The metamorphism is connected with the second Variscan deformation phase. As it is closely associated with a wide-spread contact metamorphism, the Spanish part of the Pulo do Lobo Zone appears widely underlain by synkinematic intrusions. The timing of transpression in the Pulo do Lobo Zone can be constrained by 40Ar/39Ar and 87Rb/86Sr isotope dates of synkinematic intrusions which range between 337 Ma and 328 Ma. They are interpreted as cooling ages and can be related to microstructural observations in the contact aureole of the intrusives. According to this Variscan transpression. low-grade metamorphism and magmatism occured almost contemporaneously in the late Visean and was terminated by the end of the Early Carboniferous. The Variscan deformation of the Pulo do Lobo Zone ended with left-lateral movements along previously generated thrusts during the late Carboniferous.

Keywords

Variszidesmobil belttectonometamorphismHercynian Orogenypolyphase processestranspression intrusionsigneous rockscontact metamorphismmetamorphic faciesdatingRb/SrAr/Arcooling agesViseanUpper Carboniferousleft-lateral movements