Early carboniferous wrenching, exhumation of high-grade metamorphic rocks and basin instability in SW Iberia: Constraints derived from structural geology and U–Pb and 40Ar–39Ar geochronology

Abstract

New U-Pb and 40 Ar geochronology and structural data from high-to medium grade metamorphic shear zones of the Ossa-Morena Zone, and structural data from Early Carboniferous basins (Ossa-Morena Zone and South-Portuguese Zone), place additional constraints on the Variscan tectonics in SW Iberia. A zircon U-Pb age of 465±14 Ma (Middle Ordovician) measured on migmatite from the Coimbra-Cordoba shear zone is interpreted as the age of protolith crystallization. This age determination revises the information contained in the geological map of Portugal, in which these rocks were considered to be Proterozoic in age. This paper describes the evolution of Variscan wrench tectonics related to the development of shear zones, exhumation of deep crustal rocks and emplacement of magma in the Ossa-Morena Zone basement. In the Coimbra-Cordoba shear zone (transpressional), migmatites were rapidly exhumed from a depth of 42.5 km to 16.6 km over a period of ca. 10 Ma in the Viséan (ca. 340-330 Ma), indicating oblique slip exhumation rates of 8.5 to 10.6 mm/yr (Campo Maior migmatites) and 3.2 mm/yr (Ouguela gneisses) respectively. In the Évora Massif, the gneisses of the Boa Fé shear zone (transtensional) were exhumed from 18.5 to 7.4 km depth in the period ca. 344-334 Ma (Viséan), with exhumation oblique slip rates of 2.8 to 4.2 mm/yr. At the same time, the Early Carboniferous basins of SW Iberia were filled by turbidites and olistoliths, composed mostly of Devonian rocks. The presence of olistoliths indicates significant tectonic instability during sedimentation with large-scale mass movement, probably in the form of gravity slides. Deformation and metamorphism dated at 356±12 Ma, 321±13 Ma and 322±29 Ma respectively suggests that Variscan wrench movements were active in SW Iberia during the Early Carboniferous for a period of at least 35 Ma.

The north-subducting Rheic Ocean during the Devonian: consequences for the Rhenohercynian ore sites

“…Similarly, in the Central Iberian-South Portuguese area, zircon age data from detrital zircons (Pereira et al 2012) reveal active wrench tectonics including exhumation and basin instability during the Early Carboniferous, flysch sediments being recognized over large domains. Although the Pyrite belt is not specifically discussed in this strongly juxtaposed assemblage (Pereira et al 2012), the attribution of part of this assemblage to the Rhenohercynian type of evolution at the Laurussian border should not be disregarded (see discussion above).…”

“…Although the Pyrite belt is not specifically discussed in this strongly juxtaposed assemblage (Pereira et al 2012), the attribution of part of this assemblage to the Rhenohercynian type of evolution at the Laurussian border should not be disregarded (see discussion above). Menor-Salván et al (2010) indicate well the complication of the general plate-tectonic situation, where the closure of the Rheic Ocean between the Hanseatic terrane and Ossa Morena-type microcontinents , the supposed detachment of the Rheic slab (von Raumer et al 2015) and the subsequent subduction reversal , would accelerate an orographic evolution, and a strong mobilization of thermal circulation.…”

“…Although strongly transformed during Late Variscan wrenching (Pereira et al 2012), Late Devonian rifting of the Laurussian shelf was recognized in the South Portuguese Zone (Oliveira 1990;Silva et al 1990). But the early findings of Lower Carboniferous trilobites in the South Portuguese Zone (Frischmuth 1968, Oliveira et al 1979, proving the narrow relationship with contemporaneous endemic trilobites of the Harz area in the Rhenohercynian domain, did not yet enter as a valuable argument for paleogeographic considerations: The respective South Portuguese fossil sites should be located in a Devonian reconstruction of the Rhenohercynian Zone (comp.…”

The multistage crystallization of zircon in calc-alkaline granitoids: U–Pb age constraints on the timing of Variscan tectonic activity in SW Iberia

“…The suture of the Rheic Ocean in SW Iberia, resulting from the collision of two continental blocks during the amalgamation of Pangea (Laurussia and Gondwana), is located at the boundary between the Ossa-Morena and South Portuguese zones (Eden 1991;Quesada et al 1994;Díaz Azpiroz et al 2006;Ribeiro et al 2010;Braid et al 2011;Pereira et al 2012a, b). One of the most important discussions surrounding the Variscan orogen of SW Iberia concerns the tectonic setting of Early Carboniferous Abstract CL imaging and U-Th-Pb data for a population of zircons from two of the Évora Massif granitoids (Ossa-Morena Zone, SW Iberia) show that both calcalkaline granitoids have zircon populations dominated by grains with cores and rims either showing or not showing differences in Th/U ratio, and having ages in the range ca.…”

“…Deformation of the ductile basement under high-temperature metamorphic conditions promotes partial melting and the formation of syn-kinematic crustal-derived magmas that interact with magmas of mantle origin. There is reason to argue that as a consequence of the Variscan oblique convergence, SW Iberia developed into a wrenching regime with the significant spread and formation of extensive calc-alkaline magmatism on both sides of the Rheic suture, inferred to have been emplaced after the Rheic Ocean had already closed, whose origin is not well understood (de la Rosa and Castro 2004;Romeo et al 2006;Jesus et al 2007;Pereira et al , 2009Pereira et al , 2012aMoita et al 2009Moita et al , 2013Rosa et al 2008;Braid et al 2010;Cambeses et al 2014;Gladney et al 2014).…”