Fragmentation of Neoproterozoic magmatic arcs along the northern Gondwana margin in the Late Ediacaran (SW Iberia).

Abstract

SW Iberia includes exposures of late Ediacaran rocks in the Central Iberian Zone-CIZ, and the Ossa-Morena Zone-OMZ. This study presents new LA-ICP-MS U-Pb detrital zircon data from late Ediacaran greywackes of the SW CIZ (Beiras Group, a unit from the Schist-Greywacke Complex). A comparison is made with U-Pb detrital zircon data form late Ediacaran greywackes (Série Negra) and early Cambrian arkosic sandstones of the OMZ in order to: i) characterize potential sedimentary provenances through zircon forming older events, and ii) improve paleogeographic reconstructions in the northern Gondwana active margin. The U-Pb detrital zircon data set for the Beiras Group greywackes (SW CIZ) indicates predominance of Neoproterozoic (75-78%; mainly Cryogenian and Ediacaran, and few Tonian), Paleoproterozoic, Archean (mainly Neoarchean) and few Mesoproterozoic ages. The Beiras Group greywackes present three major episodes of Neoproterozoic zircon crystallization in the source area distributed by the Cryogenian and the Ediacaran at ca. 850-700 Ma (Pan-African suture), ca. 700-635 Ma (early Cadomian arc) and ca. 635-545 Ma (late Cadomian arc). Two peaks stand out in the Cryogenian as the most significant, at ca. 840-750 Ma and at ca. 685-660 Ma. In the OMZ, the detrital zircons content from the early Cambrian arkosic sandstones overlap the ages of detrital zircon from the late Ediacaran greywackes (Linnemann et al., 2008). The U-Pb detrital zircon data set for those late Ediacaran-early Cambrian siliciclastic rocks show predominance of Neoproterozoic ages (mainly Ediacaran and Cryogenian), Paleoproterozoic and Archean. Major episodes of zircon crystallization in the source area are mainly distributed by latemost Cryogenian and Ediacaran (late Cadomian arc): ca. 640–630 Ma, ca. 615–605 Ma, ca. 590–570 Ma and ca. 560-550 Ma, but locally, older Cryogenian ages also occur in the interval ca. 700-635 Ma (early Cadomian arc). Considering the detrital zircon ages of the SW CIZ and the OMZ basins is possible to conclude that: i) Late Ediacaran basins of SW Iberia were filled by detritus resulting from erosion of three sources: the West African craton, the Pan-African belt and the Cadomian belt; ii) The Neoproterozoic age clusters may reflect the existence of a long-lived magmatic arc in the northern margin of Gondwana (Cadomian-Avalonian orogenic belt) (Pereira et al., 2011 and references therein); iii) The early Cadomian arc was the main source of the SW CIZ basin (Beiras Group) with a probable contribution of the Pan-African suture, whereas the OMZ basin (Serie Negra) had a greater contribution of the late Cadomian arc with additional contribution of the early Cadomian arc; iv) SW Iberia Ediacaran basins seem to represent back-arc basins in the northern Gondwana active margin, separated sufficiently to justify the differences in their detrital zircon content: the SW CIZ basin (Beiras Group) located close to the continent-ward passive margin with major contribution of the early Cadomian arc and the Pan-African suture, and the OMZ basin (Serie Negra) in the vicinity of the late Cadomian arc-ward margin characterized by more strongly stretched continental crust (including the early Cadomian arc) and the accumulation of predominantly arc-derived greywacke–mudstone turbidites.