Ediacaran–Cambrian paleogeography and geodynamic setting of the Central Iberian Zone: Constraints from coupled U–Pb–Hf isotopes of detrital zircons

Abstract

The combination of U–Pb geochronology and Hf isotopes in detrital zircons represents a valuable tool in the characterization of sediment sources and paleographic reconstructions. This analytical methodology has been applied to seven Neoproterozoic to Lower Cambrian metasedimentary rocks from the northern and southern domains of the Central Iberian Zone (CIZ), which represent part of the Cadomian basement in the Iberian Massif. The samples collected in the southern domain (belonging to the Schist–Greywacke Complex) yield Ediacaran maximum depositional ages (582–550Ma), whereas samples from northern areas range from 588 to 536Ma, respectively in the Talavera and the Guadarrama sectors. A relatively large fraction of zircon grains gave anomalously young ages (∼500–320Ma), probably due to post-depositional lead loss induced by thermal overprints. Zircon U–Pb age distribution provide similar broad age ranges in all cases: abundant Ediacaran–Late Cryogenian zircons (540–700Ma), and less abundant ages in the following ranges: 700–850Ma, 880–1100Ma, 1900–2250Ma and 2400–2800Ma. However, the southern CIZ samples display an important juvenile input at ca. 1000–1075Ma, 800Ma and in the range 700–570Ma, which contrasts with the mostly recycled nature of the northern CIZ zircons. The Ediacaran juvenile signatures can be derived from the nearby Cadomian arc, as sedimentation likely occurred in a back-arc setting, but Early Cryogenian and Stenian juvenile peaks are more characteristic of basement rocks from NE Africa, so the CIZ was likely situated near the Saharan Metacraton and the Arabian–Nubian Shield during Late Ediacaran. On the other hand, the predominance of evolved components in the younger northern CIZ sediments implies important geodynamic modifications in northern Gondwana at the Ediacaran–Cambrian boundary. The differences between southern and northern CIZ metasedimentary rocks can be interpreted as a transition from a back-arc scenario to development of new basins after the arc collision against the main continent. In this context, the arc signature would decline and the influence of hinterland more mature regions would increase.