serie NOVA TERRA nº 49

109 Geochemistry of the Ediacaran – Early Cambrian transition in Central Iberia: Tectonic setting and isotopic sources José Manuel Fuenlabrada a , ⁎ , Agustín P. Pieren b , Rubén Díez Fernández c , Sonia Sánchez Martínez c , Ricardo Arenas c a CAI de Geocronología y Geoquímica Isotópica, Universidad Complutense, 28040 Madrid, Spain b Departamento de Estratigrafía, Universidad Complutense, 28040 Madrid, Spain c Departamento de Petrología y Geoquímica e Instituto de Geociencias (UCM, CSIC), Universidad Complutense, 28040 Madrid, Spain a b s t r a c t a r t i c l e i n f o Article history: Received 4 August 2015 Received in revised form 12 November 2015 Accepted 18 November 2015 Available online 26 November 2015 A complete Ediacaran – Early Cambrian stratigraphic transition can beobserved in the southern part of the Central Iberian Zone (Iberian Massif). Two different stratigraphic units, underlying Ordovician series, display geochemi- cal and Sm – Nd isotopic features in agreement with an evolving geodynamic setting. Pusa Shales (Early Cambri- an) rest unconformably on greywackes of the Lower Alcudian Formation (Late Ediacaran). Both sequences present minor compositional variations for major and trace element contents and similar REE patterns, close to those ofPAAS (Post ArcheanAustralianShale). Trace element contents and element ratios suggest mixed sources, with intermediate to felsic igneous contributions for both units. Tectonic setting discrimination diagrams for the Ediacaran greywackes indicate that these turbiditic series were deposited in a sedimentary basin associated with a mature active margin (volcanic arc). However, the compositions of the Cambrian shales fi t better with a more stable context, a back-arc or retro-arc setting. ε Nd (T) and TDM ages are compatible with dominance of a similar cratonic source for both sequences, probably the West Africa Craton. ε Nd 565 values for the Ediacaran greywackes ( − 3.0 to − 1.4)along with TDMages(1256 – 1334Ma) implya signi fi cantcontribution ofjuvenilematerial,prob- ably derived from the erosion of the volcanic arc. However, ε Nd 530 values in the Cambrian shales ( − 5.2 to − 4.0) together with older TDM ages (1444 – 1657 Ma), suggest a higher contribution of cratonic isotopic sources, prob- ably derived from erosion of the adjacent mainland. Coeval with the progressive cessation of arc volcanism along the peri-Gondwanan realm during the Cambrian, there was a period of more tectonic stability and increasing arrival of sediments from cratonic areas. The geochemistry of the Ediacaran – Cambrian transition in Central Iberia documents a tectonic switch in the periphery of Gondwana, from an active margin to a more stable context related to the onset of a passive margin. © 2015 Elsevier B.V. All rights reserved. Keywords: Ediacaran-Cambrian transition Whole-rock geochemistry Sedimentary rocks Sm-Nd isotope geochemistry Iberian Massif 1. Introduction Geochemical composition and isotopic characteristics of fi ne- grained clastic rocks have proved to be a reliable record of continental crust evolution, and therefore are a useful indicator of the origin and tectonic setting in which sedimentation took place ( Condie, 1993; McLennan et al., 1990; Taylor and McLennan, 1985 ). The relative ab- sence of REE fractionation during depositional processes has allowed the use of Sm – Nd data to discriminate between sources, providing in- formation about the paleogeography of sedimentary basins as indicated by the juvenile or more evolved character of their sediment in fi ll ( Murphy and Nance, 2002 ). Over the last few years, we have focused our work on using whole- rock geochemistry and Sm – Nd isotopic features of metasedimentary series in the Iberian Massif as a potential mean of shedding light on the dynamic evolution of the peri-Gondwanan arc system and to re- construct the paleogeography of this domain during Ediacaran – Early Paleozoic times. Geochemical features and Nd model ages of metasedimentary rocks from the allochthonous complexes of NW Iberia postulated an arc-derived provenance for the metagreywackes exposed in the Órdenes Complex (Upper Units) ( Albert et al., 2015; Fuenlabrada et al., 2010 ), whereas older TDM ages obtained from metagreywackes and metapelites of the Basal Units pointed to a back- arc or retro-arc basin located in the interior of the continental domain ( Díez Fernández et al., 2010, 2012; Fuenlabrada et al., 2012 ). Ediacaran – Early Cambrian rocks crop out extensively in the Central Iberian Zone (Iberian Massif). These sequences are affected by low- grade metamorphism and exhibit a gradual and complete transition be- tween both periods. The Central Iberian Zone (CIZ) has been a source of discussion on the provenance and tectonic settings of its Neoproterozoic Tectonophysics 681 (2016) 15 – 30 ⁎ Corresponding author at: CAI de Geocronología y Geoquímica Isotópica Facultad de Ciencias Geológicas Universidad Complutense de Madrid Calle José Antonio Novais, 12 28040, Madrid España. E-mail address: jmfuenla@ucm.es (J.M. Fuenlabrada). Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto http://dx.doi.org/10.1016/j.tecto.2015.11.013 0040-1951/© 2015 Elsevier B.V. All rights reserved.