Abstract The recent discovery of Ediacaran ophiolites in the SW Iberian Massif has made it possible to pinpoint the evolution of the Cadomian basement of Europe. The Calzadilla and Mérida ophiolites (gabbroic protoliths dated at c. 600 and 594 Ma, respectively) have geochemical characteristics typical of supra-subduction zone ophiolites. They are interpreted as originating during the initial opening of a forearc basin with boninitic magmatism (Calzadilla), followed by the formation of a back-arc basin with arc-tholeiites (Mérida). Widening of the back-arc led to the rifting and drifting of a section of the active continental margin (Cadomia). Closure of these oceanic domains initiated rapid contraction, culminating in the collision of Cadomia with Gondwana ( c. 590–540 Ma). The application of a PANALESIS model to this palaeogeographic setting confirms the plausibility of Cadomian rifting and the likely opening of broad oceanic domains. It also confirms the final collision of Cadomia with Gondwana, although the synthetic and regional data disagree in the precise chronology of the convergence and collision of Cadomia with the West Africa Craton. This work shows that the evolution of the Cadomian basement is much more complex than traditionally considered.

Abstract The Variscan Orogen in Iberia and the Anti-Atlas Mountains in Morocco contains a set of ophiolites formed between Neoproterozoic and Devonian times, during the complex evolution of the NW African–Iberian margin of Gondwana. During this time interval, the margin evolved from an active margin ( c. 750–500 Ma: the Reguibat–Avalonian–Cadomian arc) to the final collision with Laurussia in Devonian times to form Pangaea. In this context, one of the oldest recognized ophiolites is the Bou Azzer Ophiolite from the Anti-Atlas Mountains, dated at c. 697 Ma and containing two types of mafic rocks, the youngest of which has a boninitic composition. To the north, in the SW Iberian Massif, the Calzadilla Ophiolite contains mafic rocks also of boninitic composition dated at c. 598 Ma. Farther north, in the NW Iberian Massif, the Vila de Cruces Ophiolite is formed by a thick sequence of mafic rocks with an arc tholeiitic composition and minor alternations of tonalitic orthogneisses dated at c. 497 Ma. In the same region, the Bazar Ophiolite has a similar age of c. 495 Ma. Also in NW Iberia, there is a group of ophiolites with varied lithologies and dominant mafic rocks with arc tholeiitic composition (Careón, Purrido and Moeche ophiolites) dated at c. 395 Ma. The composition of all these peri-Gondwanan ophiolites is of supra-subduction zone type, showing no evidence of preserved mid-ocean ridge basalt type oceanic lithosphere. Consequently, these ophiolites were generated in the peri-Gondwanan realm during the opening of forearc or back-arc basins. Forearc oceanic lithosphere was promptly obducted or accreted to the volcanic arc, but the oceanic or transitional lithosphere generated in back-arc settings was preserved until the assembly of Pangaea. Based on the ages of the described ophiolites, the peri-Gondwanan realm has been a domain where the generation of oceanic or transitional lithosphere seems to have occurred at intervals of c. 100 myr. These regularly spaced time intervals may indicate cyclic events of mantle upwelling in the peri-Gondwanan mid-ocean ridges, with associated higher subduction rates at the peri-Gondwanan trenches and concomitant higher rates of partial melting in the mantle wedges involved. The origin of the apparent cyclicity for mantle upwelling in the peri-Gondwanan ocean ridges is unclear, but it could have possibly been related to episodic deep mantle convection. Cycles of more active deep mantle convection can explain episodic mantle upwelling, the transition from low- to fast-spreading type mid-ocean ridges and, finally, the dynamic context for the episodic generation of new supra-subduction zone type oceanic peri-Gondwanan lithosphere.

Abstract EPMA and LA-ICP-MS trace-element maps have been acquired from amphibolitized eclogites from the Diego de Almagro Metamorphic Complex (Chile). Several garnet growth pulses and garnet resorption stages are revealed by major elements chemical zoning and by heterogeneous Y and rare earth element (REE) behaviour, associated with subduction and exhumation of these rocks. Distribution of REE in prograde garnet is texturally and chemically coupled with the breakdown of REE-bearing minerals while formation of epidote and titanite generations during amphibolitization is recorded by complex textures involving new garnet generation and overprinting phases. The latest overprint stage is characterized by fine-grained intergrowth between garnet and epidote micro-veins, phengite, hornblende, albite and titanite. Garnet cracks have been gradually re-equilibrated during this event witnessing short-scale dissolution–transport–precipitation. Pseudosection modelling shows that local variability in water content during amphibolitization controls garnet stability at the expense of epidote. Overprinting microstructures are explained by the effect of locally-derived aqueous fluids that trigger the ‘unlocking’ of elements from the reacting eclogite-facies paragenesis. These findings highlight the microscopic characteristics of amphibolitization processes documented in exhumed eclogite-facies terranes and shed light on the importance of thorough micro-chemical investigations while undertaking pressure–temperature (PT) estimates on rocks with strong textural disequilibrium.