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 In the Variscan Orogen, the NW Iberian Massif exposes a variety of ophiolites formed at c. 500 and c. 400 Ma that provide constraints on the Paleozoic evolution of the NNW African margin of Gondwana which culminated in the assembly of Pangaea. New U–Pb ages obtained in zircon from one of these ophiolites, the Vila de Cruces Ophiolite, confirm the previous U–Pb age of c. 500 Ma (thermal ionization mass spectrometry (TIMS)) obtained in zircon from a tonalitic orthogneiss. New samples of the same orthogneiss and related gabbros provided ages (laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS)) in the range 498–492 Ma, with the metagabbros being the youngest. Moreover, two different gabbro samples contain a scattered population of inherited zircon grains with an average age of c. 1150 Ma. These zircons probably represent xenolitic material included in the gabbros during their ascent and intrusion along the margin of Gondwana. Taking into account the architecture of the Vila de Cruces Ophiolite and its geochemical composition, this ophiolite is interpreted as a section of oceanic or transitional lithosphere formed in a back-arc basin opened in the peri-Gondwanan realm. The presence of inherited Mesoproterozoic zircons is likely to suggest the existence of a hidden unexposed Mesoproterozoic basement in the Gondwana margin during Paleozoic times.

Abstract The Buddusò Pluton in NE Sardinia (Italy) is a normally zoned intrusion composed of three units with chemical composition ranging from hornblende-bearing tonalites (SiO 2 ∼ 65 wt%) to leucocratic monzogranites (SiO 2 ∼ 76 wt%). Zircon crystals in the pluton are dated at 292.2 ± 0.7 Ma and have ε Hf values ranging from −4 to −8, with no systematic differences observed between the units. The pluton, which is isotopically homogeneous at the whole-rock scale in terms of Sr and Nd isotopes, shows textural evidence indicating local crystal–melt segregation. In this paper, we have implemented a novel approach based on path-dependent phase-equilibria modelling to test the hypothesis that the internal chemical variability of the pluton was generated by crystallization differentiation of a homogeneous parental magma. Our modelling indicates that this hypothesis is valid if the mechanism by which this occurs is compaction in a rheologically locked crystal-rich magma and if the separation occurs at 0.3 GPa from a tonalitic magma with water content >2 wt%. Finally, a subset of the magmatic enclaves in the pluton are considered to be autoliths, formed by the disruption of the compacted crystal mush and interaction between these cumulates and the felsic melt.

Abstract Convergent continental margins are the primary host of both growth and loss of continental crust. Continental growth largely occurs via subduction-driven magmatism, whereas continental loss largely occurs via subduction erosion and sediment subduction. Because the latter typically involves partial recycling into magmas, both growth and loss of continental crust can be represented in the magmatic record. The degree of crustal recycling can be estimated from the initial Hf isotope signatures in both magmatic and detrital zircon grains. Recent insights into the geodynamic evolution of the Peruvian margin, in combination with a new dataset of Hf isotopic data on zircon from the Carboniferous to Early Cretaceous, enable us to (1) compare the geodynamic history of the southern Peruvian margin with its Hf isotopic evolution, and (2) quantify the crustal growth between 500 and 135 Ma. The data exhibit a correlation with trends in isotope composition v. time and reflect the dominantly extensional regime that prevailed from the onset of subduction from 530 Ma to c. 135 Ma. This study demonstrates that the Peruvian margin experienced continental growth with juvenile input to arc magmatism of 30–45% on average, and illustrates the use of U–Pb and Hf isotopes in zircon as a tool to trace episodes of crustal growth through time. Supplementary material: Hf istopic analyses on zircon (A1 and A2) and new U–Pb zircon ages (A3) are available at http://www.geolsoc.org.uk/SUP18661.

Abstract Metamorphic and igneous rocks exposed in NW-vergent thrust sheets and their autocthonous basement in the NE Pontides were dated by the U–Pb method using zircons, supported by geochemical data for granitic rocks. Two meta-sedimentary units (Narlık schist and Karadağ paragneiss) yielded detrital zircon populations of 0.50–0.65 and 0.9–1.1 Ga, suggesting an affinity with NE Africa (part of Gondwana). The youngest concordant zircon age is Ediacaran for the schist but Devonian for the paragneiss, bracketing the paragneiss depositional age as Mid-Devonian to Early Carboniferous. Metamorphic rims of zircon cores in the paragneiss gave Carboniferous ages (345–310 Ma). The zircon rim data indicate two Variscan metamorphic events (334 and 314 Ma) separated by a hiatus (320–325 Ma). Granite emplacement took place during early Carboniferous, Early Jurassic and Late Jurassic phases. The crystallization age of the early Carboniferous granites ( c. 325 Ma) corresponds to a hiatus in the zircon age data that could reflect subduction slab break-off. The Variscan granitic rocks intruded a Gondwana-derived continental terrane that was loosely accreted to Eurasia during early–late Carboniferous time but remained isolated from Eurasian-derived terrigenous sediment. In contrast, the Jurassic granitic magmatism relates to later back-arc extension along the southern margin of Eurasia. Supplementary material: Full isotope data (8 tables) are available at http://www.geolsoc.org.uk/SUP18558

Abstract Detrital zircons of eight sandstone samples from the Triassic–Early Jurassic Section Peak Formation (Victoria Group, Beacon Supergroup) in northern Victoria Land, Antarctica, were investigated by U–Pb LA–ICPMS dating. The basin was flanked by the East Antarctic craton, and by a magmatic arc at the palaeo-Pacific margin of Gondwana. It accommodated sandstones ranging from quartzo-feldspathic to volcaniclastic in composition. The detrital zircon age spectra yield pronounced concentrations at c. 190–250 Ma, 500–700 Ma and 800–1200 Ma. The proportion of Triassic–Early Jurassic zircons increases from base to top of the formation, and correlates positively with the abundance of detrital volcanic rock fragments. The youngest zircon ages are close to the stratigraphic age of each sample, indicating contemporaneous magmatic activity along the active margin of Gondwana. Igneous rocks that formed during the Ross Orogeny ( c. 470–545 Ma) were a minor source only, suggesting that the Ross Orogen became progressively covered by sediments as the basin expanded. Pan-African ( c. 500–700 Ma) and Grenville ( c. 800–1200 Ma) age zircons may have been derived from crustal sources currently covered beneath the polar ice sheet, although recycling from Cambro-Ordovician units provides an alternative explanation. Supplementary material: The exact results of the age analyses of all samples are presented in Tables A1 to A8 at http://www.geolsoc.org.uk/SUP18624

Abstract The Guelb Moghrein Fe oxide–Cu–Au–Co deposit is located at the western boundary of the West African craton in NW Mauritania. The wall rocks to the mineralization represent a meta-volcanosedimentary succession typical of Archaean greenstone belts. Two types of meta-volcanic rocks are distinguished: (1) volcanoclastic rocks of rhyodacite–dacite composition (Sainte Barbe volcanic unit), which form the stratigraphic base; (2) tholeiitic andesites–basalts (Akjoujt meta-basalt unit). The trace element signature of both types is characteristic of a volcanic arc setting. A small meta-pelitic division belongs to the Sainte Barbe volcanic unit. A meta-carbonate body, which contains the mineralization, forms a tectonic lens in the Akjoujt meta-basalt unit. It can be defined by the high X Mg (=36) of Fe–Mg carbonate, the REE pattern and the δ 13 C values of −18 to −17‰ as a marine precipitate similar to Archaean banded iron formation (BIF). Additionally, small slices of Fe–Mg clinoamphibole–chlorite schist in the meta-carbonate show characteristics of marine shale. This assemblage, therefore, does not represent an alteration product, but represents an iron formation unit deposited on a continental shelf, which probably belongs to the Lembeitih Formation. The hydrothermal mineralization at 2492 Ma was contemporaneous with regional D 2 thrusting of the Sainte Barbe volcanic unit and imbrications of the meta-carbonate in the upper greenschist facies. This resulted in the formation of an ore breccia in the meta-carbonate, which is enriched in Fe, Ni, Co, Cu, Bi, Mo, As and Au. Massive sulphide ore breccia contains up to 20 wt% Cu. The ore fluid was aqueous–carbonic in nature and either changed its composition from a Mg-rich oxidizing to an Fe-rich reducing fluid or the two fluid types mixed at the trap site. All lithologies at Guelb Moghrein were deformed by D 3 thrusting to the east in the lower greenschist facies. The mobility of REE in the retrogressed rocks explains the formation of a second generation of hydrothermal monazite, which was dated at c . 1742 Ma. Archaean rocks of the West African craton extend to the west to Guelb Moghrein. The active continental margin was deformed and mineralized in the Late Archaean–Early Proterozoic and again reactivated in the Mid-Proterozoic and Westphalian, showing that the western boundary of the craton was reactivated several times.

Abstract The Cadomian Orogeny comprises a series of complex sedimentary, magmatic and tectonometamorphic events that spanned the period from the mid-Neoproterozoic ( c . 750 Ma) to the earliest Cambrian ( c . 540-530 Ma) along the periphery of the super-continent Gondwana (peri-Gondwana, Fig. 3.1 ). Modern data demonstrate broad continuity between Cadomian events and the later opening of the Rheic Ocean during Cambrian-Ordovician times ( Linnemann et al. 2007 ). Due to very similar contemporaneous orogenic processes in the Avalonian microcontinent, the collective terms ‘Avalonian-Cadomian’ Orogeny and ‘Avalonian-Cadomian’ Active Margin have often been used in the modern literature (e.g. Nance & Murphy 1994 ; Fig. 3.1 ). Rock units formed during the Cadomian Orogeny are commonly referred to collectively as ‘Cadomian Basement’. Peri-Gondwanan terranes, microcontinents and crustal units in Central, Western, Southern and Eastern Europe, in the Appalachians (eastern USA and Atlantic Canada), and in North Africa were affected by the Cadomian Orogeny. This orogenic event is also apparently present in Baltica because of the 'Cadomian affinity' of late Precambrian orogenic events in the Urals and in the Timanides on the margin of Baltica ( Roberts & Siedlecka 2002 ). The Cadomian Orogeny sensu stricto was first defined in the North Armorican Massif in France on the basis of the unconformity that separates deformed Precambrian rock units from their Early Palaeozoic (Cambro-Ordovician) overstep sequence (see below). This unconformity is commonly referred to as the ‘Cadomian unconformity’ (Fig. 3.2 ). However, it cannot be precluded that the youngest metasedimentary rocks affected by

Sediment provenances and magmatic events of Late Neoproterozoic (Ediacaran) and Cambro-Ordovician rock complexes from the Saxo-Thuringian zone are constrained by new laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb dating of detrital zircons from five sandstones and magmatic zircons from an ignimbrite and one tuffite. These geochronological results in combination with the analysis of the plate-tectonic setting constrained from field observations, sedimentological and geochemical data, and trends of the basin development are used to reconstruct Cadomian orogenic processes during the Late Neoproterozoic and the earliest Cambrian. A continuum between Cadomian orogenesis and the opening of the Rheic Ocean in the Cambro-Ordovician is supported by the data set. In our model, the early stage of the Cadomian evolution is characterized by a Cordilleran-type continental magmatic arc, which was established at the periphery of the West African craton between ca. 650 and 600 Ma. Subsequently, at ca. 590–560 Ma, a back-arc basin was formed behind the Cadomian magmatic arc. The back-arc basin was closed between ca. 545 and 540 Ma, leading to the development of a short-lived Cadomian retroarc basin. Subsequently, a mid-oceanic ridge was subducted underneath the Cadomian orogen. Slab break-off of the subducted oceanic plate resulted in increased heat flow, leading to voluminous magmatic and anatectic events that culminated at ca. 540 Ma. Oblique incision of the oceanic ridge into the continent caused the formation of rift basins during the Lower to Middle Cambrian. This process continued from the Middle to Upper Cambrian, finally caused the opening of the Rheic Ocean in the Lower Ordovician.