The Cadomian Orogeny and the opening of the Rheic Ocean: The diacrony of

Cadomian orogenic processes and their continuum to the opening of the Rheic Ocean were modeled by making use of new LA-ICP-MS U–Pb ages from detrital zircons of sedimentary rocks of Late Neoproterozoic (Ediacaran) and Cambro-Ordovician sediments of the Ossa-Morena Zone (Iberian Massif) compared with those from the Saxo-Thuringian Zones (Bohemian Massif). Presented data constrain a diachrony of Cadomian and related geotectonic processes along the northern realm of the Gondwana Supercontinent. Early stage of Cadomian evolution is characterized by a continental magmatic arc at the periphery of the West African Craton and a related back-arc basin opened at c. 590 to 570 Ma. Diacronic arc–continent collision was caused by oblique vector of subduction and started first in the East of Peri-Gondwana at c. 560–570 Ma and resulted at c. 543 Ma in the formation of a short-lived Cadomian retro-arc basin in the Saxo-Thuringian Zone. In contrast, more to the West in the Ossa-Morena Zone, the Cadomian back-arc basin was longer active, at least until c. 545 Ma. In that region, final magmatic pulse of the Cadomian magmatic arc at c. 550 Ma is documented by new zircon data. Closure of the Cadomian back-arc basin and arc–continent collision in the Ossa-Morena Zone occurred between c. 545 Ma and the overall onset of Cambrian plutonism at c. 540 Ma. A mid-oceanic ridge was subducted underneath the Cadomian orogen accompanied by slab break-off of the subducted oceanic plate. Oblique incision of the oceanic ridge into the continent caused the formation of rift basins during the Lower to Middle Cambrian (c. 530–500 Ma). This process continued and finally caused the opening of the Rheic Ocean documented by thick Lower Ordovician siliciclastic sediments and a final magmatic event at c. 490–485 Ma. Opening of the Cambrian rift basin and of the Rheic Ocean again was diachronic and started from the West of Peri-Gondwana and expanded eastward

Inherited arc signature in Ediacaran and Early Cambrian basins of

Geochemical data from clastic rocks of the Ossa-Morena Zone (Iberian Massif) show that the main source for the Ediacaran and the Early Cambrian sediments was a recycled Cadomian magmatic arc along the northern Gondwana margin. The geodynamic scenario for this segment of the Avalonian-Cadomian active margin is considered in terms of three main stages: (1) The 570–540 Ma evolution of an active continental margin evolving oblique collision with accretion of oceanic crust, a continental magmatic arc and the development of related marginal basins; (2) the Ediacaran–Early Cambrian transition (540–520 Ma) coeval with important orogenic magmatism and the formation of transtensional basins with detritus derived from remnants of the magmatic arc; and (3) Gondwana fragmentation with the formation of Early Cambrian (520–510 Ma) shallow-water platforms in transtensional grabens accompanied by rift-related magmatism. These processes are comparable to similar Cadomian successions in other regions of Gondwanan Europe and Northwest Africa. Ediacaran and Early Cambrian basins preserved in the Ossa-Morena Zone (Portugal and Spain), the North Armorican Cadomian Belt (France), the Saxo-Thuringian Zone (Germany), the Western Meseta and the Western High-Atlas (Morocco) share a similar geotectonic evolution, probably situated in the same paleogeographic West African peri-Gondwanan region of the Avalonian-Cadomian active margin

New U-Pb zircon dating of Late Neoproterozoic magmatism in Western Meseta (Morocco)

We present new U-Pb zircon ages from magmatic rocks of the Western Meseta, part of the Moroccan Variscan belt. The Neoproterozoic–Cambrian stratigraphy in the region of Goäida (Aguelmous massif, SE of Moroccan Central Massif) consists of limestones with conglomeratic and felsic volcaniclastic levels, pelites and mafic volcanic rocks assigned to the Cambrian which unconformably overlie rhyolites, andesites and rhyodacites and felsic tuffs associated with the Aguelmous granite of probable Neoproterozoic age. The Neoproterozoic–Cambrian stratigraphy of the region of Sidi Ali is roughly similar and also includes a volcanic-sedimentary complex with limestones, arkoses rhyolites and conglomerates. These conglomerates contain pebbles of granite and rhyolite, whose source may be the Neoproterozoic basement. In order to constrain the age of the Precambrian felsic magmatism we sampled and dated zircons (LA-ICPMS) from the Goäida granite and a pebble of granite included in the conglomeratic levels of the volcanicsedimentary complex at Sidi Ali dome (central Rehamna massif). In the Goäida granite, zircon ages are Ediacaran ranging between ~ 610 Ma and ~ 540 Ma, with a discordia upper intercept age of 598±32 Ma (MSWD=0.04) that could be interpreted as the age of intrusion. However, if we consider only the two youngest ages we obtain a Concordia age of 590±3 Ma (MSWD= 0.34). In the Sidi Ali pebble sample the majority of zircon ages are Cryogenian–Ediacaran in the range ~ 640–600 Ma, with the youngest yielding a Concordia age of 609±2 Ma (MSWD=0.04), indicating the intrusion age of the granite from which the pebble derived. These results support the existence of Cadomian/Pan-African magmatism in the Western Meseta of Morocco, characterized by the intrusion of granites at ~ 609 Ma and ~ 590 Ma. This result is important for studies of sedimentary provenance and to improve paleogeographic reconstructions of the northern margin of Gondwana during the Neoproterozoic

. U-Pb detrital zircon ages from the Beiras Group: Implications for the Neoproterozoic evolution of the SW Iberia

U-Pb detrital zircon ages from the Beiras Group greywackes (SW Central Iberian Zone - CIZ) indicate a maximum depositional age of late Ediacaran (c. 560-578 Ma). Two salient features distinguish the Beiras Group from the Série Negra greywackes (age equivalent from the Ossa-Morena Zone - OMZ): i) The presence of Tonian and Mesoproterozoic (<8%) age clusters in the Beiras Group greywackes, that are almost absent in the OMZ, imply either a distinct or an additional source of detrital zircons from the West African Craton; and 2) The higher content of Cryogenian zircon ages of the Beiras Group greywackes (mainly at c. 840-750 Ma and c. 685-660 Ma), that contrast with the dominant Ediacaran zircon ages of the Série Negra greywackes (OMZ). The Cryogenian zircon forming events that are dominant in the SW CIZ basins are probably related to a different source with early Cadomian juvenile crust (c. 700-635 Ma) and with a possible contribution of the Pan-African suture (c. 850-700 Ma). The Nd isotopic signatures support the addition of a juvenile source to pre-existent older crust for the Beiras Group metasediments. Although the Beiras Group (SW CIZ) and Serie Negra (OMZ) late Ediacaran basins have evolved together in the active margin of Gondwana, they were sufficiently separated to account for the differences in their detrital zircon content and isotopic signatures

The provenance of Late Ediacaran and Early Ordovician siliciclastic rocks in the

U–Pb geochronology of detrital zircon from Late Ediacaran (Beiras Group greywackes) and Early Ordovician (Sarnelhas arkosic quartzites and Armorican quartzites of Penacova) sedimentary rocks of the southwest Central Iberian Zone (SW CIZ) constrain the evolution of northern Gondwana active-passive margin transition. The LA-ICP-MS U–Pb data set (375 detrital zircons with 90–110% concordant ages) is dominated by Neoproterozoic ages (75% for the greywakes and 60% for the quartzites), among which the main age cluster (more significant for Beiras Group greywackes) is Cryogenian (c.840–750 Ma), while a few Mesoproterozoic and Tonian ages are also present (percentages <8%). These two features, and the predominance of Cryogenian ages over Ediacaran ages, distinguish the Beiras Group greywackes (SW CIZ) from the time-equivalent Serie Negra (Ossa-Morena Zone – OMZ), with which they are in inferred contact. The age spectra of the Beiras Group greywackes also reveal three major episodes of zircon crystallisation in the source area during the Neoproterozoic that are probably associated with a long-lived system of magmatism that developed either along or in the vicinity of the northern Gondwana margin at: (1) c. 850–700 Ma – Pan-African suture (not well represented in OMZ); (2) c. 700–635 Ma – early Cadomian arc; and (3) c. 635–545 Ma – late Cadomian arc. Comparison of Neoproterozoic ages and those of the Paleoproterozoic (c. 2–1.8 Ga) and Archean (mainly Neoarchean – 2.8–2.6 Ga, but also older) in the Beiras Group greywackes with U–Pb ages of Cadomian correlatives shows that: (1) SW CIZ, OMZ, Saxo- Thuringian Zone, North Armorican Cadomian Belt and Anti-Atlas) evolved together during the formation of back-arc basins on the northern Gondwana active margin and (2) all recorded synorogenic basins that were filled during the Ediacaran by detritus resulting from erosion of the West African craton, the Pan- African suture and a long-lived Cadomian magmatic arc. Differences in detrital zircon age populations in the greywackes of the Beiras Group (SW CIZ Cadomian basement) and the Serie Negra (OMZ Cadomian basement) are also observed in their respective overlying Early Ordovician quartzites. Since both these SW Iberia Cadomian basements evolved together along the active margin of Gondwana (but sufficiently separated to account for the differences in their detrital zircon content), this continuation of differing zircon populations into the Early Ordovician suggests that the inferred contact presently juxtaposing the Beiras Group and the Serie Negra is not pre-Early Ordovician and so is unlikely to demonstrate a Cadomian suture

Provenance analysis of the Paleozoic sequences of the northern Gondwana margin in NW Iberia: Passive margin to Variscan collision and orocline development

The Cantabrian Zone of NW Iberia preserves a voluminous, almost continuous, sedimentary sequence that ranges from Neoproterozoic to Early Permian in age. Its tectonic setting is controversial and recent hypotheses include (i) passive margin deposition along the northern margin of Gondwana or (ii) an active continental margin or (iii) a drifting ribbon continent. In this paper we present detrital zircon U–Pb laser ablation age data from 13 samples taken in detrital rocks from the Cantabrian Zone sequence ranging from Early Silurian to Early Permian in depositional age. The obtained results, together with previously published detrital zircon ages from Ediacaran– Ordovician strata, allow a comprehensive analysis of changing provenance through time. Collectively, these data indicate that this portion of Iberia was part of the passive margin of Gondwana at least from Ordovician to Late Devonian times. Zircon populations in all samples show strong similarities with the Sahara Craton and with zircons found in Libya, suggesting that NW Iberia occupied a paleoposition close to those regions of present-day northern Africa during this time interval. Changes in provenance in the Late Devonian are attributed to the onset of the collision between Gondwana and Laurussia. Additionally, the Middle Carboniferous to Permian samples record populations consistent with the recycling of older sedimentary sequences and exhumation of the igneous rocks formed before and during the Variscan orogeny. Late-Devonian to Permian samples yield zircon populations that reflect topographic changes produced during the Variscan orogeny and development of the lithospheric scale oroclinal buckling

Evidence formulti-cycle sedimentation and provenance constraints from

Laser ablation ICP-MS U–Pb analyses were conducted on detrital zircons of Triassic sandstone and conglomerate from the Lusitanian basin in order to: i) document the age spectra of detrital zircon; ii) compare U–Pb detrital zircon ages with previous published data obtained from Upper Carboniferous, Ordovician, Cambrian and Ediacaran sedimentary rocks of the pre-Mesozoic basement of western Iberia; iii) discuss potential sources; and iv) test the hypothesis of sedimentary recycling. U–Pb dating of zircons established a maximum depositional age for this deposit as Permian (ca. 296Ma),which is about sixty million years older compared to the fossil content recognized in previous studies (Upper Triassic). The distribution of detrital zircon ages obtained points to common source areas: the Ossa–Morena and Central Iberian zones that outcrop in and close to the Porto–Tomar fault zone. The high degree of immaturity and evidence of little transport of the Triassic sediment suggests that granite may constitute primary crystalline sources. The Carboniferous age of ca. 330 Ma for the best estimate of crystallization for a granite pebble in a Triassic conglomerate and the Permian–Carboniferous ages (ca. 315Ma) found in detrital zircons provide evidence of the denudation of Variscan and Cimmerian granites during the infilling of continental rift basins in western Iberia. The zircon age spectra found in Triassic strata are also the result of recycling from the Upper Carboniferous Buçaco basin,which probably acted as an intermediate sediment repository.U–Pb data in this study suggest that the detritus from the Triassic sandstone and conglomerate of the Lusitanian basin is derived fromlocal source areas with features typical of Gondwana,with no sediment from external sources from Laurussia or southwestern Iberia

Sedimentary record of the amalgamation and break-up of Pangaea: U-Pb detrital zircon geochronology and provenance of Carboniferous-Triassic siliciclastic rocks (SW Iberia)

In SW Iberia there is a basal unconformity where Triassic sandstones overlie previously deformed Carboniferous turbidites. This important stratigraphic boundary records the transition from the final stages of Pangaea’s amalgamation to the initial stages of break-up. U-Pb dating of detrital zircon from Carboniferous greywackes of the South Portuguese Zone and Triassic sandstones of the Algarve and Alentejo basins was carried out to conduct a study of sedimentary provenance. The radiometric ages obtained by LA-ICP-MS show that most of the samples have common age spectra which are typical of North Gondwana sources: Archean and Paleoproterozoic ages characteristic of the West African craton, Neoproterozoic ages associated with Cadomian and Pan-African orogenies, and Cambrian ages related to the intra-continental rifting of North Gondwana. The only exception is a Visean immature greywacke that was probably derived from Devonian magmatic arcs related to the Variscan orogeny. Very rare pre-Devonian zircons indicate faint contributions from recycled sediments. In contrast the Serpukhovian to Moscovian greywackes are derived from felsic mature source rocks and include Proterozoic and Paleozoic detrital zircons suggesting recycling of an old basement. They also include Ordovician and Silurian detrital zircons, the ages of which have no correspondence in North Gondwana, and thus indicate an external source (Laurussia?). U-Pb ages younger than Ordovician are significant in the Triassic sandstone of the Alentejo basin and resemble the zircon populations of the Serpukhovian to Moscovian greywackes from the South Portuguese Zone. U-Pb ages younger than Neoproterozoic are poorly represented in the zircon population of the Triassic sandstone from the Algarve basin, which rests unconformably upon Moscovian turbidites, pointing to Upper Devonian quartzites of the South Portuguese Zone as probable sources, since they are dominated by Precambrian detrital zircons. These differences occurring in the Triassic basins suggest that detrital zircon populations could be derived from two independent sources and paleo-drainage systems due to complex crustal-block extension architecture