J. B. Silva

Zircon U–Pb geochronology of paragneisses and biotite granites from the SW Iberian Massif (Portugal): evidence for a palaeogeographical link between the Ossa–Morena Ediacaran basins and the West African craton

Abstract Sensitive high-resolution ion microprobe U–Th–Pb age determinations on detrital and inherited zircon from the Évora Massif (SW Iberian Massif, Portugal) provide direct evidence for the provenance of the Ossa–Morena Ediacaran basins (Série Negra) and a palaeogeographical link with the West African craton. Three samples of the Série Negra paragneisses contain large components of Cryogenian and Ediacaran (c. 700–540 Ma) detrital zircon, but have a marked lack of zircon of Mesoproterozoic (c. 1.8–0.9 Ga) age. Older inherited zircons are of Palaeoproterozoic (c. 2.4–1.8 Ga) and Archaean (c. 3.5–2.5 Ga) age. The same age pattern is also found in the Arraiolos biotite granite, which was formed by partial melting of the Série Negra and overlying Cambrian rocks. These results are consistent with substantial denudation of a continental region that supplied sediments to the Ediacaran Ossa–Morena basins during the final stages of the Cadomian–Avalonian orogeny (peri-Gondwanan margin with principal zircon-forming events at c. 575 Ma and c. 615 Ma). Combined with the detrital zircon ages reported for rocks of the same age from Portugal, Spain, Germany and Algeria, our data suggest that the sediment supply to the Ediacaran–Early Palaeozoic siliciclastic sequences preserved in all these peri-Gondwanan regions was similar. The lack of Grenvillian-aged (c. 1.1–0.9 Ga) zircon in the Ossa–Morena and Saxo-Thuringia Ediacaran sediments suggests that the sediment in these peri-Gondwanan basins was derived from the West African craton.

Variability over time in the sources of South Portuguese Zone turbidites: evidence of denudation of different crustal blocks during the assembly of Pangaea

This study combines geochemical and geochronological data in order to decipher the provenance of Carboniferous turbidites from the South Portuguese Zone (SW Iberia). Major and trace elements of 25 samples of graywackes and mudstones from the Mértola (Visean), Mira (Serpukhovian), and Brejeira (Moscovian) Formations were analyzed, and 363 U-Pb ages were obtained on detrital zircons from five samples of graywackes from the Mira and Brejeira Formations using LA-ICPMS. The results indicate that turbiditic sedimentation during the Carboniferous was marked by variability in the sources, involving the denudation of different crustal blocks and a break in synorogenic volcanism. The Visean is characterized by the accumulation of immature turbidites (Mértola Formation and the base of the Mira Formation) inherited from a terrane with intermediate to mafic source rocks. These source rocks were probably formed in relation to Devonian magmatic arcs poorly influenced by sedimentary recycling, as indicated by the almost total absence of pre-Devonian zircons typical of the Gondwana and/or Laurussia basements. The presence of Carboniferous grains in Visean turbidites indicates that volcanism was active at this time. Later, Serpukhovian to Moscovian turbiditic sedimentation (Mira and Brejeira Formations) included sedimentary detritus derived from felsic mature source rocks situated far from active magmatism. The abundance of Precambrian and Paleozoic zircons reveals strong recycling of the Gondwana and/or Laurussia basements. A peri-Gondwanan provenance is indicated by zircon populations with Neoproterozoic (Cadomian-Avalonian and Pan-African zircon-forming events), Paleoproterozoic, and Archean ages. The presence of late Ordovician and Silurian detrital zircons in Brejeira turbidites, which have no correspondence in the Gondwana basement of SW Iberia, indicates Laurussia as their most probable source.

Provenance Analysis of Lower Palaeozoic Siliciclastic Rocks of Southwestern Iberia (Ossa–Morena Zone): Distal Shelf Deposition on the North Gondwana Passive Margin

U–Pb dating of detrital zircons from the lower Palaeozoic siliciclastic rocks of southwestern Iberia (the Ossa–Morena Zone or OMZ: comprising the Fatuquedo, Ossa, Colorada, and Terena formations) shows that sedimentation during the middle–late Cambrian to Early Devonian was marked by slight variations in the source areas, involving the denudation of crustal blocks with similar zircon-forming events typical of North Gondwana and the absence of volcanism younger than ca. 470 Ma. The potential source areas of the middle–upper Cambrian to Lower Devonian sediments of the basins of the OMZ could be the Neoproterozoic basement of the OMZ (the Serie Negra) intruded by Cambrian and Early Ordovician plutonic rocks, and/or the lower Palaeozoic sedimentary sequences of the OMZ with Cambrian and Early Ordovician volcanics. In the oldest siliciclastic rocks, the most relevant populations of detrital zircons have Cryogenian and Ediacaran ages (the Ossa and Fatuquedo formations). Futhermore, in the youngest siliciclastic rocks, in addition to Cryogenian and Ediacaran zircon grains there are relevant clusters of Cambrian and Tonian ages (the Colorada Formation). Cambrian and Ordovician zircon ages found in the Lower Devonian greywackes (the Terena Formation) suggest intense denudation of the OMZ during the Early Devonian. No evidence was found of sources outside North Gondwana. The lack of zircon-forming events younger than ca. 470 Ma seems to indicate that the middle–upper Cambrian to Lower Devonian siliciclastic rocks of southwestern Iberia were deposited on a distal shelf of the North Gondwana passive margin related to the opening of the Rheic Ocean and in the absence of magmatic activity.

Provenance of Cambrian-Ordovician Siliciclastic Rocks of Southwestern Iberia: Insights into the Evolution of the North Gondwana Margin

This study makes a comparison between the populations of detrital zircons of the Cambrian sandstones from the Ossa–Morena Zone (OMZ) and the Ordovician quartzites from the southern domains of the Central Iberian Zone (S-CIZ) to identify the sediment sources during the development of North Gondwana basins (southwestern Iberia). The U–Pb results obtained for the lower Cambrian sandstones of the OMZ show a remarkable similarity to the detrital zircon ages of greywackes from the underlying OMZ Ediacaran basement (the Serie Negra succession). However, there is a greater proportion of Cryogenian grains in the Cambrian rocks, whose main sources are: (1) the late Cadomian magmatic arcs (Ediacaran, ca. 635–545 Ma) which also contributed to filling the late Ediacaran basins of the OMZ; and (2) the early Cadomian arcs (Cryogenian, ca. 700–635 Ma). In the Lower Ordovician quartzites of the S-CIZ (the Armorican and Sarnelha formations), the age distribution of detrital zircons overlaps the population of detrital zircons of the underlying S-CIZ Ediacaran basement (the Beiras Group). However, there are some differences in the Sarnelhas quartzites, which have a population of detrital zircons similar to those of the Ediacaran greywackes and Cambrian sandstones of the OMZ. The Cambrian grains found in the Lower Ordovician quartzites fit the ages of magmatism representing the onset of rifting in North Gondwana that is registered in the OMZ but absent from the S-CIZ. The early Ordovician zircon grains are probably related to the magmatic event that preceded the passive margin stage of the Rheic Ocean, and are found in both the CIZ and OMZ.

Provenance Analysis of the Late Ediacaran Basins from Southwestern Iberia (Série Negra Succession and Beiras Group): Evidence for a Common Neoproterozoic Evolution

This study makes a comparison of detrital-zircon age populations from upper Ediacaran greywackes of the Ossa–Morena Zone (OMZ) with those from the southern domains of the Central Iberian Zone (S-CIZ). The results reveal that the main difference between the age spectra of both populations of detrital zircon is the Neoproterozoic, in particular the Cryogenian grains. Our new data suggest that deposition in the CIZ and OMZ Ediacaran basins was coeval, indicating a long-lived magmatic event typical of the northern Gondwana margin (Avalonian–Cadomian belt and Pan-African belt). Overall, southwestern Iberia shows the following sequence of Cryogenian and Ediacaran zircon-forming events: (1) ca. 850–700 Ma, Pan-African suture (well represented in the Beiras Group and in the Mares Formation of the Serie Negra succession); (2) ca. 700–635 Ma, early Cadomian arc (dominant in the Beiras Group and in the Mares Formation of the Serie Negra succession); and (3) ca. 635–545 Ma, late Cadomian arc (predominant in the Mosteiros and Escoural formations of the Serie Negra succession). The results emphasise that the late Ediacaran basins of southwestern Iberia evolved together in the active margin of North Gondwana in the same palaeogeographical scenario but were sufficiently separated to justify the differences identified in their Neoproterozoic detrital zircon contents. This finding shows that there is no basis for considering that the boundary between the OMZ and the S-CIZ marks a Cadomian suture.

The Significance of Changes of Source Areas During Carboniferous Turbiditic Deposition (Southwestern Iberia)

U–Pb dating of detrital zircons from the Carboniferous turbidites of southwestern Iberia (the Cabrela, Mertola, Mira, and Brejeira formations) shows that synorogenic sedimentation during the Carboniferous was marked by significant variations in the source areas, involving the denudation of different crustal blocks and a break in synorogenic volcanism. The Visean is characterized by the accumulation of immature turbidites (the Cabrela and Mertola formations and the base of the Mira Formation). These turbidites were probably formed in relation to sources (magmatic arcs) of Mid–Late Devonian age poorly influenced by sedimentary recycling, as indicated by the near-absence of pre-Devonian zircons, ages that are typical of the Gondwana basement. The presence of Carboniferous grains in Visean turbidites indicates that volcanism was active at this time. Later, Serpukhovian to Moscovian turbiditic sedimentation (the Mira and Brejeira formations) included sedimentary detritus derived from mature felsic source rocks situated far from active magmatism. The abundance of Proterozoic and Palaeozoic zircons reveals strong recycling of the pre-Carboniferous basement. A peri-Gondwanan provenance is indicated by zircon populations with Neoproterozoic (the Cadomian–Avalonian and Pan-African zircon-forming events), Palaeoproterozoic, and Archean (the West African Craton zircon-forming events) ages. The presence of Late Ordovician and Silurian detrital zircons in the Brejeira turbidites, which do not correspond to the Gondwana basement of southwestern Iberia, indicates an external source (Laurussia?).