M. F. Pereira

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 of upper Triassic sandstone, southwest Iberia (Alentejo and Algarve basins): tracing variability in the sources

Laser ablation ICP-MS U–Pb analyses have been conducted on detrital zircon of Upper Triassic sandstone from the Alentejo and Algarve basins in southwest Iberia. The predominance of Neoproterozoic, Devonian, Paleoproterozoic and Carboniferous detrital zircon ages confirms previous studies that indicate the locus of the sediment source of the late Triassic Alentejo Basin in the pre-Mesozoic basement of the South Portuguese and Ossa-Morena zones. Suitable sources for the Upper Triassic Algarve sandstone are the Upper Devonian–Lower Carboniferous of the South Portuguese Zone (Phyllite–Quartzite and Tercenas formations) and the Meguma Terrane (present-day in Nova Scotia). Spatial variations of the sediment sources of both Upper Triassic basins suggest a more complex history of drainage than previously documented involving other source rocks located outside present-day Iberia. The two Triassic basins were isolated from each other with the detrital transport being controlled by two independent drainage systems. This study is important for the reconstruction of the late Triassic paleogeography in a place where, later, the opening of the Central Atlantic Ocean took place separating Europe from North America.

Strike-slip shear zones of the Iberian Massif: Are they coeval?

Strike-slip shear zones of the Variscan orogen are used to derive the evolution of paleostrain and discuss the kinematics of the waning stages of the Gondwana-Laurussia collision during the amalgamation of Pangea. In the Iberian Massif, the recognition of three late Carboniferous deformation events related to strike-slip tectonics (D3, D4, D5) in the Trancoso-Pinhel region (Portugal) reveals that late orogenic transcurrent deformation was episodic and occurred in a short period of time (<15 m.y.). Early stages of strike-slip deformation included dextral and sinistral shear zones and orogen-parallel upright folds (D3; ca. 311 Ma). These structures followed the development of extensional shear zones (D2) during the tectonothermal reequilibration of the orogen. D3 structures were deflected and folded by the sinistral D4 Juzbado-Penalva do Castelo shear zone, dated as ca. 309–305 Ma by SHRIMP (sensitive high-resolution ion microprobe) U-Pb zircon dating of synkinematic granitoids. D3 and D4 structures were folded under east-west compression (D5) influenced by the strike-slip movement of the dextral Porto-Tomar shear zone. Variscan movement along the Porto-Tomar shear zone started ca. 304 Ma (onset of the Buçaco basin and syn-D5 granites), but ceased before ca. 295 Ma (age of the final closure of the Ibero-Armorican arc and crosscutting granites). The contrasting geometry, kinematics, and timing of these strike-slip shear zones are explained by deformation partitioning upon a rheologically inhomogeneous crust with structural and tectonothermal anisotropies generated during previous deformation. The convergence vector between Gondwana and Laurussia during D3–D5 remained the same, and was equivalent to the vector that explains the previous tectonic record (D2) in central and northwestern Iberia. LITHOSPHERE; v. 9; no. 5; p. 726–744; GSA Data Repository Item 2017250 | Published online 30 June 2017 https://doi.org/10.1130/L648.

Development of a Tourist Route around the Mining Heritage of the Estremoz Anticline

The areas of the counties of Estremoz, Borba and Vila Viçosa, traditionally and since ancient times, have been a major region for extraction of marbles for use as a dimension stone in Portugal. The geological evolution of the Iberian Peninsula allowed the formation, in Alto Alentejo, of one of the World’s most important and famous marble deposits. The Estremoz Anticline, about 42 km long and 8 km in maximum width, is an impressive place where the strength and ingenuity of Man has been used for decades to turn the “land upside down”. The 27 km2 where the marble is concentrated is a place with a high density of quarries that have an unavoidable environmental impact, leaving stone exposed or accumulated in large tips, side-by-side with the Alentejo plains of wheat fields and olive trees. It is impossible to fully rehabilitate this area either for economic or strategic reasons, but it can be considered as resource for the promotion and development of industrial and scientific tourism and artistic and cultural events. A survey of the assets of the region has been undertaken and a wide variety of organizations and the industry are collaborating in planning a route and activities for the region.

Discussion on ‘Detrital zircon geochronology of the Carboniferous Baixo Alentejo Flysch Group (South Portugal); constraints on the provenance and geodynamic evolution of the South Portuguese Zone’, Journal of the Geological Society, 172, 294–308

In their recent study, Rodrigues et al . (2014) attempted a provenance analysis of the Carboniferous Baixo Alentejo Flysch Group (SW Iberia), discussing variability over time in sources of South Portuguese Zone turbidites. The U–Pb ages presented by these authors are similar to those previously obtained by Pereira et al . (2012 a , 2013). A comparison of U–Pb age distributions obtained in turbidites from the two studies using the Kolmogorov–Smirnov test indicates that they are not significantly different at the 5% confidence level, increasing the statistical significance of the detrital zircon populations of each stratigraphic formation. However, the interpretations put forward in the two studies are considerably different. The most controversial of these concerns the sources of the Lower Carboniferous turbidites forming the Baixo Alentejo Flysch Group (Mertola and Mira formations). Rodrigues et al . (2014) argued that the detrital zircon ages of the Mertola and Mira greywackes indicate provenance from an extra-basinal source (Ossa–Morena Zone) with a minor intra-basinal contribution (South Portuguese Zone). Pereira et al . (2012 a , 2013) proposed that Early Carboniferous flysch deposition (Lower Carboniferous turbidites; immature greywackes of the Mertola Formation and the base of the Mira Formation) mainly derived from a source terrane dominated by zircon populations of Middle–Late Devonian ages ( c . 393–359 Ma) that are absent in the Ossa–Morena Zone (Fig. 1). The Middle–Late Devonian zircons (37.7%) found in the Lower Carboniferous turbidites display simple growth patterns attributable to a single magmatic event, and are idiomorphic, with no evidence of long transport and/or multiple recycling (Pereira et al . 2012 a ). The Lower Carboniferous turbidites were less markedly influenced by sedimentary recycling than the Upper Carboniferous turbidites, as indicated by the presence of a percentage of detrital zircons with ages greater than c . 393 Ma (Lower Carboniferous turbidites 32%; Upper Carboniferous turbidites 96%; Fig. 1). These Early Palaeozoic and Precambrian zircon-forming events are characteristic of the Gondwana (Ossa–Morena Zone; Pereira et al . 2008, 2012 b , 2013, and references therein) and/or Laurussia (Meguma–South Portuguese Zone, Braid et al . 2011; Pereira et al . 2012 a ; and Avalonia, Waldron et al . 2011, and references therein) basements.

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.

Deciphering a Multipeak Event in a Noncomplex Set of Detrital Zircon U–Pb Ages

The determination of U–Pb ages from detrital zircons of sedimentary rocks using LA–ICP–MS has been widely used to develop studies of provenance analysis. A problem that frequently arises is to find a population that appears to be noncomplex despite several perceptible age peaks in their spectrum. These peaks are qualitatively defined through diagrams of relative probability (probability density function or PDF), but it is difficult to quantify their statistical significance relative to a zircon-forming multipeak event. Therefore, the question arises as to whether we can decipher and characterize a multipeak event in a noncomplex set of detrital zircon U–Pb ages. This work is an attempt to answer the above question by means of a statistical analysis. The objectives are: (1) to determine the most appropriate minimum number of zircon age populations (peaks); (2) to characterize each peak in terms of age and event duration; and (3) to compare results obtained for two datasets showing similar zircon ages. The process starts by using a cluster analysis to group zircon ages into a set of consistent clusters. A Gaussian kernel function is then fitted to each cluster and summed to obtain a theoretical PDF. At the end of the process, the best modelled PDF must coincide with the original PDF at ≥95 %, and the deciphered peaks can then be characterized.

Chroniberia : the ongoing development of a geochronological GIS database of Iberia

Geochronological comparisons of large datasets are facilitated by the use of structured databases. This paper reports the ongoing development of CHRONIBERIA, a geochronological GIS database that will provide access to zircon U–Pb age datasets from Iberia. The database will also integrate relevant distinct datasets from other correlative regions around the world to construct a framework based on stratigraphic, sedimentological, and palaeogeographical data.