M. Chichorro

Variscan intra-orogenic extensional tectonics in the Ossa-Morena Zone (Évora-Aracena-Lora del Rı́o metamorphic belt, SW Iberian Massif): SHRIMP zircon U-Th-Pb geochronology

Abstract Following a Middle–Late Devonian (c. 390–360 Ma) phase of crustal shortening and mountain building, continental extension and onset of high-medium-grade metamorphic terrains occurred in the SW Iberian Massif during the Visean (c. 345–326 Ma). The Évora–Aracena–Lora del Rı́o metamorphic belt extends along the Ossa–Morena Zone southern margin from south Portugal through the south of Spain, a distance of 250 km. This major structural domain is characterized by local development of high-temperature–low-pressure metamorphism (c. 345–335 Ma) that reached high amphibolite to granulite facies. These high-medium-grade metamorphic terrains consist of strongly sheared Ediacaran and Cambrian–early Ordovician (c. 600–480 Ma) protoliths. The dominant structure is a widespread steeply-dipping foliation with a gently-plunging stretching lineation generally oriented parallel to the fold axes. Despite of the wrench nature of this collisional orogen, kinematic indicators of left-lateral shearing are locally compatible with an oblique component of extension. These extensional transcurrent movements associated with pervasive mylonitic foliation (c. 345–335 Ma) explain the exhumation of scarce occurrences of eclogites (c. 370 Ma). Mafic-intermediate plutonic and hypabyssal rocks (c. 355–320 Ma), mainly I-type high-K calc-alkaline diorites, tonalites, granodiorites, gabbros and peraluminous biotite granites, are associated with these metamorphic terrains. Volcanic rocks of the same chemical composition and age are preserved in Tournaisian–Visean (c. 350–335 Ma) marine basins dominated by detrital sequences with local development of syn-sedimentary gravitational collapse structures. This study, supported by new U–Pb zircon dating, demonstrates the importance of intra-orogenic transtension in the Gondwana margin during the Early Carboniferous when the Rheic ocean between Laurussia and Gondwana closed, forming the Appalachian and Variscan mountains.

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.

Zircon geochronology of intrusive rocks from Cap de Creus, Eastern Pyrenees

New petrological and U–Pb zircon geochronological information has been obtained from intrusive plutonic rocks and migmatites from the Cap de Creus massif (Eastern Pyrenees) in order to constrain the timing of the thermal and tectonic evolution of this northeasternmost segment of Iberia during late Palaeozoic time. Zircons from a deformed syntectonic quartz diorite from the northern Cap de Creus Tudela migmatitic complex yield a mean age of 298.8±3.8 Ma. A syntectonic granodiorite from the Roses pluton in the southern area of lowest metamorphic grade of the massif has been dated at 290.8±2.9 Ma. All the analysed zircons from two samples of migmatitic rocks yield inherited ages from the Precambrian metasedimentary protolith (with two main age clusters at c . 730–542 Ma and c . 2.9–2.2 Ga). However, field structural relationships indicate that migmatization occurred synchronously with the emplacement of the quartz dioritic magmas at c . 299 Ma. Thus, the results of this study suggest that subduction-related calc-alkaline magmatic activity in the Cap de Creus was coeval and coupled with D 2 dextral transpression involving NNW–SSE crustal shortening during Late Carboniferous – Early Permian time ( c . 299–291 Ma). Since these age determinations are within the range of those obtained for undeformed (or slightly deformed) calc-alkaline igneous rocks from NE Iberia, it follows that the Cap de Creus massif would represent a zone of intense localization of D 2 transpression and subsequent D 3 ductile wrenching that extended into the Lower Permian during a transitional stage between the Variscan and Cimmerian cycles.

Comment on “Geodynamic evolution of the SW Europe Variscides” by António Ribeiro et al.

[1] Ribeiro et al. [2007] have presented a geodynamic view of the SW Iberia Variscides based on data from Portugal. Their treatment of already published data is commendable, and the knowledge gained will surely encourage the discussion of the SW Europe Variscides. However, in our opinion, Ribeiro et al.’s modeling and interpretation of the Ediacaran–Lower Ordovician ( 560–470 Ma) geodynamic evolution are of limited value. In this regard, they based their analysis of the Ossa-Morena Zone (OMZ) on assumptions which are contradicted by recent published data. Ribeiro et al. ignored recent progress in the OMZ Ediacaran-Ordovician stratigraphy and, as a consequence they misunderstood the structure of domains overprinted by strong Variscan (Carboniferous) deformation and metamorphism. [2] The first arguable point of Ribeiro et al.’s [2007] work is the possibility of occurrence of a likely Grenville inlier in the OMZ. Until now, no rocks older than the Ediacaran have ever been proven to exist in the OMZ (Portugal and Spain). The oldest dated rocks are Ediacaran felsic dykes with 623 ± 3Ma (U-Pb TIMS/zircon, Loma del Aire rhyolites [SanchezGarcia et al., 2007]). The most representative Ediacaran rocks constitute the Serie Negra succession [Eguiluz et al., 2000; Pereira et al., 2006b]. Regardless of the Cambrian and Carboniferous high-grade metamorphism with anatexis, zircons from the Ediacaran Serie Negra metasediments preserve older cores and metamorphic overgrowths [OrdonezCasado, 1998; Fernandez-Suarez et al., 2002; Chichorro, 2006;Chichorro et al., 2006;Pereira et al., 2008; Linnemann et al., 2008]. The obtained ages for the metamorphic overgrowths are mainly Carboniferous, but also Ediacaran, Cambrian and Ordovician. U-Pb SHRIMP and LA-ICP-MS dating on detrital zircons from the Serie Negra metasediments indicate maximum age of deposition ranging from 560 to 540Ma [Ordonez-Casado, 1998; Fernandez-Suarez et al., 2002; Pereira et al., 2006a, 2008; Linnemann et al., 2008]. A common feature of all investigated samples is an ‘‘age gap’’ between 1.7 and 1.0 Ga. This age gap with no sign of Grenvillian zircon forming events is a characteristic of a Cadomian/West African provenance and distinguishes Cadomia/West Africa from Amazonia and Baltica. These Ediacaran sediments represent the infill of basins related to dismantling of a Cadomian continental arc [Pereira and Chichorro, 2004; Pereira et al., 2006b, 2007; Linnemann et al., 2008]. Ribeiro et al.’s [2007] statement that the Ediacaran calc-alkaline plutons are spatially limited to the northern margin of the Coimbra-Cordoba shear zone is not correct. It was recognized as plutonism of Ediacaran age in the northern margin of this major Variscan shear zone with 580 Ma (Pb-Pb Kober/zircon, Aljucen granodiorite [Talavera et al., 2008]), 573 ± 14 Ma (U-Pb SHRIMP/ zircon, Valle de la Serena porphyritic granitoid [OrdonezCasado, 1998]) and 555 Ma (Sm-Nd/garnet, MeridaMontoro gabbro-diorite [Bandres et al., 2002]) but also toward the south with 552 ± 10 Ma (U-Pb SHRIMP/zircon; Ahillones granite [Ordonez-Casado, 1998]). The Cadomian back-arc basin was active longer, at least until 545Ma. The final magmatic pulse of the Cadomian magmatic arc at 550 Ma (coeval with the crystallization of the Ahillones granite) is documented by newU-Pb LA-ICP-MS zircon data [Pereira et al., 2006a; Linnemann et al., 2008]. Closure of the Cadomian back-arc basin and final events of arc-continent collision in the OMZ occurred probably between 545 Ma and the overall onset of Cambrian intracontinental riftrelated plutonism at 530 Ma [Sanchez-Garcia et al., 2003, 2008]. [3] The second debatable point is the assertion of Ribeiro et al. [2007] that continental rifting on the Cambrian platform of northern Gondwana started around the Middle Cambrian. New zircon U-Pb zircon dating indicates an early igneous event with calc-alkaline signature and peraluminous tendency in the OMZ during the Lower Cambrian ( 530–515 Ma) associated with rift-related carbonate and siliciclastic deposition: 532 ± 4 Ma (U-Pb TIMS/zircon, Mina Afortunata leucogranite [Sanchez-Garcia et al., 2008]), 530 ± 3 Ma (U-Pb SHRIMP/zircon, Bodonal porphyroid [Romeo et al., 2006]) and 529–527Ma (U-Pb SHRIMP/zircon, Alcacovas and Santiago do Escoural orthogneisses [Chichorro, 2006; Chichorro et al., 2008]). This igneous event that seems to represent last residual melts of high-temperature, zirconundersaturated mafic magmas later affected by crustal contamination, while others indicate partial melting of crustal metasediments variably contaminated by basaltic liquids, is interpreted as the result of an extensional tectonic process accompanied by strong thermal rise [Sanchez-Garcia et al., 2003, 2008; Chichorro et al., 2008]. This situation was probably connected to an underlying mantle plume or simply TECTONICS, VOL. 28, TC4009, doi:10.1029/2008TC002430, 2009 Click Here for Full Article

The role of strain localization in magma injection into a transtensional shear zone (Variscan belt, SW Iberia)

This study deals with the interaction between deformation and magmatism in mid- to deep-crustal domains. The relation is analysed between migmatites and shear zones and the spatial distribution of leucogranitoid veins and dykes running through a footwall migmatite system, and reaching a transtensional shear zone operated under amphibolite- to greenschist-facies metamorphic conditions (Boa Fé shear zone, Variscan belt, SW Iberia). Statistical results show that the frequency of width and spacing of the leucogranitoid dykes conform to power-law distributions comparable with observations in volcanic systems. The fractal geometry of the distribution of leucogranitoid dykes highlights the development of a dense framework of thinner weakly or non-mineralized veins and dykes formed at higher nucleation/growth ratios in the footwall migmatite system that contrasts with the emplacement of thicker dykes associated with strongly mineralized thinner veins within the shear zone. The volume of injected leucogranitoid dykes in the shear zone is lower as compared with the footwall and is comparable with an expanding footwall shear zone with non-coaxial flow and volume increase. The Boa Fé shear zone seems to form a physical barrier to the transport of magma to the hanging wall.

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.

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.