The origin of the Variscan upper allochthons in the Ortegal Complex, northwestern Iberia: Sm–Nd isotopic constraints on the closure of the Rheic Ocean

2008 ◽  
Vol 45 (6) ◽  
pp. 651-668 ◽  
Author(s):  
J. Brendan Murphy ◽  
Gabriel Gutiérrez-Alonso
Keyword(s):  
Suture Zone ◽  
Variscan Orogeny ◽  
Common Source ◽  
Massif Central ◽  
Early Devonian ◽  
Composite Body ◽  
Mafic Rocks ◽  
Rheic Ocean ◽  
Late Cambrian ◽  
Nw Iberia

Northwestern Iberia preserves a stack of allochthons in which the vestiges of a suture zone generated during the Variscan orogeny by the late Paleozoic collision between Laurussia and Gondwana are exposed. Lower allochthons contain Ordovician ophiolite (known as the Lower Ophiolite), and are structurally overlain by Devonian ophiolite (Upper Ophiolite), which are in turn structurally overlain by allochthons containing high-grade metamorphic rocks with continental affinities and Late Cambrian – Early Ordovician protolith ages (known as the Upper Units). Geochemical and Sm–Nd isotopic data from the Upper Ophiolite and the structurally overlying Upper Units exposed in the Ortegal Complex of Galicia show that these allochthons are derived from a variety of mantle and crustal sources and indicate that the suture zone juxtaposes a variety of oceanic assemblages. The general isotopic characteristics of each assemblage are similar to allochthons in other Variscan complexes in NW Iberia suggesting that the allochthons are each derived from a common source and may be regionally extensive. One of the bodies mapped within the Upper Ophiolite (Purrido amphibolite) is a composite body that, in addition to recently identified Mesoproterozoic mafic rocks, is characterized by a juvenile signature at ca. 395 Ma that was chemically modified from coeval intra-oceanic subduction. The very high ϵNd of this Late Devonian ophiolite is typical of several penecontemporaneous ophiolites within the Variscan orogen including the Lizard Complex (Britain) and the Massif Central (France), suggesting derivation from a regionally extensive anomalous mantle characterized by time-integrated depletion in Nd relative to Sm. Paleozoic mafic rocks in the Upper Units have ϵNd values typical of Paleozoic mafic rocks in Avalonia, which are thought to have been derived from subcontinental lithospheric mantle (SCLM) that was enriched at ca. 1.0 Ga. They exhibit elevated Th/Yb and Ce/Yb relative to Ta/Yb suggesting that their composition has been contaminated by subduction zone components, although the age of this contamination is unclear. Felsic rocks in the Upper Units were derived by melting of Mesoproterozoic or older (West African?) crust. These data, when combined with other geologic constraints, including the outboard position of the Upper Units relative to the ophiolite, support the hypothesis that the Upper Units collectively represent a crustal fragment that drifted from Gondwana during the formation of the Rheic Ocean, was transferred to Laurussia in Silurian or early Devonian times, and was subsequently thrust over the Gondwanan margin during the closure of the Rheic Ocean and the Variscan orogenesis.

scholarly journals Radial anisotropy and S-wave velocity depict the internal to external zones transition within the Variscan orogen (NW Iberia) 

2021 ◽  
Author(s):  
Jorge Acevedo ◽  
Gabriela Fernández-Viejo ◽  
Sergio Llana-Fúnez ◽  
Carlos López-Fernández ◽  
Javier Olona ◽  
...  
Keyword(s):  
Shear Wave ◽  
Wave Velocity ◽  
Ambient Noise ◽  
Variscan Orogeny ◽  
Low Grade ◽  
The West ◽  
Cantabrian Zone ◽  
Variscan Orogen ◽  
Nw Iberia ◽  
Radial Anisotropy

Abstract. The cross-correlation of ambient noise records registered by seismic networks has proven to be a valuable tool to obtain new insights into the crustal structure at different scales. Based on 2- to 14-s-period Rayleigh and Love dispersion data extracted from the seismic ambient noise recorded by 20 three-component broadband stations belonging to two different temporary experiments, we present the first i) upper crustal (1–14 km) high-resolution shear wave velocity and ii) radial anisotropy variation models of the continental crust in NW Iberia. The area of study represents one of the best exposed cross-sections along the Variscan orogen of western Europe, showing the transition between the external eastern zones towards the internal areas in the west. Both the 2-D maps and an E-W transect reveal a close correspondence with the main geological domains of the Variscan orogen. The foreland-fold and thrust-belt of the orogen, the Cantabrian Zone, is revealed by a zone of relatively low shear wave velocities (2.3–3.0 km/s), while the internal zones generally display higher homogeneous velocities (> 3.1 km/s). The boundary between both zones is clearly delineated in the models, depicting the arcuate shape of the orogen grain. The velocity patterns also reveal variations of the bulk properties of the rocks that can be linked to major Variscan structures, such as the basal detachment of the Cantabrian Zone or the stack of nappes involving pre-Variscan basement; or sedimentary features such as the presence of thick syn-orogenic siliciclastic wedges. Overall, the radial anisotropy magnitude varies between −5 and 15 % and increases with depth. The depth pattern suggests that the alignment of cracks is the main source of anisotropy at < 8 km depths, although the intrinsic anisotropy seems to be significant in the West-Asturian Leonese Zone, the low-grade slate belt adjacent to the Cantabrian Zone. At depths > 8 km, widespread high and positive radial anisotropies are observed, caused by the presence of subhorizontal alignments of grains and minerals in relation to the internal deformation of rocks either during the Variscan orogeny or prior to it.

scholarly journals Two-step obduction of the Porvenir serpentinites: A cryptic Devonian suture in SW Iberian Massif (Ossa-Morena Complex)

2021 ◽  
Author(s):  
Rubén Díez Fernández ◽  
Jerónimo Matas ◽  
Ricardo Arenas ◽  
Luis Miguel Martín-Parra ◽  
Sonia Sánchez Martínez ◽  
...  
Keyword(s):  
Continental Crust ◽  
Upper Mantle ◽  
Suture Zone ◽  
Lower Plate ◽  
Late Devonian ◽  
Iberian Massif ◽  
Step Process ◽  
Nw Iberia ◽  
Thrust System ◽  
Tectonic Slice

ABSTRACT The Porvenir serpentinites are an ∼600-m-thick body of meta-peridotites exposed in SW Iberia (Variscan Orogen). The serpentinites occur as a horse within a Carboniferous, out-of-sequence thrust system (Espiel thrust). This thrust juxtaposes the serpentinites and peri-Gondwanan strata onto younger peri-Gondwanan strata, with the serpentinites occupying an intermediate position. Reconstruction of the pre-Espiel thrust structure results in a vertical juxtaposition of terranes: Cambrian strata below, Porvenir serpentinites in the middle, and the strata at the footwall to the Espiel thrust culminating the tectonic pile. The reconstructed tectonic pile accounts for yet another major thrusting event, since a section of upper mantle (Porvenir serpentinites) was sandwiched between two tectonic slices of continental crust (a suture zone sensu lato). The primary lower plate to the suture is now overlying the upper plate due to the Espiel thrust. Lochkovian strata in the upper plate and the Devonian, NE-verging folds in the lower plate suggest SW-directed accretion of the lower plate during the Devonian, i.e., Laurussia-directed underthrusting for the closure of a Devonian intra-Gondwana basin. Obduction of the Porvenir serpentinites was a two-step process: one connected to the development of a Devonian suture zone, and another related to out-of-sequence thrusting that cut the suture zone and brought upward a tectonic slice of upper mantle rocks hosted in that suture. The primary Laurussia-dipping geometry inferred for this partially obducted suture zone fits the geometry, kinematics, and timing of the Late Devonian suture zone exposed in NW Iberia and may represent the continuation of such suture into SW Iberia.

Geological setting and geochemical signatures of the mafic rocks from the Intra-Pontide Suture Zone: implications for the geodynamic reconstruction of the Mesozoic Neotethys

2015 ◽  
Vol 105 (1) ◽  
pp. 39-64 ◽  
Author(s):  
Kaan Sayit ◽  
Michele Marroni ◽  
M. Cemal Göncüoglu ◽  
Luca Pandolfi ◽  
Alessandro Ellero ◽  
...  
Keyword(s):  
Suture Zone ◽  
Mafic Rocks ◽  
Geological Setting

Stratigraphy and lithogeochemistry of rocks from the Nugget Pond Deposit area, Baie Verte Peninsula, Newfoundland

2021 ◽  
Author(s):  
C Mueller ◽  
S J Piercey ◽  
M G Babechuk ◽  
D Copeland
Keyword(s):  
Gold Mineralization ◽  
Sedimentary Rocks ◽  
Mafic Rocks ◽  
Basaltic Rocks ◽  
Late Cambrian ◽  
Lower Unit ◽  
Iapetus Ocean ◽  
Post Archean Australian Shale ◽  
Clastic Sedimentary ◽  
Ocean Ridge

Stratigraphic and lithogeochemical data were collected from selected drill core from the Nugget Pond gold deposit in the Betts Cove area, Newfoundland. The stratigraphy consists of a lower unit of basaltic rocks that are massive to pillowed (Mount Misery Formation). This is overlain by sedimentary rocks of the Scrape Point Formation that consist of lower unit of turbiditic siltstone and hematitic cherts/iron formations (the Nugget Pond member); the unit locally has a volcaniclastic rich-unit at its base and grades upwards into finer grained volcaniclastic/turbiditic rocks. This is capped by basaltic rocks of the Scrape Point Formation that contain pillowed and massive mafic flows that are distinctively plagioclase porphyritic to glomeroporphyritic. The mafic rocks of the Mount Misery Formation have island arc tholeiitic affinities, whereas Scrape Point Formation mafic rocks have normal mid-ocean ridge (N-MORB) to backarc basin basalt (BABB) affinities. One sample of the latter formation has a calc-alkalic affinity. All of these geochemical features are consistent with results and conclusions from previous workers in the area. Clastic sedimentary rocks and Fe-rich sedimentary rocks of the Scrape Point Formation have features consistent with derivation from local, juvenile sources (i.e., intra-basinal mafic rocks). The Scrape Point Formation sedimentary rocks with the highest Fe/Al ratios, inferred to have greatest amount of hydrothermally derived Fe, have positive Ce anomalies on Post-Archean Australian Shale (PAAS)-normalized trace element plots. These features are consistent with having formed via hydrothermal venting into an anoxic/ sub-oxic water column. Further work is needed to test whether these redox features are a localized feature (i.e., restricted basin) or a widespread feature of the late Cambrian-early Ordovician Iapetus Ocean, as well as to delineate the role that these Fe-rich sedimentary rocks have played in the localization of gold mineralization within the Nugget Pond deposit.

Origin and the Silurian odyssey of the Brunovistulian terrane: paleomagnetic evidence from the Brno Massif (central Europe).

2020 ◽  
Author(s):  
Jerzy Nawrocki ◽  
Jaromir Leichmann ◽  
Magdalena Pańczyk
Keyword(s):  
Central Europe ◽  
Time Characteristic ◽  
Massif Central ◽  
Northern Margin ◽  
East European ◽  
Rheic Ocean ◽  
Crustal Block ◽  
Gondwana Margin ◽  
Isotope Studies ◽  
Isotope Dating

&lt;p&gt;The Brno Massif forms a part of larger tectonostratigraphic unit named the Brunovistulian Terrane (BVT) that is one of crustal block of Europe with the Neoproterozic basement. &amp;#160;However, the Neoproterozoic orogenic belt was developed in wide area i.e. along the Gondwana margin and near the present day eastern and southern edge of the East European Craton. For more than 20 years, the problem of primary setting of the BVT inside the Neoproterozic orogenic &amp;#160;belt have been discussed. Also the path of their drift and &amp;#160;time of their final accretion have been a matter of debate. To solve these problems the paleomagnetic and isotope studies of vertical intrusions cutting the BVT basement near Brno in Moravia have been undertaken. Preliminary isotope dating of granitic and basaltic intrusions points to the early Silurian age of them. Results of demagnetization of paleomagnetic samples from three localities revealed the presence of stable components with a steep inclination, at that time characteristic for the northern margin of Gondawana but not for the Baltica paleocontinent that during the Silurian was situated between the equator and 30&lt;sup&gt;o&lt;/sup&gt;S. The Emsian &amp;#160;&amp;#8220;old red&amp;#8221; type deposits may indicate that final amalgamation of the BVT took place some-time between the Silurian and the Devonian. This time of joining of the BVT&amp;#160; to Baltica and quite high (50 &amp;#8211; 60&lt;sup&gt;o&lt;/sup&gt;S) paleolatitudes obtained from the early Silurian rocks of the Brno Massif&amp;#160; point to a rapid drift of the BVT across the Rheic Ocean during the Silurian.&lt;/p&gt;

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