150 research outputs found
Detrital zircon from a late Paleozoic accretionary complex of SW Iberia (Variscan Belt): History of crustal growth and recycling at the Rheic convergent margin
In this study we present new U-Pb ages of detrital zircons from
greywackes and quartzites of the Pulo do Lobo Anticline (PLA) that have
been interpreted to represent a Late Paleozoic accretionary complex in SW
Iberia. The PLA separates the Ossa Morena Zone, which has a North-
Gondwana affinity throughout Late Ediacaran and Early Paleozoic times,
from the South Portuguese Zone, which is considered to be underlain by
Laurussia basement. The PLA stratigraphy most likely represents a
synorogenic basin that records the closure of the Late Paleozoic Rheic
Ocean and the amalgamation of Pangaea. The youngest formations of the
PLA contain upper Devonian microfossils.The results obtained indicate that the detrital zircons from the PLA
represent a wide range of Precambrian and Paleozoic crystallization ages.
Recycling of older sedimentary units of the Late Ediacaran active margin
(Cadomian/Pan-African orogenies) as well as of the Early Paleozoic rifting
and passive margin (Rheic Ocean) stages, accounts for the older
populations with North-Gondwana affinity (Cambrian, Neoproterozoic,
Paleoproterozoic and Archean, with a gap of Mesoproterozoic-age).
However, the Mesoproterozoic detrital zircon ages found in the
greywackes of the Pulo do Lobo Formation (< 7%) that do not correspond
to any substantial source within North-Gondwana, could come from
recycled sedimentary deposits or from denudation of Grenville-age
basement (Laurussia?). The more recent formations present in the northern
limb (Ferreira-Ficalho Group) of the PLA show a significant age cluster in
the upper Devonian (c. 378 Ma), whereas on the southern limb (Chança
Group), samples have from base to top of the stratigraphic sequence: a
minor age cluster in the middle Devonian (c. 390 Ma), a significant age
cluster in upper Devonian (c. 380 Ma) and very significant age cluster in
the upper Devonian (c. 372 Ma). The presence of middle-upper Devonian
detrital zircons in combination with very low abundances of
Mesoproterozoic detrital zircon suggests that the PLA sedimentary rocks
were not derived from exotic sources but rather have a North-Gondwanan
origin. The zircon population in the interval c. 390-380 Ma has no
identified corresponding magmatic or stratigraphic source in SW Iberia.
Considering that, during the development of the upper Devonian basins of
SW Iberia, Laurussia basement was not exposed and that there was no
magmatic arc on the North-Gondwana margin, we suggest that the c. 390-
380 Ma detrital zircons are most probably derived from denudation of a
(intra-oceanic) magmatic arc related to the closure of the Rheic Ocean
Comment on ‘‘Geodynamic evolution of the SW Europe
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 SWEurope 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
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
Tectonothermal analysis of high-temperature mylonitization in the
Mylonites in pelitic and quartzofeldsphatic gneisses from the Ouguela tectonic unit (Coimbra–Córdoba shear zone, SW Iberian Massif) have
been studied as an example of high-temperature ductile deformation associated with transcurrent tectonics. Detailed microstructural and P–T
analysis indicates that ductile deformation evolved from a metamorphic peak at approximately 650–750 °C and 7.5–9.5 kbar (quartzofeldsphatic
gneisses) and 730–790 °C and 7.5–9.5 kbar (pelitic gneisses) to retrograde conditions at 500–575 °C and 4.5/5.5–6.5/7.5 kbar (quartzofeldsphatic
gneisses) and 525–600 °C and 3.5/4.5–5.5/7.5 kbar (pelitic gneisses). Following the metamorphic peak, exhumation was very fast. The P–T
trajectory, which does not reach the curve for granite melting, is distinct that of isothermal decompression. Instead, the progressive and
contemporaneous decrease in pressure and temperature was a direct response to strong heat dissipation along the contacts between the ascending
slice and the adjacent blocks. The horizontal component of exhumation path, calculated for middle and shallower crustal levels, sum to ca. 57 km
to 94 km (for the pressure peak). Assuming this offset acted in the Viséan during a time interval of ca. 9 Ma, the estimated exhumation horizontal
slip rate is in the order of 6.3 to 10.4 mm/yr, which corresponds to an exhumation oblique-slip exhumation rate of 6.6 to 10.7 mm/yr (for ductile
deformation). These values indicate that the transcurrent tectonic displacements accommodated by these mylonitic are similar to those of modern
intra-continental shear zones, such as the still active Karakoram Fault (8.3 mm/yr) in the Himalayas. The Coimbra–Córdoba shear zone is
therefore a typical intra-continental transcurrent zone with ten-to-one hundred kilometre along-strike mass movement of material that aided the
exhumation of deep crustal rocks. Study of this large-scale structure in the SW Iberian Massif is therefore central to models of orogenic
deformation during the amalgamation of Pangea
Deciphering a multi-event in a non-complex set of detrital zircon U–Pb ages from Carboniferous graywackes of SW Iberia
The determination of U–Pb ages from detrital zircons of sedimentary rocks using LA-ICP-MS has been widely
used for the purpose of provenance analysis. One problem that frequently arises is finding a population that
appears to be non-complex despite several perceptible age peaks in its spectrum. These peaks are qualitatively
defined by means of relative probability diagrams, or PDFs, but it is difficult to quantify their statistical significance
relative to a zircon forming multi-event. Thus, can a multi-event in a non-complex set of detrital zircon
U–Pb ages be deciphered and characterized?
The aim of this study is to attempt to provide an answer to this question by means of statistical analysis. Its
objectives are: a) to determine the best minimum number of zircon age populations (peaks), BmPs, b) for the
characterization of each peak in terms of age and event duration; c) to compare the results obtained from two
datasets showing similar zircon ages; and d) to demonstrate the usefulness of deciphering these BmPs. First,
cluster analysis is carried out, aimed at grouping zircon ages into a set of consistent clusters. A Gaussian Kernel
function is then fitted to each cluster and summed to obtain a theoretical PDFm (modeled probability density
function). Finally, the selected modeled PDFm (that built on the BmPs) is that which reports the lowest number
of peaks for which the difference as compared with the original gPDF (global probability density function) is
equal to or below 5%. Deciphered BmP peaks can be characterized and used for characterizing and providing
an understanding of related event(s).
A geological interpretation, based on the results obtained, is attempted. This includes a robust measure for
maximum age of deposition for both Cabrela and Mértola graywackes
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 D2 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 D2 transpression and subsequent D3 ductile wrenching that extended into the Lower Permian during
a transitional stage between the Variscan and Cimmerian cycles
Zircon U–Pb geochronology of paragneisses and biotite
Sensitive high-resolution ion microprobe U–Th–Pb age determinations on detrital and
inherited zircon from the E ´ vora Massif (SW Iberian Massif, Portugal) provide direct evidence for
the provenance of the Ossa–Morena Ediacaran basins (Se´rie Negra) and a palaeogeographical link
with the West African craton. Three samples of the Se´rie 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 Se´rie 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
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
Exhumation of high-pressure rocks in northern Gondwana during the Early
The Coimbra–Córdoba shear zone (CCSZ) represents a major intra-continental shear zone of the European
Variscan orogen. The shear criteria found in metamorphic rocks of the CCSZ are consistent with sinistral
transcurrent movements. Isoclinal and open folds with axes parallel to the stretching lineation are
responsible for dip variations in the mylonitic foliation, but are related to the same kinematics. In selected
outcrops of the Campo Maior unit (SW Iberian Massif, Portugal), boudins of high-pressure mafic granulites,
high-grade amphibolites and felsic gneisses with long-axes parallel to the stretching lineation in the
surrounded metamorphic rocks, were sampled together with the host migmatites for petrographic,
geothermobarometric and U–Th–Pb SHRIMP in-situ zircon geochronology analysis. The results show that
decompression associated with shearing and partial melting in the CCSZ began under granulite facies
conditions during the Variscan orogeny (early Carboniferous: c. 340 Ma.). Peak metamorphic conditions in
the mafic granulites (850–880 °C and 14.5–16.5 kbar), were followed by symplectitization at 725–750 °C and
12.5–14.5 kbar. Peak P–T conditions were 615–675 °C and 9.5–11.5 kbar in the high-grade amphibolites,
750–850 °C and 11.5–15.5 kbar in the weakly deformed gneisses, and 675–725 °C and 9–11.5 kbar in the
sheared migmatites. Subsequently, temperatures and pressures decreased during amphibolite facies
metamorphism coeval with mylonitization. Retrograde P–T conditions were 550–700 °C and 7–9 kbar in
the high-grade amphibolite, 620–640 °C and 6–8 kbar in the gneisses, and 560–610 °C and 5–6.5 kbar in the
migmatites. Zircon dating of the migmatites and gneisses indicate Ediacaran (c. 590 Ma) and Ordovician
(c. 488–479 Ma) ages for the protoliths, and show that these rocks were part of the northern Gondwana
margin with a West Africa Craton signature dominated by Paleoproterozoic (c. 2–1.8 Ga) and Neoproterozoic
(c. 664–555 Ma) ages, and a characteristic lack of Mesoproterozoic (c. 0.9–1.7 Ga) ages. These rocks were
probably subducted, and subsequently exhumed during the complex processes of Pangea formation. The
high temperature–high pressure rocks of the Campo Maior unit were likely displaced by large-scale
transcurrent movements within the CCSZ in the early Carboniferous. The CCSZ appears to represent a major
shear zone in the SW Iberian Massif connected in some way to the Variscan suture zon
Relative timing of transcurrent displacements in northern Gondwana: U-Pb laser ablation ICP-MS zircon and monazite geochronology of gneisses and sheared granites from the western Iberia Massif (Portugal)
The Variscan belt of Western and Central Europe was formed by the oblique subduction of the Rheic Ocean and the collision of Laurussia with Gondwana during the Late Palaeozoic. We present field relationships and new U–Pb LA-ICP-MS zircon and monazite ages for Variscan gneisses and granites from a key section of the western Iberian Massif. The Martinchel section records the interplay of two kilometre-scale Variscan transcurrent shear zones active in the Gondwana basement of Pangaea: the Porto–Tomar fault zone (PTFZ) and the Coimbra–Córdoba shear zone (CCSZ). Different kinematic models have been invoked to explain the formation and evolution of these major Variscan structures mainly based on assumptions made in the absence of reliable radiometric ages. We show that: (1) ductile deformation and metamorphism were active in the CCSZ during the Visean–Serpukhovian (c.335–318 Ma) and created conditions for amphibolite facies metamorphism and coeval emplacement of granites; and (2) later ductile–brittle deformation related to dextral movements along the PTFZ overprinted the earlier foliation and folds derived from the CCSZ deformation, and deformed the previously intruded granites. U–Pb dating of zircon and monazites yield c.335 Ma ages for the ductile deformation developed under amphibolite facies metamorphic conditions in the Martinchel gneisses of the CCSZ. The gneisses were intruded by granites at c.335–318 Ma, and both were later deformed under ductile–brittle conditions by dextral motion on the PTFZ. The geometry of the Martinchel gneisses (typical of the CCSZ) changed from one of thrusting to one of normal faulting by refolding of the early foliation, stretching lineation and asymmetric structures related to the later PTFZ dextral shear episode. This pattern of interference is not fully considered in previous models and may lead to incorrect tectonic interpretations. According to our data and recently published ages, we suggest that the PTFZ was active after the Serpukhovian–Kasimovian since the c.318–308 Ma granites are deformed by north–south (170°) dextral shear planes. These data are critical to the interpretation of large-scale Carboniferous transcurrent displacements in northern Gondwana (Iberian Massif), and bear upon global models of crustal deformation that emphasize the importance of long-lived dextral movements during the collision between northern Gondwana and Laurussia following the closure of the Rheic Ocean
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