148 research outputs found
The Cadomian Orogeny and the opening of the Rheic Ocean: The diacrony of
Cadomian orogenic processes and their continuum to the opening of the Rheic Ocean were modeled by
making use of new LA-ICP-MS U–Pb ages from detrital zircons of sedimentary rocks of Late Neoproterozoic
(Ediacaran) and Cambro-Ordovician sediments of the Ossa-Morena Zone (Iberian Massif) compared with
those from the Saxo-Thuringian Zones (Bohemian Massif). Presented data constrain a diachrony of Cadomian
and related geotectonic processes along the northern realm of the Gondwana Supercontinent. Early stage of
Cadomian evolution is characterized by a continental magmatic arc at the periphery of the West African
Craton and a related back-arc basin opened at c. 590 to 570 Ma. Diacronic arc–continent collision was caused
by oblique vector of subduction and started first in the East of Peri-Gondwana at c. 560–570 Ma and resulted
at c. 543 Ma in the formation of a short-lived Cadomian retro-arc basin in the Saxo-Thuringian Zone. In
contrast, more to the West in the Ossa-Morena Zone, the Cadomian back-arc basin was longer active, at least
until c. 545 Ma. In that region, final magmatic pulse of the Cadomian magmatic arc at c. 550 Ma is
documented by new zircon data. Closure of the Cadomian back-arc basin and arc–continent collision in the
Ossa-Morena Zone occurred between c. 545 Ma and the overall onset of Cambrian plutonism at c. 540 Ma. A
mid-oceanic ridge was subducted underneath the Cadomian orogen accompanied by slab break-off of the
subducted oceanic plate. Oblique incision of the oceanic ridge into the continent caused the formation of rift
basins during the Lower to Middle Cambrian (c. 530–500 Ma). This process continued and finally caused the
opening of the Rheic Ocean documented by thick Lower Ordovician siliciclastic sediments and a final
magmatic event at c. 490–485 Ma. Opening of the Cambrian rift basin and of the Rheic Ocean again was
diachronic and started from the West of Peri-Gondwana and expanded eastward
Provenance study of Pliocene–Pleistocene sands based on ancient detrital zircons (Alvalade basin, SW Iberian Atlantic Coast)
Pliocene–Pleistocene sand of the Alvalade basinwas taken fromthe sea-cliffs of SWIberia coast for a provenance
study using radiometric dating. The U–Pb ages obtained revealed a wide interval ranging from Cretaceous to
Archean, with predominance of Paleozoic, Neoproterozoic and Cretaceous zircon ages. Cretaceous ages
interpreted to indicate a Sines Massif provenance are dominant in sands close to Cape Sines but are absent in
sand sampled 12 km north. Carboniferous ages younger than ca. 315 Ma suggesting a possible contribution
from the Central-Iberian Zone originally; however, these zircons may be multi-cyclic, having been reworked
from Eocene–Miocene siliciclastic deposits related to transport from central Iberia (Lower Tagus basin drainage
evolution). These signatures provide important constraints on the location and extent of the Pliocene–Pleistocene
topography and drainage system that were probably controlled by: i) Miocene to Pleistocene landscape
rejuvenation driven by Alpine movements along major faults; and ii) residual reliefs related to inherited
Variscan structure. The U–Pb ages obtained also trace the pre-Cenozoic paleotectonic evolution of SWIberia
recorded in their sources: i) the North Gondwana accretion and breakup; ii) the Gondwana and Laurussia
collision; and iii) the Pangea breakup and opening of the Atlantic Ocean
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
Tracing the geodymamic evolution of the North Gondwana
Pliocene–Pleistocene sand of the Alvalade basin was sampled from the sea-cliffs of SW Iberia
close to Cape Sines for a provenance study using LA-ICPMS U-Pb dating of detrital zircons.
The results are used to compare age distributions and to trace potential source areas based
on existing knowledge of zircon-forming events in the pre-Cenozoic basement of SW Iberia.
The 492 U-Pb ages obtained span a wide interval ranging from Cretaceous to Archean, with
predominance of Paleozoic (31–71%; mainly Carboniferous), Neoproterozoic (19–20%;
mainly Cryogenian–Ediacaran) and Cretaceous (21–39%). Two important features were
noted: i) Carboniferous ages younger than ~ 315 Ma infrequent or absent in SW Iberia, and
ii) Cretaceous ages are interpreted to indicate a Sines massif provenance. The data obtained
confirm previous studies that indicate a sediment source in the Paleozoic basement of SW
Iberia (Ossa-Morena and South Portuguese zones) but also suggest a more complex history
of drainage than previously documented, involving other sources located to the north. The
detrital zircons age populations also enable recognition of previously undocumented sources
of Pliocene–Pleistocene sands and decipher their paleotectonic meaning: i) Neoproterozoic to
Early Paleozoic meta-sedimentary and meta-igneous rocks of the Ossa-Morena and Central-
Iberian zones formed in North Gondwana during the Cadomian orogeny and opening of the
Rheic Ocean, and ii) Devonian to Carboniferous sedimentary and igneous rocks of the Ossa-
Morena, Central Iberian, and South Portuguese zones formed when Gondwana and Laurussia
collided (Variscan orogeny)
Provenance of Holocene beach sand in the Western Iberian
Detrital zircons from Holocene beach sand and igneous zircons from the Cretaceous syenite forming Cape Sines (Western Iberian margin) were dated
using laser ablation – inductively coupled plasma – mass spectrometry. The
U–Pb ages obtained were used for comparison with previous radiometric
data from Carboniferous greywacke, Pliocene–Pleistocene sand and Cretaceous syenite forming the sea cliff at Cape Sines and the contiguous coast.
New U–Pb dating of igneous morphologically simple and complex zircons
from the syenite of the Sines pluton suggests that the history of zircon crystallization was more extensive (ca 87 to 74 Ma), in contrast to the findings of
previous geochronology studies (ca 76 to 74 Ma). The U–Pb ages obtained in
Holocene sand revealed a wide interval, ranging from the Cretaceous to the
Archean, with predominance of Cretaceous (37%), Palaeozoic (35%) and
Neoproterozoic (19%) detrital-zircon ages. The paucity of round to subrounded grains seems to indicate a short transportation history for most of
the Cretaceous zircons (ca 95 to 73 Ma) which are more abundant in the
beach sand that was sampled south of Cape Sines. Comparative analysis
using the Kolmogorov–Smirnov statistical method, analysing sub-populations separately, suggests that the zircon populations of the Carboniferous
and Cretaceous rocks forming the sea cliff were reproduced faithfully in
Quaternary sand, indicating sediment recycling. The similarity of the pre-
Cretaceous ages (>ca 280 Ma) of detrital zircons found in Holocene sand, as
compared with Carboniferous greywacke and Pliocene–Pleistocene sand, provides support for the hypothesis that detritus was reworked into the beach
from older sedimentary rocks exposed along the sea cliff. The largest percentage of Cretaceous zircons (<ca 95 Ma) found in Holocene sand, as com-
pared with Pliocene–Pleistocene sand (secondary recycled source), suggests
that the Sines pluton was the one of the primary sources that became progressively more exposed to erosion during Quaternary uplift. This work
highlights the application of the Kolmogorov–Smirnov method in compar-
ison of zircon age populations used to identify provenance and sediment
recycling in modern and ancient detrital sedimentary sequences
New U-Pb zircon dating of Late Neoproterozoic magmatism in Western Meseta (Morocco)
We present new U-Pb zircon ages from magmatic rocks of the Western Meseta, part of the
Moroccan Variscan belt. The Neoproterozoic–Cambrian stratigraphy in the region of Goäida
(Aguelmous massif, SE of Moroccan Central Massif) consists of limestones with conglomeratic
and felsic volcaniclastic levels, pelites and mafic volcanic rocks assigned to the Cambrian which
unconformably overlie rhyolites, andesites and rhyodacites and felsic tuffs associated with the
Aguelmous granite of probable Neoproterozoic age. The Neoproterozoic–Cambrian stratigraphy
of the region of Sidi Ali is roughly similar and also includes a volcanic-sedimentary complex
with limestones, arkoses rhyolites and conglomerates. These conglomerates contain pebbles of
granite and rhyolite, whose source may be the Neoproterozoic basement. In order to constrain
the age of the Precambrian felsic magmatism we sampled and dated zircons (LA-ICPMS) from
the Goäida granite and a pebble of granite included in the conglomeratic levels of the volcanicsedimentary
complex at Sidi Ali dome (central Rehamna massif). In the Goäida granite, zircon
ages are Ediacaran ranging between ~ 610 Ma and ~ 540 Ma, with a discordia upper intercept
age of 598±32 Ma (MSWD=0.04) that could be interpreted as the age of intrusion. However,
if we consider only the two youngest ages we obtain a Concordia age of 590±3 Ma (MSWD=
0.34). In the Sidi Ali pebble sample the majority of zircon ages are Cryogenian–Ediacaran in the
range ~ 640–600 Ma, with the youngest yielding a Concordia age of 609±2 Ma (MSWD=0.04),
indicating the intrusion age of the granite from which the pebble derived. These results support
the existence of Cadomian/Pan-African magmatism in the Western Meseta of Morocco,
characterized by the intrusion of granites at ~ 609 Ma and ~ 590 Ma. This result is important
for studies of sedimentary provenance and to improve paleogeographic reconstructions of the
northern margin of Gondwana during the Neoproterozoic
. U-Pb detrital zircon ages from the Beiras Group: Implications for the Neoproterozoic evolution of the SW Iberia
U-Pb detrital zircon ages from the Beiras Group greywackes (SW Central Iberian Zone - CIZ) indicate a maximum
depositional age of late Ediacaran (c. 560-578 Ma). Two salient features distinguish the Beiras Group from the SĂ©rie
Negra greywackes (age equivalent from the Ossa-Morena Zone - OMZ): i) The presence of Tonian and
Mesoproterozoic (<8%) age clusters in the Beiras Group greywackes, that are almost absent in the OMZ, imply either a
distinct or an additional source of detrital zircons from the West African Craton; and 2) The higher content of Cryogenian
zircon ages of the Beiras Group greywackes (mainly at c. 840-750 Ma and c. 685-660 Ma), that contrast with the
dominant Ediacaran zircon ages of the SĂ©rie Negra greywackes (OMZ). The Cryogenian zircon forming events that are
dominant in the SW CIZ basins are probably related to a different source with early Cadomian juvenile crust (c. 700-635
Ma) and with a possible contribution of the Pan-African suture (c. 850-700 Ma). The Nd isotopic signatures support the
addition of a juvenile source to pre-existent older crust for the Beiras Group metasediments. Although the Beiras Group
(SW CIZ) and Serie Negra (OMZ) late Ediacaran basins have evolved together in the active margin of Gondwana, they
were sufficiently separated to account for the differences in their detrital zircon content and isotopic signatures
The provenance of Late Ediacaran and Early Ordovician siliciclastic rocks in the
U–Pb geochronology of detrital zircon from Late Ediacaran (Beiras Group greywackes) and Early Ordovician
(Sarnelhas arkosic quartzites and Armorican quartzites of Penacova) sedimentary rocks of the
southwest Central Iberian Zone (SW CIZ) constrain the evolution of northern Gondwana active-passive
margin transition. The LA-ICP-MS U–Pb data set (375 detrital zircons with 90–110% concordant ages) is
dominated by Neoproterozoic ages (75% for the greywakes and 60% for the quartzites), among which the
main age cluster (more significant for Beiras Group greywackes) is Cryogenian (c.840–750 Ma), while a
few Mesoproterozoic and Tonian ages are also present (percentages <8%). These two features, and the
predominance of Cryogenian ages over Ediacaran ages, distinguish the Beiras Group greywackes (SW
CIZ) from the time-equivalent Serie Negra (Ossa-Morena Zone – OMZ), with which they are in inferred
contact. The age spectra of the Beiras Group greywackes also reveal three major episodes of zircon crystallisation
in the source area during the Neoproterozoic that are probably associated with a long-lived
system of magmatism that developed either along or in the vicinity of the northern Gondwana margin
at: (1) c. 850–700 Ma – Pan-African suture (not well represented in OMZ); (2) c. 700–635 Ma – early
Cadomian arc; and (3) c. 635–545 Ma – late Cadomian arc. Comparison of Neoproterozoic ages and those
of the Paleoproterozoic (c. 2–1.8 Ga) and Archean (mainly Neoarchean – 2.8–2.6 Ga, but also older) in the
Beiras Group greywackes with U–Pb ages of Cadomian correlatives shows that: (1) SW CIZ, OMZ, Saxo-
Thuringian Zone, North Armorican Cadomian Belt and Anti-Atlas) evolved together during the formation
of back-arc basins on the northern Gondwana active margin and (2) all recorded synorogenic basins that
were filled during the Ediacaran by detritus resulting from erosion of the West African craton, the Pan-
African suture and a long-lived Cadomian magmatic arc. Differences in detrital zircon age populations in
the greywackes of the Beiras Group (SW CIZ Cadomian basement) and the Serie Negra (OMZ Cadomian
basement) are also observed in their respective overlying Early Ordovician quartzites. Since both these
SW Iberia Cadomian basements evolved together along the active margin of Gondwana (but sufficiently
separated to account for the differences in their detrital zircon content), this continuation of differing
zircon populations into the Early Ordovician suggests that the inferred contact presently juxtaposing
the Beiras Group and the Serie Negra is not pre-Early Ordovician and so is unlikely to demonstrate a
Cadomian suture
Provenance analysis of the Paleozoic sequences of the northern Gondwana margin in NW Iberia: Passive margin to Variscan collision and orocline development
The Cantabrian Zone of NW Iberia preserves a voluminous, almost continuous, sedimentary sequence that ranges from Neoproterozoic to Early Permian in age. Its tectonic setting is controversial and recent hypotheses include (i) passive margin deposition along the northern margin of Gondwana or (ii) an active continental margin or (iii) a drifting ribbon continent. In this paper we present detrital zircon U–Pb laser ablation age data from 13 samples taken in detrital rocks from the Cantabrian Zone sequence ranging from Early Silurian to Early Permian in depositional age. The obtained results, together with previously published detrital zircon ages from Ediacaran– Ordovician strata, allow a comprehensive analysis of changing provenance through time. Collectively, these data indicate that this portion of Iberia was part of the passive margin of Gondwana at least from Ordovician to Late Devonian times. Zircon populations in all samples show strong similarities with the Sahara Craton and with zircons found in Libya, suggesting that NW Iberia occupied a paleoposition close to those regions of present-day northern Africa during this time interval. Changes in provenance in the Late Devonian are attributed to the onset of the collision between Gondwana and Laurussia. Additionally, the Middle Carboniferous to Permian samples record populations consistent with the recycling of older sedimentary sequences and exhumation of the igneous rocks formed before and during the Variscan orogeny. Late-Devonian to Permian samples yield zircon populations that reflect topographic changes produced during the Variscan orogeny and development of the lithospheric scale oroclinal buckling
Idades U-Pb dos zircões detrĂticos do Grupo das Beiras - implicações para a evolução do SW da IbĂ©ria durante o NeoproterozĂłico
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