Gondwana Margin Research Articles

Permian-Triassic volcanic and plutonic records of the Argentine Frontal Cordillera: A review with new U–Pb and Hf-isotope zircon data

The Permian-Triassic magmatism of western Argentina and Chile represents one of the most outstanding silicic magmatic events of the southwestern Gondwana margin, notably marked by the development of the Choiyoi Magmatic Province (CMP). We provide a comprehensive review of its volcanic and plutonic record in the Argentine Frontal Cordillera. The volcanic rocks form three distinct sequences. The oldest is depicted by the Las Lozas volcanic sequence of the northern Frontal Cordillera for which new U–Pb zircon data (288 ± 2 Ma and εHft values ranging from −3.97 to +0.73) reassigns these outcrops to the early Cisuralian, aligning with volcanic records of northern Chile (ca. 297-288 Ma). The middle sequence, deposited during the late Cisuralian-late Guadalupian interval, is ascribed to the Choiyoi Group, which is characterised by a transition from andesitic (ca. 280-270 Ma) to rhyolitic compositions (ca. 270-262 Ma), including a remarkable mid-Guadalupian ignimbrite flare-up event (ca. 265 Ma). The upper sequence, composed of andesite-dacite-rhyolite, is associated with the Guanaco Sonso basin situated in the westernmost region, deposited from the late Lopingian to the middle Triassic period (ca. 254-240 Ma). The volcanic successions were developed in an extensional/transtensional setting, with facies and thickness variations controlled by normal faults, some of them active during the eruption of caldera-forming ignimbrites. Regarding the plutonic component, it comprises over sixty granitoid bodies forming the Colangüil and El Portillo batholiths and scattered stocks throughout the Frontal Cordillera. The early-stage plutons (ca. 285-272 Ma) exhibit calc-alkaline tonalite-diorite to granodiorite-monzogranite compositions and overlap in age with the lower andesitic section of the Choiyoi Group, while the late-stage plutons (ca. 265-252 Ma) display syenodiorite and alkali granite compositions and are in most cases younger than the Choiyoi Group succession. Compiled U–Pb zircon geochronological data reveal a distinctive Permian-age phase characterized by a rapid expansion of magmatism from the Gondwana margin towards its interior, followed by a slower westward shift of the main magmatic belt, predominantly recorded in the Frontal Cordillera of Argentina.

Late Cambrian-early Ordovician extensional magmatism, uplift and sedimentation of the N Gondwana margin: new field, geochemical and geochronological data from the peri-Gondwanan Central Sakarya Terrane, Sakarya Zone, NW Turkey

ABSTRACT The timing and mechanism of the crustal extension that caused blocks of continental crust to rift and drift northwards from the northern Gondwana margin during the Early Palaeozoic are currently debated. Here, we present new field, geochemical and U-Pb zircon data from peri-Gondwanan Central Sakarya terrane in NW Turkey to shed light on the timing and petrogenesis of extensional magmatism and subsequent passive margin development of the northern Gondwana margin. Our 1/25.000 scale mapping of a selected area (250 km2) has revealed a south-vergent thrust stack comprising three slices. The Middle Slice of the thrust stack consists of a gneiss-amphibolite complex that has a basement-cover type stratigraphy (before regional metamorphism). The basement unit consists of meta-granitoids, intruded by meta-syenite and banded amphibolites, with geochemical characteristics reflecting I&S-type, A-type and within plate alkaline/transitional-type magmatism, respectively. The cover meta-clastic sediments have amphibolite lenses with MORB and back-arc basin-type geochemistry. Meta-granitoid bodies yielded zircon U-Pb ages of 507 ± 16 Ma, 503 ± 12 Ma, and 488 ± 2.6 Ma (Late Cambrian-Early Ordovician), whereas the meta-syenites gave ages of 473.9 ± 5.7 Ma and 471.4 ± 2.5 Ma (late Early Ordovician); also, age of the banded amphibolite was dated as 473.3 ± 4 Ma. The youngest detrital zircon population in the pebbly quartzite sample within cover meta-sediments (458 ± 3 Ma) constrains the maximum depositional age to around early Late Ordovician. The late Early Ordovician bimodal magmatism, a key focus of our study, is interpreted to have formed in response to the Rheic Ocean rifting along the northern margin of Gondwana. The Late Cambrian granitic magmatism, a precursor to bimodal magmatism, can be explained by lithospheric thinning and upwelling of the asthenosphere to form anatectic silicic melts. The unconformity between the basement and cover units may represent the initiation of a new phase of extension at the N Gondwana margin that culminated in the northward drifting of the Central Sakarya and the opening of Palaeotethys, associated with back-arc basic magmatism in Late Ordovician-Silurian.

Late Triassic to Middle Jurassic magmatism in the Eastern Pontides (NE Türkiye): Evidence for divergent double subduction of Paleo-Tethys lithosphere

The Meso-Cenozoic geodynamic evolution of northern Türkiye remains a subject of debate primarily due to a lack of systematic geological, geochemical, geochronological, and geophysical investigations. This paper presents comprehensive geochronological and geochemical data on the Late Triassic andesite porphyry, andesite breccia and quartz diorite porphyry, and the Middle Jurassic diabase, and amphibole-poor and amphibole-rich andesite porphyry from Çevrepınar Igneous Complex (Gümüşhane) in the southern part of the Eastern Pontides Orogenic Belt (EPOB), a well-preserved continental arc in the Alpine-Himalayan orogen. Zircon U–Pb geochronology indicates crystallization ages of ∼208–202 Ma (Rhaetian) for the Late Triassic rocks and ∼175–172 Ma (Aalenian) for the Middle Jurassic rocks. Whole-rock geochemical and Sr-Nd-Pb isotopic data, and zircon εHf(t) data indicate that both the Late Triassic and the Middle Jurassic rocks originated by low-degree melting of a spinel lherzolite lithospheric mantle source modified by subduction-related fluids and/or melts. Based on the new and published data, we suggest that the Late Triassic to Middle Jurassic arc magmatism in EPOB occurred as a result of southward subduction of the Paleo-Tethys oceanic lithosphere beneath the northern margin of Gondwana-Land. Late Triassic to Jurassic arc magmatism and basin evolution occurred synchronously in the northern and southern peripheries of the present-day Eastern Black Sea Basin, indicating divergent double subduction of the Paleo-Tethys oceanic lithosphere beneath the northern margin of Gondwana and southern margin of Laurasia during the Early Mesozoic.

Validity of Myobradypterygius hauthali von Huene, 1927 (Ichthyosauria: Ophthalmosauria) from the Early Cretaceous of Chile and Argentina

Abstract Early Cretaceous ichthyosaurs were globally distributed pelagic marine reptiles, but many remains are fragmentary, creating a Northern Hemisphere diversity bias. A rich Hauterivian locality near the Tyndall Glacier inside Torres del Paine National Park in southern Chile yields important new data regarding ichthyosaurian diversity along the Pacific margin of Gondwana. These new data will contribute to clarifying questions regarding ichthyosaur taxonomy and the palaeobiogeographical relationships between the southern Gondwanan and Northern Hemisphere ichthyosaur groups during the Early Cretaceous. Here, we describe three new ichthyosaur specimens from this locality. Two of them are referred to Myobradypterygius hauthali, expanding the distribution of this species from the Barremian of Argentina to the Hauterivian of the Chilean Patagonia. This material shows that M. hauthali differs from Platypterygius platydactylus in forefin construction and scapular morphology, supporting its classification as a separate genus within Platypterygiinae. The third specimen is a large-bodied indeterminate ophthalmosaurine ichthyosaur. This record represents the southernmost record of Ophthalmosaurinae and the first occurrence of this group from the Cretaceous of the Southern Hemisphere. These discoveries show that ophthalmosaurines and platypterygiines continued to occur sympatrically in southernmost Gondwana during the Early Cretaceous, expanding the pattern documented in Europe to the Pacific region.

Sedimentary provenance supports a mid-paleozoic tectonic connection between the Junggar and Altai terranes in central Asia

The provenance of Precambrian detritus in the Junggar and Altai terranes provides crucial constraints on the peri-Siberian accretionary tectonic evolution in the middle Paleozoic. The Precambrian detrital zircons have no coeval magmatic equivalents in the Junggar terrane but show U–Pb age spectra and εHf(t) values comparable to those in the Altai terrane. The correlations suggest that the old detrital materials in the Junggar and Altai terranes were most likely derived from the Siberia craton and adjacent Tuva-Mongolian microcontinent. Paleozoic zircons in the Junggar terrane display a εHf(t) pattern from large spread to dominantly positive values at ca. 420–410 Ma. Such an abrupt change points to an accretionary tectonic transition from an advancing to retreating mode during mid-Paleozoic time, synchronous with similar tectonic switch occurring in the Altai terrane. Taking into account the temporal and spatial relations in sedimentation, tectonism and arc magmatism, we propose that the Junggar terrane had once collided onto the peri-Siberian Altai terrane to receive abundant old detritus from the Siberian continent in the Silurian–early Devonian. They were subsequently separated at ca. 420–410 Ma, possibly due to the slab rollback of the subducting Paleo-Asian Ocean (PAO) plate. These results constrain an Early Paleozoic tectono-paleogeographic boundary of the CAOB along the North Tianshan–Solonker suture zone, and also imply a long-lived PAO subduction was responsible for the Neoproterozoic to Paleozoic accretionary orogenesis at the margins of southern Siberia, eastern Kazakhstan, and northern Gondwana.

Reply to “Comment on: Passive continental margin subducted to mantle depths: Coesite-bearing metasedimentary rocks from the Neoproterozoic Brasília Orogen, West Gondwana margin” by Schönig (2024)

Reply to “Comment on: Passive continental margin subducted to mantle depths: Coesite-bearing metasedimentary rocks from the Neoproterozoic Brasília Orogen, West Gondwana margin” by Schönig (2024)

Pre-Choiyoi volcanism at Cordillera del Viento, southwestern margin of Gondwana (∼37° S): Geological characterisation, geochronology and regional implications

This contribution provides a comprehensive description and classification of fifteen lithofacies within the Carboniferous rocks attributed to the Arroyo del Torreón Formation and Sofía Dacite in order to understand the eruptive dynamics and the evolution of volcanism associated with the onset of the Gondwanan Orogenic Cycle at the Cordillera del Viento (Argentina). The outcrops of Carboniferous volcanic and volcaniclastic rocks linked to this cycle are restricted to the Cordilllera del Viento range. In consequence, they offer a valuable opportunity to research the volcanic arc products of the pre-Choiyoi magmatism.The Arroyo del Torreón Formation lithofacies interpretation and its distribution indicate that the volcanism commenced with local andesitic lava flows followed by a volcano-sedimentary sequence formed by multiple pumice-rich quasi-steady concentrated pyroclastic density currents alternating with episodes of epiclastic sedimentation. The latter are covered ignimbrites developed by boiling over eruptions. Crystal-rich tuffs overlie the ignimbrites and represent a transition to a plinian eruption. This eruption style continued, developing concentrated pyroclastic density currents with highly unsteadiness conditions and scarce fall-out deposits. Conversely, the Sofía Dacite, represent a notable shift in the volcanism style, characterized by dacitic to rhyolitic lava flows with minor pyroclastic beds. Zircon dating of a pumice-rich lapilli-tuff from the Arroyo del Torreón Formation yielded a minimum age of 327.77 ± 1.79 Ma that was interpreted as the maximum depositional age for this unit. Similarly, zircons from a porphyritic dacite representing the Sofía Dacite provided a Concordia age of 329.11 ± 1.31 Ma that was interpreted as the crystallisation age of this rock.Consequently, we propose a new stratigraphic division of the Arroyo del Torreón Formation into two members. The Cerro San Pedro Member will comprise the pyroclastic rocks at the base of the sequence, while the Sofía Member will include the upper lava-dominated portion of the unit. In this sense, the whole lithostratigraphic unit represents a distinctive Carboniferous basin associated with arc-related volcanism representative from the pre-Choiyoi magmatism.

The Early Carboniferous age of the Arrayán Formation in the Choapa Accretionary Complex: Implications for its fossil floral content, tectonic setting and evolution of the southwestern Gondwana margin (north-central Chile)

The Arrayán Formation comprises a thick, well stratified, strongly folded turbiditic succession, exposed along the Chilean coast, between 31°30′ and 32°S. It is interpreted as the frontally accreted portion of the Choapa Accretionary Complex formed in the south western Gondwana margin in Carboniferus and Permian times. The Arrayán deposits are unconformably overlain by the marine neritic and richly fossiliferous late Early to early Late Permian Huentelauquén Formation. The contact with the basally accreted metamorphic units of the Choapa Accretionary Complex is tectonic. A previous and a recently dated sample from the Arrayán Formation (PS-8, this paper) yielded mid-Early Carboniferous (Visean) maximum depositional ages of 340 ± 5 Ma and 342 ± 4 Ma respectively. This age (i) contradicts the so far accepted Late Devonian to Early Carboniferous age assigned to the Arrayán Formation, based on its fossiliferous content, (ii) indicates that the age of the psylophytal remains found in this formation are post-Visean, (iii) confirms that the Arrayán Formation is younger than the mid-Early Carboniferous and older than the late Early to early Late Permian Huentelauquén Formation, and (iv) locates its stratigraphic position between the late Early Carboniferous and Early Permian. Previous dates from the compositionally similar metaturbiditic Agua Dulce Formation, which is exposed next to the Arrayán Formation and grades into a mélange, yielded a similar age, which indicates that deposition of both units was coeval. The maximum depositional ages of the Arrayán and the Agua Dulce formations are older than the maximum depositional ages obtained from the metamorphic units in the Choapa Accretionary Complex, which confirms that frontal accretion began earlier than basal accretion, and that the latter reached the Permian and was coeval with deposition of the Huentelauquén Formation. The stratigraphic position and the zircon age distribution pattern suggest that the El Toco, Sierra El Tigre and the Arrayán formations are correlative. The Las Tórtolas Formation is somewhat younger than these formations. The age distribution pattern of the zircon grains in sample PS-8 shows two well-defined age peaks at ∼340 (Visean) and ∼480 Ma (Ordovician) and a barren interval between them. The barren interval coincides partly with the Middle Ordovician to Late Carboniferous passive stage proposed for this region of Gondwana. The Ordovician peak is present in all analyzed samples, between 22° and 39°20′S, whereas the Visean peak and the barren interval are less developed in samples north of 27°S. This similarity indicates the existence of roughly similar sources of sediments along this large section of the western Gondwana margin and, possibly, a rather similar paleogeographic context and depositional environment in the trench. A peak younger than 300 Ma in the Llano de Chocolate Beds indicates a younger depositional age and a different paleogeographic setting in the active continental margin, possibly in the forearc basin, like the Huentelauquén Formation. Broken-formation or mélange facies occur in all complexes, except in the El Toco Formation, and affect the frontally accreted portion of the subduction wedge. The much higher metamorphic grade shown in the Chañaral mélange compared to the much lower grade present in the Agua Dulce mélange suggests formation at different levels along a mega-splay or thrust rooted in the subduction channel. The overall mid-Mississippian age of the frontally accreted deposits analyzed in this study indicates that subduction was active already in the Early Carboniferous. The age, the tectonic setting and the geographic location of these deposits is totally different from the mostly older, marine deposits exposed to the east, in the Frontal Cordillera, which accumulated during a passive stage, in the Middle/Late Ordovician to late Early Carboniferous.

Early Devonian bioclastic contourites in the High Atlas: a plastered drift recording the convergence between Gondwana and Laurussia (Sub-Meseta Zone, Morocco)

Bottom-current deposits, formed in association with ancient oceanic gateways, preserve records of past global oceanic circulation, plate-tectonic events, and palaeogeographic change. This outcrop study showcases a Pragian to early Emsian carbonate-contourite drift, plastered on the northern upper margin of Gondwana, which recorded the convergence of Laurussia and Gondwana and the (temporal) closure of the nearby American Gateway. The results underscore the value of bed-scale features, including microfacies and primary sedimentary structures, in interpreting bottom-current dynamics. The documented contourite-specific bi-gradational sequences, 40 to 80 cm thick, are largely complete and well-comparable with the standard facies sequence identified in modern oceans. The bioclastic contourites show subtle variations in grain size and mud content in the lower part, distinct sequences of inverse-to-normal grading at different scales in the middle, and a large variety of traction structures in the upper part of the succession. This long-term coarsening-upward trend was caused by a successive increase in maximum bottom-current strength during drift growth. Traction structures such as current ripples and dunes document a westward-directed palaeocirculation of surface or intermediate water during the Pragian-Emsian. They provide evidence of enhanced bottom-current episodes towards the top of the drift, before the contourites were buried by late Emsian shales.

Metamorphic and chronological records of early Permian continental collision in Beni Bousera granulites (Internal Rif belt): implications for the tectonic evolution of northern Morocco during Pangea construction

This work presents petrological data obtained from shielded mineral inclusions within large garnets from the Beni Bousera metamorphic unit (internal Rif belt, northern Morocco) combined with in situ U‒Th‒Pb dating of monazite inclusions. In the considered Beni Bousera metapelites, the occurrence of mineral inclusions of kyanite + rutile + plagioclase + K-feldspar + quartz ± graphite trapped in garnet cores is diagnostic of high-pressure granulite-facies metamorphism. Geothermobarometry combined with thermodynamic modelling yields minimal P ‒ T conditions of approximately 1.37–1.5 GPa and 780–795°C, consistent with the development of palaeogradients during crustal thickening in collision belts. A concordant U/Pb age of 281 ± 3 Ma is obtained from associated monazite inclusions in the same garnet cores. These new petrochronological data attest to the early Permian continental collision in northern Morocco. The obtained dataset is constrained with petrological and geochronological parameters available on other neighbouring segments of the Variscan belt. Within the framework of recently unified full plate reconstruction models, the presented tectonic scenario suggests that all these orogenic segments were built during the late Carboniferous–early Permian crustal thickening at the northwestern Gondwana margin in response to the convergence of Laurentia and Gondwana during the late stages of Pangaea construction.

Ediacaran to Jurassic geodynamic evolution of the Alborz Mountains, north Iran: geochronological data from the Gasht Metamorphic Complex

The Alborz Mountains in north Iran underwent several tectono-metamorphic events during opening and closure of the Palaeotethys and Neotethys Oceans. These events are recorded by rare and discontinuously exposed metamorphic rocks, such as the HP-LT Asalem-Shanderman Complex and the Gasht Metamorphic Complex (GMC), that are considered to have been metamorphosed during the closure of the Palaeotethys Ocean. The GMC comprises poorly exposed metasediments and amphibolites metamorphosed under greenschist- to amphibolite-facies conditions, along with smaller volumes of granites. Different dating methods were applied to selected samples of the GMC basement to constrain the geological evolution of this part of the Alborz Mountains. A metagranite yielded two LA-ICP-MS U–Pb zircon ages of 638.4 ± 4.1 Ma and 590.3 ± 4.8 Ma that possibly date protolith crystallisation and later deformation and metamorphism, respectively, and a granite yielded a late Ediacaran 551 ± 2.5 Ma U–Pb zircon crystallisation age. A northern provenance from the basement to the South Caspian Basin can neither be established nor ruled out because no age data are available for this unit. Derivation of the GMC from Turan Block basement is unlikely, as this has a different crustal makeup and is probably composed of Paleoproterozoic and early Neoproterozoic material. The zircon ages are similar to published ages from the Arabian-Nubian Shield, indicating that this part of the Alborz basement may have belonged to the northern margin of Gondwana in the Neoproterozoic before rifting and drifting away along with other Iranian blocks (the Cimmerian terranes) during opening of the Neotethys Ocean. Chemical Th-U-total Pb ages for metamorphic monazites from two metapelite samples yielded a very large range of spot ages, of which c. 80% falls between 200 and 250 Ma, that do not allow to distinguish between Eo-Cimmerian and Main Cimmerian events in the GMC. However, they may indicate that the amphibolite-facies peak metamorphism of the GMC occurred sometime in the Triassic, in any case much later than the Carboniferous metamorphism in the neighbouring Asalem-Shanderman Metamorphic Complex to the north. Peak-metamorphic amphibole from amphibolite, retrograde white mica and foliation-defining biotite from metapelites and magmatic white mica from granite yielded much younger 175.1 ± 0.5 to 177.0 ± 0.4 Ma 40Ar/39Ar plateau ages. The Toarcian 40Ar/39Ar ages for minerals with different nominal closure temperatures reflect very rapid cooling of GMC basement below the Shemshak Group due to extension-triggered uplift. This late Toarcian to Aalenian extension event can be correlated with the regional Mid-Cimmerian unconformity of mid-Bajocian age (c. 170 Ma) that resulted from the tectonic movements causing rapid uplift and erosion. Extension probably started in the western Alborz Mountains in the Toarcian, migrated eastward, and culminated in the Aalenian in the eastern Alborz with the formation of a deep-marine basin. It was probably triggered by the onset of the subduction of Neotethys oceanic crust beneath the Central Iranian Microcontinent.

Lower Cretaceous iguanodontian dinosaurs from the southwestern margin of Gondwana

The early evolutionary and biogeographical history of Gondwanan iguanodontian dinosaurs is poorly understood due to their scarce Lower Cretaceous fossil record. In South America, the Lower Cretaceous iguanodontian osteological record is very fragmentary and most published reports cannot be used to discard or confirm hadrosauroid affinities. The single exception is Tietasaura from Brazil, whose incomplete femur shows a combination of traits found only in non-hadrosauroid iguandontians. Furthermore, no skeletal remains whatsoever of Lower Cretaceous iguanodontians have been reported from the western margin of South America. Here, we describe an isolated ornithopod caudal centrum (SGO.PV.22900) from the Lower Cretaceous Quebrada Monardes Formation in the Atacama Desert, northern Chile. Although incomplete, SGO.PV.22900 presents iguanodontian traits, such as the sub-hexagonal contour of the articular faces, the rectangular profile in lateral view and the absence of transverse processes below the neurocentral suture. We were also able to use quantitative measurements to explore taxonomic affinities, by carrying out a Linear Discriminant Analysis (LDA) and a Principal Components Analysis (PCA) using measurements of caudal centra of several iguanodontian species. The results of both analyses are consistent with those of our comparisons and suggest that SGO.PV.22900 belongs to a non-hadrosauroid iguanodontian ornithopod. This specimen represents one of the most compelling and best documented pieces of osteological evidence of Lower Cretaceous non-hadrosauroid iguanodontian dinosaurs in South America and provides further support for the presence of iguanodontians in the southwestern margin of Gondwana since at least the Early Cretaceous, as previously suggested based on footprints.

Estimating the relationship between South China and Gondwana based on big data analysis

The situation of South China within the ancient continent Gondwana remains a topic of discussion and controversy. The reconstruction of South China's linking to East Gondwana is crucial for understanding the evolutionary history of the Gondwana breakup, the growth of Asia, and the development of the Paleo-Tethys Ocean. Based on the newly established database of detrital zircons (https://repository.deep-time.org), this study integrates UPb age dating and Hf isotope analysis of detrital zircons from Early Paleozoic strata in South China. The aim is to investigate potential sources, provenance, and reconstruct its tectono-paleogeography during the early Paleozoic on the northeast margin of Gondwana. The UPb age distributions from detrital zircon populations predominantly reveal peaks in the Paleoproterozoic, Mid/Neoproterozoic, and Early Paleozoic eras, ca. 2480, 960, 830, 520, and 440 Ma, with a minor peak at approximately 1750 Ma. The significant temporal shift in the major age populations from approximately 2480 to 440 Ma reveals the global continental crustal growth of South China through a sequence of subduction and collision processes. These geodynamic processes contributed to the development of the mid-Paleozoic Kwangsian intraplate orogeny in the late early Paleozoic (ca. 440 Ma) in South China, ultimately leading to the final assembly of Gondwana. The latter orogeny is marked by regional unconformity between pre-Devonian and Devonian strata, accompanied by prevalent magmatic and metamorphic events. These earliest Proterozoic and mid-Paleozoic detrital zircon grains match with ages from India- Tethyan Himalaya region, but different from those from East Antarctica, western margin of Australia and Lhasa Block where Australia and India were detached by the Kuunga Ocean, suggesting assembly and paleogeographical constraints of South China. It fully shows that South China is located on the northeast margin of the Indian during early Paleozoic, which is of great significance for the reconstruction of the tectonic paleogeography of the continent.

From onset of Cadomian subduction to forearc-arc-continent collision: Insights from the Neoproterozoic Calzadilla Metaophiolite (SW Iberia)

ABSTRACT The Cadomian basement of SW Iberian Massif appears in the so-called Ossa-Morena Complex, which contains several allochthonous terranes that are key for deciphering the evolution of the Gondwana margin during the Neoproterozoic. In this context, the Calzadilla Ophiolite represents the remnants of an oceanic Ediacaran lithosphere formed by a boninitic mafic crust that overlies a serpentinized ultramafic section, the latter being the biggest outcrop of Cadomian serpentinites in Europe. In this work, the petrological and geochemical nature of the mantle section is analysed, as well as the tectonothermal evolution of both, the mafic crust and the ultramafic section. The algebraic analysis of whole rock geochemistry of the serpentinites indicates that the protolith corresponds to harzburgite. The geochemical fingerprint of the serpentinites, as well as the primary spinel crystals composition, shows that peridotites underwent high degrees of partial melting (up to 16%) in a forearc setting. Thermodynamic modelling and average PT methods were applied to serpentinites and mafic dykes from the ultramafic section as well as to amphibolites from the mafic section, both of which experienced progradation up to 4–5 kbar and 525–575°C. According to the data, the protoliths of the Calzadilla Ophiolite were formed in a peri-Gondwanan arc system developed during the subduction of oceanic crust below Gondwana. The roll-back of the subducting slab would induce the opening of the forearc basin at c. 602 Ma, protolith age of amphibolites from crustal section, with an extensive partial melting of the mantle wedge and the formation of boninitic-affinity magmas that constitute the mafic section of the Calzadilla Ophiolite. Metasomatism and serpentinization of the mantle wedge would occur via melt- and fluid–rock interactions. A compression-dominated stage at c. 540 led to the emplacement of the Calzadilla Ophiolite onto the Cadomian magmatic arc, with the development of the amphibolite-facies metamorphic imprint.

Garnet Pyroxenite Cumulates from Cretaceous Alkaline Intraplate Magmas Underplate the Zealandia Mantle Lithosphere

Abstract The elemental and isotopic properties of garnet pyroxenites can yield information on lithospheric mantle composition, thermal state, and evolution. The 34Ma Kakanui Mineral Breccia in New Zealand contains spectacular but little-studied mantle peridotite and pyroxenite xenoliths that yield new insights into the evolution of a portion of the underlying mantle lithosphere of a former Gondwana margin. The moderately depleted and metasomatized spinel peridotites, as judged from spinel and olivine compositions and bulk rock major and platinum group element abundances, give mineral equilibration temperatures <1020°C and are derived from the middle to shallow (~35 to 50 km) lithospheric mantle when projected onto a 70 mW∙m−2 geotherm. These residues have low Re/Os and Re-depletion 187Os/188Os model ages that range from Eocene (0.05 Ga) to Paleoproterozoic (1.9 Ga), consistent with extraction from a lithospheric mantle comprising fragments with complex depletion histories. Although the peridotites have restricted δ18O (olivine +5.2 to 6.2), evidence for an isotopically heterogeneous mantle column in addition to the 187Os/188Os is seen in clinopyroxene 87Sr/86Sr (0.70244 to 0.70292), εNd (+4.1 to 18.8), 206Pb/204Pb (17.8 to 20.3), and εHf (+10 to +101). Higher metamorphic equilibrium temperatures of the garnet pyroxenites (Fe–Mg exchange of >1150°C) compared to the peridotites indicate their Eocene extraction was from towards the base of this isotopically heterogeneous mantle lithosphere. Pyroxenite bulk compositions point to cumulate origins, and the mineral isotope ratios of 87Sr/86Sr (0.70282 to 0.70294), εNd (+5.5 to 8.0) and 206Pb/204Pb (18.1 to 19.3) match many of the Zealandia metasomatized mantle peridotite xenoliths as well as the primitive intraplate basalts but not the garnet pyroxenite host magmas. In contrast to many global pyroxenite studies, the garnet pyroxenite 87Sr/86Sr and δ18O (+5.2 to 5.8) data provide no evidence for subducted crustal material in the primary magma source region, and Sm–Nd and Lu–Hf isotope data yield mid-Cenozoic ages that are probably related to isotope closure during eruption. An exception is one sample that yields a Lu–Hf isochron age of 111.9 ± 9.1 Ma, which corresponds to the convergence of the Lu–Hf isotope evolution curves of three other samples. Liquids calculated to have been in equilibrium with these cumulates have trace element compositions comparable to primitive alkaline intraplate basalts like those found at the surface of Zealandia. The new data, therefore, indicate that a pulse of intraplate magmatism occurred during or directly after the cessation of long-lived subduction on the former Zealandia Early Cretaceous forearc Gondwana margin, despite any volcanic surface exposure having been long eroded away. The lower lithospheric mantle emplacement of the garnet pyroxenites suggests that the source of the alkaline parent magmas was probably the convecting mantle, which supports conclusions that intraplate magmas in Zealandia have asthenospheric and lithospheric mantle sources.

Reviewing the geochemistry and petrology of the subcordilleran plutonic belt, early Jurassic magmatic arc in northern Patagonia

The Subcordilleran Plutonic Belt, formed in Early Jurassic times, is a short-lived batholith that developed during Gondwana dismembering. In our study, we analyzed both well-known and potential new localities of the belt of plutonic rocks, considering new whole-rock geochemical data as well as data from the literature. Additionally, new geochemical data from five samples and one U-Pb zircon date are presented from the diorites and granodiorites from the Cerro Catedral, at Bariloche (41°S). The obtained U-Pb age in zircon is Early Jurassic (192 ± 1.5 Ma).The granitoids of the Subcordilleran Plutonic Belt are calc-alkaline, of medium to high potassium. They exhibit similar alkali content than the granitoids of the Middle Jurassic-Cretaceous Patagonian Batholith, and lower alkalis than the volcanic and plutonic rocks of the Jurassic Chon Aike Silicic Large Igneous Province. These findings suggest that the Subcordilleran Plutonic Belt represents, in the region comprised within 39° and 45° S, an early stage of the Andean arc. Our observations indicate that while some granitoids in potential localities, such as Cerro Catedral, can now belong to the Subcordilleran Plutonic Belt, other sites, like Pilcaniyeu and Pilahue, may be associated with the plutonic facies of the Chon Aike Province.The Subcordilleran Plutonic Belt is an example of arc magmatism that developed during a period of major plate reorganization, along with the subduction of mature sea floor beneath the Gondwana margin, and the latter promoted slab roll-back processes and a subduction with extension tectonic regime.

Berthierine/Chamosite ooids and associated phases in Devonian deposits of the Parnaíba Basin, Brazil: Genesis and diagenetic evolution

The Parnaíba Basin Devonian deposits comprise ironstones and organic-rich sediments deposited in an epeiric sea margin under greenhouse conditions, enhanced weathering, reduction of oceanic circulation, and black shale deposition. This study aims to contribute to the understanding of the interrelation between depositional and shallow diagenetic processes of the Devonian units of the Parnaíba Basin, specifically regarding the genesis of iron minerals that constitute the ooidal ironstones of the Itaim and Pimenteiras formations. These units comprise bioturbated heterolites, ooidal ironstones, intraclastic conglomerates, and shales, deposited in a deltaic to neritic, storm-dominated platform. This research investigates drill cores from two wells in the southern part of the Parnaiba Basin, focusing on depositional and diagenetic processes. The proposed model suggests deposition of sediments originally rich in organic matter, under low siliciclastic input, and initially anoxic, transgressive conditions. Under these conditions, ferroan precursor clays were deposited, alternating with periods of reworking by storm currents, generating concentric ooids of berthierine. Bioturbation processes resulted in the concentration of previously formed berthierine ooids during intervals of low sedimentation rates. Eodiagenetic microcrystalline siderite formed along the redox boundary between oxic/bioturbated and anoxic zones. Chamosite formed by partial transformation of berthierine, upon incipient mesodiagenetic evolution of the ooidal ironstones. This study contributes to the understanding of depositional and diagenetic conditions in the Devonian section of the Parnaíba Basin, and of similar deposits, offering insights into the formation and development of berthierine ooids under the influence of sea level fluctuations, oxic/anoxic conditions, sedimentation rate, and bioturbation in sedimentary environments, providing key insights into the paleoenvironmental conditions along the northern Gondwana margin during this period.

Alkaline Devonian magmatism of the Menorca Island: Tracking the mantle isotopic sources in the realm of the western Paleo-Tethys Ocean

Several locations with alkaline magmatism are recognised in Silurian-Devonian basins along the southern variscan autochthon units (e.g. Central Iberian Zone) of the northern Gondwana margin. The origin of the Devonian basins and their magmatism has not been studied in the context of the passive margin of Gondwana. The basement of Menorca, Balearic Islands, consists of a deep Devonian-Carboniferous basin with mafic igneous rocks, the Tramuntana Gabbros. In this study, we trace the geodynamic setting and isotopic sources of the Tramuntana Gabbros through elemental geochemistry, isotopic geochemistry (SrNd) and UPb geochronology in zircons. These gabbros are the product of an intraplate alkaline magmatism with immobile trace element and REE contents similar to those of Ocean Island Basalts. Average 87Sr/86Sr(370) of 0.708456 and εNd(370) of +4.0 indicate a source similar to a Type-2 enriched mantle with average TDM of 665 Ma, suggesting a relatively old metasomatized mantle. Concordant UPb ages of c. 597 Ma (Ediacaran, radiometric age) from a single population of 31 zircons separated from the Tramuntana Gabbros (Devonian, biostratigraphic age) reinforce the presence of older units in the corresponding lithospheric mantle. The Tramuntana Gabbros and the Devonian-Carboniferous sequences of Menorca limit the westward extension of the Paleo-Tethys Ocean, whose development never reached these westernmost regions. Assuming a common sublithospheric mantle source for the peri-Gondwanic Devonian alkaline rocks and considering the previous Cadomian (Neoproterozoic) subduction to be the most favourable origin of the separated zircons, the bulk rock TDM and zircon UPb data obtained from the Tramuntana Gabbros track the mixing, recycling, and mantle accretion in this peri-Gondwanic section from Precambrian to Devonian times.

Neoproterozoic tectonic shift along the western margin of the Yangtze block, South China craton: Implications for its paleogeographic position during the Rodinia−Gondwana transition

Neoproterozoic igneous rocks in the Panxi region from the western margin of the Yangtze block, South China craton, record major changes in tectonic evolution and crustal thickness, and provide important constraints on the paleogeographic position and climate changes of the block in Rodinia and Gondwana reconstructions. Our new dating results show that the Pengguan complex from the Panxi region comprises ca. 743 Ma adakitic granodiorites and ca. 741 Ma A-type granites. The former is plagioclase-rich and has relatively low SiO2 content (65.22−67.36 wt%), high Mg# values (44.40−52.33), Sr/Y (23.22−61.94) and (La/Yb)N ratios (normalized to chondritic values; 13.41−24.00), with positive whole-rock εNd(t) values (+1.17 to +5.15), and high zircon εHf(t) (+5.53 to +8.83) and low zircon δ18O (4.21‰ to 5.51‰) values, indicating derivation from the partial melting of thickened juvenile mafic lower crust. By contrast, the granites are rich in K-feldspar and have high SiO2 contents (78.71−84.62 wt%), 10,000 × Ga/Al ratios (>2.6), and Fe/Mg ratios, with negative εNd(t) values (−1.47 to −0.02), high zircon εHf(t) (+2.94 to +9.77), and high saturation temperatures (828−920 °C). These features indicate fractional crystallization and partial melting of continental crust in a low-pressure, high-temperature, extensional environment. Integrated with the zircon Eu anomalies (Eu/Eu*) data and Hf-O isotopic data on pre-740 Ma magmatism in the Panxi region, we find that a rapid increase of crustal thickening (820−740 Ma) was synchronous with a gradual decrease of zircon δ18O and an increase of zircon εHf(t) values. These variations indicate an increase in the degree of hybridization between mantle sources and subducting slab-derived materials, thus generating a great number of low-δ18O melts, and continuous crustal thickening through the underplating of juvenile magmas. In addition, thinning of the crust since ca. 740 Ma, along with the gradual increase in zircon δ18O and εHf(t), suggests a shift from regional compression to extension. This documented shift, together with regional data from along the western margin of the Yangtze block, indicates a change from the well-developed and long-established Panxi convergent plate margin arc system (at least ca. 870−750 Ma) to passive-margin development and Cryogenian (720−635 Ma) glaciation. The long-lived subduction zone requires a peripheral location of South China that was linked to northern India during the Rodinia breakup. The subsequent and contemporaneous passive-margin development of the Yangtze block and India, along with the similar tectonic affinities of the Cathaysia block with India, indicate that South China lay outboard of the northern Gondwana margin during the assembly of this supercontinent.

Contourite-drift archive links Late Devonian bioevents with periodic anoxic shelf water cascading

Analysis of a Devonian contourite depositional system in the eastern Anti-Atlas of Morocco reveals the formation of widespread erosional hiatuses and organic-rich bioclastic contourites (ORCs) coinciding with the expansion of an anoxic water mass during Frasnian bioevents, ultimately culminating in the Kellwasser crisis (Frasnian-Famennian extinction event). The identified contourite terrace formed on the uppermost slope of the northern passive margin of Gondwana. Its inner part was bounded by an along-slope contourite channel and a small mounded drift at its downslope margin. Facies- and drift-scale contourite features evidence northwest-directed bottom currents driven by repeated overflows of dense, highly saline, anoxic water originating from the northern Gondwana Epicontinental Sea. These periodic overflows were channeled through the Ougarta trough, then deflected westward over the Tafilalt contourite terrace by the Coriolis force and cascaded downslope until reaching a density equilibrium level, probably forming an intermediate water mass. The cascading of dense, anoxic shelf water supports the photic-zone eutrophication (top-down) model proposed for the Kellwasser crisis and related Devonian anoxic events. We propose a direct link between the anoxic overflows and the Devonian evolutionary events.