Western Gondwana Research Articles

The unusual record of Nereites, wrinkle marks and undermat mining trace fossils from the late Silurian–earliest Devonian of central-western margin of Gondwana (Argentina)

Silurian–Devonian deposits of central western Argentina contain one of the most diverse Palaeozoic records of Nereites in Gondwana. Nereites cf. cambrensis, Nereites irregularis?, Nereites jacksoni, Nereites macleayii, Nereites missouriensis and Nereites pugnus are documented in a turbidite-like succession of the San Rafael Block (Río Seco de los Castaños Formation). The succession, historically assumed as deep-sea deposits or interpreted as a deltaic system, also contains abundant microbially induced sedimentary structures (MISS) such as wrinkle marks. Glossy surfaces indicate abundant biofilms, where the fossil record comprises N. macleayii, N. missouriensis, scarce arthropod trackways (Diplichnites?) and undermat-miner structures. Nereites, the pioneer colonizer, is in some cases cross-cut by Dictyodora, while undermat miners are the late bioturbators. The succession is dominated by gravity flow deposits, including storm related, turbidite-like deposits and abundant wrinkle marks. These suggest deposition on the shelf rather than the deep sea. This study shows that microbial mats were not restricted to marginal marine environments during the mid Palaeozoic, and documents one of the most diverse Palaeozoic records of Nereites in western Gondwana. It also contributes to the record of Nereites, Dictyodora, and Zoophycos found, for first time, in association with microbial mat structures in the late Silurian−earliest Devonian, the Nereites ichnofacies together with abundant wrinkle marks.

First occurrence of a mawsoniid coelacanth in the Early Jurassic of Europe

ABSTRACT—Coelacanths form a clade of sarcopterygian fishes (lobe-finned vertebrates) that today is represented by a single genus, Latimeria. This genus belongs to a lineage of marine coelacanths, the latimeriids, whose fossils are common in the Jurassic and the Cretaceous deposits of Europe and North America. During the same periods, another lineage of fresh/brackish water coelacanths, the mawsoniids, occurred in South America, Africa, and Madagascar. Mawsoniids are supposed to have originated during the Triassic in North America and were assumed to have subsequently dispersed to South America during the Jurassic, before reaching western Africa during the Early Cretaceous. Previous hypotheses advocated that mawsoniid coelacanths reached Europe during the Late Cretaceous, suggesting the dispersal of freshwater organisms from Africa to Europe during this period. We here reevaluate this scenario based on the reexamination of the coelacanth Trachymetopon from the Early Jurassic of Germany. Although this genus is known from remarkably well-preserved material, its relationships to other Mesozoic coelacanths remained unsolved. An anatomical investigation shows that Trachymetopon shares many common features with the Late Jurassic–Late Cretaceous mawsoniids Mawsonia and Axelrodichthys from western Gondwana, such as the absence of a descending process of the supratemporal, the presence of ossified ribs, and the skull roof and cheek bones ornamented by conspicuous coarse rugosities and ridges. A phylogenetic analysis of morphological characters places Trachymetopon within Mawsoniidae. We suggest that mawsoniid coelacanths were already present in Europe from the Early Jurassic onwards, challenging previous paleobiogeographic scenarios.SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

The Early Cretaceous megaflora of the Springhill Formation, Patagonia. Paleofloristic and Paleonvironmental inferences

The Springhill Formation (Lower Cretaceous) is considered as one of the most important oil producer units within the Austral Basin in Argentina. However, knowledge regarding this formation's plant community composition is still very scarce. In this work a thorough survey of the outcrops of the Springhill Formation was conducted regarding both geological and palaeobotanical aspects. Abundant fossil plant material was collected at the Estancia El Salitral and Río Correntoso localities in Santa Cruz Province, Argentina, consisting primarily of compressions of leafy shoots with several branching orders, completely to partially preserved fronds, simple to pinnate leaves and fertile scales. Analysis of these materials made it possible to describe 20 new taxa which add to the already known flora of the Springhill Formation; nine of these taxa were found in Estancia El Salitral, and the remaining 11 were found in Río Correntoso. The floristic diversity of the unit has increased through the discovery of the first megafossils of ginkgophytes (Baiera sp. 1 and Baiera? sp. 2), whereas new taxa have been described within the divisions of Pteridophyta, Pteridospermophyta, Cycadophyta and Pinophyta. Analysis of the vegetative and reproductive epidermal structures of the various groups included within this megaflora has been demonstrated to be a valuable tool for the recognition and differentiation of species. Analysis has also made it possible to suggest paleoenvironmental inferences. Finally, the megaflora from the Springhill Formation was compared to other relevant plant assemblages from the Lower Cretaceous of Western Gondwana. The megaflora was determined to be most similar to that of the well-known Patagonian Anfiteatro de Ticó Formation and most different from that of the Brazilian Crato Formation.

Structural evolution of the Sarandí del Yí Shear Zone, Uruguay: kinematics, deformation conditions and tectonic significance

The Sarandi del Yi Shear Zone is a crustal-scale shear zone that separates the Piedra Alta Terrane from the Nico Perez Terrane and the Dom Feliciano Belt in southern Uruguay. It represents the eastern margin of the Rio de la Plata Craton and, consequently, one of the main structural features of the Precambrian basement of Western Gondwana. This shear zone first underwent dextral shearing under upper to middle amphibolite facies conditions, giving rise to the reactivation of pre-existing crustal fabrics in the easternmost Piedra Alta Terrane. Afterwards, pure-shear-dominated sinistral shearing with contemporaneous magmatism took place under lower amphibolite to upper greenschist facies conditions. The mylonites resulting from this event were then locally reactivated by a cataclastic deformation. This evolution points to strain localization under progressively retrograde conditions with time, indicating that the Sarandi del Yi Shear Zone represents an example of a thinning shear zone related to the collisional to post-collisional evolution of the Dom Feliciano Belt that occurred between the Meso- to Neoproterozoic (>600 Ma) and late Ediacaran–lower Cambrian times.

Early Cretaceous palynostratigraphy of the Abu Tunis 1x borehole, northern Western Desert, Egypt, with emphasis on the possible palaeoclimatic effect upon the range of Dicheiropollis etruscus in North Africa

Recent hydrocarbon exploration in the northern sector of the Western Desert in Egypt has revealed relatively rich hydrocarbon accumulations, mainly of gas, and demonstrate promising future prospects. In order to improve our understanding of this area and to provide a biostratigraphical framework for the hitherto poorly dated Lower Cretaceous successions, a palynological analysis was carried out on 57 ditch cutting samples from the Abu Tunis 1x borehole. Palynostratigraphic investigation on these samples has enabled the identification of three new palynostratigraphically defined age divisions with three corresponding palynozones defined by first uphole occurrences of gymnosperm and angiosperm pollen and dinoflagellate cysts. Spore and pollen grains recovered from the Abu Tunis 1x borehole show the characteristics of the pre-Albian Dicheiropollis/Afropollis Phytogeographical Province. Discrepancies in the reported range of Dicheiropollis etruscus, when compared with earlier (Berriasian) appearances in West Africa and later (late Hauterivian) appearances in East Africa, may be attributed to palaeoecological factors. Dicheiropollis etruscus is accepted as having a cheirolepidiacean conifer affinity and is regarded as having been produced by a thermophilous plant. Here, we suggest that Dicheiropollis etruscus was adapted to arid conditions. Dicheiropollis etruscus thus first appeared in hot, dry palaeo-subtropical African regions, but as Western Gondwana broke up and the African Plate moved northeast during/after the Late Jurassic, the region that is now present-day Egypt, Libya and Sudan had moved by the late Hauterivian into a subtropical position; the ensuing increased aridity thus allowed Dicheiropollis etruscus to migrate into these areas.

Lithogeochemistry of the meta-igneous units from Arroio Grande Ophiolitic Complex, southernmost Brazil

Ophiolites are defined as slices of genetically-related upper mantle serpentinized peridotites and oceanic crustal rocks, tectonically displaced from its primary igneous origin of formation by plate convergence and associated (meta) sedimentary rocks of marine origin. From this premise, a meta-ultramafic-mafic-sedimentary complex (Cr-rich magnesian schists - upper mantle or crustal ultramafic cumulate candidates; epidote amphibolites, metadiorites and metagabbros - oceanic crust candidates; metasedimentary schists, quartzites and marbles - marine sedimentary rocks candidates), located in southeastern Dom Feliciano Belt (southernmost Brazil), started to be interpreted as possible slices of an ophiolitic complex related to the closure of a paleo-ocean during Brasiliano/Pan-African orogenic cycle and was called Arroio Grande Ophiolitic Complex. The present research fills the lack of geochemical data from previous studies and tests the hypothesis of an oceanic setting for the meta-igneous units of this complex from a lithogeochemistry point of view. The meta-ultramafics were interpreted as peridotites (mantle or crustal cumulates) that were subsequently serpentinized (probably in the ocean floor) and posteriorly metasomatized (probably in a continental setting). The meta-mafics were interpreted as oceanic gabbros/basalts formed in a back-arc basin. The results, together with field relationships, rock associations and petrographic evidences, support an oceanic origin for the protoliths of the meta-igneous units. The hypothesis that these rocks represent metamorphosed slices of an ophiolitic complex is still the most reasonable one. This work updates the geologic knowledge of the area and supports discussions about the evolution of Dom Feliciano Belt and Western Gondwana paleocontinent.

High-pressure whiteschists from the Ti-N-Eggoleh area (Central Hoggar, Algeria): A record of Pan-African oceanic subduction

The Ti-N-Eggoleh area (Sérouènout Terrane, Central Hoggar, Algeria) comprises mainly a high-pressure Neoproterozoic metamorphic formation consisting of talc–kyanite–quartz whiteschists, chlorite schists, marbles, sulphide ores, partially serpentinized peridotites and partially amphibolitized eclogites, and reminiscent of an ophiolitic mélange that was metamorphosed and strongly deformed under eclogite-facies conditions. Major and trace elements indicate that the whiteschists underwent intense hydrothermal alteration, with Mg enrichment and leaching of alkalis and Ca, prior to high-pressure metamorphism. The main talc–kyanite–quartz paragenesis is stable within a large multivariant P–T field extending from high-P amphibolite to eclogite facies; the Tschermak substitution in talc marginally constrains peak pressure conditions to P>11kbar and 600<T<~800°C. The subsequent development of cordierite±sapphirine±corundum symplectites and coronae at the contact between talc and kyanite was due to isochemical and almost univariant reactions (Tlc+Ky+Qtz→Crd; Tlc+Ky→Crd+Crn; Tlc+Ky→Crd+Spr) which combined together to produce hybrid microstructures. These metamorphic reactions indicate a thermal overprint under granulite-facies conditions (T=650–860°C; P<12kbar). The peraluminous Mg-rich sapphirine observed in the symplectites reaches one of the highest degrees of Tschermak substitution ever reported for this mineral (with n=3.6), intermediate between the 13:19:5 (n=3) and 3:5:1 (n=4) theoretical compositions. The neighbouring eclogites record a similar P–T evolution: after the eclogite-facies metamorphic peak, they underwent partial amphibolitization and a subsequent high-temperature overprint under granulite-facies conditions that led to partial dehydration. The Ti-N-Eggoleh series is interpreted as the product of the thermal alteration of oceanic rocks that were subducted prior to the continental collision that formed the West Gondwana orogenic belt during the Neoproterozoic Pan-African orogeny. The decompression associated with the early phase of exhumation was followed by an important increase in temperature towards granulite-facies conditions, possibly determined by the intrusion of abundant mafic rocks in this region due to delamination of the lithospheric mantle. The Ti-N-Eggoleh area and its high-pressure meta-ophiolitic series apparently belong to the Sérouènout Terrane, which stretches along the eastern margin of the Western Gondwana orogenic belt and consists mainly of oceanic metasediments; they are possibly markers of an ancient, yet unidentified, subduction and suture zone.

Combined U–Pb and Lu–Hf isotope study from the Las Lozas volcanics, northwestern Argentina: Evidence of juvenile Cryogenian-derived, lower Pennsylvanian volcanism in western Gondwana

Las Lozas volcanic succession, cropping out in the southwestern Puna, Catamarca province, Argentina, consists of an intracontinental volcanic sequence of Carboniferous age. The lavic members are predominantly rhyolites, and subordinated andesites and basalts. The volcaniclastic layers consist of monomictic and polymictic breccias with structures denoting processes of lava fragmentation. They constitute a bimodal suite, ranging from basalts to high silica rhyolites. A new U–Pb SHRIMP age of 320 ± 2 Ma for a rhyolite allows extending the Mississipian magmatism of the region to the lower Pennsylvanian. Hf data point to juvenile sources of Cryogenian age with no evidence of older reworked crustal contamination.

Magnetotelluric characterization through the Ambargasta-Sumampa Range: The connection between the northern and southern trace of the Río de La Plata Craton – Pampean Terrane tectonic boundary

The South American Platform was part of the Western Gondwana, a collage of plates of different ages assembled in late Neoproterozoic to Cambrian times. The Transbrasiliano Lineament, a continental shear belt that transversely intersects this platform from NE to SW, has its southern expression in the tectonic boundary between the Río de La Plata Craton and the Pampean Terrane. Magnetotelluric long-period data in a W–E profile (29°30′ S) that crosses the Ambargasta-Sumampa Range and the Chaco-Pampean Plain were obtained to connect information of this mostly inferred tectonic boundary. A 2-D inversion model shows the Chacoparanense basin, Río Dulce lineament, Ambargasta-Sumampa Range and Salina de Ambargasta in the upper crust. At mid-to-lower crust and 40 km to the east of the Ambargasta-Sumampa Range, a discontinuity (500–2000 Ω m) of 20-km-wide separates two highly resistive blocks, the Río de La Plata Craton (6000–20,000 Ω m) in the east, and the Pampean Terrane (5000–20,000 Ω m) in the west. This discontinuity represents the tectonic boundary between both cratons and could be explained by the presence of graphite. The geometry of the Pampean Terrane suggests an east-dipping paleo-subduction. Our results are consistent with gravimetric and seismicity data of the study area. A more conductive feature beneath the range and the tectonic boundary was associated with the NE–SW dextral transpressive system evidenced by the mylonitic belts exposed in the Eastern Pampean Ranges. This belt represents a conjugate of the mega-shear Transbrasiliano Lineament and could be explained by fluid–rock interaction by shearing during hundreds of years. The eastern border of the Ambargasta-Sumampa Range extends the trace of the Transbrasiliano Lineament. The electrical Moho depth (40 km to the west and 35 km to the east) was identified by a high electrical contrast between the crust and upper mantle. The upper mantle shows a resistive structure beneath the Río de La Plata Craton that could have been originated by stationary delamination by the presence of hydrated lithosphere.

Diachronic collision, slab break-off and long-term high thermal flux in the Brasiliano–Pan-African orogeny: Implications for the geodynamic evolution of the Mantiqueira Province

The Mantiqueira Province in SE Brazil and Uruguay is a complex Brasiliano/Pan-African orogenic province that has recently been the topic of conflicting geodynamic models focusing the possibility of multiple collision events during the Ediacaran–Ordovician assembly of the Western Gondwana Supercontinent. A thorough compilation of recently obtained high-resolution U–Pb geochronology and geochemistry in the Mantiqueira Province provided the following results: (a) considering the entire orogenic system, the pre-, syn- and post-orogenic periods range from >670–595Ma, 620–550Ma and 560–490Ma, respectively, and are coeval with the intrusion of 3 distinct magmatic sequences within each geographic segment of this large orogenic province, evolving from: (i) calc-alkaline, arc-related magmatism; to (ii) syn-orogenic, anatectic magmatism coeval with the metamorphic climax; to (iii) calc-alkaline/alkaline (including shoshonitic and A-type affinity) magmatism that reveal significant lower crust and mantle contribution; (b) progressively younger ages are present from south to north, irrespective of the orogenic period considered, implying that the different geographic sectors were, from south to north, diachronically amalgamated into the Mantiqueira Province. Results provide important evidence for the existence of a single diachronic collision event at 630–590Ma, followed by the thermal climax at 615–560Ma, slab break-off, asthenospheric upwelling and mantle underplating that sustained long-term high heat flux conditions until 540–490Ma, when post-tectonic calc-alkaline/alkaline granitoids (probably admixed with small amounts of slab melts formed at depth) intruded the middle/low-crust magmatic and metamorphic sequences during the final stages of thermal and orogenic collapse of the Mantiqueira Province belts.

Prograde and retrograde evolution of eclogite from Adrar Izzilatène (Egéré-Aleksod terrane, Hoggar, Algeria) determined from chemical zoning and pseudosections, with geodynamic implications

Adrar Izzilatène in the Egéré-Aleksod terrane of the LATEA metacraton (Hoggar, Algeria) exposes one of the best preserved examples of eclogite facies metamorphism in Hoggar. Three distinct stages of metamorphic development are recognised, namely, the pre-peak stage (M1), characterised by garnet, amphibole, epidote, quartz and rutile, the peak eclogite facies stage (M2), consisting of omphacite, garnet, edenite, epidote, quartz and rutile, and the retrograde stage (M3), where initial decompression resulted in the appearance of plagioclase, the development of pargasite+plagioclase kelyphites and finally the formation of anhydrous plagioclase+diopside coronas. Porphyroblastic omphacite has a jadeite content of up to XJd=0.36, which is the highest yet observed for eclogite facies rocks from the Tuareg Shield. Garnet growth zoning patterns are characterised by flat profiles in the cores (XAlm=0.55–0.60; XPrp=0.12–0.16; XGrs=0.26–0.30) before showing a decrease in almandine to XAlm=0.45, coupled to an increase in pyrope to XPrp=0.29 and decrease in grossular to XGrs=0.26 at the rims. Calculated P–T–MH2O pseudosections show that the prograde M1 assemblage equilibrated at 13–14kbar and 580°C, before pressure and temperature increased to 19kbar and 650–700°C at fluid-saturated conditions during peak metamorphism. Retrogression involved near-isothermal decompression to 8–9kbar and 700–750°C at fluid-undersaturated conditions. Prograde-to-peak metamorphism of the Izzilatène eclogite could have involved either oceanic or continental subduction, followed by exhumation as the area was obducted towards the LATEA metacraton during the Pan-African orogeny and the assembly of Western Gondwana.

The late Neoproterozoic Sierra de las Ánimas Magmatic Complex and Playa Hermosa Formation, southern Uruguay, revisited: Paleogeographic implications of new paleomagnetic and precise geochronologic data

A new paleomagnetic study was carried out on 15 sites of the Sierra de las Ánimas complex, exposed in SE Uruguay. A paleomagnetic remanence was isolated at six sites, and this direction is likely to be pre-tectonic and primary. Mean directions from five of these sites are consistent with the SA2 paleomagnetic pole, which is based on a previous paleomagnetic study of the complex. A new paleomagnetic pole was computed for compiled results from the Sierra de las Ánimas complex (SAn, 12.2°S, 258.9°E, A95: 14.9°). A geochronological study based on U/Pb (SHRIMP) dating of magmatic zircons was also carried out on three samples of the Sierra de las Animas Complex, yielding a grand mean age of 578±4Ma. The new SAn paleomagnetic pole anchors the Early Ediacaran segment of the Apparent Polar Wander Path for the Río de la Plata craton. Consistency of the new pole with other Ediacaran poles from the Congo-São Francisco craton suggests a coherent Central Gondwana by this time. These data also support a wide Clymene Ocean between the conjoined Río de la Plata–Congo-São Francisco blocks and Western Gondwana, i.e., Amazonia–West Africa. Additional sampling (sixteen samples) was carried out on the basal levels of the glaciogenic Playa Hermosa Formation, exposed in the same area. These samples retain a paleomagnetic component consistent with the previously reported data, confirming the original pole position. U–Pb (SHRIMP) dating of detrital zircons reveal a bimodal distribution of Paleoproterozoic (around 2.1Ga) and late Neoproterozoic ages. The youngest zircons are interpreted as pre-dating to penecontemporaneous with deposition and points to a ≤594Ma age for deposition of the older levels of the Playa Hermosa Formation. A minimum age of deposition is set by the 578±4Ma age of the Sierra de las Ánimas magmatism. The low inclination of magnetization indicates that low-latitude (13.0°+9.5°/−5.5°), Gaskiers-aged glacial deposits are preserved atop the Rio de la Plata craton.

Sedimentology and sequence stratigraphy from a mixed (carbonate–siliciclastic) rift to passive margin transition: The Early to Middle Cambrian of the Argentine Precordillera

Through the analysis of facies stacking patterns, geometries and bounding surfaces, sequence stratigraphy provides a framework to understand sedimentary facies distribution and evolution. Many published works contain noteworthy examples where a sequence stratigraphy approach was applied to Middle–Late Paleozoic, Mesozoic and Cenozoic sedimentary units. Early Paleozoic examples, however, remain relatively scarce. This article addresses the exceptionally well-exposed Early to Middle Cambrian La Laja Formation and equivalent units of the Argentine Precordillera carbonate bank. Given the stratigraphic and faunal similarities, the Precordillera is considered as a Laurentia-derived allochthonous terrane, which detached from the Laurentia margin during Early to Middle Cambrian and docked against western Gondwana margin by Mid-Ordovician time. This work focuses on the lowermost mixed carbonate–siliciclastic interval that records the early evolution of the Precordillera terrane, providing a link to better understand the paired Laurentia margin and Lower Paleozoic paleogeography. The study was built upon a detailed facies and microfacies analysis where thirteen facies associations were recognized, representing a spectrum of environments ranging from deep subtidal, shallow subtidal to inter-supratidal sabkha facies. Our analysis allowed the recognition of three sequences (S1, S2 and S3) separated by unconformities (D1, D2, D3 and D4). A reciprocal sedimentation model is suggested to explain coarse-grained siliciclastic intervals overlying and bypassing subtidal limestones in S1 and S2. However the fine-grained siliciclastic-rich carbonate intervals within S2 seem to represent transgressive deposits, probably influenced by higher accommodation and climatically driven siliciclastic influx. Lateral facies and thickness variations suggest that the La Laja Formation developed during the progressive margin stabilization with differential subsidence. The sequence boundary separating the La Laja Formation from the overlying Zonda Formation represents an important architectural change from differential subsidence during the late rift stage in the Early–Middle Cambrian La Laja Formation to the passive margin stage with flat-lying remarkably continuous rhythmic dolomites in the overlying Upper Cambrian Zonda Formation. Further strengthening the stratigraphic similarities with the Lower Paleozoic of Laurentia, a similar transition has also been suggested for coeval units of the Laurentia paired margin. This work highlights the importance that Precordillera stratigraphy is linked to the early evolution of the paired margins during the Iapetus Ocean opening and Lower Paleozoic paleogeography reconstructions.

Geological context of the dinosauriform-bearing outcrops from the Triassic of Southern Brazil

The Triassic of the western Gondwana (southern Brazil and northwestern Argentina) records the oldest dinosaurs. The Southern Brazilian Triassic fauna is subdivided into four assemblage zones (AZ's), recorded in alluvial (channel and floodplain) deposits, split into three third-order sequences that comprise the Santa Maria Supersequence. These deposits record dinosauriforms in three of these AZ's, mostly in near-channel environments (channel deposits, crevasse splays, distal floodplains) with different types of preservation (mostly partly articulated, with little carbonate deposition). There is faciological homogeneity within the Dinodontosaurus, Santacruzodon and Hyperodapedon AZ's, whereas change in fluvial style is recorded at the Riograndia AZ. So, further stratigraphic studies must include the recognition of post-depositional tectonism, in order to better understand the autogenic and allogenic mechanisms of deposition. It is suggested here that there is a lateral change on main channel deposition, with the areal restriction of the Santacruzodon AZ probably linked to reactivation on the Passo do Sobrado lineament and modification of the basin floor.

Early seed plants from Western Gondwana: Paleobiogeographical and ecological implications based on Tournaisian (Lower Carboniferous) records from Argentina

The oldest seed occurrences in Western Gondwana have been recognized in a new stratigraphic section located in Western Argentina (Precordillera Basin). Palynological evidence indicates an Early Mississippian (probably Tournaisian) age for this new succession. The two identified early seeds generas, Pseudosporogonites cf. hallei and Warsteinia sancheziae n. sp. were up to now considered as restricted to the Devonian of Laurussia. This finding suggests a dispersal of earliest spermatophytes between Laurussia and Gondwana during Devonian/Tournaisian times, thus accounting for the Rheic Ocean as a surmountable biogeographic barrier for continental biotas. Alternatively, contrasting biogeographic hypotheses dealing with early spermatophytes rising in paleotropics and then displacing herbaceous communities of non-spermatophytes typical from cool high latitudinal regions, are explored for explaining the recognized paleobiogeographical pattern. The new information supports a weak impact of the Devonian/Carboniferous biotic crisis on earliest seed plant diversity. Based on preliminary evidences of niches differentiation and ecological dynamics probably affected by wildfires, Tournaisian Gondwanan plant communities from high latitudes are interpreted as being more complex than previously thought, and more similar to those reported from Laurussia. In addition, their discovery in a sedimentary environment associated to glacigenic deposits, show that this new record might be linked to the coeval glacial age widely recorded elsewhere in Gondwana.

U–Pb age of the coesite-bearing eclogite from NW Borborema Province, NE Brazil: Implications for western Gondwana assembly

The Late Neoproterozoic assembly of western Gondwana played an important role in the subduction of oceanic and continental lithospheres. Such event was also a source of arc magmatism, reworking of cratonic margins and development of ultra-high pressure (UHP) suture zones. In the Borborema province, NE Brazil, we have described for the first time UHP rocks enclosed within gneiss migmatite and calc-silicate rocks. They bear coesite included in atoll-type garnet from metamafic rocks, identified by petrographic study and Raman microspectroscopy analysis. U–Pb zircon dating of the leucosome of the migmatites and the calc-silicate rock displays, concordant ages of 639±10Ma and 649.7±5Ma, respectively, here interpreted as the minimum age of the eclogitization event in the region. U–Pb zircon dating of the coesite-bearing rock defined a concordia age of 614. 9±7.9Ma that comprised the retrograde eclogitic conditions to amphibolite facies. The UHP rocks, mostly retrograded to garnet amphibolites, occur enclosed in the Paleoproterozoic continental block composed of calc-silicate rocks, migmatized sillimanite gneiss, mylonitic augen gneiss and granitic and tonalitic gneiss along a narrow N–S oriented belt between the Santa Quitéria magmatic arc and the Transbrasiliano lineament. This block was involved in the subduction to UHP eclogite depths, and was retrogressed to amphibolite during its exhumation and thrusting. Our data indicate an important Neoproterozoic transcontinental suture zone connecting the Pharusian belt with Borborema Province, and probably with the Brasília belt in central Brazil.

Palynological record of the Silurian/Devonian boundary in the Argentine Precordillera, western Gondwana

Fil: Garcia, Victoria Josefina. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Mendoza. Instituto Argentino de Nivologia, Glaciologia y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivologia, Glaciologia y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivologia, Glaciologia y Ciencias Ambientales; Argentina. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Mendoza; Argentina

Early Ordovician Trilobites from the Nazareno Area, Northwestern Argentina

The upper part of the Santa Rosita Formation (Ordovician, Tremadocian) in the Nazareno area, Cordillera Oriental, northwestern Argentina, records the vertical passage of high-energy, shallow water platform environments to offshore settings. Eighteen trilobite species are described from this locality for the first time. Although the taxa from the lower part of the succession (Leptoplastidessp.,Asaphellussp.) are scarce and non-age diagnostic, those from the upper part include diverse assemblages partially assigned to the late TremadocianNotopeltis orthometopaZone. Systematic descriptions of several species (Geragnostus nesossiiHarrington and Leanza,G. callaveiformisHarrington and Leanza,Asaphellus jujuanusHarrington,Notopeltis orthometopa[Harrington],Mekynophrys nannaHarrington,Ceratopyge forficuloidesHarrington and Leanza,Apatokephalus tibicenPřibyl and Vanĕk) are improved, the genusNileusDalman (includingN. australisn. sp.) is first reported from the Tremadocian of western Gondwana, and new species ofAsaphellusCallaway (A. nazarenensisn. sp.),ConophrysCallaway, andApatokephalusBrøgger are described. The trilobites have their closest affinities with faunas from Norway and Sweden.Notopeltis orthometopaandMekynophrys nannaare restricted to the uppermost part of the succession, well above the first records of most other trilobites recognized.

Early Ordovician Trilobites from the Nazareno Area, Northwestern Argentina

The upper part of the Santa Rosita Formation (Ordovician, Tremadocian) in the Nazareno area, Cordillera Oriental, northwestern Argentina, records the vertical passage of high-energy, shallow water platform environments to offshore settings. Eighteen trilobite species are described from this locality for the first time. Although the taxa from the lower part of the succession (Leptoplastides sp., Asaphellus sp.) are scarce and non-age diagnostic, those from the upper part include diverse assemblages partially assigned to the late Tremadocian Notopeltis orthometopa Zone. Systematic descriptions of several species (Geragnostus nesossii Harrington and Leanza, G. callaveiformis Harrington and Leanza, Asaphellus jujuanus Harrington, Notopeltis orthometopa [Harrington], Mekynophrys nanna Harrington, Ceratopyge forficuloides Harrington and Leanza, Apatokephalus tibicen Přibyl and Vanĕk) are improved, the genus Nileus Dalman (including N. australis n. sp.) is first reported from the Tremadocian of western Gondwana, and new species of Asaphellus Callaway (A. nazarenensis n. sp.), Conophrys Callaway, and Apatokephalus Brøgger are described. The trilobites have their closest affinities with faunas from Norway and Sweden. Notopeltis orthometopa and Mekynophrys nanna are restricted to the uppermost part of the succession, well above the first records of most other trilobites recognized.

New evidence of an Ediacaran age for the Bambuí Group in southern São Francisco craton (eastern Brazil) from zircon U–Pb data and isotope chemostratigraphy

Extensive carbonate–siliciclastic successions of the Bambuí Group, which overlie Neoproterozoic glaciogenic diamictites, cover most of the southern São Francisco craton (eastern Brazil). This group records sedimentation in a foreland setting related to the diachronic orogenic processes that formed the Brasília and Araçuaí marginal belts. The lowermost unit of the Bambuí Group, the Sete Lagoas Formation, comprises two shallowing-upward sequences of carbonate rocks with subordinated pelitic intercalations, overlying the glaciogenic diamictites in the southern São Francisco Craton. This study combines isotope chemostratigraphy (C, O, Sr) and U–Pb dating of zircon detrital grains retrieved from marls of the Sete Lagoas Formation. The basal sequence comprises low organic matter limestones and dolostones with δ13C values around 0‰, positioned above cap carbonates dated at around 740Ma (Pb–Pb whole-rock isochron). The U–Pb ages obtained for this sequence show several age peaks between 1270–870Ma and 625–550Ma. The upper sequence includes dark limestones with δ13C values as high as +10‰, best preserved 87Sr/86Sr ratios of around 0.7075 and U–Pb ages ranging from 625Ma to 550Ma. Our geochronological data suggest that the Araçuaí orogen is the main source of sediment for the Sete Lagoas Formation, and the youngest zircon population sets the maximum depositional age for its upper part at around 557Ma. This suggests that the studied section of the Sete Lagoas Formation is not related to either the Sturtian or the Marinoan glacial events. Also, the 87Sr/86Sr ratios obtained from Sete Lagoas carbonates contrast with Sr evolution curves available in the literature, especially with those for the Ediacaran–Cambrian limit, when ratios higher than 0.7080 would be expected. The same discrepancy is reported for other Ediacaran carbonate successions, pointing to local disturbances in Sr composition of marine basins rather than global processes. Interbasinal correlations and blind dating based on isotope chemostratigraphy should proceed carefully, especially for Ediacaran marine deposits located on the inner parts of large palaeocontinental regions, such as those found in western Gondwana.