Gondwana Margin Research Articles

Cambrian Age 3 small shelly fossils from the Terrades inlier, southern Pyrenees, Spain: Biostratigraphic and paleobiogeographic implications

AbstractThe Cambrian stratigraphic succession of the Pyrenees (SW Europe) has undergone a complex Variscan and Alpine tectonothermal history leading to marked metamorphism and development of cleavage networks, which might partly explain the lack of Cambrian fossiliferous beds. This gap has traditionally precluded its paleobiogeographic and biostratigraphic relationships with other neighboring peri-Gondwanan units. Correlations are only based on lithostratigraphic comparisons and radiometric constraints. In this general scheme, the Terrades inlier (Gerona Province, Spain) provides the only significant and indisputable ‘early Cambrian’ fossil record of the Pyrenees. This predominantly siliciclastic outcrop consists of multiple patch reefs and bioherms having yielded archeocyaths dated at Cambrian Epoch 2, Age 3. This paper describes, for the first time, the microfossil assemblage included in the archeocyathan-microbial reefal complex that crops out in the Terrades inlier to clarify its age and affinities with surrounding tectonostratigraphic units. Reefal flanks of patch reefs have yielded bradoriids, brachiopods, molluscs, tommotiids, chancelloriids, hyoliths, and the problematic fossil (and chronostratigraphically significant)Rhombocorniculum cancellatumCobbold, 1921. In addition to confirming the previously assigned age of the succession, the recovered fauna emphasizes strong affinities with the surrounding Occitan Domain (Montagne Noire, southern Massif Central, France) and Sardinia (Italy). Along with lithostratigraphic comparison and tectonic considerations, this further supports the recent reconstructions positioning the Pyrenean domain between the Montagne Noire (to the southwest) and Sardinia (further to the northeast) on the Gondwana margin during Cambrian times.

A new locality with ctenochasmatid pterosaurs (Pterosauria: Pterodactyloidea) in the Atacama Desert, Northern Chile

We describe a new locality with ctenochasmatid pterosaurs found in a tidal estuarine paleoenvironment of the Quebrada Monardes Formation (Lower Cretaceous). The new locality, which is named “Cerro Tormento”, is in Cerros Bravos in the northeast Atacama region, Northern Chile. Here, we describe four cervical vertebrae, one of them belonging to a small individual, the impression of a right scapulocoracoid, a left coracoid, an impression of a left humerus, an incomplete left humerus, a distal fragment of the right humerus, and impressions of a left femur and tibiotarsus. The presence of three humeri and a cervical vertebra belonging to a small pterosaur indicate that these materials represent more than one individual. The cervical vertebrae present diagnostic traits shared with ctenochasmatid pterosaurs, such as elongated vertebral centra, with integrated neural arch, low neural spines, and dorsally located neural canal. It is currently not possible to determine if there are one or more species represented. This finding is the second geographic occurrence of pterosaurs of the clade Ctenochasmatidae in the Atacama region, although it is currently uncertain if ctenochasmatids from both locations were contemporaneous. This suggests that the clade Ctenochasmatidae was widespread in what is now northern Chile. In addition, the presence of bones belonging to more than one individual preserved in Cerro Tormento suggest that pterosaur colonies were present at the southwestern margin of Gondwana during the Early Cretaceous.

The unique Cambro-Ordovician silicic large igneous province of NW Gondwana: Catastrophic melting of a thinned crust

Cambro–Ordovician silicic magmatism in the Central Iberian Zone of the Iberian Massif (Ollo de Sapo Formation, OSF) constitutes a voluminous and geochemically atypical magmatic event that formed preceding the breakup of the northern margin of Gondwana. To date, and due to uncommon geochemical signatures, such as a high Fe, Mg content compared to anatectic melts and the departing from the calc-alkaline trends, the origin of such magmatic event is not fully understood. Herein, we report a data-analysis of geochemistry linking magmas and source compositions. The analysis of the combined data from multiple studies ascribes the geochemistry of the OSF rocks to a combination of extensive melting of Ediacaran metasiliciclastic rocks and a Ca-rich component. It is hypothesized that fluids released by crystallization of mafic magmas contributed to partial melting of a thick metasedimentary pile represented by Ediacaran siliciclastic rocks. Such melting event gave rise to a mobile nebulite or migma, which was able to extrude and form the super-eruption or “flare-up” that characterizes Cambro-Ordovician silicic magmatism at the Gondwana margin. Fast, catastrophic crustal melting with large-scale restite entrainment, triggered by the influx of mafic magma-derived fluids, are considered the main cause of the unique features of this Cambro-Ordovician atypical silicic large igneous province of Gondwana.

Ordovician submarine to subaerial volcanism along the western Gondwana margin: records of the Famatinian belt evolution, north-western Sierras Pampeanas, Argentina

Ordovician submarine to subaerial volcanism along the western Gondwana margin: records of the Famatinian belt evolution, north-western Sierras Pampeanas, Argentina

Evaluating sediment recycling through combining inherited petrogenic and acquired sedimentary features of multiple detrital minerals

AbstractAn implicit assumption of most sedimentary provenance analyses is a direct link between source and sink. However, recycling of sedimentary detritus from pre‐existing strata interrupts the direct source‐to‐sink link and can result in incorrect interpretations of paleogeography and paleodrainage. Detrital zircon is the favoured proxy of contemporary provenance studies, but its physiochemical resilience makes it particularly prone to recycling. In this study, we integrate geochemical (age, isotope, and trace elements) and grain roundness data of multiple detrital minerals with different physiochemical stabilities (zircon, tourmaline, rutile, and apatite) to evaluate the importance of recycling in an ancient sedimentary basin. We focus on the early Cambrian Lalun Formation of Iran, which forms part of a laterally extensive sandstone‐rich succession deposited along the northern margin of Gondwana. The Lalun Formation preserves a distinct change of compositional maturity between lower arkose and shale units and an upper unit of quartz‐rich sandstone. Detrital zircon, rutile, and apatite data demonstrate that all units of the Lalun Formation share a common source in the Arabian‐Nubian Shield. Whole‐rock geochemical data further indicate that all units have similar chemical alteration indices, suggesting the change in compositional maturity is not a product of differential weathering of the source region. Analysis of grain roundness reveals that detrital zircon, rutile, and tourmaline in the upper quartz‐rich unit are typically more rounded than those in the underlying arkose and shale units. In contrast, detrital apatite grains are nearly all angular in the quartz‐rich unit but mostly rounded in the lower arkose and shale. Together, the detrital mineral provenance, whole‐rock geochemistry, and morphological data are consistent with recycling of the lower arkose and shale units of the Lalun Formation into the uppermost quartz‐rich unit, with the latter also receiving a component of first‐cycle detritus represented by angular detrital apatite. Our findings demonstrate that integrating the features of detrital minerals acquired during a sedimentary cycle (grain rounding and diversity of mineral assemblages) with features inherited from their ultimate source rocks (age, isotopic, and geochemical proxies) can assist in recognising sediment recycling in ancient strata.

Late Paleozoic-Early Mesozoic paleotectonics of the northern Arabian Plate (SE Turkey) and its role in the Paleozoic petroleum system

The Carboniferous compressional deformation of the Arabian Plate has been causally linked with the Hercynian orogeny. However, this epeirogenic movement occurred far from the Arabian plate. Instead, the Gondwana-directed subduction of the Paleo-Tethys oceanic crust may be the controlling factor of the Carboniferous structural development of southeast Turkey (the northern Arabian Plate). The tectonic activity resulted in uplift and widespread erosion of a large area and produced granitic intrusions, metamorphism, folding, and faulting. The compressional event was intermitted by theback-arc spreading of the Neo-Tethys Ocean (Middle Permian-Triassic) that occurred because of the detachment of the Cimmerian supercontinent from the Gondwana margin. The spreading of the Neo-Tethys caused the Arabian Plate to be subjected to extensional tectonism. The tectonic overprints of these epeirogenic events on the structural fabric of southeast Turkey have not been studied in detail. Innovations in subsurface imaging techniques, wireline logs, and sufficient-quality data enable the depiction of the traces of the Carboniferous compression and the subsequent Permian–Triassic regional extension. This study explores the structural remnants of the Carboniferous compression and the Permian–Triassic extension in southeast Turkey and their impact on the Paleozoic petroleum system. In conclusion, the Carboniferous, SW- and EW-oriented compressional forces led to major uplift, inversion, and substantive erosion in southeast Turkey. The regional uplift confined the Silurian-sourced oil exploration activities to a narrower area due to the denudation of the Silurian units towards the Carboniferous Arches. Subsequently, the opening of the Neo-Tethys yielded en-echelon NW and WNW-trending horsts and grabens bounded on both sides by normal faults, resulting in the northeastward thickening of the syn-rift units. The horst structures allow hydrocarbon accumulation in the Ordovician Bedinan Formation sandstones tectonically juxtaposed with the Silurian source rock.

Distribution pattern of Middle Permian fusulinids in the Lhasa Block, Tibet and their paleogeographic implications

The paleobiogeographic affinity of fusulinids in the Lhasa Block remains contentious, despite its significance for deciphering the complex Paleozoic history of the Tibetan Plateau. Our new collection from the Lhunzhub area and literature data recently accumulated necessitate an up-to-date overview of the Middle Permian fusulinids in this block from both taxonomic and paleobiogeographic perspectives. Systematic identification reveals Lepidolina multiseptata multiseptata, Colania ozawai, Verbeekina cf. tenuispira, V. americana etc., indicative of a Midian age, in the new materials from the Luobadui Formation. Overall, the Middle Permian fusulinids of this block could be distinguished into two types: 1) characterized by verbeekinids and neoschwagerinids; 2) dominated by Nankinella and Chusenella . Furthermore, these fusulinids as a whole were compared, at the generic rank, with their counterparts from other Gondwana-derived blocks, as well as Cathaysian South China, both qualitatively and quantitatively (cluster analysis and principal ordination analysis). Results show that these fusulinids of the Lhasa Block, is closely affiliated with those of the South Qiangtang, both of which paleobiogeographically belong to the Cimmerian Province, especially based on the occurrence of Rugososchwagerina ( Xiaoxinzhaiella ) and smaller foraminifer Shanita . Meanwhile, the Lhasa Block also bears resemblance with Cathaysian South China by sharing some genera of verbeekinids and neoschwagerinids, including the Lepidolina which more commonly occurs in the eastern Tethys and Panthalassa. It is remarkable that the Lhasa Block accommodates more diversified Middle Permian fusulinids than the eastern Cimmerian blocks do, given its lacking Early Permian fusulinids. Such contrast between the Early and Middle Permian fusulinids reflects considerable climatic amelioration of the Lhasa Block, which is interpreted to mainly result from its rifting from Gondwana before the Guadalupian time and northward drifting afterwards. The Lhasa Block was possibly located near to the South Qiantang Block, and at lower paleolatitudes than the eastern Cimmerian blocks during the Middle Permian. • Two types of Mid-Permian fusulinid faunas in Lhasa Block, Nankinella- dominated or not. • Lepidolina , more commonly in eastern Tethys and Panthalassa, confirmed in Lhasa Block. • It is part of Cimmerian Province due to endemic taxa and less diversity than S. China . • It might be near S Qiantang and at lower palaeolatitudes than E. Cimmeriarn blocks. • Our study supports that it rifted from the Gondwana margin before the Guadalupian.

Evidence for Late Cambrian Arc-Related Magmatism Along the Northern Margin of Gondwana: Zircon Geochronological and Geochemical Data from the Leszczyniec Metaigneous Complex, Sudetes, Poland

Evidence for Late Cambrian Arc-Related Magmatism Along the Northern Margin of Gondwana: Zircon Geochronological and Geochemical Data from the Leszczyniec Metaigneous Complex, Sudetes, Poland

Hf Isotopes in Detrital Zircon Point to a Mesoproterozoic Orogenic Belt Bordering the Western Margin of the Río de la Plata Craton

Published U-Pb-Hf values for detrital zircons of Mesoproterozic age in Cambrian and Ordovician strata of the Argentina Precordillera and San Rafael Block of the Cuyania Terrane overlap extensively with values from Grenville/Appalachian detritus and the Granite-Rhyolite Province of Laurentia and with values from Sierra de Maz and Sierra de Pie de Palo of the Western Sierras Pampeanas (WSP) of western Argentina. These data have been interpreted as evidence of the Laurentian origin of Cuyania and the WSP. However, U-Pb and corresponding εHf values presented here for Mesoproterozoic zircons from Ediacaran sandstones deposited on the Río de la Plata Craton (RPC) are markedly similar to those of Mesoproterozoic zircons from Laurentia, Cuyania, and the WSP, thus contradicting the previous interpretation. Given the geotectonic history of southwestern Gondwana, the U-Pb-Hf values from the Mesoproterozoic zircons of the Piedras de Afilar and Cerro Largo Formations of the RPC are consistent with a proto-Andean Mesoproterozoic belt on the western margin of the RPC. This belt would have served as the basement and thus the provenance of Mesoproterozoic detrital zircons in Cuyania and the WSP. The existence of this Mesoproterozoic belt introduces a different paradigm for interpreting the late Neoproterozoic to early Paleozoic geotectonic history of the proto-Andean margin of Gondwana and renders Neoproterozoic–early Paleozoic transfers of a Sierra de Maz–Arequipa–Rio Apa Terrane and the Cuyania Terrane from Laurentia unnecessary.

Cambro–Ordovician ferrosilicic magmatism along the northern Gondwana margin: constraints from the Cézarenque–Joyeuse gneiss complex (French Massif Central)

It is well-acknowledged that the northern margin of the Gondwana supercontinent was affected by a major magmatic event at late Cambrian (Furongian) to early Ordovician (Tremadocian–Floian) times. However, an accurate assessment of its extent, origin, and significance is partly hampered by the incomplete characterization of the numerous gneiss massifs exposed in the inner part of the Variscan belt, as some of them possibly represent dismembered and deformed Furongian–Lower Ordovician igneous bodies. In this study, we document the case of the “Cézarenque–Joyeuse” gneiss complex in the Cévennes parautochthon domain of the French Massif Central. The gneisses form decametre- to kilometre-thick concordant massifs interlayered within a pluri-kilometric sequence of mica- and quartz schists. They encompass two main petrological types: augen gneisses and albite gneisses, both typified by their blue and engulfed quartz grains with the augen facies differing by the presence of centimetre-sized pseudomorphs after K-feldspar and the local preservation of igneous textures. Whole-rock geochemistry highlights that many gneisses have magmatic ferrosilicic (acidic with anomalously high FeOt and low CaO) compositions while others are akin to greywackes. Collectively, it is inferred that the bulk of the Cézarenque–Joyeuse gneisses represents former rhyodacite lava flows or ignimbrites and associated epiclastic tuffs. Volumetrically subordinate, finer grained, and strongly silicic leucogneisses are interpreted as microgranite dykes originally intrusive within the volcanic edifices. LA–ICP–MS U–Pb dating of magmatic zircon grains extracted from an augen gneiss and a leucogneiss brackets the crystallization age of the silicic magmas between 486.1 ± 5.5 Ma and 483.0 ± 5.5 Ma which unambiguously ties the Cézarenque–Joyeuse gneisses to the Furongian–Lower Ordovician volcanic belt of SW Europe. Inherited zircon date distributions, Ti-in-zircon and zircon saturation thermometry demonstrate that they formed by melting at 750–820 °C of Ediacaran sediments. Zircon Eu/Eu* and Ce/Ce* systematics indicate that the melts were strongly reduced (fO2 probably close to the values expected for the iron–wüstite buffer), possibly because they interacted during ascent with Lower Cambrian black shales. This would have enhanced Fe solubility in the melt phase and may explain the peculiar ferrosilicic signature displayed by many Furongian–Lower Ordovician igneous rocks in the northern Gondwana realm. We infer that crustal melting resulted from a combination of mantle-derived magma underplating in an intracontinental rift setting and anomalously elevated radiogenic heat production within the Ediacaran sedimentary sequences.

Neoproterozoic and Early Paleozoic magmatism in the eastern Lhasa terrane: Implications for Andean-type orogeny along the northern margin of Rodinia and Gondwana

Precambrian and Early Paleozoic magmatic activities in the Lhasa terrane provides important context for distinguishing between the North and South Lhasa terranes, and deciphering the nature and geotectonic history of the northern margin of the Rodinia and Gondwana supercontinents. Here, we conducted a detailed petrological, geochemical, and geochronological study of mafic to granitic meta-intrusive rocks of the Bomi complex in the eastern segment of the Lhasa terrane. Zircon U–Pb dating indicates that these rocks record Neoproterozoic and Cambrian magmatic events, with protolith crystallization ages of ca. 805 Ma, ca. 505 Ma, and ca. 503 Ma for dioritic gneisses, amphibolites (meta-gabbros), and granitic gneisses, respectively. These rocks were later metamorphosed during the Late Jurassic, with zircon U–Pb ages of ca. 158 Ma obtained from the amphibolites. All rocks are enriched in Rb, U and LREEs, and depleted in Nb, Ta, Sr and HREEs, showing affinity with arc-related magmas. With positive zircon εHf(t) values of +1.4 to +6.9, the Neoproterozoic dioritic gneisses are interpreted to have formed by fractional crystallization of mantle-derived magmas. Cambrian amphibolites have εHf(t) values of –3.0 to +6.0, differing from those of coeval granitic gneisses (+0.3 to +3.2). These amphibolites are interpreted to have been derived from partial melting of enriched continental lithospheric mantle with contamination of subducted sediments, whereas the granitic gneisses probably originated from anatexis of lower crust that was induced by coeval emplacement of mantle-derived magma. These rocks are interpreted to be formed in a continental arc setting. When comparing these data with those of other Precambrian and Cambrian magmatic rocks in the Lhasa terrane and other blocks, we propose that the North Lhasa terrane and South Lhasa terrane have distinctive Precambrian basements. The North Lhasa terrane diachronically witnessed an Andean-type orogeny along the northwestern margin of Rodinia during the Neoproterozoic and the northeastern margin of Gondwana during the Early Paleozoic.

Lower Triassic conodont biostratigraphy of the Guryul Ravine section, Kashmir

The Guryul Ravine section in Kashmir, northern India is an important reference section for global Lower Triassic stratigraphy. Once a candidate Global Stratotype Section and Point (GSSP) for the Permian-Triassic Boundary (PTB), the section has attracted intense attention for the PTB beds, but few studies have concerned the entire Lower Triassic stratigraphy. As one of the most continuous marine Lower Triassic successions on the northern margins of Gondwana, the Guryul Ravine section provides an important conodont biostratigraphic yardstick for worldwide correlations. This study presents recent results of the Lower Triassic conodont zonation from the Guryul Ravine section. A total of 6500 elements including 1600 P1 elements were recovered. Ten conodont zones were recognized from the Griesbachian to Spathian: Hindeodus parvus, Isarcicella staeschei, Clarkina planata, Neoclarkina krystyni, Neospathodus dieneri, Ns. pakistanensis, Novispathodus waageni, Scythogondolella mosheri, Nv. pingdingshanensis, and Nv. abruptus – Nv. brevissimus zones. The PTB is placed within Bed 52 in Unit E2, 80 cm above the base of Unit E2, based on the first occurrence (FO) of H. parvus. The Induan–Olenekian boundary (IOB) is tentatively placed at the base of Member G according to the positive maximum δ13Ccarb values, but it could be slightly higher based on conodonts. The Smithian-Spathian boundary (SSB) is drawn at the top of Member H based on the FO of Nv. pingdingshanensis and the carbon mdpt(N3-P3). Several conodont taxa are newly recognized near the top of the study section where the conodont fauna is dominated by segminiplanate elements of the Neogondolellinae. Many of these age-diagnostic species have been identified in other sections around the world, and their occurrence in Guryul Ravine supports their potential for worldwide correlation. The conodont sequence erected from the study section corresponds well to those of South China (e.g. Meishan) and elsewhere worldwide. The newly established conodont zonation from Kashmir provides a high precision time-frame to consider biotic evolution and environmental change during the Early Triassic, a crucial period of Earth history.

Detrital zircon ages and provenance of a Cambrian succession in the Sierra Albarrana Domain (SW Iberian Massif)

This study presents the first U-Pb geochronological data on detrital zircon grains from the metasedimentary successions of the Sierra Albarrana Domain (SW Iberian Massif) obtained by SHRIMP and LA-ICP-MS. We provide new information on the Maximum Depositional Ages (MDA) during the middle Cambrian through a systematic study, together with age comparisons of the successions, using Kernel Density Estimates (KDE) diagrams, Cumulative Age Distributions (CADs) and the Kolmogorov–Smirnov (K-S) test.On the other hand, the U-Pb zircon data presented in this study have been compared with all the existing data from detrital zircon grains in Neoproterozoic and Cambrian rocks of the Iberian Massif (20 samples, 2706 U–Pb zircon ages). For the comparison, in addition to the KDEs, CADs and K-S test, we apply 3D multidimensional scaling techniques (3D-MDS). The results, together with the geochemical and isotopic characterisation from previous studies, suggest that this domain is likely to be part of the autochthonous section of the Central Iberian Zone (CIZ). Zircon age patterns indicate a probable sedimentary provenance from the Saharan Metacraton with, or without, minor input from the Tuareg Shield. The most important local detrital source corresponds to Cadomian magmatism developed during a magmatic event (535–515 Ma) followed by back-arc extension and early Paleozoic rifting (535–460 Ma). The remaining Mesoproterozoic, Paleoproterozoic and Archean zircon grains would have been provided by the Paleoproterozoic basement and/or the older continental crust recycled in the western sections of the Gondwana margin.

Annelid Borings on Brachiopod Shells From the Upper Ordovician of Peru. A Long-Distance Co-migration of Biotic Partners

The Recent planktonic larvae of the polychaete spionids are some of the most widespread and abundant group of coastal meroplankton worldwide. To study the possible co-migration of biotic partners and determine whether they were host-specific, the type of biotic relationship between hosts and borers of an Upper Ordovician Peruvian brachiopod collection from the Proto-Andean margin of Gondwana was re-exanimated and compared with material from Wales (Avalonia). The species list studied is composed of Colaptomena expansa (41%), Heterorthis retrorsistria (24%), Horderleyella chacaltanai (19%), Drabovinella minuscula (13%), and Dinorthis cf. flabellulum (3%) and coincides closely with that of the Dinorthis community described in the Caradoc series of North Wales. The borings attributed to these spionids have been identified as Palaeosabella prisca only present in the valves of Colaptomena expansa and Heterorthis retrorsistria. All the studied valves are disarticulated, with very low fragmentation and are randomly oriented in a context below the fair-weather wave base. The settling larvae would feed on their brachiopod host soft parts at an early stage, being the biotic interaction initially of the parasitic type. Since Palaeosabella borings from Peru and Wales are identical, as well as the species specificity of their producers with their brachiopod hosts, it can be concluded that the same spionid annelid species produced them. The Southern Westerlies current that connected the Proto-Andean margin of Gondwana with Avalonia must have been responsible for transporting the larvae of annelids and brachiopods in what had to be a successful biotic relationship over a great transoceanic distance.

Highly refractory dunite formation at Gibbs Island and Bruce Bank, and its role in the evolution of the circum-Antarctic continent

ABSTRACT The continental margin is of profound importance as it records continental growth by accretion of orogenic magmas and following continental rifting. A high degree of mantle melting due to hydrous fluid input is expected to simultaneously stimulate continental growth and lower the intrinsic density of the mantle than more fertile mantle, which in turn isolates the continental lithosphere from the convective mantle. The mantle peridotites from Gibbs Island (South Shetland Islands) and Bruce Bank in the Drake Passage provide us an insight into the tectonic history in the circum-Antarctic region. To elucidate the continental growth of Antarctica, we present geochemical data of eight dunites from Gibbs Island and one dunite from Bruce Bank, including Re–Os isotope and highly siderophile element compositions. The dunites are severely affected by serpentinization as evidenced by antigorite + brucite or lizardite (loss on ignition = LOI ranging from 3 to 34 wt.%) but contain primary euhedral to subhedral chromites with or without spherical inclusions. The chromites rarely form lens-shaped aggregates. A dunite from Gibbs Island contains fresh olivine grains filling a fracture in the chromite with low LOI (3 wt.%), indicating a deserpentinization origin from a precursor serpentinized dunite. The dunites show highly depleted bulk-rock major element compositions (Mg/Si = 1.4–1.6 and Al/Si = 0.004–0.01 for Gibbs Island dunites, Mg/Si = 0.66 and Al/Si = 0.008 for Bruce Bank dunite), overlapping a compositional field defined by forearc peridotites. The positive correlation in Re/Ir–LOI space corroborates Re input during the later serpentinization process. The 187Os/188Os ratios of the dunites range from 0.11907 to 0.14493. Phanerozoic Re-depletion (melt depletion) ages of ca. 535–129 Ma are recorded in the Gibbs Island dunites, except for one with a Mesoproterozoic Re-depletion age of ca. 1.2 Ga. Since there exists serpentinization-related perturbation of Re, the ages provide minimum time estimates for melt depletion events. The early Paleozoic melt depletion is inferred to have occurred at a very early stage of Antarctic Peninsula formation in response to plate convergence along the margin of Gondwana, whereas the Mesoproterozoic Re-depletion age reflects convecting mantle heterogeneity unrelated to any nearby crust-forming events. The petrographic characteristics of the chromites and highly depleted nature of the dunites are attributed to melt–peridotite reaction in a subduction zone setting. A feasible interpretation for the dunite formation is that the mantle had experienced two stages of melting with the final stage occurring along the Gondwana continental margin in the subduction zone setting. Resultant highly refractory lithospheric mantle was later displaced and dispersed during the Gondwana breakup. Widespread existence of the dunite may be attributed to multi-stage melt depletion along the continental margin.

Constraints of rare detrital V-rich tourmaline and rutile on late Devonian palaeogeographic reconstruction in the Azarbaijan district, NW Iran

• New geochemical data on detrital tourmaline and rutile from the Devonian sedimentary rocks in NW Iran. • Specific V-bearing tourmaline and rutile compositions indicate metaultramafic source rocks. • Evidence for far-distant transport of sediment from the Gondwanan-East African Belt to the distal margin of Gondwana . This paper presents the first geochemical data on detrital tourmaline and rutile from sedimentary rocks of the upper Devonian Ilanqareh Formation (Fm.) which is extended in the Azarbaijan area in NW Iran. The deposits have typically mature character with ultra-stable heavy minerals predominantly derived from the metamorphic rocks. The detrital tourmaline has mainly dravitic, uvitic and Mg-foititic composition, thus indicating metaultramafic source rocks and common metapelites and metapsammites. Detrital rutile geochemistry indicates the presence of lower-up-to-upper amphibolite facies metapelitic source rocks, and part of this rutile most likely underwent granulite facies metamorphism. Unique detrital V-bearing tourmaline with high V 2 O 3 content up to 8.93 wt% indicates its primary derivation from far-distant source rocks in the East African Belt and adjacent areas. This is confirmed by both the exceptional geochemistry and the shape and roundness of the tourmaline which indicate recycled detrital origin and multiple reworking. The less frequent V-bearing rutile also indicates the same sources. These results confirm the wide circulation of sediments distributed by the Gondwana super-fan system from the inner parts of this super-continent to the Gondwana and peri-Gondwanan terrain continental margins, and also their later re-distribution during the Devonian era. The results highlight the significant contribution from Gondwanan sources to the supply of upper Devonian siliciclastic formations in NW Iran compared to those from the Iranian plate.

Three stages of early Paleozoic magmatism in the Tibetan-Himalayan orogen: New insights into the final Gondwana assembly

Earth’s evolution involves deep, hot rocks rising upward by convection to near-surface environments, as well as the unique scenarios such as the formation and breakup of Gondwana and Pangea . These processes have been well recorded by the widely distributed magmatism . To understand the tectonic evolution and Gondwana assembly, we compiled early Paleozoic zircon U–Pb ages (n = 67), in-situ zircon Hf isotopes (n = 1011), and bulk-rock elemental compositions (n = 293) and Sr–Nd isotopes of magmatic rocks in the Tibetan–Himalayan orogen. Three stages of Paleozoic magmatism were identified here, including early (>490 Ma) to middle (490–470 Ma) to late (<470 Ma) stages, in response to the final assembly of the Gondwana supercontinent . Early-stage magmatic rocks are characterized by highly variable SiO 2 (48.0–80.0 wt%) and MgO (0.02–9.65 wt%) contents with Mg# range from 5.4 to 78, K 2 O/Na 2 O ratios ranging from 0.14 to 2.81, and whole-rock Sr–Nd ( 87 Sr/ 86 Sr(i) (0.7035–0.7340), ε Nd (t) (-9.5–+1.0)) and zircon Hf (ε Hf (t) = –15 to + 8.0) isotopic compositions with significant mantle contributions; they could be generated in an Andean-type arc setting along the active northern continental Gondwana margin. Middle-stage magmas were dominated by fertile continental crustal signatures (SiO 2 > 70 wt%, A/CNK > 1.10, MgO < 2.69 wt%) in a crustal thickening setting. Late-stage magmas also have highly variable Si, Mg and isotopic components, with coeval mantle-derived magmas developing in an extensive setting. Importantly, the widespread presence of early Cambrian to late Ordovician peraluminous high-K calc-alkaline magmatism in the Tibetan–Himalayan orogen indicates that the reworking of ancient continental crustal materials played a key role in reconstructing and stabilizing the final Gondwana assembly.

Sedimentary architecture of a Middle Ordovician embayment in the Murzuq Basin (Libya)

The Hawaz Formation comprises a siliciclastic, shallow to transitional marginal marine succession, deposited on the north-western cratonic margin of Gondwana during the Middle Ordovician. This unit is well documented in the north central part of the Murzuq Basin, where it is often dramatically truncated by Late Ordovician glaciation unconformities, generating a major discontinuity, not only in the studied area but also across the whole Saharan Platform of North Africa. The Hawaz Formation is particularly relevant as one of the two major oil-bearing reservoirs in the Murzuq Basin. However, Late Ordovician erosion ensures that its present configuration bears very little relation to the original sedimentary architecture and, consequently, there is a need for a detailed large-scale correlation and sedimentary reconstruction of the Hawaz in order to improve subsurface management of this reservoir unit. The present study was developed from a previous sedimentological characterization of the Hawaz Formation, based on subsurface data provided by 35 wells. This sedimentological background provided the basis for the reconstruction of the sedimentary architecture of this unit by means of eight correlation panels oriented along both sedimentological dip (NNW-SSE) and strike (WSW to ENE). In addition, a series of Gross Depositional Environment (GDE) maps were also generated with the aim of providing insight into the lateral distribution of facies belts within the framework of a sequence stratigraphic-based reservoir zonation. The results of this study suggest that the Hawaz Formation was deposited in a relatively protected or embayed shoreline with multiple bays/estuaries as the main entry points for sediment into the basin, most likely influenced by the effects of pre-existing north-northwest to south-southeast Pan-African structures controlling local accommodation space and reactivated during Ordovician times. Correlation panels and GDE maps also show the Hawaz Formation to be an extensive and continuous reservoir across the studied area, deposited in a broadly extensive subtidal to intertidal paralic environment, with very few or possibly no modern sedimentary analogues.

Metamorphism of the Sierra de Maz and implications for the tectonic evolution of the MARA terrane

Abstract The Mesoproterozoic MARA terrane of western South America is a composite igneous-metamorphic complex that is important for Paleozoic paleogeographic reconstructions and the relative positions of Laurentia and Gondwana. The magmatic and detrital records of the MARA terrane are consistent with a Laurentian origin; however, the metamorphic and deformation records lack sufficient detail to constrain the correlation of units within the MARA terrane and the timing and mechanisms of accretion to the Gondwana margin. Combined regional mapping, metamorphic petrology, and garnet and monazite geochronology from the Sierra de Maz of northwest Argentina suggest that the region preserves four distinct litho-tectonic units of varying age and metamorphic conditions that are separated by middle- to lower-crustal ductile shear zones. The Zaino and Maz Complexes preserve Barrovian metamorphism and ages that are distinct from other units within the region. The Zaino and Maz Complexes both record metamorphism ca. 430–410 Ma and show no evidence of the regional Famatinian orogeny (ca. 490–455 Ma). In addition, the Maz Complex records an earlier granulite facies event at ca. 1.2 Ga. The Taco and Ramaditas Complexes, in contrast, experienced medium- and low-pressure upper amphibolite to granulite facies metamorphism, respectively, between ca. 470–460 Ma and were later deformed at ca. 440–420 Ma. The Maz shear zone that bounds the Zaino and Maz Complexes records sinistral oblique to sinistral deformation between ca. 430–410 Ma. The data suggest that at least some units in the MARA terrane were accreted by translation, and the Gondwana margin of northwest Argentina transitioned from a dominantly convergent margin to a highly oblique margin in the Silurian.

Re-initiation of plutonism at the Gondwana margin after a magmatic hiatus: The bimodal Permian-Triassic Longwood Suite, New Zealand

The Cambrian to Cretaceous Tuhua Intrusives, New Zealand, preserve an igneous record of Phanerozoic subduction and crustal growth at the margin of Gondwana. Within the Tuhua Intrusives, the coeval gabbroic and trondhjemitic intrusions of the c. 261–243 Ma Longwood Suite stand out as being isotopically more primitive and chemically distinct from all other New Zealand plutonic suites. We present new U-Pb crystallization ages, trace element analyses and Sr-Nd isotope compositions of the Longwood Suite. U-Pb SHRIMP zircon ages of 258.5 ± 2.5 Ma, 256.0 ± 1.8 Ma, 247.8 ± 2.7 Ma and 243.2 ± 2.4 Ma obtained from plutons on Ruapuke Island, and a dike at Bluff, affirm the restricted time range and expand the known areal extent of the Longwood Suite. Longwood Suite granitoids are I-type and sodic (K/Na < 0.4), with distinctive low Rb and Nb/Ta, flat rare earth element patterns (La/YbN < 10), unradiogenic 87Sr/86Sr(t) (0.7029 to 0.7032) and radiogenic ε143Nd(t) (+6.3 to +8.2), compared to the nearby, calc-alkaline, Late Triassic Darran Suite I-type plutons of the Tuhua Intrusives. Stable Nd isotope ratios of Longwood Suite samples are highly variable (δ146/144Nd = 233 ppm) compared to global plutonic rocks (δ146/144Nd = 44 ppm) and reflect the removal of phosphate minerals. Collectively, these geochemical characteristics are consistent with generation of the granitoids by shallow (garnet-absent) melting of an amphibolitic residue, from which we infer relatively thin lithosphere. The Longwood Suite has a maximum areal addition rate of 43 km2/Ma, substantially less than the subsequent plutonic suites when the magmatic arc was fully established. We suggest a petrotectonic model whereby Gondwana continental margin crust was tectonically underplated by Permian intra-oceanic island arc crust and mantle lithosphere, which subsequently melted to generate the isotopically primitive gabbro and trondhjemite plutons of the Longwood Suite.