Breakup Of Gondwana Research Articles

Role of the Kerguelen mantle plume in breakup of eastern Gondwana: Evidence from early cretaceous volcanic rocks in the eastern Tethyan Himalaya

To better understand the breakup mechanism of eastern Gondwana, U Pb Zircon geochronological and whole-rock geochemical studies were performed on the Zhela and Weimei formations volcanic rocks in the Zhuode area of the eastern Tethyan Himalaya. Sensitive high-resolution ion microprobe (SHRIMP) U Pb dating of zircons yielded 206 Pb/ 238 U ages of ~135–130 Ma, indicating that the volcanic rocks from Zhela and Weimei formations erupted during the Early Cretaceous. Whole rock analyses of major and trace elements show that the volcanic rocks are characterized by high content of TiO 2 (1.84 wt%–4.03 wt%) and P 2 O 5 (0.29 wt%–0.61 wt%), highly fractioned in LREE/HREE [(La/Yb) N = 9.12–14.53], and most of them have no obvious anomaly of Eu. The characteristic, highly enriched HFSE with no obvious anomaly of Nb and Ta, is similar to those of alkali basalts originated from the Kerguelen mantle plume. Our geochemical and geochronological results, combined with recent paleomagnetic results show that the original erupted position of the Zhela and Weimei formations volcanic rocks was just located in the center of the presently Kerguelen mantle plume, and support that the Kerguelen mantle plume led to the breakup of eastern Gondwana. • The volcanic rocks from Zhela and Weimei Formation (Fm) erupted at ~130–135 Ma. • The volcanic rocks from Zhela and Weimei Fm originated from the Kerguelen plume. • The Kerguelen plume played an active role in the breakup of eastern Gondwana.

Petrogenesis of Mesozoic giant dike swarms and geodynamical insights about EMI-Gough flavors in the Equatorial Atlantic Magmatic Province

The Rio Ceará-Mirim magmatism produced Mesozoic giant arcuate dike swarms during the initial stage of the West Gondwana breakup near the Equatorial Atlantic Ocean (NE Brazil), where they assemble tholeiites and subordinate intermediate rocks (SiO2 > 57 wt%). A multidata approach based on petrography, mineral chemistry, whole-rock geochemistry, and Sr-Nd-Pb isotopes enables the discrimination of four geochemical groups (two high-Ti and two low-Ti). High-Ti groups are composed of (1) basalts and basaltic andesites with TiO2 generally higher than 2.0 wt%, MgO > ~3 wt% (named the HT group) and rich in incompatible elements and (2) (trachy)andesites and trachytes with lower TiO2 (<2.5 wt%; Mg < 3 wt%) interpreted as fractionated magmas from the HT group, therefore named evolved HT. Both these groups have similar 87Sr/86Sr(127) (~0.706) and εNd(127) (−2.90 to −2.56) and moderately radiogenic 206Pb/204Pb(m) ratios between 18.66 and 18.21. Conversely, low-Ti tholeiites (TiO2 < 2 wt%) are (3) LT1 composed of evolved basaltic andesites, with ~Ti/Zr < 70, MgO <6 wt%, and Al2O3 > 15 wt%; and (4) LT2 composed of subalkaline (to transitional) basalts and basaltic andesites but ~Ti/Zr >70, and MgO > 6 wt%. These two low-Ti groups are, respectively, less radiogenic (LT1 εNd(127) -4.65 to −4.40) and slightly more radiogenic (LT2 εNd(127) -1.44 to −1.08) in Nd relative to the broad HT group. The geochemical groups present different degrees of enrichment in large ion lithophile and light rare-earth elements coupled with depletion in high-field-strength elements (NbTa) combined with isotopic (Sr-Nd-Pb) signatures compatible with enriched mantle sources. We showed that the EMI (Gough-like) isotope compositions of Rio Ceará-Mirim magmatism may be explained by mixing DMM with enriched SCLM-derived melts plus minor crustal assimilation or by the involvement of OIBs derived from mantle plumes of the South Atlantic area mixed with lithospheric sources. In a geodynamical perspective, given the narrow similarity of HT Rio Ceará-Mirim isotope compositions with EMI-Gough type and high-Ti magmas of Transminas swarm, a genetic connection between Equatorial Atlantic and Paraná-Etendeka provinces must be considered.

Structural development and tectonostratigraphic evolution of the Sylhet Trough (northeastern Bengal Basin) in the context of Cenozoic Himalayan Orogeny: Insights from geophysical data interpretation

The Sylhet Trough of the Bengal Basin in the northeastern Indian Plate had started to develop since the Late Mesozoic Gondwana break-up and subsequent onset of the Cenozoic Himalayan Orogeny. However, very little is known about evolution of the Cenozoic Sylhet Trough in response to the latter tectonic event. Therefore, we aim at providing insights into the nature of basement, variations in structural style, kinematics and possible timing of structural activation(s) and tectonostratigraphic evolution of the Sylhet Trough based on geophysical data interpretation. Results show that the Sylhet Trough is mostly underlain by an attenuated continental or oceanic crust, bounded by a NE-SW-trending geophysical feature in the northwest that corresponds to the boundary with the Precambrian continental crust. The overlying Cenozoic sediments show variations in style of deformations and can be divided into three structural domains. Domains 1 and 2 in the southeastern and northeastern parts of the Sylhet Trough, respectively, are characterized by anticlines that are highly deformed to the east and are mostly associated with west- or northwest-dipping reverse faults. However, the anticlinal trends are deviated from a generalized NS-orientation in the Domain 1 to a dominant NE-SW-orientation in the Domain 2. Many of these anticlines resemble positive flower structures, indicating possible impacts of regional transpression associated with the Dauki and Kaladan faults to the north and east; respectively. In contrast, the Domain 3 to the west shows NE-SW-trending extensional structures. A southward thickening of the Oligocene to Miocene pre-kinematic packages indicates an early episode of foreland basin subsidence, although there is no evidence of a major structural activation. Rather, the presence of the Pliocene syn-kinematic packages indicates a major structural reactivation in response to the E-W and N-S crustal shortening since the Pliocene. This younger episode of structural activation can be directly linked with the late Cenozoic subduction related processes of the Indian Plate to the north and to the east, which is also responsible for the transformation of the Sylhet Trough into a remnant ocean basin. This latter (i.e., Pliocene and onward) episode of crustal shortening played a significant role in structural trap formation and preservation of hydrocarbons in the Sylhet Trough.

Mesozoic to Cenozoic alkaline and tholeiitic magmatism related to West Gondwana break-up and dispersal

For over 50 years, Mesozoic tholeiites, kimberlites and carbonatites from the South American platform have been enabled the understanding of melting processes in the Earth’s upper mantle. However, the genetic relationship between alkaline and tholeiitic magmatism remains unknown. In this context, an extensive review, based on a compilation of published geochemical and isotopic data, shows an integrated evolution for mantle-derived magmatism in South America. The K-rich alkaline-carbonatite intrusions occur widespread through time at 255–209 Ma, 146–106 Ma and 91–71 Ma. Moreover, the Na-rich magmatic episodes are also documented at 130–120 Ma and 66–32 Ma. Tholeiitic basaltic lavas and dikes are recorded at ∼200 Ma in Northern Brazil and mainly between 134 and 131 Ma in the Paraná Magmatic Province. Simultaneous tholeiitic lavas and carbonatitic complexes are related to near isothermal decompression of enriched asthenospheric and lithospheric mantle sources at different depths (80–200 km). Likewise, the 267–226 Ma kimberlites in the Amazonian Craton, ∼128 Ma Rosário kimberlite in the Rio de la Plata Craton and 88–80 Ma Alto Paranaíba kimberlites in the western edge of the São Francisco Craton provide evidence for deeper (>200 km) metasomatized mantle sources. Compiled numerical, geophysical and geological data support the proposal that the thickness of the lithosphere, extension rates and the presence of previous weak crustal structures contributed to the generation and emplacement of multiple tholeiitic and alkaline intrusions from 250 to 30 Ma. We propose that several crustal extension events induced repeated thermal convection cells in the metasomatized asthenosphere, which triggered partial melting in the previously enriched and heterogeneous lithospheric mantle. The local mantle composition, depth and crustal extension ratios controlled the magma composition. Thus, West Gondwana break-up and dispersal played a crucial role in the Mesozoic to Paleogene melting processes of the metasomatized mantle in South America.

The Elusive Southern Margin of the Congo Craton During Gondwana Breakup: Insights from Lithospheric Mantle Structure and Heat-Flow Beneath the 85 Ma Xaudum Kimberlite Province in Nw Botswana

The Elusive Southern Margin of the Congo Craton During Gondwana Breakup: Insights from Lithospheric Mantle Structure and Heat-Flow Beneath the 85 Ma Xaudum Kimberlite Province in Nw Botswana

Transtensional Tectonics During the Gondwana Breakup in the Interior of Northeast Brazil: Early Cretaceous Paleostress Inversion in the Araripe Basin

Transtensional Tectonics During the Gondwana Breakup in the Interior of Northeast Brazil: Early Cretaceous Paleostress Inversion in the Araripe Basin

Unravelling the Jurassic Intraplate Tectono-Magmatic Evolution of Northern Patagonia During the Initial Breakup of Southwestern Gondwana

Unravelling the Jurassic Intraplate Tectono-Magmatic Evolution of Northern Patagonia During the Initial Breakup of Southwestern Gondwana

The Signature of Metasomatized Subcontinental Lithospheric Mantle in the Basaltic Magmatism of the Payenia Volcanic Province, Argentina

AbstractThe Payenia region of Argentina (34.5–38°S) is a large Pliocene‐Quaternary volcanic province of basaltic compositions in the Andean Cordillera foothills representing the northernmost extent of back‐arc volcanism in the Andean Southern Volcanic Zone (SVZ). Although the chemical diversity of the Payenia basalts has been characterized previously, the processes and sources responsible for such variation remain controversial. Here, we report new whole‐rock major and trace element concentrations, Sr‐, Nd‐, Hf‐, and Pb‐isotope ratios and high‐precision olivine oxygen‐isotope ratios in a suite of 35 alkaline basalts from Payenia. These lavas have major and trace elements that define a compositional range from arc‐influenced to intraplate signature. Variable crustal contamination and/or recent slab‐derived inputs inadequately account for elemental and isotopic systematics and spatial compositional variations of Payenia lavas. We present a simple forward model indicating that early metasomatism and subsequent melting of the metasomatized subcontinental lithospheric mantle (SCLM) has significantly contributed to the Payenia lava compositional range. Isotopic ingrowth calculations of radiogenic Sr, Nd, Hf, and Pb suggest that the SCLM metasomatism occurred at 50–150 Ma, consistent with the timing of the breakup of Gondwana and the development of the proto‐Pacific Andean arc. Variations in δ18Oolivine values from modeled melts indicate that the metasomatism and melting within the SCLM can fractionate oxygen isotopes even when the metasomatizing melt has MORB‐like δ18O values, providing a different explanation for the low‐δ18O signatures observed in continental arc settings.

U-Pb zircon xenocrysts dating as a proxy to assess volcanic assimilation and the underlying crust, Cretaceous Jaguarão Formation, RS, Brazil

The Early Cretaceous volcanic Jaguarão Formation formed duringthe early stages of Gondwana breakup. These felsic rocks characteristically enclose numerous crustal xenoliths. Fieldwork, petrographic studies, and laser ablation multicollector ion coupled plasma mass spectrometer (LA-ICP-MS) U-Pb analyses were carried out to evaluate the xenoliths of the Jaguarão Formation. A base-to-top distribution of xenoliths in the different flows of the volcanic pile indicates that earlier flows incorporated centimeter-sized fragments of local underlying gneiss, granite, and quartz veins, with limited assimilation. In the middle part of the volcanic pile, the size of the xenoliths is smaller and assimilation textures are more frequent. At this same level, reequilibrated orthopyroxene and plagioclase antecrysts were recognized, suggesting fractionation coeval with assimilation in a lower magmatic chamber prior to extrusion. At the top, no visible xenoliths occur. U-Pb ages of the zircon xenocrysts served as a proxy for assessing the crust underlying the volcanic rocks. Most of the ages obtained are similar to the ages of the Eastern Dom Feliciano Belt cropping out nearby, ranging from 680 to 620 Ma, with a subordinate group ranging from 580 to 570 Ma, suggesting a shallow crust contribution. The presence of few Paleoproterozoic and Mesoproterozoic (1.0-1.2 Ma) zircons, however, suggeststhat the underlying terrain may be more complex..

First report of a bothremydid turtle, Sindhochelys ragei n. gen., n. sp., from the early Paleocene of Pakistan, systematic and palaeobiogeographic implications

We report the discovery of remains of a large chelonian from the base of the early Paleocene Khadro Formation exposed in the Ranikot Fort area (Ranikot Group, Sindh Province, Southern Pakistan). This formation already yielded the snake Gigantophis Andrews, 1901, studied by our friend Jean-Claude Rage. The chelonian specimens consist of a large carapace and a shell fragment of Bothremydidae, a family of Gondwanan origin. A new genus and species, Sindhochelys ragei n. gen., n. sp. is identified from the first specimen and named in honor of Jean-Claude Rage. It is the first report of a Bothremydidae in Southern Pakistan. Its affinities with Cretaceous and Paleocene representatives of the family are discussed. The association of characters such as the shape of the shell, anterior plastral scute pattern and strongly marked decoration characterize the taxon and, despite some similarities, allows excluding close phylogenetic affinities with Taphrosphyini and Carteremys group; other well-documented bothremydids are also excluded. The shell fragment, also strongly decorated, is left undetermined. The discovery of two new littoral bothremydid specimens in the early Paleocene of Pakistan fills a geographic and stratigraphic gap in our knowledge of the family, which is known since the continental early Cretaceous of Africa, diversifying in the world up to the Miocene deposits of the Neotethys. A particular diversification during the Maastrichtian-Paleocene is recognized along the neotethyan coasts, and occasional dispersals across this ocean were possible. Sindhochelys ragei n. gen., n. sp. may have colonized the Indian subcontinent by this time, or may represent an older diversification before the Gondwana breakup.

Exhumation of the passive margin of the DR Congo during pre- and post- Gondwana breakup: Evidence from low-temperature thermochronology, geology and geomorphology

The Lower Congo region encompasses the South Atlantic passive margin of the Democratic Republic of Congo (Central Africa). It is host to the lowermost course of the Congo River, cutting through the Central African Atlantic Swell (CAAS). The region is underlain by litho-structural units of the Pan-African West Congo Belt, which consists of different tectono-metamorphic domains. The Precambrian basement is covered to the west by marine deposits of the South Atlantic Ocean and to the east by continental deposits of the Congo Basin. In this study we aim to constrain the timing of uplift and exhumation of the CAAS by using apatite fission track (AFT) thermochronology in combination with an updated overview of the geology and geomorphology of the Lower Congo region. AFT ages vary widely between 108 and 312 Ma. Short track lengths (11–12 μm) and broad, complex track length distributions indicate mixed ages between multiple thermal events. We derive the timing of exhumation from inverse thermal history models, that show that the Lower Congo region experienced a first exhumation event before Gondwana breakup during the Carboniferous to Middle Jurassic. This event is probably related to compressive forces at the boundaries of Gondwana. Both rifting and subsequent opening of the South Atlantic Ocean do not seem to have had a pronounced effect on the CAAS region. During the Late Cretaceous to Palaeogene, a slight reheating is suggested and could be due to subsidence and consequential modest reburial of the Lower Congo rocks. A second phase of exhumation initiated around the Palaeogene–Neogene and eventually emplaced the sampled rocks at surface temperatures. The multi-phased differential denudation results from reactivation of fault-bounded tectono-metamorphic blocks of the Precambrian basement, controlled by the combination of two systems of faults related to the Cretaceous South Atlantic Ocean opening and Pan-African orogeny respectively. Differential denudation of the Lower Congo region is also well-marked by independent qualitative indicators of the present-day geomorphology including distinct knickpoints and steep valleys along the course of the Lower Congo River, reworking of erosion surfaces and associated laterite crust and contrasting karst morphology.

A stratigraphic record from syn to post subduction sedimentation in Marlborough, New Zealand, and implications for Gondwana breakup

Sediments that accumulate within marine basins at convergent plate margins can provide important insights into the evolution and tectonic stability of the subduction wedge. Long-term sediment dispersal and stratigraphic patterns can also be influenced by geodynamic reconfigurations of the plate margin, such as changes in plate dip or velocity, or the subduction of anomalous oceanic crust. An understanding of tectonic context and complexity is therefore critical for any interpretation of convergent margin stratigraphy. Unfortunately, ongoing subduction and wedge deformation tends to obscure or destroy the stratigraphic record of such events as sediments become progressively incorporated into the wedge.The Awatere Valley in New Zealand's South Island contains well-preserved forearc stratigraphic record that accumulated within coastal-plain and marine basins during the final stages of East Gondwana subduction. We assign seven facies associations (FA) to characterise post-subduction units, showing that a transition from syn to post-subduction deposition is contained within largely marine strata from the New Zealand Urutawan to Ngaterian stages (c. 108 to 95 Ma). In contrast to previous accounts of deposition within nascent rift-basins, restored basin geometries are most consistent with those of terraced subduction wedges. We find no evidence that extension affected deposition prior to 95 Ma. Our results instead document a transition from highly deformed sedimentary wedge (pre 108 Ma) to largely undeformed sedimentary cover units (∼100 Ma) that is predominantly gradational.Up to 2 km of basin fill documents a relative sea-level (RSL) rise and fall between 108 and 95 Ma. A period of relative sediment starvation during the Urutawan (108–103 Ma) was abruptly terminated by synchronous mass-wasting, including the remobilisation of metre-scale blocks and rafts, that signals a dramatic increase in depositional energy relative to preceding patterns. Within the context of a large RSL fluctuation, diminishing compressional tectonism and forearc volcanism (from 98 Ma), this increase is anomalous and appears to signal a significant change in underlying subduction dynamics.

Ecological changes have driven biotic exchanges across the Indian Ocean

The Indian Ocean has a complex geological history that has drawn the attention of naturalists for almost a century now. Due to its tectonic history, many geological elements and processes have been evoked to explain the exchange of species between landmasses. Here, we revisited previous studies on twenty-three taxa to investigate trends across time since the Gondwana breakup. We investigated these datasets by applying a time-calibrated Bayesian framework to them and reconstructing their ancestral ranges. We conclude that ecological transformations have presented opportunities for the establishment of migrants. The role of donating and receiving migrants has shifted several times according to these transformations. Time-specific trends show weak evidence for the stepping-stones commonly suggested as physical routes between landmasses. However, before its collision with Asia, India may have served as an intermediary for such exchanges.

The geology of the Capiru Group, Curitiba Terrane, Southern Ribeira Belt (Brazil)

ABSTRACT The Capiru Group is a Neoproterozoic metasedimentary unit that crops out in the Curitiba terrane, Southern Ribeira Belt, Brazil. A detailed geological mapping of this unit was performed during 2016 to 2020. Original interpretations and the new geological map were compiled and integrated with the past field data of the state (MINEROPAR) and national (CPRM) geological surveys, and to unpublished and recent published studies. This compilation represents the most updated synthesis of the Capiru Group, and provides the basis for recognition and individualization of lithostratigraphic units. The lithostratigraphic units reflect depositional/tectonic events, which involved: a) passive continental margin deposition on the Curitiba microplate, b) compressional episodes during the Brasiliano - Pan African Orogeny (Late Neoproterozoic), c) extensional processes that followed the collision stage (Cambrian), and finally, the d) extensional tectonics related to the Gondwana break-up during the Lower Cretaceous, that culminated to the Proto-Atlantic Ocean opening.

Sedimentary architecture of a Late Cretaceous under‐filled rift basin, Canterbury Basin, New Zealand

AbstractThe Canterbury Basin in southeastern Zealandia was initiated during the late Albian (ca. 105 Ma) as a rift system, prior to the onset of seafloor spreading between Zealandia and eastern Gondwana at ca. 85 Ma. Basin‐fill architecture and sediment types have been determined from interpretation of 2D and 3D seismic‐reflection lines tied to five wells and compared to outcrop data from the literature. These data show that Cretaceous syn‐rift basin‐fill architecture was controlled by normal faulting, which produced basin and range topography that persisted for more that ca. 30 Myr after the cessation of faulting. Initial sedimentation was dominated by short drainage systems sourced from within the basin to produce alluvial fans along fault scarps, which inter‐fingered with axial‐flowing braided river conglomerates, coal measures and mudstone‐rich lake deposits in more central portions of the basins. Marine incursion of the basin from the east commenced during rifting and onset of Gondwana breakup, with maximum water depths achieved in the Oligocene. Post‐rifting, detrital sediments were mainly sourced locally from structural highs and augmented by pelagic sedimentation, which collectively draped and eventually buried most of the earlier‐formed horsts by the Early Eocene. The temporal persistence of basin and range topography reflected the low rates of erosion of horst blocks compared to the rates of fault displacement. The lack of substantial sediment input from outside of the rift basin was a key factor in the under‐filling of the Canterbury Basin. This research emphasises the key role played by sediment supply in rift basin filling. Despite an abundance of active faulting at initiation, some rift basins may fill slowly over tens of million years after rift cessation.

Global dispersal and diversification in ground beetles of the subfamily Carabinae

The origin and diversification process of lineages of organisms that are currently widely distributed among continents is an interesting subject for exploring the evolutionary history of global species diversity. Ground beetles of the subfamily Carabinae are flightless except for one lineage, but nevertheless occur on all continents except Antarctica. Here, we used sequence data from ultraconserved elements to reconstruct the phylogeny, divergence time, biogeographical history, ancestral state of hind wings and changes in the speciation rate of Carabinae. Our results show that Carabinae originated in the Americas and diversified into four tribes during the period from the late Jurassic to the late Cretaceous, with two in South America (Celoglossini) and Australasia (Pamborini) and two in Laurasia (Cychrini and Carabini). The ancestral Carabinae were inferred to be winged; three of four tribes (Cychrini, Ceglossini and Pamborini) have completely lost their hind wings and flight capability. The remaining tribe, Carabini, diverged into the subtribes Carabina (wingless) and Calosomina (winged) in the Oligocene. Carabina originated in Europe, spread over Eurasia and diversified into approximately 1000 species, accounting for around 60% of all Carabinae species. Calosomina that were flight-capable dispersed from North America or Eurasia to South America, Australia, and Africa, and then flightless lineages evolved on oceanic islands and continental highlands. The speciation rate increased in the Cychrini and Carabini clades in Eurasia. Within Carabini, the speciation rate was higher for wingless than winged states. Our study showed that the global distribution of Carabinae resulted from ancient dispersal before the breakup of Gondwana and more recent dispersal through flight around the world. These patterns consequently illustrate the causal relationships of geographical history, evolution of flightlessness, and the global distribution and species diversity of Carabinae.

Dispersal, not vicariance, explains the biogeographic origin of iguanas on Madagascar

Lizards of the clade Iguanidae (sensu lato) are primarily a New World group. Thus, the remarkable presence of an endemic lineage of iguanas (family Opluridae) on the isolated Indian Ocean island of Madagascar has long been considered a biogeographic anomaly. Previous work attributed this disjunct extant distribution to: (1) vicariance at about 140–165 Ma, caused by the breakup of Gondwana and the separation of South America, Africa, and Madagascar (with subsequent extinction of iguanas on Africa, and potentially other Gondwanan landmasses), (2) vicariance at about 80–90 Ma, caused by the sundering of hypothesized land-bridge connections between South America, Antarctica, India, and Madagascar, or (3) long-distance overwater dispersal from South America to Madagascar. Each hypothesis has been supported with molecular divergence dating analyses, and thus the biogeographic origin of the Opluridae is not yet well resolved. Here we utilize genetic sequences of ultraconserved elements for all Iguania families and the majority of Iguanidae (s.l.) genera, and morphological data for extant and fossil taxa (used for divergence dating analyses), to produce the most comprehensive dataset applied to date to test these origin hypotheses. We find strong support for a sister relationship between the Opluridae (Madagascar) and Leiosauridae (South America). Divergence of the Opluridae from Leiosauridae is dated to between the late Cretaceous and mid-Paleogene, at a time when Madagascar was already an island and was isolated from all other Gondwanan landmasses. Consequently, our results support a hypothesis of long-distance overwater dispersal of the Opluridae lineage, either directly from South America to Madagascar or potentially via Antarctica or Africa, leading to this radiation of iguanas in the Indian Ocean.

Sawflies out of Gondwana: phylogenetics and biogeography of Argidae (Hymenoptera)

ABSTRACTWe present the first phylogenetic hypothesis for the cosmopolitan family Argidae based on both molecular and morphological data. Furthermore, we present a biogeographic scenario based on a dated phylogeny and interpret the evolutionary history of the family. Information from sequences of eight genes is analysed to reconstruct the phylogeny of the Argidae based on maximum likelihood and Bayesian inference. Total evidence Bayesian analyses are also conducted, combining the molecular dataset with data from adult and larval morphology. For the historical biogeographic reconstruction, divergence times are estimated using node dating with uncorrelated relaxed‐clock analysis, and ancestral biogeographical distributions are estimated applying a Dispersal Extinction Cladogenesis model and a Bayesian binary model. Argidae s.s. is retrieved as monophyletic in all analyses and the clade Zenargidae + (Argidae + Pergidae) is better supported when combining molecular and morphological characters, and when excluding the saturated third codon position in the molecular analysis. Within Argidae, two large clades corresponding to the subfamilies Arginae and Sterictiphorinae sensu Benson are retrieved as monophyletic. The ancestral distribution of Arginae and Sterictiphorinae is estimated to be the Australian and Neotropical regions. Divergence of Argidae‐Pergidae and Arginae‐Sterictiphorinae is estimated to occur in the middle‐upper Jurassic before or during the east‐west Gondwana breakup. Diversification of Argidae may be associated with the radiation of angiosperms in the Early Cretaceous, especially for the Neotropical Sterictiphorinae after the separation of South America and Africa. Arginae were probably introduced to the northern hemisphere by dispersal to Africa and/or India and subsequent continental collision with Eurasia in the Cenozoic. The occurrence of Sterictiphorinae in the northern hemisphere is more difficult to explain. Maternal care and brood guarding behaviour evolved independently in Argidae and Pergidae, with a single origin in a subclade (Dielocerini) of Sterictiphorinae.

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.

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.