Continental Rift Setting Research Articles

Geology of the Liuyuan Complex, NW China: A Permian back-arc basin ophiolite at the southern edge of the Central Asian orogenic belt

Located in the southern margin of the Central Asian orogenic belt, the stratigraphy and tectonic setting of the mafic 290−280 Ma Liuyuan Complex have been controversial for decades, with workers arguing for a forearc ophiolite or a continental rift setting. Here, we present the results of a detailed field study, where the Liuyuan Complex was subdivided into troctolite, melatroctolite, layered gabbro, varitextured olivine gabbro, hornblende gabbro, plagiogranite, sheeted dike, and mafic tectonite, in addition to previously identified and studied basalt and chert. All contacts between the igneous facies are intrusive, with gabbroic rocks separated from the overlying basalt by a well-developed and laterally continuous sheeted dike complex. Based on their geochemical affinities, two groups of basalt were identified: group I (low-TiO2) and group II (high-TiO2). A modeled liquid line of descent, assuming perfect mineral fractionation, with a liquidus temperature of 1212 °C, pressure of 1 kbar, fO2 at the quartz-fayalite-magnetite (QFM) buffer, and initial melt H2O of 0.5 wt%, provides an excellent fit to group I lavas, with group II basalts interpreted as having formed from a distinct arc source. The stratigraphy, extended trace-element patterns, and tectonic fingerprinting of the lavas suggest the Liuyuan Complex formed as an ophiolite in a fast-spreading back-arc basin, a setting inconsistent with previously proposed tectonic models for the southern Central Asian orogenic belt. We expand on a tectonic model that proposes the Liuyuan Complex formed as a back-arc to the recently identified Ganquan arc. The back-arc basin was then consumed in a north-dipping subduction zone beneath the active margin of composite Siberia. The magmatic center of this arc migrated southward, likely caused by slab roll-back, with the Liuyuan Complex becoming the basement of an arc. Exhumation of the Liuyuan Complex took place by 267 Ma, as constrained by the age of a subaerial dacite that unconformably overlies the basalts of the Liuyuan Complex.

Petrogenesis and metallogenesis of the Qieganbulake carbonatite-related phosphate deposit associated with the mafic–ultramafic–carbonatite complex in the Kuluketage block, northeastern Tarim Craton

AbstractThe Qieganbulake deposit associated with a mafic–ultramafic–carbonatite complex in the Kuluketage block is not only the world’s second-largest vermiculite deposit, but also a medium-size carbonatite-related phosphate deposit. Field observations, radiometric dating results and Sr–Nd–Hf isotopes reveal that the parental magmas of the carbonatite and mafic–ultramafic rocks are cogenetic and formed synchronously at c. 810 Ma. Geochemical characteristics and Sr–Nd–Hf–S isotopes ((87Sr/86Sr)i = 0.70581–0.70710; ϵNd(t) = −0.20 to −11.80; ϵHf(t) = −7.5 to −10.3; δ34S = +0.7 ‰ to +3.0 ‰ (some sulfides with high δ34S values (+3.2 to +6.6) were formed by late hydrothermal sulfur)), in combination with mineral compositions and previous research, strongly indicate that the Qieganbulake mafic–ultramafic–carbonatite complex formed via extensive crystal fractionation/cumulation and liquid immiscibility of a carbonated tholeiitic magma, possibly derived from partial melting of an enriched subcontinental lithospheric mantle previously modified by slab-released fluids and sediment input in a continental rift setting. The coupled enriched Sr–Nd isotopic signatures, in combination with previous research, suggest that the enriched subcontinental lithospheric mantle could have been metasomatized by asthenospheric mantle melts to different degrees. The Qieganbulake carbonatite-related phosphate ores were the products of normal fractional crystallization/cumulation of P–Fe3+ complex enriched carbonatite magma in high oxygen fugacity conditions, which was generated by liquid immiscibility of CO2–Fe–Ti–P-rich residual magma undergoing high differentiation.

Discovery of a >1,000 km Cambrian Eclogite‐Bearing High‐Pressure Metamorphic Belt in the Central Asian Orogenic Belt: Implications for the Final Closure of the Pan‐Rodinian Ocean

AbstractThe immense Central Asian Orogenic Belt (CAOB) records the transition from the Pan‐Rodinian Mirovoi Ocean to the Paleo‐Asian Ocean, but the final closure of the Mirovoi Ocean remains unexplored. Here we document two new eclogite occurrences, located between CAOB microcontinents and the Ediacaran–Cambrian island arc in Mongolia. Pseudosection modeling and geothermobarometry constrain peak conditions of ∼560°C and ∼2.2 GPa for the Urgamal eclogite and ∼715°C and ∼2.0 GPa for the Tsengel eclogite. Zircon U‐Pb dating results suggest the Urgamal eclogite‐facies metamorphism occurred at ∼522 Ma, and the granite gneiss enclosing the eclogite yielded ∼811 Ma crystallization age and ∼519 Ma metamorphic age. Detrital zircon age spectra of the paragneisses associated with the eclogite suggest a probable provenance from the adjacent Zavkhan microcontinent and the Neoproterozoic arc superimposed on the microcontinent. The Tsengel eclogite has a protolith age of ∼853 Ma and an eclogite‐facies metamorphic age of ∼522 Ma, and the associated migmatite gneiss record a ∼520 Ma migmatization event. The rock associations and geochemical affinities indicate that their protoliths represent mafic rocks formed in a continental rift setting and the Early Neoproterozoic subduction–accretion complex, respectively. Their similar tectonic position and metamorphic age suggest that these rocks are part of a single >1,000 km long Cambrian high‐pressure metamorphic belt. The rocks record closure of the Mirovoi Ocean within the CAOB and constrain this crucial event to ∼520 Ma along the western margin of the microcontinents in Mongolia.

Late Permian High-Ti and Low-Ti Basalts in the Songpan–Ganzi Terrane: Continental Breakup of the Western Margin of the South China Block

Although the Emeishan Large Igneous Province (ELIP) has been thoroughly researched, the role of the ELIP in the tectonics of the Songpan–Ganzi extensional basin in the eastern Tibetan Plateau has long been argued without any corroborated and robust evidence. We have investigated the basalt succession of the Dashibao Formation along Xindianzi and Xuecheng sections in the southeast margin of the Songpan–Ganzi Terrane (SGT). New SIMS zircon U-Pb ages and geochemical features of the Dashibao Formation are reported in this paper. Zircons of basalt sample XDZ02-1 yielded a weighted mean age of 259.1 ± 1.66 Ma, which is in alignment with the period when the main eruption of the ELIP occurred. Zircons from two tuff samples XDZ05-1 and XC05-2, overlying the basalt succession, were dated at 251.8 ± 1.57 Ma and 251.5 ± 0.27 Ma, respectively. These new dating results revealed a ~10 Ma eruption for the Dashibao basalts. The Dashibao basalts are geochemically classified into alkaline basalts (Group 1) and tholeiitic basalts (Group 2), which are a part of the Emeishan basalt. We thus propose that the Dashibao basalts erupted in a continental rift setting, located at the margin of the ELIP. The temporal and spatial coincidence of the Dashibao basalt, ELIP, and continental rifting in the western margin of the South China Block suggest that the continental breakup is a response to the Permian mantle plume that triggered the separation of the SGT in the eastern Tibetan Plateau.

Geochemistry of Permian carbonaceous shales from Raniganj sub-basin, Damodar Valley, India: Implications for provenance, weathering, tectonics and source of organic matter

Raniganj sub-basin, the easternmost depository of Damodar Valley encompasses the entire succession of Gondwana Supergroup, the lower and upper Gondwana sequences. The present investigation of Permian carbonaceous shales belonging to Barakar, Barren Measure and Raniganj formations of lower Gondwana sequence from this basin is an integrated study of inorganic and organic geochemical characteristics viz. major/trace elements, stable isotopic composition and kerogen type to ascertain source rock attributes such as provenance, tectonic setting, prevailing weathering/climatic conditions as well as source and type of organic matters. Trace element indices, such as La/Sc, Th/Co, Th/Sc, (Gd/Yb)CN, Eu/Eu*, Cr/V, Y/Ni, reveal that the shales are originated from felsic/granitic source rock. Prominent Eu-anomaly, enrichment of chondrite normalised light rare earth element (LREE) values and a flat heavy rare earth elements (HREE) pattern reflect post-Archean felsic source for the shales. The major element composition, especially relative enrichment of Al over readily soluble elements, Ca, Na and K yielding high CIA (74.9–84.6%) and CIW (87.1–97.5%) as well as low ICV (0.64–1.32) values, establishes significantly weathered and matured nature of the shales. The distribution of redox-sensitive elements (U/Th, V/Cr, Ni/Co, Ce/Ce*) as well as low ICV, depleted K2O/Al2O3 ratios and high CIA values point towards an oxic environment and warm-humid climatic condition during deposition of these shales. The conventional method for determining tectonic setting using major and trace element composition fails to clearly delineate the tectonic setting for the current shales. The samples show wide scattering across active and passive continental margin in the K2O/Na2O–SiO2 diagram, whereas the samples are concentrated near the active in the continental margin in the Th–Sc–Zr/10 diagram contradicting the geological framework of Raniganj sub-basin. Discriminant functions-based approach which categorises the tectonic setting into continental/oceanic arc, continental rift and continental collision settings has been proved to be suitable in the present case and reveals continental-rift setting for the shales from Raniganj sub-basin. The carbon isotopic signature (δ13Corg) of the shales reveals a range (−24.5 − −22.9‰) intermediate between that of marine and terrestrial (C3 plants) organic matters indicating possible mixed type of organic matters. The fluvio-marginal marine depositional condition obtained from K2O/Al2O3 and MgO/Al2O3 ratios further implies the possibility of mixed nature of organic matter. The geochemical parameters of the organic matter (Rock-Eval pyrolysis parameters S1, S2, TOC, HI, OI etc.) indicate that the studied shales contain thermally matured mixed Type III/II gas-prone kerogen and are derived predominantly from terrestrial sources with significant marine input.

Grain-size-evolution controls on lithospheric weakening during continental rifting

Variation in the effective strength of the lithosphere allows for active plate tectonics and is permitted by different deformation mechanisms operating in the crust and upper mantle. The dominant mechanisms are debated, but geodynamic models often employ grain-size-independent mechanisms or evaluate a single grain size. However, observations from nature and rock deformation experiments suggest a transition to grain-size-dependent mechanisms due to a reduction in grain size can cause lithospheric weakening. Here, we employ a two-dimensional thermo-mechanical numerical model of the upper mantle to investigate the nature of deformation and grain-size evolution in a continental rift setting, on the basis of a recent growth law for polycrystalline olivine. We find that the average olivine grain size is greater in the asthenospheric mantle (centimetre-scale grains) than at the crust–mantle boundary (millimetre-scale grains). This grain-size distribution could result in dislocation creep being the dominant deformation mechanism in the upper mantle. However, we suggest that along lithospheric-scale shear zones, a reduction in grain sizes due to localized deformation causes a transition to diffusion creep as the dominant deformation mechanism, causing weakening of the lithosphere and facilitating the initiation of continental rifting.

Post-UHP tectonic evolution of the East Kunlun orogenic belt, northern tibetan plateau: insight from the regional-scale crustal anatexis

ABSTRACT Extensive crustal anatexis is common during the decompressional exhumation of deeply subducted continental crust and the delamination of a collision‐thickened orogenic lithosphere, indicating that determining the timing of partial melting can provide insights into the possible tectonic evolution during orogenic processes. An integrated study of petrology, zirconology and whole-rock geochemical compositions from migmatites and pegmatites in the Nageng, Langmuri and Wulonggou area of the East Kunlun orogenic belt, is presented in this study. The protoliths of these migmatites include Tonian sedimentary rocks and ca. 440 Ma arc-related dioritic rock. Together with previous studies, an extensive crustal anatexis event recorded by anatectic zircons occurred after ca. 410 Ma. In addition, hafnium isotope analyses indicate that the anatectic zircons may or may not inherit their Hf isotope compositions from protolith zircons during partial melting, depending on whether or not the garnet Hf contributes to the Hf budget of anatectic melt. Thus, the most reasonable method to trace the source of granite is to compare the Hf isotope composition of anatectic zircon in migmatite and magmatic zircon in contemporaneous granite. By such method, anatectic melt produced by migmatites in this study could account for both penecontemporaneous A- and I-type granites with high magmatic zircon εHf(t) values and S-type granites with low magmatic zircon εHf(t) values. As the extensive crustal anatexis is later than both metamorphic peak and amphibole-facies retrogression of high-pressure and ultrahigh-pressure (HP-UHP) metamorphic rocks, it is reasonable that post-collision orogenic collapse of the collision‐thickened orogenic lithosphere as a result of delamination and asthenospheric upwelling occurred after ca. 410 Ma, which is also supported by contemporaneous granulite-facies metamorphism, molasse deposition, and magmatic events including Xiarihamu mafic-ultramafic intrusions, A-type granites, and mafic dykes formed in the continental rifting setting.

Geophysical and Geochemical Evidence for a New Mafic Magmatic Province Within the Northwest Shelf of Australia

AbstractThe formation of mafic magmatic provinces are significant geological events that can drive mass extinctions and continental rifting and can influence basin evolution, petroleum prospectivity and mineralization. Buried magmatic provinces, however, are rarely identified and difficult to define. The Northwest Shelf of Australia contains large volumes of potentially interconnected mafic igneous material across several sedimentary basins. However, limited study and a lack of surface exposure have prevented detailed description and classification of these rocks. In this study, the distribution and composition of these mafic igneous rocks are described using an integrated geophysical and geochemical approach, which included over 10,000 km line length of 2D seismic data, well log data and chemical analysis of samples from 14 wells across the Browse, Roebuck, Canning and North Carnarvon basins. Using this combined data set, we demonstrate interconnectivity of buried mafic igneous rocks across the Northwest Shelf and calculate a total surface area exceeding 280,000 km2 and a cumulative minimum volume of ∼140,000 km3. Petrology and geochemistry of samples indicate they are basaltic and doleritic with alkaline and sub‐alkaline compositions and formed in a continental rift setting. Collectively, the igneous rocks meet the criteria for classification as a mafic magmatic province (MMP) and closely match the criteria required for classification as a large igneous province (LIP). Emplacement of the newly defined Northwest Shelf MMP may represent hotspot magmatism that could have initiated rifting of the Cimmerian Block from NW Australia during the Permian and could have potential for future, large scale CO2 sequestration and storage.

Triassic trachytic volcanism in the Bangong–Nujiang Ocean: geochemical and geochronological constraints on a continental rifting event

AbstractThe Bangong–Nujiang suture zone (BNSZ), which separates the Gondwana-derived Qiangtang and Lhasa terranes, preserves limited geological records of the Bangong–Nujiang Ocean (BNO). The timing of opening of this ocean has been hotly debated due to the rare and complicated rock records in the suture zones, which span over 100 Ma from Carboniferous–Permian to Early Jurassic time, based on geological, palaeontological and palaeomagnetic data. A combination of geochemical, geochronological and isotopic data are reported for the Riasairi trachytes, central BNSZ, northern Tibet, to constrain its petrogenesis and tectonic settings. Zircon U–Pb dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yields mean ages of 236 Ma. Geochemically, these rocks are high-K calc-alkaline with moderate SiO2 (59.1–67.5 wt%) and high K2O + Na2O (8.1–11.6 wt%) contents. They are enriched in light rare earth elements with negative Eu anomalies, and show enrichments in high-field-strength elements with positive ‘Nb, Ta’ anomalies, similar to the intra-continental rift setting-related felsic lavas from the African Rift System. The high positive zircon ϵHf(t) and bulk ϵNd(t) values, as well as high initial Pb isotopes, imply a heterogeneous source involving both asthenospheric and subcontinental lithospheric mantle. The field and geochemical data jointly suggest that the Riasairi trachytes within the Mugagangri Group were formed in a continental rift setting. We interpret that the continental-rift-related Riaisairi trachytic lavas as derived from the southern margin of the Qiangtang terrane, implying that the BNO would have opened by Middle Triassic time, well after the commonly interpreted break-up of the Qiangtang terrane from Gondwana.

Late Permian ultrapotassic rhyolites in SE Thailand: evidence for a Palaeotethyan continental rift basin

The volcanic rocks in the Chanthaburi zone are rarely reported, but are important in investigating the tectonic evolution of the Palaeotethyan Ocean in SE Thailand. Four rhyolitic samples from Ko Chang Island yield zircon ages of 258–254 Ma, confirming the presence of Late Permian volcanic rocks in SE Thailand. These rocks consist of Group 1 rhyolites and Group 2 rhyolitic ignimbrites and have high K 2 O contents of 4.92–7.10 wt% and A/CNK values of 1.10–1.69. They are enriched in the light rare earth elements, Rb, Th, U, Zr and Y and show negative anomalies of Ba, Sr, Nb, Ta and Ti, with clear Eu anomalies. Their whole-rock ε Nd ( t ) values range from −1.7 to −3.1. Zircon in situ ε Hf ( t ) and δ 18 O values range from 0.0 to +5.6 and from 8.2 to 9.6‰, respectively. They belong to the group of peraluminous, ultrapotassic A-type rhyolites and were derived from partial melting of a mixed source of Mesoproterozoic metasedimentary rocks with a component of juvenile mafic crust. These ultrapotassic rhyolites formed in a continental rift setting in response to the rollback of a subducted Palaeotethyan oceanic slab beneath the Indochina Block. Combining these results with previous geological observations, we propose that there were some sporadically distributed continental rift basins along the Eastern Palaeotethyan domain during the Permian. Supplementary material: Major oxides, trace element analytical and Nd isotopic data, LA–ICP–MS and SIMS zircon U–Pb ages and Zircon in-situ Hf–O isotopic compositions are available at https://doi.org/10.6084/m9.figshare.c.5635390

Petrogenesis of Late Carboniferous–Early Permian mafic igneous series in the Baoshan block: Implications to birth of Neo-Tethys and generation of magmatic sulfide deposit

The petrogenesis of the rift-related Late Carboniferous–Early Permian mafic igneous rocks (basaltic lavas, dolerite dykes and mafic–ultramafic intrusive rocks) from the Sibumasu block in the eastern end of Cimmeria was still ambiguous. This petrogenesis is related to understand the drive for the formation of Neo-Tethys which was developed along the margin of Cimmeria, and to the generation of magmatic sulfide deposit via partial melting of previously subduction-modified mantle induced by rifting of continental margin. Most of the mafic rock samples belong to the subalkaline series, with lithologies varying from basalt to basaltic andesite. Magmatic zircons from the doleritic dykes yield weighted mean 206Pb/238U ages of 308.7 ± 2.5 Ma. The εHf(t) values of zircon are ranging from 3.2 to 11.8, which are much lower than those of the coeval depleted mantle (εHf(t) ≈ 16 at 305 Ma). The igneous series share similar features, such as elevated light rare earth elements, negative Nb-Ta anomalies and enriched Sr-Nd-Hf isotopes. Based on geochemical analyses of the mafic igneous series in the northern part of Sibumasu, we suggest that the primary melts of the mafic igneous rocks were derived from a fertile subcontinental lithospheric mantle (SCLM) which was previously modified by oceanic subduction, and these rocks were intruded and erupted in a passive plate margin during plate extension. The subsequent melting-induced softening of SCLM triggered the evolution of continental margin from the rift to the open of Neo-Tethys Ocean. The proposed model emphasizes that the melt generated from previously-modified SCLM in a continental rift setting is beneficial for the generation of magmatic sulfide deposit.

The ca. 1.18–1.14 Ga A-type granites in the southwestern Yangtze Block, South China: New evidence for late Mesoproterozoic continental rifting

The late Mesoproterozoic tectonic history of the Yangtze Block, South China in the context of the supercontinent cycles remains poorly understood due to the scarce preservation of geological records of this age. In this study, we report petrology, whole-rock geochemistry, zircon U-Pb ages and Hf-Nd isotopes for newly identified late Mesoproterozoic monzogranites from the southwestern Yangtze Block. SHRIMP and LA-ICP-MS zircon U-Pb dating results reveal that these monzogranites crystallized at ca. 1.18–1.14 Ga. All the studied samples possess typical geochemical signatures of A1-type granite, such as relatively high SiO2 (73.31–77.50 wt.%), Na2O + K2O (7.14–8.09 wt.%) and Zr + Nb + Ce + Y (362–445 ppm) contents, zircon saturation temperatures (825–851 °C), and low Y/Nb ratios (0.78–1.04). They have negative whole-rock εNd(t) values from −6.9 to −5.7 and zircon εHf(t) values from −7.4 to −2.5, with consistent Nd and Hf model ages of 2.4–2.1 Ga. The parental magmas for these monzogranites were generated by partial melting of the early Paleoproterozoic crustal materials at low pressure and high temperature. Taking into account of the coeval within-plate mafic magmatism in the southwestern Yangtze Block, these ca. 1.18–1.14 Ga A1-type monzogranites were most likely emplaced in a continental rift setting. Available data indicate continental rifting may have occurred extensively in the Yangtze Block at 1.2–1.1 Ga, followed by oceanic plate subduction starting at 1.1 Ga. We propose the Yangtze Block was an isolated plate during 1.2–1.1 Ga and started to drift toward the Rodinia supercontinent at ca. 1.1 Ga, favoring the proposal that a coherent supercontinent did not yet exist in the late Mesoproterozoic.

Paleo-Mesoproterozoic magmatism in the Tarim Craton, NW China: Implications for episodic extension to initial breakup of the Columbia supercontinent

The Tarim Craton located in the northwest of China, where the Paleo-Mesoproterozoic extensional magmatism related to the Columbia supercontinent is widely developed. However, the process and time of the extension and initial breakup of the Tarim Craton are still controversial during Paleo-Mesoproterozoic era. In this contribution, we present a systematic petrographic, geochemical, and zircon U-Pb-Hf investigation on Aketashitage granite dykes, Wulan granitic gneisses and Astingbulake metadiabases from the Tarim Craton. Results from our study indicate that the ca. 1.88–1.86 Ga Aketashitage granite dykes belong to I-type granites, which were probably derived from low degree partial melting of thickened Archean crust during early post-collisional setting. The ca. 1.55 Ga Wulan granitic gneisses also show the characteristics of I-type granites, which mainly came from partial melting of mafic lower crust with minor mantle-derived magma input during the rift setting. The ca. 1.55 Ga protoliths of Astingbulake metadiabases show affinity to ocean island basalts and/or continental flood basalts and were produced by partial melting of enriched continental lithospheric mantle under continental rifting setting. In conjunction with previous studies, it can be concluded that the Tarim Craton experienced episodic extension to initial breakup: the South Orogen underwent early post-collisional extension during ca. 1.90–1.85 Ga, followed by late post-collisional extension during ca. 1.80–1.73 Ga; the North Orogen experienced early post-collisional extension at ca. 1.80–1.73 Ga and then entered the late post-collisional extension at ca. 1.67–1.60 Ga; finally, the South Orogen and North Orogen simultaneously entered the initial breakup stage during ca. 1.55–1.47 Ga. Based on a comparison of the global Mesoproterozoic (ca. 1.58–1.45 Ga) magmatic activities as a response to the initial break up of the Columbia supercontinent, we suggest that the Tarim Craton (including the Oulongbuluke Block) and North China Craton were close to India, Australia, Yangtze, Siberia, Congo, São Francisco and West African.

Petrogenesis of silica-rich carbonatites from continental rift settings: a missing link between carbonatites and carbonated silicate melts?

Petrogenesis of silica-rich carbonatites from continental rift settings: a missing link between carbonatites and carbonated silicate melts?

A non-arc tectonic setting for the evolution of Archean gabbro anorthosite Complexes: Evidence from the Singhbhum Craton, eastern India

Understanding the petrogenetic evolution of Archean gabbroic cumulates provides a window to the mantle geodynamics operated in the early Earth. Gabbroic cumulates are associated with various tectonic settings including convergent plate boundaries, mantle plumes, mid-ocean ridges, anorogenic, post-orogenic, continental rift settings and oceanic plateaus. The exposures of preserved gabbroic suites in the Archean are limited. The Mesoarchean Mayurbhanj Gabbro Anorthosite Complex (GAC) is a well-preserved mafic body situated along the eastern margin of the craton along with concomitant anorogenic Mayurbhanj granite. These rocks are devoid of deformational structures and/or mineral recrystallization. Based on their mineralogical characteristics, GAC rocks are mainly grouped into gabbronorite, leucogabbronorite, leucogabbro and gabbro. The minimal abundance of magmatic amphibole and the variation in the anorthite content (An36-An84) rules out the role of a hydrous mantle source in the evolution of these rocks. The studied samples display tholeiitic trend transitioning to calc-alkaline field, major and trace element data indicates the fractional crystallization of olivine and pyroxene. The trace element systematics of the Mayurbhanj GAC show LILE enrichment over HFSE depletion, with negative Nb-Ta and mixed Zr-Hf and Ti anomalies. The geochemical signatures such as elevated Th/Yb, Nb/Yb, variable Dy/Yb and Dy/Dy* and varying Gd/YbCN ratios suggest that parental magmas of the Mayurbhanj GAC originated from a heterogeneous depleted mantle source. The studied rocks' Sr-Nd isotopic systematics reveal that their parental magma has undergone assimilation by a Paleoarchean felsic crust. We argue that the ‘arc-like’ geochemical signatures displayed by these rocks resulted from crustal contamination and may not be due to subduction origin. Consolidating the field, geochemical and isotopic evidence suggests that the Mesoarchean Mayurbhanj GAC possibly evolved from an anhydrous heterogeneous mantle source in a non-compressive tectonic regime.

The NW Amazonian Craton in Guainía and Vaupés departments, Colombia: Transition between orogenic to anorogenic environments during the Paleo-Mesoproterozoic

In order to improve our understanding of the NW-Amazonian Craton evolution, we present new petrographic, geochemical and geochronological analyses of 27 samples from the geotectonic Rio Negro-Juruena Province in eastern Colombia (Guainía and Vaupés departments). New LA-ICP-MS zircon U-Pb ages suggest that the oldest known rocks in Colombia are metamorphic rocks (migmatitic gneisses) with ages between ~ 1850 and ~ 1800 Ma, and gneisses and granitoids with ages between ~ 1800–1720 Ma which form part of the Mitú Complex, interpreted as the result of Statherian collisional and orogenic events (Querari Orogeny). Detrital zircons in low-grade meta-sedimentary sequences of the Tunuí Group (sandstones, conglomeratic sandstones and mudstones), that crop out over almost the entire basement, indicate older than ~ 1770 Ma source rocks. Intrusions of different suites of granitic rocks with syn- to post-collisional affinities suggest a termination of the collisional events between ~ 1600–1500 Ma which had affected the whole region, occasionally metasomatically overprinting parts of the Tunuí meta-sedimentary sequence. The recognizable metamorphic and magmatic-processes finish with ~ 1400–1340 Ma anorogenic granites without signs of tectonic deformation, resembling anorogenic granites in the Western Amazonian Craton in Brazil and the Parguaza Batholith in Venezuela. This study allows us to conclude that the basement records the collision of a continental arc (Rio Negro-Juruena Province) against the NW-Amazonian Craton (Ventuari-Tapajos Province) and its subsequent transition to anorogenic conditions in a continental rift setting long before the actual stable craton conditions.

Combined petrography, noble gas, stable isotope and fluid inclusion chemistry of carbonatites from Uganda: Implications for the origin of the carbonatite melt in continental rift setting

An integrated petrographic, fluid inclusion, noble gas, and stable isotope study was performed for the Late Oligocene, spatially associated Sukulu and Tororo carbonatite complexes in SE Uganda in order to understand their genesis and provide additional constraints on the evolution of the East African Rift System (EARS). Both carbonatite complexes comprise mainly ring-shaped calcio‑carbonatites with accessory primary magmatic phases, including olivine, phlogopite, aegirine and pyrochlore. Absent or low Na deficiency on the A-site and high F contents (0.4–1.2 apfu) reveal the magmatic origin of pyrochlore. In contrast, the corroded rims of olivine, Sr-enrichments of the rims of apatite, as well as the occurrence of Sr- and Mg-rich lamellae in calcite, coupled with the presence of barite, REE-witherite, siderite, pyrite, bastnäsite and galena as secondary phases imply a hydrothermal overprint of the primary magmatic mineral assemblage.Fluid inclusions found in the cores of clustered apatite crystals are primary, two-phase H2O + NaCl aqueous inclusions in Sukulu-, and two-, or multi-phase H2O + NaCl ± NaHCO3 ± other solid inclusions in Tororo Complex. In the gas phase of some inclusions small amounts of CO2 or CH4 have been detected by micro-Raman analysis. The estimated trapping temperature of 330–425 °C and pressure of 465–1330 bars using the accepted denudation rates calculated for the EARS as independent barometers, reveal subsolidus crystallization conditions for the intercrystalline clustered apatite aggregates. Stable C-O isotope compositions of both carbonatite complexes suggest ultimately a mantle origin for the carbonate phase.Helium, neon and argon isotope compositions, liberated by crushing of bulk rock, apatite and calcite samples, show a systematic variation between the analyzed phases. Relatively low 3He/4He ratios of apatite and pyroxene separates (<5.3 Ra) are accounted for by radiogenic production of 4He in these minerals, whereas the air-corrected values in calcite reach as high as 9.6 Ra, characteristic for plume sources. Whole-rock Ne isotope ratios provide evidence for a depleted MORB mantle (DMM) trend. Combined (21Ne/22Ne)EX (air-corrected 21Ne/22Ne ratio extrapolated to Ne-B composition Holland and Ballentine, 2006; Trieloff et al., 2000) versus 4He/3He are indicative of mixing of a plume-like and a DMM-like component in the source region of the parental carbonatite magmas, which is in contrast to some recent interpretations of recent/sub-recent magmas in the EARS. Based on new noble gas data from this study we argue that in the early phase of the magmatic activity of the EARS, a low-viscosity carbonatite magma may have intruded into the shallow crust rapidly, without any significant contamination by the sub-continental lithospheric mantle.Combined fluid inclusion data and noble gas results collectively imply that the carbonatites worldwide are carbothermal (hydrothermal) rather than purely magmatic products and that they crystallized at sub-solidus temperatures from dominantly magma-derived fluids.

Formation of Continental Microplates Through Rift Linkage: Numerical Modeling and Its Application to the Flemish Cap and Sao Paulo Plateau

AbstractContinental microplates are enigmatic plate boundary features, which can occur in extensional and compressional regimes. Here we focus on microplate formation and their temporal evolution in continental rift settings. To this aim, we employ the geodynamic finite element software ASPECT to conduct 3D lithospheric‐scale numerical models from rift inception to continental breakup. We find that depending on the strike‐perpendicular offset and crustal strength, rift segments connect or interact through one of four regimes: (1) an oblique rift, (2) a transform fault, (3) a rotating continental microplate or (4) a rift jump. We highlight that rotating microplates form at offsets &gt;200 km in weak to moderately strong crustal setups. We describe the dynamics of microplate evolution from initial rift propagation, to segment overlap, vertical‐axis rotation, and eventually continental breakup. These models may explain microplate size and kinematics of the Flemish Cap, the Sao Paulo Plateau, and other continental microplates that formed during continental rifting worldwide.

Bimodal volcanic rocks in the northeastern margin of the Yangtze Block: Response to breakup of Rodinia supercontinent

Mid-Neoproterozoic protoliths of high–ultrahigh-pressure metamorphic rocks in the northeastern margin of the Yangtze Block preserve one of the best examples of 18O-depleted magmatic provinces worldwide, which is the significant record for lithospheric extension associated with continental rifting. In this contribution, we report the depleted 18O signature of felsic meta-volcanic rocks from the Zhangbaling Uplift (ZBLU), a segment of the Tan–Lu fault zone, which is located between the Dabie and Sulu orogenic belts. The low-δ18O signature of these rocks provides crucial information for understanding their origin and tectonic setting. Zircons yield UPb protolith ages of 771 ± 5 Ma to 761 ± 5 Ma and show a narrow range in δ18O values of 1.97‰ to 3.11‰. The typical syn-magmatic characteristics of these zircons indicate that they record inheritance from low-δ18O magmas during the mid-Neoproterozoic, which suggests the occurrence of high-temperature surface-water–rock interaction in a continental rift setting. Quartz keratophyres in the ZBLU exhibit A-type granite geochemical compositions with high contents of high-field-strength elements (Zr + Nb + Ce + Y > 350 ppm) and a zircon saturation temperature of 829 °C. The zircon grains have slightly depleted εHf(t) values of −3.4 to +1.9, which suggest Mesoproterozoic juvenile crustal formation in response to the amalgamation of the Yangtze Block with the Rodinia supercontinent. A few coeval spilites (758 Ma) crop out sporadically in the ZBLU. Their geochemical characteristics include high SiO2 contents (51.06–57.03 wt%), low Mg# values (42–53), and arc-like trace-element distribution patterns. The variable zircon εHf(t) values for the spilite samples can be divided into two groups: negative εHf(761 Ma) values of −16.4 to −11.1, and positive εHf(744 Ma) values of +5.9 to +8.5, which supports source mixing between enriched mantle and residual oceanic crustal material. In conclusion, the ZBLU spilite–keratophyre sequence is composed of a typical bimodal rock association, providing new petrological evidence for the origin of low-δ18O magmas and for a continental rift setting related to the separation of the Yangtze Block from the Rodinia supercontinent at ca. 771 Ma.

The formation of Neoarchean continental crust in the south-east Superior Craton by two distinct geodynamic processes

The formation of the continental crust in the early Earth, and the geodynamics that drove it, are fundamental to understanding the evolution of our planet, but remain intensely debated. Here, we analysed 148 archived zircon separates of magmatic rocks for in-situ zircon U-Pb-Hf-O-trace element data, and compiled published geochronological, whole-rock geochemical and Sm-Nd isotopic data from across the south-east Superior Craton, Canada. We combine these data spatially and temporally to investigate the crustal evolution of this part of the craton in the Neoarchean, with a view to understanding its tectonic setting. In terms of zircon data, at >2704–2695 Ma, the central and north-west Abitibi demonstrate more juvenile εHf, light to mantle-like δ18O, lower (Eu/Eu*)/Y (drier/shallower crust), reduced ΔFMQ, less continental initial-U (Ui)/Yb, and more mantle-like Ui/Nb, relative to surrounding crust, which contains older, ca. 2800–2750 Ma inherited and magmatic zircon ages. Furthermore, whole-rock Sr/Y and La/Sm demonstrate the presence of a high Sr/Y TTG component (mainly intrusive) surrounding zones of low Sr/Y (mainly volcanic) component, the latter of which shows contamination trends with Mesoarchean crust. We interpret this to represent a continental-rift setting, driven by plume magmatism as represented by multiple komatiite suites. At ca. 2704–2695 Ma, there is a marked transition in multiple datasets, including; increases in δ18O, (Eu/Eu*)/Y, ΔFMQ, Ui/Yb and Ui/Nb data, together with more distinct arc-like trace element trends, suggesting a transition to north-dipping subduction. This process closed the rift system and initiated orogenesis. Subsequently, this study constrains the geodynamic setting which formed the majority of Neoarchean continental crust in the south-east Superior Craton, and the timing at which it transitioned to subduction. If these findings are replicated in other cratons, it suggests that plate tectonics was active by, or started at, ca. 2.7 Ga.