Enrichment In LILE Research Articles

Petrogenetic and geochemical constraints on ca. 1.89–1.88 Ga Bastanar mafic dyke swarm, Bastar craton, India: Insights into MORB- and OIB-type contributions and interactions with metasomatized subcontinental lithospheric mantle

The Bastar craton of the Indian Shield hosts several generations of mafic dyke swarms of various trends, compositions, and ages, which span from ca. 2.7 Ga to 1.42 Ga. This study focuses on geochemical attributes of the ca. 1.89–1.88 Ga NNW-trending Bastanar swarm, aiming to address a perceived discrepancy between its arc-like geochemistry and the influence of a heterogeneous mantle source. To resolve the intra-swarm geochemical variations, we conducted a comprehensive geochemical characterization and petrogenetic interpretation of the ca. 1.89–1.88 Ga Bastanar swarm. The samples from this swarm are categorized into two distinct groups, primarily based on their geochemical composition. The Group 1 samples exhibit higher TiO2 (1.06–1.86 wt%), (La/Yb)N (7.2–8.6), (Gd/Yb)N (2- 2.27), Nb (14.6–16.6 ppm), Th (1.23–3.03 ppm) and Zr (104–118.72 ppm) concentrations than the Group 2 samples. Furthermore, rare-earth element patterns and variations in high-field strength element contents in the Group 1 samples suggest derivation from a deeper, less depleted mantle source resembling an OIB/less MORB-type. This inference is further supported by higher TiO2/Yb, Zr/Nb, and Nb/Y ratios. In contrast, the Group 2 samples indicate derivation from a shallower, more MORB/less OIB-type depleted mantle source, as evidenced by lower TiO2/Yb, Zr/Nb, and Nb/Y ratios. Variations in Dy/Yb and Gd/Yb ratios confirm the involvement of variable mantle sources, implying the derivation of the Group 1 and 2 samples from garnet-rich and spinel-rich lherzolite mantle sources, respectively. The absence of consistent negative Nb-Ta-Ti anomalies in the Group 1 samples suggests an uncontaminated nature, ruling out any role of crustal contamination. On the other hand, the Group 2 samples display negative Nb-Ta-Ti anomalies with enriched LREE and LILE patterns, indicating the involvement of crustal components in their genesis. A trace-element modelling suggests that the ca. 1.89–1.88 Ga mafic dyke swarm exhibits significant intra-swarm variability, with at least two distinct source components contributing to its genesis – a depleted MORB-type and an enriched OIB-type mantle. Notably, the geochemical characteristics of the Group 2 samples suggest interaction with a metasomatized mantle source, possibly enriched by fluids from an earlier subducted slab. Geochemical evidence presented in this work supports Archean subduction-related processes for the crustal growth of the Bastar craton and highlight the enduring influence of a metasomatized sub-continental lithospheric mantle on subsequent magmatism over millions of years.

Petrogenesis of Late Mesozoic trachydacites in the Jiaolai Basin, Eastern China: Implications for the destruction of the North China Craton

The North China Craton has witnessed a large-scale lithospheric thinning event in the Mesozoic. The volcanic rocks of the Qingshan Group formed during the destruction of the North China Craton, therefore retaining important information to the deep processes in this region. Zircon U-Pb dating and geochemical analyses have been carried out on volcanic rocks of the Qingshan Group collected from southeastern margin of the Jiaolai Basin. In combination with previously published data on the volcanic rocks of the Qingshan Group, the newly acquired data show that: (1) the volcanic rocks of Qingshan Group in Mashan have SiO2 content of 62.82 ∼ 66.3 wt% and total alkali content (K2O + Na2O) of 7.64 ∼ 9.34 wt%. According to TAS diagram, they belong to trachydacite which experienced strong fractionation between LREEs and HREEs with depletion of HFSEs and enrichment of LILEs. (2) The crystallization age of trachydacite in Mashan is 116.4 ± 2.3 Ma (MSWD = 1.9, n = 9). The magma originated from the partial melting of the ancient lower crust and the enriched lithospheric mantle metasomatized by the fluids/melts released from the subducted slab of the Yangtze Craton. (3) The formation age of magmatic rocks in western Shandong is generally earlier than that in eastern Shandong, while the lower crust delamination in western Shandong preceded that in eastern Shandong. The occurrence of magmatism in eastern Shandong is probably attributed to the delamination process in western Shandong as well as the ongoing tectonic activities along the Tan-Lu fault zone. The lithospheric thinning of the North China Craton was mainly controlled by the change of the subduction angle of the Paleo-Pacific plate, and is probably influenced by the continuous activity of the Tan-Lu fault zone in eastern Shandong.

Constraints on the source and petrogenesis of early Ediacaran shoshonitic mafic magmatism and high-K calc-alkaline granitoids in the Sergipano Orogenic System, Borborema Province, Brazil

The early Ediacaran Capela pluton (ca. 630 Ma) is intruded into the metasedimentary succession of the Sergipano Orogenic System along the oriental segment of the Macururé Domain, southeastern Borborema Province, NE Brazil. This body is an uncommon example of mantle-derived shoshonitic mafic rocks associated with high-K calc-alkaline granitoids emplaced in a pre- to early-collisional context. Here, we aim to unravel the petrogenesis of this plutonic association to determine the role of crustal contribution to magmatism and its geodynamical significance during the early stages of the Brasiliano/Pan-African Orogeny. The Capela pluton comprises cumulate hornblendites, gabbronorites, gabbros, diorites, granodiorites, quartz monzonites, and granites. Shoshonitic mafic rocks and high-K calc-alkaline granitoids show significant enrichments in LILE and LREE compared to HREE and HFSE. Ultramafic and mafic rocks display a conspicuous crustal signature, yielding subchondritic εNd(t) and zircon εHf(t) values, along with variable radiogenic Sr–Pb isotopic compositions. The dominant factors controlling the geochemical features of the mafic magmas were the enriched signature of the lithospheric mantle source and the degree of partial melting. Mantle source metasomatism is thought to have occurred by incorporating subducted sediments into the mantle wedge before the onset of the Brasiliano Orogeny. Geochemical modeling demonstrated that low partial melting degrees (5–10%) of phlogopite-bearing lithospheric mantle in the spinel stability field metasomatized by 5–10% subducted sediments may account for the main characteristics of the mafic rocks. Introducing sedimentary material into the mantle caused a decrease in Sm/Nd and Lu/Hf ratios while increasing the Rb/Sr, U/Pb, and Th/Pb ratios, which led to the time-integrated evolved isotopic signature observed in the mafic rocks. The coeval granitoids are metaluminous to slightly peraluminous and display Nd–Pb isotopic compositions similar to those of mafic–ultramafic rocks. The petrological and isotopic features of the granitoids are consistent with derivation from an ancient mafic lower crustal source. We argue that the ascent and emplacement of hydrous mantle-derived magmas as successive sills into the lower crust produced a deep hot zone beneath the Sergipano Orogenic System during the onset of the Brasiliano/Pan-African Orogeny. Aqueous fluids released from such potassic mafic magmas favored the partial melting of surrounding rocks, producing contemporary felsic magmatism.

Early Paleozoic metamorphism and anatexis of lower retro−/back-arc crust in the western Central Qilian belt, NE Tibet

The tectonic-thermal mechanism of crustal melting provides vital information on continental differentiation and orogenic evolution. This study aims to understand the evolution and tectonic setting of newly discovered lower crustal leucosome-rich granulites of the Qilian Orogen, NW China. Tonalitic leucosome within garnet amphibolite occurs in net-like arrays or as stringers and has diffuse boundaries with the host, with enriched LILE, and high Sr and Sr/Y ratios, similar to that of adakite, formed by water-fluxed melting. However, granitic leucosome within garnet granitic gneiss shows sharp contacts with the host, and has low Sr, Ba, K and Pb but high Rb, Rb/Sr, with negative Eu anomalies, consistent with dehydration melting. Petrographic observations, geochemical data and pseudosection modelling suggest that garnet amphibolite experienced water-fluxed melting and garnet granitic gneiss underwent dehydration melting, respectively. LA-ICP-MS zircon and titanite dating indicate the anatectic event proceeded at ∼480–470 Ma. Phase equilibrium modelling suggests the garnet amphibolite and garnet granitic gneiss share a similar P-T path involving decompression heating from ∼12.1 kbar/711–746 °C to reach peak temperatures of 798–836 °C at 10.4 kbar, and subsequently decompression cooling to 4.6–7.6 kbar/600–672 °C. The calculated P-T conditions suggest Barrovian-type metamorphism with thermal gradients of 24.2 °C/km for the Central Qilian lower crust. We interpret a retro−/back-arc setting resulting from the southward subduction of the North Qilian oceanic lithosphere.

Neoproterozoic granitoids of northwest Vietnam and their tectonic implications

ABSTRACT A combined study of whole-rock geochemical and Sr-Nd isotopic data, and zircon U-Pb geochronological and Hf isotopes has been carried out for several Neoproterozoic intrusions, including the Lung Thang, Posen and Sin Quyen in the Phan Si Pan Zone, northwest Vietnam to constrain their age, petrogenesis and tectonic implications. The Lung Thang and Posen intrusions, mainly composed of granodiorite, were formed at 803–777 Ma and are characterized by moderate SiO2 (64.43–66.65 wt.%), and K2O (4.05–4.89 wt.%), with A/CNK and A/NK values of (0.94–1.03) and (1.72–2.11), respectively. They have negative whole-rock εNd(t) (−6.16 to −3.73) and zircon εHf(t) values (−7.9 to −4.1), which suggest that the Lung Thang and Posen intrusions were generated by partial melting of ancient, K-rich crustal rocks. The Sin Quyen intrusion, occurring as dykes, is composed of monzodiorite and was emplaced at 742 ± 3 Ma. The Sin Quyen intrusion has high alkalies (K2O+Na2O = 7.42–7.47 wt.%), and low MgO (<1.31 wt.%) and Ni (6.55–6.93 ppm), with A/CNK and A/NK values of (0.67–0.68) and (1.64–1.68), respectively. Their whole-rock εNd(t) and zircon εHf(t) values are −6.15 to −5.92 and −5.8 to + 8, respectively. These geochemical characteristics suggest that the Sin Quyen intrusion was produced by the partial melting of ancient crustal sources mingled with mantle-derived components. Geochemically, the Lung Thang, Posen and Sin Quyen intrusions are medium- to high-K, calc-alkaline in nature and show enrichment in LILE (Th, U, K, Rb) and LREE, and strong negative anomalies of Nb, Ta, and Ti. Such geochemical characteristics suggest that they formed in a subduction-related tectonic environment. The geochronological and geochemical correlation of these intrusions with those along the southwestern margin of the Yangtze Block in South China suggests that the Phan Si Pan zone in northwestern Vietnam is a constituent of the SW Yangtze Block. Additionally, these intrusions show a significant correlation with other contemporaneous magmatic rocks in the northeast Indochina Block, Lhasa Block, the northwestern margin of Greater India as well as those in Seychelles and northern Madagascar. This correlation suggests a similar history and synchronous episode of crustal growth/recycling in an Andean-type arc system along the western and northern margin of the Rodinia supercontinent during the Neoproterozoic.

Magmatic and Geochemical Studies of Early Neoproterozoic Koraput Anorthosite Massif, Eastern Ghats Mobile Belt and their Implications for Rodinia Assembly

Abstract This paper reports new mineralogical and geochemical information on the Neoproterozoic Koraput Anorthosite pluton in domain-2 of the Eastern Ghats Mobile Belt. The lenticular-shaped, NE-SW trending small Koraput Anorthosite Pluton is dominated by gabbro-anorthosite, anorthosite sensu-stricto, and norite-diorite. Isolated patches of ferrodiorite (residual melt following crystallization of anorthosite) occur at the pluton margin. The estimated magmatic P-T conditions for anorthosite emplacement are ~1000°C and ~1.1GPa. In addition, the intermediate Anorthite (48-52 mole %) presence in plagioclase of gabbro-anorthosite and anorthosite indicates plagioclase crystallisation and accumulation at 30-40 km (circa 1 GPa) depth near the crust-mantle boundary. Furthermore, the geochemical traces suggest Koraput anorthosite pluton crystallized from a less fractionated basaltic magma. The REE and trace element systematics of anorthosite and norite-diorite exhibit enriched LILEs over depleted HFSE with negative Nb, Ta, Th, and Zr anomalies. Gabbro-anorthosite and ferrodiorite, on the other hand, exhibit positive Nb, Ti, and negative Zr and Th anomalies. Nb/Th versus Nb/La diagram implies lithodemic units in Koraput Anorthosite pluton are variably contaminated by crustal sources. Integration of geochemical data with available monazite age suggests that the Koraput anorthosite pluton in isotopic domain-2 of the Eastern Ghats Mobile Belt was emplaced during the collision between proto-India and Antarctica during the Rodinia assembly.

Late Palaeozoic tectonic transformation of the North Qinling Orogenic Belt: Insight from the geochronology, geochemistry and Sr‐Nd‐Hf isotopes of Carboniferous magmatic rocks

The North Qinling Orogenic Belt (NQB) records the pivotal geological information for understanding the Palaeozoic evolution of the Qinling Orogenic Belt (QOB). Previous studies mainly focused on the subduction–collision process along the Shangdan suture zone before the Early Devonian. However, as the significant interim period between the Early Palaeozoic and Mesozoic orogeny, the Late Palaeozoic tectonic evolution of the NQB remains poorly understood. Fortunately, the Carboniferous magmatic rocks discovered this time provide substantial geological evidence for revealing the Late Palaeozoic tectonic history of the Qinling Orogenic Belt. This paper will provide a detailed analysis of zircon U–Pb ages, geochemical characteristics and Sr‐Nd‐Hf isotopic compositions of Carboniferous rocks. The Carboniferous magmatic rocks are categorized as highly differentiated S‐type granite and monzodiorite, formed at 350 ± 2.4 Ma and 353 ± 5.2 Ma, respectively. The granite shows weak peraluminous (A/CNK = 1.02–1.11) and shoshonite nature. Based on the isotopic composition and trace element characteristics, we propose that the Qinling Group paragneiss is the primary source of the Carboniferous granite. Monzodiorite is characterized by enriched LREEs and LILEs (Ba, K and Pb), depleted HFSEs (Th, Nb, Ta and Ti) and enriched Sr‐Nd isotopic composition. Monzodiorite magma source region consists of continental crustal material and lithospheric metasomatized mantle. Carboniferous magmatic rocks are the product of an extensional tectonic setting, which indicates the NQB tectonic regimes transition from compression to extension during the Carboniferous.

The December 2018 eruption at Etna volcano: a geochemical study on melt and fluid inclusions

This study focus on the Mt Etna December 2018 eruption with the aim of investigating the geochemical characteristics of the feeding magma. New data on major and trace element geochemistry of olivine-hosted melt inclusions (MI) in volcanic products are presented together with the noble gas geochemistry of fluid inclusions (FI) in olivines. The noble gas geochemistry of fluid inclusions (FIs) in olivines was also investigated. The major element composition of MIs is variable from tephrite/trachybasalt to phonotephrite/basaltic trachyandesite, with SiO2 = 45.51–52.72 wt%, MgO = 4.01–6.02 wt%, and CaO/Al2O3 = 0.34–0.72. Trace element patterns of MIs present a typical enrichment in LILE and LREE, depletion in HFSE, and relatively fractionated REE patterns: (La/Lu) N= 18.8–41.08, with Eu/Eu* = (0.5–1.8). Positive anomalies in Sr (Sr/Sr* = 0.8–2.3) and Ba can be ascribed to the assimilation of plagioclase-rich cumulates in the magmatic reservoir. The variable Ba/La (9.8–15.8), K/Nb (260–1037), Ce/Nb (1.9–3.4), Rb/La (0.4–1.6), and Ba/Nb (10.8–25.8) ratios reveal mixing between two types of end-member magmas comparable to those emitted from 1) the 2001 Upper Vents and 2002–03 Northern Fissures (Type-1) and 2) the 2001 Lower Vents and 2002–03 Southern Fissures (Type-2), respectively. Type-2 represents a magma that was under the influence of a crustal component, whereas Type-1 is compatible with a HIMU–MORB-type heterogeneous mantle source. It appears that the 2018 MIs have captured the two different types of magmas, and the lack of homogenization may imply a very fast ascent (a few months). Compatible with the contemporary presence of primordial HIMU–MORB and crust-contaminated end-members are the data on noble gases from FI that highlighted an 3He/4He value of 6.5–6.6Ra. The hypothesis of two different types of magmas, identified by the trace element geochemistry in MIs, is, thus, reinforced by helium isotopic data on FI of the 2018 eruption together with data from other Etnean eruptions and allows the inference of a bicomponent magma mixing.

Late Cretaceous basalts in the eastern South China Block: The result of slab tearing during retreating subduction of the Paleo-Pacific plate

Cretaceous arc-like, ocean island basalt (OIB)-like, and transitional basalts erupted on the eastern South China Block (SCB), and their petrogenesis was possibly related to the subduction of the Paleo-Pacific plate. However, the detailed petrogenesis in terms of slab subduction, magma sources, and the spatial distribution of the different types in the coastal and inland regions is poorly understood. Here we conduct a comparative study on the Late Cretaceous basalts from both coastal and inland regions. Our compiled age data demonstrate that the basalts in the coastal regions erupted in the period of 104–81 Ma. They show low Nb/La (<0.8), Nb/Y (<0.7), and other typical arc-like signatures, e.g., negative Nb-Ta anomalies, variable enrichment of LILE and LREE, and more radiogenic Sr-Nd isotopic compositions. In contrast, the basalts in the inland regions erupted at 86–71 Ma. They are characterized by high Nb/La (>0.9), slightly negative to positive Nb-Ta anomalies; they mostly possess positive εNd(t) values and geochemical features transitional to OIB-like basalts. The systematic geochemical differences may imply the spatial heterogeneity in their magma sources. The three end-member mixing model suggests that the isotopic signatures of arc-like basalts largely resulted from partial melting of the mantle wedge metasomatized by the subducted sediments, while those of transitional basalts likely resulted from depleted mantle source metasomatized by melts from altered oceanic crust (AOC) with minor sediments. We suggest that the retreating of the Paleo-Pacific plate promoted the melting of sediment-hybridized mantle wedge, and then triggered the opening of a slab window and asthenosphere-slab interactions.

Geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes of the Late Jurassic granitoids from the Shuangyuangou deposit, South Qinling, China: Implications for petrogenesis and porphyry Cu metallogeny

The Shuangyuangou copper deposit is located in the southern segment of the Qinling orogenic belt in central China. The ore‐related intrusions include quartz diorite porphyry, granite porphyry and lamprophyre. In this study, we present whole‐rock geochemistry, zircon U–Pb geochronology, trace elements and Lu–Hf isotope data on the Shuangyuangou intrusions which provide critical constraints on the emplacement age, magma origin, petrogenesis and implications for porphyry Cu mineralization in this newly discovered deposit. Zircon U–Pb dating constrains the emplacement age of quartz diorite porphyry, lamprophyre and granite porphyry at 150–147 Ma. The quartz diorite porphyry and granite porphyry display high‐K calc–alkaline I‐type granite affiliation. The rocks show enrichment of LREE and LILE (Rb, Ba), depletion in HREE and HFSE (Nb, Ta, Ti) and absence of Eu anomaly. Zircon εHf(t) values of quartz diorite porphyry, lamprophyre and granite porphyry range from −7.5 to −1.7 with TDM2 ages of 1.3–1.6 Ga. Our data suggest that the parental magma was derived from partial melting of juvenile lower crust and metasomatic lithospheric mantle, with more involvement of mantle material for the lamprophyre. The zircon Eu/Eu* values of 0.55–0.78 suggest high magmatic water content which suppresses the fractionation of plagioclase. In addition, the ΔFMQ values calculated from zircon trace elements range from −0.90 to 2.96 which is a favourable factor for porphyry Cu mineralization at Shuangyuangou. The Late Jurassic Shuangyuangou intrusion and associated Cu mineralization were formed within an intracontinental setting after the collisional assembly of the Qinling orogenic belt.

Deeply Derived Magma Controlling the Polymetallic Mineralization at Shuikoushan, South China: Constraints from Mineral U–Pb Dating and Whole-Rock Geochemistry

Various magmatic–hydrothermal activities have resulted in different styles of polymetallic mineralization in South China. Shuikoushan is a large Fe-Cu-Pb-Zn-Au-Ag orefield situated in fold-and-thrust belts within the South China Block. Two types of granodiorite have been identified in recent drilling work. The early-stage, coarse-grained granodiorite has developed magnetite-bearing skarns in the deep level. The late-stage, fine-grained granodiorite is associated with garnet-hematite–magnetite–pyrite–sphalerite–chalcopyrite-bearing skarns in its contact zone. Away from the garnet-bearing skarn are calcite–quartz–pyrite–sphalerite–galena veinlets in faulted breccia. Fieldwork has identified iron mineralization in both skarns, whereas copper mineralization was only discovered in the garnet-bearing skarns. Lead, zinc, gold, and silver mineralization were observed in the garnet-bearing skarns and faulted breccia. Zircon U–Pb analyses suggested the emplacement of two granodiorite at 167.8 ± 0.8 Ma (MSWD = 1.1, N = 31) and 163.6 ± 0.7 Ma (MSWD = 1.3, N = 32). Apatite and garnet U–Pb dating further indicated the magnetite-bearing skarns of 166.2 ± 1.9 Ma (MSWD = 4.5, N = 27), the hematite–magnetite–sulfide-bearing skarns of 158.6 ± 2.8 Ma (MSWD = 1.3, N = 34), and the calcite–quartz–sulfide veinlets of 159.5 ± 5.2 Ma (MSWD = 1.7, N = 24). The time–space relationship between the two intrusions and hydrothermal activities suggests that the fine-grained granodiorite is responsible for polymetallic mineralization. Whole-rock geochemistry analyses demonstrated the enrichment of LILEs and the depletion of Nb and Ta in two granodiorites, with a slight enrichment in LREEs and flat HREE patterns. These granodiorite bodies therefore belong to high-K calc-alkaline magma generated via the crust’s partial melting. The fine-grained granodiorite generally has a lower HREE and higher Dy/Yb, Sr/Y ratios than coarse-grained granodiorite, corresponding to the source of magma in garnet stable lower crust. The residual garnet keeps ferric iron in melts, leaving the fine-granodiorite more oxidized for copper and gold concentration. Through these analyses and our drilling work, a continuous skarn–hydrothermal–epithermal system has been identified for Cu-Pb-Zn-Au-Ag targeting in Shuikoushan.

Neoarchean to Paleoproterozoic crustal evolution of the central Aravalli-Banded Gneissic Complex (NW India): Evidence from zircon U-Pb-Hf isotope systematics and whole-rock composition of Neoarchean sanukitoids, Paleoproterozoic granitoids and metasedimentary rocks

The central Aravalli-Banded Gneissic Complex in NW India is an Archean nucleus that was extensively reworked during the late Paleo- to Neoproterozoic. New results of combined zircon U-Pb-Hf isotopes and whole-rock geochemical data reveal that the basement was affected by felsic magmatism during the Neoarchean and Paleoproterozoic, followed by a passive margin stage. The results provide evidence for coeval emplacement of sanukitoids and TTGs at ca. 2540 Ma, and for granitoid magmatism at ca. 1875 Ma. Similar REE patterns of the synchronous sanukitoids and TTGs, coupled with high SiO2 contents (59.2–70.9 wt%), subchondritic εHf2.55 Ga values (−5.0 to −2.6) and Hf-model ages (3246–3123 Ma) of the Masuda sanukitoids indicate that the sanukitoid magmas were formed mainly by interaction of TTG magmas (generated by the melting of subducted Paleo- to Mesoarchean oceanic crust) with the overlying enriched peridotite. The calc-alkalic and magnesian compositions, enrichment in LREE and LILE along with pronounced depletion in HFSE (Nb, P and Ti) in ca. 1875 Ma granitoid gneisses imply subduction-related magmatism during Paleoproterozoic. The predominance of subchondritic εHf1.87 Ga values, ranging from −4.4 to +1.7, suggests that the parental magmas mainly resulted from the reworking of heterogeneous Meso- to Neoarchean felsic crust. Age-Hf isotope data of detrital zircon grains provide evidence that clastic sedimentary rocks covering the central Aravalli basement were deposited in a passive margin setting at < 1730 Ma, and the detritus was mainly sourced from adjoining Neoarchean and Paleoproterozoic granitoids. Comparison of our new data with other Indian and worldwide cratons indicate that the Aravalli orogen represents a unique terrane, where the Neoarchean TTGs and sanukitoids with subchondritic Hf isotopic signatures were emplaced nearly coeval in a close spatial relationship. The Aravalli Archean basement was subsequently reconfigured by voluminous felsic magmatism during Paleoproterozoic at 1875–1810 Ma. Furthermore, the compiled zircon U-Pb age and Hf-Nd isotope data for the late Paleoproterozoic subduction-related magmatic rocks provide evidence for substantial crustal reworking during assembly of the Columbia supercontinent.

The Neoarchean mafic-ultramafic São José do Jacuípe Suite in the Itabuna-Salvador-Curaçá orogen, Brazil: New U-Pb ages and geochemical data

The São José do Jacuípe Suite (SSJJ) comprises an association of mafic and ultramafic rocks that crops out in the northern portion of the Itabuna Salvador-Curaçá Orogen (ISCO), São Francisco Craton. They are represented by small NW-SE elongated bodies comprising metamorphosed anorthosites, leucogabbros, gabbronorites, gabbros, ferrogabbros, and subordinate pyroxenites and serpentinites. The rocks are part of an orogenic belt that comprises orthogranulites and orthogneisses of the Caraíba complex, and metasedimentary rocks of the Tanque Novo-Ipirá complex, which appear to be amalgamated into a folded mélange. Petrographical analysis of the SSJJ rocks revealed relict cumulus textures in the pyroxenites. Moreover, inclusion relationships allowed the identification of two metamorphic phases; the first phase likely occurred at amphibolite facies conditions and was overprinted by granulite facies metamorphism that represents the peak metamorphic conditions recorded by the SSJJ rocks. Analytical results for major and trace elements indicate cogeneticity between the mafic rocks, and arc-related mafic cumulate field compositions. In the geotectonic environment diagrams the rocks plot in the MORB and IAT fields, indicating an origin involving melting of different sources or magma mixing. The characteristic signature of the studied rocks is represented by negative HFSE anomalies (Nb, Ti, Zr and Hf), depletion in HREE and enrichment in LILE (Ba, Rb, and Cs), indicating their derivation from metasomatic mantle in a suprasubduction setting. These data corroborate previous isotope data that point to mildly enriched mantle sources. Incompatible trace elements signatures are similar to ophiolites generated in supra-subduction zone (SSZ) environments worldwide. New U/Pb LA-ICP-MS ages indicate igneous crystallization at 2549 ± 23 Ma and 2563 ± 13 Ma for SSJJ gabbronorites, thus indicating that they postdate the other members of the magmatic arc that formed the ISCO by 20–30 Ma.

Geochemistry, Chronology and Tectonic Implications of the Hadayang Schists in the Northern Great Xing’an Range, Northeast China

The Late Paleozoic tectonic evolution of the Xing’an block in the eastern Central Asian orogenic belt has long been the subject of debate. In this paper, a comprehensive study of U-Pb zircon ages, Lu-Hf isotopes and whole-rock elemental analyses was carried out on Hadayang schists. Representative samples of the epidote-biotite-albite schist and biotite-albite schist yielded the weighted mean 206Pb/238U ages of 360 ± 2 Ma and 355 ± 3 Ma, respectively. This indicated the presence of Late Devonian–Early Carboniferous intermediate-basic rocks in the eastern Xing’an block. The Hadayang schists exhibited a Na-rich, tholeiitic and calc-alkaline affinity in composition with low Mg# (35.2–53.0), Cr (23.7–86.5 ppm), Ni (21.1–40.0 ppm) and Co (12.1–30.6 ppm). They were characterized by enrichment of LILEs, depletion of HFSEs and highly positive zircon εHf(t) values (the average values were +8.93 and +9.29, respectively). The magma source of the Hadayang schists was a mantle that consisted of both spinel and garnet lherzolite, with a partial melting degree of 1%–5%, and it had undergone fractional crystallization of olivine, orthopyroxene and plagioclase. The Hadayang schists, together with other Late Devonian–Early Carboniferous intermediate-basic magmatic rocks in the eastern Xing’an block, were formed in an intracontinental extension tectonic setting similar to that of the North American Basin and Range basalt. Moreover, Late Devonian–Early Carboniferous ophiolite under a similar tectonic background in the western Xing’an block has been reported. We believe that the Xing’an block would - have been in the stage of intracontinental extension during the Late Devonian–Early Carboniferous.

Late-orogenic potassic to ultrapotassic dykes from the central Grenvillian hinterland: Trace element and Sr-Nd-Pb-O isotopic perspective

New whole-rock Sr-Nd-Pb-O isotope data are combined with previously published results for Sr and Nd isotopes and major and trace element chemistry to examine the petrogenetic and tectonic environments of a suite of ca. 980 Ma potassic to ultrapotassic dykes (PUD) from the Canyon domain, central Grenville Province in Canada. The PUD suite can be divided into two geochemically distinct groups that exhibit characteristic source mineralogy, depth of partial melting, and multi-stage metasomatism of heterogeneous mantle sources. Major and trace element data suggest involvement of both fertile and depleted sources involving spinel and garnet peridotite, while trace element and isotope data indicate long-term LREE and LILE enrichments and HFSE depletions in ancient subduction settings. Low time-integrated Rb/Sr, Sm/Nd, and U/Pb ratios suggest enriched mantle EM-I metasomatized by ancient subduction of continental upper crust and/or carbonate sediments. A latest phase of fluid/melt metasomatism is attributed to a late-Grenvillian intracontinental subduction and limited lithospheric extension.The late-Grenvillian tectonic setting is proposed here to be characterized by shallow-angle underthrusting of the foreland beneath the Grenvillian hinterland, where SCLM, initially of Proterozoic age and eventually of Late Archean age, became transported beneath the orogenic core. The intracontinental collision was short-lived and probably terminated because of buoyancy constraints related to the refractory character of the subducted Archean lithospheric mantle.

Cryogenian-Ediacaran crustal growth and evolution of the active margin of the Dahomeyide belt, Ghana

AbstractThe study presents detailed petrographical, geophysical, structural and geochemical data of the internal nappes zone to establish the deformational history, origin and tectonic setting and constrain the crustal growth and evolution of the active margin of the Dahomeyide belt. Two main lithological units, (i) deformed meta-granitoids (migmatites and gneisses) and (ii) undeformed granitoids, dominate the internal nappes zone. The granitoids are generally I-type, metaluminous to weakly peraluminous, low-K tholeiite to high-K calc-alkaline and of tonalite, granodiorite and granite affinity. The overall trace element patterns of the studied granitoids characterized by the enriched LILE and depleted HFS, with negative peaks of Nb-Ta, Sr, P and Ti, are indications of arc-related magmatism. Structural analysis reveals four deformation phases (D1-D4). D1 represents Northwest-Southeast (NW-SE) Pan African shortening associated with a continent-continent collision, resulting in westward nappe stacking. Progressive NW-SE shortening resulted in D2 and D3 top-to-the-NW dextral and sinistral thrusting events during the Pan-African orogeny. D4 is an extensional event likely associated with the orogenic collapse phase. The gneisses and migmatites, with dominant axial planar foliations, point to their formation in a collisional setting or influence by the Pan-African collisional processes. Continental-arc signatures in these rocks imply continental subduction during their protolith formation. The intrusive granitoid and pegmatite are undeformed, meaning late- to post-orogenic emplacement. These findings suggest that the internal nappes zone archived the subduction-collision and post-collisional phase of the Pan-African orogeny and recorded large-scale migmatization and granitoid emplacement due to partial melting of thickened lower crust between Mid-Cryogenian and late Ediacaran.

Heterogeneous Archean oceanic protoliths in Neoproterozoic retrogressed eclogites from Northeast Brazil: Petrological, geochemical and Sr–Nd–Pb–Hf isotopic constraints

Eclogites are reliable indicators of high to ultrahigh-pressure (HP-UHP) conditions for oceanic and continental crust and are essential rocks to understand the lithospheric convergence from subduction to post-collisional setting. Furthermore, the origin and nature of eclogite precursors may reveal crucial constraints on the crustal history before HP-UHP metamorphism. Here, we provide petrological, geochemical and Pb-Sr-Nd-Hf isotopic constraints from the Campo Grande retrogressed eclogites, Northeast Brazil, to determine the source and evolution of their Archean protoliths. Eclogites were formed under HP conditions (18 ± 1.0 kbar and 680 ± 20 °C) followed by retrogression in the amphibolite facies, which is characterized by Mg-rich garnet porphyroblasts surrounded by clinopyroxene + plagioclase + amphibole + orthopyroxene symplectites (stable at <6.0 kbar). These retrogressed eclogites have chemical compositions corresponding to enriched-type tholeiites such as E-MORB protoliths (G1 eclogitic group). A second group of retrogressed eclogites with higher amphibole content (G2 eclogitic group) shows similar chemical patterns to G1, however with higher enrichment in LILE, REE and incompatible elements. The G1 eclogite is characterized by higher modal concentration of garnet and clinopyroxene, positive εNd(t) (+2.1 to +5.8) with low 87Sr/86Sr(i) ratios (0.7024 to 0.7131), whereas G2 displays dominant coarse-grained amphibole, positive and negative εNd(t) (−7.5 to +8.5) with high radiogenic 87Sr/86Sr(i) ratios (0.7109 to 0.7255). The chemical and isotopic G1 values show typical E-MORB signatures, whereas G2 indicate an E-MORB enriched by crustal components in a supra-subduction zone setting. A LuHf isochron provided the age of 2667.9 ± 30.7 Ma, which is close to the published ∼2.65 Ga crystallization age for the G1 and G2 oceanic protoliths. Therefore, Hf isotopes in whole rock and cogenetic minerals were useful for constraining the crystallization time of metamafic protoliths. In contrast, SmNd isochron age of 606.9 ± 24.8 Ma falls within the range of 623 to 590 Ma HP metamorphism recorded in the G1 and G2 rocks. Lastly, a third type of eclogite group (G3 eclogitic group) is represented by a banded Mn-rich-garnet (spessartite) and Al-rich clinopyroxene with detrital zircon cores from 3.42 to 2.15 Ga and recrystallized rims at 607.7 ± 5.4 Ma. Mineral texture and composition, geochemistry, strongly negative εNd(t) (−4.3 to −1.5) and higher radiogenic 87Sr/86Sr(i) ratios (0.7434–0.7501) suggest a different Mn-rich protolith for the G3 eclogites. Integrated data reveal that retrogressed eclogites retain the memory of the original chemical and isotopic patterns of Archean oceanic protoliths and its diachronous reworking processes at deep crustal levels.

Early Cretaceous magmatism in the northern Gyeonggi Massif and the northcentral Yeongnam Massif, Korean Peninsula: Its implications for the Cretaceous tectonic evolution of the Northeast Asia

Most Cretaceous igneous rocks in the northeastern North China Cratons (NCC) and southeastern South China Cratons (SCC) in Northeast Asia are interpreted to have formed in the extensional tectonic environment caused by subducted slab roll-back. On the other hand, the Cretaceous magmatism in the proto-Southwest Japan was confirmed to have occurred in the compressional state produced by the paleo-Pacific plate subduction. The granitoids and enclaves in the northern Gyeonggi Massif intruded at ca. 110–109 Ma and 112 Ma respectively, and the granite and pyroclastic rock in the northcentral Yeongnam Massif were formed at 106 Ma and 103 Ma respectively. The enclaves in the Ganghwa area which located in northern Gyeonggi Massif have high whole-rock Sr isotope composition (0.70748–0.71181) and low whole-rock εNd(t) (−14 to −15) value and low zircon εHf(t) (−19.2 to −14.7) values with enrichment of LILE, suggesting their origin from enriched lithospheric mantle metasomatized by subducted slab sediment. The Early Cretaceous granitoids on the northern Gyeonggi and northcentral Yeongnam massifs have adakite like geochemical composition which are derived from the lower crust instead of subducted oceanic crust derived melt. The geochemical features of the Early Cretaceous igneous rocks in the Korean Peninsula including the study area imply that the Early Cretaceous magmatism on the Korean Peninsula mainly occurred in the extension regime caused by slab roll-back of the paleo-Pacific plate as in the NCC. On the other hand, the igneous rocks formed at 80–66 Ma in the Korean Peninsula have geochemical characteristics similar to that in the Cretaceous igneous rocks in proto-Southwest Japan, indicating that the 80–66 Ma magmatism in the Korean Peninsula were produced by the subduction of the paleo-Pacific plate. The Cretaceous igneous rocks in the northeastern NCC and on the Korean Peninsula show a southeastward-younging trend from 145 Ma to 80 Ma, indicating that southeastward slab roll-back had started at 145 Ma and continued until 80 Ma. Arc-related igneous activity occurred from 116 Ma in southwestern Japan and propagated northward, resulting arc related magmatism in the northern margin of proto-southwestern Japan and the southeastern margin of the Korean Peninsula at 80–66 Ma.

Precambrian tectono-magmatic evolution of the western margin of the Yangtze block, South China: evidence from zircon U-Pb-Lu-Hf isotopes, REE and trace elements of Yuanmou-Miyi complexes

ABSTRACT Here new evidence of the isotope geochronology and the geochemistry from the Yuanmou-Miyi complexes reveal the Precambrian tectono-magmatism geodynamic process in the study area. The Yangtze Block basement with 1.81- ~1.77 Ga and negative ε Hf(t) values of −7.8 to −0.7 corresponded to the assembly of the Columbia-supercontinent. However, the rifting magmatic event of ~ 1.70 Ga, which caused the continental crust extension-thinning in the study area, may exert influence on the subduction-magmatism in the Grenvillian period. The Yuanmou-Miyi complexes of 1.19- ~1.00 Ga are characterized by (La/Sm)N＞1, depletion in HFSEs, negative anomalies of Nb-Ta, and enrichment of LILEs, which show a genetic link with the subduction-magmatism. The temporal tendency that the magmatism from Yuanmou to Miyi gradually became younger, may be responsible for the subducting direction. The subduction caused the reactivation of the Anninghe-Lvzhijiag fault zone, and created the rupture windows of the subduction slab, leading to upwelling of the deep magma. These granitoids were derived from the partial melting of the mantle wedge, while the small-scale gabbro dikes intruded into the granitoids were likely spillover products from the asthenosphere mantle melts through the rupture windows.

Carbonatite and ultrabasic magmatism at Toro Ankole and Virunga, western branch of the East African Rift system

The western branch of the East African Rift hosts four main Neogene-Quaternary volcanic provinces, characterized by products with wide chemical and mineralogical variability. This study focuses on the two northernmost volcanic provinces, Toro Ankole where ∼0.2 Ma-old carbonatites, melilitites and kamafugites, erupted together with foidites, and Virunga where ∼13–9 Ma-old strongly and mildly alkaline rocks, from nephelinite up to trachytes crop out. Petrographic and whole/rock geochemical characteristics (low SiO2, high CaO and K2O, coupled with LILE enrichment) of 49 samples from Toro Ankole and 5 samples from Virunga (tephrites, trachybasalts and trachyandesites) point out enriched and heterogeneous sub-lithospheric mantle sources. The nature of the ultramafic nodules occasionally associated to the investigated samples highlights the presence of metasomatic veins variably enriched in clinopyroxene, phlogopite, carbonate, apatite, Ti-magnetite and titanite. 143Nd/144Nd ratios of Toro Ankole rocks are below ChUR (0.51249–0.51260), and all but two carbonatite lavas have 87Sr/86Sr ˃ BSE (0.7046–0.7056). Virunga lavas display less radiogenic 143Nd/144Nd (0.51235–0.51249) and more radiogenic 87Sr/86Sr ratios (0.7058–0.7071). Lead isotopes are all above the NHRL (206Pb/204Pb mostly in the range 19.12–19.63). δ11B values cover the whole OIB range (−8.3 to −3.3 ‰), but with the exception of some Toro Ankole samples with notably heavier compositions (−1.9 to 6.6), here linked to the carbonate enrichment. The differences in the isotopic features observed between Toro Ankole and Virunga products may reflect a north to south change of the lithospheric mantle source in terms of composition, mineralogy and depth of melting. Toro Ankole phlogopite-bearing mantle is more intensely metasomatized by a 87Sr- and 11B-rich component (as altered oceanic crust or subducted carbonates) compare to that one beneath Virunga, in which the metasomes might contain amphibole rather than phlogopite. Low degree of partial melting of the metasomatized mantle produces magmas of carbonatitic and ultrabasic/ultrapotassic compositions of the western rift branch.