Nd Hf Isotope Research Articles

Formation of Paleoarchean-Mesoarchean Na-rich (TTG) and K-rich granitoid crust of the Singhbhum craton, eastern India: Constraints from major and trace element geochemistry and Sr-Nd-Hf isotope composition

The Singhbhum craton in eastern India is underlain by Paleo- to Mesoarchean Tonalites-Trondhjemites-Granodiorites (TTGs) and granites that formed in two pulses at 3.45–3.44 Ga and 3.35–3.32 Ga, followed by potassic granite activity at 3.1 Ga. The TTGs are metaluminous to slightly peraluminous and highly evolved with low Ni, Cr, and MgO. Low heavy-REE concentrations, Sr/Y and Eu/Gd, and high Nb, Ta, and Y concentrations indicate they belong to the low HREE, medium- to low-pressure TTG type. Their compositions are similar to TTGs derived from chondritic source with amphibolitic residue, consistent with rutile-free and garnet-bearing metabasic source. The granites have similar REE and multielement patterns as the TTGs, albeit with pronounced negative Eu, Sr, and Ti anomalies. High K2O/Na2O, higher heavy-REE concentration, low Sr/Y and Eu/Gd, and negative Eu anomalies of the 3.05 Ga Mayurbhanj potassic granites are consistent with shallow plagioclase-bearing crustal sources. The increasing K2O/Na2O of the granitoids from 3.45 Ga to 3.05 Ga is related to gradual thickening of the continental crust and increasing contributions of intracrustal melts in younger granitoids. Neodymium and Hf isotope data indicate the 3.45–3.44 Ga granitoids have juvenile as well as Eoarchean TTG crust-like signature. The 3.35–3.32 Ga granitoids were produced by partial melting of the same mafic crust that gave rise to the 3.45–3.44 Ga TTGs with some components requiring up to 50% crustal contribution. The Mayurbhanj granites represent intracrustal melts of more potassic members of the Paleoarchean TTGs. The trace-element characteristics of the granitoids reflect melt generation at different depths in a tectonic setting producing both TTGs as well as granites contemporaneously. Plausible geodynamic settings could be very hot subduction or “dripduction” regime involving shallow melting of delaminated mafic crust producing the 3.45 Ga juvenile TTGs and minor granites. Large scale delamination and melting of mafic lower crust and felsic proto-crust in a thickened crustal pile in a subduction-like regime could have produced granitoids with juvenile as well as recycled isotopic signatures. The Nd-Hf isotope data indicate that the Paleoarchean upper mantle was moderately depleted in response to a spurt in Paleoarchean continental growth.

Geochemistry of Late Mesozoic volcanic rocks in the central Great Xing’an Range, NE China: petrogenesis and crustal growth in comparison with adjacent areas

ABSTRACTThe central Great Xing’an Range contains widespread exposures of Mesozoic igneous rocks, yet the origin of its Late Mesozoic volcanic rocks has long been controversial. They are composed of andesite, trachyte, trachyandesite, trachydacite, rhyolite, and minor basaltic andesite. The intermediate rocks are enriched in large ion lithophiles and light rare earth elements (LILEs and LREEs) and are depleted in high field strength elements (HFSEs) such as Nb, Ta, and Ti, with positive εHf(t) values (+6.2 to +13.4) and Hf single-stage model ages (TDM1 = 556–227 Ma). This indicates that they were derived from the partial melting of a mantle wedge that was modified by previously subducted slab-derived fluids was likely introduced with limited crustal contamination. The felsic rocks show positive εHf(t) values (+4.3 to +10.2) and Hf two-stage model ages (TDM2 = 910–544 Ma), with more enrichment of LILEs and LREEs and more depletion of HFSEs and negative Eu anomalies. This indicates that felsic rocks were mainly derived from partial melting of Neoproterozoic accreted quartz-amphibolite facies mafic crust. We summarise the geochronological and Hf-Nd isotope data for Mesozoic igneous rocks within the central Great Xing’an Range (CGXR), the northern Xing’an block (NXB), and the Erguna block (EB), with the aim of constraining the geochronological framework for Mesozoic volcanism and comparing the growth of continental crust. The evolution of Late Mesozoic magmatism in the three areas reached its climax ~125 Ma ago. Our data, together with regional isotope evidence, suggest that crustal accretion of the CGXR occurred during the Neoproterozoic–Phanerozoic. However, that of the EB occurred during the Meso- to Neoproterozoic. The EB exhibits more lateral and vertical heterogeneities and ancient continental crust than the CGXR and NXB in the northern magma source region.

Lithospheric Architecture and Metallogenesis in Liaodong Peninsula, North China Craton: Insights from Zircon Hf-Nd Isotope Mapping

The Liaodong Peninsula is an important mineral province in northern China. Elucidating its lithospheric architecture and structural evolution is important for gold metallogenic research and exploration in the region. In this study, Hf-Nd isotope maps from magmatic rocks are constructed and compared to geological maps to correlate isotopic signatures with geological features. It is found that gold deposits of different age periods in Liaodong are located in areas with specific εHf(t) and εNd ranges (Triassic: from −8 to −4 and from −12 to −8, Jurassic: from −22 to −8 and from −14 to −8, Cretaceous: from −12 to −10 and from −22 to −20), respectively. This may reflect that when the Paleo-Pacific plate was subducted beneath the North China Craton, the magma was derived from the juvenile lower crust and the ancient lower crust, and formed the low-to-moderate hydrothermal Au-(Ag) and Pb-Zn deposits in the Triassic. In the Jurassic, continued subduction may have led to lithospheric thickening. Subsequently, the magma from the ancient lower crust upwelled and formed low-to-moderate hydrothermal Au deposits and porphyry Mo deposits. In the Cretaceous, crustal delamination may have taken place. The magma from the ancient lower crust upwelled and formed various low-to-moderate hydrothermal Au deposits.

Revisiting the Lushan-Taihua Complex: New perspectives on the Late Mesoarchean-Early Neoarchean crustal evolution of the southern North China Craton

The Taihua Complex in the Lushan area was divided into gneisses series and supracrustal rocks. The former is mainly composed of TTG gneisses and amphibolites, and records the Late Mesoarchean-Early Neoarchean crustal evolution of the southern North China Craton (NCC). Late Mesoarchean amphibolites (>2.84 Ga) are enriched in light rare-earth elements (LREEs) and large-ion-lithophile elements (LILEs) with negative Nb-Ta and Ti anomalies, derived from partial melting of a metasomatic mantle source. Late Mesoarchean TTG gneisses (>2.81 Ga) have high La/Yb and Sr/Y ratios and low MgO, Cr and Ni contents, and are interpreted as resulting from partial melting of a thickened lower crust with residual garnet and amphibole in the source. Early Neoarchean amphibolites (2.80–2.73 Ga) show flat REE patterns but slightly negative Nb-Ta and Ti anomalies due to the fractionation of Ti-rich minerals and have depleted Hf-Nd isotopes, and were derived from large degree partial melting of a depleted mantle source. Most of the Early Neoarchean TTG gneisses show pronounced positive Eu anomalies and have relatively lower trace element concentrations and more depleted Nd-Hf isotopes than the Late Mesoarchean TTG gneisses, resulted from the partial melting of thickened juvenile lower crust with residual garnet and amphibole in the source and subsequent plagioclase accumulation. In addition, some of the Early Neoarchean TTG gneisses have high MgO, Cr and Ni contents, which were likely derived from partial melting of delaminated lower crust or a thickened lower crust interacting with mafic melt.Late Mesoarchean amphibolites and TTG gneisses are restricted in the center of Lushan area, denoting an ancient continental nucleus. In contrast, Early Neoarchean amphibolites and TTG gneisses outcrop in peripheral of the ancient nucleus, exhibiting as an elongated belt along the Dangzehe River, which is evidence for the subduction. Overall, the Lushan amphibolites and TTG gneisses show slightly negative to positive whole-rock εNd(t) and εHf(t) values and zircon εHf(t) values, indicating a dominant crustal accretion with subordinate crustal reworking. Therefore, the continental growth recorded in the Lushan-Taihua Complex was caused by the subduction-accretion process, suggesting a convergent margin in the southern NCC during the Late Mesoarchean and Early Neoarchean.

Early Paleoproterozoic magmatism in the Yangtze Block: Evidence from zircon U-Pb ages, Sr-Nd-Hf isotopes and geochemistry of ca. 2.3 Ga and 2.1 Ga granitic rocks in the Phan Si Pan Complex, north Vietnam

Our understanding of the early evolution of the Yangtze Block is limited by the sparsely dispersed nature of pre-Neoproterozoic exposures. New, integrated petrographic, zircon U-Pb age and Hf-Nd isotope analyses, and whole-rock geochemical data for early Paleoproterozoic granites in the Phan Si Pan Complex provides new insights into the evolution of the Yangtze Block as well as its role in the Pre-Nuna supercontinent. LA-ICP-MS zircon U-Pb dating of magmatic zircons from quartz monzonite and gneissic granite yielded 207Pb/206Pb ages of 2306 ± 12 Ma and 2096 ± 15 Ma, respectively. Zircons from the quartz monzonite have εHf(t) values ranging from −4.1 to −2.1, corresponding to TDM2 model ages of 3002–2890 Ma, whereas zircons in the gneissic granite have εHf(t) values between −0.95 and +1.72 and corresponding TDM2 model ages of 2660–2516 Ma, which are consistent with their whole-rock Nd isotope values. Geochemically, the quartz monzonites are I-type granites. Combined with their relatively high Sr/Y ratios and low Y concentrations, as well as fractionated REE patterns with relatively high LREE but low HREE concentrations, they were probably generated by partial melting of the thickened middle-lower crust under elevated temperature. Geochemical and isotopic signatures suggest that the ca. 2.1 Ga gneissic granites are high-K calc-alkaline, ferroan A-type granites formed by partial melting of juvenile crustal source at high temperature and low pressure with little involvement of ancient crustal material. The Phan Si Pan complex has a distinct early Paleoproterozoic crustal evolution history compared with the other crustal provinces of the Yangtze Block, suggesting independent histories that were not unified until the late Paleoproterozoic during the assembly of Nuna. Moreover, the magmatism and tectonic evolution of the north Vietnam region is broadly similar to that of the Arrowsmith Orogen of the Rae craton in Laurentia suggesting a potential spatial linkage. The geologic record of the Yangtze Block does not support an early Paleoproterozoic shutdown of plate tectonics.

Geochemistry, zircon U-Pb and Lu-Hf systematics of high-grade metasedimentary sequences from the South Muya block (northeastern Central Asian Orogenic Belt): Reconnaissance of polymetamorphism and accretion of Neoproterozoic exotic blocks in southern Siberia

High-grade metasedimentary rocks of the South Muya block (Baikal-Muya Foldbelt) have been studied with respect to whole-rock elemental and Nd isotopic compositions, as well as the U-Pb and Lu-Hf systematics of detrital zircon and in-situ grown metamorphic grains in order to reveal the initial source provenance for sedimentation and the age of major metamorphic modification. Based predominantly on trace-elemental compositions, the sedimentary protolith is most likely a poorly sorted and immature greywacke, such as those formed in island arc or active continental margin settings. The island arc-related chemical signature coupled with zircon U-Pb age data supports a Neoproterozoic (>760 Ma) sedimentation age, whereas Nd-Hf isotope data of precursor sediments are linked to a relatively immature Tonian (∼940–780 Ma) continental arc involving Paleoproterozoic crustal basement. The limited provenance information and metamorphic history of the Muya block suggests it was exotic to Siberia and originally related to another continent, e.g. – subduction-accretion complexes of South China. The polyphase metasediments experienced high-grade metamorphism in the Tonian during the 764–754 Ma arc accretion to an enigmatic continental block and then in the Ediacaran during the 622–608 Ma accretion of the Baikal-Muya belt to the arc situated at the southern Siberia margin. Metamorphic overprinting during the introduction of supracrustal rocks to deeper-crustal levels led to protracted recrystallization of primary igneous zircon population and growth of new granulite-facies grains that did not involve notable juvenile input. Tonian sedimentary provenance and two metamorphic events suggest a protracted evolution for the Baikal-Muya belt associated with the formation of precursor complexes beyond the southern margin of Siberia and their later accretion to Late Cryogenian – Ediacarian arc complexes of the southern Siberian, which proceeded only after the opening of the Paleoasian Ocean (670–630 Ma).

The Neoproterozoic southern passive margin of the São Francisco craton: Insights on the pre-amalgamation of West Gondwana from U-Pb and Hf-Nd isotopes

Sedimentary basins are, in many cases, the only preserved record of the geological processes active through geological time. These deposits preserve age, isotopic and geochemical signatures of the source rocks, providing insights into tectonic setting. In this context, we investigate the lithostratigraphy, provenance and tectonic environment of the Carrancas Group, a meta-sedimentary sequence characterised by a basal psammitic unit (Lower Unit) passing up into interbedded psammo-pelites (Intermediate Unit), which are overlain by a further psammitic unit (Upper Unit). This succession, along with the Canastra, Paranoá and Vazante groups (Brasília orogen), comprises a mature passive margin developed at the western-southern margin of the São Francisco craton during the Neoproterozoic. Detrital zircon U-Pb ages and Nd-Hf isotope data enable revised stratigraphic correlations across the basin, highlighting variations in the source of detritus. Deposition began after 0.92 Ga, with the sedimentation of the Lower Unit, with input of juvenile ∼2.1 Ga detritus (εNdt = −0.1 to +3.2) derived from the São Francisco craton. A marine transgression characterises the transition to the Intermediate Unit, which gradually covered over juvenile ∼2.1 Ga terrains and rendered them unavailable as possible source rocks for detrital zircons. Evolved Archaean-Palaeoproterozoic basement rocks, the Espinhaço Supergroup and evolved Neoproterozoic granitoids, all belonging to the São Francisco craton, acted as sources for the Intermediate and Upper units (U-Pb = 2.7 Ga to 0.9 Ga; εNdt = −10.4 to +7.1). The Goiás massif might also have acted as a source, providing juvenile Mesoproterozoic detritus. The sediments of the Upper Unit register a significant shift in source, with incoming of juvenile Mesoproterozoic and crustal 1.9 – 1.7 Ga detritus. This change in the ages of the detrital material may be related to exhumation of the Espinhaço Supergroup during the evolution of the passive continental margin basin. The whole-rock Nd TDM model ages become progressively younger from the base to the top of the sequence (Nd TDM = 2.6–1.8 Ga). The collisional events between Amazonian and São Francisco cratons and the Paranapanema block set the minimum age of deposition along this passive continental margin at around 0.67 Ga.

Cretaceous (~100 Ma) high-silica granites in the Gajin area, Central Tibet: Petrogenesis and implications for collision between the Lhasa and Qiangtang Terranes

High-silica granites that occur as isolated plutons are widely distributed in collisional orogenic belts and are considered a hallmark of continental collision processes. The late Mesozoic collision between the Lhasa and Qiangtang Terranes along the Bangong–Nujiang Suture Zone was key event in the evolution of the Tibetan Plateau; however, the timing of this collision remains debated. Here, we report the ages and geochemical characteristics of granites from the Gajin area in the north-central Lhasa Terrane, central Tibet. Zircon UPb dating reveals that the Gajin granites formed at ~100 Ma. These granites exhibit uniformly high SiO2 contents (74.9–78.4 wt%), are weakly peraluminous compositions, and have high K2O, moderate Na2O and Al2O3, and low Fe2O3T, MgO, and CaO contents. They are characterized by enrichments in Rb, Cs, and U, and by depletions in Zr, Hf, Nb, and Ti. These characteristics, in combination with high Rb/Sr ratios and pronounced negative Eu anomalies, indicate that the Gajin granites are highly fractionated rocks. The granites have enriched whole-rock Nd (εNd[t] = −4.3 to −2.9) and relatively depleted and slightly variable zircon Hf (εHf[t] = −1.0 to +5.5) isotope compositions. The Gajin granites were probably generated by high degrees of crystal fractionations of magma produced by partial melting of mature crustal sources. The Cretaceous magmatic rocks in the north-central Lhasa Terrane record a magmatic lull between ~100 and 85 Ma, which corresponds to a transition in both geochemical composition (e.g., HfNd isotopes) and rock association at ~100 Ma. Combining the above characteristics with the development of a regional angular unconformity around the time of the formation of the granites, we suggest that the initial continental collision between the Lhasa and Qiangtang Terranes probably occurred between 110 and 90 Ma. Therefore, the ~100 Ma high-silica granites in the Gajin area were formed during the continuing continental convergence that followed the initial collision between the Lhasa and Qiangtang Terranes. The generation of discrete and monotonous high silica granites may constitute evidence for a significant tectonothermal transition in evolution of the continental crust.

The 825 Ma Yiyang high–MgO basalts of central South China: Insights from Os–Hf–Nd data

High–MgO basaltic lavas, including komatiites and picrites, have long been used as probes of both the chemical and thermal evolution of the mantle through time. The ca. 825 Ma Yiyang high–MgO basalts in the central South China Block (SCB) were considered to be the first evidence for >1500 °C mantle melts by a Rodinian mantle plume. However, later studies proposed that they may have been generated within an arc setting and can be classified as boninites. Here we present a comprehensive Os–Hf–Nd isotopic study of the Yiyang basalts. The Yiyang basalts have high MgO (>10%), Ni (>182 ppm), Cr (>667 ppm) and Os contents (mostly >0.3 ppb) with radiogenic Os isotopes ((187Os/188Os)i = 0.134 to 0.282). They have relatively uniform whole–rock Nd and Hf isotopes with εNd(t) and εHf(t) values ranging from −3.7 to −1.3 and from +2.9 to +4.3, respectively. Our new data indicate that although crustal assimilation and fractional crystallization (AFC) may have played a role in the geochemical diversity of the Yiyang basalts, the enrichment in light rare earth elements (LREE) and large–ion lithophile elements (LILE) relative to high field strength elements (HSFE) most likely are features of the primary magma. The Yiyang high–MgO basalts, along with contemporary high–MgO basalts in the central SCB, have lower SiO2 contents, and Al2O3/TiO2 ratios, but higher TiO2, Zr, Nb, and Nd contents and Th/U ratios than typical boninites. The presence of negative Ba anomalies and depletion in middle REE (MREE) to heavy REE (HREE) in these high-MgO basaltic rocks are also distinct from what is observed in typical boninites. Therefore, the primary magma of the Yiyang basalts had similar whole–rock geochemical compositions to that of komatiites and plume–derived basalts. Their decoupled HfNd isotopes indicate that recycled oceanic sediments may have been incorporated into their source. They have primitive mantle–like Al2O3/TiO2 ratios (21−23) and flat HREE patterns. However, unlike other Al–undepleted or Munro–type komatiites, their mantle source did not undergo earlier melting events. Our new results support the view that the Yiyang basalts are komatiitic and not boninitic, and that they were likely derived from a Rodinian mantle plume.

Deciphering the origin of the Cenozoic intracontinental rifting and volcanism in eastern China using integrated evidence from the Jianghan Basin

Intracontinental rifting and low-volume volcanism are a globally common phenomenon, yet the underlying driving mechanisms and whether they can be explained through classic plate tectonic concepts, remain hotly debated. A prominent example is the Cenozoic rift and volcanic province in eastern China. Using an integration of geological, geophysical and geochemical data, we unravel the spatial and temporal variations of the rifting and volcanism in the Jianghan Basin. Both rifting and volcanism in the Jianghan Basin show two intense-to-weak cycles (65–50 Ma and 50–26 Ma, respectively) with significant enhancement in activity during the late rift phase. Moreover, rifting and depocentres progressively migrated eastward. The Jianghan basalts all share an asthenospheric origin while the source of the late phase basalts is slightly more enriched and heterogenous in Nd-Hf isotopes than that of the early phase basalts. The late phase basalts also display a smaller extent of partial melting even under a thinner lithosphere, likely indicating a significant decrease of volatile content in the mantle source. Based on regional tectonic correlations, the main stages of tectonic evolution of the Jianghan Basin and eastern China are not synchronous with changes in Pacific plate motion, while they are coincident with India-Asia collision processes. These observations lead us to propose that the asthenospheric flow driven by India-Asia collision rather than the rollback of the subducted Pacific slab has caused the widespread rifting and volcanism in eastern China. The variations of rifting and volcanism in the Jianghan Basin suggest a multiphase and eastward asthenospheric flow beneath eastern China driven by India-Asia collision, with an intense upwelling when passing through the North-South Gravity Lineament (NSGL). The much more intense rifting and volcanism during the late rift phase may indicate a much larger scale of volatile-poor asthenospheric flow than the early rift phase which could result in a more intense erosion of ancient enriched lithospheric mantle and the volatile content in the mantle source dropping sharply. This study provides an improved model based on our multidisciplinary observations for asthenospheric flow which may be an alternative driving mechanism for intracontinental rifting and low-volume volcanism in the regions where there are step changes in lithospheric thickness globally.

Hf-Nd Isotopes in Archean Marine Chemical Sediments: Implications for the Geodynamical History of Early Earth and Its Impact on Earliest Marine Habitats

The Hf-Nd isotope systems are coupled in magmatic systems, but incongruent Hf weathering (‘zircon effect’) of the continental crust leads to a decoupling of the Hf-Nd isotope systems in low-temperature environments during weathering and erosion processes. The Hf-Nd isotope record was recently dated back from the Cenozoic oceans until the Archean, showing that both isotope systems were already decoupled in seawater 2.7 Ga ago and potentially 3.4 Ga and 3.7 Ga ago. While there might have existed a hydrothermal pathway for Hf into Archean seawater, incongruent Hf weathering of more evolved, zircon-bearing uppermost continental crust that was emerged and available for subaerial weathering accounts for a significant decoupling of Hf-Nd isotopes in the dissolved (<0.2 µm) and suspended (>0.2 µm) fractions of Early Earth’s seawater. These findings contradict the consensus that uppermost Archean continental crust was (ultra)mafic in composition and predominantly submerged. Hence, Hf-Nd isotopes in Archean marine chemical sediments provide the unique potential for future research to trace the emergence of evolved continental crust, which in turn has major implications for the geodynamical evolution of Early Earth and the nutrient flux into the earliest marine habitats on Earth.

Hafnium isotopic constraints on the origin of late Miocene to Pliocene seamount basalts from the South China Sea and its tectonic implications

Post-spreading intraplate volcanism has widely affected the South China Sea (SCS) region including Indochina, the northern margin of the SCS, and the SCS basin itself. In the SCS basin, several off- and on-fossil spreading center seamounts formed between 3.8 and 7.9 Ma. Based on previously published geochemical and Sr-Nd-Pb isotopic data, the intraplate volcanism is widely related to the Hainan mantle plume, whose existence has been evidenced by recent geophysical studies. To test this petrogenetic model, new Hf isotope data have been obtained from volcanic rocks from a suite of compositionally representative seamounts in the SCS basin. Compared to published Nd isotope ratios (0.512675–0.512965, 5 εNd units), 176Hf/177Hf ratios span a much larger range (0.282876–0.283097, 8 εHf units), even within individual seamounts (e.g., Zhangzhong seamount). These features, combined with previous studies, clearly confirm mantle heterogeneity beneath the SCS region. Similar to the trends in Sr-Nd-Pb isotope space, Nd-Hf isotope ratio show a relatively narrow, elongate mixing trend between a depleted Indian MORB-type mantle endmember and an enriched EMII-type mantle end member. We propose that this narrow trend is inconsistent with the origin of the enriched end member from a heterogeneous sub-continental lithospheric mantle (SCLM) and instead suggests a plume-related origin. As a conceptual model for the post-spreading tectonic scenario of the Hainan plume affecting the SCS region, we propose that a plume ascends to the bottom of the lithosphere beneath Hainan and its northern Leizhou peninsula at the northern margin of SCS, from where it migrates along sloping rheologic boundary layers to lithospheric faults under an extensional setting towards the central SCS where the magmas erupt at the young spreading centers.

Impact of residual zircon on Nd-Hf isotope decoupling during sediment recycling in subduction zone

The subduction factories in convergent plate margins exert crucial control on recycling terrestrial components and returning to the overlying crust. The Nd and Hf isotopic systems provide potential tracers to evaluate these processes. Here we present a case where these isotopic systems are decoupled in a suite of granites from the Chinese Altai, showing a wide range of εHf(t) values (from −4.7 to +10.8) in contrast to a limited range of εNd(t) values (from −5.8 to −1.9). The zircon xenocrysts occurring frequently in these rocks show markedly negative εHf(t) values (from −34.3 to −6.5) and positive δ7Li values (from +12.5 to +18.2). We propose a model to explain the observed relationship between residual zircon and Nd–Hf isotope decoupling. We suggest that the Altai granites originated from partial melting of subducted slab components under relatively low temperature conditions which aided the residual zircon from oceanic sediments to inherit and retain a significant amount of 177Hf in the source, thereby elevating the 176Hf/177Hf ratio of the melt, and decoupling from the 143Nd/144Nd ratio during the subsequent magmatic processes. Our study illustrates a case where sediment recycling in subduction zone contributes to decoupling of Nd and Hf isotopic systems, with former providing a more reliable estimate of the source characteristics of granitic magmas.

Plume-stagnant slab-lithosphere interactions: Origin of the late Cenozoic intra-plate basalts on the East Eurasia margin

Intra-plate basalts of ~35–0Ma in East Eurasia formed in a broad backarc region above the stagnant Pacific Plate slab in the mantle transition zone. These basalts show regional-scale variations in Nd-Hf isotopes. The basalts with the most radiogenic Nd-Hf center on the Shandong Peninsula with intermediate Nd-Hf at Hainan and Datong. The least radiogenic basalts occur in the perimeters underlain by the thick continental lithosphere. Shandong basalts possess isotopic signatures of the young igneous oceanic crust of the subducted Pacific Plate. Hainan and Datong basalts have isotopic signatures of recycled subduction materials with billions of years of storage in the mantle. The perimeter basalts have isotopic signatures similar to pyroxenite xenoliths from the subcontinental lithospheric mantle beneath East Eurasia. Hainan basalts exhibit the highest mantle potential temperature (Tp), while the Shandong basalts have the lowest Tp. We infer that a deep high-Tp plume interacted with the subducted Pacific Plate slab in the mantle transition zone to form a local low-Tp plume by entraining colder igneous oceanic lithosphere. We infer that the subducted Izanagi Plate slab, once a part of the Pacific Plate mosaic, broke off from the Pacific Plate slab at ~35Ma to sink into the lower mantle. The sinking Izanagi slab triggered the plume that interacted with the stagnant Pacific slab and caused subcontinental lithospheric melting. This coincided with formation of the western Pacific backarc marginal basins due to Pacific Plate slab rollback and stagnation.

Protracted post-collisional magmatism during plate subduction shutdown in early Paleoproterozoic: Insights from post-collisional granitoid suite in NW China

Tectonic regimes display different styles worldwide during the early Paleoproterozoic ranging from plate tectonic slowdown or shutdown to vigorous subduction-related magmatism. Here we report a suite of early Paleoproterozoic granitoids from the Quanji Massif in northwestern China that provides potential insights on the tectonic regime in this period. The granitoids formed at 2.37–2.34Ga as evidenced by LA-ICP-MS zircon U-Pb ages. These dioritic to granitic rocks with amphibole and biotite as the major mafic minerals display a wide range of SiO2 (55.8–66.2wt%), low to moderate MgO (1.4–4.1wt%), low to high K2O (1.6–5.9wt%) and Al2O3 (16.3–21.5wt%), with Mg# values in the range of 36–47 and relatively low Cr (7.7–79.2ppm) and Ni (2.3–19.3ppm) concentrations. Geochemically they correspond to high‑potassium calc-alkaline I-type granitoids, with some showing dominantly adakitic but minor A-type affinities. Both the I-type and A-type granitoids show moderately fractionated LREE to HREE and moderately to weakly negative Eu anomalies. However, the adakitic granitoids display strongly fractionated REE patterns and weak negative to positive Eu anomalies. All the rock types from this suite show enrichment in LILEs and LREEs and depletion in HFSEs (Ta, Nb). The Nd-Hf isotope data suggest that the granitoids were sourced from Mesoarchean to Neoarchean metamafic rocks in the lower crust. The coeval nature of the complex genetic types of rocks in this suite suggest that their precursor magmas formed in an extensional tectonic regime similar to those previously reported for the ~2.39–2.37Ga granitoids in central to eastern domains of the Quanji Massif. Our new results in conjunction with those from previous studies suggest protracted post-collisional magmatism lasting for >50Myr along the periphery Quanji Massif following the cessation of plate subduction in the early Paleoproterozoic.

A newly identified Precambrian terrane at the Pamir Plateau: The Archean basement and Neoproterozoic granitic intrusions

A number of Precambrian terranes are distributed in the Tibetan Plateau, including the Lhasa, Sibumasu, Qiangtang, Qaidam, Qilian and Tianshuihai terranes. These terranes exhibit distinct features in rock associations and tectonothermal history. They are important for understanding the evolution of the Tethys Ocean and the assemblage and breakup of the Precambrian supercontinents. In this contribution, we identify a Precambrian terrane at the southeastern section of the Pamir Syntax and we name it as Mazar terrane. Systematic zircon U-Pb dating shows that the basement, mainly composed of metamorphic bimodal volcanic rocks and gneiss/schist, deposited at ca.2.5 Ga and was overprinted by ca.2.0 Ga amphibolite-facies metamorphism. The Neoproterozoic granitic intrusions into the basement, emplaced at 840–835 Ma. Geochemistry of the granitic intrusions defines two distinct types of granitic rocks, including the trondhjemite-tonalites and granodiorites. The sodic trondhjemite- tonalites show TTG-like or adakitic signatures such as enrichment of LREE and LILEs, depletion of HREE and HFSE, resulting in their high Sr/Y and (La/Yb)N ratios. The granodiorites exhibit typical signatures of the calc-alkaline I-type granites. Both trondhjemite-tonalites and granodiorites have coupled unradiogenic whole-rock Nd and zircon Hf isotope compositions. εNd(T) and εHf(T) values of the trondhjemite- tonalites range from −9.5 to −8.7 and from −14 to −10, respectively. The granodiorites have slightly more radiogenic Nd-Hf isotope compositions with εNd(T) values ranging from −5.5 to −4.4 and εHf(T) values ranging from −10 to −7. According to elemental and isotope signatures of the granitic intrusions, the trondhjemite-tonalites were derived from partial melting of a mafic crust at depth ≥30 km with a garnet amphibolite residue whereas the granodiorite was formed by partial melting of a mafic crust at a shallower level and mantle-derived magma could be involved in the genesis of the granodiorite. In line with their geochemistry and the fact that the granodiorite is slightly younger than that of the trondhjemite-tonalites, we propose that those granitic intrusions were most likely formed in an extensional environment, and that they could be part of the earliest igneous activity related to the initial breakup of the Rodinia supercontinent. Our study, in combination with previous works, suggests that the Mazar terrane, as well as the Qilian, Qaidam, Tianshuihai terranes, is most likely continental fragments drifted from the Yangtze Block during the breakup of the Rodinia supercontinent.

The prevalence of kilometer-scale heterogeneity in the source region of MORB upper mantle.

The source regions of mid-ocean ridge basalts (MORB) are heterogeneous, consisting of chemically and lithologically distinct domains of variable size. Partial melting of such heterogeneous mantle sources gives rise to diverse isotopic compositions of MORB and abyssal peridotites. Variations in radiogenic isotope ratios in MORB are attributed to mixing of melts derived from enriched and depleted mantle components. However, melt mixing alone cannot fully account for the difference between the average 143Nd/144Nd in abyssal peridotites and their spatially associated MORB. We show that the more depleted Nd isotope composition in abyssal peridotites is a natural consequence of melt migration-induced mixing or smearing in the melting column. Sub-kilometer scale enriched mantle components or heterogeneities are significantly damped or homogenized in both the residue and erupted melt during their transit through the melting region. Heterogeneities with larger size and higher incompatible trace element abundance are more resistive to the mixing processes. The size-sensitive mixing depends on a parameter called the enrichment strength, which is the product of the heterogeneity size and the ratio between incompatible trace element abundance in the enriched and depleted mantle sources. Observed Nd-Hf isotope variations in MORB and abyssal peridotites can be reproduced if the enrichment strength is 20 to 60 km. These heterogeneities could be on the kilometer scale and have similar isotope ratios to but less incompatible trace element abundances than recycled oceanic crust.

Meso- and Neoarchean Banded Iron Formations and Genesis of High-Grade Magnetite Ores in the Anshan-Benxi Area, North China Craton

AbstractThe Anshan-Benxi area in the North China craton has numerous occurrences of Algoma-type banded iron formations (BIFs) with subordinate high-grade magnetite ores. These ores provide insight into iron metallogenesis and early evolution of the North China craton. In this paper, we present Sm-Nd-Fe-O isotope, mineralogical, and structural data for four BIF-type iron deposits to place constraints on their depositional ages and formation mechanism.Previous SIMS and LA-ICP-MS zircon U-Pb dating results indicated a Mesoarchean age (ca. 3.10 Ga) for the Dagushan BIF and a Neoarchean age (ca. 2.55 Ga) for other regional BIFs (Dai et al., 2012, 2013, 2014).This is confirmed by Sm-Nd isochron ages of these BIFs, high-grade magnetite ores, and host metavolcanics, which yield two regression lines and match apparent ages of 3149 ± 85 Ma (MSWD = 1.2) for Dagushan, and 2671 ± 120 Ma (MSWD = 3.0) for the other three deposits. Our new chronological data thus suggest Meso- and Neoarchean BIF deposition and potentially significant BIF-type iron deposits at depth.The regional high-grade magnetite ores are all hosted in the BIFs that occur in the same orientation and have transitional boundaries between them. They also show similar Sm-Nd isotope compositions and magnetite rare earth elements + yttrium (REY) profiles, indicating that the Anshan-Benxi BIFs were most likely the source beds. The high-grade magnetite ores contain abundant pyrite and actinolite, with systematically lower δ56Fe values (0.67–0.40‰) when compared to the BIFs (1.88–0.64‰), suggesting a hydrothermal origin. In the field, some high-grade orebodies with schistose textures are adjacent to undeformed granitic plutons. This geologic relationship implies that the high-grade magnetite ores were formed earlier and probably did not result from magmatic hydrothermal fluids. Therefore we suggest that the Anshan-Benxi high-grade magnetite ores were most likely produced by infiltration of metamorphic fluids into primary BIFs, based on the following: (1) magnetite δ18O values within the high-grade magnetite ores (+2.5 to −0.6‰) are significantly lower than those in the BIFs (9.2–2.6‰); (2) magnetite (avg 0.39 ppm) and pyrite (avg 0.098 ppm) in the high-grade magnetite ores have much lower REY abundances than magnetite in the BIFs (avg 14.6 ppm); (3) skeletal quartz in the high-grade magnetite ores shows systematically higher FeOtolal contents (1.36–0.56 wt %) than those in laminated chert bands (0.06–0.00 wt %); and (4) hydrothermal zircons within the Nanfen BIF yield a U-Pb age of 2480 Ma, which is comparable to ca. 2.48 Ga regional metamorphism (Zhu et al., 2015).Furthermore, microstructural textures indicate a maximum regional deformation temperature of up to 500°C, which is lower than the plastic flow temperature (>600°C) of magnetite. Finite strain measurements and electron backscatter diffraction analyses suggest a general flattening deformation and similar crystallographic preferred orientation for all magnetite crystals. These structural features reveal that magnetite in the high-grade magnetite ores never experienced a separate tectonic event. Our microscopic studies also show that microfractures at the interfaces of BIF bands contain fragmented quartz crystals and are filled with abundant metamorphic minerals (e.g., actinolite and chlorite). Considering that the Anshan-Benxi high-grade magnetite ores are commonly adjacent to weak structural planes (e.g., faults), we propose that macro- and microscopic fractures probably provided channels for metamorphic fluids.Recent zircon U-Pb geochronology has indicated widespread BIF formation at ca. 2.55 Ga in the North China craton, corresponding to a pronounced peak in BIF deposition of other Precambrian cratons. It is thus implied that a global geologic event triggered the extensive occurrence of BIFs. We correlate the Neoarchean tectonic evolution of the North China craton with the 2.7 to 2.5 Ga Kenorland supercontinent. Significantly, planar distribution signatures of the North China craton BIFs indicate ca. 2.5 Ga cratonization through the amalgamation of at least seven microblocks that were welded by several Neoarchean greenstone belts. Hf-Nd isotope studies have highlighted the Archean episodic crustal evolution of the North China craton, and the Meso- and Neoarchean BIF deposition could have benefitted from these geologic processes. The Anshan-Benxi high-grade magnetite ores that formed at ca. 2.48 Ga were closely related to important metamorphic events during the North China cratonization process.

Late Neoarchean magmatism and tectonic evolution recorded in the Dengfeng Complex in the southern segment of the Trans-North China Orogen

The Dengfeng Complex exposed in Songshan area in the southern segment of the Trans-North China Orogen (TNCO) mainly consists of Late Neoarchean tonalite-trondhjemite-granodiorite (TTG) gneiss, metadiorite and amphibolite. The Sanhuangzhai amphibolite records the earliest mafic magmatism (2658±5Ma) in the region, which is enriched in LREE and LILE with negative Nb, Ta and Ti anomalies and positive initial εNd(t) and εHf(t) values, suggested to be the remnant of an arc-related juvenile mafic crust. The Shipaihe amphibolite at the end of Neoarchean (2492±7Ma) is characterized by flat REE pattern, slightly negative Nb, Ta and Ti anomalies and positive initial εNd(t) and εHf(t) values, which is interpreted as large-degree partial melt of metasomatised mantle source in a back-arc basin. Zircon U-Pb dating shows two episodes of TTG magmatism (2.57–2.55Ga and 2.55–2.50Ga) within the Dengfeng Complex. All TTG gneisses have high SiO2 and Na2O, but low Mg#, Cr, Ni, [Yb]N and Y with high [La/Yb]N and Sr/Y. The early TTG gneiss (2.57–2.55Ga) in the eastern Songshan area has variable whole-rock Hf-Nd isotopes (εHf(t)=−2.14–4.64; εNd(t)=0.11–3.35; respectively), and its protolith is interpreted as the result of partial melting of a thickened lower crust with residual garnet and amphibole in the source. But the later TTG gneiss (2.55–2.50Ga) in the western Songshan area has depleted whole-rock Hf-Nd isotopes (εHf(t)=5.59–6.08; εNd(t)=2.42–3.86; respectively), suggesting an overall juvenile source, and its protolith is interpreted as the partial melt of flat-subducting oceanic crust. The Shipaihe metadiorite is characterized by high Mg#, Cr, Ni, Sr, Ba, highly fractionated REE patterns and strong negative Nb, Ta and Ti anomalies, resembling to sanukitoid suite, which is interpreted as the partial melt of a mantle wedge metasomatised by subduction-derived fluids/melts. Integrated geochronology and geochemical data of the Dengfeng Complex in Songshan area reveal the Late Neoarchean crustal accretion process from subduction to arc-continental collision (accretionary orogeny) followed by post-collisional extension (back-arc basin), implying an active continental margin arc environment in the southern segment of the North China Craton.

Rift–plume interaction reveals multiple generations of recycled oceanic crust in Azores lavas

We present 176Hf/177Hf isotope ratios on 41 previously well-characterized subaerial and submarine samples from the Azores islands of São Miguel, Terceira, Graciosa, Faial, Pico and the João de Castro seamount (on the Terceira Rift). In εNd–εHf isotope space all Azores lavas fall below the mantle array reference line and do not overlap the proximal Atlantic MORB. Lavas from São Miguel and João de Castro form two distinct and well defined arrays extending below the mantle array, which has not been previously documented in other oceanic magmatic provinces. The Nd-Hf isotope compositions of João de Castro overlap those of HIMU type lavas, yet they lack the characteristically radiogenic Pb isotope ratios of HIMU. The combined Nd-Hf-Pb-Sr isotope systematics of both São Miguel and João de Castro endmembers can be explained by recycling of a single package of heterogeneous oceanic crust ranging from D-MORB to E-MORB in composition, with an age between 2.5 and 3.0Ga, with no requirement for parent-daughter ratio modification during subduction. In contrast the Nd-Hf-Pb isotope systematics of lavas from São Jorge, Terceira, Graciosa, Pico and Faial are consistent with the presence of younger (<700Ma) recycled crust that underwent low-temperature alteration and dehydration during subduction. There is no evidence in the erupted lavas for direct mixing between these two generations of recycled material within the plume. These data suggest that old recycling age and absence of sediments along with recycled oceanic crust are both required to develop isotopic compositions below the mantle array in εNd–εHf space. Our modeling shows that the compositional variability of erupted MORB is large enough that, given enough time, they can generate a wide range of isotope compositions such as observed in OIB. Lastly, lava compositions along the Terceira rift can be explained by a westward asthenospheric flux along a tilted lithosphere/asthenosphere boundary, where fertile components are exhausted by partial melting after ∼70km of transport along the Terceira Rift. While this observation is broadly consistent with the plume source–ridge sink model, it also suggests that the lithosphere/asthenosphere boundary geometry can smear the view of the plume heterogeneity.