Crust-mantle Interaction Research Articles

Syn-exhumation magmatism in an active continental margin above a continental subduction zone: Evidence from Late Triassic mafic igneous rocks in the southeastern North China Block

AbstractSyn-subduction arc magmatism is absent above continental subduction zones, providing important constraints on the nature of petrogenetic processes during continental collision. Nevertheless, we have identified syn-exhumation mafic igneous rocks in an active continental margin above a continental subduction zone, where the South China Block (SCB) was deeply subducted beneath the North China Block (NCB) for collisional orogeny in the Triassic. These mafic igneous rocks occur in the southeastern margin of the NCB, showing consistent Late Triassic zircon U-Pb ages of ca. 219–218 Ma, coeval with exhumation of the deeply subducted continental crust. These rocks are categorized into two series of sub-alkaline and alkaline, all exhibiting arc-like trace element distribution patterns, highly enriched radiogenic Sr-Nd-Hf and high zircon O isotope compositions. In particular, they exhibit two-stage whole-rock Nd and Hf model ages and zircon Hf model ages of Paleoproterozoic, which are comparable to those of ultrahigh-pressure metamorphic rocks with the SCB affinity in the Dabie-Sulu orogenic belt. Such geochemical features indicate that these mafic igneous rocks were derived from partial melting of ultramafic metasomatites generated by reaction of felsic melts from the subducted SCB with the mantle wedge peridotite beneath the NCB. The geochemical differences in element and isotope compositions between the two series igneous rocks can be mainly ascribed to different proportions of the crustal component in the metasomatites, which is verified by quantitative modellings of the geochemical transfer in the continental subduction zone. The systematic variations in some geochemical variables such as Fe/Mn, Zn/Fe, and Nb/Ta ratios indicate pyroxenite-rich and hornblendite-rich lithologies, respectively, for the sub-alkaline and alkaline series igneous rocks. With the tectonic extension for exhumation of the deeply subducted continental crust in the Late Triassic, the fertile and enriched metasomatites in the mantle wedge underwent partial melting for the syn-exhumation mafic magmatism in the southeastern NCB. Therefore, the mafic igneous rocks in the active continental margin not only record the crust-mantle interaction in the continental subduction zone, but also witness the generation of syn-exhumation magmatism in the late stage of continental collision.

The origin of arc basalts: New advances and remaining questions

Whether arc magmatism occurs above oceanic subduction zones is the forefront of studies on convergent plate margins. The most important petrologic issue related to the evolution of arc systems is the origin of arc magmatism, among which arc basalts are the most important one because they provide insights into mantle enrichment mechanism and crust-mantle interaction at oceanic subduction zones. Fluids or melts released either by dehydration or by melting of subducting oceanic slab infiltrate and metasomatize the overlying mantle wedge at varying depth, leading to the formation of source regions of arc basalts. Such processes make most of arc basalts commonly enriched in large ion lithosphile elements and light rare earth elements, but depleted in high-field strength elements and heavy rare earth elements. Small amounts of arc basalts are characterized by relatively high Nb contents or by Nb enrichment. Rare basalts with compositions similar to ocean island basalts or mid-ocean ridge basalt also occur in arc systems. For these peculiar rocks, it remains debated whether their source is affected by subduction-related components. During their ascent and before their eruption, arc basaltic magmas are subjected to crystal fractionation, mixing and crustal contamination. In addition to the contribution of subducting slab components to the mantle source of arc basalts, the materials above the subducting slab at forearc depths would have been transported either by drag or by subduction erosion into the subarc mantle and into the source of arc magmas. Heats and materials brought by corner flows also play important roles in the generation of arc basalts. Despite the important progresses made in recent studies of arc basalts, further efforts are needed to investigate subarc mantle metasomatism, material recycling, the formation of arc magma sources, geodynamic mechanism in generating arc basalts, and their implicationd s for the initiation of plate tectonics on Earth.

Lower crust-mantle interactions in the massif-type anorthosite formation: New evidence from zircon U-Pb-Hf-O isotopes of the Neoproterozoic Kadavur Complex, southern India

The Kadavur massif-type anorthosite, which intruded the basement of the Madurai Block in southern India, comprises anorthosite, leucogabbro, and (noritic) gabbro with minor Fe-Ti oxide ores. The U-Pb zircon SIMS dating indicates that the magmatism occurred during the Neoproterozoic period (Tonian), i.e., noritic gabbro (790.4 ± 5.1 Ma), gabbro (793.0 ± 4.1 Ma), leucogabbro (784.9 ± 4.1 Ma), and anorthosite (790.7 ± 4.3 Ma). Combined with the published age data from the coeval charnockite and A-type granite, we conclude that the anorthosite-charnockite-rapakivi granite suite was emplaced during ca. 785–805 Ma. In-situ zircon Hf-O isotopic data of the Kadavur gabbros and anorthosite provide insights on the source and evolution of the parental magmas of the massif-type anorthosites. Zircons from the anorthosite are characterized by ancient crustal εHf(T) values (−11.4 to −6.5) and higher δ18O values (5.92 to 6.4‰). In contrast, zircons from the coarse-grained gabbro have relatively primitive εHf(T) values (−6.3 to −2.3) and mantle-like δ18O values (4.86–5.73‰). Moreover, zircons from the noritic gabbro and leucogabbro have εHf(T) values (−11.1 to −3.7) and δ18O values (5.35 to 6.77‰) distributed between those of the anorthosite and coarse-grained gabbro. The new zircon Hf-O data demonstrate that the parental melt of the anorthosite was subjected to crustal contamination during the early evolution stages, producing an evolved magma with crustal isotope and trace element signatures. In contrast, the gabbros show less crustal influence and like to represent original magma source of the Kadavur Complex. The zircon Hf-O isotope compositional array from the primitive gabbros to the more-evolved anorthosite demonstrates that the parental magmas were derived from partial melting of the upper mantle with varying crustal input, which can be up to 30–40% for the anorthosite formation. Contamination of the ponded basaltic magmas by the felsic crust can effectively increase the SiO2, Al2O3, Na2O, and Sr contents in the magmas, which was likely essential for enormous plagioclase fractionation in the massif-type anorthosites.

Crust-mantle interaction during syn-collisional magmatism – Evidence from the Oamikaub diorite and Neikhoes metagabbro (Damara orogen, Namibia)

The early-syntectonic 563.7 ± 6.1 Ma old Oamikaub diorite (Damara orogen, Namibia) consists of metaluminous, magnesian, calc-alkalic to calcic diorites, granodiorites and granites. Associated gabbro-diorites and gabbros belong to the Neikhoes metagabbro. Linear major and trace element variations imply that the rock suite evolved through fractional crystallization processes involving amphibole, biotite, Fe-Ti oxides, zircon and apatite. Initial Sr (87Sr/86Sr: 0.7058–0.7123) and Nd (ε Nd: −2.1 to −18.8) isotopic compositions are highly variable and negatively correlated indicating that assimilation of crustal components occurred. Unradiogenic initial 206Pb/204Pb (16.23–17.23) and 207Pb/204Pb ratios (15.50–15.57) suggest derivation from or interaction with ancient crust with low U/Pb. Two gabbro-diorites have MgO, Ni and Cr abundances that are compatible with derivation of these rocks from upper mantle lithologies. Their initial ε Nd values (−2.1 and – 7.4) and 87Sr/86Sr ratios (0.7058 and 0.7076) imply derivation from an aged metasomatized lithospheric mantle. Other mafic samples have MgO abundances and compatible element concentrations that exceed the values commonly accepted for primary mafic melts implying some accumulation of clinopyroxene and amphibole. The granodiorites form a homogenous group in which the isotope data (initial ε Nd: −12.4 to −14.1; initial 87Sr/86Sr: 0.7083–0.7096) imply a lower crustal source. The granites are also magnesian and calc-alkaline but two of them are strongly peraluminous. Their isotope data (initial ε Nd: −13.2 to −18.8; initial 87Sr/86Sr: 0.7099 to 0.7123) imply derivation from more ancient sources, alternatively these samples gained their isotope systematics through extensive AFC processes from parental granodiorites. A common subduction zone environment as suggested from negative Nb-Ta anomalies in multi-element diagrams seems unlikely for all samples because of a lack of isotopically depleted signatures. The data from the Oamikaub diorite and other mafic complexes are better explained by a “flat” subduction model involving mainly continental mantle lithosphere and crust with limited, if any, melting of asthenospheric mantle.

Geochronology and petrogenesis of the composite Zuluhong Granite, North Xinjiang Province of China: Implications for the crust-mantle interaction and continental crustal growth in Western Tianshan Orogen

The mineralogy, whole rock geochemistry, zircon U-Pb-Hf isotopic compositions, and Sr–Nd isotopes were analysed from the composite Zuluhong Granite at the Alataw Mountains in NW China. The aim is to determine the granite's nature and tectonic setting, and gain a better understanding of the continental growth of the Western Tianshan Orogen (WTO), which is commonly regarded as part of the Central Asian Orogen (CAO). The Zuluhong Granite is divided into a monzogranite and porphyritic granodiorite. The monzogranite yields a zircon U–Pb date of 320 ± 2 Ma interpreted as its emplacement age, and the porphyritic granodiorite yields a zircon U–Pb date of 309 ± 1 Ma interpreted as its emplacement age. The monzogranite is characterised by its high SiO2 content (72.7–74.4 wt%), moderate 10,000 × Ga/Al values(2.59–2.81), strongly negative Eu anomaly (0.39–0.43), and relatively low zircon saturation temperatures (Tzr) of 726–807 °C, which are indicative of a highly fractionated monzogranite. The combined arc-like geochemical signatures (e.g. enrichment in LILE and LREE relative to HFSE, negative anomalies in Nb and Ta), and relatively high positive εNd(t) values of +4.4 to +4.8, indicate that the monzogranite originated from the partial melting of arc rocks in the lower crust. In contrast, the granodiorite has higher Al2O3 assays averaging 16 wt% and Sr/Y ratios averaging 40, and lower Yb assays of 1.38–1.49 ppm with a weak Eu anomaly, which are characteristics of adakite-like rocks. The combination of the granodiorite's relatively lower SiO2 content of 63.7–65.3 wt%, higher Mg# values of 53–57, higher εHf(t) values of +10.9 to +13.4, and complex oscillatory zoned plagioclase phenocrysts are indicative of a source derived from material derived from the mantle and lower crust. Our data on the Zuluhong Granite, coupled with the results of previous studies of the Late Carboniferous to Early Permian magmatism in the WTO, indicate that a regional tectonic transition from compression to extension took place in the CAO during the end of the Early Carboniferous (ca. 320 Ma). Furthermore, substantial post-collisional reworking of the Precambrian rocks along the northeastern margin of the Yili Terrane is more pronounced than the West Junggar Terrane to the east. The generation of widespread magmatism and continental growth in the CAO in a post-collisional tectonic setting including the recycling of earlier juvenile-arc complexes, and underplating of mantle-derived magma.

Two stages of crust-mantle interaction during oceanic subduction to continental collision: Insights from mafic-ultramafic complexes in the North Qaidam orogen

Subducted sedimentary rocks contribute to different reservoirs in the mantle, which results in crust-mantle interaction; however, the metasomatic processes involved in this sedimentary contribution to the mantle still remain poorly understood. Chromite is a robust mineral and together with its inclusions, occurring in orogenic peridotite, it is an important candidate for decoding both the petrogenetic and tectonic settings as well as crust-mantle interaction. This study presents an integrated petrological and geochemical study of the chromitite-bearing Luofengpo mafic-ultramafic complex (CLMC) in the North Qaidam orogen, China. The CLMC consists of amphibole pyroxenite, serpentinized peridotite, and serpentinized pyroxenite. Electron probe micro-analysis and laser ablation inductively coupled plasma mass spectrometry analysis demonstrated that the studied chromite grains display compositional zoning with the cores preserving primary igneous compositions. Chromitites with high Cr#(100*Cr/(Cr+Al)) signatures are enriched in IPGE (Os, Ir, and Ru) and depleted in PPGE (Rh, Pt, and Pd) indicating that they formed in the supra-subduction zone mantle wedge. Core compositions of chromite grains indicate that the parental melts of the chromitites were similar to typical boninitic melts, formed in a forearc setting related to subduction initiation. Petrographic, geochronological, and mineralogical analyses suggest that the CLMC underwent at least two stages of crust-mantle interaction. The first stage (metasomatic agent) occurred at 483.1 ± 3.5 Ma. The metasomatic agent was a hydrous melt derived from a mixed source of carbonates and sediments, subducted with the south Qilian oceanic slab. The second stage (metasomatic fluids) occurred at 434.2 ± 2.1 Ma and the metasomatic fluids originated from subduction and exhumation of the deeply subducted continental crust. These results suggest that the CLMC represents a supra-subduction zone mantle wedge slice, metasomatized by at least two stages of subduction crustal fluids during the transition from oceanic subduction to continental collision in the North Qaidam ultrahigh pressure metamorphic belt.

Dating mafic magmatism by integrating baddeleyite, zircon and apatite U–Pb geochronology: A case study of Proterozoic mafic dykes/sills in the North China Craton

Direct dating of mafic dykes/sills is crucial for understanding mantle geodynamic evolution and crust-mantle interaction, but is also challenging, especially in ancient samples. Here, we provide a reliable approach for dating Proterozoic mafic dykes/sills through an integrated investigation of U–Pb isotopes and elemental distribution maps of the coexisting baddeleyite, zircon and apatite. This study focuses on Paleo- to Neo-Proterozoic mafic dykes/sills in the North China Craton.In the studied samples, baddeleyite is undoubtedly magmatic, with sensitive high resolution ion microprobe (SHRIMP) 207Pb/206Pb ages that may record crystallization ages of the dykes/sills. Common lead incursion along rims and fractures of baddeleyite is identified via laser ablation quadrupole inductively coupled plasma mass spectrometry (LA-Q-ICP-MS) elemental mapping as the likely origin of age uncertainties. In contrast, magmatic zircons from the Proterozoic mafic intrusions are somewhat affected by metamictization as roughly indicated by high alpha dose (Dα), and thus show intricate U–Pb isotopic features. Intensely metamict zircons are susceptible to Pb loss due to fluid infiltration, as clearly illustrated by opposing enrichment trends of Pb and U in zircon maps. Where metamict zircons are unaffected by fluid alteration, they are able to preserve the original 207Pb/206Pb ratios and thus give high precision age constraints on emplacement of the ancient dykes/sills.As for apatites, abundant euhedral coarse-grained crystals were separated from the mafic rocks, rendering them suitable for LA-Q-ICP-MS measurement. U–Pb data define Tera-Wasserburg isochrons with lower intercept ages that match crystallization of the host rocks. Age uncertainties of this method lie in instrumental analysis errors caused by low concentrations of U, Pb and a low proportion of radiogenic Pb in the total Pb, with these parameters commonly showing consistent enrichment trends from crystal cores to rims. By combining U–Pb dating and elemental mapping, this study provides a novel approach to the geochronology of mafic rocks, by highlighting the potential of ubiquitous apatite, offering additional means for error analysis and age interpretation of baddeleyite and zircon.

Geodynamic mechanism and classification of basins in the Earth system

The sedimentary basin is a basic geological unit in the Earth system: its formation and evolution are controlled mainly by plate tectonics and geodynamics, which have produced a variety of basin types. The development and evolution of various basins depend not only on the tectonic effect of lithospheric plate but also on sublithospheric, and Earth surface actions. Basin classification schemes based on the theories of platform–geosyncline or plate tectonics could neither include all basin types nor reflect the complex processes of basin formation and evolution. From the viewpoint of Earth system, we proposed a new basin classification scheme based on geodynamic mechanism—that is, lithospheric plate, sublithospheric, and Earth surface actions. In our geodynamic classification, we employed a series of geodynamic settings of basin formation and evolution, including the lithospheric plate, sublithospheric, and Earth surface environments, the dynamic regimes of basin formation and evolution, such as tectonic setting (extension, compression and shearing), the thermal processes and gravity effects, the various natures of basin basement and crustal types (continental, oceanic, or transitional crust), and the history of basin sedimentary filling and tectonic paleogeography successively, as the controlling factors for basin classification.This new geodynamic classification scheme highlights both the role of the lithosphere and the effects of the sublithospheric and the Earth surface actions on controlling the basin formation and evolution. The basic dynamic types of basin classification include the prototype basin forming under a single tectonic, thermal, or gravity regime and the superimposed basin as well as residual basin forming under multistage thermal and/or tectonic regimes. Thus, this classification scheme could provide an evolving understanding of lithosphere, crust–mantle interaction, and geodynamic processes as well as of hydrocarbon accumulation and metallogenic mechanisms in sedimentary basins and would help in the synthetic exploration of various resources.

Mantle and crust interaction scenario at the crust-mantle transition zone: Depicted from inter-layered pyroxenite-granulite xenolith in Hannuoba area, North China Craton

Mantle-crust interaction occurring in various ways has been proposed for a long time, but the details of its mechanism at the crust-mantle transition zone and the lower crust are poorly known. This study reports a layered pyroxenite-granulite xenolith from the Cenozoic basalts of Hannouba in the North China Craton. Petrological and mineral chemical investigations reveal that the pyroxenite layers are cumulates from underplating basaltic magmas, while the granulite portions represent pre-existing lower crust. The magmas were crustally contaminated by interaction with the granulite. Orthopyroxene and plagioclase in the granulite involving reaction with exotic melts are transformed to K-feldspar, clinopyroxene, Fe- and Mg-oxide, melt and/or sulfide. The mineral chemical similarities and overlapping temperature estimates suggest that the pyroxenites and the granulites had been incrementally equilibrated with each other, both physically and chemically, via significant chemical and thermal exchange. The phase transformation and interaction could represent crust anatexis to generate more felsic magmatism and related ore deposits for the middle to upper crust and subsequently increase crustal maturity. This is also an efficient way of forming crust-mantle transition zone below the lower crust and crustal reworking in the Hannuoba area and may be applicable to other places worldwide.

Critical metal resources in the Middle-Lower Yangtze River Valley metallogenic belt

Critical metals are strategic key mineral resources for global competition, with research on mineralization characteristics and metallogenic regularity attracting attention. The Middle-Lower Yangtze River Valley metallogenic belt (MLYB) is a part of the circum-Pacific magmatic belt. Under complex continental dynamic setting, such as the Mesozoic transformation of structural systems and intensive crust-mantle interactions, a large amount of porphyry-skarn Cu-Fe-Au deposits have been formed. During large-scale mineralization processes, critical resources, including Cr, Co, Se, Re, and Te, are produced from these deposits, in addition to bulk metals such as Cu, Ag, Fe, and S-rich ores. These critical metals are urgently needed in China’s strategy. Therefore, research should receive emphasis to achieve new breakthroughs. In recent years, large mining companies in the Yangtze River economic belt have paid special attention to the comprehensive use of critical metals. Therefore, studies on critical metals in the belt have important theoretical and economic significance. This paper has systematically reviewed and summarized the research outcomes and literature related to the exploration and research work of critical metals in MLYB and preliminarily investigated the enrichment characteristics of the critical metals in the deposits in the belt. The preliminary investigation indicates that if the dominant metals in skarn deposits differ, the subsequent byproduct critical metals also differ. Most critical metals are hosted in sulfides in these skarn deposits. This paper shows a spatial distribution map of the critical metal deposits. Based on this map, the relationships between the dominant metal and the associated critical metal elements are summarized. Moreover, this paper has reviewed and summarized the deportment and enrichment mechanism of Cr, Co, Se, Re, and Te in the porphyry-skarn Cu-Fe-Au deposits in the belt and clarified the key host minerals and their abundances by referring to the five critical elements. Based on comprehensive analyses of previous studies on critical metals in the belt, we have raised three key scientific points that should be the focus of future studies on related topics. First, the primary research clue about sources and enrichment mechanisms should consider the systematic evolution of the porphyry-skarn Cu-Au deposits in the intracontinental setting. The second is a key question and challenging issue regarding the recovery and comprehensive utilization of the critical metals in specific sulfides. Finally, the full use of microscale to nanoscale lab analyses and related theories about the synthesized host minerals bearing critical metals may provide important evidence that can help interpret the enrichment mechanism of the critical metals. This point should be based on research on the crystallinity relationships and interface correlations between the host and critical metal elements.

Synchronous late Neoarchean Na- and K-rich granitoid magmatism at an active continental margin in the Eastern Liaoning Province of North China Craton

The appearance of voluminous K-rich granitoid rocks during Late Archean marks gradual maturation and stabilization of the continental crust. While most K-rich granitoid magmatism followed TTG magmatism, they occur synchronously sometimes, and this relationship is crucial for our understanding of the evolution of late Archean continental crust and its geodynamic setting.In this study, a series of ~2.57–2.52 Ga coeval and diverse granitoid rocks, including quartz dioritic-trondhjemitic and granodioritic to monzo-/syenogranitic gneisses, were identified in the southern Fushun area of Eastern Liaoning Province, North China Craton. The ~2.57 Ga quartz dioritic gneisses show moderate MgO (≤ 3.96 wt.%) and moderate to high Mg# (37.3–75.5). Geochemical modeling, together with mildly fractionated REE patterns and negative Eu anomalies and depleted zircon ƐHf(t2) (+2.1 − +7.4), suggest that they were differentiated from a depleted mantle source that was metasomatized by slab-derived fluids. The younger trondhjemitic gneisses (~2.55–2.52 Ga) are divided into two subgroups, i.e., an older subgroup (~2547–2540 Ma) characterized by mildly fractionated REE patterns and negative Eu anomalies, and a younger subgroup (~2533–2517 Ma) with strongly fractionated REE patterns and positive Eu anomalies. The trondhjemitic gneisses lack evidence for magma differentiation, and they are inferred to have been formed by reworking of amphibolites/greywackes at diverse crustal levels, with some inputs of mantle materials in the earlier subgroup. The coeval ~2550–2529 Ma granodioritic and monzo-/syenogranitic gneisses are characterized by high K2O/Na2O (0.67–2.45) but low MgO and Mg# (mostly <2 wt.% and < 50, respectively). Geochemical modeling data indicate that the granodioritic gneisses are less differentiated, which could have been derived from the partial melting of high-K mafic rocks (e.g., regional late Neoarchean calc-alkaline meta-basaltic rocks), as further supported by the constant A/CNK (1.00–1.14) and zircon ƐHf(t2) (+2.0 − +9.3) values. In comparison, the monzo-/syenogranitic gneisses show variable A/CNK (0.78–1.32) and zircon ƐHf(t2) (−2.4 − +7.8) values. They are explained to be formed by the partial melting of mixed sources of high-K mafic and sedimentary rocks, with the primary magmas showing plagioclase and apatite fractionation.Taken together, the above granitoid rocks record gradually decreasing zircon ƐHf(t2) values and increasing crustal thickness. Considering the petrogenetic information, regional geological data and the presence of some ~3.45–2.70 Ga crustal materials, the late Neoarchean granitoid magmatism of southern Fushun were likely generated via complex crust-mantle interaction processes at an active continental margin. It is further emphasized that Archean active continental margins are key sites for the initial maturation of the crust, and this tectonic scenario acted as a trigger for granitoid diversification during both the subduction and subsequent collisional stages.

Integration of zircon and apatite U–Pb geochronology and geochemical mapping of the Wude basalts (Emeishan large igneous province): A tool for a better understanding of the tectonothermal and geodynamic evolution of the Emeishan LIP

Radiogenic isotopic dating and Lu–Hf isotopic composition using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) of the Wude basalt in Yunnan province from the Emeishan large igneous province (ELIP) yielded timing of formation and post-eruption tectonothermal event. Holistic lithogeochemistry and elements mapping of basaltic rocks were further reevaluated to provide insights into crustal contamination and formation of the ELIP. A zircon U–Pb age of 251.3 ​± ​2.0 ​Ma of the Wude basalt recorded the youngest volcanic eruption event and was consistent with the age span of 251–263 ​Ma for the emplacement of the ELIP. Such zircons had εHf(t) values ranging from −7.3 to +2.2, identical to those of magmatic zircons from the intrusive rocks of the ELIP, suggesting that crust-mantle interaction occurred during magmatic emplacement, or crust-mantle mixing existed in the deep source region prior to deep melting. The apatite U–Pb age at 53.6 ​± ​3.4 ​Ma recorded an early Eocene magmatic superimposition of a regional tectonothermal event, corresponding to the Indian–Eurasian plate collision. Negative Nb, Ta, Ti and P anomalies of the Emeishan basalt may reflect crustal contamination. The uneven Nb/La and Th/Ta values distribution throughout the ELIP supported a mantle plume model origin. Therefore, the ELIP was formed as a result of a mantle plume which was later superimposed by a regional tectonothermal event attributed to the Indian–Eurasian plate collision during early Eocene.

Geochemistry and zircon ages of the Yushigou diabase in the Longshoushan area, Alxa Block: implications for crust–mantle interaction and tectonic evolution

AbstractThe North Qilian orogenic belt in North China has been defined as a subduction–collision zone between the Alxa Block and the Qilian Block. We present petrography, zircon U–Pb geochronology, major- and trace-element geochemistry, and Sr–Nd–Pb–Hf isotope analysis for the Yushigou diabase from the Longshoushan area, which is located SW of the Alxa Block, aiming to understand its petrogenetic link to subduction processes. The Yushigou diabase belongs to the tholeiite series, and shows enrichment in light rare earth and large-ion lithophile elements, and a depletion in heavy rare earth and high-field-strength elements. Laser ablation – inductively coupled plasma – mass spectrometry U–Pb zircon dating yielded an emplacement age of 414 ± 9 Ma, with an ϵHf(t) value in the range of −10.3 to 1.8. The whole-rock initial 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of the diabase range over 16.811–17.157, 15.331–15.422 and 37.768–37.895, respectively. The (87Sr/86Sr)i ratios vary between 0.7086 and 0.7106, and ϵNd(t) values vary between −14.4 and −13.4, which are significantly higher than the ϵHf(t) value (Nd–Hf decoupling). An interpretation of the elemental and isotopic data suggests that the Yushigou diabase was derived from partial melting of an enriched mantle I (EM-I) -type lithospheric mantle in the spinel–garnet transitional zone. Based on the geochemical features and previous regional geological data, we propose that the Silurian magmatism was most likely triggered by slab break-off after the closure of the North Qilian Ocean, and ancient continental materials from the subduction slab metasomatized the overlying lithospheric mantle during exhumation.

Geochemistry of high-pressure to ultrahigh-pressure granitic melts produced by decompressional melting of deeply subducted continental crust in the Sulu orogen, east-central China

The production of granitic melts by decompressional melting of deeply subducted crustal rocks under high-pressure (HP) to ultrahigh-pressure (UHP) conditions is important for the crust-mantle interaction at convergent plate boundaries. However, little is known on the geochemical composition of natural HP to UHP granitic melts from collisional orogens. This problem is partially resolved here through an integrated study of the Yangkou granitic veins in the Sulu orogen, which provide the excellent records of HP to UHP granitic melts from the deeply subducted continental crust. In trace element composition, these granitic veins show enrichment in LREE and LILE but depletion in HFSE with remarkable negative anomalies of Nb, Ta, P and Ti. They have low Sr/Y (5.92–26.3) and (La/Yb)N ratios (12.7–45.6), which are primarily controlled by the solubility of accessory minerals such as apatite and epidote/allanite in anatectic melts. Phengite in the granitic veins have high Si (3.29–3.42 p.f.u.) and Mg (0.16–0.35 p.f.u.) with Mg# of 37.7–68.1 and calculated pressures of 1.7–3.1 GPa, indicating their crystallization under HP to UHP conditions. Geochemical variables such as K2O/Na2O, A/CNK and LREE for whole-rock exhibit a positive correlation with pressure, indicating the significant pressure effect on the mineral stability that in turn controls the composition of anatectic melts. Furthermore, the granitic veins can be divided into two groups based on their formation pressures and geochemical compositions. Group I granitic veins formed at low pressures of 1.7–2.4 GPa and exhibit higher SiO2 and Na2O contents, but lower contents of K2O, MgO, TiO2, P2O5 and most trace elements (e.g., Rb, Ba, LREE) with lower K2O/Na2O ratios and A/CNK values compared to Group II granitic veins that formed at high pressure of 2.6–3.1 GPa. Zircon from the granitic veins give concordant U–Pb ages of 755 ± 10 to 769 ± 9 Ma with steep HREE patterns for relict magmatic cores and 216 ± 4 to 222 ± 4 Ma with steep to flattened HREE patterns for anatectic rims. In comparison to concordant U–Pb ages of 220 ± 3 to 227 ± 3 Ma for newly grown zircon in the host eclogites, it appears that these granitic veins were crystallized from anatectic melts produced by crustal anataxis during the exhumation of the deeply subducted continental crust. The anatectic rims are characterized by the absence of not only oscillatory zoning in CL images but also Eu anomalies and low U contents, indicating their growth from anatectic melts under HP to UHP conditions. The granitic veins show different Sr–Nd–O isotope compositions from the host UHP eclogites, indicating that the former did not originate from partial melting of the latter. This is also supported by the distinct δ18O and εHf(t) values for the Triassic zircon from the granitic veins and eclogites. Instead, there are comparable isotopic compositions between the granitic veins and regional UHP granitic gneisses, indicating that the granitic veins would be derived from partial melting of the underlying UHP granitic gneisses. Group II granitic veins have higher εNd(t) values than Group I ones and the granitic gneisses by 3–5 εNd unit, indicating that the anatectic melts forming Group II granitic veins are in Nd isotope disequilibrium with their source rocks. This can be caused by the behavior of accessory minerals like apatite and epidote/allanite during crustal anataxis at different pressures. The contributions of apatite together with garnet can also account for the increased Hf isotopes in the anatectic zircon. Therefore, the accessory minerals play a key role in constraining the geochemical features of incompatible-element enriched granitic melts during the crustal anatexis under HP to UHP conditions.

Magma mixing in Early Jurassic granites in the Lesser Xing'an Range, NE China: Evidence from petrology, geochronology, and Lu–Hf isotopes

The contribution of the mantle to the formation of granites with mantle‐like or juvenile crustal properties having positive εHf(t) and εNd(t) values remains ambiguous. We have carried out petrologic, geochemical, and zircon U–Pb–Hf–O isotope composition analyses on the Woniuhe and Yuantoupiao granites and associated enclaves with such properties in the northern and southern Lesser Xing'an Range, NE China to determine the crust–mantle interactions involved in the petrogenesis of these host granites and enclaves. The two host granites and their enclaves have identical crystallization ages of ~180–175 Ma within analytical error. The petrological, mineral microstructural, geochronological, Hf–O isotopic, and whole‐rock geochemical data imply that mixing between felsic and mafic magmas occurred in both the Woniuhe and Yuantoupiao plutons. The primary magmas of the enclaves in the two plutons most likely are both mantle‐derived. Hence, mantle materials were directly involved in the genesis of both the Woniuhe and Yuantoupiao host granites through the emplacement of mantle magmas. Combined with the published petro‐geochemical data, our data indicate that large‐scale underplating and emplacement of mantle magmas following crustal growth can occur and may be responsible for the magmatism in the region in the Early Jurassic. This underplating and emplacement might be related to the subduction of the oceanic plate between the Songliao and Jiamusi Massifs beneath the former.

Petrography and geochemistry of Neoproterozoic charnockite–granite association and metasedimentary rocks around Okpella, southwestern Nigeria

The Neoproterozoic charnockite–granite association of Okpella intrudes metasedimentary and migmatite-gneiss complex rocks in the eastern Igarra Schist Belt, southwestern Nigeria. In order to unravel the complex processes involved in the formation and tectonic evolution of the Igarra Schist Belt during the Neoproterozoic, detailed field, petrographic and whole-rock geochemical study of the charnockite–granite association and metasedimentary rocks around Okpella was conducted. Published data on the metasedimentary rocks and Pan-African granitoids in different sectors of the belt were also compiled for detailed interpretations. The charnockites and granites of Okpella show primary magmatic mineralogy and geochemical characteristics. They are silicic (> 63 wt% SiO2), metaluminous to peraluminous, high-K calc-alkaline, ferroan, post-collisional granitoids. The garnet-biotite schist, calc-silicate gneiss and quartzite in the area are low–medium grade metasedimentary rocks. The mineralogy and geochemistry of the charnockites suggest that the charnockitic melt was derived from mafic lower continental crust through partial melting and assimilation-fractionation processes. The granites probably originated from mixed melts derived from lower- to mid-crustal tonalites-trondhjemites-granodiorites and/or subducted metagreywackes and mantle-derived magmas (probably the charnockitic melt and/or its progenitor). The granites and charnockites are coeval and were presumably emplaced during the post-collisional stages of the Pan-African Orogeny. The garnet-biotite schist, calc-silicate gneiss and quartzite represent metamorphosed immature to slightly mature sedimentary rocks, probably greywacke, marl and subarkose, respectively, that were sourced from intermediate–acid rocks which underwent low–moderate chemical weathering with minor contribution from recycled sediment sources and deposited in active continental environments. The Neoproterozoic evolution of the Igarra Schist Belt, therefore, involved deposition and infolding of sediments in active continental margin during the early Pan-African followed by upwelling of basaltic magma from the mantle which underplated and crystallized in the lower continental crust and was subsequently partially melted to generate the charnockitic and granitic melts through mantle-crust interaction during the late Pan-African.

Syn-exhumation melting of the subducted continental crust: Geochemical evidence from early Paleozoic granitoids in North Qaidam, northern Tibet

Syn-exhumation partial melting of the deeply subducted continental crust commonly occurs in collisional orogenic settings. The resulting felsic melts not only form granitoids, but can also modify the composition of the mantle wedge above continental subduction zones. This study investigates crustal anatexis and its role in granitoid formation and crust–mantle interaction during continental collision using new whole-rock geochemical, Sr–Nd–Hf isotope, and zircon U–Pb geochronological and Lu–Hf isotope data for early Paleozoic granitoids of the North Qaidam area of northern Tibet, a collisional orogen that contains ultrahigh-pressure (UHP) metamorphic rocks. Zircon U–Pb dating of the granitoids in this area yielded ages between 416 ± 6 Ma and 393 ± 2 Ma, coeval with the exhumation of the deeply subducted continental crust. This indicates that these granitoids formed as a result of syn-exhumation magmatism during the continental collision. Relict zircons in the granitoids yield concordant Neoproterozoic and early Paleozoic U–Pb ages that are consistent with the protolith and metamorphic ages of UHP metaigneous rocks in the orogen, respectively. The granitoids have calc-alkaline to high-K calc-alkaline compositions, with total alkali contents of 4.07–8.09 wt%. They are also metaluminous to weakly peraluminous with A/CNK ratios of 0.86–1.14. They are enriched in large ion lithophile elements (LILE) and depleted in high field strength elements (HFSE), yielding arc-like trace element patterns. They also have high (87Sr/86Sr)i ratios (0.7058–0.7195) and negative εNd(t) values (−7.1 to −1.4) and have whole-rock and zircon εHf(t) values of −2.8 to +2.7 and − 8.6 to +4.7, respectively. These data indicate that the granitoids are compositionally similar to the UHP metaigneous rocks in the collisional orogen, suggesting a genetic link. This in turn indicates that the granitoids in the study area formed from magmas generated by the partial melting during the exhumation of deeply subducted continental crust. The resulting granitic melts not only generated the syn-exhumation granitoids in this area, but also caused metasomatism associated with crust–mantle interaction in continental subduction channels.

The study of subduction channels: Progress, controversies, and challenges

The subduction channel is defined as a relatively thin and weak zone with coherent kinematics between the descending and overriding plates during subduction. The materials in the channel, showing the characteristics of melange, consist of low-density, low-viscosity, highly sheared metasediments and/or serpentinite-rich matrix with relatively rigid blocks. The channel materials flow downwards and upwards, forming a kind of channel convection. Based on geophysical and geological observations on active oceanic subduction zones, combined with numerical simulations, the subduction channel model was initially set to shallow depths With the development of numerical simulation technology and fossil subduction zone investigations, the subduction channel model was extended to depths of 80–100 km to describe deeper geodynamic environments of oceanic subduction zones and explore the exhumation of high to ultrahigh pressure metamorphic rocks. It is also extended to continental subduction zones in order to explain the exhumation of metamorphic rocks in different grades as well as fluid activity and crust-mantle interaction during continental subduction. Although there are a series of differences in material composition and slab/mantle interface property between the upper and lower plates, continental subduction channels exhibit many similarities in structure and dynamics to oceanic subduction channels. Existing studies have provided a fundamental understanding of the structural and dynamic characteristics of subduction channels, but many problems still need to be solved and clarified. In particular, the fundamental characteristics of subduction channels obtained by numerical simulation need to be confirmed and modified through geological investigations of fossil subduction channels.

Mantle influx compensates crustal thinning beneath the Cathaysia Block, South China: Evidence from SINOPROBE reflection profiling

The crustal architecture of the Cathaysia Block, South China, is a key element in understanding the late Mesozoic tectono-magmatic history of East Asia, and includes a spectacular (∼260,000 km2) basin and igneous province that forms a natural laboratory for unravelling the interplay of extension, magmatism and crust-mantle interaction. Here we analyze the whole crustal architecture of East Cathaysia through a ∼300-km-long SINOPROBE deep seismic reflection profile. This profile reveals that East Cathaysia is underlain by two seismically distinct terranes (i.e., the Wuyishan and Coastal terranes), separated by a low-angle, northwest-dipping set of mantle-penetrating reflectors interpreted as the Zhenghe-Dapu fault zone, which documents Neoproterozoic terrane amalgamation and Cretaceous extensional reactivation. The seismic image of the Coastal terrane shows large-scale low reflectivity truncated by arrays of high-reflective subhorizontal bands of layered reflectors, corresponding to an extension-related crustal architecture dominated by extensive Cretaceous magmatism. The layered reflectors are interpreted to represent mantle-derived mafic sills, which document significant magmatic underplating associated with asthenospheric upwelling beneath the terrane. This interpretation accounts for the localized distribution of Cretaceous magmatic rocks in the Coastal terrane. We suggest that the magmatic underplating added large volumes of mantle-derived material to the crust, compensating for the crustal thinning. Such a magma-compensating process might have formed an integral part of crustal reworking and growth in creating the magma-dominated crust and the broadly flat Moho of the Coastal terrane during the Cretaceous.

Crust-mantle geodynamic origin of ~2.7 Ga granitoid diversification in the Jiaobei terrane, North China Craton

The crust-mantle geodynamic origin of global late Archean granitoid diversification remains enigmatic. A series of early Neoarchean diversified granitoids were newly identified in the Jiaobei terrane of the North China Craton, including ~2749–2739 Ma Na-rich quartz dioritic, tonalitic, and trondhjemitic (DTTG) gneisses, and ~2718–2701 Ma K-rich quartz monzodioritic, granodioritic, and monzogranitic gneisses.The quartz dioritic and TTG gneisses commonly show transitional field relationships. They have low K2O/Na2O (0.19–0.59), and are characterized by decreasing MgO, FeOT, TiO2, P2O5, and CaO, changing Eu anomalies from negative to positive, and more fractionated REEs, with increasing SiO2. Both the quartz dioritic and TTG gneisses possess depleted zircon εHf(t2) values (+1.4 to + 7.0). Through quantitative petrogenetic modeling, a fractional crystallization model is invoked, i.e., the quartz dioritic gneisses were derived from partial melting of a depleted mantle source metasomatized by slab-derived melts, whereas TTG gneisses (a low silica and a high silica group) formed by different degrees of fractionation from the quartz dioritic magmas, with inferred liquidus minerals of hornblende, clinopyroxene, plagioclase, and apatite. In comparison, the K-rich granitoids show higher K2O/Na2O (0.69–2.87), with some containing euhedral muscovites. The quartz monzodioritic and granodioritic gneisses have higher MgO and Mg#, but lower Fe* (FeOT/(FeOT + MgO)) than the monzogranitic gneisses. The zircon εHf(t2) values (-0.5 to + 5.6) are lower than those of DTTG gneisses. It is suggested that these K-rich granitoids formed by partial melting of either a metasomatized mantle source (with recycled sediments) or metasedimentary rocks at diverse crustal levels.Early Neoarchean crust-mantle interactions of the Jiaobei terrane are reconstructed, including (1) earlier slab-mantle wedge interactions under a continental marginal arc, forming quartz dioritic and the derivative tonalitic and trondhjemitic rocks; (2) ongoing convergence led to high relief of the arc mountain, facilitating erosion and reworking/recycling of sediments; and (3) partial melting of coupled crust-mantle system following accretion between the above arc system and regional ~2.9 Ga or older continental nucleus generated different K-rich granitoids. It is emphasized that the construction and erosion of thickened arcs are prone to the formation of both high pressure TTGs and K-rich granitoids, leading to the late Archean granitoid diversification globally.