Subcontinental Lithosphere Research Articles

Geochemistry of Mafic Rocks From the Nagrota–Kathindi Section, Himachal Pradesh, Northwestern Himalaya: A Probable Example of Plume–Lithosphere Interaction

ABSTRACTThe Northwest Himalayan region has a record of several phases of mafic magmatic activity spanning from Precambrian to Cenozoic in a dynamic tectonic setting. Here, we studied detailed petrography and new whole‐rock geochemistry of mafic volcanic and dykes from the Nagrota–Kathindi Section (NKS), Himachal region of the NW Himalaya, to understand the petrogenesis and possible tectonic setting. Both rock types have comparable mineralogical compositions (clinopyroxene + plagioclase + actinolite‐tremolite + chlorite + iron oxides ± hornblende ± epidote ± quartz ± carbonates) overprinted by greenschist to lower amphibolite facies metamorphism. The mafic volcanic and dykes of NKS exhibit subalkaline basalts to basaltic andesites and a typical tholeiite compositional character. The chondrite‐normalized rare earth element pattern exhibits similar LREE‐enrichment and strong HREE‐fractionation, whereas primitive mantle‐normalized multi‐element patterns show pronounced LILE‐enrichment of Rb, Ba, Th, LREE, and HFSE depletion of Nb, K, P, and Ti. The Zr–Y–Nb–Th relationships indicate that both rock types were derived from the plume source, whereas low Nb/La (< 1), similar high large‐ion lithophile element concentrations, and pronounced negative Nb, Zr, P, and Ti anomalies suggest that components other than mantle plume must have been involved in the generation and evolution of both rock types, that is, most likely plume and subcontinental lithosphere mantle (SCLM) interaction. The genesis of parent magma for the NKS volcanic and dykes was derived by 4%–6% and 10%–20% partial melting from the spinel + garnet lherzolite stability field. The majority of the studied samples correspond to spinel + garnet peridotite melting on (Gd/Yb)N versus CaO/Al2O3 diagram, thereby corroborating residual garnet in the mantle restite. All the basalts and dykes from the NK section did erupt/intrude in an intracontinental rift setting based on geochemical discrimination. The key petro‐tectonic processes attributed to the formation of these rocks are as follows: (i) the melting of the ascending plume by adiabatic decompression; (ii) the partial melting of this plume–SCLM source in the melting regime, which produces basaltic magma with a tholeiitic composition; and (iii) the release of heat that provides the thermal condition for melting of SCLM and interaction between upwelling mantle plume and subduction metasomatized SCLM.

Molybdenum isotope evidence for recycled oceanic crust in the mantle sources of continental intraplate mafic lavas, Emeishan Large Igneous Province

The chemical and lithological heterogeneities of Earth's mantle in oceanic settings have long been attributed to the presence of recycled materials such as subducted crust, oceanic or subcontinental lithosphere mantle and sediment, as evidenced by Mid-Ocean Ridge Basalts (MORB) and Ocean Island Basalts (OIB). However, the extent to which recycled materials contribute to mantle heterogeneity in continental settings remains uncertain. Here we present systematic Sr-Nd-Pb-Mo isotope data for the Pingchuan picritic porphyries and associated mafic rocks (basalts and gabbros) in the central Emeishan Large Igneous Province, southwestern China, to resolve this issue. In contrast to the gabbro samples that show continental crust contamination in variations of δ98/95Mo (relative to NIST SRM 3134, −0.35 ± 0.03 to 0.03 ± 0.02 ‰), the Pingchuan picritic porphyries and basalts show a large range of δ98/95Mo (−0.32 ± 0.01 to −0.15 ± 0.06 ‰) and broadly negative trend between δ98/95Mo and Ce/Mo ratios, which cannot be explained by post-magmatic alteration, crustal contamination, fractional crystallization, olivine accumulation, metasomatism or fluid-activity in the mantle source. They were most likely derived from incongruent melting of a heterogeneous mantle source, i.e., a deep mantle with chondritic-like δ98/95Mo (−0.15 ± 0.01 ‰) containing recycled oceanic crust (+ sediment) with light δ98/95Mo (< −0.32 ‰). The basalts display low δ98/95Mo (−0.32 ± 0.01 to −0.24 ± 0.04 ‰), high 87Sr/86Sri (0.7046 to 0.7059), TiO2, Sm/Yb, Dy/Yb and Ce/Mo, pointing to characteristics of melts produced by MORB-type eclogite (+ sediment) at relatively low degrees of partial melting. With increasing degrees of partial melting, melts produced by peridotite (87Sr/86Sri = 0.7035 to 0.7043, chondritic-like δ98/95Mo = −0.18 ± 0.03 to −0.15 ± 0.06 ‰) come to dominate, resulting in low TiO2, Sm/Yb, Dy/Yb and Ce/Mo of the Pingchuan picritic porphyries. From a global perspective, continental basalts also plot on the negative trend defined by the OIB datasets (δ98/95Mo vs. Ce/Mo). As such, Mo isotope may provide a robust tracer of recycled oceanic crust in shaping mantle heterogeneities that occur in both oceanic and continental settings.

Post-collisional calc alkaline-adakites from a metasomatized subcontinental lithospheric mantle: Sr-Nd isotopes and bulk-rock geochemistry of Miocene volcanic rocks from the Varzaghan area, NW Iran

ABSTRACT The NW Iran is located at the western termination of the Alborz Mountain range and continuation of the Sanandaj-Sirjan magmatic-metamorphic zone at the NW. The volcanic rocks in the Varzaghan area are distributed over different compositional ranges from high-K basalt, andesite, dacite, and trachyandesite, dacite, to rhyolite with adakitic signatures. Chondrite-normalized REE diagrams show flat MREE and HREE patterns for calc-alkaline volcanic rocks and enrichment in LREE, indicating a garnet-free source. Adakitic rock sample chondrite-normalized REE patterns are relatively steep with significant enrichment in LREE and depletion in HREE, indicating a garnet-bearing source or garnet fractionation process. The arc trace element signatures, narrow range of initial87Sr/86Sr(i) = 0.70425–0.70487, different initial143Nd/144Nd(i) = 0.51264–0.51286 and the presence of adakitic rocks reveal that the volcanic rock sources were not the same and can be related to the partial melting of metasomatized mantle and lower continental crust sources. Slab break-off and upwelling of hot asthenosphere in a post-collisional tectonic regime resulted in the melting of the subcontinental lithosphere and lower continental crust which produced calc-alkaline-adakitic volcanic rocks in the Varzaghan area. The main petrological processes involved in the volcanic rocks petrogenesis were fractional crystallization and crustal assimilation.

Remobilization of carbon in the lithospheric mantle during decratonization

The sub-continental lithosphere mantle (SCLM) contains considerable amounts of carbon and is responsible for significant CO2 emissions via magmatism during major heating and rifting events. However, the role of SCLM in Earth's carbon cycle during other geological processes, such as decratonization, a process of major removal and replacement of cratonic SCLM, remains poorly understood. The North China Craton is an archetype of Archean craton having been decratonized during the Mesozoic with extensive magmatism developed. Here we show that the primary melt of Early Cretaceous Yixian basalts contained abundant CO2 (0.6 wt%), through investigating melt inclusions trapped in olivine crystals. The melt inclusions are alkalic and chemically distinct from their sub-alkalic host basalts, which could be explained by that the primary alkalic melt was reactively modified during magma ascent in the SCLM. The primary melt has geochemical characteristics consistent with partial melting of a veined carbon-rich amphibole clinopyroxenite source in the SCLM formed metasomatically during circum-craton subduction events. The findings highlight that the carbon introduced into the cratonic SCLM during subduction could be subsequently reactivated and transferred to the crust and atmosphere through decratonization-related magmatism, which might represent a major process regarding the SCLM in regulating the terrestrial carbon cycle.

Gondwanan flood basalts linked seismically to plume-induced lithosphere instability

Delamination of the continental lithospheric mantle is well recorded beneath several continents. However, the fate of the removed continental lithosphere has been rarely noted, unlike subducted slabs reasonably well imaged in the upper and mid mantle. Beneath former Gondwana, recent seismic tomographic models indicate the presence of at least 5 horizontal fast-wavespeed anomalies at ~600 km depths that do not appear to be related to slab subduction, including fast structures in locations consistent with delamination associated with the Paraná Flood Basalt event at ~134 Ma and the Deccan Traps event at ~66 Ma. These fast-wavespeed anomalies often lie above broad slow seismic wavespeed trunks at 500 to 700 km depths beneath former Gondwana, with slow wavespeed anomalies branching around them. Numerical experiments indicate that delaminated lithosphere tends to stagnate in the transition zone and mid-mantle above a mantle plume where it shapes subsequent plume upwelling. For hot plumes, the melt volume generated during plume-influenced delamination can easily reach ~2 to 4 × 106 km3, consistent with the basalt eruption volume at the Deccan Traps. This seismic and numerical evidence suggests that observed high-wavespeed mid-mantle anomalies beneath the locations of former flood basalts are delaminated fragments of former continental lithosphere, and that lithospheric delamination events in the presence of subcontinental plumes induced several of the continental flood basalts associated with the multiple breakup stages of Gondwanaland. Continued upwelling in these plumes can also have entrained subcontinental lithosphere in the mid-mantle to bring its distinctive geochemical signal to the modern mid-ocean spreading centers that surround southern and western Africa.

Pliocene to Pleistocene REE-P Metasomatism in the Subcontinental Lithosphere beneath Southeast Asia—Evidence from a Monazite- and REE-rich Apatite-bearing Peridotite Xenolith from Central Vietnam

Abstract Mantle rocks usually contain rare earth elements (REEs) in very low concentrations. Here, we document an occurrence of monazite associated with REE-rich apatites in a carbonate-bearing wehrlite xenolith from central Vietnam. The xenolith displays an equigranular matrix of rounded olivine grains while texturally primary orthopyroxene, clinopyroxene and spinel are notably absent. Scattered within the olivine matrix, two types of domains are present: domain-I contains blocky clinopyroxene grains within a matrix of quenched silicate melt and is associated with a second generation of olivine, small euhedral spinel, and rare grains of carbonates. Domain-II contains irregularly shaped patches of carbonate associated with silicate glass, secondary olivine, spinel, and clinopyroxene. Monazite and apatite occur only in domain-I: very small rounded to elongate monazite I grains are included in primary olivine, partly crosscut by fine glass veinlets, monazite II as large grains up to 300 × 200 μm in size and monazite III as small euhedral and needle-like crystals in silicate glass pools. Apatite I forms lath-shaped to rounded crystals up to 200 × 50 μm in size, whereas apatite II is present within silicate melt pools where it forms euhedral needle-like to equant grains. Monazites show compositional variation mainly with respect to ∑REE2O3 (63–69 wt %) and ThO2 (1.1–5.3 wt %) and only minor variations in P2O5 (29–32 wt %), SiO2 (&amp;lt;0.05–0.4 wt %), and CaO (0.2–0.4 wt %). Apatites are characterized by strongly variable and high REE2O3 and SiO2 contents (4–27 wt % ∑REE2O3, 0.6–6.8 wt % SiO2) as well as with significant Na2O (0.3–1.5 wt %), FeO (0.1–1.8 wt %), MgO (0.2–0.6 wt %) and SrO (0.2–0.9 wt %) contents. F and Cl contents are in the range 1.9–3.0 wt % and 0.2–0.8 wt %, respectively. The textures observed in this wehrlite xenolith are thought to be the result of an interaction of depleted (harzburgitic) mantle with cogenetic silicate and carbonatite melts formed by fractionation-driven immiscibility within a parental SiO2 undersaturated melt characterized by high P, CO2, and REE contents. The immiscibility occurred in the shallow subcontinental lithosphere at T of 700–800 °C and a depth of ~30 km and the melt–rock interaction occurred in two successive and most likely nearly simultaneous events: an initial stage of metasomatism was triggered by the P-REE-CO2-rich agent with low aH2O resulting in the co-precipitation of carbonates as patches and along micro-veins and of phosphates in a peridotitic assemblage. A second stage is characterized by pervasive infiltration of an alkali-rich basaltic melt into the carbonate + phosphate-bearing assemblage. Based on 232Th and 208Pb contents of monazite, a young age of ~2 Ma can be calculated for the timing of the monazite-forming metasomatic imprint. Based on 39Ar-40Ar extrusion ages of the xenolith-hosting alkali basalts of 2.6–5.4 Ma, this indicates that both carbonatite and basaltic melt infiltration must have occurred no more than a few hundred thousand years before extraction of the xenolith to the surface.

Decoding subcontinental lithosphere processes: The key role of fractional crystallization in Central Andes monogenetic volcanism - Insight from El Negrillar volcanic field, Chile

Subduction zones are complex geodynamic settings in which volcanic arcs form due to the interaction between subducting slabs, water, and continental materials. Investigating this extensive Earth recycling system offers valuable insights into the exchange of materials between the crust and mantle. While previous research has primarily focused on analyzing polygenetic volcanoes to explore this interaction, monogenetic volcanic fields (MVF) present an opportunity to delve into the mechanisms operating within the upper and lower mantle. Monogenetic volcanoes are a class of volcanoes that are typically smaller in size but showcase a range of eruptive styles and compositions. These volcanoes can be found in various locations along the arc, including the main Central Volcanic Zone (CVZ) (arc-front volcanoes), and even far from the tectonic plate boundaries in the back-arc region (intraplate volcanism).We carried out a detailed geochemical study of El Negrillar (EN) MVF, located in the Central Andes main-arc, on the Altiplano-Puna Plateau's southwestern border. The volcanic field has 35 eruptive centers and a total of 86 eruptive phases originating from its three clusters: Northern, Central, and Southern. EN magmas range in composition from basaltic andesite to dacite and have produced over 6.8 km3 (DRE) of lava and pyroclastic material, which makes it the most voluminous volcanic field in the Central Andes, and therefore, an excellent example to study MVF in arc-systems. We have conducted an extensive study encompassing major and trace elements, as well as isotopic analysis using whole rock Sr-Nd-Pb. We also present an analysis of our results with available geochemical data from the back-arc and main-arc volcanoes, including other monogenetic volcanoes near EN (<80 km, EN's neighbors). The results revealed that EN magmas and their neighbors are strongly related and exhibit a clear similarity in major and trace element compositions and isotopic ratios, indicating a common source and origin of their melts. Our comparison across the arc surprisingly reveals similarities between EN and back-arc monogenetic volcanism. Both regions indicate geochemical signatures that do not support melting via slab metasomatism, typical of subduction zones. Instead, they show low Th/Ce (<0.1 ppm) and Ba/La (<30 ppm) and high Ce/Pb (>6 ppm), Ba/Th (>100 ppm), high 143Nd/144Nd > 0.51240, and La/Yb ratios at a given La/Sm compared to the main-arc polygenetic volcanoes. Interestingly, EN also exhibits adakite-like signatures with increasing SiO2 content (high Sr > 600 ppm, Sr/Y > 40, and high La/Yb > 20), a term used for igneous rock suites with chemical characteristics that are identical to those of adakites but produced through petrogenetic processes that do not include a slab component. We explore how fractional crystallization and crustal assimilation participate in the development of monogenetic volcanoes, and our findings underscore the significant influence of fractional crystallization on the monogenetic volcanic processes in the Salar de Atacama region. The geochemical changes observed in this area can be attributed to a combination of varying degrees of fractional crystallization within magmas generated through the partial melting of a garnet-enriched environment, which could be primary lower crust or partial melting of a peridotite mantle that has later equilibrated with garnet-bearing crustal melts.

Survived carbonates in orogenic dunites from recycling of subduction-related sediments

The carbon recycling between the Earth's surface and its interior is accomplished through plate subduction and magmatic processes. The orogenic mantle peridotites document multi-stage metasomatism in the subduction zone and play a significant role in revealing the material transportation and migration between the Earth's different spheres. However, how the migration process of carbon-bearing species influenced the subcontinental lithosphere mantle remains a mystery. To address these issues, we conducted detailed petrography, whole-rock and mineral geochemical compositions, and Sr–C–O isotope studies on two types of peridotites (silicate melt pocket-bearing and carbonate melt pocket-bearing dunites) of the Maowu ultramafic massif in the Dabie orogen. The silicate-melt pockets were captured earlier than the carbonate-melt pockets, and were derived from a large proportion of silicate melts during the formation of garnet pyroxenites. The subsequent carbonate metasomatism was mainly divided into two stages: the stage 1 reflects the reaction between dolomite melts and orthopyroxenes to generate clinopyroxenes with high CaO content and Mg# value (> 92); the stage 2 is the injection of Ca-rich fluids into dunites, forming dolomite veins and volatile-rich assemblages of tremolite, barite and monazite. The clinopyroxenes display high 87Sr/86Sr (0.7079–0.7097) and (La/Yb)N ratios, low Ti/Eu ratios, and different enrichment in Th, U, Sr and light rare earth elements (LREEs), indicating a carbonate metasomatic origin. The equilibrium melts of them are similar to sedimentary limestones with positive Sr anomaly and depleted high field strength elements (HFSEs). The Sr-O isotope mixing simulation shows a mixing of 80–90% carbonate sediments with the upper mantle peridotite. The C-O isotopes of the carbonate minerals have similar variation ranges (δ13CV-PDB: 19.7–15.6 ‰, δ18OV-SMOW: 18.1–28.1 ‰) within the range of sedimentary organic carbon. The 87Sr/86Sr ratios of limestone reservoir (0.708–0.709) suggest the addition of sedimentary carbonates into the mantle wedge at ∼370–500 Ma. Therefore, we propose that the sedimentary carbonates and organic carbon migrated into the deep mantle-wedge through the subducted Tethyan-Ocean slab in the Early Paleozoic, which experienced complex metasomatism and formed as carbonate minerals in the Maowu dunites to achieve the carbon recycling between the sedimentary carbon reservoir and the mantle wedge beneath the North China Craton.

Continental Deep Subduction Versus Subduction Cessation: The Fate of Collisional Orogens

AbstractThe contrasting fates of collisional orogens, that is, continental deep subduction or subduction cessation, are widely recognized by petrological, paleomagnetic and geophysical observations. However, the mechanisms of such different collisional modes, especially the dynamics of continental deep subduction, are controversial. In this study, we integrate the phase transition‐induced density evolutions into a thermo‐mechanical numerical model. Combing the systematic petrological‐thermo‐mechanical models with force balance analysis, we find that the high metamorphic transformation degree, mildly depleted mantle composition of the subcontinental lithosphere, and a long preceding oceanic slab, increase the driving force for continental deep subduction. Additionally, the rheologically weak continental crust and asthenospheric mantle decrease the resistance force and promote deep subduction. Otherwise, the continental subduction cessation mode is favored. The calculations of slab negative buoyancy indicate that the phase transition‐induced metamorphic densification of the subducted continental crust and the mildly to moderately depleted lithospheric mantle can provide a great slab pull force to sustain the continued continental deep subduction; however, the positive buoyancy of highly depleted Archean lithospheric mantle impedes deep subduction and causes subduction cessation. Based on systematic numerical models, we also evaluate the crustal mass balance or deficit in continental collisional system, which indicates that ∼12%–47% of pre‐collisional felsic crust could be subducted deeply with the sinking slab in the regime of continental deep subduction. In contrast, the recycled felsic crust is negligible in the regime of subduction cessation. Thus, the different modes of continental collision play a crucial role in the global crustal recycling and related mantle heterogeneities.

Deciphering mantle heterogeneity associated with ancient subduction-related metasomatism: Insights from Mg-K isotopes in potassic alkaline rocks

Melts and fluids derived from subducting/subducted oceanic lithosphere contribute significantly to mantle compositional heterogeneity. Historically metasomatized lithospheric mantle modified by varying materials, especially volatile-rich slab fluids, has been understood to be crucial for the formation of giant ore deposits, thus providing potential prospective areas for mineral exploration. However, it is not straightforward to identify superimposed effects of slab melts and fluids on the sub-continental lithosphere (SCLM) that has experienced long-lived and/or multiple metasomatism. In this study, we present a novel study using Mg and K isotopes, together with radioactive isotopes on mafic post-collisional alkaline potassic rocks (PARs) with the aim of deciphering varying agents contributing to the heterogeneous metasomatism of the SCLM beneath the southeastern Tibetan Plateau. The PARs, formed at ∼36 Ma, are characterized by high abundances of large ion lithophile elements (LILE) but depleted in high field strength elements (HFSE) with high Rb/Sr ratios, indicating a phlogopite-bearing mantle source. We find that features of light Mg (δ26Mg = −0.52 to −0.25‰) and heavy K (δ41K = −0.56 to +0.08‰) isotopes observed in the PARs are largely inherited from their mantle sources with negligible effects of kinetic fractionation, fractional crystallization and crustal level processes. Our modelling reveals that the notable divergence of Sr–Nd–Pb–Mg–K isotope compositions between PAR suites in different tectonic blocks are best understood as resulting from varying contributions of slab-derived components into the overlying mantle lithosphere, including sediment-derived melts, ocean crust-derived supercritical fluids, and marine carbonates. Particularly, their high Th/U ratios and negative correlations between δ26Mg and δ41K values suggest that slab-derived supercritical fluids can dissolve and transport Mg-rich (dolomitic) carbonate into the mantle wedge.

Time-space variations in the East African Rift magmatism: the role of different mantle domains

The East African Rift System (EARS) is the classic example of an active continental rift where extensional tectonics and lithospheric thinning have been closely associated to the generation of large volumes of magmas and represents the environment with the largest range of erupted magma types all over the world. The geochemical signature of erupted magmas testifies the involvement of different mantle domains and depths (i.e., subcontinental lithosphere, asthenosphere and deeper mantle sources). Our aim is to investigate the variable contribution of different mantle domains in the genesis of the EARS magmas through space and time, considering not only the geochemical signature of erupted magmas but also the geochemical message of mantle xenoliths. The main goal is to provide a large-scale view of the common process driving the origin of magmas in the EARS beyond the local peculiarities linked to specific settings. To this aim, we screened an exhaustive geochemical database of basalts and mantle xenoliths from the EARS, and we report original trace element and Sr-Nd isotope data of new samples collected from the Main Ethiopian Rift and Turkana depression. The data were subdivided according to spatial and temporal criteria. From a spatial point of view, the samples were ascribed to five groups, namely: Afar, Ethiopia, Turkana, Eastern Branch, and Western Branch; from a temporal point of view, the magmatic activity of the EARS was subdivided into three main temporal intervals: 45-25 Ma, 25-10 Ma and 10-0 Ma. The geochemical and radiogenic isotope (Sr, Nd, Pb) signature of the selected basalts denotes the variable contributions of a mantle plume, a more depleted asthenospheric mantle (DMM), and different SubContinental Lithospheric Mantle (SCLM) domains, depending on their temporal and spatial distribution. The geochemistry of the selected basalts shows a marked correspondence with the compositional heterogeneity of mantle xenoliths, whose isotopic systematics (Sm-Nd, Re-Os) indicates the formation of the local SCLM in the Archean and during the Pan-African orogeny. Both SCLM domains contributed significantly to magma genesis in the Western Branch (whose signature points towards a contribution of the Pan-African lithosphere) and Eastern Branch (which is also affected by Archean SCLM domains) magmas. The contribution of the SCLM generally increases with time, possibly related to an increase of the geothermal gradient in response to the arrival and flattening of the plume head at the base of the lithosphere and later extension, thinning and shallower melting. Our interpretation supports a pivotal role of the different SCLM domains in magma genesis that is able to fully explain the large compositional heterogeneity of the EARS basalts and represents a reasonable alternative to the putative presence of multiple mantle plumes or a heterogeneous mantle upwelling.

A Mantle Plume Connection for Alkaline Lamprophyres (Sannaites) from the Permian Tarim Large Igneous Province: Petrological, Geochemical and Isotopic Constraints

AbstractThe origin of lamprophyres associated with large igneous provinces (LIPs) remains controversial, particularly whether they are derived by direct melting of mantle plumes, or from previously metasomatized domains in thermally perturbed subcontinental lithosphere. Here, we report the petrological and geochemical characteristics of a recently identified suite of alkaline lamprophyres (sannaites) that represent the final pulse of magmatism in the Permian Tarim LIP in NW China. The sannaites display porphyritic texture with phenocrysts of olivine, clinopyroxene, hornblende, phlogopite, and titanomagnetite in a groundmass of plagioclase, clinopyroxene, nepheline, hornblende, biotite, and titanomagnetite with minor pyrite and apatite. Carbonate ocelli and almost pure albite in the groundmass are interpreted to have crystallized from immiscible carbonate and hydrous fluids, respectively, produced by late-stage magmatic segregation. The rocks show low to moderate SiO2 (37.7–49.3 wt.%) and MgO (2.74–9.91 wt.%), together with high Fe2O3T (up to 22.7 wt.%) and alkali contents (up to 9.02 wt.% Na2O + K2O). They are characterized by high incompatible element abundances, especially a marked enrichment in large-ion lithophile elements (Rb and Ba) and light rare-earth elements (e.g. La and Ce) relative to P and high-field-strength elements (e.g. Ti). They show a relatively restricted range of δ66Zn values between 0.22‰ and 0.46‰ with an average of 0.37 ± 0.04‰ (2SE, n = 10), which is marginally heavier than that of MORBs (0.27 ± 0.05‰). Their (87Sr/86Sr)t values range from 0.7035 to 0.7061, εNd(t) from −0.97 to +5.62, and δ26Mg from −0.36‰ to −0.17‰ (n = 8), the latter being consistent with those of global MORBs. Based on the correlation between Zn isotopes and TiO2–FeO concentrations, we infer that the heavy Zn isotopes in some of the sannaites resulted from fractional crystallization of Fe–Ti oxide minerals. The whole rock geochemical features of these rocks (negative K anomalies and enrichment in large-ion lithophile elements) and rhyolite–MELTS simulations suggest that the primary magmas of the sannaites were derived from an amphibole-bearing enriched lithospheric mantle source. Metasomatism and related formation of amphibole-bearing metasomatized mantle may be linked to sublithospheric melts/fluids derived from the Tarim plume in the earlier stages of plume activity, rather than slab-derived fluids or carbonate melts as suggested in previous studies for other alkaline mantle-derived magmas. Partial melting may have been triggered by the thermal input from the Tarim plume during a later stage. This study suggests that exotic, alkali-rich magmas can be produced during the multi-stage evolution of large mantle plumes, involving complex cycles of lithospheric mantle metasomatism and later melting of previously enriched domains.

Geochemistry and zircon U–Pb ages of early Ordovician syenites from the Inexpressible Island, Antarctica and tectonic implications

The Ross Orogenic Belt is in the Antarctica Transantarctic Mountains. North Victoria Land Granite Harbour Intrusive complex (GHI) records the tectonic-magmatism evolution of Ross orogeny. Extensively developed post-collisional granites around this margin of early Paleozoic magmatism can provide insights into the growth of continental crust through accretionary orogenesis. We provide geochemical and geochronological data from syenites from Terra Nova Bay, north Victoria Land in order to constrain its tectonic evolution and setting. The syenite belongs to the potassium-alkaline, calc-alkaline series and is characterized by high concentrations of rare Earth elements and large ion lithophile elements (LILE), and low content in high field strength elements (Nb, Ta, P, Ti). The petrographic and geochemical signatures show a possible island-arc granite affinity. LA-ICP-MS zircon U-Pb dating results suggest that the Inexpressible Island syenite was emplaced at ca. 471.8 ± 1.8 Ma and 477.3 ± 1.7 Ma, respectively. Zircon εHf(t) values range from −7.4 to −9.1; average −8.2 and whole-rock εNd (t) values range from −8.5 to −10.3, indicating that formed by the partial melting of the lithospheric mantle enriched with subduction slab fluids and subcontinental lithosphere. Whereas, the syenite has a strong positive Eu anomaly and a positive Sr anomaly, suggesting that plagioclase cumulate crystallization occurred in the magma source area. Furthermore, through integration with previous studies, we suggest that syenite is a result of the melting zone of an older previously subduction enriched layer of the subcontinental lithospheric mantle (SCLM). To enable syenite emplacement we suggest a tectonic-magmatic model that invokes alternating phases of extension and contraction in the overriding plate. Finally, we report the youngest age of (post-orogenic) magmatism occurred during extension in the overriding plate ca. 478–471 Ma.

A Jurassic volcanic passive margin in Iran and Turkey

AbstractBroadly similar Early to Middle Jurassic stratigraphic sequences including bimodal igneous rocks of the Sanandaj–Sirjan Zone of Iran and the Sakarya Zone of Turkey suggest that these formed in a common tectonic setting in an extensional basin that evolved from a terrestrial magmatic rift to a marine shelf and passive continental margin. Whole‐rock chemistry and Sr–Nd isotope signatures indicate derivation of mafic melts from partial melting of the subcontinental lithosphere. Decompression associated with extension led to 5%–30% partial melting of spinel–garnet lherzolite with minor involvement of continental crust, producing tholeiitic to transitional basaltic magma. Extensional basins inverted during the Mid‐Late Jurassic. These relationships suggest the Early to Middle Jurassic formation of a volcanic rifted margin on the SW Eurasian margin, similar to that of offshore Norway.

Trends and rhythms in carbonatites and kimberlites reflect thermo-tectonic evolution of Earth

Abstract Earth's thermo-tectonic evolution determines the way the planet's interior and surface interact and shows temporal changes in both trends and periodic rhythms. By sampling the subcontinental lithospheric mantle that represents the interface between the convecting mantle and the crust, carbonatite and kimberlite should be ideal rock types for documenting this evolution. The first-order secular rise of kimberlites over time has been noted by researchers, but there is much debate over how to interpret this trend, and their second-order variability has received less attention. We compiled a comprehensive global carbonatite database and compared it with an existing kimberlite one. We find that the numbers of carbonatites and kimberlites have similar increasing secular trends, with accelerated growth after ca. 1 Ga, and show the same periodic rhythms that have been synchronized to the supercontinent cycle since ca. 2.1 Ga. We link these trends and rhythms to the long-term change of Earth and the supercontinent cycle, both of which have altered the temperature of, and the subduction-recycled volatile flux into, the subcontinental lithosphere. Such consistent records in carbonatite and kimberlite behavior provide critical evidence for the synchronous thermo-tectonic evolution of the entire subcontinental lithosphere.

Tracing Recycled Crustal Materials in the Subcontinental Lithospheric Mantle Using Thallium Isotopes

AbstractHere we report the first set of thallium (Tl) isotope data in alkaline rocks from the North China Craton to constrain the nature of recycled materials in the metasomatized subcontinental lithospheric mantle. Samples from the Hekanzi and Saima alkaline complexes display Tl isotope compositions (ε205Tl) identical to the present‐day upper mantle and continental crust, suggesting that neither the recycled low‐temperature altered oceanic crust nor the pelagic sediments were involved in their sources. The volcanic rocks from the Liaodong‐Jinan region, whose source was previously proposed to contain recycled low‐temperature altered oceanic crust, also display modern mantle‐like Tl isotope composition, suggesting a significant Tl‐loss in the recycled oceanic crust during the subduction due to the dehydration process. Our Tl isotope data provide complementary constraints to the Sr‐Nd‐Hf‐O isotopes on the nature of metasomatic agent in the subcontinental lithospheric mantle.

Batholith recorded mesozoic multistage tectonic evolution of the South china block: A case study of the guandimiao intrusions

South China is a well-known grand felsic igneous rocks province. However, it is still controversial and not well understood whether the Mesozoic tectono-magmatic pattern is dominated by the subduction of the paleo-Pacific oceanic plate. In this study, we address this question by concentrating on the long-term evolutionary Guandimiao batholith, which has complex lithofacies with different formation ages and can be a superb record of the Mesozoic tectonic evolution in South China. Geochronologically, four stages of magmatism can be identified combined with previous reports: granodiorite (G1, 239 Ma), biotite monzogranite (G2-1) and two-mica monzogranite (G2-2) (230–203 Ma), granite porphyry (G3, 211–190 Ma), and lamprophyre (L4, 121 Ma). G1 and G2-1 have an affinity with I-type granite and were derived from metabasaltic to metatonalitic sources, whereas G2 and G3 show S-type granite characteristics and were derived from the para-metamorphic basement of the Cathaysia block. The L4 was derived from partial melting of garnet and spinel lherzolite and underwent mixing between Mesoproterozoic pelagic and/or terrigenous sediments and the subcontinental lithosphere mantle (SCLM) of South China. The granitoids of the Guandimiao batholith underwent intensely fractional crystallization of feldspar, Ti-bearing minerals, allanite and monazite. The zircon U–Pb dating of L4 in the Guandimiao batholith completely records the six stages of pre-Mesozoic tectonic events in the SCB. During the Mesozoic, the main body of the Guandimiao batholith (G1, G2-1 and G2-2) recorded the closure of the paleo-Tethys Ocean in the Triassic and the subsequent regional extension of the postcollision. G-3 and L4 of the Guandimiao batholith documented the transition of tectonic and dynamic regimes in the early Yanshanian and the rollback and steep subduction of the paleo-Pacific Ocean in the late Yanshanian.

Decoupled Zn-Sr-Nd isotopic composition of continental intraplate basalts caused by two-stage melting process

Ocean island basalts (OIBs) with Zn isotopic ratios higher than the normal mantle (δ66Zn = 0.17 ± 0.08‰) or mid-ocean ridge basalts (MORBs; δ66Zn = 0.27 ± 0.06‰) generally also have an enriched Sr-Nd isotopic signature, suggesting carbonate-bearing eclogites, whose protolith is inferred to be subducting altered oceanic crust, in their mantle source. On the contrary, continental intraplate basalts with high δ66Zn usually show depleted Sr-Nd isotopic signatures (i.e., decoupled Zn-Sr-Nd isotopic composition). To elucidate the origin of the decoupled Zn-Sr-Nd isotopic composition in continental intraplate basalts, we report the discovery of both coupled and decoupled Zn-Sr-Nd isotopic data for a suite of Cenozoic continental intraplate basalts from the Zhejiang province, Southeast China. These basalts display clear spatial and temporal geochemical variations, with early-stage inland low-silica samples presenting moderately enriched Sr-Nd isotopic signatures and high δ66Zn (coupled Zn-Sr-Nd isotopic composition, similar to OIBs), and later-stage coastal high-silica samples that display a pronounced δ66Zn decrease with increasing SiO2 and 87Sr/86Sr and with decreasing alkali contents and 143Nd/144Nd (decoupled Zn-Sr-Nd isotopic composition). The early-stage basalts with coupled high Zn-Sr-Nd isotopic signatures are also more enriched in incompatible elements than any other basalts from eastern China reported so far. We explain the spatial and temporal geochemical variations of these basalts as the result of two main melting events: 1) the low-silica early-stage magmatism mostly occurs inland and results from high-pressure partial melting of a carbonated eclogite-bearing asthenospheric mantle. Because of the presence of a thick lithosphere limits the melting of the depleted mantle component, the signature of the Zn-Sr-Nd isotopically enriched, and more fusible carbonated eclogite is preserved. 2) At the later stage, magmatism mostly occurs on the coast where the subcontinental lithosphere is thinner. Hence, decompression melting progresses to shallower depth, resulting in an increase of the contribution from the depleted peridotite matrix and a dilution of the signal from the isotopically enriched fusible component. Further upwelling and in-situ melting at the base of the subduction-modified sub-continental lithospheric mantle (SCLM) explains both the decoupled Zn-Sr-Nd isotopic signature of the coastal basalts and their major and trace element variability. We further propose that decompression melting is driven by small-scale convection resulting from variations of lithospheric thickness. Our data highlight the importance of dynamic melting of carbonated eclogite-bearing asthenosphere and subsequent lithospheric melting in preservation and destruction of the coupled enriched Zn-Sr-Nd isotopic signature of carbonated eclogite component and generation of the apparent decoupled Zn-Sr-Nd isotopic signal commonly observed in continental intraplate basalts.

CO2 storage in the Antarctica Sub-Continental Lithospheric Mantle as revealed by intra- and inter-granular fluids

The investigation of the role played by CO2 circulating within the mantle during partial melting and metasomatic/refertilization processes, together with a re-consideration of its storage capability and re-cycling in the lithospheric mantle, is crucial to unravel the Earth's main geodynamic processes. In this study, the combination of petrology, CO2 content trapped in bulk rock- and mineral-hosted fluid inclusions (FI), and 3D textural and volumetric characterization of intra- and inter-granular microstructures was used to investigate the extent and modality of CO2 storage in depleted and fertile (or refertilized) Sub-Continental Lithospheric Mantle (SCLM) beneath northern Victoria Land (NVL, Antarctica). Prior to xenoliths entrainment by the host basalt, the Antarctic SCLM may have stored 0.2 vol% melt and 1.1 vol% fluids, mostly as FI trails inside mineral phases but also as inter-granular fluids. The amount of CO2 stored in FI varies from 0.1 μg(CO2)/g(sample) in olivine from the anhydrous mantle xenoliths at Greene Point and Handler Ridge, up to 187.3 μg/g in orthopyroxene from the highly metasomatized amphibole-bearing lherzolites at Baker Rocks, while the corresponding bulk CO2 contents range from 0.3 to 57.2 μg/g.Irrespective of the lithology, CO2 partitioning is favoured in orthopyroxene and clinopyroxene-hosted FI (olivine: orthopyroxene = 0.10 ± 0.06 to 0.26 ± 0.09; olivine: clinopyroxene = 0.10 ± 0.05 to 0.27 ± 0.14). The H2O/(H2O + CO2) molar ratios obtained by comparing the CO2 contents of FI to the H2O amount retained in pyroxene lattices vary between 0.72 ± 0.17 and 0.97 ± 0.03, which is well comparable with the values measured in olivine-hosted melt inclusions from Antarctic primary lavas and assumed as representative of the partition of volatiles at the local mantle conditions. From the relationships between mineral chemistry, thermo-, oxy-barometric results and CO2 contents in mantle xenoliths, we speculate that relicts of CO2-depleted mantle are present at Greene Point, representing memory of a CO2-poor tholeiitic refertilization related to the development of the Jurassic Ferrar large magmatic event. On the other hand, a massive mobilization of CO2 took place before the (melt-related) formation of amphibole veins during the alkaline metasomatic event associated with the Cenozoic rift-related magmatism, in response to the storage and recycling of CO2-bearing materials into the Antarctica mantle likely induced by the prolonged Ross subduction.

The composite Triassic\u2013Eocene Poshteh pluton, eastern Iran, an Eo-Cimmerian element south of the main Paleotethys suture

The composite Poshteh Pluton, at the northeastern margin of the Central Iranian Microplate near Taybad in eastern Iran, is positioned at a critical tectonic junction, south of the inferred main Paleotethys suture and along the major regional Doruneh Fault system. It consists of two distinct intrusions. Quartz monzonite is dated in this study by zircon U–Pb ID-TIMS to 215.8 ± 0.5 Ma, an age that coincides with the time of closure of the Paleotethys during the late collisional stages of the Eo-Cimmerian Orogeny. It is geochemically very similar to coeval plutons present along and north of the Paleotethys suture, where they intruded Carboniferous-Permian arc sequences, ophiolites and flysch. The Poshteh quartz monzonite is located south of the suture in a position similar to the Anarak and related complexes further west, which previously have been interpreted as reflecting Mesozoic and Cenozoic disruption of the Eo-Cimmerian Orogen by extensional and transtensional processes. The Triassic quartz monzonite was subsequently invaded by granite at 41.23 ± 0.31 Ma. The emplacement was in part structurally controlled by the Doruneh Fault system and associated to hydrothermal alteration and Fe mineralization. The granite is thus a coeval member of a widespread late Eocene to Oligocene plutonic suite in the region, and likely the result of delamination and melting of the subcontinental lithosphere.