Enriched Mantle Research Articles

Middle Triassic basaltic pyroclastic rocks from the Mt. Medvednica ophiolitic mélange (NW Croatia): petrology, geochemistry and tectono-magmatic setting

Hectometric blocks of Middle Triassic mafic pyroclastic rocks, represented by volcanic agglomerates/breccias and lapilli tuffs, form part of the ophiolitic mélange of Mt. Medvednica, situated in the southwestern segment of the Zagorje-Mid-Transdanubian Zone. These rocks share petrochemical characteristics with pyroclastic derivatives of alkali, within-plate basaltic lavas of Mts. Medvednica, Samoborska Gora, and Kalnik, indicating the occurrence of explosive events preceding the dominant effusive submarine volcanism during the Middle Triassic (Illyrian-Fassanian?) stages. The formation of these pre-ophiolitic pyroclastics is associated with an intracontinental rift setting and reflects melts derived from an OIB-type enriched mantle plume source. These pyroclastics represent uncontaminated melts that erupted through a highly thinned continental crust. In geodynamic terms, the formation of pyroclastites occurred during the Late Anisian-Early Ladinian along the continental margin of Palaeotethys through the proto back-arc rifting of continental lithosphere (Adria Plate), leading to the formation of the Maliak/Balkan Neotethys Rift, in the emerging northwestern segment of Neotethys. The investigated pyroclastic rocks of Mt. Medvednica document the extension in an evolved intracontinental rift basin, which immediately preceded the generation of the initial Neotethyan oceanic lithosphere during the Upper Triassic.

Early Paleozoic tectonic evolution in the central Vietnam: evidence from geochronological and geochemical constraints

ABSTRACT Magmatic rocks distributed along the western part of the Po Ko fault, which documented tectono-thermal evolution of the Indochina block, when and how they formed are less constrained. Our new geochronological and geochemical data reveal that they were generated at 488–457 Ma and display a variety of SiO2 concentrations (SiO2 = 50.7–72.8 wt%). They also generally show slightly negative Eu anomalies (Eu/Eu* = 0.65–0.98) and enrich in LILEs (e.g. Rb and K) but deplete in HFSEs (e.g. Nb and Ta), consistent with arc-related signatures. Combined with their relatively high Mg# values (Mg# = 56–40) and initial87Sr/86Sr ratios (0.7085–0.7167) but negative εNd(t) values (−2.5 to −3.7), we consider those magmatic rocks originated from an enriched mantle source (e.g. EMII), which was metasomatized in the past. In comparing our data with tectono-thermal events in the southern Truong Son belt, we infer that there existed an ocean (a branch of the proto-Tethys Ocean) between the Truong Son belt and the Kontum massif during Late Cambrian, which subsequently underwent southeast subduction beneath the Kontum mass if initiated at ~488 Ma and terminated at ~457 Ma.

Protracted magmatism and crust–mantle interaction during continental collision: insights from the Variscan granitoids of the external western Alps

Variscan granitoids and associated mafic rocks exposed in the External Crystalline Massifs (ECM) of the Western Alps document the Variscan stages from the early Carboniferous collision to the early Permian post-collisional setting. Our study focuses on the Central part of the ECM, synthesizing newly acquired and existing geochronological, whole-rock geochemical and isotopic data. We identified two distinctive magmatic series: (i) high-K calc-alkaline granitoids, which range from magnesian (MgG) to ferro-magnesian (FeMgG) rocks; (ii) ultra-high-K metaluminous (UHKM) rocks (“durbachites”). These series were emplaced roughly simultaneously between ca. 350 and 300 Ma, with two main episodes during the Visean (ca. 348–335 Ma) and the late Carboniferous (305–299 Ma), with a more limited activity in between. A younger Permian event at ca. 280–275 Ma has also been identified in one granitoid pluton. Contemporaneous emplacement of these two series reflects concomitant crustal anatexis and melting of LILE–LREE-rich metasomatized lithospheric mantle. Trace elements and Nd–Sr isotopes reveal significant hybridization between these two magmatic end members, by magma mixing, or assimilation of crystallized mafic ultrapotassic enclaves in the high-K calc-alkaline granitoids. Granitoid composition evolves over time, especially SiO2, Mg#, Sr/Y, La/Yb and Nb/Ta, possibly explained by increasing differentiation of magmas over time, changes in the crust versus mantle sources mass-balance, and decrease in melting pressure due to the orogenic collapse. The εNdi values of both high-K calc-alkaline granitoids and durbachites decreases from [− 3.8; − 2.9] to [− 6.4; − 5.2] between 345 and 320 Ma, possibly indicating an increasing influence of subducted/relaminated crustal material contaminating the lithospheric mantle source. εNdi values then rise to [− 3.7; − 0.5] during the late Carboniferous, possibly due to progressive exhaustion of the enriched mantle source, or advection of the asthenosphere during the post-collisional stage.Graphic abstractPossible geodynamic scenario along the central-eastern segment of the Variscan Belt, which may account for the temporal evolution of Variscan magmatism in the External Western Alps.

Petrographic and Geochemical Evidence for a Complex Magmatic Plumbing System beneath Bagana Volcano, Papua New Guinea

Abstract Bagana is a persistently active stratovolcano located on Bougainville Island, Papua New Guinea. Characteristic activity consists of prolonged lava effusion over months to years, with occasional shifts to explosive vulcanian or sub Plinian eruptions that threaten surrounding communities. Satellite observations have shown that Bagana is a major SO2 emitter, particularly during eruptive intervals. Despite persistent and potentially hazardous activity, no previous geophysical, petrological, or geochemical studies have constrained the magma storage conditions and reservoir processes at Bagana. To address this knowledge gap, we present new bulk rock major, trace element, and radiogenic isotope data, plus mineral phase major element compositions, for Bagana lavas erupted in 2005 and 2012 and ash erupted in 2016. We use our new data to understand the magmatic processes controlling the typical effusive activity and provide the first estimates of magma storage conditions beneath Bagana. The basaltic andesite bulk rock compositions (56–58 wt% SiO2) of our Bagana lavas reflect accumulation of a plagioclase + clinopyroxene + amphibole + magnetite + orthopyroxene crystal cargo by andesitic-dacitic (57–66 wt% SiO2) carrier melts. Constraints from clinopyroxene and amphibole thermobarometry, amphibole hygrometry, and experimental petrology suggest that the high-An plagioclase + clinopyroxene + amphibole + magnetite assemblage crystallizes from basaltic-basaltic andesite parental magmas with >4 wt% H2O, over a temperature interval of ~1100–900°C, at pressures of ~130–570 MPa, corresponding to ~5–21 km depth. Continued crystallization in the magma storage region at ~5–21 km depth produces andesitic to dacitic residual melts, which segregate and ascend towards the surface. These ascending melts entrain a diverse crystal cargo through interaction with melt-rich and mushy magma bodies. Degassing of carrier melts during ascent results in crystallization of low-An plagioclase and the formation of amphibole breakdown rims. The radiogenic isotope and trace element compositions of Bagana lavas suggest that parental magmas feeding the system derive from an enriched mantle source modified by both slab fluids and subducted sediments. Our findings suggest that the prolonged lava effusion and persistently high gas emissions that characterise Bagana’s activity in recent decades are sustained by a steady state regime of near-continuous ascent and degassing of magmas from the crustal plumbing system. Our characterisation of the Bagana magmatic plumbing system during effusive activity provides a valuable framework for interpreting ongoing monitoring data, and for identifying any differences in magmatic processes during any future shift to explosive activity.

Continental crust recycling in collisional zones: insights from Li isotope compositions of the syn-exhumation and post-collisional mafic magmatic rocks

Syn-exhumation and post-collisional mafic magmatism in continental collision orogenic belts may provide insights into the nature of orogenic lithospheric mantle and recycled continental components in continental subduction zones. Lithium and its isotopes have emerged as potentially valuable tools for shedding light on the origin of these magmas, given the contrast Li contents and isotopic compositions between the subducting continental crust and the mantle. Here, we present high-precision Li isotopes data for representative orthogneiss, continental eclogite, syn-exhumation and post-collisional mafic magmatic rocks from the North Qaidam orogen. The syn-exhumation mafic magmatic rocks have relatively higher Li contents (26.5–50.0 ppm) and lower δ7Li values (−1.01‰–1.48‰) than those of the post-collisional mafic magmatic rocks (Li = 11.1–22.7 ppm, δ7Li = 1.20‰–3.38‰), which are comparable to those of orthogneiss and continental eclogite, respectively. Dehydration and melting modelling results show that these mafic magmatic rocks have similar Li contents and δ7Li values to the continental eclogite- and orthogneiss-derived melts but are different from their derived fluids. Monte Carlo simulation for Li-Nd isotopes suggests the syn-exhumation and post-collisional mafic rocks could be derived from an enriched mantle source that contains ∼3–8% continental crustal components dominated by the orthogneiss and continental eclogite. The calculated results are consistent with the results from the previous study simulated by trace elements. Therefore, our results highlight Li isotopes as a potential tool to trace the nature of the continental crustal components recycling in continental subduction zones.

Late Neoproterozoic alkaline intrusives in Huqf sediments; a new reservoir target in southern Oman

Abstract Recent drilling in Block 56 on the southern Oman coast NE of Salalah has revealed the presence of intrusive rocks in Huqf sediments. The igneous bodies are found as both discrete and composite, decametre scale sills separated by screens of Huqf host rock. Compositionally they form a cogenetic suite of monzonites and syenites that can be related by fractional crystallization to a gabbro–diorite parental magma. Enrichment in alkalis and other large ion lithophile elements points to either an enriched mantle source or magma-crust interaction on ascent. Geochemical profiles match closely those of the latest Neoproterozoic alkaline eruptives now found as exotic volcanic blocks entrained in the salt diapirs of the Ghaba Salt Basin. Limited geochronological data indicate that the Block 56 intrusives are contemporaneous and derived from the same magma source. Porosities in excess of 10% have been recorded in the intrusives; it is a dissolution porosity created by volume loss during the conversion of primary ferromagnesian minerals to biotite and ferroan dolomite. Porosity creation took place during a short lived, fault-controlled Cretaceous hydrothermal event contemporaneous with uplift and alkaline volcanic activity in the Masirah Ophiolite. The intrusives contain a Huqf-sourced oil with light to medium viscosity appearance. Charging is believed to be coming from the cooler and shallower parts of the South Oman Salt Basin to the north and from beneath thick Cenozoic cover in the basin to the south.

The Genesis of A‐Type Granites in Lower Yangtze River Belt: Evidence From Calcium Isotopes

AbstractA‐type granite is a favorable rock for the study of crustal evolution, crust‐mantle interaction and metallogenesis. However, the origin of A‐type granite remains controversial. In this study, we report high‐precision Ca isotopic compositions of a co‐genetic suite of A‐type granites from the Lower Yangtze River Belt (LYRB) in eastern China. Our results show that the δ44/40Ca of the A‐type granites ranges from 0.55 ± 0.11‰ to 1.44 ± 0.06‰, and is negatively correlated with CaO, Sr and Ba contents and Eu/Eu*, indicating that the Ca isotope fractionation of A‐type granites is dominated by plagioclase and potassium feldspar. There is kinetic fractionation of Ca isotopes during A‐type granite magma evolution. Sample BSL‐7, the least evolved sample, revealed that it had a low δ44/40Ca of 0.59‰, which is significantly lower than that of the upper continental crust (δ44/40Ca = 0.71–0.72‰) and the bulk silicate earth (δ44/40Ca = 0.94‰). The low δ44/40Ca can be best explained by partial melting of the enriched mantle metasomatized by subduction material. Combined with the geodynamic background of the LYRB, we propose that the formation of A‐type granites in the LYRB originates from partial melting of the enriched mantle, triggered by early Cretaceous ridge subduction of the Pacific and Izanagi plates.

Tectonic evolution of the South Pamir Orogen: Insights from the Permian to cretaceous magmatism

The Pamir orogen originated through the amalgamation of various arc terranes. Despite undergoing numerous studies, the subduction polarity and time regarding the accretion processes remain inadequately constrained. To address these issues, we carried out a detailed study of zircon and apatite U-Pb ages, zircon Hf isotopes, and whole-rock geochemistry for the Permian to Cretaceous gabbro, volcanic rocks and granitoids from the Southern Pamir and the Rushan-Pshart suture. Three stages of magmatism are identified, including 279–266 Ma gabbro and andesite to dacite, 209–208 Ma granitoids, and 108–103 Ma gabbro and granitoids, respectively. The Permian gabbro and basaltic andesite to dacite in Pshart Range are enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), but are depleted in Ta, Nb, and Ti, suggesting formation from an enriched mantle source in a subduction zone. The Late Triassic I-type granitoids are typical arc calc-alkaline magmatic rocks with εHf(t) values varying from −8.7 to −3.5, which mainly sourced from the partial melting of mixed crustal sources predominately consisting mafic rocks with minor metasedimentary rocks. The Early Cretaceous Pshart granites are I-type granites with relatively low εHf(t) values ranging from −14.6 to −2.9, suggesting that they were crust-derived rocks. The Early Cretaceous Bazardara granites are identified as A2-type granites with the εHf(t) values ranging from −18.3 to +5.3, and were derived from mixing of mantle-derived and crust-derived magmas, or interaction between mantle melts and old crustal rocks. The early Cretaceous gabbro exhibits -MORB-like geochemical characteristics with flat REE pattern (La/YbN = 4.96), moderate enrichment of LILEs and weak depletion of Nb, Ta and Ti (Nb/LaPM = 0.94). Thus, the early Cretaceous A2-type granite, E-MORB-like gabbro and volcanic rocks were formed in an extensional setting. Our new data indicate that the southward subduction of the Rushan-Pshart oceanic slab started in the Early Permian and continued to the Late Triassic. Moreover, combining the previous data, we conclude that the Early Cretaceous granites and gabbro in the Southern Pamir formed in an extensional environment due to the southward rollback after northward near-flat subduction of the Neo-Tethyan oceanic lithosphere.

Early Triassic high-K granitoids and enclaves of the Daheba pluton, West Qinling (China): Implications for relative contributions of crust and mantle

Early Triassic granitoids are widespread in the northwestern West Qinling Orogen, China, but their petrogenesis and geodynamic implications remain unclear. In this study, we integrated new field and petrological observations, mineralogical compositions, zircon U-Pb dating, Hf isotopic and whole-rock geochemical analyses for the early Triassic granitic pluton in the Daheba area to determine its magma source and geodynamic scenario. The pluton is composed of granodiorite and syenogranite and carries microgranular enclaves (MEs) which formed at ca. 253–249 Ma. The granitoids show wide SiO2 contents of 63.73–77.49 wt% (av. 69.89), high K2O contents of 3.4–5.4 wt% (av. 4.1) and moderate Mg# of 16–51 (av. 36), belonging to high-K, calc-alkaline I-type granites. These rocks have high radiogenic but uniform Hf isotopic compositions with 176Hf/177Hf and ɛHf(t) of 0.282511–0.282658 and −3.85 – +1.25, respectively. The MEs hosed within the granite are characterized by high Mg# of 35–58 (av. 46) and variable εHf(t) of −4.64 – +9.35, which likely represent a hybridized melt derived from a slab-modified mantle and lower crust. In combination with coeval magmatic rocks in the western Gonghe-East Kunlun area, we propose a genetic model where mafic magmas derived from an enriched mantle and underplated beneath the overlying lower crust are considered to have produced the high-K felsic magma. Further, the hybridized melt ascended to shallower crustal levels to generate a series of rocks ranging from dioritic MEs to granodiorite to syenogranite. Mass balances modeling suggests that the generation of these rocks involved 36 % of the lower crustal-derived melt and 64 % of the SCLM (R2 = 0.9). Our new data, in tandem with published results suggest that the Daheba pluton formed during the subduction stage of the Paleo-Tethyan Ocean and that a local extensional episode occurred at 253–249 Ma in the western Gonghe area.

Petrogenesis of volcanic rocks from the Quaternary Eifel volcanic fields, Germany: detailed insights from combined trace-element and Sr–Nd–Hf–Pb–Os isotope data

Quaternary rocks from the East and West Eifel volcanic fields in western Germany are a key suite of intraplate volcanic rocks that can provide insights into volcanism of the Central European Volcanic Province (CEVP) and into continental intraplate volcanism in general. We present a comprehensive dataset for Eifel lavas including isotope as well as major and trace element data for 59 samples covering representative compositions of the different volcanic fields. In line with previous studies, the lavas are all SiO2-undersaturated, alkaline-rich and mainly comprise primitive basanites, melilitites, and nephelinites (Mg# ≥ 57). Geochemical compositions of samples from both volcanic subfields display distinct differences in their trace-element as well as radiogenic isotope compositions, largely confirming previous subdivisions. Coupled trace-element and radiogenic Sr–Nd–Hf–Pb–Os isotope compositions can now provide firm evidence for spatially heterogeneous mantle sources and compositionally distinct magmatic pulses. Within the West Eifel Field, Sr–Nd–Pb isotope compositions of the younger (≤80 ka), ONB-suite (olivine-nephelinite-basanite) are similar to FOZO (FOcal ZOne) or the EAR (European Asthenospheric Reservoir) and resemble compositions that have been previously reported from plume-sourced ocean island basalts (OIB). In marked difference, older (700 Ma to 80 ka) volcanic rocks from the F-suite (Foidite) in the West Eifel field and from the entire east Eifel Field tap a more enriched mantle component, as illustrated by more radiogenic Sr isotope (86Sr/87Sr up to 0.705408) and variable Pb isotope compositions (206Pb/204Pb = 18.61–19.70, 207Pb/204Pb = 15.62–15.67 and 208Pb/204Pb = 38.89–39.76). Combined trace-element compositions of ONB-suite samples are in good agreement with results from batch melting models suggesting a hybrid composition of Eifel magmas formed through mixing 10% of a FOZO-like melt with 90% of a DMM-like melt, similar to melts from the Tertiary HEVF. However, radiogenic Sr–Nd–Pb isotope compositions of F-suite and EEVF and some ONB lavas require the admixture of melts from lithospheric mantle sources. Elevated Nb/Ta and Lu/Hf ratios in combination with variable 187Os/188Os ratios can now demonstrate the presence of residual carbonated eclogite components, either in the lithosphere or in the asthenospheric mantle. Finally, by combining geochemical and temporal constraints of Tertiary and Quaternary volcanism it becomes evident that CEVP volcanism in central and western Germany has resulted from compositionally distinct magmatic pulses that tap separate mantle sources. Although the presence of a mantle plume can neither be fully confirmed nor excluded, plume-like melt pulses which partially tap carbonated eclogite domains that interact to variable extents with the lithosphere provide a viable explanation for the temporal and compositional cyclicity of CEVP volcanism.

Petrography, geochemistry and monazite geochronology of crustal xenoliths hosted by Afonso Cláudio Intrusive Complex, Araçuaí-West Congo orogen (southeast Brazil): Insights about contamination, magma sources and evolution of the post-collisional magmatism

The Afonso Cláudio Intrusive Complex (ACIC) is a typical post-collisional intrusion from Araçuaí-West Congo orogen (AWCO). The ACIC intruded allanite-bearing orthogneisses from AWCO pre-collisional tectonic stage (Rio Doce arc related rocks) and paragneisses from Nova Venécia Complex and it is composed of two lowered off-centered monzogabbroic and monzodioritic cores surrounded by quartz monzonite high hills, showing intense magma mingling and mixing between these domains. Xenoliths attributed to the enclosing gneisses have been commonly found widespread in the quartz monzonitic rocks, while are occasional in the monzogabbroic and monzodioritic domain. This paper presents the first detailed investigation of the xenoliths hosted by an AWCO post-collisional intrusion, focusing on petrography, major and trace element geochemistry and U–Th–Pb monazite dating of the main xenolith types. For comparison, major and trace element data investigation of the ACIC enclosing rocks has also been conducted. The correlated xenoliths and ACIC enclosing rocks show partially similar geochemical features, however both show different REE and multi-element normalized patterns compared to the ACIC main rocks. These results suggest that correlated xenoliths and enclosing rocks are cogenetic, however both could not represent the main sources of magmas that generated the ACIC. Geochemical modeling of igneous processes of previous ACIC dataset considering the investigated xenoliths and enclosing rocks as the main assimilant material suggests that mixing was responsible for both ACIC main rocks similar geochemical patterns and linear trends in the bimodal diagrams. This modeling also indicates that the evolution of the ACIC main rocks was ruled by different processes. Fractional crystallization with minor influence of assimilation of enclosing rocks controlled the magmatic evolution of monzogabbro and monzodiorite related to an enriched mantle source, while coupled assimilation and fractional crystallization ruled the evolution of quartz monzonite possibly associated to lower crust magmas mainly contaminated by enclosing rocks. U–Th–Pb monazite dating of two xenoliths showed that these rocks preserved similar main ages related to different stages of ACIC and AWCO post-collisional evolution. Both xenoliths showed main older ages of 478 and 477 Ma which are related to ACIC intrusion during AWCO post-collisional stage, while younger ages of 431 and 427 Ma are probably related to the intrusion of syenogranite dykes that crosscut the ACIC, representing an extension of AWCO post-collisional magmatism or a later unrelated thermal event. Due to the similarities between the typical AWCO post-collisional intrusions, the insights brought about the evolution of ACIC magmas could be considered to the typical AWCO post-collisional mantle and crustal magmas. This first investigation of xenoliths hosted by an AWCO post-collisional intrusion has showed that these strange rock pieces could bring new and important insights about the AWCO evolution, which shows an amazing and long-lasting magmatism with common occurrence of poorly investigated xenoliths.

Deciphering mantle source heterogeneity in space and time in the back-arc of a contemporary subduction system: A regional study of the Sredinny Range, Kamchatka

We present here the first regional set of Sr-Nd-Pb isotopic compositions, major and trace element compositions and KAr ages for a representative suite of back-arc samples of the Sredinny Range (SR) of Kamchatka. Together with previously published analyses, this unique dataset allowed us to trace the source heterogeneity and invoke new mechanisms to explain the variably enriched geochemical signatures. Our results indicate that the Sr isotopic ratios and the LILE content of the studied rocks were mainly influenced by the subduction fluid. Neodymium isotopes, as well as the HFSE and REE distributions in the rocks, require a more complex explanation, depending on the geographical location of the studied samples and the age of their eruption. Melts representing Miocene rocks of the northern part of the SR and most of the rocks from the eastern and central flanks of the SR (from Miocene until present) were produced from a depleted MORB-like metasomatized mantle, and were then exposed to varying degrees of crustal assimilation and fractional crystallization. Higher HFSE contents and lower 143Nd/144Nd ratios in the western flank lavas, as well as high HFSE contents in the Quaternary lavas of the northern part of the SR and some of the lavas from the eastern flank of the southern part of the SR, require an alternative source for their enrichment. Delamination of the lithosphere explains the unusual, enriched signature of these rocks, whereas their variably enriched neodymium isotope ratios identify the various ages of the separation of the lithosphere from the mantle. The Nd isotopic composition of the rocks together with their HFSE content, therefore, serves as an unusual tracer for the enriched mantle domain, showing the presence of the older lithospheric blocks and indicating the timeframes for this source involvement in magma generation.

Continental Arc Accumulation Mafic Rocks in the Mid-Upper Crust: Constraints From the Early Eocene Hornblende Gabbro–Diorite in the Tengchong Block, Southeastern Extension of Tibet

Abstract Magmatic activity in the syn-collision stage is key for net crustal growth. To understand the mechanism of accretion–differentiation and compositional change of the continental crust, it is important to focus on the magmatic activity during the syn-collision stage. Early Eocene mafic–ultramafic rock assemblages found in the western part of the Tengchong Block resulted from a continuous series of arc magmatic evolution, thoroughly recording the continental arc magmatic system during the subduction of the Neo-Tethys Ocean and syn-collision of the Indian-Asian continents. Early Eocene hornblende gabbro–diorite in the Tengchong Block formed at 53 Ma, and the primitive magma was derived from an enriched mantle source due to the enriched Nd–Hf isotopes. The amphibole and biotite thermobarometer measurements indicate that the mafic magma reservoirs in the Tengchong Block occurred at a mid-upper crust. Petrography, amphibole Fe/Mg exchange coefficient (KD), Rayleigh fractionation, and equilibrium melt calculation indicate that the Early Eocene hornblende gabbro–diorite in the Tengchong Block was created due to plagioclase-dominated accumulation at the mid-upper crust level. Based on the calculation, the corresponding amphibole equilibrium melt is more silicic (dacitic–rhyolitic in composition) than the bulk rocks, indicating a more evolved composition in the mid-upper crust. Three types of plagioclases reveal the multi-recharging and dissolution–reprecipitation promoting the further evolution of these mafic rocks. Therefore, this study concludes that magma recharge and plagioclase-dominated accumulation processes may be important mechanisms for the formation and evolution of mafic magma and the further crustal differentiation at the mid-upper crust level in a continental margin arc.

Geochemistry, Sr-Nd Isotope Compositions, and U-Pb Chronology of Apatite from Kimberlite in Wafangdian, North China Craton: Constraints on the Late Magmatic Processes

Diamondiferous kimberlites occur in the Wafangdian area in the eastern part of the North China Craton (NCC). In order to better constrain their magmatic source and emplacement time, we have investigated apatite from two kimberlites, i.e., the #110 dike kimberlite and the #50 root-zone kimberlite by measuring in situ their U–Pb and Sr–Nd isotopic compositions. The crystallization ages of the #110 and #50 apatites are 460.9 ± 16.8 Ma and 455.4 ± 19.3 Ma, respectively. For the #50 apatite, 87Sr/86Sr = 0.70453–0.70613 and εNd(t) = −2.74 to −4.52. For the #110 apatite, 87Sr/86Sr = 0.70394–0.70478 and εNd(t) = −3.46 to −5.65. Based on the similar distribution patterns of the rare earth elements (REEs) and the similar Sr-Nd isotope compositions of the apatite, it is believed that the #110 and #50 kimberlites have the same source region and the kimberlite magmas in Wafangdian were derived from an enriched mantle source (EMI). The primary magmatic composition has little effect on the emplacement pattern. It is more likely that the geological environment played an important role in controlling the retention and removal of volatile components (H2O and CO2). This led to the different evolutionary paths of the kimberlite magma in the later period, resulting in differences in the major element compositions of the apatite. High Sr concentrations may be associated with hydrothermal (H2O-rich fluid) overprinting events in the later magmatic period; the higher light rare earth element (LREE) concentration of the #50 apatite reflects the involvement of the REE3+ + SiO44− ⇔ Ca2+ + PO43− replacement mechanism. Two emplacement patterns of the #110 dike kimberlite (#110 apatite, low Sr, and high Si) and the #50 root-zone (#50 apatite, high Sr, and low Si) kimberlites were identified via major element analysis of the #110 apatite and #50 apatite.

Chronological, Petrogenetic, and Tectonic Significance of Paleoproterozoic Continental Crust within the North China Craton during the Global Tectono-Magmatic Lull

Abstract The 2.45–2.20 Ga period during the early Paleoproterozoic era is considered to have witnessed a global “Tectono-Magmatic Lull (TML)” and thus marks a relatively quiescent period. Our study unveils a 2.45–2.20 Ga magmatic suite from the Xiong’ershan area in the southern North China Craton, offering some key constraints on localized active tectonics during the TML. Zircon U-Pb dating shows Paleoproterozoic ages for the meta-basalt (2.31, 2.28 Ga), Na-rich meta-andesite (~2.33 Ga), tonalite-trondhjemite-granodiorite (TTG) gneisses (2.36, 2.30 Ga), K-rich granodiorite (~2.29 Ga), and monzogranite (2.33, 2.27 Ga). The meta-basalts geochemically and petrographically belong to calc-alkaline basalts and show distinctive Nb, Ta, and Ti contents and primitive mantle normalized patterns from different places in the Xiong’ershan area. Combined with their enriched εHf(t) values, the magmas were derived from subduction-related enriched mantle sources within a convergent plate boundary. The meta-andesites display high MgO content (average 4.5 wt%) and Mg# (44–57), strongly fractionated rare-earth pattern, calc-alkaline affinity, and negative Nb, Ta, and Ti anomalies. The TTG gneisses are of high SiO2 type (>62 wt%), high (La/Yb)N (17.5, 39.2), and Sr/Y (50.2, 104.3) and mostly display positive Eu anomalies and high-pressure type. Zircons from these rocks show a relatively narrow range of δ18O isotope values (5.35‰, 6.79‰) with εHf(t) isotope characteristics (−9.3, −3.3), suggesting derivation from partial melting of a thickened mafic lower crust. The youngest K-rich granodiorite and monzogranite show high K2O/Na2O ratios (0.65, 2.45). Variable molar ratio Al2O3/(CaO+Na2O+K2O) (A/CNK) and low zircon εHf(t) values suggest that the K-rich granitoids formed from the partial melting of different levels of crust. The presence of meta-basalt to andesite assemblages and diverse intermediate to felsic magmatic rocks implies magmatic activity within a convergent plate boundary tectonic environment with potential influence from plume-triggered extensional processes, supported by evidence of slab rollback and upwelling of mantle material.

Nd isotope systematics of Late Paleozoic granitoids from the Western Transbaikalia (Russia): Petrological consequences and plume model testing

Late Paleozoic granites of the Angara-Vitim batholith (AVB) occupy an area of 200,000 km2 in Western Transbaikalia (the eastern part of the Central Asian fold belt). Batholith granitoids form a sheet-like body with an average thickness of 7–10 km and a volume of about 1 million km3. The granitoid massifs that make up the batholith are composed of high-potassium calc-alkaline and subalkaline quartz monzonites, quartz syenites, amphibole-biotite granodiorites, and biotite granites of autochthonous and allochthonous facies. An extremely high heterogeneity of the batholith isotopic structure was established, which basically corresponds to the parameters of uneven-aged crustal metaterrigenous protoliths. There are significant variations in εNd(Т) and accordingly T(DM-2) in granitoids of different complexes. It is assumed that the isotopic heterogeneity of AVB was formed due to the melting of a limited protoliths number that are maximally contrasting in isotopic and lithological composition: the Paleoproterozoic continental crust with εNd(Т) ≈ -20 ÷ -22 and T(DM-2) = 2.9–2.5 Ga and Neoproterozoic mafic granulites of increased potassium alkalinity, enriched in the juvenile component (εNd(Т) ≈ -3.0; T(DM-2) = 1.2–1.3 Ga). The latter were the main magmas source of postbatholitic alkali granites. Melts from these contrasting protoliths were only in some cases complementary to the salic melts sources. The main mechanism that determined the isotopic composition of AVB granitoids was the mixing of isotopically contrasting magmas in different proportions. Mingling dikes, mafic inclusions in granitoids, and synplutonic mafic intrusions in the AVB indicate an additional mixing component. It was mafic magma from an enriched mantle reactivated in the Late Paleozoic under the mantle plume influence on the heated plastic crust of a young (Hercynian) orogen.

Alkaline–Salicic Kurung Complex of the Ket-Kap–Yuna Igneous Province (Aldan Shield, Russia): Petrogenesis and Geodynamic Conditions of Formation

Abstract —Integrated geological data, petrogeochemical characteristics of magmatites of the Kurung complex (the Ket-Kap–Yuna igneous province of the Aldan Shield), and previously published isotopic data along with the results of geochronology and geochemistry research provide evidence for this complex evolving in the Late Cretaceous in a setting of riftogenesis (final phase) of continental margins. Similar to the Late Jurassic–Early Cretaceous polyformational volcano-plutonism of the Aldan Shield, the Late Cretaceous alkaline magmatism in the Ket-Kap–Yuna province is associated with the tectonomagmatic activation which manifested itself mainly as continental margin rifting associated with deep reorganization of the southwestern, southern and southeastern framing of the Siberian craton triggered by gravitational sliding of the lithospheric plates. Results of the petrogeochemical analysis of the studied magmatites revealed that the heterogeneous rock groups identified within this rock complex are not related to each other by fractional or other type of differentiation, thus suggesting their possible derivation as a result of fluid syntexis-type interaction between mantle-derived alkali–basite melt and felsic crustal melts. Alkali–basite parent melts are shown to have formed during partial melting of variably enriched mantle which is widely spread beneath the Aldan Shield and is varied in composition (from BSE enriched to nearly EM-I). Origination of such deep-seated melts (magma sources) was associated with the rifting processes (strike-slip tectonics) triggered by the setting of gravitational sliding of lithospheric plates. The formation of alkaline–salic magmas of the Kurung complex is associated with large-scale selective assimilation of crustal material by high-temperature fluidized mantle magmas during their ascent to the surface, possibly as a result of fluid syntexis of alkaline–basite mantle magmas and crustal smeltings formed under their influence.

Contribution of carbonatite and recycled oceanic crust to petit-spot lavas on the western Pacific Plate

Abstract. Petit-spot volcanoes, occurring due to plate flexure, have been reported globally. As the petit-spot melts ascend from the asthenosphere, they provide crucial information of the lithosphere–asthenosphere boundary. Herein, we examined the lava outcrops of six monogenetic volcanoes formed by petit-spot volcanism in the western Pacific. We then analyzed the 40Ar/39Ar ages, major and trace element compositions, and Sr, Nd, and Pb isotopic ratios of the petit-spot basalts. The 40Ar/39Ar ages of two monogenetic volcanoes were ca. 2.6 Ma (million years ago) and ca. 0 Ma. The isotopic compositions of the western Pacific petit-spot basalts suggest geochemically similar melting sources. They were likely derived from a mixture of high-μ (HIMU) mantle-like and enriched mantle (EM)-1-like components related to carbonatitic/carbonated materials and recycled crustal components. The characteristic trace element composition (i.e., Zr, Hf, and Ti depletions) of the western Pacific petit-spot magmas could be explained by the partial melting of ∼ 5 % crust bearing garnet lherzolite, with 10 % carbonatite flux to a given mass of the source, as implied by a mass-balance-based melting model. This result confirms the involvement of carbonatite melt and recycled crust in the source of petit-spot melts. It provides insights into the genesis of tectonic-induced volcanoes, including the Hawaiian North Arch and Samoan petit-spot-like rejuvenated volcanoes that have a similar trace element composition to petit-spot basalts.

Plume-lithosphere interaction in the Comei Large Igneous Province: Evidence from two types of mafic dykes in Gyangze, south Tibet, China

Two suites of mafic dykes, T1193-A and T1194-A, outcrop in Gyangze area, southeast Tibet. They are in the area of Comei LIP and have indistinguishable field occurrences with two other dykes in Gyangze, T0902 dyke with 137.7±1.3 Ma zircon age and T0907 dyke with 142±1.4 Ma zircon age reported by Wang YY et al. (2016), indicating coeval formation time. Taking all the four diabase dykes into consideration, two different types, OIB-type and weak enriched-type, can be summarized. The “OIB-type” samples, including T1193-A and T0907 dykes, show OIB-like geochemical features and have initial Sr-Nd isotopic values similar with most mafic products in Comei Large Igneous Provinces (LIP), suggesting that they represent melts directly generated from the Kerguelen mantle plume. The “weak enriched-type ” samples, including T1194-A and T0902 dykes, have REEs and trace element patterns showing within-plate affinity but have obvious Nb-Ta-Ti negative anomalies. They show uniform lower εNd(t) values (–6––2) and higher 87Sr/86Sr(t) values (0.706–0.709) independent of their MgO variation, indicating one enriched mantle source. Considering their closely spatial and temporal relationship with the widespread Comei LIP magmatic products in Tethyan Himalaya, these “weak enriched-type” samples are consistent with mixing of melts from mantle plume and the above ancient Tethyan Himalaya subcontinental lithospheric mantle (SCLM) in different proportions. These weak enriched mafic rocks in Comei LIP form one special rock group and most likely suggest large scale hot mantle plume-continental lithosphere interaction. This process may lead to strong modification of the Tethyan Himalaya lithosphere in the Early Cretaceous.©2024 China Geology Editorial Office.

Extensive H2O degassing in deeply erupted submarine glasses inferred from Samoan melt inclusions: The EM2 mantle source is damp, not dry

Submarine glasses erupted at intraplate volcanic hotspot settings sampling enriched mantle (EM)—characterized by high 87Sr/86Sr—exhibit lower H2O/Ce than glasses representing less enriched mantle domains, leading to the interpretation that the EM mantle is H2O-poor (“dry”). We test whether low H2O/Ce observed in pillow glasses of EM lavas resulted from degassing of higher H2O/Ce primary melts by measuring H2O/Ce and 87Sr/86Sr in olivine-hosted melt inclusions from two deeply-erupted (3950 and 2190 m below sea level (mbsl)) Samoan submarine lavas from Vailuluʻu and Malumalu seamounts. Vailuluʻu (H2O/Ce = 161–275) and Malumalu (180–215) melt inclusions with 87Sr/86Sr data have an average H2O/Ce (197 ± 58 2SD, N = 15) that is nearly twice as high as H2O/Ce in pillow glasses from these two seamounts (average H2O/Ce = 106 ± 51, N = 65), and comparable to pillow glasses from non-EM hotspots. We show that lower H2O/Ce in submarine Samoan glasses compared to melt inclusions results from greater closed-system degassing, and concomitant loss of H2O, because EM melts have higher initial concentrations of CO2. We show that the lower H2O/Ce in global EM pillow glasses compared to non-EM pillow glasses can be modeled to be the result of more extensive degassing of H2O in EM melts, which owes to higher CO2 in primary melts (20,000–90,000 ppm) of EM sources compared to non-EM melts (300–50,000 ppm CO2). Instead of originating from a dry mantle, we conclude that EM lavas derive from a damp mantle, but EM melts lose more H2O by degassing than non-EM melts.