Metasomatic Component Research Articles

Identification of metasomatic components in lithospheric mantle under multiple tectonic regimes: Insights from Early Cretaceous mafic rocks in the northeastern North China Craton

Identification of mantle metasomatic components under multiple tectonic regimes is crucial for reconstructing the subduction history of multiple plates. Representative Early Cretaceous (125–122 Ma) gabbroic diabase from the northeastern North China Craton (NCC), provides a geochemical record to explore the genetic relationship between mantle metasomatic agents and tectonic evolution. Samples from gabbroic diabase exhibit low SiO2 and high MgO contents as well as arc-like trace element patterns, suggesting a sub-continental lithospheric mantle (SCLM) origin. Their inherited zircons and isotope variations indicate the existence of two crustal components with different tectonic affinities that were recycled into the SCLM: Paleo-Asian oceanic crust and Yangtze Craton continental crust. The metasomatic signatures of the two crustal components are previously imprinted on the geochemical compositions of the studied mafic rocks. Subsequently, the melting of the metasomatic SCLM is triggered by the subduction of the Paleo-Pacific Plate. Based on the identification of mantle metasomatic components in this study and previous geological evidence, the spatial extents of the subduction influence of the Paleo-Asian oceanic plate and Yangtze Craton on the SCLM beneath the northeastern NCC are broader than previously considered. These new findings signify a critical role of the mantle metasomatic components that can effectively identify the geological relics of the previous plate subduction. Thus we believe that the former mantle metasomatism and latter mantle melting for mafic rocks suggest two geological implications.

Major-, trace-element and Sr-Nd-Hf isotope geochemistry of diamondiferous dykes from Tonguma and Koidu, Sierra Leone: Highly micaceous kimberlites formed by assimilation of metasomatised lithospheric mantle rocks

The Man Craton in West Africa hosts two clusters of highly micaceous kimberlites at Koidu and Tonguma with mineralogical and geochemical features transitional towards those of cratonic lamproites. To develop a better understanding of the genesis of these peculiar rocks, which have also been documented elsewhere (e.g., South Africa, China, Russia, India), we have undertaken a geochemical and isotopic study of representative fresh samples from Koidu and Tonguma. New mica RbSr dating by isotope dilution confirms the Jurassic age of Koidu (∼145 Ma) and establishes that the Tonguma dykes are marginally younger (∼138 Ma). Bulk-rock analyses of samples from both localities show strong enrichment in K2O (up to 4.4 wt%), but also CaO and CO2 (up to 24 wt% and 18 wt%, respectively), which is entirely consistent with the micaceous nature of these rocks and their variable abundance of calcite. Despite their highly potassic compositions, the incompatible trace element systematics (e.g., Ba/Nb) of these rocks are typical of archetypal kimberlites and unlike those of cratonic lamproites. The Sr-Nd-Hf isotopic composition of the bulk samples range between moderately depleted values (age-corrected 87Sr/86Sr ∼ 0.7032; εNd = +3.8; εHf = +3.9) resembling the common depleted (Prevalent Mantle or PREMA-like) kimberlite component and more geochemically enriched compositions (87Sr/86Sr ∼ 0.7058; εNd = −0.4; εHf = +0.3). The direct relationship between bulk-rock Nd isotopes and average Mg# of magmatic olivine, combined with the previously reported correlation between average Mg# of magmatic and xenocrystic olivine of lithospheric mantle derivation in these kimberlites, suggests that the observed spread in radiogenic isotope compositions stems from the following petrogenetic scenario. Primary kimberlite melts of similar asthenospheric origin with unradiogenic Sr and moderately radiogenic Nd and Hf isotopes interacted with variably metasomatised (i.e., containing phlogopite) and hence geochemically enriched lithospheric mantle wall rocks. Mass balance calculations suggest that the transfer of incompatible elements to the asthenospheric melts was mediated by incongruent melting of metasomatic components in the lithospheric mantle rather than their bulk addition. This study reinforces previous suggestions that interaction between carbonate-rich asthenospheric melts and the sub-continental lithospheric mantle can generate the range of potassic magmas observed within cratons.

Syn-collisional extension and Ni-Cu sulfide-bearing mafic magma emplacement along the Irtysh Shear Zone in the Central Asian Orogenic Belt

The Central Asian Orogenic Belt was characterized by a long-lived accretionary history from Neoproterozoic to Paleozoic, followed by a collisional phase of orogeny in the latest Paleozoic with the closure of the Paleo-Asian Ocean. However, Ni-Cu sulfide mineralization associated with mantle-derived mafic magmas in the Central Asian Orogenic Belt was relatively short-lived. Thus, the coherence between the protracted accretionary and collisional history of the Central Asian Orogenic Belt and the transient Ni-Cu sulfide-bearing mafic magmatism is difficult to understand. We investigated both Ni-Cu sulfide-bearing and sulfide-barren mafic intrusions along a major suture zone of the Irtysh Shear Zone within the Central Asian Orogenic Belt to understand the origin of short-lived Ni-Cu sulfide mineralization in the context of the collision of two arc systems of the Chinese Altai and the East Junggar regions. Zircon U-Pb dating results for the Ni-Cu sulfide-bearing mafic intrusions show that they were only emplaced briefly from 290 Ma to 280 Ma after subduction terminated (ca. 312 Ma), substantially earlier than Ni-Cu sulfide-barren mafic intrusions (ca. 274−271 Ma). Zircons from all Ni-Cu sulfide-bearing/barren mafic intrusions yield arithmetic mean δ18O values ranging from 6.0 to 7.4‰, higher than the typical mantle value (∼5.3‰), which indicates their derivation from a metasomatized mantle with inherited subduction signatures. The δ11B values of the rocks from these intrusions range from −17.2‰ to −2.3‰, significantly lower than those of typical volcanic arcs (up to +18‰), which suggests that the metasomatic components in the mantle source were dominated by sediment melts rather than fluids. The pressure calculated using amphibole thermobarometry shows that the parental magmas of the Ni-Cu sulfide-bearing mafic intrusions were emplaced shallower than those of Ni-Cu sulfide-barren mafic intrusions. This is supported by the fact that the country rocks of the Ni-Cu sulfide-bearing mafic intrusions are shallow crustal lithologies of sedimentary rocks, whereas the country rocks of the barren intrusions are relatively deeper crustal lithologies of high-grade metamorphic rocks and granites. Olivine from the Ni-Cu sulfide-barren intrusions has forsterite (Fo) contents ranging from 51.1 mol% to 76.2 mol% and Ni from 196 ppm to 1312 ppm. These values are much lower than those of the Ni-Cu sulfide-bearing intrusions (Fo = 72.6−79.4 mol%; Ni = 1022−1925 ppm), which indicates that the parental magmas of the barren intrusions may have experienced extensive olivine crystallization. Notably, the emplacement of the Ni-Cu sulfide-bearing mafic intrusions (290−280 Ma) coincidently overlapped with a transient period of orogen-parallel extension (ca. 295−280 Ma) in response to the collision of the Chinese Altai and East Junggar. Syn-collisional extension may lead to the rapid ascent and emplacement of mantle-derived mafic magmas at a shallower crustal level, preventing significant loss of Ni from olivine crystallization at depths, and thus providing metal-rich mafic magmas for potential Ni-Cu sulfide mineralization. Our results highlight that syn-collisional extension is an essential geodynamic mechanism that controls the emplacement of Ni-Cu sulfide-bearing mafic intrusions in orogenic belts.

Two episodes of Late Paleozoic mafic magmatism in the western Tianshan Orogen: From Carboniferous subduction to Permian post-collisional extension

The evolution of the western Tianshan Orogen in the southern Central Asian Orogenic Belt involved prolonged accretion and collision processes in response to the closure of the Paleo-Asian Ocean during the Late Paleozoic. However, the timing for the tectonic transition from subduction to post-collision is still poorly constrained. Herein, we address this issue based on geochronological, geochemical and Sr-Nd-Hf isotopic investigations on representative mafic dikes from the Awulale Mountains in the heart of the western Tianshan. Zircon U-Pb dating suggest two episodes of mafic magmatism during the Late Carboniferous (ca. 324–310 Ma) and Early Permian (ca. 299–290 Ma). The Late Carboniferous gabbro dikes exhibit large variations in MgO (2.79–10.97 wt%) and Ni (7.58–244 ppm), and arc-like trace element patterns with high Th/Yb (0.55–2.26) ratios, relatively low εNd(t) (+0.2 to +2.5) and variable εHf(t) (+1.3 to +10.4) values, indicating that they were derived from fractional crystallization of a peridotite-bearing mantle source metasomatized by sediment-derived hydrous melts. In contrast, the Early Permian diabase dikes have slightly variable MgO (5.88–7.78 wt%) and Ni (59.90–104 ppm) with apparently low Th/Yb ratios (0.20–0.34, except for one sample: 0.86) and depleted εNd(t) (+5.0 to +7.8) and εHf(t) (+8.0 to +14.6) values, which indicates derivation from partial melting of a juvenile lithospheric mantle source with contributions from early subduction metasomatic components. In context with relevant published data, our study indicates that the Late Carboniferous magmatism of the Awulale Mountains was likely induced by breakoff of the North Tianshan oceanic slab during ongoing subduction; conversely, the Early Permian magmatism may have originated from delamination of the lower crust and lithospheric mantle in the subsequent post-collisional setting. Furthermore, the tectonic transition from Carboniferous subduction to Permian post-collisional extension of the western Tianshan suggests that terminal closure of the North Tianshan Ocean took place at Carboniferous-Permian boundary at ∼ 300 Ma.

Multi-stage garnet formation and destruction in Kimberley harzburgitic xenoliths, South Africa

Thirty-nine garnet harzburgites from Kimberley in the Kaapvaal Craton (South Africa) were studied to constrain the origin, age and evolution of sub-cratonic lithospheric mantle (SCLM). In order to avoid chemical overprinting by recent metasomatism, only garnet harzburgites that appeared clinopyroxene-free to the naked eye were sampled. The majority of garnets were, however, in equilibrium with clinopyroxene (24 of 39). Whole rock and mineral major–trace element geochemistry and garnet Sr–Nd–Hf isotope data are presented. Equilibration pressures range from 3.8–6.1 GPa, indicating the harzburgites were derived from a large portion of the SCLM (~115–185 km). High olivine Mg# (~93.4, n = 39) and low whole rock heavy rare earth elements (HREE) contents are consistent with large degrees of partial melting (>45%) and garnet exhaustion leaving a dunitic residue with olivine ≥90%, orthopyroxene ≤10% and HREE <0.01 times chondrite. Mineral modes, whole rock Al2O3 (0.5–3.2 wt%) and SiO2 (43.1–49.1 wt%), however, indicate heterogeneous re-introduction of garnet (≤13%) and orthopyroxene (≤50%). Harzburgites with high garnet and relatively low orthopyroxene modes (mostly ~7–13% and ~ 9–30%; n = 6) are characterised by mildly sinusoidal garnet REE patterns (Tb-Dy minimum and high HREE) and Archaean depleted Hf TDM ages (2.7–3.3 Ga; εHfe: +190 to +709). In contrast, harzburgites with high orthopyroxene and relatively low garnet and modes (~1.5–7.5% and ~ 25–50%; n = 19) are characterised by highly sinuous REE patterns (Ho-Yb minimum and low HREE) and Proterozoic enriched Hf TDM ages (0.7–1.6 Ga; εHfe: −16 to +6). It is inferred that Archaean G10 garnet re-introduction caused a significant increase in HREE, making melt depletion models based on HREE inaccurate. Orthopyroxene addition, a few hundred million years later, most likely at ~2.7 Ga and associated with Ventersdorp magmatic activity, caused partial consumption of garnet and olivine, and changed garnet compositions leading to: 1) Cr/Al ratio increase; 2) HREE decrease; 3) more sinusoidal REE patterns; and 4) un-radiogenic 176Hf/177Hf. Garnets define a Lu-Hf isochron age of 2702 ± 64 Ma (εHfi = +44, n = 31), which is interpreted as a consequence of partial isotopic equilibrium within the SCLM and mixing of the garnet- and orthopyroxene-rich metasomatic components. The low LILE contents and absence of Nb-Ta anomalies are consistent with modal metasomatism caused by intra-plate magmatism. In addition, the REE signatures of metasomatic agents in equilibrium with the garnets suggest that carbonatitic melts and SiO2-rich hydrous melts were responsible for re-introduction of garnet and orthopyroxene, respectively. Sr-Nd isotope systematics were disrupted associated with kimberlite magmatism (Nd isochron: 217 ± 58 Ma, εNdi = +4; n = 34), consistent with recent G10 garnet transformation into G9 garnets (Ca + Fe-enriched). This event may have caused garnet addition (up to 1%), suggesting that garnet was formed or destroyed in at least 4 different events: i) initial extensive polybaric melting, ii) asthenospheric melts re-introducing the bulk of the garnet, iii) orthopyroxene addition and garnet loss, all in the Archaean, and iv) minor garnet addition possibly related to recent kimberlite magmatism prior to eruption.

Nitrogen and noble gases reveal a complex history of metasomatism in the Siberian lithospheric mantle

The Siberian flood basalts (SFB) erupted at the end of the Permian period (∼250 Ma) in response to a deep-rooted mantle plume beneath the Siberian Sub-Continental Lithospheric Mantle (SCLM). Plume-lithosphere interaction can lead to significant changes in the structure and chemistry of the SCLM and trigger the release of metasomatic material that was previously stored within the stable craton. Here, we investigate the nature of the Siberian-SCLM (S-SCLM) by measuring nitrogen abundances and isotopes (δ15N) in 11 samples of two petrologically-distinct suites of peridotitic xenoliths recovered from kimberlites which bracket the eruption of the SFB: the 360 Myr old Udachnaya and 160 Myr old Obnazhennaya pipes. Nitrogen isotope (δ15N) values range from -5.85 ± 1.29‰ to +3.94 ± 0.63‰, which encompasses the entire range between depleted Mid-Ocean Ridge Basalt (MORB) mantle (DMM; -5 ± 2‰) and plume-derived (+3 ± 2‰) endmembers. In addition, we present neon (n=7) and argon (n=8) abundance and isotope results for the same two suites of samples. The 20Ne/22Ne and 21Ne/22Ne range from atmospheric-like values of 9.88 up to 11.35 and from 0.0303 to 0.0385, respectively, suggesting an admixture of DMM and plume-derived components. Argon isotopes (40Ar/36Ar) range from 336.7 to 1122 and correlate positively with 40Ar contents. We show that volatile systematics of Siberian xenoliths: (1) exhibit evidence of ancient metasomatic and/or recycled signatures, and (2) show evidence of subsequent plume-like re-fertilization, which we attribute to the emplacement of the SFB. Metasomatic fluids are highly enriched in radiogenic gases and have elevated Br/Cl and I/Cl values, consistent with an ancient subducted crustal component. The metasomatic component is marked by light N isotope signatures, suggesting it may be derived from an anoxic Archean subducted source. Taken together, these N2-Ne-Ar isotope results suggest that mantle plume impingement has profoundly modified the S-SCLM, and that N, Ne and Ar isotopes are sensitive tracers of metasomatism in the S-SCLM. Metasomatic fluids that permeate the S-SCLM act to archive a “subduction-fingerprint” that can be used to probe relative volatile-element recycling efficiencies and thus provide insight into volatile transport between the surface and mantle reservoirs over Earth history.

Asthenosphere-induced melting of diverse source regions for East Carpathian post-collisional volcanism

The occurrence of post-subduction magmatism in continental collision zones is a ubiquitous feature of plate tectonics, but its relation with geodynamic processes remains enigmatic. The nature of mantle sources in these settings, and their interaction with subduction-related components, are difficult to constrain using bulk rocks when magmas are subject to mixing and assimilation within the crust. Here we examine post-collisional magma sources in space and time through the chemistry of olivine-hosted melt inclusions and early-formed minerals (spinel, olivine and clinopyroxene) in primitive volcanic rocks from the Neogene–Quaternary East Carpathian volcanic range in Călimani (calc-alkaline; 10.1–6.7 Ma), Southern Harghita (calc-alkaline to shoshonitic; 5.3–0.03 Ma) and the Perșani Mountains (alkali basaltic; 1.2–0.6 Ma). Călimani calc-alkaline parental magma compositions indicate a lithospheric mantle source metasomatised by ~ 2% sediment-derived melts, and are best reproduced by ~ 2–12% melting. Mafic K-alkaline melts in Southern Harghita originate from a melt- and fluid-metasomatised lithospheric mantle source containing amphibole (± phlogopite), by ~ 5% melting. Intraplate Na-alkaline basalts from Racoș (Perșani) reflect small-degree (1–2%) asthenosphere-derived parental melts which experienced minor interaction with metasomatic components in the lithosphere. An important feature of the East Carpathian post-collisional volcanism is that the lithospheric source regions are located in the lower plate (distal Europe-Moesia), rather than the overriding plate (Tisza-Dacia). The volcanism appears to have been caused by (1) asthenospheric uprise following slab sinking and possibly south-eastward propagating delamination and breakoff, which induced melting of the subduction-modified lithospheric mantle (Călimani to Southern Harghita); and (2) decompression melting as a consequence of minor asthenospheric upwelling (Perșani).

Geology, geochronology and geochemistry of the Miocene Sulutas volcanic complex, Konya-Central Anatolia: genesis of orogenic and anorogenic rock associations in an extensional geodynamic setting

ABSTRACT Subduction-related environments are complex geological sites where various magmas occur in close spatial and temporal association. To better understand geological processes led to the generation of compositionally distinct magmas, we need to know the origin and evolution of magmas formed in these settings. Our purpose here is to determine the genesis of the Sulutas Volcanic Complex (SVC), located near Konya (Central Anatolia, Turkey) and to understand its relation to the subduction-related processes. Based on a detailed field study of the SVC, we present here Ar-Ar geochronology, mineral chemistry, whole-rock major, trace element and Sr-Nd-Pb isotope data. The SVC can be subdivided into three distinct chemical groups. The oldest group (~16 Ma) characterized by bimodal association is composed of Na-alkaline basaltic rocks and adakitic dacite. The Na-alkaline basaltic rocks were derived from an OIB-like enriched asthenospheric mantle source, but the adakitic dacites from a mafic lower crust. Geochemistry of potassic lamprophyres (~13.5 to 12.5 Ma), the second group, can be best explained by melting of a depleted lithospheric mantle source that was metasomatized by slab-derived sediment melts. This metasomatic component generated phlogopite-rich veins in their source. The youngest group (~13 to 11 Ma), high-K calc-alkaline rocks (HKCA) is composed of basalt, andesite and mafic enclave-bearing dacite. Parental magmas to the HKCA are consistent with the derivation from a lithospheric mantle source metasomatized by fluid-like subduction components. Geochemical variations in the HKCA can be explained by magmatic processes (e.g. assimilation-fractional crystallization for basalt and dacite, fractional crystallization for andesite) from a common parental magma similar to the mafic enclave. The SVC composed of orogenic and anorogenic rocks occurred in a graben-like extensional basin in the Central Anatolia, and this extensional tectonics is related to the retreating subduction zone along the Cyprus arc within the convergence system of the African-Eurasian plates during Miocene.

40Ar-39Ar dating and petrology of monzonite ejecta in tephra from Quaternary Gölcük volcano (Isparta, SW Turkey): tear-related contrasting metasomatic symptoms in extensional mantle-derived magmas

Eruption of (74-26 ka old) pyroclastic (pumice fall) flow deposits of the Quaternary Gölcük volcano, Isparta, SW Turkey, carried a cargo of Western Anatolian’s youngest monzonitic-syenitic ejecta, 40Ar-39Ar hornblende ages range 313+64−268+43 ka, and indicate that a shallow level felsic plutonic complex existed underneath the crater. These ejecta represent some of the youngest plutonic rocks known. The cooling time of this complex was coeval with the earlier main pyroclastic flow deposits (440 - 148 ka) of the multi-cycle eruptive center which also had a long-lived magma chamber. The monzosyenite ejecta contain plagioclase, K-feldspar, clinopyroxene, amphibole, mica, and accessory apatite, titanite and magnetite. In terms of Nb-Y-3Ga and Nb-Y-Ce, the ejecta, with strong shoshonitic affinity, plot in the field of A1 subtype granites, and represent A1-type magma generation in a back-arc extensional setting. Low Sr and high Nd isotopic ratios of monzosyenites exhibit isotopic similarity to enriched mantle-derived Gölcük-Isparta shoshonitic-ultrapotassic magmas. REE patterns show no significant Eu anomaly, but a strong enrichment of the LREE over the HREE. In multi-element diagrams, however, all samples have negative anomalies of Nb_Ta, Zr_Hf, P and Ti and strong enrichments of Ba, Sr, U, and Th. Moreover, variable (Hf/Sm)N and low (Ta/La)N ratios reflect subduction-related metasomatism in their mantle source. Gölcük monzosyenites and shoshonitic-ultrapotassic rocks overlap with mantle-derived A1-type alkali syenites in trace element abundances. This observation suggests that not only subduction–related fluids/melts, but also intraplate sublithospheric metasomatic agents played a crucial role on their mantle source. It is contended that, due to the percolation of both sublithospheric alkali silicate- and subduction-released-melts/fluids, the metasomatized mantle-derived parental magmas may illustrate a chemical paradox, carrying geochemical imprints of genetically distinct types of mantle sources. Such a metasomatic component in continental lithosphere defined in the genesis of Gölcük monzonitic/syenitic magmas can be explained by tearing of the African oceanic slab during Miocene, where slab-derived and intraplate melts/fluids concurrently modified the lithospheric mantle. Metasomatism of wall-rock peridotites occurred prior to the generation of Gölcük magmas during Miocene slab-tearing. Plio-Quaternary Gölcük potassic magmas were possibly derived from partial melting of carbonated mantle metasomes under an extensional tectonic regime.

K-rich hydrous mantle lithosphere beneath the Ontong Java Plateau: Significance for the genesis of oceanic basalts and Archean continents

Olivine-free metasomatic mantle-derived xenoliths, frequently recovered from kimberlite and lamprophyre intrusions on the continents, are seldom described from oceanic settings. We report the mineralogy, geochemistry and Sr-Nd-Hf-Pb-Os isotopic compositions of a unique K-rich, hydrous mantle nodule sourced from a 34 Ma ultramafic lamprophyre (alnöite) pipe on the island of Malaita, Ontong Java Plateau, SW Pacific. The Ontong Java Plateau, the most voluminous oceanic large igeous province, has a >100 km thick lithospheric root reminiscent of thick lithospheres that characterize cratonic settings. The phlogopite, amphibole, clinopyroxene and ilmenite-rich nature of the Malaita metasomatic nodule bears striking similarity to cratonic mica-amphibole-rutile-ilmenite-diopside (MARID) suite xenoliths, and it provides a tangible example of heavily overprinted oceanic mantle lithosphere. The nodule phlogopite 40Ar/39Ar age of 44.7 ± 1.8 Ma (95% confidence level) predates the 34 Ma alnöites, but is contemporaneous with 44 Ma alkali basalts on Malaita. Geodynamic reconstructions of the Ontong Java Plateau position within the Pacific realm demonstrate that alnöite magma and K-rich metasomatic nodule formation occurred within a strictly oceanic environment during the Eocene, away from subduction zones. The elevated incompatible trace element concentrations coupled with low highly siderophile element contents suggest that the K-rich metasomatic nodule formed by olivine-absent crystallisation from low-volume mantle-derived melt comparable to alnöite, but not the Malaita alkali basalts. A genetic link between the Malaita metasomatic nodule and alnöite is further suggested by overlapping Sr-Nd-Hf-Pb isotopic compositions (87Sr/86Sr45Ma = 0.70419–0.70423; εNd45Ma = +3.5; εHf45Ma = +5.3; 206Pb/204Pb45Ma = 18.66–18.71; 207Pb/204Pb45Ma = 15.61). These isotopic compositions are generally more enriched than those of mantle-derived peridotites and 122 Ma plateau-building basalts at Ontong Java, but share similarities with pyroxenite xenoliths from Malaita previously interpreted to represent ancient recycled crustal material. Mixing models between melts derived from fertile mantle and the more enriched pyroxenite, as well as recycled sedimentary material, can account for the composition of the K-rich metasomatic nodule. Extremely low contents of highly siderophile elements and high 187Os/188Os45Ma (0.1824–0.1997) can also be reconciled with the involvement of recycled crustal components in the complex origin of the K-rich hydrous nodule. K-rich hydrous metasomatized mantle lithosphere could represent an enriched source component that is required for the formation of many oceanic intraplate basaltic provinces. As such, strongly metasomatized oceanic lithosphere provides an alternative to recycled crust components within the convecting mantle to explain enriched mantle signatures observed in oceanic basalts worldwide. However, constraining the volumetric abundance of strongly metasomatized oceanic lithosphere, possibly revealed by seismic mid-lithospheric discontinuities, is required before its importance can be established. The existence of K-rich metasomatic components within oceanic plateau lithosphere may also have implications for Archean continental crust formation models. The K-rich mantle component required to explain sanukitoid magma formation associated with continental crust building TTG magmatism may not be restricted to a subduction channel environment, but it can equally reside within overthickened oceanic lithosphere. Thus, oceanic plateaus away from subduction zones could have served as nucleation points for growth of continental crust during the Archean.

The Thickness of the Mantle Lithosphere and Collision-Related Volcanism in the Lesser Caucasus

The Lesser Caucasus mountains sit on a transition within the Arabia–Eurasia collision zone between very thin lithosphere (<100 km) to the west, under Eastern Anatolia, and a very thick lithospheric root (up to 200 km) in the east, under western Iran. A transect of volcanic highlands running from NW to SE in the Lesser Caucasus allows us to look at the effects of lithosphere thickness variations on the geochemistry of volcanic rocks in this continental collision zone. Volcanic rocks from across the region show a wide compositional range from basanites to rhyolites, and have arc-like geochemical characteristics, typified by ubiquitous negative Nb–Ta anomalies. Magmatic rocks from the SE, where the lithosphere is thought to be thicker, are more enriched in incompatible trace elements, especially the light rare earth elements, Sr and P. They also have more radiogenic ⁸⁷Sr/⁸⁶Sr, and less radiogenic ¹⁴³Nd/¹⁴⁴Nd. Across the region, there is no correlation between SiO₂ content and Sr–Nd isotope ratios, revealing a lack of crustal contamination. Instead, ‘spiky’ mid-ocean ridge basalt normalized trace element patterns are the result of derivation from a subduction-modified mantle source, which probably inherited its subduction component from subduction of the Tethys Ocean prior to the onset of continent–continent collision in the late Miocene. In addition to the more isotopically enriched mantle source, modelling of non-modal batch melting suggests lower degrees of melting and the involvement of garnet as a residual phase in the SE. Melt thermobarometry calculations based on bulk-rock major elements confirm that melting in the SE must occur at greater depths in the mantle. Temperatures of melting below 1200°C, along with the subduction-modified source, suggest that melting occurred within the lithosphere. It is proposed that in the northern Lesser Caucasus this melting occurs close to the base of the very thin lithosphere (at a depth of ∼45 km) as a result of small-scale delamination. A striking similarity between the conditions of melting in NW Iran and the southern Lesser Caucasus (two regions between which the difference in lithosphere thickness is ∼100 km) suggests a common mechanism of melt generation in the mid-lithosphere (∼75 km). The southern Lesser Caucasus magmas result from mixing between partial melts of deep lithosphere (∼120 km in the south) and mid-lithosphere sources to give a composition intermediate between magmas from the northern Lesser Caucasus and NW Iran. The mid-lithosphere magma source has a distinct composition compared with the base of the lithosphere, which is argued to be the result of the increased retention of metasomatic components in phases such as apatite and amphibole, which are stabilized by lower temperatures prior to magma generation.

Some remarks on the nature of mantle metasomatism beneath western Anatolian–Aegean region: Contrasting isotopic signatures recorded in the Miocene lavas from the Söke Basin

The presence of a subduction signature with variable isotopic patterns in the melt products of mantle origin within the Aegean–Anatolian domain reflects the differential effects of slab‐derived material on the chemical composition of the supraslab mantle region. Here, we use Sr–Nd–Pb isotopic compositions and major and trace element abundances to evaluate the characteristics of the source components and possible roles of fluid/melt metasomatism in the genesis of the Middle to Late Miocene mafic–intermediate lavas from the Söke Basin in western Anatolia. This volcanic suite comprises a number of individual lava flows in which the dominant rock types are potassium‐rich basaltic andesites and andesites with trace element relative abundances indicative of melt derivation from subduction‐modified sources. These samples have significantly heterogeneous Sr and Nd isotopic ratios, with 87Sr/86Sr and 143Nd/144Nd values ranging from 0.70475 to 0.70927 and from 0.51218 to 0.51270, respectively; they also yield a range of Pb isotope ratios, with 206Pb/204Pb = 18.963–19.344, 207Pb/204Pb = 15.672–15.733, and 208Pb/204Pb = 39.096–39.264. Remarkably large variations in isotopic ratios with relatively limited changes in major elements suggest the involvement of heterogeneously enriched sources with diverse isotopic signatures. The modelling of trace element and isotopic data indicates that parts of the lithospheric mantle in this region are isotopically enriched, with geochemical signatures that reflect the interaction of variably depleted mantle peridotites with melts from subducted sediments of continental crustal origin, while other parts host phlogopite‐rich metasomatic components with signatures of devolatilization reactions from ancient subduction events that led to the preferential loss of Pb and Rb to create metasomatic domains with high U/Pb and low Rb/Sr values that ultimately generated highly radiogenic 206Pb/204Pb and relatively unradiogenic 87Sr/86Sr values. The observed isotopic diversity in relatively short‐time‐ and length‐scales may be interpreted to have resulted largely from the variable extent of melt contributions from a number of discrete lithological domains with contrasting long‐term isotopic evolution and is consistent with the view that the continental lithosphere in this region comprises a number of disparate lithospheric fragments with different petrogenetic histories.

Vestiges of older greenstone in Mesoarchaean Chakradharpur granite gneiss, Singhbhum Craton, India: Implications for plume‐lithosphere interaction at rifted cratonic margin

The Chakradharpur granite gneiss (CGG) represents an uplifted Archaean basement within the North Singhbhum Orogen (NSO) of Singhbhum‐Orissa Craton in the eastern Indian shield and shares a Mesoarchaean 3.12 Ga age equivalent to the emplacement of Singhbhum granite (SBG) phase III. The CGG hosts numerous enclaves of tonalitic gneisses, metamorphosed ultramafic and mafic volcanics, metagabbros, metasediments, and conglomerates that collectively represent vestiges of an older greenstone belt preserved within it. The mineralogical composition of the ultramafic volcanic enclaves of CGG has been overprinted by greenschist facies metamorphism presenting a talc‐tremolite‐phlogopite‐actinolite assemblage with prominent development of crenulation cleavage, pressure solutions and relict spinifex texture. These are characterised by restricted low SiO2 (39.7–46.8 wt.%) at consistently high MgO (20.7–28 wt.%) contents, and geochemically classified as: (a) subalkaline (Na2O + K2O &lt; 2) komatiite and meimechite and (b) an alkaline (Na2O + K2O &gt; 2) type picrobasalt‐foidite‐basanite assemblage. Depleted LILE and LREE chemistry, Al2O3/TiO2 versus (Gd/Yb)N and HFSE abundances suggest that the studied Al‐undepleted Munro‐type komatiite and meimechite were derived by high degree (30%–50%), shallow level (&lt;90 km) partial melting of a hot, anhydrous mantle plume carrying depleted components. It is suggested that the komatiite‐meimechite melts were deflected by an Archaean cratonic keel and erupted through thinned continental lithosphere at a rifted cratonic margin. Geochemical evidence of crustal contamination preserved in these komatiites are equated with minor input from reworking of older continental materials and interaction between plume‐derived asthenospheric melts and metasomatized subcontinental lithospheric mantle (SCLM). Zr/Hf, Zr/Sm, Nb/Ta, Zr/Nb, and Nb/Th ratios for the picrobasalt, high Mg basalt, foidite and basanite rocks corroborate their generation through variable degrees of partial melting of a heterogeneous lithospheric mantle source carrying depleted plume‐derived melts and enriched components from metasomatized lithosphere. Mantle processes involved assimilation of metasomatic components of SCLM by ascending melts of plume incubating at the base of lithosphere ensued by melt generation at different mantle depths extending from garnet to spinel peridotite domains. Therefore, the geochemical signatures of ultramafic enclaves in CGG invoke distinct magmatic episodes during plume‐craton interaction, melt generation at thinned craton margin to rifted continental interior and melting of metasomatized continental lithosphere that collectively contributed to the evolution of Archaean mantle lithosphere system, volcanism with diverse chemical compositions and greenstone generation in the Singhbhum Craton.

Olivine in ultramafic lamprophyres: chemistry, crystallisation, and melt sources of Siberian Pre- and post-trap aillikites

We studied olivines from the Devonian pre-trap (the Ilbokich occurrence) and the Triassic post-trap (the Chadobets occurrence) carbonate-rich ultramafic lamprophyres (UMLs) in the southwestern portion of the Siberian craton. On the basis of detailed investigations of major, minor, and trace-element distributions, we have reconstructed the main processes that control the origins of these olivines. These include fractional crystallisation from melt, assimilation, and fractional crystallisation processes with orthopyroxene assimilation, melt-reaction diffusive re-equilibration, alkali enrichment, and CO2 degassing of the melt. Furthermore, we inferred the composition of the sources of the primary UML melt and their possible correlations with proto-kimberlitic melts, as well as the influence of the Triassic Siberian plume on the composition of the lithospheric mantle. The main differences between olivines from the Ilbokich and the Chadobets aillikites were that the olivines from the former had more magnesium-rich cores (Mg# = 89.2 ± 0.2), had Mg- and Cr-rich transition zones (Mg# = 89.7 ± 0.2 and 300–500 ppm Cr), had lower Ni (up to 3100 ppm) and Li (1.4–1.5 ppm), and had higher B (0.8–2.6 ppm) contents, all at higher Fo values (90–86), relative to the olivines from the latter (Mg# = 88–75; 200–300 ppm Cr; up to 3400 ppm Ni; 1.4–2.4 ppm Li; 0.4–2.2 ppm B). The Siberian aillikite sources contained a significant amount of metasomatic material. Phlogopite-rich MARID-type veins provided the likely metasomatic component in the pre-trap Devonian Ilbokich aillikite source, whereas the Triassic Chadobets aillikitic post-trap melts were derived from a source with a significant carbonate component. A comparison of UML olivines with olivines from the pre-trap and post-trap Siberian kimberlites shows a striking similarity. This suggests that the carbonate component in the aillikitic source could have been produced by evolved kimberlite melts. The differences in the lithospheric metasomatic component that contributed to pre-trap and post-trap aillikitic melts can be interpreted as reflections of the thermal impact of the Siberian Traps, which reduced phlogopite-bearing metasomes within the southwestern Siberian sub-continental lithospheric mantle.

Seismic evidence for depth-dependent metasomatism in cratons

The long-term stability of cratons has been attributed to low temperatures and depletion in iron and water, which decrease density and increase viscosity. However, steady-state thermal models based on heat flow and xenolith constraints systematically overpredict the seismic velocity-depth gradients in cratonic lithospheric mantle. Here we invert for the 1-D thermal structure and a depth distribution of metasomatic minerals that fit average Rayleigh-wave dispersion curves for the Archean Kaapvaal, Yilgarn and Slave cratons and the Proterozoic Baltic Shield below Finland. To match the seismic profiles, we need a significant amount of hydrous and/or carbonate minerals in the shallow lithospheric mantle, starting between the Moho and 70 km depth and extending down to at least 100–150 km. The metasomatic component can consist of 0.5–1 wt% water bound in amphibole, antigorite and chlorite, ∼0.2 wt% water plus potassium to form phlogopite, or ∼5 wt% CO2 plus Ca for carbonate, or a combination of these. Lithospheric temperatures that fit the seismic data are consistent with heat flow constraints, but most are lower than those inferred from xenolith geothermobarometry. The dispersion data require differences in Moho heat flux between individual cratons, and sublithospheric mantle temperatures that are 100–200 °C less beneath Yilgarn, Slave and Finland than beneath Kaapvaal. Significant upward-increasing metasomatism by water and CO2-rich fluids is not only a plausible mechanism to explain the average seismic structure of cratonic lithosphere but such metasomatism may also lead to the formation of mid-lithospheric discontinuities and would contribute to the positive chemical buoyancy of cratonic roots.

Temporal constraints on magma generation and differentiation in a continental volcano: Buckland, eastern Australia

The eastern margin of the Australian continent hosts a large number of Cenozoic intraplate volcanoes along a 2000 km long track. Here, we study mafic lavas from the Buckland volcano, Queensland, located in the northern (older) segment of this track, to assess magma generation and differentiation through time. The rocks are aphanitic to microporphyritic basalts, trachy-basalts and basanites. Incompatible element geochemistry together with Sr-Nd-Pb isotope ratios indicate that magmas formed from an enriched mantle I (EMI)-like garnet-bearing source with variable degrees of crustal contamination. Whole rock elemental variations suggest fractionation of olivine, plagioclase, clinopyroxene and/or magnetite. There is no petrographic or geochemical evidence of magma mixing in the studied rocks (e.g., lack of recycled minerals), suggesting a relatively quick ascent from the source to the surface without major storage at shallow levels.40Ar/39Ar geochronology reveals two stages of volcanism: 30.3 ± 0.1 Ma and 27.4 ± 0.2 Ma. The Old Buckland (30.3 ± 0.1 Ma) melts have negative K anomalies, and incompatible element ratios suggest the occurrence of residual hydrous minerals in a metasomatised mantle source. We therefore infer that at the onset of volcanism, deep-mantle-derived magmas interacted with metasomatised sub-continental lithospheric mantle (SCLM). Major and trace element data, clinopyroxene thermobarometry and thermodynamic modelling indicate magma evolution by assimilation and fractional crystallisation (AFC) during ascent through the crust. Following a hiatus in volcanic activity of ~2.5 Ma, eruption of Young Buckland (27.4 ± 0.2 Ma) lavas marked a shift towards more alkaline compositions. Trace element compositions indicate lower degrees of partial melting and a lack of interaction with metasomatic components. Young Buckland lavas become progressively more SiO2-saturated up stratigraphy, suggesting an increase in the degree of partial melting with time. Young Buckland lavas also have more radiogenic 87Sr/86Sr and 207Pb/204Pb ratios and less radiogenic 143Nd/144Nd ratios up stratigraphy. These isotopic variations, together with coupled increases in Pb and K and decreases in Ce/Pb (27.22 to 11.09) and Nb/U (68.30 to 29.96), suggest that crustal contamination also increased with time.By placing absolute age and stratigraphic constraints on the Buckland lavas, we have been able to ascertain differentiation signatures imposed on mantle-derived melts during ascent through the continental lithosphere over 3 Ma. Our study provides new constraints on magma generation and differentiation in continental intraplate volcanic systems.

Variation of olivine composition in the volcanic rocks in the Songliao basin, NE China: lithosphere control on the origin of the K-rich intraplate mafic lavas

Lithospheric thickness and the heterogeneity of the mantle lithosphere are two major parameters that play a role in determining the final composition of the mafic melts and their minerals. The Songliao basin in northeast China represents an ideal natural laboratory to study the effect of these two parameters on early Pliocene to Holocene K-rich mafic lavas (K2O>4wt.%; K2O/Na2O>1). A series of Cenozoic volcanic edifices (Erkeshan, Wudalianchi, Keluo and Xiaogulihe) are tentatively divided into three groups (Group 1 — thin, Group 2 — middle, and Group 3 — thick) according to the lithosphere thickness. They are located in the northern region of the Songliao basin extending in a near north–south direction along a broad zone where the lithosphere thickness increases gradually. We present a detailed petrographical and geochemical study on olivine macrocrysts in combination with new geochemical data on their host lavas, including major and trace element abundances as well as Sr, Nd, and Pb isotopic signatures. Our ultimate aim is to quantitatively and qualitatively determine the role of lithospheric mantle thickness (named as “lid effect”) and composition in the variation of mafic lavas and olivine composition.When corrected to Mg#=0.72, a number of major elements in the lavas correlate with increasing lithospheric thickness (L): Si72 and Al72 decrease, whereas Mg72, Fe72, Ti72 and P72 increase. Sm/Yb ratios in the lavas increase, implying that lithospheric thickness exerts an important control. Group 3 mafic lavas are ultrapotassic (showing lamproite affinity) with K2O/Na2O>4: their La/Sm and Pb isotope ratios deviate from the above correlations, indicating that the lavas from the thickest part of the basin exhibit the highest extent of metasomatic enrichment of the mantle source.Several parameters (e.g. [Ni], Ni/Mg, Ni/(Mg/Fe), Mn/Fe and Ca/Fe) in melt-related olivine from Group 1 and Group 2 lavas are controlled by variable lithosphere thickness. Olivine from Group 3, however, is heavily influenced by the composition of the lithosphere, which disturbs the lid effect. Trace elements (e.g. Al, V, Zn and Li) in olivine are mostly invariable across all three groups. The low-Ni olivine and olivine crystal fractionation model indicates magma mixing in the source region of Group 3 lavas, with melts derived from a phlogopite-bearing wehrlitic metasomatic component and surrounding peridotitic mantle. Al and Cr concentrations in olivines indicate that the mantle sources are highly refractory harzburgite instead of lherzolite, and that the SCLM under the basin is stratified.Our new olivine data offer additional support for the lithospheric mantle origin of the Songliao K-rich mafic lavas, with a clear affinity towards an EM1 mantle component. The identification of phlogopite in the source strongly implies a lithospheric mantle source, and high P (200–600μg/g) in olivine suggests that metasomatism by volatile-rich melts occurred prior to melting.

Melt evolution beneath a rifted craton edge: 40Ar/39Ar geochronology and Sr–Nd–Hf–Pb isotope systematics of primitive alkaline basalts and lamprophyres from the SW Baltic Shield

A new high-precision 40Ar/39Ar anorthoclase feldspar age of 176.7±0.5Ma (2-sigma) reveals that small-volume alkaline basaltic magmatism occurred at the rifted SW margin of the Baltic Shield in Scania (southern Sweden), at a time of global plate reorganization associated with the inception of Pangea supercontinent break-up. Our combined elemental and Sr–Nd–Hf–Pb isotope dataset for representative basanite and nephelinite samples (>8wt.% MgO) from 16 subvolcanic necks of the 30 by 40km large Jurassic volcanic field suggests magma derivation from a moderately depleted mantle source (87Sr/86Sri=0.7034–0.7048; εNdi=+4.4 to +5.2; εHfi=+4.7 to +8.1; 206Pb/204Pbi=18.8–19.5). The mafic alkaline melts segregated from mixed peridotite–pyroxenite mantle with a potential temperature of ∼1400°C at 2.7–4.2GPa (∼90–120km depths), which places ultimate melt generation within the convecting upper mantle, provided that the lithosphere–asthenosphere boundary beneath the southern Baltic Shield margin was at ⩽100km depth during Mesozoic–Cenozoic rifting. Isotopic shifts and incompatible element enrichment relative to Depleted Mantle reflect involvement of at least 20% recycled oceanic lithosphere component (i.e., pyroxenite) with some minor continent-derived sediment during partial melting of well-stirred convecting upper mantle peridotite. Although pargasitic amphibole-rich metasomatized lithospheric mantle is excluded as the main source of the Jurassic magmas from Scania, hydrous ultramafic veins (i.e., hornblendite) may have caused subtle modifications to the compositions of passing sublithospheric melts. For example, modeling suggests that the more radiogenic Hf (εHfi=+6.3 to +8.1) and Pb (206Pb/204Pbi=18.9–19.5) isotopic compositions of the more sodic and H2O-rich nephelinites, compared with relatively homogenous basanites (εHfi=+4.7 to +6.1; 206Pb/204Pbi=18.8–18.9), originate from minor interactions between rising asthenospheric melts and amphibole-rich metasomatic components. The metasomatic components were likely introduced to the lithospheric mantle beneath the southern Baltic Shield margin during extensive Permo-Carboniferous magmatic activity, a scenario that is supported by the geochemical and isotope compositions of ca. 286Ma lamprophyres from Scania (87Sr/86Sri=0.7040–0.7054; εNdi=+2.0 to +3.1; εHfi=+6.1 to +9.0; 206Pb/204Pbi=17.8–18.2).Strong variations in lithosphere thickness and thermal structure across the southern Baltic Shield margin may have caused transient small-scale mantle convection. This resulted in relatively fast and focused upwellings and lateral flow beneath the thinned lithosphere, where mafic alkaline magmas formed by low degrees of decompression melting of sublithospheric mantle. Such a geodynamic scenario would allow for enriched recycled components with low melting points to be preferentially sampled from the more depleted and refractory convecting upper mantle when channeled along a destabilizing craton edge. Similar to the ‘lid effect’ in oceanic island volcanic provinces, lithospheric architecture may exert strong control on the mantle melting regime, and thus offer a simple explanation for the geochemical resemblance of continental and oceanic intraplate mafic alkaline magmas of high Na/K affinity.

Petrogenesis of orogenic lamproites of the Bohemian Massif: Sr–Nd–Pb–Li isotope constraints for Variscan enrichment of ultra-depleted mantle domains

During convergence of Gondwana-derived microplates and Laurussia in the Palaeozoic, subduction of oceanic and continental crusts and their sedimentary cover introduced material of regionally contrasting chemical and isotopic compositions into the mantle. This slab material metasomatised the local mantle, producing a highly heterogeneous lithospheric mantle beneath the European Variscides. The eastern termination of the European Variscides (Moldanubian and Saxo-Thuringian zones of Austria, Czech Republic, Germany and Poland) is unusual in that the mantle was modified by material from several subduction zones within a small area. Orogenic lamproites sampled this lithospheric mantle, which has a chemical signature reflecting extreme depletion (low CaO and Al2O3 contents and high Mg-number) followed by strong metasomatic enrichment, giving rise to crust-like trace element patterns, variable radiogenic 87Sr/86Sr(330) (0.7062–0.7127) and non-radiogenic Nd isotopic compositions (εNd(330)=−2.8 to −7.8), crustal Pb isotopic compositions, and a wide range of δ7Li values (−5.1 to +5.1). This metasomatic signature is variably expressed in the lamproites, depending on the extent of melting and the nature of the source of the metasomatic component. Preferential melting of the metasomatically enriched (veined) lithospheric mantle with K-rich amphibole resulted in lamproitic melts with very negative, crust-like δ7Li values, which correlate positively with peralkalinity, HFSE contents and lower εNd. Both the higher degree of melting and progressive consumption of the metasomatic component reduce the chemical and isotopic imprints of the metasomatic end member. The very positive δ7Li values of some lamproites indicate that the source of these lamproites may have been modified by subducted oceanic lithosphere. Fresh olivine from the Brloh (Moldanubian) lamproitic dyke shows very high Fo (up to 94%) and very high Li contents (up to 25ppm), demonstrating that the extremely depleted and later enriched lithospheric mantle may have contributed significantly to the Li budget of the lamproites. The regional distribution of lamproites with contrasting chemical and isotopic fingerprints mimics the distribution of the different Variscan subduction zones.

Dating thin zircon rims by NanoSIMS: the Fengtien nephrite (Taiwan) is the youngest jade on Earth

Nephrite in the Fengtien area of the eastern part of the Central Mountain Range, Taiwan, is associated with antigorite-serpentinite within the Yuli belt, a late Cenozoic subduction–accretionary complex related to the eastward subduction of the South China Sea plate forming the Luzon arc. Diopsidite and clinozoisite rock are two other metasomatic components accompanying nephrite between serpentinites and the greenschist-facies country rock (carbonaceous material-)quartz-mica schist. Detrital zircons were separated from one clinozoisite rock sample, formed through metasomatic replacement after mica-quartz schist at temperatures of 320–420°C or slightly lower, which is lower than the metamorphic temperature conditions of the Yuli belt. Most of the detrital zircons have thin zircon rims less than 15–20 μm wide. These zircon rims, considered as newly formed during metasomatism leading to nephrite/diopsidite/clinozoisite rock formation, were dated by a high lateral resolution secondary ion mass spectrometer (CAMECA NanoSIMS NS50). The resulting 238U/206Pb-204Pb/206Pb inverse isochron gave an age of 3.3 ± 1.7 Ma. The collision of the Eurasian continental margin with the Luzon arc has been suggested to have begun at ca. 6.5 Ma in the Taiwan area. The nephrite formation processes therefore clearly post-dated South China Sea plate subduction. The present date, substantiated by the metamorphic and metasomatic temperature information, demonstrates that the fluid–rock interaction forming Fengtien nephrite would have taken place during a Barrovian-type metamorphic overprint resulting from arc-continent collision, leading to the exhumation of the Yuli belt. This conclusion on nephrite formation with regard to regional tectonics can serve as a working model for future studies on other nephrite deposits with similar occurrences, mostly embedded within Mesozoic or older subduction-accretionary complexes. The Fengtien nephrite deposit is therefore the youngest one of its kind exposed on Earth’s surface.