Subcontinental Lithospheric Mantle Research Articles

Major, trace element and Sr-Nd isotope evidence for a sublithospheric mantle source for the Umkondo large igneous province

The Mesoproterozoic (1.11 Ga) Umkondo large igneous province (LIP) in southern Africa and Antarctica was emplaced in < 5 Myr and is dominated by low-Ti tholeiitic doleritic-gabbroic sills. It is of particular interest because it is the least studied LIP in southern Africa with both sublithospheric and lithospheric mantle sources proposed and it coincides with the early assembly of Rodinia, so it has importance in understanding the nature of magmatism and tectonics in and around the Kalahari craton during the Mesoproterozoic. In this study, we compiled a large database of existing (∼750) and new (∼100) major and trace element data for the Umkondo province, as well as 42 new Sr-Nd isotopic measurements, to provide constraints on its magma sources and geochemical evolution. Major element compositional variations in the low-Ti tholeiites are explained by low-pressure (1 kbar) three-phase fractional crystallisation (olivine, clinopyroxene and plagioclase) of a parent magma with ∼ 10 wt.% MgO in oxidising conditions (QFM + 1). Inverse models show that the low-Ti tholeiitic magmas were derived as residual melts after the crystallization of 12%–33% olivine from primary komatiitic-basaltic magmas (up to ∼ 20 wt.% MgO) in equilibrium with mantle olivine (Fo90). Low Sm/Yb and TiO2/Yb-Nb/Yb indicate that the primary magmas were derived by 2%–20% shallow (40–50 km) partial melting of spinel lherzolite. High Sm/Yb is restricted to dyke swarms and may imply limited magma production from deeper (up to ∼ 70 km) garnet lherzolite-like sources. The low-Ti tholeiites of the Umkondo province are enriched in large ion lithophile elements (Rb-Sr-Cs-K) and depleted in high-field strength elements (Zr-Hf-Nb-Ta), indicating the involvement of crustal material and/or the subcontinental lithospheric mantle. This is supported by covariations in Th/Nb, Nb/Yb, Nb/La and Ce/Sm with generally negative ΔNb. Sr-Nd isotopes lend support to the notion that the Umkondo magmas were derived from depleted and/or enriched sublithospheric mantle sources and subsequently contaminated by enriched lithospheric material during emplacement (initial (at 1.11 Ga) 87Sr/86Sr between 0.704820 and 0.737464 and εNd between −8.9 and +5.3). The Vredefort sills are significant as they display the most depleted Sr-Nd isotopic signatures (average initial 87Sr/86Sr of 0.705342 and average εNd of 0.4) and are the least contaminated magma suite in the Umkondo province. Because of (i) the large volume of low-Ti magmas, (ii) evidence of a primary hot and MgO-rich (komatiitic) magma, and (iii) the short duration of magmatism, we suggest that the Umkondo province was formed by plume-induced melting of the sublithospheric mantle beneath the Kalahari craton in an extensional setting. This contrasts with previous suggestions that the heat source developed in response to the “thermal insulation” of the mantle beneath a thickened Kalahari craton in the absence of a mantle plume. There is further evidence from the elevated Zn/Fe that the sublithospheric mantle was lithologically heterogeneous and consisted of mixed peridotite and pyroxenite domains. There is a general lack of ultramafic cumulates in the low-Ti magma suite that may imply there was deeper ponding and storage of the primary magmas that fractionated large quantities of ultramafic rocks. There is also a paucity of high-Ti rocks in the Umkondo province that may reflect limited direct melting of the lithospheric mantle or that they are simply not as well-preserved in this province compared to the Karoo province. The similar trace element and Sr-Nd isotopic compositions of the Umkondo sills in southern Africa with the Borgmassivet sills in Antarctica support the concept that the Kalahari craton and Grunehogna terrane were adjoined at 1.11 Ga. The timing of the Umkondo province indicates there was localised lithospheric extension and upwelling asthenospheric mantle during a time of dominantly compressional tectonics on Earth at the end of the ‘boring billion’.

Petrology, Age, and Rift Origin of Ultramafic Lamprophyres (Aillikites) at Mount Webb, a New Alkaline Province in Central Australia

AbstractDiamond exploration over the past decade has led to the discovery of a new province of kimberlitic pipes (the Webb Province) in the Gibson Desert of central Australia. The Webb pipes comprise sparse macrocrystic olivine set in a groundmass of olivine, phlogopite, perovskite, spinel, clinopyroxene, titanian‐andradite and carbonate. The pipes resemble ultramafic lamprophyres (notably aillikites) in their mineralogy, major and minor oxide chemistry, and initial 87Sr/86Sr and εNd‐εHf isotopic compositions. Ion probe U‐Pb geochronology on perovskite (806 ± 22 Ma) indicates the eruption of the pipes was co‐eval with plume‐related magmatism within central Australia (Willouran‐Gairdner Volcanic Event) associated with the opening of the Centralian Superbasin and Rodinia supercontinent break‐up. The equilibration pressure and temperature of mantle‐derived garnet and chromian (Cr) diopside xenocrysts range between 17 and 40 kbar and 750–1320°C and define a paleo‐lithospheric thickness of 140 ± 10 km. Chemical variations of xenocrysts define litho‐chemical horizons within the shallow, middle, and deep sub‐continental lithospheric mantle (SCLM). The shallow SCLM (50–70 km), which includes garnet‐spinel and spinel lherzolite, contains Cr diopside with weakly refertilized rare earth element compositions and unenriched compositions. The mid‐lithosphere (70–85 km) has lower modal abundances of Cr diopside. This layer corresponds to a seismic mid‐lithosphere discontinuity interpreted as pargasite‐bearing lherzolite. The deep SCLM (&gt;90 km) comprises refertilized garnet lherzolite that was metasomatized by a silicate‐carbonatite melt.

Geochronology, petrogenesis, and tectonic setting of amphibolitic rocks from the Tutak metamorphic Complex, Sanandaj-Sirjan Zone, Iran

Amphibolitic rocks of the Tutak Metamorphic Complex (TMC), in the southern part of the Sanandaj Sirjan Zone (SaSZ), are associated with meta-dolerites, felsic intrusions, mica-schists, and marbles as bodies and interlayers. Rutile grains from an amphibolite (ES-18-1) yield a U-Pb age of 56.08 ± 1.13 Ma (Late Paleocene-Early Eocene) while Zr-in-rutile thermometry indicates peak temperatures ∼ 616 to 674 °C. These data are inferred to reflect the timing of medium-pressure/medium-temperature (MP/MT) amphibolite facies metamorphism in Late Paleocene-Early Eocene rather than upper Cretaceous time in the TMC complex. Geochemically, the amphibolites are from an Nb-enriched basalt-andesite to basalt protolith with sub-alkaline to alkaline affinity. The amphibolites have moderate 87Sr/86Sr(t) ratios (0.702451 to 0.712720), 143Nd/144Nd(t) ratios (0.512122 to 0.512209), positive ƐNd(535 Ma) (3.38 to 5.08) values and two-stage Nd model ages (TDM) between 0.84 and 1.3 Ga. The geochemical trends and (La/Yb)cn> 3, Nb/La> 0.6, Sm/Nd< 0.3, Zr/Y (4.42 to 7.47, avg. 5.72) ratios with moderate 87Sr/86Sr(535) ratios and positive ƐNd(535 Ma) values indicate that the mafic protoliths of the amphibolites are probably derived from the low partial melting of subcontinental lithospheric mantle in a spinel lherzolite field, with enriched mantle (E-MORB/OIB-like) nature. Subsequently, they experienced minor crustal contamination in the early stages of the development of intra-continental-type back-arc basin (BABB). Lack of subduction signals in the amphibolites with E-MORB/OIB-like features may result from the roll-back of subducting Proto-Tethys oceanic slab during the Cambrian similar to other peri-Gondwana terranes.

Petrogenesis, Sampling Depth, Thermal and Redox Evolution of Spinel Peridotite Xenoliths From Jiaohe, NE China: Insights From Trace Elements in All Rock‐Forming Silicate Minerals

AbstractOlivine and orthopyroxene (Opx) are the most abundant minerals in lherzolitic to harzburgitic peridotites, and their compositions represent an important—and sometimes only—source of information for the origin and physicochemical evolution of the subcontinental lithospheric mantle (SCLM). We report a high‐quality in situ trace element data set for Opx and olivine of well‐characterized spinel‐peridotite xenoliths, including eight lherzolites and four harzburgites, which were entrained in the Cenozoic basalts from Jiaohe, NE China. While the lherzolites and harzburgites show contrasting evolution involving various metasomatic agents, reflected in different trace element relationships, both experienced early cooling followed by recent (pre‐entrainment) heating, as unraveled by a combination of mineral thermometers with inherently different diffusion rates. The sampling depth of spinel‐facies peridotites is obtained by olivine‐spinel thermobarometry, taking into account post‐melting subsolidus processes and pre‐emplacement thermal disturbance, using an empirical correction for the Ca content in olivine, which yields a pressure range of 1.1–2.0 GPa. The V contents (92–116 μg/g) and V/Sc ratios (5.7–6.4) in lherzolitic Opx are higher than those in harzburgitic Opx (V: 56–76 μg/g; V/Sc: 3.4–5.9), qualitatively reflecting the reducing and oxidizing effect, respectively of refertilization versus metasomatic interaction with a small‐volume melt. Thus, comparative thermobarometry, together with trace element systematics of Opx and olivine can be effective tools to reveal the architecture, chemical, thermal, and redox evolution of the shallow SCLM, as sampled by spinel peridotite xenoliths.

Early Permian passive-margin magmatism in the Sibumasu Block, northern Thailand: A product of the slab-pull force of the Paleo-Tethys Ocean?

Continental rift-related magmatism provides important clues to continental rifting and breakup, but the petrogenesis and geodynamic processes remain debatable. Here we report a detailed study on zircon and apatite UPb dating, whole-rock geochemistry, and Sr-Nd-Hf isotopes for the hornblende gabbros in northern Thailand. Zircon and apatite UPb age data reveal emplacement of the hornblende gabbros at ca. 290 Ma. The hornblende gabbros are alkaline in composition and characterized by slight negative Nb-Ta-Zr-Hf and marked negative Ti anomalies. They have enriched Sr-Nd-Hf isotopic values [whole-rock (87Sr/86Sr)i = 0.705970–0.706756; εNd(t) = +0.1 to +0.7; zircon εHf(t) = +0.2 to +9.2]. Their geochemical affinities to within-plate basalts suggest a continental rifting setting. These combined features reveal that the hornblende gabbros in northern Thailand originated from a subcontinental lithospheric mantle previously modified by subducted sediments. Our results combined with published data indicate that the continental rift-related magmatism in the Sibumasu Block took place over a long period of ca. 309–282 Ma. We suggest that the slab pull force of the Paleo-Tethys oceanic lithosphere is a likely mechanism for the Late Carboniferous-Early Permian magmatic event in the passive margin of the Sibumasu Block.

Feeding system and mantle sources of the southern and western sector of the Madagascar flood basalt province, and comparisons with Southwest Indian Ridge “anomalous” basalts

The flood basalts of Cretaceous age in southwestern Madagascar range from moderately incompatible-element-enriched, tholeiitic, high-Ti ferrobasalt/basaltic andesite lavas (MgO = 4–5 wt%, Fe2O3t = 14–16 wt%; TiO2 = 3–3.6 wt%, Nb =16–22 ppm, Zr = 236–269 ppm; Lan/Ybn = 7–8) in the southernmost outcrops (Lavanono, Tulear), to incompatible element-poor basaltic dikes (MgO = 4.2–4.9 wt%, Fe2O3t = 12–13 wt%; TiO2 = 1.6 wt%, Nb = 8–9 ppm, Zr = 90–100 ppm; Lan/Ybn = 2.3) around Morondava. The two magma types, at the same degree of magmatic evolution, have markedly different Sr-Nd-Pb isotopic composition (e.g., 87Sr/86Sri = 0.7121–0.7123, 208Pb/204Pb = 40.9–41.3 for the Tulear-Lavanono lavas vs 87Sr/86Sri = 0.7033–0.7040, 208Pb/204Pb = 38.7 for the Morondava dikes), that suggest markedly different sources, magmatic history, and variable interplay between Madagascan Precambrian crust and mantle-derived melts, some with a marked enriched mid-ocean ridge basalt-like composition. The geochemical and isotopic data show no involvement of ancient basement crust in the genesis of the Morondava dikes. The Tulear-Lavanono lavas have high Ba/Nb and La/Nb ratios and Sr-Nd-Pb isotopic characteristic of magmas that have assimilated crustal material. The chemical and isotopic composition of the contaminated tholeiites of southern Madagascar matches some Late Cretaceous group B1 basalts of the Androy volcanic complex and tholeiites of the Ejeda-Bekily dike swarm, and therefore they could derive from the same feeding system or were fed by similar parental magmas. Basalts from the Southwest Indian Ridge between 39 and 41°E section (anomalous magmatic sectors) with low 206Pb/204Pb and 143Nd/144Nd have been interpreted to contain a component of either recycled lower continental crust and/or subcontinental lithospheric mantle. A detailed review of the isotopic and geochemical compositions of the Mesozoic -and Cenozoic- magmatism in Madagascar, and of the lower and upper crustal domains point out that the sources of the anomalous magmatic sectors of the Southwest Indian Ridge do not contain lower crustal (or lithospheric mantle) components of Madagascan origin.Comparison between the samples from the northern and southern parts of the Madagascar flood basalts province indicates that several different parental magmas and mantle sources were involved in the petrogenesis of the Madagascan basalts, and that chemical contributions from a “Marion” hotspot (as a magma source or source component) to the Madagascar primary magmas is not identifiable.

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

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

A craton‐like subcontinental lithospheric mantle beneath the Trans‐North China Orogen revealed by the ca. 1.54 Ga kimberlites

AbstractAncient orogens within the supercontinent like Columbia can remain stable evolution as long as the cratons. What kind of lithospheric mantle was beneath those orogens and how it evolved into a stable state are still enigmatic. The Trans‐North China orogen (TNCO) is one of the typical collisional orogens within the Columbia supercontinent and was formed at ca. 1.85 Ga. Our work reveals that a cluster of kimberlites intruded the orogenic belt at ca. 1.54 Ga. These rocks were originally generated under a thick lithosphere (&gt;200 km). Their entrained olivine cores show a composition of overlapping olivines from refractory mantle peridotites. The results suggest a thick and refractory lithospheric mantle beneath the TNCO at ca. 1.54 Ga. Such craton‐like property may result from large volume melt extraction from the lithospheric mantle, possibly caused by the ca. 1.78 Ga large igneous event, which eventually induces the long‐term stability of the TNCO during the subsequent supercontinent cycle.

Post-collisional ultrapotassic volcanic rocks and ultramafic xenoliths in the Eslamieh Peninsula, NW Iran: Petrological and geochemical constraints on mantle source and metasomatism

The Eslamieh Peninsula in NW Iran exposes Miocene potassic to ultrapotassic volcanic rocks that are often associated with a variety of ultramafic xenoliths. Based on geochemical features, the rocks investigated in this study can be subdivided into two main groups: 1) MgO-rich (8.0–13.9 wt%) basic (SiO2 = 46.1–50.9 wt%) rocks, which comprise mafic lamprophyres and pyroxenite xenoliths, and 2) MgO-poor (1.5–6.4 wt%) basic to intermediate (SiO2 = 48.2–61.2 wt%) rocks, mainly consisting of trachytes, felsic lamprophyres, and analcime-bearing tephriphonolites and phonotephrites. Rocks in both groups have variable but generally low Ni (10–115 ppm; mostly <60 ppm) and Cr (6–628 ppm; mostly <200 ppm), and display enrichment in LREE over HREE (e.g., La/Yb = 17–55), with a large overlap between the MgO-rich and MgO-poor types. These features are coupled with enriched LILE/HFSE ratios (e.g., Ba/Nb mostly 16–116), negative Nb-Ta-Ti anomalies and positive Pb peaks in primitive mantle-normalised diagrams (e.g., Nb/Nb* mostly <0.7). Both high-and poor-MgO samples have strongly radiogenic (87Sr/86Sr)i ratios (0.7078–0.7086), (143Nd/144Nd)i below the Chondritic Uniform Reservoir (CHUR) = 0.51235–0.51244, and ɛHfi ranging from −3.1 to −11.1‰, suggesting involvement of a lower continental crust component in their genesis. The major oxide and trace element contents the high-MgO and high-CaO rocks indicate derivation from a subcontinental lithospheric mantle source, variably metasomatised by fluids and melts released by a subducted slab in a mantle wedge. Primitive melts evolved via mafic mineral fractionation and possible assimilation of lower crust, generating the SiO2 higher and CaO-MgO-poorer group. We suggest that small degree partial melting of the enriched mantle sources could have been triggered by Neo-Tethys slab roll-back during the Arabian-Iranian collision.

Zircon U–Pb geochronology, Nd isotopes and geochemistry of mafic granulites from the Central Indian Tectonic Zone: isotopic constraints on Proterozoic crustal evolution

Abstract The Central Indian Tectonic Zone (CITZ) comprises northern and southern Indian cratonic blocks and is a tectonic window that is suitable for investigating the Proterozoic crustal evolution because of the presence of a wide variety of lithologies. Geochemical and geochronological data on mafic granulites by previous workers do not ascertain the possibility of mafic protoliths and their coeval link to other CITZ units. Thus, determining the precise timing of the formation of mafic granulites may indicate a connection between metamorphism and fragmentation of the Columbian supercontinent. This study presents zircon U–Pb ages, Nd isotopes and the geochemistry of mafic granulites to evaluate their genesis and timing of metamorphism. The results show the tholeiitic affinity and primary magmatic differentiation of the parental melt. Depletion of Nb, P, Zr and Ti and positive enrichment of Ba, U and Pb indicate the derivation of mafic granulites from a variably enriched subcontinental lithospheric mantle (SCLM) source. The zircon U–Pb ages (1564 ± 8 to 1598 ± 9 Ma) are interpreted as a period of granulite-facies metamorphism. The T DM (depleted-mantle) model ages (2.9–3.4 Ga) of mafic granulites indicate the timing of mafic protolith extraction. The mineral isochron age c. 1.0 Ga indicates that these rocks underwent some events during an early Neoproterozoic period. Protolith of mafic granulites could be related to the evolution of melts derived from metasomatized SCLM through fractional crystallization processes.

Noble and base metal geochemistry of late- to post-orogenic mafic dykes from central Spain

The post-tectonic and post-orogenic mafic rocks from the Spanish Central System (SCS) (Iberian Massif) include dyke swarms of shoshonitic (microgabbros) and alkaline (lamprophyres and diabases) geochemical affinity, which register the nature of the metasomatic lithospheric mantle under central Spain. Such magmas sometimes show a direct (or indirect) relationship with the formation of orogenic and intrusion-related gold deposits, which are relatively abundant in the Iberian Massif. The noble and base metal composition of these intrusions shows Primitive Mantle-normalized patterns characterized by positive Au and Co anomalies and fractionated platinum group elements (PGE): from lower Ir-group PGE (IPGE; Ir–Ru) to higher Pd-group PGE (PPGE; Rh–Pt–Pd). The low contents of PGE, together with the base metal contents of pyrite (which is the dominant sulphide phase in the alkaline dykes), is in accordance with low degrees of mantle partial melting and the early segregation of sulphides during magma differentiation. The scarcity of PGE mineral deposits in the Iberian Massif could be explained in part by the apparent lack of PGE enrichment in the Iberian lithospheric mantle. On the contrary, the positive Au anomaly of the SCS mafic dykes represents relatively high Au contents, similar to (and higher than) those of mafic rocks derived from metasomatized subcontinental lithospheric mantle underlying Au-endowed cratons. Several geochemical features point to subduction-related metasomatism of either oceanic or continental nature as the main source of Au enrichment. The Au re-fertilization of the lithospheric mantle under central Spain makes it a potential source in the formation of gold mineralizations.

Extent and significance of the Racklan–Forward Orogen in Canada: far-field interior reactivation during Nuna assembly

Abstract Mesoproterozoic orogenesis is well established on the western and southern flanks of Laurentia in the well-known Racklan–Forward and Mazatzal orogens, but its significance within the previously assembled interior of the supercontinent Nuna has not been established. We examine regional isotopic and structural evidence for Mesoproterozoic deformation in the c. 1.7–1.63 Ga Hornby Bay, Elu, Thelon and Athabasca intracontinental basins, and present evidence for Mesoproterozoic reactivation of Paleoproterozoic structures in the Wopmay and Trans-Hudson orogens. The Racklan–Forward Orogeny in the interior of north Laurentia comprises north–south-trending, high-angle, east-vergent folds and thrusts that occur across a region 1660 km wide and over 1000 km long, stretching from the Yukon to near Hudson Bay and from Banks Island to below the Western Canada Sedimentary Basin. The structures progress from ductile amphibolite and greenschist facies in the Racklan type area to sub-greenschist facies and ultimately brittle or brittle-ductile in the far foreland, showing a predominant thick-skinned style typical of many intracontinental orogens. We present compiled low-temperature thermochronological data, including ages of spatially associated uraninite mineralization, to characterize the scope of reactivation of basement structures in the Archean Rae craton in Nuna's interior. We compare the nature of widespread far-field reactivation in the Racklan–Forward Orogen with other orogens of Nuna's assembly to show it is unusual for Nuna's peripheral margin. We suggest that c. 1.6 Ga continent–continent collision of North Australia with NW Laurentia propagated stresses far into the interior as a result of combined favourable pre-existing structural grain and a weak subcontinental lithospheric mantle in the Rae craton due to repeated episodes of refertilization across 500 Ma of accretion and intrusion. Cratons that experience the complex, two-sided collision and protracted upper plate setting during supercontinent assembly noted herein may be particularly susceptible to extensive foreland propagation of peripheral orogens.

Early Paleozoic subduction imprints of the Proto-Tethys Ocean: Evidence from the Appinite–Diorite–Granodiorite complex in East Kunlun, Northern Tibet

The East Kunlun Orogenic Belt (EKOB), on the northern margin of the Tibetan Plateau, forms a part of the western segment of the orogenic system in Central China. The Paleozoic tectonic evolution of the EKOB, along the western margin of Gondwana, is closely related to Paleozoic subduction and closure of the Proto-Tethys and is key to the palinspastic reconstruction of the Central Asia and Tethys domains. Numerous studies have been conducted on the Early Paleozoic granitoids in the EKOB. However, a convincing magmatic proxy to indicate the timing of oceanic subduction during the Early Paleozoic is yet to be well constrained, partially because the arc geochemical signature of the calc-alkaline granitoids is not a reliable indicator of oceanic subduction. Herein, we present new zircon U–Pb ages, whole-rock major and trace element analyses, and Nd–Pb and zircon Hf isotopic data for Early Ordovician appinite (mafic), diorite (intermediate), and granodiorite (felsic) intrusive rocks in the Dundegou (DDG) area of the EKOB, northern Tibet. The zircon U–Pb ages suggest this complex was emplaced at approximately 477–472 Ma. Abundant 557–510 Ma zircon xenocrysts and/or inherited zircons were found in the granodiorite and appinites, suggesting two earlier magmatic episodes during the late Ediacaran–Cambrian. Appinites and diorites are characterized by depletion in high field strength elements, with negative initial εNd(t) values (appinites: −6.5 to −9.4; diorites: −8.2 to −9.1) and negative zircon εHf(t) values (appinites: −2.5 to −10.3; diorites: −1.0 to −9.1). These features suggest that appinites and diorites were most likely derived from the partial melting of enriched mantle wedge peridotites with the addition of sediment-derived melts from the sedimentary veneer of the subducting slab, along with variable degrees of fractional crystallization and a limited extent of crustal contamination. The granodioritic magma was contributed by (a) the ancient mafic lower crust-derived magma, (b) the derivative felsic magma from a dioritic parent, and (c) a minor mantle-derived magma component, resulted in the formation of granodiorites with highly unradiogenic initial Nd and Hf isotopic compositions (εNd(t) values of −8.5 and εHf(t) values from −0.6 to −7.3). Our study suggests that the subcontinental lithospheric mantle beneath the EKOB is likely heterogeneous in terms of its isotopic composition. The DDG complex is the oldest appinite-bearing complex found so far in the EKOB and a good indicator of proto-Tethys oceanic subduction.

He Isotopic Composition of Alkaline Intrusions of the Hovsgol Area, Northwestern Mongolia

The He isotopic composition of fluid inclusions in magnetite of alkaline and subalkaline intrusion of the Hovsgol region, Northwestern Mongolia, is studied. The measured 4He content varies from 6.6 × 10–7 to 114 × 10–7 cm3/g. The 3He/4He isotope ratio of most samples is 0.23–0.59 Ra indicating the presence of He from different sources in magmatic fluid. The maximum of mantle He (2.51 Ra) is observed in magnetite of subalkali gabbro. The mantle He component was ~40–60% during the generation of parental magma with possible involvement of subcontinental lithospheric mantle material or a plume-like reservoir less enriched in 3He. The mantle He component did not exceed 10–15% during the evolution of the foidoitic and alkali-syenitic melts because of mixing with crustal radiogenic He. We suggest the interaction of intrusions with fragments of the Precambrian accretionary–collision complexes of the Tuva–Mongolian Terrane. This mixing of He isotopes in melts and fluids can serve as indirect evidence of the development of mantle magmatism at the active continental margin.

Refertilized continental root controls the formation of the Mianning–Dechang carbonatite-associated rare-earth-element ore system

Rare earth element ore deposits associated with carbonatite derived from Earth’s mantle supply half of the world’s rare earth element. However, the formation of carbonatite and initial enrichment and transport of rare earth element in the mantle, is unclear. Here, we image the lithospheric architecture of a Cenozoic rare earth element ore belt in southwestern China by integrating seismic tomography with geochemical data. The subduction of the Indian continent caused vertical upwelling and lateral flow of the asthenosphere, which triggered the melting of the overlying subcontinental lithospheric mantle to generate carbonatites. Such a mantle source that previously metasomatized by fluids from recycled marine sediments is a precursor process critical for forming a giant rare earth element system. For the studied ore belt, three key factors are prerequisites to generating ore-forming carbonatites: thick lithosphere with a continental root; prior fertilization of the subcontinental lithospheric mantle; and trans-lithospheric weakness for magma ascent.

Early Ordovician seamounts preserved in the Canadian Cordillera: Implications for the rift history of western Laurentia

Abstract The breakup of the supercontinent Rodinia and development of the western Laurentian rifted margin are in part recorded by Neoproterozoic to mid-Paleozoic igneous and sedimentary rock successions in the Canadian Cordillera. New bedrock mapping and volcanic facies analysis of Early Ordovician mafic rocks assigned to the Menzie Creek Formation in central Yukon allow reconstruction of the depositional environment during the volcanic eruptions, whole-rock geochemical data constrain the melting depth and crust-mantle source regions of the igneous rocks within the study area, and zircon U-Pb age studies provide determination of the precise timing of submarine eruptions. Menzie Creek Formation volcanic rocks are interlayered with continental slope strata and show lithofacies consistent with those of modern seamount systems. Representative seamount facies contain several kilometers of hyaloclastite breccia and pillow basalt with rare sedimentary rocks. Menzie Creek Formation seamounts form a linear array parallel to the Twopete fault, an ancient extensional or strike-slip fault that localized magmatism along the nascent western Laurentian margin. Zircon grains from two volcanic successions yielded high-precision chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) dates of ca. 484 Ma (Tremadocian), which are interpreted as the age of eruption. Menzie Creek Formation rocks are alkali basalt and have oceanic-island basalt–like geochemical compositions. The whole-rock trace element and Nd-Hf isotope compositions are consistent with the partial melting of subcontinental lithospheric mantle at ~75–100 km depth. Post-rift, Early Ordovician seamounts in central Yukon record punctuated eruptive activity along a rift-related fault, the separation of a continental fragment from western Laurentia, or the oblique post-breakup kinematics from the counterclockwise rotation of Laurentia that facilitated local extension in the passive margin.

Evidence of South American lithosphere mantle beneath the Chile mid-ocean ridge

Numerous geochemical studies on mid-ocean ridge basalts have established the presence of both compositional and lithological heterogeneities in the upper mantle. These studies have successfully constrained the composition and age of the chemical heterogeneities within the Earth's upper mantle. However, the origin of those heterogeneities remains highly debated. The Chile Mid-Ocean Ridge in the southeast Pacific Ocean, located far away from any hotspot, is colliding and subducting under the South America plate promoting the formation of a slab window. Comprehensive geochemical data (major, trace, volatile element contents and Sr, Nd, Hf and Pb isotope ratios) on Chile Ridge submarine glasses collected over the 1000 km ridge length demonstrate significant mantle compositional variability. Four main mantle components have been recognized: the typical Pacific MORB mantle, an enriched mantle (e.g., EM-1), a Subduction Modified mantle, and an anciently depleted mantle with unusually high Hf isotope ratios. Surprisingly, despite the large compositional variability, all glasses – including those with a subduction signature – have volatile-refractory element ratios within the range of the Pacific normal MORB. We propose that the Patagonia sub-continental lithospheric mantle, variably metasomatized since the Early Paleozoic, is eroded and incorporated into an asthenosphere with a south Atlantic MORB mantle composition that is flowing westward across the slab window from South America to the Chile Ridge. This scenario contrasts with a well-accepted geodynamic model that predicts the opposite direction of mantle flow. We speculate that the westward mantle flow across the slab window might be a counter-flow response to the opening of the Drake passage and the eastward movement of Pacific mantle since ∼30 Ma ago.

Continuous crustal thinning of the North China Craton in the Early Cretaceous: Evidence from A-type granites

Widespread crust-derived granitoids in North China provide evidence of Mesozoic lithospheric thinning and destruction of the North China Craton. However, the link between lower crust and lithospheric mantle reactivation remains poorly understood. We present whole-rock geochemistry and zircon O-Hf isotopes for Early Cretaceous peralkaline A-type granites (i.e., Xiwanzi, Xiangshan, and Yansehu) from the Yanshan Mountains of the eastern block of the North China Craton. The Xiwanzi granite has a U-Pb age of 134.3 ± 0.4 Ma, with homogeneous whole-rock Nd isotopes [εNd(t) = −7.7 to −7.5] but highly variable zircon O-Hf isotopes [δ18O = 5.1‰ to 7.2‰ and εHf(t) = −7.7 to +9.6]. Such isotopic variations likely resulted from a hybrid juvenile source consisting of partial melting of newly underplated basaltic rocks derived from both the metasomatized ancient and juvenile sub-continental lithospheric mantle. In contrast, the Xiangshan (117.4 ± 1.0 Ma) and Yansehu (116.3 ± 1.4 Ma) granites have relatively uniform isotopic compositions [whole-rock εNd(t) = −12.2 to −10.8, zircon δ18O = 5.2‰ to 6.4‰, and εHf(t) = −17.1 to −4.7], close to those of the Proterozoic mafic granulite xenoliths from the ancient lower crust. The isotopic variations of the Early Cretaceous A-type granites suggest a continuous change in the melting model from the underplated juvenile lowermost crust to the overlying ancient lower crust at high temperatures in an extensional environment. Our results indicate that A-type granites can be used to probe the nature and reactivation of the lower crust of an Archean craton. Significantly, large-scale continuous thinning of lower continental crust provides key insights into the generation of the widespread Mesozoic crust-derived granitoids in North China.

Petrogenesis of the Hoy lava field, a long-lived continental mafic volcanic province in eastern Australia

Small-scale continental basaltic fields can erupt with little warning and bring deep undegassed magmas to the surface rapidly. To explore the lifetime, petrogenesis and plumbing system architecture feeding such basaltic lava fields and compare them with large-scale shield volcanoes, we have focused on the Hoy lava field, central Queensland, Australia. 40Ar/39Ar geochronology, elemental and isotopic whole-rock geochemistry and mineral chemistry on selected Hoy samples reveal long-lived volcanism of ca 50 Ma and magma storage at mantle depths, notably different from the comparatively short duration (3–5 Myr) and crustal magma storage depths of shield volcanoes. In this study, four Hoy lava-field eruptive intervals spanning ca 50 Ma were investigated: 67.5 ± 0.3 Ma, 32.3 ± 0.6–31.6 ± 0.7 Ma, 21.9 ± 0.5 Ma and 18.1 ± 0.3 Ma. In all four eruptive events, samples are porphyritic alkali basalts and trachybasalts (11.41–6.45 wt% MgO) with incompatible element concentrations and Sr–Nd–Pb isotope ratios dominantly derived from a metasomatised sub-continental lithospheric mantle (SCLM) source with an enriched mantle I (EMI) signature. Complex crystal populations show major- and trace-element variations reflecting fractional crystallisation, magma recharge, magma mixing and mantle xenocryst entrainment. Clinopyroxene–melt thermobarometry indicates magma storage in SCLM reservoirs at ∼30–47 km depths. The nearby larger but shorter-lived (3–5 Myr) Buckland central volcano has similar source compositions; however, magma storage is limited and concentrated in the crust, resulting in increased crustal contamination. The results suggest that basaltic centres of contrasting scale and longevity are linked to distinct magma production mechanisms, fluxes, ascent and differentiation.

Geochronology, geochemistry, and Sr–Nd–C–O isotopic composition of the Qieganbulake carbonatites at the northeastern margin of the Tarim Craton, northwest China

Carbonatites are the predominant source of rare earth element (REE) resources globally, but their origins and petrogenesis remain unclear. Currently, >600 carbonatites have been found worldwide, but only a few (<14%) of these carbonatites contain REE deposits. Some carbonatites are rich in REEs but do not host high-grade REE deposits. The Qieganbulake carbonatites at the northeastern margin of the Tarim Craton, China, are ideal for investigating why some carbonatites do not host REE mineralization. We undertook a detailed petrographic, geochronological, and isotopic study of the Qieganbulake carbonatites. In previous studies, a carbonatite at Qieganbulake was dated to ca. 811 Ma. However, in this study, ages of ca. 926 Ma were obtained for phlogopite and baddeleyite in carbonatite dykes rather than pyroxenites or phlogopitelites-bearing carbonatite complex. Such ages of ca. 926 Ma are consistent with a previously published apatite U–Pb age. 40Ar/39Ar dating of phlogopite yielded a plateau age of 928 ± 17 Ma and secondary ion mass spectrometry U–Pb dating of baddeleyite yielded a concordia age of 926 ± 8 Ma, implying the carbonatite was emplaced at ca. 926 Ma. Laser ablation–inductively coupled plasma–mass spectrometry U–Pb dating of monazite yielded a 206Pb/238U intercept age of 811 ± 27 Ma, indicating that the main REE mineralization stage was coeval with later carbonatite emplacement at ca. 811 Ma. The studied Qieganbulake calcio-carbonatite samples are REE-rich (864–1495 ppm) and consist of calcite (62.18–93.92 vol%), apatite (1.10–24.82 vol%), dolomite (0.37–10.48 vol%), and lesser amounts of phlogopite, olivine, monazite, magnetite, and vermiculite. The calcite, apatite, and monazite (<1 vol%) have REE contents of 757–1111 ppm, 5127–11,556 ppm, and 45.35–59.88 wt%, respectively, and account for the high REE contents of the carbonatites (883–1539 ppm). The small amount of monazite in this carbonatite suggests it is a prospective rather than a confirmed ore deposit. Petrographic studies, oxygen values of 5.74 and 7.51 ‰ and characteristics of trace element for apatites with less monazite around suggest they have magmatic origin with less alteration. The 926 Ma Qieganbulake carbonatite has high Sr isotope ratios (0.70580–0.70639) and negative εNd(t) values (−7.6 to −11.8), indicative of an enriched mantle source. δ13CPDB and δ18OVSMOW values of the carbonatites vary from –3.3‰ to –4.1‰ and 9.7‰ to 12.0‰, respectively, indicative of a contribution from recycled crustal C that was probably derived from a subducted oceanic slab. Above characteristics of carbonatite with ∼926 Ma are different with those of carbonatite occurred at ∼811 Ma, suggesting two carbonatite emplacements in Qieganbulake. Based on our results and the regional geology, we infer that the 926 Ma Qieganbulake carbonatite was derived by partial melting of a subcontinental lithospheric mantle source that had been metasomatized by early Neoproterozoic subduction during circum-Rodinia subduction in the northern Tarim Craton. Early crystallization of apatite sequestered the REEs, resulting in limited REE mineralization. In addition, the limited hydrothermal alteration and ligands available, such as F− and SO42−, hindered REE transportation and precipitation.