Partial Melting Of Mantle Research Articles

Origin of Early to Middle Triassic ultrapotassic syenites in South China: A record of block amalgamation in East Asia

Alkaline intrusions such as ultrapotassic syenites are typically formed in extensional tectonic settings including continental rift, post-orogenic and post-collisional regimes. Here we present two early Mesozoic ultrapotassic syenite plutons (Tieshan and Yangfang) in South China that were formed in a continent collisional setting. LA-ICP-MS zircon UPb chronological, major and trace elemental, and SrNd isotopic data of these syenites are used to explore their origin and to further constrain the geodynamics of early Mesozoic magmatism in South China. Our new data indicate that the Tieshan and Yangfang plutons were formed in Early Triassic (249 Ma) and Middle Triassic (240 Ma), respectively. The Tieshan and Yangfang syenites have SiO2 contents of 50.84–56.95 wt% and 59.13–68.34 wt%, respectively, both are enriched in alkalis with very high K2O (5.60–9.68 wt%) and K2O/Na2O (1.8–15.7) and in LILE (Ba = 2464–8813 ppm; Sr = 652–3499 ppm), REE (498–1355 ppm) and HFSE (Zr = 264–766 ppm). They show enriched SrNd isotopic compositions with 87Sr/86Sr (t) and εNd (t) of 0.7096–0.7102 and − 5.2 to −6.5, respectively for Tieshan and of 0.7112–0.7119 and − 8.8 to −9.7, respectively for Yangfang. Geochemical data suggest that the studied ultrapotassic syenites were derived by partial melting of ancient subduction-modified lithospheric mantle including slab-derived melt hybridization and slab-derived carbonate fluid metasomatism, with the source depth of 60–90 km for Tieshan and at least 90 km for Yangfang. We further suggest that their origins are related to the transtension of the regional NE–trending strike-slip faults caused by continental collisions during the early Mesozoic amalgamation in East Asia.

Geochemical Signatures of Platinum Group Elements in Ultramafic Rocks of Chotanagpur Gneissic Terrain, Eastern India and Their Genetic Control

Ultramafic rocks occur as intrusives in the form of lensoid bodies in the northwestern part of Chotanagpur Gneissic Terrain and consist mainly of olivine, orthopyroxene and clinopyroxene and are devoid of plagioclase. Based on distinct geochemical characteristics, the rocks have been identified as komatiites. The rocks are geochemically analogous to Al-undepleted Munro type (Al2O3/ TiO2=17.88-54.73) with distinctly high MgO (26.2-35.62 wt%), Ni (958-1902 ppm) and Cr (21.32-3320 ppm) contents. These rocks are characterised by low CaO/Al2O3, (Gd/Yb)n, (La/Yb)n with positive Zr, Hf, Ti anomalies suggesting high degree partial melting of mantle under anhydrous conditions at shallow depth with garnet as a residual phase in the mantle restite. These high MgO volcanic rocks having elevated concentrations of Ni and Cr are potential hosts for Platinum Group Elements (PGE) owing to their primitive mantle origin and eruption at high temperatures. These rocks have low ΣPGE (29-269.02ppb) content with Ir (0.1-0.8ppb) and Ru (1.05-5.78ppb) among Iridium group PGE (IPGE); and Pt (5.04-18.72ppb), Pd(3.5-18.0ppb), Rh (0.22-0.84ppb) among Platinum group PGE (PPGE). The PGE abundances in komatiites were controlled by olivine fractionation. The Major, trace, REE and PGE composition of the rock suggest melting under anhydrous condition at shallow depth above the garnet stability field under S-undersaturated condition. Anhydrous melting associated with mantle plume activity gave rise to the rock which subsequently contaminated by lower crustal materials during magma ascent and emplacement. Keywords: Chotanagpur Gneissic Terrain, Komatiite, Platinum Group Elements, Sulphur Undersaturation, Plume

Early Cambrian forearc ophiolite-hosted VMS-type Cu deposit in the North Qaidam belt, northern Tibetan Plateau

The North Qaidam belt is an important polymetallic metallogenic belt in the northwestern region of China. However, the tectonic setting and age of related VMS deposits remain debated. Here we performed an integrated analysis of field relationship, geochemistry, and geochronology for hosting rocks of the Lüliangshan VMS-type Cu deposit and surrounding mafic-ultramafic rocks. These rocks, including serpentinite, pyroxenite, chromitite, mafic dykes with associated meta-plagiogranite, lava, chert, and limestone, constitute a relatively complete ophiolite complex, indicating that the Lüliangshan Cu deposit can also be introduced as an ophiolite-hosted VMS deposit. Geochemical data show that the meta-dolerite and ore-hosting lava exhibit geochemical features similar to tholeiitic forearc basalt and are probably generated by partial melting of a depleted mantle source metasomatized by hydrous fluids. Some lavas have boninitic compositions and are formed by partial melting of residual mantle after extraction of forearc basalt. Some ore-hosting lavas also have geochemical affinities to island arc tholeiites as a result of more SSZ components involved in their magma source. The chert samples have remarkably high Fe2O3T contents and are classified as iron-rich one of hydrothermal origin, which is deposited in a ridge-proximal environment. These rocks, together with chromitites with subducted-related geochemical features, collectively indicate that the ophiolite-hosted VMS-type Cu deposit was formed in the forearc setting. Meta-gabbros intruding the ore-hosting lavas yield zircon U-Pb ages mainly ranging from 527 Ma to 518 Ma. The new ages of forearc ophiolite and the oldest age of island-arc rocks (514 Ma) suggest that the Lüliangshan Cu deposit formed in the early Cambrian during early-stage subduction of Proto-Tethys Ocean.

The factors controlling along-arc and across-arc variations of primitive arc magma compositions: A global perspective

Arc volcanism is a key process influencing Earth’s climate, continental growth, and the formation of mineral deposits. Therefore in this contribution, we have compiled whole-rock geochemistry of frontal arc and rear-/back-arc basalts, crustal thickness, and slab parameters (e.g., slab age, slab dip, and convergence velocity) from global convergent margins to investigate the factors controlling along-arc and across-arc variations. Crustal thickness or lithosphere thickness plays a dominant role in elements immobile in aqueous fluids (e.g., Zr/Yb and Nb/Yb). The effect is imposed through mantle partial melting for both frontal arc and rear-/back-arc rocks. Slab thermal structure also affects these immobile elements and gives rise to along-arc variations. Both slab sediment and altered oceanic crust can melt especially in hot subduction zones which yield across-arc variations (e.g., Nd isotope). Aqueous fluids (represented by element ratios such as Ba/Nb and Sr/Nd) also show across-arc variations as they decrease toward rear-/back-arc. This meanwhile decreases mantle wedge melting as rear-/back-arcs show higher Zr/Yb and Nb/Yb. However, no correlations between aqueous fluids and slab parameters suggest aqueous fluids in arc rocks are controlled by complex processes. We summarize factors such as slab alteration, slab dehydration, and mantle metasomatism might impose an effect on the content of fluid mobile elements in arc rocks.

A changing thermal regime revealed from shallow to deep basalt source melting in the Moon.

Sample return missions have provided the basis for understanding the thermochemical evolution of the Moon. Mare basalt sources are likely to have originated from partial melting of lunar magma ocean cumulates after solidification from an initially molten state. Some of the Apollo mare basalts show evidence for the presence in their source of a late-stage radiogenic heat-producing incompatible element-rich layer, known for its enrichment in potassium, rare-earth elements, and phosphorus (KREEP). Here we show the most depleted lunar meteorite, Asuka-881757, and associated mare basalts, represent ancient (~3.9 Ga) partial melts of KREEP-free Fe-rich mantle. Petrological modeling demonstrates that these basalts were generated at lower temperatures and shallower depths than typical Apollo mare basalts. Calculated mantle potential temperatures of these rocks suggest a relatively cooler mantle source and lower surface heat flow than those associated with later-erupted mare basalts, suggesting a fundamental shift in melting regime in the Moon from ~3.9 to ~3.3 Ga.

Petrogenesis of the Eocene Yulong potassic intrusion in non‐subduction setting in the Sanjiang Tethys

The giant Yulong porphyry Cu (‐Mo‐Au) deposit was formed in a post‐collisional setting in eastern Tibet, which is associated with Eocene potassic to ultrapotassic felsic intrusive rocks. The origin of the Yulong potassic intrusion remains highly disputed. Hence, we present new zircon U–Pb ages, whole‐rock geochemistry and zircon Lu‐Hf isotopes for the Yulong intrusion. Zircon U–Pb dating from the porphyritic monzogranite and porphyritic quartz monzonite yield ages of 41.1 ± 0.2, 40.8 ± 0.1 and 40.5 ± 0.2 Ma, respectively. Samples from the Yulong intrusion exhibit high K2O (4.13–4.85%), and high K2O/Na2O (1.00–1.25), [La/Yb]n (30.37–40.37) and Sr/Y (57.86–87.68) ratios, low Mg#, MgO, Ni, and Cr contents, enrichment in light rare earth elements (LREEs) and depletion of high‐field‐strength elements (HFSEs, e.g., Nb, Ta, and Ti). Zircon grains from this study have mostly positive εHf(t) values (+1.6 to +2.9), and crustal model ages (TDMC) of 0.90–1.19 Ga. These features suggest that the Yulong intrusion is derived from a thickened juvenile lower crust. During the Permian to late Triassic, Jinshajiang‐Ailaoshan Palaeo‐Tethyan oceanic subduction from the eastern margin of the North Qiangtang produced metasomatic domains within the continental lithospheric mantle and lower crust. During the late Eocene, the continuing India‐Asia continental collision may have preferentially overthickened the crust. Convective removals in the lower part of the thickened lithosphere resulted in the hot asthenosphere upwelling along the Jinshajiang‐Ailaoshan Suture, which induced the partial melting of the residual metasomatized lithospheric mantle, as well as the thickened lower crust in the Eocene, which formed the Yulong intrusion.

Triassic volcanism on the North margin of the North China Craton: Insights for lithospheric modification during closure of Paleo-Asian Ocean

As a key tectonic region linking the northern margin of the North China craton (NCC) and eastern Central Asian Orogenic Belt (CAOB), western Liaoning, with extensively distributed magmatic activity, provides natural samples to investigate the evolutionary history of the Paleo-Asian Ocean plate subduction to closure. The Middle Triassic (ca. 241 Ma) trachyandesites and Late Triassic (ca. 223 Ma) basaltic andesites were first identified from the Tiaojishan Formation, western Liaoning. They have variable SiO 2 contents (52.48–60.31 wt%) and alkaline or high-K calc-alkaline to shoshonite compositions, and belong to the metaluminous series. Middle Triassic trachyandesites are enriched in large ion lithophile elements (LILEs), high field strength elements (HFSEs), and light rare earth elements (LREEs) with high Nb/La ratios (0.63–0.76). They have homogeneous whole-rock Sr Nd ( 87 Sr/ 86 Sr i = 0.704684–0.704808, ε Nd (t) = −4.6 to −4.0) and variable zircon Hf isotopic signatures (ε Hf (t) = −10.3 to 8.2), indicating that they probably originated from the partial melting of the subcontinental lithospheric mantle with the addition of asthenosphere materials. In contrast, the Late Triassic basaltic andesites show arc-related geochemical affinities, such as LILEs and LREEs enrichment and HFSEs depletion. They have more enriched whole-rock 87 Sr/ 86 Sr i (0.704895–0.705394), ε Nd (t) (−16.1 to −14.0), and zircon ε Hf (t) (−13.1 to −8.8) than the Middle Triassic trachyandesites, suggesting that they probably represent partial melts of the subduction-related fluids-modified lithospheric mantle. The modification of the northern margin of the NCC lithospheric mantle is closely related to the final subduction of the Palaeo-Asian oceanic plate and the subsequent post-orogenic extension. • Triassic volcanic rocks are founded in Tiaojishan Formation, Western Liaoning. • Study area was in a post-orogenic extensional setting in the Middle-Late Triassic. • Ocean Subduction modified the North China Craton lithosphere in the Triassic.

The Formation of the Jiaodong Gold Province

AbstractGold deposits in Jiaodong, termed Jiaodong‐type, are tectonically located in the southeastern margin of the North China Craton. Their major features are reviewed in this paper to highlight the differences between Jiaodong deposits and other genetic types of gold deposits. The mineralization was synchronized with asthenosphere upwelling indicated by syn‐ore OIB‐like mafic dike and large‐scale crustal thinning suggested by decrease of Sr/Y from pre‐ore to syn‐ore granites. Asthenosphere upwelling induced by the roll‐back of Paleo‐Pacific Plate drove partial melting of lithospheric mantle and devolatilization, which induced the release of the ore‐forming fluids. In concomitant with magmatic records, mineralization migrated from the western Jiaobei terrane (133–127 Ma) to the eastern Sulu orogenic belt (114–108 Ma), corresponding to the eastward roll‐back of Paleo‐Pacific Plate. Gold mineralization in Jiaodong formed in the transitions of ductile to brittle deformation, rapid to slow crustal uplift, and regional compression to extension. In the regional‐scale, the gold deposits in the Jiaobei terrane are mostly situated at intersections between NE‐trending faults and EW‐trending basement faults, and gold orebodies dominantly controlled by the lithologic contacts between Precambrian metamorphic rocks and Mesozoic granites. The mineralization was dominated by the disseminated‐veinlet ores related to quartz–sericite alteration in strong cataclasite‐breccia zone, with subsidiary thick quartz‐sulfide veins developed in secondary fault zones. The ore‐forming fluids belong to a H2O–CO2–NaCl±CH4system and show minor variations in salinity among different types of ore. Structure‐fluid feedback involving fluid‐rock reaction and hydrofracturing triggered the fluid phase separation and resultant gold deposition. The Jiaodong gold deposits are distinct from orogenic and intrusion‐related gold deposits in terms of tectonic setting, origin of ore‐forming fluids, and mechanism of gold deposition.

Miocene transition from an active continental margin to a trench–arc–basin system in Northeast Asia: Constraints from compositional variations of basaltic rocks in the Russian Far East

The timing and mantle processes of the Cenozoic transition from an active continental margin to a trench–arc–basin system in northeast Asia remain controversial. Here we present major and trace element, and Sr–Nd–Pb–Hf isotope data for Cenozoic basaltic rocks from the Russian Far East, which provide new constraints on the deep mantle processes during the transition. The basaltic rocks formed mainly in four stages. The results show that middle Eocene basaltic rocks have arc-like geochemical features, such as depletions in high-field-strength elements, indicating an origin by partial melting of a mantle wedge that had been modified by slab-derived fluids and recycled sediments. Early and middle Miocene basaltic rocks also have arc-like geochemical features, and have relatively low contents of fluid-mobile elements and high contents of fluid-immobile elements, as compared with the middle Eocene basaltic rocks. This suggests that these magmas were derived by the melting of previously melted lithospheric mantle that had been modified by slab-derived fluids. In contrast, late Miocene basalts have ocean island basalt-like geochemical features, such as enrichments in high-field-strength elements, indicating their derivation by partial melting of the asthenospheric mantle. The temporal variations in the mantle sources of the Cenozoic basaltic rocks in the Russian Far East, along with an abrupt change in Sr Nd isotopic compositions at ca. 20 Ma, demonstrate that the transition from an active continental margin to a trench–arc–basin system in northeast Asia occurred at ca. 20 Ma. • The basalts in the Russian Far East formed mainly at ca. 45, 20, 13, and 7 Ma. • The temporal variations in the sources of the Cenozoic basalts were determined. • Transition from active continental margin to trench-arc-basin occurred at 20 Ma. • The rollback of Pacific Plate and trench retreat control the tectonic transition.

Petrogenesis of Paleozoic high Ba Sr granitoids from the Qinling orogen, Central China: Implications to the formation of Archean sanukitoids and TTGs

Modern analogues of Archean granitoids are particularly important, for they offer an opportunity to investigate processes of early crustal formation and tectonic transition. High Ba Sr granitoids are usually considered as the Phanerozoic equivalents of Archean sanukitoids, however, their petrogenesis is still equivocal. Here, we collected a suite of high Ba Sr granitoids from the Banshanping pluton in the Qinling orogen, central China, and carried out an elaborated study including mineral chemistry, whole rock elemental and Sr Nd isotopic compositions, as well as zircon U Pb, Lu Hf and O isotopes, to constrain its petrogenesis. Our new secondary ion mass spectroscopy (SIMS) zircon U Pb geochronology suggests the emplacement age of the Banshanping pluton at 435 ± 2 Ma (MSWD = 0.37). The rocks have variable SiO 2 , elevated Mg# (50–53), Cr, and Ni concentrations, as well as strong enrichment of Ba and Sr (Ba + Sr = 1310–3040 ppm), but depletion of high field strength elements (HFSE). Some trace element ratios are positively (e.g., La/Yb, Sr/Y) or negatively (e.g., Dy/Yb) correlated with their SiO 2 concentrations, indicating that amphibole fractionation might play a crucial role in their formation. They also exhibit restricted and relatively high initial 87 Sr/ 86 Sr (0.70437–0.70515) and chondritic to slightly positive ε Nd (t) values (0.29–1.32), but high zircon ε Hf (t) (8.0–9.9) and normal mantle zircon δ 18 O (5.45 ± 0.13 ‰) values. Isotopic modeling constrains incorporation of about 4% oceanic crust-derived fluid and 0.3% sediment melt into their mantle source. Combined with regional geological evolution, the Banshanping pluton is inferred to originate from a two-stage process: its mantle source was metasomatized by the subducted proto-Tethyan oceanic slab during the early stage and partial melting of the metasomatites generated the high Ba Sr granitoids induced by slab roll-back. The high Ba Sr granitoids geochemical signatures overlap with Archean sanukitoids in many aspects, thus offering essential constraints on their formation. Compared with Archean sanukitoids, our results suggest that the sanukitoids derived from fluid-dominated metasomatic mantle might have the potential to evolve into TTG through amphibole-dominated fractional crystallization. • The Banshanping pluton represents a suite of high Ba-Sr rocks geochemically similar to Archean sanukitoids. • They were produced by partial melting of a fluid metasomatic mantle and undergo amphibole-dominated fractional crystallization. • Fluid-dominated metasomatic mantle-derived Archean sanukitoids may have the potential to evolve into TTGs.

Architectural and Compositional Diversity of Early Earth Ocean Floor Evidenced by the Paleoarchean Nondweni Greenstone Belt, South Africa

Abstract The nature of the early Archean ocean floor remains a topic of important debate. There are relatively few well-preserved occurrences worldwide where such terrains may be studied in detail because of structural dismemberment, metamorphic overprinting and pervasive early stage hydrothermal alteration to recent weathering. The 3.41-Ga dominantly mafic formations of the Nondweni Greenstone Belt (NGB) covering 270 km2 in the south-eastern Kaapvaal Craton comprise submarine volcanics that exhibit a wide range of textural features, including pillows, chill zones and brecciated flow tops, and various spinifex textures, including the rare platy pyroxene type, cumulate layers, and tuffs. Channelized subaqueous lava lakes that underwent fractionation are capped by thick spinifex-textured units and pillows. Early stage seafloor alteration is regionally variable, ranging from intense to minimal, with preservation of original mineralogy in many areas. Mafic volcanic rocks of the NGB contrast with those of the Barberton Greenstone Belt both in the style of volcanism and in the associated compositional range of komatiitic basalt to basalt with a complete absence of high-Mg komatiites. Olivine-phyric rocks, or derivatives thereof, are largely absent and pyroxene is the main controlling phase with orthopyroxene in the most primitive komatiitic basalts and clinopyroxene in the evolved lava lake sequences. The abundance of orthopyroxene typifies the long-standing silica-enriched character of the Kaapvaal Craton. Three exceptionally well-preserved and well-exposed sequences were studied utilizing hand-drilled samples and deep coring providing unprecedented stratigraphic and textural detail and field controls for more than 400 samples. A unifying feature of the mafic volcanics of the NGB is the range of compositions and ratios of incompatible elements most clearly illustrated by a series of high- and low-Ti compositional lineages reflecting differing sources or degrees of mantle partial melting. Sharp boundaries between high- and low-Ti flow successions indicate sudden changes in the melting regimes or the interaction of flow sequences from different volcanic centres. Th/Nb ranges from 0.1 to 0.2 and reveals crustal contamination of primitive lavas. The primary magma that gave rise to the most primitive komatiitic basalts with 19.5% MgO was derived from partial melting of a mantle plume source in the garnet stability field. Trace element modeling shows that the sequences studied in detail have been modified by fractionation and crustal contamination with the most likely contaminant being the Ancient Gneiss Complex (3.43–3.66 Ga), which is extensively exposed in Eswatini and probably underlies the Paleoarchean terrains in the southern Kaapvaal Craton. The geotectonic setting was likely that of a submerged felsic crustal platform as enclaves within an oceanic plateau.

Geochemistry of volcanic rocks and dykes from the Remeshk-Mokhtarabad and Fannuj-Maskutan Ophiolites (Makran Accretionary Prism, SE Iran): New constraints for magma generation in the Middle East neo-Tethys

The Remeshk-Mokhtarabad and Fannuj-Maskutan ophiolites represent two major ophiolitic units in the North Makran Domain (Makran Accretionary Prism). Volcanic rocks and dykes of these ophiolites mainly consist of basalts and rare basaltic andesites, andesites and dacites. No chemical distinction can be seen in basalts from these two ophiolitic units, or between volcanic rocks and dykes. Basaltic rocks show a broad MORB-type nature but variable chemical composition (e.g., SiO2= 42.64–52.63 wt%; TiO2= 0.98–2.43 wt%; Mg# = 71–50). They show both N-MORB (Type 1) and E-MORB (Type 2) compositions (MORB: mid-ocean ridge basalt; N: normal; E: enriched). Type 1 rocks are very rare in both ophiolitic units, whereas Type 2 rocks are predominant. Type 1 rocks show low Th (0.10–0.16 ppm), Nb (1.86–2.82 ppm), Ta (0.09–0.17 ppm) abundance and low (La/Yb)N (0.50–0.75), (La/Sm)N (0.48–0.72) ratios. Compared to N-MORBs, Type 2 basalts show slight enrichment in Th (0.42–1.60 ppm), Nb (6.09–14.6 ppm), and Ta (0.227–0.792 ppm), as well as (La/Yb)N and (La/Sm)N ratios >1 like those observed in typical E-MORB. Trace element petrogenetic models indicate that primitive basalts derived from partial melting of a heterogeneous sub-oceanic mantle variably metasomatized by plume-type (OIB-) components. Type 1 basalts derived from partial melting of mantle regions with no enrichment in OIB-type components, whereas Type 2 basalts derived from partial melting of DMM sources variably enriched by OIB-components. These rocks formed in an oceanic basin that was strongly affected by mantle plume activity and different extents of plume-ridge interaction.

Late Jurassic syn-rift deposition in the Flemish Pass basin, offshore Newfoundland: Evidence for Tithonian magmatism and Appalachian-Variscan sediment sources from quantitative mineral and detrital zircon U–Pb-Hf isotope studies of Mizzen discovery strata

Quantitative mineral and detrital zircon U–Pb-Hf isotope studies of syn-rift sandstone units in the Mizzen F-09 delineation well, northern Flemish Pass basin, were conducted to test competing models for Grand Banks paleogeography and Late Jurassic development of the Newfoundland-Iberia magma-poor rift system. Bodhrán formation (informal nomenclature) strata yield ca. 145–155 Ma detrital zircon grain populations that indicate a late Tithonian (146 ± 1 Ma) maximum depositional age for syn-rift fluvial to tide-modulated fluvial strata in Mizzen F-09 and evidence for Grand Banks rift magmatism to have occurred throughout the Late Jurassic. Late Tithonian tectonic subsidence and magmatism in the northern Flemish Pass basin region were coincident with the onset of necking processes and extreme crustal thinning within the Newfoundland-Iberia rift system. The chondritic to superchondritic Hf isotope compositions of Late Jurassic detrital zircon grains indicate repeated partial melting of lithospheric mantle during rift development, perhaps along master detachment faults or transfer zone systems. Bodhrán formation strata have chemically unstable minerals (plagioclase, potassium feldspar, garnet, muscovite, rutile, staurolite) and late Paleozoic to Paleoarchean detrital zircon age populations that together show provenance from Appalachian-Variscan basement and cover assemblages. A positive correlation between the abundance of metamorphic minerals and unique-aged, mid-Permian (ca. 271–280 Ma) detrital zircon grains from post-Variscan igneous rocks indicate Iberian provenance contributions to Bodhrán formation strata. The new results from Mizzen F-09 are consistent with models for WNW-directed fluvial discharge into the northern Flemish Pass basin and sediment derivation from upper Paleozoic foreland basin or cover assemblages located west of Variscan front, some which may have been stripped off Beothuk Knoll, Flemish Cap, or adjacent highlands.

Sulfur in Archean komatiite implies early subduction of oceanic lithosphere

Komatiite is a magnesium-rich volcanic rock that erupted almost exclusively during the Archean era, requiring high partial melting of Earth's upper mantle to form. Archean komatiites, therefore, provide the most reliable information on the mantle geochemistry of the early Earth. However, it remains unclear whether komatiite compositions reflect only remnants of an early deep mantle or may provide evidence of recycling of subducted components. Sulfur mass-independent fractionation (S-MIF) could be useful to distinguish the two scenarios because S-MIF is produced only by photochemistry in anoxic atmospheres. We report here multiple sulfur isotope analyses of the 2.7 Ga Belingwe komatiites and related volcanics. Some Belingwe komatiites show clear S-MIF signatures (Δ33S from −0.20‰ to −0.12‰). Petrology, trace element chemistry, and Sr-Nd-Pb isotope compositions demonstrate that the samples contain Ni-poor pristine igneous sulfides and crustal contamination is minimal. The observed S and initial Nd isotope covariation can be reconciled if the komatiite source involved an early depleted shallow mantle which acquired S-MIF from seawater (Δ33S < 0‰) via seafloor alteration. We suggest that the Belingwe komatiites were derived at least partly from a past subducted oceanic lithosphere that stagnated in the mantle from the Archean era to the present. Therefore, the sulfur isotopes in Archean komatiite provide new constraints on the history of subduction and the recycling of crustal material.

Quaternary post-collisional high Nb-like basalts from Bijar-Qorveh, NW Iran: A metasomatized lithospheric mantle source

The Quaternary Bijar-Qorveh mafic volcanic rocks (BQMVR) occur in the Sanandaj-Sirjan Zone (SSZ) of Iran, a segment of the Alpine-Himalayan Orogeny with a Gondwana-affinity basement. Phases 1 and 2 of BQMVR are silica-undersaturated (basanite, tephrite, and phonotephrite) and form a highly alkaline or HA suite. The last phase, phase 3, of BQMVR is silica-saturated (alkali olivine basalt and hawaiite) and is called the mildly alkaline or MA suite. BQMVR share well-known subduction-related negative Nb Ta anomalies, and their Nb abundances are high enough to be classified as ‘high-Nb like basalts’. However, they do not show geochemical characteristics of typical high-Nb basalts. Trace element abundances and patterns of the MA suite mimic garnet-bearing spinel peridotite partial melts, whereas the HA suite is too rich in incompatible trace elements to match normal mantle partial melts. Enriched trace element patterns of the HA suite are akin to hornblendite experimental partial melts, and are likely derived from a garnet-amphibole peridotite source. Sr-Nd-Pb isotopic characteristics of the BQMVR indicate a subcontinental lithospheric mantle affinity. The HA suite tends toward the EM1 array, whereas the MA suite follows the EM2 array. The highly radiogenic Sr Nd isotopes of the MA suite indicate significant crustal contamination. Lithospheric delamination beneath the SSZ that followed Arabia-Eurasia collision, triggered garnet-amphibole peridotite partial melting that interacted with 10–30% of the asthenospheric component to have formed the HA suite. Mantle melting at shallower lithosphere depths followed and produced the MA suite. MELTS modeling shows the BQMVR underwent fractional crystallization of clinopyroxene ± olivine. Whole-rock thermobarometric calculations suggest derivation of the primary melts at a depth of ~97 km, however the Cpx-thermobarometer indicates that such melts equilibrated at lower crust depths at a depth of 45 km close to the Moho. The present Quaternary volcanostratigraphic study provides substantial clues for major post-collisional alkaline high-Nb mafic volcanism bearing signatures of two distinct magmatic stages; an early phase (phases 1 and 2) rooted in a deeper garnet amphibole peridotite followed by phase 3 rooted in a garnet-poor spinel peridotite. • Quaternary alkaline mafic volcanism in western Iran is post collisional. • Highly alkaline (HA) suite is derived from juvenile amphibole rich mantle. • Mildly alkaline (MA) suite is derived from shallower, ancient lithospheric mantle. • EM1 and EM2 affinities of the HA and MA suites respectively comply with the SCLM. • MELTS and isotopes revealed FC and AFC roles for HA and MA suites respectively.

Highly evolved miaskitic syenites deciphering the origin and nature of enriched mantle source of ultrapotassic magmatism in the Variscan orogenic root (Bohemian Massif, Moldanubian Zone)

A detailed study of alkali-rich syenites from the Gföhl unit of the Moldanubian Zone in the Bohemian Massif provides constraints on the exact origin and character of mantle source of ultrapotassic magmatism in the Variscan orogenic root in Central Europe. The syenites are characterized by highly alkaline composition (K2O 11.3–12.7 wt%; K2O/Na2O 6.8–7.5), crust-like isotopic signatures (87Sr/86Sr335 ∼ 0.7116; εNd335 ≤ −7.8), and significant trace element enrichment (Th, U, Zr, Hf, LILE, LREE). They host an extraordinary amount of zircon (0.5–5 vol%) along with subordinate titanite and apatite, reflecting their miaskitic affinity. Mineral chemistry suggests that syenites formed through fractional crystallization and accumulation from highly reduced, metaluminous to slightly peraluminous mantle-derived alkaline magmas. Whole-rock geochemistry and Sr-Nd isotopes provide direct evidence that they were produced by partial melting of metasomatic phlogopite-bearing vein network (glimmerites) in the lithospheric mantle, generated by the interaction of (U)HP fluids/melts derived from deeply subducted crustal material (Moldanubian granulites) with wall-rock peridotites. Low-degree partial melting of pure vein component produced unusual geochemical signatures of miaskitic syenites, compared to the composition of common ultrapotassic rocks in the Bohemian Massif, reflecting relatively higher degrees of partial melting of mixed glimmerite-peridotite mantle source.The emplacement of alkali-rich syenites was almost contemporaneous with the mantle source enrichment and closely followed by initial fast cooling dated by 40Ar/39Ar amphibole-biotite ages ranging between 329.8 ± 1.6 and 331.4 ± 0.7 Ma, corresponding to rapid exhumation of the Variscan orogenic root in Central Europe. The U-Pb apatite age of 305.9 ± 5.3 Ma likely reflects further cooling to lower temperatures. The syenite emplacement was linked to the early impulse of ultrapotassic magmatism associated with the Andean-type subduction of the Saxothuringian domain beneath the Moldanubian block. The close temporal association of K-rich magmatism in the Saxothuringian and Moldanubian Zone is indicated by identical cooling ages of miaskitic syenites and other Saxonian ultrapotassic rocks, as revealed by 40Ar/39Ar dates around 330 Ma.

AN UNUSUAL EARLY EOCENE, SYNCOLLISIONAL CARBONATITE COMPLEX AND RELATED RARE EARTH ELEMENT DEPOSIT IN THE INDIA-ASIA COLLISION ZONE, NORTHWESTERN VIETNAM

Abstract Carbonatite-related rare earth element (REE) deposits, the most significant source of REEs globally, are normally generated in extensional settings, such as intracontinental rifts, mantle plume-related environments, or postcollisional orogens. Syncollisional orogens represent overall compressional regimes, so carbonatites and related REE deposits are rarely identified in such a setting. However, this study reports an anomalous syncollisional carbonatite-related REE deposit, Dong Pao, in the India-Asia collision zone in northwestern Vietnam. The Dong Pao deposit is dated at ca. 52 to 51 Ma through zircon and bastnäsite Th-U-Pb chronometers. The ore-hosting carbonatites were emplaced as stocks with associated syenite. The carbonatite-syenite complex is significantly enriched in light REEs, Ba, and Sr and depleted in high-field strength elements, and has high (87Sr/86Sr)i ratios (&amp;gt;0.707) and low εNd(t) values (–6.5 to –5.6). These geochemical signatures imply that the carbonatite-syenite complex was derived from partial melting of subcontinental lithospheric mantle previously metasomatized and fertilized by REE- and CO2-bearing fluids. Timing of the REE-rich carbonatite-syenite complex indicates that it was related to a far-field stress within the early Eocene main-collision stage at 52 to 51 Ma rather than the late-collision stage at 42 to 35 Ma as previously thought. Collisional tectonism involving block rotation and fault activation are interpreted to have induced disturbance of the lithosphere mantle and created localized, transtensional/extensional environments oblique to the trend of the orogen that facilitated emplacement of the REE-rich carbonatitic magmas. Dong Pao appears to be the first identified, high-tonnage REE deposit that formed in the syncollisional geodynamic setting. Such a finding highlights that tectonic disturbance of an REE-rich lithosphere mantle distal to collision sutures has the potential to generate REE deposits, even during prominent convergence and collision of continents. As such, it defines additional search spaces for exploration of other REE orebodies of this style in complex collisional orogens.

No measurable calcium isotopic variations of back-arc lavas across the Okinawa Trough

Calcium (Ca) is an essential element closely connecting to the recycling of marine carbonate at subduction zones. Here, we report Ca isotopic compositions (denote as δ44/40Ca with respect to NIST SRM915a) of back-arc lavas collected across the Okinawa Trough. The δ44/40Ca values showed a narrow range with an average of 0.81 ± 0.08‰ (2SD, n = 13), almost identical to reported values of arc magmas, mid-ocean ridge basalts (MORBs), and other mafic rocks. The MORB-like δ44/40Ca values were dominantly attributed to the mantle partial melting. The accumulation of clinopyroxene and plagioclase has played a negligible role on the Ca isotopes, despite the collected lavas exhibited distinctive rock types ranging from basalt to dacite. The metasomatism by subduction components (slab-derived fluids, sediment melts) and/or crustal assimilation have imprinted trace elemental and HfNd isotopic signals on the Okinawa Trough lavas, while the Ca isotopes have not been significantly modified. Our work suggests that thermal structures and tectonic stages are not pivotal factors constraining the Ca isotopic characteristics of arc and back-arc magmas at subduction zones. The opportunity and possibility to discriminate carbonate Ca isotopic signals in subduction zones are obscured by the highly variable Ca isotopes and carbonate contents of subducted sediments, the homogenization effect of the mantle wedge, variable degrees of sediment melting, and the survival of sediment melts during petrogenesis.

Late Permian-Triassic ridge subduction beneath the northeastern margin of the North China Craton: Evidence from the Kaiyuan appinitic to high-Mg intermediate intrusions in northern Liaoning, North China

Recognizing terminal ridge subduction is of first-order importance in chronicling the tectonic timeline of any accretionary orogens, but remains elusive in the final suturing zone of the Paleo-Asian Ocean along the Solonker−Xar Moron−Changchun−Yanji swath. Here we report two generations of ultramafic-mafic to intermediate intrusions from northern Liaoning, North China, linking eastern Central Asian Orogenic Belt and northern North China Craton. The ca. 259 Ma appinites show variable enrichment in large-ion lithophile elements (LILEs) but depletion in high-field strength elements (HFSEs), and variably enriched isotopic compositions with εNd(t) of −11.8 to −2.4 and zircon εHf(t) of −15.7 to +5.0. These petro-geochemical traits argue for their derivation from partial melting of subduction-modified lithospheric mantle. By contrast, the high-Mg diorites of 226−223 Ma display decoupling features between LILEs and HFSEs and juvenile isotopic signatures with εNd(t) of −0.3 to +2.6 and zircon εHf(t) of +5.4 to +11.4. These characters are consistent with a magmatic origin from partial fusion of sediment-fertilized asthenospheric mantle. Synthesizing the two mantle-melting events with regional coeval igneous activities leads to the identification of Late Permian (265−250 Ma) and Middle-Late Triassic (241−218 Ma) magmatic flare-ups with a 10−15 m.y. lull along the northern North China Craton. This magmatic tempo tends to record a salient mantle replacement from hydrous metasomatized lithosphere to anhydrous asthenosphere. In combination with ensuing progressive crustal growth, simultaneous metamorphism, and structural activities, these diverse manifestations make a compelling case of ridge subduction and concomitant slab window formation during the eventual closure of the Paleo-Asian Ocean along the Solonker−Xar Moron−Changchun−Yanji swath.

Paleo-Tethys subduction and arc-continent collision: Evidence from zircon U-Pb chronology, geochemistry and Sr-Nd-Hf isotopes of eclogites in western Yunnan, bangbing area, southeastern Tibetan Plateau

The Changning-Menglian suture zone (CMSZ) in the southeastern Tibetan Plateau is a newly discovered HP-UHP metamorphic zone. The eclogites therein are the key evidence constraining the main suture of the Proto- and Paleo-Tethys Ocean in western Yunnan. Targeting the weakly studied Bangbing eclogites, we developed a comprehensive study on the whole-rock compositions, Sr-Nd isotope and zircon U-Pb ages, zircon trace elements and Lu-Hf isotope to reveal the subduction and arc-land collision. The eclogites occur as massive blocks or lenses and embedded in garnet phengite quartz schists of Lancang Group, Early Paleozoic accretionary complex. Their geochemistry is similar to E-MORB, and exhibit isotopic εNd(t) values of 3.14–4.49 and εHf(t) of 14.64–16.41, respectively. The Nb-enriched mafic protoliths suggested they were probably generated by partial melting of the enriched oceanic mantle within the spinel stability field and emplaced or erupted as mid-ocean ridge in the Paleo-Tethys Ocean. By LA-ICP-MS zircon U-Pb age testing, the magmatic zircon grains separated from the eclogites yield a wide range of ages, which may be capture zircon ages rather than protolith crystallization. We infer the age of eclogite-facies metamorphism to be 238 ± 2 Ma based on CL images, zircon trace element analysis, and that this metamorphism marks the collision between the Eastern Lincang magmatic arc, the Simao block and the Western Baoshan block. Thus, exhumation of the eclogites occurred only 7 to 23 Ma later, according to age 231–215 Ma for post-collisional volcanic and granitic rocks east of the CMSZ. Conclusively, the continued subduction of the Paleo-Tethys oceanic crust occurred during the Early-Middle Triassic, and rapid exhumation in the Late Triassic. The Changning-Menglian suture zone is a typical oceanic subduction-accretionary orogeny belt.