Partial Melting Of Mantle Research Articles

Post-tholeiite rifting and the genesis of ultramafic lamprophyres around 65 Ma in the Deccan large igneous province: Implications for the separation of India and Seychelles

This study examines the mineral and whole-rock compositions and paleomagnetic dating of lamprophyres from the West Coast Alkaline Complex (WCAC) within the Deccan Large Igneous Province. We aim to elucidate their connection with the Réunion plume, Deccan tholeiite magmatism, the onset of rifting and separation of Seychelles, and the role of lithospheric thinning.WCAC lamprophyres contain olivine and phlogopite phenocrysts/macrocrysts set in a groundmass of carbonate, clinopyroxene, nepheline, spinel, and melilite. They are characterized by Ti-Al phlogopite and Al-enriched spinels with high Fe2+/(Fe + Mg) ratios. They are undersaturated in silica and rich in MgO, TiO2, and LREEs, resembling the global ultramafic lamprophyres (UMLs).They originated from an enriched garnet lherzolite mantle, metasomatized by silicate and carbonate veins. Trace-element modeling suggests the derivation of melt by ∼1% partial melting of phlogopite-bearing garnet lherzolite mantle, corresponding to moderate pressure depths (3–4 GPa) and lithospheric mantle thickness of 90–100 km. Paleomagnetic investigations corroborate their intrusions, primarily during chron C29n, approximately at 65 Ma, which coincided with amplified plate velocities in the Indian Ocean.Seismic tomography indicates a current lithospheric thickness of about 50 km under the western Indian margin, suggesting a lithospheric delamination of about 40–50 km following the intrusion of lamprophyres at ca. 65 Ma. The initiation of passive rifts in the region, culminating in the separation of the Indian subcontinent and the Seychelles, prompted the UML magmatism. Lithospheric thinning continues along with continental rifting in response to greater plate-tectonic stresses within regions of persistent lithospheric weakness.

Constraints on the source of Siberian Trap magmas from Mo isotope evidence

The relative contributions of asthenospheric mantle, lithospheric mantle, and continental crust in the genesis of the Siberian Traps Large Igneous Province (ST-LIP) remain poorly constrained. Most models invoke partial melting of asthenospheric mantle within a mantle plume with an inventory of recycled crustal material, with or without melting of subcontinental lithospheric mantle, and crustal contamination during ascend through the continental lithosphere. A greater understanding of this topic is of fundamental importance because the ST-LIP basalts are associated with the large Permian-Triassic extinction and the world’s largest magmatic Ni-Cu-platinum group element sulfide resources (Ni-Cu-PGE), the Norilsk-Talnakh mining camp. The ∼250 ± 2 Ma Siberian Traps at Norilsk contain a classic sequence of basaltic rocks that provide a spatial and stratigraphic context for changing chemistry of the eruptive products. We present a detailed geochemical and Sr-Nd-Hf-Mo isotopic investigation of ST-LIP volcanic rocks and associated sedimentary rocks, including coal and anhydrite, from the Norilsk area. The 98Mo/95Mo isotope ratios (reported as δ98Mo ratio relative to NIST SRM 3134) vary significantly from −0.62 to 0.07 ‰ for the older basalt formations and −0.41 to 0.03 ‰ for the younger basalt formations. The range of δ98Mo and its correlation with the other geochemical tracers cannot be explained by post-magmatic alteration, magmatic differentiation, or sulfide fractionation as Mo behaves as a lithophile element under these magmatic conditions. We suggest that the δ98Mo range can be explained by interaction of the plume with subcontinental lithospheric mantle modified by subduction processes. This is particularly prominent in the earlier Ivakinsky to Gudchikhinsky formations where light δ98Mo values coupled with low Mo/Ce can be explained by contributions from a dehydrated eclogitic component, whereas more rare heavy δ98Mo values and high Mo/Ce likely require contributions from a fluid metasomatized mantle source. Later magmas of the Nadezhdinsky formation show clear evidence of crustal contamination in their combined Mo-Sr-Nd-Hf isotope and trace element systematics, while the later more voluminous Morongovsky type magmas are shallower, large degree melts with limited crustal interaction. Our data shows the usefulness of Mo isotopes in deciphering magma sources of large igneous provinces eruptions.

Cu-Au-Platinum Group Element Mineralization in the Mbesa Prospect, Southern Tanzania: Unconventional Magmatic Sulfides

Abstract High-grade Cu mineralization was recently discovered in the Mbesa prospect in southern Tanzania, but its origin is poorly constrained. Herein, we present new major element, trace element, rare earth element (REE), and platinum group element (PGE) geochemistry and petrographic data to constrain the origin of the mineralization. The sulfide mineralization is dominated by chalcopyrite and bornite and characterized by high Cu/Ni over 100 in the massive and disseminated ores. The mineralization lacks pyrrhotite and pentlandite, which are found commonly in magmatic Ni-Cu-PGE sulfide deposits. Platinum group minerals present are michenerite (PdBiTe) and sudburyite (PdSb). PGE concentrations expressed as Pd + Pt + Au are up to 1.31 and 1.04 ppm in the massive and disseminated ores, respectively. The δ34S of chalcopyrite separates range from –3.9 to –0.6‰, bornite separates vary from –1.8 to 1.1‰, and bornite-chalcopyrite mixtures yield –1.3 and 0.6‰, mostly consistent with a magmatic sulfur origin. The sulfides are hosted by coarse-grained plagioclase amphibolite and hornblende gneiss with recrystallization textures suggestive of metamorphism. Both rocks are characterized by enrichment in large ion lithophile elements (LILEs), with negative anomalies of Zr and Nb, consistent with crustal contamination of magmas derived from partial melting of subcontinental lithospheric mantle (SCLM). An SCLM origin is further suggested by the close association of Cu sulfides with carbonates and apatite. It is suggested that the sulfides crystallized from the fractionated Cu-rich sulfide melt enriched in intermediate solid solution (iss) at shallow crustal levels. Sulfide were likely mobilized as droplets attached to CO2 vapor phase as expressed by close association between carbonates and apatite with the Cu-rich sulfides. Nickel-rich sulfides may remain undiscovered at deeper portions around the Mbesa prospect.

First identification of Mid-Miocene north–south trending dikes in the eastern Qiangtang terrane, eastern Tibet: Mantle melting and implications for plateau uplift

The uplift of the Tibetan Plateau during the Miocene is crucial to understanding continental deformation processes and global climatic events. However, the eastern Tibetan Plateau remains poorly investigated. Mantle-derived magmatism provides crucial insights into the deep dynamic processes and surface uplift of the plateau. In this paper we report the first discovery of north–south trending lamprophyre dikes from the Aduo Basin in the eastern Qiangtang terrane, eastern Tibet. Our new age data show that these lamprophyre dikes were generated in Mid-Miocene (15–13 Ma). This new discovery has bridged the Mid-Miocene mantle-derived magmatic gap in the Qiangtang terrane. Trace element and Nd–Sr isotopic data indicate that they were derived by partial melting of enriched subcontinental lithospheric mantle. Our study implies that the eastern Tibetan Plateau had its attained near-maximum elevation by the Mid-Miocene. Combined with previous research results, we propose that the entire plateau almost simultaneously reached its near-maximum elevation by the Mid-Miocene.

Petrogenesis of submarine volcanic arc rocks from Andaman subduction zone, Northeast Indian Ocean: Constraints from slab components and mantle wedge characteristics

The present study provides new petrological and geochemical data of the dredged rocks from submarine volcanoes along the Andaman arc and describes the petrogenetic evolution of the arc system in terms of mantle wedge characteristics, nature and quantitative input of subducted slab components, and fractionation processes of precursor magma. The studied rocks include basaltic andesite, andesite, dacite and rhyolite. These volcanic rocks exhibit LILE, LREE enrichments and HFSE depletion, corroborating their generation through subduction processes. High abundances of Th/Nd, La/Sm(N), LREE/HFSE than LILE/HFSE, LILE/LREE suggest a substantial contribution of sediments from the subducting slab over slab-dehydrated aqueous fluids to the mantle wedge. The 87Sr/86Sr-Ba/La mixing model suggests 0.6–0.8% addition of slab fluid (90:10 AOC: sediment fluid) to account for the fluid signature, whereas the 143Nd/144Nd-La/Sm(N) mixing model envisages ∼3–4% addition of sediment melt to the mantle source, reconciling the sediment signature in Andaman submarine volcanic rocks. The presence of N-MORB type mantle is attributed to the absence and/or inefficient convection of asthenospheric material from the Andaman back-arc basin to the mantle wedge. This ineffective convection can be equated with the flat subduction of the Indian Plate, caused by the convergence of the aseismic Ninety East Ridge. The non-modal batch-melting model suggests that 13–24% partial melting of the spinel lherzolite mantle beneath the Andaman submarine volcanic arc formed the parent magma. The crystallization model invokes up to 60–70% of fractionation of olivine, plagioclase, clinopyroxene, orthopyroxene, sanidine and magnetite in all the rock types with subordinate proportions of amphibole, biotite, apatite, ilmenite, and sanidine in rhyolites. The basaltic andesites, andesites and dacites do not show upper crustal input, while rhyolites indicate crustal contamination from an upper crust and/or arc crust.

Petrogenesis of the Early Cretaceous Tietonggou Diorite and Its Geological Implications

The Tietonggou pluton is mainly composed of gabbroic diorite and diorite. The petrology, zircon U-Pb age, and geochemistry of the Tietonggou diorite have been studied to determine its petrogenesis and metallogenic significance. The diorite samples have 56–58 wt% SiO2 and 11–14 wt% Al2O3 and are peraluminous and sodic (Na2O/K2O = 1.29–2.07). All the samples are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs; e.g., Rb, Ba, and Sr) but depleted in heavy rare earth elements (HREEs) and high field strength elements (HFSEs; e.g., Zr, Nb, and Ta), suggesting subduction-related affinities. The rocks have narrow ranges of (206Pb/204Pb)t (18.5–19.0), (207Pb/204Pb)t (15.71–15.75), and (208Pb/204Pb)t (38.4–39.0) ratios, respectively. Zircons from the Tietonggou diorite yielded a weighted average U-Pb age of 132.86 ± 0.92 Ma (MSWD = 0.48), whilst those from the nearby Laowa diorite yielded 129.72 ± 0.61 Ma (MSWD = 1.05). This suggests that the rocks represent Early Cretaceous plutons, coeval with the peak lithospheric thinning in eastern North China Craton (NCC). The magma likely originated from partial melting of the enriched lithospheric mantle and was contaminated by ancient lower NCC crustal materials. Our study clarifies the tectonic background of the Tietonggou pluton and provides support for the study of the genesis of Fe–skarn deposits in western Shandong.

Facies architecture, geochemistry and petrogenesis of Middle Triassic volcaniclastic deposits of Mt. Ivanščica (NW Croatia): evidence of bimodal volcanism in the Alpine-Dinaridic transitional zone

During the Middle Triassic, intensive volcanic activity took place along the eastern margin of Pangea, including the Greater Adria promontory, due to the Neotethyan oceanization. This resulted in the formation of various volcanic and volcaniclastic rock types. The region of NW Croatia, acting as a transition zone between the Southern Alps and the Dinarides, showcases the outcrops of these rocks. The present study investigates the facies of volcaniclastic rocks, the distribution of those facies, formation processes, as well as the genesis of the primary magma to gain a better understanding of the complex geodynamics of this region during the Middle Triassic. Six profiles across the Vudelja quarry front were surveyed using drone imaging and samples were collected for detailed petrographic and geochemical analyses. Two groups of volcaniclastic rocks were identified—mafic and intermediate/felsic. The former is represented by (I) autoclastic effusive facies and (II) resedimented autoclastic facies, while the latter is represented by (III) secondary pyroclastic facies. Mafic volcaniclastics were generated through basaltic effusions in marine environments, fragmentation in contact with seawater, mixing with shallow marine carbonate clasts, and subsequent redeposition in deeper marine areas. The secondary pyroclastic facies (III) consists of a regionally distributed felsic Pietra Verde tuff whose deposits may be related to pyroclastic density currents and syn-eruptive resedimentation by turbidite-like currents. Geochemical data indicate that parental magmas responsible for generating the mafic volcaniclastics had a calc-alkaline composition and originated in ensialic and mature arc settings of an active continental margin. The observed chemical composition is likely inherited from older, arc-related lithologies, associated with the subduction of the Paleotethys Ocean. Parental magmas are thought to have formed during continental rifting of the passive Middle Triassic margins of the Greater Adria through (i) partial melting of the heterogeneous lithospheric mantle, which had been metasomatized during an earlier Hercynian subduction, and (ii) subordinate processes related to the melting of the upper continental crust and subsequent fractionation. Ar/Ar dating on plagioclase separates yielded an age of 244.5 ± 2.8 Ma for mafic volcaniclastics. This aligns well with biostratigraphic ages of felsic tuffs which crop out on a broader regional scale of the Dinarides, the Southern Alps, and the Transdanubian Range. The overlapping ages obtained from radiometric dating of mafic volcaniclastics and biostratigraphic ages of the felsic Pietra Verde tuffs strongly suggest that the Greater Adria region experienced concurrent bimodal volcanism during the Middle Triassic.

Tracing Deep Carbon Cycling by Zinc Isotopes in a Peralkaline‐Carbonatite Suite

AbstractSedimentary carbonates are known to be carried into the deep mantle by subducted slabs, and studies on mantle‐derived magmas have attempted to trace the recycled carbonate in their mantle source. However, the final depth of storage of recycled carbonate and the role of recycled carbonate in the partial melting of mantle remain controversial. Peralkaline‐carbonatite suites are considered to have been derived from a carbonated mantle source and are windows to evaluate carbon in the mantle. In this study, we report the Zn isotopic compositions of a peralkaline‐carbonatite suite from the Tarim Large Igneous Province (Tarim LIP). The peralkaline‐carbonatite suite has heavier δ66Zn than normal mantle with δ66Zn of 0.34–0.40 ‰ for nephelinite, 0.35–0.47 ‰ for aillikite, 0.51–0.55 ‰ for nepheline syenite, 0.58–0.67 ‰ for calciocarbonatite and 0.38–0.56 ‰ for magnesiocarbonatite. The heavy Zn isotopic compositions of the peralkaline‐carbonatite suite in the Wajilitag complex suggest the incorporation of recycled carbonate‐bearing materials into the deep mantle. We infer that the calciocarbonatite was produced by the initial partial melting of subducted MgSiO3/MgO + C‐bearing carbonated eclogite, whereas the magnesiocarbonatite, aillikite, and nephelinite are considered as reacted melts between carbonated eclogite‐derived melts and peridotite. The heavy Zn isotopic compositions of the nepheline syenite are attributed to fractional crystallization from nephelinite magma in the magma reservoir. Our study highlights the incorporation of carbonated eclogite as an important agent of recycled carbon in the deep mantle and interactions between carbonated eclogite‐derived melts and peridotite lead to the complex lithological heterogeneities in the peralkaline‐carbonatite suite in Tarim LIP.

Late Paleozoic potassic igneous rocks of the Adyrtor intrusions in the eastern Kyrgyz Tien Shan: geochemistry, isotope U-Pb zircon geochronology, and related W-Mo(-Cu-Au) mineralization

ABSTRACT The Adyrtor multiphase pluton is situated in the easternmost part of the ‘Main Structural Line of Tien Shan’, or the ‘Nikolaev Line’, and intrudes the Paleoproterozoic metamorphic sequence representing the oldest (1.8–2.5 Ga) continental basement fragment in the Tien Shan orogenic belt. The intrusions comprise mafic to intermediate (monzodiorite, monzonite, quartz syenite) and silicic (quartz monzonite, monzogranite, and leucogranite-alaskite) rocks. Geochemical signatures of the igneous rocks correspond to high-K calc-alkaline to shoshonitic series intrusions, with a strong A-type granite affinity, emplaced in post-collisional setting. Based on the isotopic U-Pb data, quartz syenite from the Adyrtor intrusions crystallized at 330.7 ± 4.3 Ma, and quartz monzonite – at 329.5 ± 5.8 Ma. Although the ages identified correspond to those of the subduction-related magmatism in the western part of the Middle Tien Shan, geochemical characteristics of the rocks support rather post-collisional setting of the plutons, principally due to a ‘scissor-like’ closure of the Turkestan paleoocean, starting from the east. Magmatic evolution included a generation of shoshonitic magma by low-degree partial melting of the metasomatically-enriched upper mantle, followed by amphibole fractionation in a deep (lower crustal ?) magma chamber. Then, under the influence of mantle-supplied fluids and heat, granitic magmas in the crustal protolith could have been generated, with further mixing/mingling of the mantle-derived mafic (shoshonitic) magma and mantle-induced crustal granitic magma, followed by the magma fractionation and emplacement at shallower crustal levels. Assimilation of an old continental crust is also envisioned, consistent with the presence of the xenocrystic zircons dated at ca. 1.6 Ga to 2.5 Ga and the A-type granitoid affinity of the igneous rocks. The associated skarn-porphyry W-Mo(-Cu-Au) mineralization is consistent with post-collisional setting of the productive intrusions. This mineralization complements the group of similar deposits and occurrences associated with high-K calc-alkaline to shoshonitic series intrusions in the Middle Tien Shan and globally. The high endowment in W and Mo can be explained by the fertilization of subduction-modified subcontinental lithospheric mantle in these metals, with its further involvement in the magma generation in post-collisional setting. Also, a stronger W-Mo mineralization can reflect a greater role of a crustal protolith in the magma generation, as revealed by the distinct A-type affinity of the igneous rocks.

Subduction zone decoupling and rapid forearc spreading at an active continental margin: Evidence from an SSZ-type ophiolite in Qilian Orogen, NW China

The forearc supra-subduction zone (SSZ) ophiolites are of great significance for understanding the geological processes of subduction systems. They are generally considered to mark the initiation of oceanic subduction. Here we present a well-preserved forearc sequence in the Ebo area, North Qilian Accretionary Belt, NW China. Three rock types are identified from bottom to top, including the forearc basalts (FABs), boninites, and calc-alkaline arc rocks, which are similar to those from the Izu-Bonin-Mariana (IBM) area. The FABs display similar compositions to N-MORB but with lower Ti/V ratios (17.16–20.92) and they were derived from 5 to 10 % melting of depleted MORB mantle in the spinel-stability field at P–T conditions of 1.5–2.3 GPa/1442 ± 56 °C after < 2 % melt extraction in the garnet-stability field. The boninites are characterized by significant geochemical characteristics of subduction-zone fluid/melt involvement, which were the products of flux melting of refractory harzburgitic mantle that interacted with slab-derived fluids/ carbonate melt under the P–T conditions of 0.2–1.2 GPa /1316 ± 38 °C. The calc-alkaline rocks show typical arc geochemistry, suggesting the partial melting of the well-established enriched subarc mantle. Zircon U-Pb dating of the boninite and the arc gabbro yielded ages of 507 ± 10 Ma and 483 ± 4 Ma, respectively. Combined with regional data, we infer that the Ebo FABs and boninites are not the products of subduction initiation but formed during subduction-zone decoupling-retreat and rapid forearc spreading following earlier subduction at the North Qilian continental margin, which is subsequently followed by typical continental arc development.

The Esquinzo Ultra-Alkaline Rock Suite of Fuerteventura Basal Complex (Canary Islands): Evidence for Origin of Carbonatites by Fractional Crystallization

The origin of the carbonatites that appear on Earth is one of the most controversial current topics in the petrogenesis of igneous rocks. Situated in the northern sector of the Basal Complex of Fuerteventura (Canary Islands), the Miocene Esquinzo ultra-alkaline plutonic rock complex is composed of pyroxenites, melteigites-ijolites-urtites, malignites-nepheline syenites, carbonatites, silicocarbonatites, nephelinites and nepheline phonolites. This work tries to establish the genesis of this massif of ultra-alkaline rocks with associated carbonatites from Fuerteventura (which are very rare in the oceans). The geochemical characteristics of these rocks and the minerals that are included in them have allowed us to establish their origin. This complex was generated by three successive magmatic events associated with differentiation of melanephelinite magmas emplaced in the oceanic crust. Silicocarbonatite and calciocarbonatite (sövites) dykes are related to the first magmatic event and were formed by fractional crystallization of H2O- and CO2-rich ijolite magmas. The melanephelinite magmas that formed these plutonic ultra-alkaline rocks were apparently generated as partial melts of asthenospheric mantle, which assimilated enriched lithospheric mantle material as they ascended. The upwelling of this large body of anomalous asthenospheric Miocene material exceeded the deformation associated with plate motions and led to an oceanic rifting event in Fuerteventura.

Reworking and maturation of continental crust in collision zones: Insights from Early Cretaceous compositionally diverse magmatic rocks in central Tibet

The reworking of continental crust in collision zones is a key process in the chemical maturation of crustal rocks. However, the mechanisms that cause crustal reworking remain highly controversial. The compositionally diverse magmatic rocks along the Lhasa–Qiangtang collision zone provide a favorable opportunity for studying the mechanism of crustal reworking and maturation in continental collision zones. Here we report zircon UPb ages and Hf isotopes, mineral major elements, and whole-rock geochemistry and SrNd isotopes for the Momu intrusive complex in the Lhasa–Qiangtang collision zone, central Tibet. Zircon UPb ages reveal that the intrusive complex formed during the late Early Cretaceous (115–113 Ma). Diorites from the intrusive complex have high Mg# values (49.7–51.2) and initial 87Sr/86Sr ratios (0.7082 to 0.7083), and low εNd(t) values (−7.3 to −6.9), with negative to slight positive zircon εHf(t) values (−3.8 to +0.8), suggesting that these rocks were likely generated by the partial melting of the enriched lithospheric mantle. Granodiorites from the intrusive complex have high SiO2 (64.4–67.9 wt%) and Al2O3 (14.8–16.1 wt%) and low MgO (1.4–1.8 wt%) contents. These rocks are also characterized by high initial 87Sr/86Sr ratios (0.7091–0.7098) and whole-rock negative εNd(t) (−9.9 to −8.4) and zircon εHf(t) (−7.9 to −2.2) values. The associated granites have the highest SiO2 (68.7–71.5 wt%) contents among the intrusive complex rocks, high initial 87Sr/86Sr (0.7089–0.7093) ratios, and negative whole-rock εNd(t) (−10.1 to −7.7) and zircon εHf(t) (−5.3 to −0.2) values. These geochemical characteristics indicate that the granodiorites were most likely derived from the partial melting of ancient continental crust and that the coeval granites were formed by the fractional crystallization of granodioritic magmas. Integrating our new results with data for the coeval magmatic rocks of the Lhasa–Qiangtang collision zone, we suggest that the Momu intrusive complex formed in a post-collisional slab breakoff setting. Asthenosphere upwelling caused by slab breakoff could have provided a substantial source of heat for crustal reworking, which gradually contributed to the maturation of continental crust in the Lhasa–Qiangtang collision zone.

Petrological and geochemical characteristics of the Low- and High-Hf Nam Meng dioritoid, northwest Vietnam: Implication for the mantle partial melting, mixing, and magmatic differentiation

This paper investigated the petrological, elemental, and isotope geochemical characteristics of the Nam Meng dioritoid to clarify the magma source and process. The Nam Meng massif comprises large amounts of dioritoids (gabbro-diorite and quartz diorite) and lesser amounts of granitoids (granodiorite and granite). The Nam Meng gabbro-diorite is a porphyritic texture with fine-grained plagioclase, amphibole, K-feldspar, and phenocryst biotite. The Nam Meng quartz diorite comprises coarse-grained plagioclase, amphibole, biotite, quartz, and K-feldspar. The Nam Meng gabbro-diorite contains higher plagioclase and lower biotite and quartz than the Nam Meng quartz diorite. The variation in petrography and mineralogy with the negative correlation between SiO2 contents with Al2O3, MgO, Al2O3 + MgO, T-Fe2O3, and CaO contents suggest a magmatic differentiation process. All the Nam Meng dioritoids have low ACNK (mostly less than 1.0), total alkaline (Na2O + K2O ≤ 6 wt.%) with Na2O/K2O ≥ 1, and negative anomalies of Ta, Nb, and Ti. Combined with the U-Pb zircon age of 290 Ma (Hieu et al. (2017), the Nam Meng dioritoid is thought to be an I-type granitic rock formed in the subduction stage of the Indosinian amalgamation event. The low La/Yb ratios (1.14-3.21) suggest that the mantle wedge that was melted to form the Nam Meng magma had a spinel peridotite composition. The εNd (290 Ma) of the Nam Meng dioritoid is close to bulk earth silicate at 290 Ma (-4.43 to 2.34). The 87Sr/86Sr (290 Ma) of the Nam Meng dioritoid varies in a wide range from low to intermediate (0.6987 to 0.7088), and the calculated Nd model age of the Nam Meng dioritoid is 1,258 ± 47 Ma. The Sr and Nd isotope data suggest that the Nam Meng spinel peridotite was a result of mixing between an ancient ocean crust, EM1, and EM2 that occurred in a paleo-subduction zone at Meso-Neoproterozoic Rodinia supercycle. The Hf contents of the low-Hf and high-Hf Nam Meng dioritoid series are 0.38-1.02 and 2.60-8.08, respectively. The LILE/Hf and HSFE/Hf ratios in the low-Hf Nam Meng dioritoids are high and have a strongly negative correlation with Hf. On the other hand, those ratios in the high-Hf Nam Meng dioritoids are low and have a weakly negative correlation with Hf. The Hf contents positively correlate with the degree of partial melting of the mantle wedge in the subduction zone. Therefore, the low Hf, high LILE/Hf, and HSFE/Hf ratios of the Nam Meng dioritoids could be derived from a low degree of partial melting of the mantle wedge. In contrast, the high Hf, low LILE/Hf, and HSFE/Hf ratios of the Nam Meng dioritoids could be produced by a high degree of partial melting of the mantle wedge.

Calcium isotope variability among ocean islands reveals the physical and lithological controls on mantle partial melting

Major geodynamic processes, including mantle convection and plate tectonics, govern the exchange of mass between Earth's interior and its surface. These processes drive mass-transfer mechanisms such as partial melting and crustal recycling, which have generated a lithologically and compositionally heterogeneous mantle over time. Ocean island basalts (OIB) directly sample these heterogeneities, making them excellent tools to constrain the mantle’s dynamic geochemical evolution. In this study, we probe the connection between crustal recycling, mantle mineralogy, and partial melting using the stable Ca isotope compositions of OIB, ultimately contextualizing their Ca isotope variability using conventional mantle “endmember” components including DMM, HIMU, EM-1, and EM-2. Using the state-of-the-art Nu Sapphire collision cell MC-ICP-MS, we measured the Ca isotope compositions of 27 well-characterized OIB samples from 11 different ocean island groups. By supplementing our new data with additional high-precision Ca isotope measurements from the literature, we create an expanded dataset consisting of 15 ocean islands, facilitating a global-scale investigation of Ca isotope variability among hotspots. Our findings reveal that island-averaged Ca isotope compositions form two distinct groups on the basis of their radiogenic isotope compositions. The first group, consisting of OIB with un-enriched radiogenic isotope compositions (non-EM-type), exhibits robust negative correlations with both lithospheric thickness and primitive TiO2 contents, providing compelling evidence that Ca isotopes are sensitive to the degree of partial melting (F) as well as to the effects of crustal recycling. This joint signal appears to trace the process of decompression melting beneath oceanic lithosphere of variable thickness, as recycled lithologies rich in garnet and pyroxene (i.e., pyroxenites)—which generate large solid-melt Ca isotope fractionations—disproportionally control the Ca isotope composition of the final aggregate melt when F is limited by thick overlying lithosphere. In contrast, the island-averaged Ca isotope compositions of OIB with enriched radiogenic isotope compositions (EM-type) form a distinctly light negative correlation with primitive TiO2 contents without exhibiting a relationship with lithospheric thickness. In addition to lithology-dependent partial melting effects, the Ca isotope compositions of EM-type OIB appear to be sensitive to the compositional effects of crustal recycling, which may explain their decoupling from the physical controls of partial melting. Calcium isotopes show exceptional potential for elucidating the origin of both enriched and ultra-depleted mantle sources, emphasizing the need to prioritize localities with these signatures in future studies.

Structure and equation of state of Ti-bearing davemaoite: new insights into the chemical heterogeneity in the lower mantle

Abstract Davemaoite (CaSiO3 perovskite) is considered the third most abundant phase in the pyrolytic lower mantle and the second most abundant phase in the subducted mid-ocean ridge basalt (MORB). During the partial melting of the pyrolytic upper mantle, incompatible titanium (Ti) becomes enriched in the basaltic magma, forming Ti-rich MORB. Davemaoite is considered an important Ti-bearing mineral in subducted slabs by forming a Ca(Si,Ti)O3 solid solution. However, the crystal structure and compressibility of Ca(Si,Ti)O3 perovskite solid solution at relevant pressure and temperature conditions were not systematically investigated. In this study, we investigated the structure and equations of state of Ca(Si0.83Ti0.17)O3 and Ca(Si0.75Ti0.25)O3 perovskites at room temperature up to 82 GPa and 64 GPa, respectively, by synchrotron X-ray diffraction (XRD). We found that both Ca(Si0.83Ti0.17)O3 and Ca(Si0.75Ti0.25)O3 perovskites have a tetragonal structure up to the maximum pressures investigated. Based on the observed data and compared to pure CaSiO3 davemaoite, both Ca(Si0.83Ti0.17)O3 and Ca(Si0.75Ti0.25)O3 perovskites are expected to be less dense up to the core-mantle boundary (CMB), and specifically ~1-2 % less dense than CaSiO3 davemaoite in the pressure range of the transition zone (15-25 GPa). Our results suggest that the presence of Ti-bearing davemaoite phases may result in a reduction in the average density of the subducting slabs, which in turn promotes their stagnation in the lower mantle. The presence of low-density Ti-bearing davemaoite phases and subduction of MORB in the lower mantle may also explain the seismic heterogeneity in the lower mantle, such as large low shear velocity provinces (LLSVPs).

Whole-Rock and Apatite Geochemistry of Late Triassic Plutonic Rocks in the Eastern Songpan-Ganzi Orogenic Belt: Petrogenesis and Implications for Tectonic Evolution

Abstract To constrain the late Triassic tectonic evolution of the Songpan-Ganzi orogenic belt, we present new whole-rock and in situ apatite geochemistry for plutonic rocks in its eastern margin. The Taiyanghe pluton can be classified into two rock types: dioritic and granitic rocks. The former exhibits low SiO2 and MgO contents but high Al2O3, Th, LREE contents, and Th/Yb and Th/Nb ratios, as well as low Ba/La and Ba/Th ratios and enriched Sr-Nd isotopic compositions, which, together with apatite geochemistry and Nd isotopes, indicate that they were derived from low degrees of partial melting of lithospheric mantle metasomatized by sediment-derived melts. The latter is characterized by high Sr and low Y and Yb, with elevated Sr/Y and (La/Yb)N ratios, implying an adakitic affinity. Notably, their similar Sr-Nd isotopic compositions indicate an origin from partial melts of a newly underplated lower crust. The Maoergai granitic rocks, characterized by high Sr and low Y and Yb contents with high Sr/Y and (La/Yb)N ratios, are indicative of adakitic rocks. In combination with the enriched whole-rock Sr-Nd isotopes and the apatite Nd isotopic data, we suggest that they were generated by the partial melting of the ancient thickened mafic lower crust. The Markam and Yanggonghai felsic granitoid rocks are peraluminous and similar to typical S-type granitoids, indicating an origin from remelting of the Triassic metasedimentary rocks. Based on the temporal-spatial relationship of the late Triassic plutonic rocks in the orogenic belt, we suggest that these rocks were formed in association with the roll-back and subsequent break-off of a subducted slab of the Paleo-Tethys Ocean. During the subduction, the formation of the Maoergai adakitic rocks was triggered by slab roll-back, whereas the magmatic “flare up” (ca. 216–200 Ma) was likely caused by slab break-off. This indicates that the final closure of the Paleo-Tethys Ocean happened in the end of the Triassic or Early Jurassic.

Identifying recycled oceanic components and volatiles in the Lizhuang carbonatite-associated REE deposit in southwest China: New constrain from in situ Hf–O isotopes

Carbonatite-associated rare earth element (REE) deposits (CARD) are mainly found at the edges of cratons in the rift or post-collisional settings, but only a minority carbonatite can form large to giant REE deposit. Fluids rich in volatiles (such as F, Cl, S) are critical for the extraordinary enrichment of REE, but the manner in which volatiles and REE concentrate in the mantle source of CARD remains unclear. This study focuses on the Lizhuang syenite from the Mianning–Dechang REE belt, southwest China, to investigate its petrogenesis, source characteristics, and volatiles evolution, through in situ Hf–O isotope analysis, coupled with mineral chemistry, bulk-rock geochemistry, and geochronology. The Lizhuang syenite was generated in a post-collisional setting at about 27 Ma, resulting from partial melting of an enriched lithospheric mantle, followed by fractional crystallization and immiscibility. When contrasted with barren carbonatite-syenite complexes worldwide, the syenite in REE deposit has a relatively higher content of F and S, with no significant difference in Cl content, and there is no clear correlation between magma oxygen fugacity and REE mineralization. The Hf–O isotopic signatures of zircon and apatite, along with bulk-rock Sr–Nd isotopic modeling, reveal that the mantle source of the Mianning–Dechang CARD experienced metasomatic processes related to the melts/fluids derived from both marine sediments and altered oceanic crust during subduction. The cratonic root, enriched in volatiles and REE, is reactivated during the upwelling of asthenosphere, and the released volatiles are able to participate in the formation of CARD.

Geochemical and Sr–Nd–Hf isotopic evidence for Cenozoic partial melting of mantle beneath Natanz, Central Iran

The products of Cenozoic continental arc magmatism in Iran provide an outstanding natural laboratory for investigating subduction-related processes. Here we present whole rock, SrNd isotopic, zircon U–Pb–Hf age, and mineral composition data for Cenozoic intrusive rocks from the Natanz area, central Urumieh–Dokhtar magmatic arc (UDMA), with a view to re-evaluate spatial–temporal variations in arc magmatism, magma petrogenesis and mantle source characteristics. The suite investigated in this study ranges from less evolved (gabbro) to highly evolved (granite) rocks with calc-alkaline affinity and an age range from 23.2 to 19.7 Ma. Their εNd(t) values vary from −0.32 to 2.73, initial 87Sr/86Sr values range from 0.7049 to 0.7075, and zircon εHf(t) values range from −2.2 to +8.9. Combined modelling of trace element and isotopic data suggests that the most mafic Natanz magmas could have formed through ∼5% melting of a source comprising 99% depleted mantle +1% sediment, with the melts modified subsequently by ∼3% contamination by Cadomian upper crustal components. We postulate that Natanz magmatism originated during a flare-up event instigated by retreat of the subducted slab prior to or in conjunction with the collision between the Arabian and Eurasian plates. The spatial–temporal associations in central UDMA (i.e., 33° 30′ N to 34° N) mirror three distinct flare-up episodes that occurred between 57 and 36 Ma, 26–15 Ma, and < 10 Ma; these events have not been reported previously from the northwestern and southeastern portions of the UDMA. Our results argue against any lull in magmatic activity in central UDMA during Eocene to Miocene times.

Geochronology, geochemistry, and geological significance of early Jurassic intrusive rocks in the Lesser Xing'an- Zhangguangcai Range, northeast China.

The Lesser Xing'an-Zhangguangcai Range of northeast China is located in the eastern segment of the Central Asian Orogenic Belt (CAOB), which records intense magmatism during the Mesozoic. The petrogenesis and geodynamic setting of the Early Jurassic intrusive rocks in this region are unclear. In this paper, we present new zircon U-Pb age and whole-rock geochemical data for these intrusive rocks to investigate their origins and tectonic setting. Zircon U-Pb dating suggests these intrusive rocks were emplaced during the Early Jurassic (197-187 Ma). The granites are enriched in silica and alkali, and depleted in MgO and CaO. They are metaluminous to weakly peraluminous, and have high A/CNK values and low zircon saturation temperatures (TZr ~ 779°C), suggesting they are highly fractionated I-type granites derived by partial melting of lower crustal materials. The granites exhibit negative Nb, Ta, P, Eu, and Ti anomalies due to fractional crystallization. The diorites and gabbros have low SiO2 contents and high Mg# values, and are enriched in light rare earth and large-ion lithophile (Ba, K, and Sr) elements, and depleted in heavy rare earth and high field strength (Nb, Ta, and Ti) elements. The geochemical characteristics show that the mafic magmas were derived by partial melting of mantle that had been metasomatized by subduction-related fluids. Based on the geochemical characteristics of coeval intrusive rocks and the regional geological setting, we suggest the Early Jurassic intrusive rocks in the Lesser Xing'an-Zhangguangcai Range were formed along an active continental margin, possibly as a result of bidirectional subduction of the Mudanjiang Oceanic plate between the Jiamusi and Songnen-Zhangguangcai Range massifs.

Permian−Triassic magmatism above a slab window in the Eastern Tianshan: Implications for the evolution of the southern Altaids

Permian−Triassic metaluminous−peraluminous granitoids, mafic−ultramafic plutons, and Ni-Cu and Au deposits are prominent features in the Eastern Tianshan of the southern Altaids. However, the genetic relationship between coeval granitoids and mafic−ultramafic intrusions, and the geodynamics of magmatism and related mineralization, remain ambiguous. To address these ambiguities, we present petrological, geochemical, and bulk-rock Sr-Nd-Fe and zircon U-Pb-Hf isotope analyses of granitoids from the Shuangchagou Complex and gabbros from the Huangshandong Complex in the Eastern Tianshan. Zircon U-Pb ages demonstrate that the Huangshandong gabbro was emplaced at ca. 277.8 ± 1.4 Ma. In contrast, U-Pb ages determined from zircons in the granitic rocks of the Shuangchagou Complex suggest that the complex crystallized from three stages of magmatism: (1) strongly peraluminous S-type granitic magma represented by early-stage gneiss and granitic veins (ca. 289 Ma), (2) metaluminous to weakly peraluminous I-type granitic magmas represented by the intermediate-stage granitoids (ca. 283−261 Ma), and (3) late-stage granitoids (ca. 250−241 Ma). The intermediate- and late-stage granitoids (ca. 283−241 Ma) show clear enrichments in the light rare earth elements and large ion lithophile elements (e.g., Rb, Th, and U), and depletions in high field strength elements (e.g., Nb, Ta, and Ti), similar to arc magmas, which indicates that the North Tianshan oceanic plate was still subducting during the Middle Triassic. Considering the diversity of magmatic rocks (e.g., mid-oceanic-ridge−type mafic rocks, and I-, S- and A-type igneous rocks), mineralization styles (e.g., Alaskan-type Ni-Cu sulfide deposits and orogenic gold deposits), and the dextral strike-slip faults (e.g., Kanggur Fault) that occurred concurrently in the Eastern Tianshan during the Early Permian to Middle Triassic, we suggest that splitting of the subducted portion of the North Tianshan oceanic plate created a slab window that allowed the upwelling and partial melting of asthenospheric mantle to form the mafic−ultramafic intrusions and related Ni-Cu sulfide deposits. Sustained migration of magma provided the heat necessary to induce partial melting, devolatilization, and desulfurization of crustal materials, producing the Permian−Triassic, high-K to calc-alkaline I- and S-type granitoids, and associated orogenic gold deposits. By integrating the results of this study with published work regarding the Kanggur Accretionary Complex, we suggest that the subduction of the North Tianshan Ocean may have lasted until the Late Triassic.