Low Degrees Of Partial Melting Research Articles

Neoproterozoic (ca. 830 Ma) carbonatite dykes from Qianlishan in the western North China Craton: Petrogenesis and metallogenic implications

Carbonatite and the associated alkaline intrusive complexes are the primary sources of light rare earth element (REEs). Our field investigations in the western margin of the North China Craton (NCC) have firstly identified over 20 carbonatite dykes and minor coeval carbonate-rich lamprophyre dykes intruding the Paleoproterozoic Qianlishan Group in the southern Qianlishan area. Zircon LA-ICP-MS U-Pb dating on six carbonatite samples yielded crystallization ages ranging from 835.2 ± 3.7 Ma to 828.9 ± 3.8 Ma, indicating emplacement of the carbonatite dykes at ca. 830 Ma during the Neoproterozoic. Geochemical data show that most of the carbonatite dykes are rich in light REEs with total light REEs > 600 ppm to 4932 ppm, and the carbonate-rich lamprophyre dykes are characterized by high Cr (1219–1267 ppm) and Ni concentrations (857–1076 ppm). Petrological and scanning electron microprobe (SEM) analytical results show that the dominant REE-bearing minerals in the carbonatites are monazite, and other REE-bearing minerals are parisite and bastnäsite. Geochemical and carbon–oxygen isotopic results indicate that the ca. 830 Ma carbonatite dykes were produced by low-degree partial melting of a previously metasomatised subcontinental lithospheric mantle with assimilation of crustal material. Emplacement of the ca. 830 Ma carbonatite and carbonate-rich lamprophyre dykes in the southern Qianlishan area and the previously reported ca. 810 Ma mafic dykes in the middle Qianlishan area occurred in a continental rift environment. Simultaneous development of the Neoproterozoic (830 − 810 Ma) continental rifting events in the western margin of the NCC and the eastern margin of the Alxa Block as previously suggested may indicate a connection between the eastern Alxa Block and the western NCC during and before the middle Neoproterozoic.

Petrogenesis and zircon U-Pb ages of the Late Paleozoic high-potassic Sonkul and Kokturpak plutons in Kyrgyz Tien Shan, with implications for related skarn-porphyry W-Au(-Cu-Mo) mineralization

ABSTRACT The Sonkul and adjacent Kokturpak multiphase plutons are situated along the ‘Major Structural Line of Tien Shan’ (known also as the ‘Nikolaev Line’) that extends for over 500 km and divides the terranes of the Middle and Northern Tien Shan. The plutons comprise a broad variety of high-potassic igneous rocks including the early olivine gabbro, intermediate monzonite to quartz monzonite, and later granodiorite, monzogranite, and leucogranite-alaskite, in turn followed by the late mafic dikes. U-Pb dating of zircon grains yields ages of 299 ± 2 Ma (olivine gabbro), 300 ± 3 Ma to 298 ± 3 Ma (monzonite to quartz monzonite), 299 ± 3 Ma to 298 ± 4 Ma to 297 ± 4 Ma (granodiorite), and 289 ± 4 Ma to 285 ± 2 Ma (monzogranite). These consecutive intrusive phases appear to have been produced by partial re-melting of a partially crystallized (mush) magma batch at deeper levels, which is evident by the presence of zircon antecrysts dated at some 306–311 Ma (up to 323 Ma?). Geochemical signatures of the igneous rocks correspond to high-K calc-alkaline to shoshonitic series intrusions emplaced in a transitional subduction-related to post-collisional setting. A low-degree partial melting of the metasomatically enriched upper mantle can be envisioned as the leading process of shoshonitic magma generation; this was followed by amphibole fractionation in a deep (lower crustal ?) magma chamber and possibly a mantle-induced generation of granitic magmas in the crustal protolith. Further mixing/mingling of the mafic magma and crustal granitic magma was followed by the magma fractionation and emplacement at shallow crustal levels. Assimilation of the old continental crust is consistent with the presence of ancient xenocrystic zircons (dated at ca. 1.5 to 2.5 Ga) in the studied rocks. The Late Carboniferous to Early Permian age and high-K calc-alkaline to shoshonitic composition are similar to those of highly mineralized intrusions of the other segments of the Middle Tien Shan. In this regard, several magmatic pulses of high-potassic intrusions and corresponding metallogenic pulses can be suggested in this region. Whereas the early (ca. 337 to 315 Ma) igneous suites accompanied by dominantly porphyry Cu-Au-Mo mineralization, the later (ca. 305 to 285 Ma) suites including the Sonkul and Kokturpak plutons are accompanied by W- and/or Au-dominant mineralization. This mineralization appears to pre-date more substantial intrusion-related to orogenic-type Au mineralization.

Comparison of the major- and trace-element compositions of the Kang-dian and Rössing-Husaib uraniferous leucocratic rocks

The Rössing-Husaib area is well-known for its low-grade and large-reserve U deposits hosted by leucogranites. The leucocratic dykes in the Kang-dian area of China resemble the Rössing-Husaib uraniferous leucogranites in occurrences and mineral assemblages, however, it remains disputable if the Rössing-Husaib and Kang-dian leucocratic rocks have consistent U-mineralizing processes. A comparison of the major and trace element data of the leucocratic rocks from both areas may help to better understand the ore-forming mechanisms and provide implications for exploration in the Kang-dian area. The positive correlation between Rb and P2O5 contents for the Rössing leucogranite samples is typical of S-type granitoids in spite of the low A/CNK ratios (∼1.0). The Rössing-Husaib uraniferous leucogranites are generally rich in K2O and Rb, and have high Rb/Sr and Rb/Ba ratios. In contrast, the Kang-dian leucocratic dykes have lower K2O and Rb contents, and lower Rb/Sr and Rb/Ba ratios, suggesting a clay-poor source. The U contents of the Rössing-Husaib and Kang-dian dyke samples are positively correlated with TiO2 contents, but show no correlation with Rb/Sr ratios, which is inconsistent with fractional crystallization. Low-degree partial melting of a U-rich protolith may account for primary U enrichment in the leucocratic rocks. The variable major- and trace-element compositions of the Kang-dian dyke samples may reflect wall-rock assimilation and/or alteration to varying degrees, which further enhanced U enrichment. Relatively low Th/U ratios (∼0.1) favored the final crystallization of uraninite. The uraniferous leucogranites in the Kang-dian and Rössing-Husaib areas are commonly undeformed and emplaced along regional large faults and/or domes at the late stage of orogeny. We propose that decompression and uplift of basement rocks facilitated the generation of uraniferous melts, and regional large faults provided a pathway for the ascent of leucocratic magma from depth.

Petrogenesis of the eudialyte-bearing syenite and indication for Nb-Zr-REE mineralization in Bashisuogong, Northwest China

The Bashisuogong alkaline igneous intrusion is located between the South Tianshan Orogenic Belt and the Kepingtage foreland thrust belt at the northern margin of the Tarim Craton and hosts economically significant Nb–Ta–Zr–rare earth element mineralization. Eudialyte has recently been discovered in the syenite unit of this intrusion, which is the primary host mineral for high-field-strength and rare earth elements. Based on detailed petrographic investigations, this study investigated the behavior of trace elements during magma evolution and the enrichment of ore-forming elements using whole-rock geochemical analyses of eudialyte-bearing syenite and compositional analyses of aegirine–augite and different types of eudialyte. We also undertook in situ titanite UPb dating. The Bashisuogong syenite comprises aegirine–augite and aegirine–augite–eudialyte syenite, the latter of which have higher total rare earth element contents. Both types of syenite exhibit slight enrichments in light rare earth elements, significant enrichments in high-field-strength elements (Nb, Ta, Zr, Hf, and Th), depletions in large-ion lithophile elements (Ba and Sr), and large negative Eu anomalies. There are two types (I and II) of eudialyte. Eudialyte I has higher Mn/Fe, Zr/Hf, and Nb/Ta ratios but lower Th/U ratios compared with eudialyte II but similar Y/Ho ratios. Eudialyte II crystallized earlier than eudialyte I, and aegirine–augite, titanite, and apatite crystallized earlier than eudialyte I. Both aegirine–augite and eudialyte have similar trace element characteristics to the host rocks, exhibiting enrichments in highly incompatible high-field-strength and rare earth elements, as well as marked negative Eu anomalies and depletions in Sr. These characteristics suggest that the Bashisuogong alkaline rocks formed by low-degree partial melting of geochemically enriched, metasomatized lithospheric mantle, which formed an alkaline basaltic parental melt that then underwent protracted magma evolution and fractionation in the shallow crust. The eudialyte syenite yield a titanite UPb age of 277.3 ± 1.2 Ma, which is consistent with the ages of different types of alkaline igneous rocks in the study area, indicating they were the products of the same magmatic event. Temporally and spatially, the Bashisuogong ore-bearing alkaline rocks were associated with the Tarim mantle plume, and the mantle plume provided the heat for their formation.

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.

Bimodal volcanism in the Hegau region (SW Germany): Differentiation of primitive melilititic to nephelinitic rocks produces evolved nosean phonolites

Peculiar bimodal volcanism in the Hegau region (SW Germany) comprises two contrasting SiO2-undersaturated rock suites.(I)Primitive olivine melilitites and melilite-bearing olivine nephelinites (12–9 Ma) are characterized by high MgO, CaO, Fe2O3, TiO2, Ni, V, F, moderate alkalis, Al2O3, P2O5, Ba, Nb, Zr, and low SiO2, Rb, Pb, and U concentrations. The rocks are composed of forsteritic olivine, diopsidic clinopyroxene, melilite, perovskite, Cr-bearing oxyspinel, F- and Ba-rich mica, and fluorapatite. In rare cases, they contain coeval coarse-grained ijolite patches generated by rapid in-situ fractionation in small melt pockets.(II)Evolved nosean phonolites (14–11 Ma) comprise high alkalis, Al2O3, SiO2, Rb, Nb, Zr, U, Pb, S, and low MgO, CaO, Fe2O3, TiO2, P2O5, Ba, Ni, and V concentrations, and contain abundant Ba-bearing alkali feldspar and nosean-haüyne-sodalitess macrocrysts, aegirine-augitic clinopyroxene, as well as accessory apatite, titanite, zircon, and pyrochlore.The melilititic–nephelinitic rocks formed by low degrees of partial melting of a carbonated amphibole ± phlogopite-bearing garnet wehrlite in the uppermost asthenosphere or in the thermal boundary layer and occur also in neighbouring regions. However, their coexistence with evolved nosean phonolites is a unique and so far, unexplained feature in the southern Central European Volcanic Province.Thermodynamic modelling implies that removal of 11–19% oxyspinel, 4–10% olivine, 42–57% clinopyroxene, <3% mica, <9% feldspathoids, <8% feldspar from melilititic–nephelinitic parental melts at upper crustal conditions (∼200 MPa) results in significant amounts of phonolitic residues that are compositionally similar to the exposed nosean phonolites. Strongly negative P and Ti anomalies and a trough of MREE in primitive mantle-normalized trace element patterns of phonolites indicate additional fractionation of titanite and apatite, consistent with the mineralogy of coarse-grained (nepheline) syenitic cumulates, which are present as enclaves in both rock suites. The modelling results suggest crystallization of the (nepheline) syenite cumulates between 1050 and 800 °C and ascent and eruption of the phonolite residues at no less than ∼900 °C with complete solidification of the observed assemblage at >750 °C. Significant crustal assimilation during fractionation appears unnecessary to explain the mineralogical, mineral chemical, and geochemical characteristics of the phonolites. Prolonged upper crustal differentiation of the magmas in one case (nosean phonolites) and fast ascent of primitive melts in the other (olivine melilitites and melilite-bearing olivine nephelinites) can be explained by stress field changes from an extensional to a more compressive regime, the magma ascent becoming thereby increasingly structurally controlled and supported by brittle deformation.

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.

Constraints on the source and petrogenesis of early Ediacaran shoshonitic mafic magmatism and high-K calc-alkaline granitoids in the Sergipano Orogenic System, Borborema Province, Brazil

The early Ediacaran Capela pluton (ca. 630 Ma) is intruded into the metasedimentary succession of the Sergipano Orogenic System along the oriental segment of the Macururé Domain, southeastern Borborema Province, NE Brazil. This body is an uncommon example of mantle-derived shoshonitic mafic rocks associated with high-K calc-alkaline granitoids emplaced in a pre- to early-collisional context. Here, we aim to unravel the petrogenesis of this plutonic association to determine the role of crustal contribution to magmatism and its geodynamical significance during the early stages of the Brasiliano/Pan-African Orogeny. The Capela pluton comprises cumulate hornblendites, gabbronorites, gabbros, diorites, granodiorites, quartz monzonites, and granites. Shoshonitic mafic rocks and high-K calc-alkaline granitoids show significant enrichments in LILE and LREE compared to HREE and HFSE. Ultramafic and mafic rocks display a conspicuous crustal signature, yielding subchondritic εNd(t) and zircon εHf(t) values, along with variable radiogenic Sr–Pb isotopic compositions. The dominant factors controlling the geochemical features of the mafic magmas were the enriched signature of the lithospheric mantle source and the degree of partial melting. Mantle source metasomatism is thought to have occurred by incorporating subducted sediments into the mantle wedge before the onset of the Brasiliano Orogeny. Geochemical modeling demonstrated that low partial melting degrees (5–10%) of phlogopite-bearing lithospheric mantle in the spinel stability field metasomatized by 5–10% subducted sediments may account for the main characteristics of the mafic rocks. Introducing sedimentary material into the mantle caused a decrease in Sm/Nd and Lu/Hf ratios while increasing the Rb/Sr, U/Pb, and Th/Pb ratios, which led to the time-integrated evolved isotopic signature observed in the mafic rocks. The coeval granitoids are metaluminous to slightly peraluminous and display Nd–Pb isotopic compositions similar to those of mafic–ultramafic rocks. The petrological and isotopic features of the granitoids are consistent with derivation from an ancient mafic lower crustal source. We argue that the ascent and emplacement of hydrous mantle-derived magmas as successive sills into the lower crust produced a deep hot zone beneath the Sergipano Orogenic System during the onset of the Brasiliano/Pan-African Orogeny. Aqueous fluids released from such potassic mafic magmas favored the partial melting of surrounding rocks, producing contemporary felsic magmatism.

Transition from oceanic subduction to continental collision: Insights from volcanogenic-sedimentary rocks of the Tannuola terrane (northern Central Asian Orogenic Belt)

Magmatism associated with oceanic subduction plays a dominant role in crustal growth during the Earth’s evolution. The Tannuola terrane, situated in the northern Central Asian Orogenic Belt (CAOB), is a key area to understanding oceanic subduction and initial collision processes in the northern CAOB. In order to investigate the switch from subduction to collision settings, detailed field mapping, zircon SHRIMP U-Pb geochronological and whole-rock geochemical studies of volcanogenic-sedimentary rocks from the Tannuola terrane were carried out. Zircon U-Pb ages indicate multi-stage volcanism lasted at least 30 Ma from ∼540 to ∼510 Ma, that can be divided into three events: the late Ediacaran (before ∼540 Ma), the early Cambrian (∼520 Ma) and the middle Cambrian (∼510 Ma). These ages are interpreted to the initial, main and final stages of oceanic subduction during the late Proterozoic – early Paleozoic. During the late Ediacaran, tholeiitic basalts with high εNd(t) values (from +7.4 to +8.5) were emplaced. Likely forming by the 10 %–30 % partial melting of spinel – garnet mantle source during slab subduction. During the early Cambrian, transitional from tholeiitic to calc-alkaline basaltic rocks with εNd(t) value (+5.6) and coeval intermediate–felsic volcanic rocks with similar εNd(t) values (+5.9 and +6.5) formed. The early Cambrian basaltic rocks are interpreted to be derived by 10 %–30 % partial melting of a depleted mantle source metasomatized by slab-derived fluids released from the subducting oceanic slab. The middle Cambrian calc-alkaline basaltic rocks with εNd(t) value of +6.2 might be emplaced as a result of low (5 %–10 %) degree partial melting of a metasomatized mantle followed by fractional crystallization of clinopyroxene and plagioclase. Associated intermediate-felsic volcanic rocks with εNd(t) values from +6.0 to +6.8 were formed through fractionation of the juvenile Neoproterozoic sources. The middle Cambrian volcanism is interpreted to be triggered by the slab break-off during the transition to a collisional setting.

Mineral chemistry and thermobarometry of the pre-rift Upper Cretaceous to Paleocene melilite-bearing dykes from the northern part of the Bohemian Massif (Ploučnice River region): Implications for compositional variations of spinels from ultracalcic melts

The Ploučnice River region (polzenite group) is uniquely characterized by its melilite-bearing subvolcanic rocks located in the northern section of the Bohemian Massif. They are the crystallization products of ultracalcic melts during the pre-rift evolution of the Ohře/Eger Rift, which is the easternmost part of the European Cenozoic Rift System. The melt was produced by low-degree partial melting of carbonate-bearing garnet peridotite and pyroxenite at a depth of approximately 100 km (P ~ 3.0 Gpa). The rapid ascent of the ultracalcic melts through the lithosphere was accompanied by fractional crystallization of olivine + spinel ± clinopyroxene mainly within the upper to middle crustal storage zone at depths between 12 and 24 km (0.3–0.6 Gpa). Notably, olivine crystallized generally at higher temperatures (1257–1356 °C) compared to clinopyroxene (1156–1203 °C) and plagioclase (1099–1112 °C). The calculated oxygen fugacity during fractional crystallization (perovskite, −4.8 to +3.9 ΔNNO) decreases at the late-stage of crystallization due to residual magma exsolving oxidizing fluids and decreased fO2 (oxygen fugacity) of the magmas from which monticellite was crystallized (ΔNNO −6.0 to −3.9). The rounded shapes and chemical composition (Cr/(Cr + Al) 0.52–0.82) of partially resorbed chromite xenocrystic cores in subhedral to euhedral spinel grains indicate that they originated in the mantle. The first stage of magmatic evolution for the studied rocks is related to the Cr-spinel (Cr/(Cr + Al) 0.35–0.50) crystallization, which successively changed to a high-alumina composition (Cr/(Cr + Al) 0.25–0.30). Magnetite (magnetite–ulvöspinel solid solution) forms an atoll texture or small euhedral crystals in the groundmass. Both textural types of magnetite crystallized during the late-stage magmatic evolution of the ultracalcic melt. Carbonate or quartz-rich xenoliths were incorporated during magma emplacement under the upper crust. Sr-Nd isotopic data, mineral composition, and whole-rock chemical composition all verified that the assimilation of the xenoliths only affected the chemical composition of the host magma in the immediate neighborhood of the contract (up to a few millimeters around the xenolith).

Petrogenesis of the late Cenozoic potassium-rich volcanism in the NW Tibetan Plateau: Constraints on the mantle evolution through post-orogenic events

The late Cenozoic Heishi Beihu volcanic rocks represent potassium-rich magmatic events at the northwesternmost margin of the Tibetan Plateau following the Indian–Asian continental collision. The petrogenesis of this juvenile igneous activity plays a crucial role in revealing the geochemical features of the deep mantle and contributes to our understanding of orogenic geodynamic evolution. This study, identified three eruption episodes at Heishi Beihu volcano (7.1–6.2, 4.3–1.4, and 1.3–1.0 Ma) using SHRIMP zircon U-Pb dating. Geochemically, the three episodes are characterized by shoshonitic affinities (K2O = 2.96–4.36 wt%) and crust-like trace-element signatures (negative Ta, Nb, and Ti anomalies), with enrichment in light rare earth (LREE) and large-ion lithophile elements (LILE), depletions in high-field-strength elements (HFSE), and small negative Eu anomalies (0.74–0.86). Their enriched whole-rock Sr ([87Sr/86Sr]i = 0.7072–0.7090), Nd (εNd(t) = −6.91 to −6.24), and zircon Hf (εHf(t) –5.64 to −9.13) isotopic compositions, and high zircon δ18O values (+7.4‰ to +10.6‰), are indicative of an enriched and heterogeneous mantle source (EMII-type) that contained recycled continental crust. The Heishi Beihu volcanic rocks were derived by low-degree (2%) partial melting of a mantle source with dominant pyroxenite and minor peridotite under conditions of the garnet-eclogite-facies. Results obtained from clinopyroxene–liquid geothermobarometry are indicative of diverse magma reservoirs, ranging from deep crustal levels (41–56 km) during Episode I to shallower depths for Episodes II (26–35 km) and III (17–29 km). In combination with the geological development of the area, we infer that magmatism at Heishi Beihu was genetically related to asthenospheric upwelling and induced by underthrusting of the Indian lithosphere beneath Western Kunlun.

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.

Age, clinopyroxene geochemistry and petrogenesis of post-collisional magmatic rocks in the Jonob-e-Sechangi area, Lut block, eastern Iran

The Lut-Sistan region, eastern Iran has an extensive record of Eocene-Oligocene magmatism the origin of which is not fully understood. In this study, we analyze a set of volcanic rocks in the Jonob-e-Sechangi area of the Lut block. U-Pb zircon analyses demonstrate that magmatism can be divided into two age groups. Group 1 formed at ∼45 Ma and involved different felsic (calc-alkaline rhyolitic to dacitic) and intermediate (high-K calc-alkaline andesitic) rocks. All samples from Group 1 display nearly identical initial 87Sr/86Sr values (87Sr/86Sri = 0.7045-0.7056). However, the felsic rocks show higher ԐNdt (+2.21 to +2.73) and lower TDM (0.51-0.61 Ga). In contrast, intermediate rocks have ԐNdt values of +1.27 to +2.23 and TDM values between 0.63 and 0.74 Ga. Additional geochemical variations suggest that the felsic and intermediate rocks of Group 1 are not co-genetic. The felsic extrusive rocks in the Jonob-e-Sechangi area likely formed from the partial melting of the amphibolitic arc crust. In contrast, the high-K calc-alkaline andesitic rocks of Group 1 may have originated from low-degree partial melting of a spinel-peridotite mantle source.Group 2 formed basaltic andesite to andesitic rocks at37-40 Ma, showing high-K calc-alkaline to shoshonitic compositions, relatively high 87Sr/86Sri (0.7047- 0.7076) and relatively low ԐNdt (0.48 to-5.86). These geochemical features require magma derivation from a mantle source that is not only elementally but also isotopically enriched, followed by variable fractionation and crustal contamination. The enriched mantle source was most likely resided in the lithosphere and formed by metasomatism by continental crust-derived materials. Clinopyroxene trace elements compositions display marked variations between different magmatic rocks. Clinopyroxene in dacitic samples of Group 1 shows relatively high contents of Sr, Rb, and Ba whereas that in Group 2 rocks has relatively high contents of Sc, V, Ga, Nb, Zr, Hf, Y and rare earth elements (REE). The findings of this study show that the intricate interactions between the lithosphere and asthenosphere, stemming from post-collisional delamination processes following the closure of the Sistan Ocean and the collision of the Lut-Afghan blocks, led to differing levels of partial melting in the region's heterogeneous metasomatized mantle. This phenomenon explains the diverse range of volcanic rocks found not just in the Jonob-e-Sechangi area but also in other regions of the Lut Block.

Geochemical constraints on the origin of the rare metal mineralization in granite-pegmatite, evidence from three-kilometer scientific drilling core in the Jiajika Li deposit, eastern Tibetan Plateau

Although the metallogenic mechanism of the Jiajika super-large Li deposits in the eastern Tibetan Plateau has been proposed closely linked to development of the Majingzi granite pluton, the petrogenesis and evolutionary linkages between granite–pegmatites and Li mineralization still remain controversial. Here we investigated Li-Nd isotopes together with major and trace elemental data from a 3-km scientific drillcore and surface samples (including the Li-rich/poor pegmatites, the associated two-mica granite and the metasedimentary rocks) to reveal the magmatic evolution and Li-enrichment mechanism of the Jiajika deposit. In the drillcore, the Li-poor pegmatites overall exhibit low Li isotope values (δ7Li = −1.38 ‰ to +1.98 ‰, mean of +0.34 ‰) and low Li contents (67 to 45 ppm), while the Li-rich pegmatites show slightly higher δ7Li values (+0.9 ‰ to +1.1 ‰) and high Li contents (3078 to 6012 ppm). Additionally, the Li-poor pegmatites have εNd(t) values of −17.1 – −2.5 (mean of −10.34) and low Nd concentration (<10 ppm), similar to the two-mica granite (εNd(t)avg = −11.18 and Ndavg = 7.08 ppm), whereas the schists and calc-silicate rocks in the drillcore have relatively higher εNd(t) values (−11.9 to −2.4 with a mean of −7.3) and Nd contents (>20 ppm). These data, together with consistently low Nb/Ta、Zr/Hf of the deep-drilled pegmatites and two-mica granites, suggest that the Jiajika pegmatites are not directly derived from low degrees of partial melting of the Triassic metasediments (e.g., biotite shists or calc-silicate rocks), but more likely from extreme magmatic fractional crystallization of the Majingzi two-mica granite pluton. For the surface samples, the δ7Li values of the surface Li-poor pegmatite samples (−0.61 ‰ to +2.56 ‰ with a mean of +0.36 ‰) are slightly higher than that of the Li-rich pegmatites (−0.52 ‰ to −0.27 ‰ with a mean of −0.39 ‰), and the two-mica granites samples show relatively variation in δ7Li (−1.25 ‰ to +3.33 ‰ with a mean of +0.73 ‰). The surface granites and pegmatites are characterized by high A/CNK values (>1.1) and low K/Rb values (<200), and particularly, the degree of REE tetrad effect gradually increases from granites to pegmatites. We suggest that the Majingzi two-mica granites exposed by the Jiajika gneiss dome is analogous to the deeper granitic sheet implied by the 3 km deep drilling, which both are strongly peraluminous S-type granites with a highly fractionation. Based on the degree of evolution of the Li-rich, Li-poor pegmatites and the two-mica granites and other geochemical evidence, we used a Rayleigh fractionation model to reproduce Li isotope variations in different types of rocks around the Jiajika deposit, suggesting an extreme magmatic differentiation without significant fluid exsolution as the origin for Li enrichments in pegmatites.

Rifting of the upper Yangtze carbonate platform: Insight from middle Permian carbonate olistostromes in the NE Sichuan basin, SW China

Rifting of the upper Yangtze carbonate platform in the middle Permian gave rise to Kaijiang-Liangping (KL) and Deyang-Wusheng (DW) rifts in the NE Sichuan basin, SW China. These rifts are well-endowed in gas, hosting eight out of nine discovered gas fields in the Sichuan basin. However, the initiation time and geodynamics driving the rifting are poorly understood, which restricts petroleum evaluation and exploration in the KL and DW regions. Our field surveys found that the basalt and underlying volcano-carbonate olistostrome in Huayingshan Mountain (Mt.) in the NE Sichuan basin are the interlayers within the limestone of the middle Permian Maokou Formation (Fm.). Stratigraphically, the Maokou Fm. and the late Permian sedimentary rocks above it form a mega-sequence from rifting to subsidence. Within the Maokou Fm., the rifting sequence comprises the basalt, volcano-carbonate olistostrome (tephra-rich polymictic breccia limestone) and limestone oligomictic breccia of the slope facies. A new data set composed of stratigraphic, petrologic, geochemical, isotopic and zircon U-Pb analyses shows that the basalt is sub-alkaline basalt and displays OIB-like geochemical signatures. The tephra block is basaltic tuff with abundant lath-shaped plagioclase crystals. Parent magma of the basalt originated from low-degree partial melting of the slightly enriched asthenospheric mantle at the garnet-spinel peridotite transition zone. The basalt and volcano-carbonate olistostrome reveal syn-rifting magmatism in ca. 267 ± 3 Ma. The KL and DW rift basins developed on the substratum of the upper Yangtze carbonate platform, and were triggered by asthenosphere upwelling during the Wordian stage.

Exceptional eruptive CO2 emissions from intra-plate alkaline magmatism in the Canary volcanic archipelago

Alkaline mafic magmas forming intra-plate oceanic islands are believed to be strongly enriched in CO2 due to low-degree partial melting of enriched mantle sources. However, until now, such CO2 enhancement has not been verified by measuring CO2 degassing during a subaerial eruption. Here, we provide evidence of highly CO2-rich gas emissions during the 86-day 2021 Tajogaite eruption of Cumbre Vieja volcano on La Palma Island, in the Canary archipelago. Our results reveal sustained high plume CO2/SO2 ratios, which, when combined with SO2 fluxes, melt inclusion volatile contents and magma production rates at explosive and effusive vents, imply a magmatic CO2 content of 4.5 ± 1.5 wt%. The amount of CO2 released during the 2021 eruptive activity was 28 ± 14 Mt CO2. Extrapolating to the volume of alkaline mafic magmas forming La Palma alone (estimated as 4000 km3 erupted over 11 Ma), we infer a maximum CO2 emission into the ocean and atmosphere of 1016 moles of CO2, equivalent to 20% of the eruptive CO2 emissions from a large igneous province eruption, suggesting that the formation of the Canary volcanic archipelago produced a CO2 emission of similar magnitude as a large igneous province.

Geochronological Constraints on the Evolution and Petrogenesis of the Malwa Plateau Subprovince of the Deccan Traps

AbstractThe eruptive history of the Malwa Plateau Subprovince of the Deccan Traps is addressed by dating 21 lavas spanning the exposed stratigraphic extent, using the 40Ar/39Ar method applied to plagioclase separates. Major, minor, and trace element geochemistry was determined for each of the dated lavas and four additional ones. Dating results indicate that the eruptions began prior to 66.8 Ma, at least 400 ka before the oldest known lava in the more extensively studied Western Ghats, representative of the main Deccan province, to the south. Eruption rates peaked from 66.4 to 66.3 Ma and then diminished until 65.6 Ma. The peak in eruption rates coincides with the well‐documented Late Maastrichtian Warming event. Malwa lavas show some major and trace element affinities with geochemically defined lava flow formations of the Western Ghats, but are generally out of the stratigraphic sequence manifest in the Western Ghats. The distinct geochemical evolution of Malwa Plateau lavas compared with those of the Western Ghats is at least in part a consequence of differences in crustal composition between the two subprovinces. Modeling of REE concentration patterns of Malwa lavas suggests that they were derived by slightly lower degrees of partial melting, at lower mantle temperatures and depths, than those in the Western Ghats. The Malwa Plateau thus appears to record an earlier, cooler stage of the Deccan plume's evolution and continued to erupt through a large part of the lifetime of the main Deccan province.

Genesis of Carbonatite at Oldoinyo Lengai (Tanzania) from Olivine Nephelinite: Protracted Melt Evolution and Reactive Porous Flow in Deep Crustal Mushes

Abstract Carbonatites, carbon-rich magmatic rocks, are thought to form by low-degree partial melting of a relatively carbon-poor mantle followed by protracted differentiation and immiscibility. However, the nature of parental magmas and the characteristics of the early stages of differentiation that shape the subsequent crystal and liquid lines of descent remain poorly constrained. To provide new constraints, deep crustal cumulative xenoliths from Oldoinyo Lengai (East African Rift), the only active volcano erupting carbonatite magmas, were studied. We use major and volatile elements in primitive olivine-hosted melt inclusions, as well as major and trace elements in crystals, to reconstruct the conditions of formation and evolution of cumulates (pressure, temperature, composition). Xenoliths are composed of olivine, diopside, phlogopite, amphibole and accessory minerals. One remarkable feature is the presence of diopside and phlogopite oikocrysts enclosing roundish olivine chadacrysts. Melt inclusions do not have vapor bubble and have major element compositions resembling olivine nephelinite (7–10 wt % MgO after corrections for post-entrapment crystallization). The absence of vapor bubbles implies that the concentrations of volatile components (i.e. CO2, H2O, S) were not compromised by well-known post-entrapment volatile loss into the vapor bubble. Based on the melt inclusion study by SIMS, the volatile concentrations in olivine nephelinite magmas (early stage of differentiation) at Oldoinyo Lengai were 20–130 ppm S, 390–4500 ppm F, 50–540 ppm Cl, up to 6074 ppm CO2 and up to 1.5 wt % H2O. According to the calculated CO2-H2O saturation pressures and geophysical data, xenoliths from Embalulu Oltatwa document a mushy reservoir in the lower crust. Primitive olivine nephelinite melt inclusions have higher H2O contents than olivine nephelinite lavas from other further South volcanoes from the North Tanzanian Divergence (0.2–0.5 wt % H2O), suggesting that the lithospheric mantle source beneath the Oldoinyo Lengai is more hydrated than the mantle beneath the rest of North Tanzanian Divergence. We present a model in which resorption features observed in olivine chadacrysts, together with the LREE enrichments in olivine grains, are the consequences of reactive porous flows in a deep crustal mushy reservoir. We provide constraints on the major, trace and volatile element composition of the parental magmas of carbonatite series and demonstrate with Rhyolite-MELTS models that phonolites and related natrocarbonatites from Oldoinyo Lengai can be produced by protracted differentiation of olivine nephelinite melts.

Bulk composition and O–C isotope constraints on the petrogenesis of Richat carbonatites (Mauritania)

In the northwestern Mauritanian Taoudeni Basin, the famous Richat structure is the only occurrence known of carbonatite. Despite several studies on this unique geological feature, the mineralogy and petrogenesis of the Richat dome carbonatites are underdocumented.Major, trace element whole rocks and carbon-oxygen isotope data are reported for Cretaceous carbonatites from Richat dome in Mauritania. More than 45 carbonatite dykes have been mapped, they are oriented N°15-N°80. Based on color, four groups have been identified, dark, gray, red breccia, and brown carbonatites. They exhibit different mineralogical and texture characteristics. The studied carbonatites are mainly composed of dolomite, calcite, and ankerite, with lesser amounts of apatite and quartz. Accessory minerals such as pyrochlore, spinel, pyrite, chalcopyrite, magnetite, hematite, baryte, phlogopite, and zircon are also present. Other REE-bearing minerals (britholite, monazite, and bastnaesite) were revealed by X-ray diffraction.Three types of carbonatites are identified: magnesiocarbonatites (dominants), ferrocarbonatites and calciocarbonatite. The presence of spinel and pseudomorphed olivine, as well as high contents of Ni (380 ppm), Co (34 ppm), and Cr (840 ppm) in the dark and brown carbonatites, suggests that these rocks are the result of the low-degree partial melting of a spinel-bearing peridotite. Furthermore, the Zr/Hf (48–80) and Nb/Ta (26–48) ratios show significantly higher values than those observed in the primitive mantle, they are interpreted as formed by partial melting of the metasomatized mantle. The high values of δ18O in addition to the presence of baryte, ankerite, and quartz in the Richat carbonatites suggest that they are the result of secondary processes involving a low-temperature fluid-rock interaction.The carbonatites from the Richat dome are enriched in LREE (La = 258–1070 ppm) and LILE as Sr, Ba, Th, U, and depleted in HREE (Yb = 2–14 ppm) and HFSE such as Zr, Hf, Ti. These characteristics are like those of Cretaceous peri-atlantic alkaline carbonatites such as Barra do Itapirapua in Brazil, Ondurakorume and Kalkfeld in Namibian, lupangola in Angola, and Twihinate in Morocco. Reactivation of a deep pan-African NNE-SSW lineament would have been a fundamental factor in the formation of the Richat carbonatites.

Mantle Metasomatism and REE Enrichment in the Genetic Source of the Dalucao Carbonatite Complex (Sichuan, China): Insights from Elemental Geochemistry and In-Situ Sr Isotopes of Two Calcite Types

Abstract Carbonatites possess the highest rare earth element (REE) concentrations among all magmatic rocks, yet the mechanisms governing the enrichment of REEs in carbonatites remain enigmatic. Carbonatite-hosted calcites provide crucial mineralogical and geochemical insights into addressing this matter. In this study, we present a dataset comprising major and trace elemental geochemistry, as well as in-situ Sr isotopic data, for two types of calcites from the Dalucao carbonatite complex in Sichuan Province, China. Our primary objective is to shed light on the origin of fertile carbonatites. These calcites are believed to have originated at different stages of carbonatitic evolution. Type I calcites, characterized by a euhedral granular texture and a homogeneous surface, crystallized at the early stage of carbonatite magmatism. In contrast, type II calcites, closely associated with bastnäsite in a paragenesis, formed within a fluid setting of hydrothermal overprinting. Both calcite types exhibit highly radiogenic (87Sr/86Sr)i ratios, ranging from 0.7059 to 0.7060 for type I calcites and 0.7059 to 0.7068 for type II calcites. The high Sr concentrations (3646–7315 µg/g for type I calcites and 6566–13,427 µg/g for type II calcites) and uniform Pb isotopic compositions (206Pb/204Pb ratios of 18.23–18.27) in the Dalucao calcites refute the hypothesis of crustal assimilation as their genetic source. Instead, the radiogenic Sr isotopic compositions suggest that subducted marine sediments have contaminated the lithospheric mantle that generated the carbonatitic magma. Mineralogical evidence indicating the presence of abundant silicate minerals in the Dalucao carbonatites, combined with a compilation of published C-O isotopic data, suggests that these carbonatites may have resulted from low-degree partial melting (melting proportion &amp;lt;1%) of a metasomatized lithospheric mantle. Finally, we propose a comprehensive model for REE enrichment in the Dalucao carbonatite complex and explore the significance of tectonism in the genesis of fertile carbonatites.