LREE Enrichment Research Articles

Timing of Transition from Proto- to Paleo-Tethys: Evidence from the Early Devonian Bimodal Volcanics in the North Qaidam Tectonic Belt, Northern Tibetan Plateau

The transition from the Proto- to the Paleo-Tethys is still a controversial issue. This study reports a new petrology, zircon U–Pb geochronology, and whole-rock geochemistry of volcanic rocks from the Maoniushan Formation in the Nankeke area, northern Qaidam (NQ) of the Tibetan Plateau, to provide new evidence for the transition from the Proto- to the Paleo-Tethys oceans. The volcanic suite consists mainly of rhyolitic crystal lithic tuff lavas and minor basalts. Zircon U–Pb data indicate that the bimodal volcanic rocks were formed during the Early Devonian (ca. 410–409 Ma). Geochemically, the basalts have low contents of SiO2 (48.92 wt.%–51.19 wt.%) and relatively high contents of MgO (8.94 wt.%–9.99 wt.%), TiO2 (1.05 wt.%–1.29 wt.%), K2O (2.35 wt.%–4.17 wt.%), and K2O/Na2O ratios (1.04–2.56), showing the characteristics of calc-alkaline basalts. Their rare earth element (REE) patterns and trace element spider diagrams are characterized by enrichments in LREEs (LREE/HREE = 18.31–21.34) and large ion lithophile elements (LILEs; Rb, Th, and K) and depletion in high-field-strength elements (HFSEs; Nb, Ta, P, and Ti), with slight negative Eu anomalies (Eu/Eu* = 0.82–0.86), which are similar to Etendeka continental flood basalts (CFB). These features suggest that the basalts were most likely derived from low degree (1%–5%) partial melting of the asthenospheric mantle, contaminated by small volumes of continental crust. In contrast, the felsic volcanics have high SiO2 (68.41 wt.%–77.12 wt.%), variable Al2O3 (9.56 wt.%–12.62 wt.%), low MgO, and A/CNK ratios mostly between 1.08 and 1.15, defining their peraluminous and medium-K calc-alkaline signatures. Their trace element signatures show enrichments of LREEs and LILEs (e.g., Rb, Th, U, K, and Pb), depletion of HFSEs (e.g., Nb, Ti, Ta, and P), and negative Eu anomalies (Eu/Eu* = 0.22–0.66). These features suggest that the felsic volcanics were derived from partial melting of the middle crust, without interaction with mantle melts. Considering all the previous data and geochemical features, the Maoniushan Formation volcanic rocks in NQ formed in a post-collisional extensional setting associated with asthenospheric mantle upwelling and delamination in the Early Devonian. Together with the regional data, this study proposed that the Proto-Tethys Ocean had closed and evolved to the continental subduction/collision orogeny stage during the Middle to Late Ordovician, evolved to the post-collisional extensional stage in the Early Devonian, and finally formed the Zongwulong Ocean (branches of the Paleo-Tethys Ocean) in the Late Carboniferous, forming the tectonic framework of the Paleo-Tethys Archipelagic Ocean in the northern margin of the Tibetan Plateau.

Highly Unradiogenic Pb Isotopes Revealed by Plagioclase‐Hosted Melt Inclusions in E‐MORB From the South China Sea

AbstractMid‐ocean ridge basalts (MORBs) can record heterogeneity of the upper mantle. Early crystallized minerals and their melt inclusions commonly record greater isotopic variability than whole‐rocks and are powerful tools to investigate the mantle source. In this study, we develop a new in situ Pb analytical protocol for Pb‐poor materials (sub‐ppm level) and apply it to the plagioclase‐hosted melt‐inclusions from IODP Expedition 349 Site U1433, South China Sea (SCS). Together with element mapping, in situ major and trace element analyzes and in situ Sr isotopes of plagioclase phenocrysts, we show that unlike the low and uniform whole‐rock 87Sr/86Sr ratios, the plagioclase phenocrysts record extreme Sr isotopic heterogeneity. Strontium isotopic heterogeneity is observed between crystals even in a single thin section. Based on the high an contents of plagioclase phenocrysts and high Mg# values of their melt inclusions, we propose they crystallized early in the magma chamber and were most likely formed in different batches of mantle‐derived melts. The plagioclase‐hosted melt inclusions show highly variable Pb isotopes, exhibiting a typical linear variation in 207Pb/204Pb versus 206Pb/204Pb. Such a mixing trend indicates an end‐member with highly unradiogenic Pb isotopes. The LREE enrichment and negative correlations between La/Sm and Pb isotopes of the melt inclusions, indicate that ancient cratonic lower continent crustal materials, with low time‐integrated U/Pb ratios, are likely present in the sub‐ridge mantle of the southwest Sub‐basin, SCS.

Silica cycling in Neoproterozoic oceanic lithosphere: A case study from Wadi Igla carbonate-serpentinite (southern Eastern Desert of Egypt)

Serpentinites play a pivotal role in carrying fluids and different elements into the Earth’s mantle. However, their role in exchanging silica (Si) between the marine environment and the mantle remains a matter of investigation. The Wadi Igla serpentinite (southern Eastern Desert of Egypt) formed at the expense of abyssal harzburgite by ∼15–22 % melting. It contains abundant Cr-spinel with sub-microscopic serpentine and chlorite-rich pores providing a base to explore Si cycling during serpentinization-carbonatization processes. The low-grade metamorphism of the harzburgite protolith started on the ocean floor, forming lizardite and chlorite (250–300 °C). Increasing the temperature (400–450 °C) caused the formation of brucite and antigorite. With the subduction in the fore-arc and the interaction with subducting sediments-related CO2-rich fluid, the Wadi Igla serpentinite underwent metasomatism, producing chlorite (300 °C), antigorite, tremolite, dolomite, and ferritchromite rims around Cr-spinel (Type 1), with brucite loss. In the upper greenschist-lower amphibolite facies (ca. 500 °C), CO2-rich hydrothermal fluids (with XCO2 of ∼0.55) penetrated a large volume of the protolith leading to full serpentinization together with abundant magnesite replacement. The resultant silica-rich fluids percolated in the Type 1 Cr-spinel from the outward to cores through microfractures and pores, producing Type 2 and Type 3 Cr-spinel with serpentine ± chlorite along cleavages, diminished Al-cores and growing outer ferritchromite zone and/or Cr-magnetite to magnetite zones. The suprachondritic NbN/LaN (up to 39.35) and NbN/BaN (up to 13.37) of whole rock implies for HFSEs metasomatism by subduction sediments input components, while slight enrichment in LREEs (LaN/YbN = 2.5–3) and FMEs (Li, Pb, Sr, and Ba) may have resulted from serpentinization-related hydrothermal alteration. The Wadi Igla serpentinite indicates silica cycled in a closed system, suggesting that the altered Neoproterozoic oceanic lithosphere may not have shared their main components with the surrounding environment whether to the ocean floor or the subduction zone.

Ca. 830 Ma alkaline mafic dykes in the southeastern Guizhou Province, South China: New constraints on the Neoproterozoic evolution of the Yangtze Block

The Neoproterozoic alkaline mafic rocks in the Yangtze Block are critical to investigate the magmatic and tectonic evolution of South China. However, these rocks are rarely exposed. We here present a detailed study of ca. 830 Ma alkaline mafic dykes newly identified from a drilled borehole at the Nage area in the southern Yangtze Block. The dykes have SIMS zircon U-Pb ages of 827 ± 8 Ma. They show coherent variations in major and trace elements, characterized by enrichment in LREE and incompatible elements and no significant Nb-Ta and Zr-Hf anomalies, suggesting that their parental magmas have experienced varying degrees of fractionation crystallization with negligible crustal contamination. Modeling calculations based on their εNd(T) values (-0.55–3.23), SiO2 contents (48.38–53.61 wt%) and elemental ratios (e.g., La/Ta and Nb/La) suggest that the Nage mafic dykes were probably generated from an asthenospheric mantle with previously input of a plume-derived enrich component occurred within the source region rather than during ascent. Rocks from the Nage dykes exhibit OIB-like trace element and REE patterns as well as trace elemental compositions, which are different from arc-related igneous rocks but similar to alkaline basalts and mafic dykes formed in mantle plume related rifting systems around the world. In combination with tectonostratigraphic evidences, the Nage dykes in this study are suggested to have been generated in an intraplate rift which was probably induced by a mantle plume beneath the Yangtze Block.

Petrological and geochemical evidence for partial melting and melt-rock interaction in mantle rocks from the eastern part of the Sabzevar ophiolite, NE Iran

ABSTRACT The Sabzevar ophiolite is the most extensive and best-exposed section of oceanic lithosphere in NE Iran. In this study, we examined the mantle section of the eastern part of the Sabzevar ophiolite, where mantle peridotites are the most widespread rock type and are crosscut by pyroxenite veins and mafic dykes. Major and trace element compositions (minerals and whole-rock) of lherzolites are akin to those of abyssal peridotites from mid-ocean ridges (MOR), whereas those of harzburgites evidence formation in a supra-subduction zone (SSZ) setting. Rare-earth element (REE) patterns of whole-rock and clinopyroxene of lherzolites and harzburgites suggest polybaric melting, with initial melting in the garnet peridotite field (~5–6%), followed by ~5–15% melting for lherzolites and ~10–20% for harzburgites in the spinel peridotite field. Harzburgites show LREE enrichment incompatible with partial melting models, indicating significant interaction with SSZ-related melts, whereas analysed lherzolites experienced limited melt-rock interaction. Pyroxene dissolution and olivine precipitation in residual peridotites resulting from reaction with island arc tholeiite and/or boninite melts led to the formation of dunites. These melts eventually intruded as dykes and veins in the mantle and crustal sections of the ophiolite. In addition, boninite melts are inferred to be responsible for forming olivine pyroxenite veins within the harzburgites. We conclude that the mantle section of the Sabzevar ophiolite formed in an intra-oceanic arc setting and experienced a geochemical evolution from a MOR-like to SSZ setting. The intra-oceanic subduction of the Neotethys oceanic lithosphere and shifting conditions of melting during the development of subduction beneath the incipient island arc were responsible for the geochemical heterogeneity of mantle peridotites and the generation of SSZ-related magmas within the Sabzevar oceanic lithosphere.

Unravelling magmatic-hydrothermal processes at Salobo and GT-46 IOCG deposits, Carajás mineral province, Brazil: Constraints from whole-rock geochemistry and trace elements in apatite

The iron oxide-copper-gold (IOCG) deposits of the northern portion of the Carajás Province, Amazonian Craton, Brazil, were primarily formed at the 2.5 Ga IOCG event. These deposits commonly exhibit wide Fe–K hydrothermal alteration halos (i.e., almandine-grunerite-biotite-magnetite) and display a temporal and spatial association with the 2.5 Ga granitic bodies, particularly in the GT-46 and in the world-class Salobo deposit. However, this association still lacks investigation concerning the metallogenic influence of the 2.5 Ga granitic bodies in the IOCG formation. Within these deposits, ca. 2.5 hydrothermally altered granites are recognized and include the Old Salobo, at the Salobo deposit, and G1 and G2, at the GT-46 deposit. Whole-rock geochemistry combined with petrography has pointed out that the G1 and Old Salobo are represented by granites to monzogranites, ferroan, metaluminous to weakly peraluminous with negative Eu anomaly. On the contrary, the G2 granite seems to represent a distinct body with granite composition, ferroan, metaluminous to weakly peraluminous with positive Eu anomaly. This granite exhibit pegmatitic textures, high Sr/Y ratios and listric-shaped normalized REE patterns, which suggest crystallization under high water contents. These granites primarily plots in the field of the volcanic arc granites, though some shifts towards to syn-collisional granites are observed. Effects of hydrothermal alteration are commonly seen in these granites both in textural aspects and geochemistry. Apatite mineral chemistry studies shows that magmatic signature in apatite is preserved in the G2 granite (G2core apatite), Old Salobo granite (OSA) and in the GT-46 ore zone (OZGT-A apatite), represented by enrichment in LREE and negative Eu anomaly. Hydrothermally altered apatite is recognized in the Old Salobo (OSB) and G2 granite (G2rim) and in the ore zone of GT-46 deposit and (OZGT-B) with depletion in LREE, which suggest that the same fluid might have affected both deposits. Truly hydrothermal apatite crystals from the Salobo and GT-46 ore zones shows high (Eu/Eu*)N values, indicative of high oxygen fugacity and alkaline fluids. Strong HREE depletion of the apatite grains from the central zone of the deposits suggest coeval almandine crystallization in the Fe–K alteration halos and evolved hydrothermal fluids. These results indicate that magmatic and hydrothermal process coexist in the IOCG deposits from the northern sector of Carajás Province. This might suggest that the 2.5 Ga IOCG deposits from the Carajás Province can have formed during the development of wide magmatic-hydrothermal systems.

Early Paleoproterozoic tectonic evolution of the Yinshan Block in the North China Craton: Constraints from the geochronology and geochemistry of basic to felsic magmatic rocks in the Guyang area

The earliest Paleoproterozoic magmatic rocks have recorded the transition from the Neoarchean cratonization of the NCC to its post-Archean tectonic evolution, which will provide insight into the onset of the modern-style subduction. This study focuses on the early Paleoproterozoic granodiorite, monzonitic diorite, alkali feldspar granite and gabbro in the Guyang area to decipher the early Paleoproterozoic tectonic evolution of the Yinshan Block. Zircon U–Pb dating indicates that these rocks were emplaced at ca 2.46 – 2.41 Ga. The ∼ 2.46 Ga granodiorite samples have high Sr contents, low Y and Yb contents with high Sr/Y ratios, similar to the characteristics of adakitic rocks. Based on their high Mg# values, Na2O/K2O ratios, Cr, Co and Ni concentrations, and positive εNd(t) values, we propose that the granodiorite was likely produced by partial melting of subducting oceanic slab. The contemporary monzonitic diorite samples are characterized by high Nb concentration, which are similar to those of Nb-enriched basalt/andesite. The monzonitic diorite samples are characterized by LREE-enriched patterns with negative Eu anomalies and have negative εNd(t) values (-0.4 – −1.4), which suggest that they were derived from partial melting of fertile mantle peridotite metasomatized by adakitic melts. The ∼ 2.44 Ga alkali feldspar granite samples have negative εNd(t) values (-4.0 – −1.9) and show geochemical characteristics of A-type granites, indicating an extension background. The distinct geochemical characteristics of ca. 2.46 – 2.44 Ga Guyang adakatic granodiorite, Nb-enriched monzonic diorite and A-type alkali feldspar granite suggest that a slab window was opened in the southern margin of the Yinshan Block. The gabbros were emplaced at ∼ 2.41 Ga revealed by U-Pb dating. The enriched LREEs and relatively flat HREEs patterns with negative Nb-Ta anomalies of gabbros indicate a subducting tectonic setting. Their high Ba/Th and low Nb/Y ratios imply that the mantle source was metasomatized by slab-derived fluids. This special rock association not only constrains the tectonic evolution in the southern margin of the Yinshan Block, but also indicates modern-style subduction operated already in the early Paleoproterozoic.

Geological Significance of Neoproterozoic Intrusive Rocks in the South Section of the Ailaoshan Orogenic Belt, SW China: Insights from Petrology, Geochemistry, and Geochronology

The Ailaoshan orogenic belt is one of the most significant orogenic belts in the southeastern margin of the Qinghai–Tibet Plateau. The widely developed magmatic rocks in this belt preserve the multi-stage tectonic evolution records of the South China Plate. As an important response to the Rodinia breakup tectonic event, the study of Neoproterozoic magmatic rocks in the area is of great significance for reconstructing the Neoproterozoic tectonic process of the Ailaoshan orogenic belt and the tectonic evolution of the South China Plate. Petrology, geochemistry, zircon U-Pb, and Lu-Hf isotopes of the Daping pluton in the Ailaoshan orogenic belt are studied in this paper. The Daping pluton is mainly divided into gabbros and granites. Gabbros and granites belong to the sub-alkaline series, which are relatively enriched in large ion lithophilic elements and depleted in high-field strength elements. The ΣREE contents of the gabbro are low with enrichment in LREEs and depletion in HREEs, and the degree of differentiation of light and heavy rare earth is low, with positive δEu and weak negative δCe anomalies. The ΣREE contents of the granite are low with enrichment in LREEs and depletion in HREEs, and the degree of light and heavy rare earth differentiation is high, with medium–weak negative δEu and weak positive δCe anomalies, suggesting an A2-type granite with A1-A2 transition characteristics. The weighted average age of the gabbro is 816.1 ± 4.1 Ma (MSWD = 0.11), with zircon εHf(t) values of −7.5–5.5. The magma source is a mixture of an ancient crust source and a new mantle source; the weighted average age of the syenogranite is 783.7 ± 8.1 Ma (MSWD = 1.4), with zircon εHf(t) values of −4.3–0.4. The magma source is mainly ancient crustal material (Ailaoshan Group), mixed with a small amount of mantle-derived material; the weighted average age of the monzogranite is 754.8 ± 6.1 Ma (MSWD = 3.0), with positive zircon εHf(t) values of 1.65–10.36. The magma source is a mixture of a large number of mantle-derived materials and a small number of crust-derived materials (Ailaoshan Group). The Daping pluton was formed in the transitional tectonic environment from post-collision to intraplate continental margin rift, corresponding to the Rodinia breakup process in the western margin of the South China Plate.

Variation in Trace Element and Isotope Composition of Neoarchean Mafic Granulites of the Southwest Siberian Craton: A Consequence of Various Mantle Sources or Crustal Contamination

The paper presents geochemical and isotopic characteristics of Neoarchean (2.7–2.66 Ga) mafic granulites of the Sharyzhalgay uplift in the southwestern Siberian craton. Mafic and predominant felsic granulites compose fragments of the metamorphic complex among the Neoarchean and Paleoproterozoic granitoids. Mafic granulites are characterized by the mineral association Cpx + Pl ± Hbl ± Opx ± Qz and include two types with different major and immobile trace element contents. The dominant rocks of the first type have a wide range of Mg# and concetrations of TiO2 and immobile trace elements (REE, Zr, Nb, and most positive εNd(Т) va-lues. The first type of mafic granulites show elevated (La/Sm)n and enrichment in Th and LREE relative Nb which is typical of basalts of subduction origin or crustal contaminated basalts. The absence of negative correlation between (La/Sm)n and εNd(Т) and a clear positive correlation of TiO2 with Nb testify against the effect of crustal contamination on the composition of the mafic granulites. The magmatic protoliths of first type of mafic granulites are suggested to form by the melting of depleted peridotites of the subcontinental mantle which metasomatized by melts formed from basalts or terrigenous sediments of the subducting plate. Mafic granulites of the second type have a narrower range of Mg#, TiO2 content, positive εNd(Т), flat rare earth patterns and no subduction signatures, which indicates an asthenospheric depleted mantle source. Ma-fic granulites contaminated by the Paleoarchean crust are characterized by increased (La/Sm)n, depletion of Nb relative to Th and LREE, and negative εNd(Т) values. Post magmatic influence of granitoids lead to the enrichment of mafic granulites in biotite and apatite, an increased in concentrations of K2O, P2O5, a signi-ficant enrichment of Zr, Nb, Th, LREE, and negative εNd(Т) values. The difference between mafic granulites of the first and second types is not resulted from crustal contamination, but is due to the melting of two types of sources: asthenospheric and subcontinental lithospheric mantle. The subcontinental lithospheric mantle of the Irkut block was isotopically depleted for the Neoarchean time (∼2.7 Ga), and its enrichment in incompatible trace elements, presumably by felsic melts generated from the rocks of subducting plate, immediately preceded mafic magmatism.

Petrogenesis and Tectonic Implications of the Cryogenian I-Type Granodiorites from Gabgaba Terrane (NE Sudan)

The widely distributed granitic intrusions in the Nubian Shield can provide comprehensive data for understanding its crustal evolution. We present new bulk-rock geochemistry and isotopic (zircon U-Pb and Lu-Hf) data from the Haweit granodiorites in the Gabgaba Terrane (NE Sudan). The dated zircons presented a 206Pb/238U Concordia age of 718.5 ± 2.2 Ma, indicating that they crystallized during the Cryogenian. The granodiorites contain both biotite and amphibole as the main mafic constituents. The samples exhibit metaluminous (A/CNK = 0.84–0.94) and calc-alkaline signatures. Their mineralogical composition and remarkable low P2O5, Zr, Ce, and Nb concentrations confirm that they belong to I-type granites. They exhibit subduction-related magma geochemical characters such as enrichment in LILEs and LREEs and depletion in HFSEs and HREEs, with a low (La/Yb)N ratio (3.0–5.9) and apparent negative Nb anomaly. The positive Hf(t) values (+7.34 to +11.21) and young crustal model age (TDMC = 734–985 Ma) indicates a juvenile composition of the granodiorites. The data suggest that the Haweit granodiorites may have formed from partially melting a juvenile low-K mafic source. During subduction, the ascending asthenosphere melts might heat and partially melt the pre-existing lower crust mafic materials to generate the Haweit granodiorites in the middle segment of the Nubian Shield.

Cadomian arc recycling along the northern Gondwana margin: Source-inherited composition of Miaolingian rift-related rhyolitic rocks (Ossa-Morena Zone, SW Iberia)

Rhyolites and rhyolitic tuffs of the Freixo-Segóvia Volcanic-Sedimentary Complex of the Cambrian of the Ossa-Morena Zone (Variscan belt, SW Iberia) were analyzed for petrography, major and trace element geochemistry, Sm–Nd isotopes and U–Pb zircon geochronology in order to deduce magma sources. New U–Pb zircon age data indicate that Freixo-Segóvia rhyolitic rocks, previously assigned to the Terreneuvian, formed during the Miaolingian (ca. 509-505 Ma). These rhyolitic rocks exhibit calc-alkaline signature, LREE enrichment, nearly flat HREE patterns, negative Eu, Nb and Ti anomalies, and are chemically similar to the bulk continental crust. Freixo-Segóvia rhyolitic rocks have negative to slightly positive εNd(T) values (−2.8; 0.5) resulting in TDM model ages (1.0–1.3 Ga) that overlap the range defined by Terreneuvian Malcocinado andesites, formed in the transition of the Cadomian (West-African) arc to continental rifting in northern Gondwana margin. Based on the Sm–Nd isotopic data, the Freixo-Segóvia rhyolitic rocks may have resulted from of partial melting of andesitic crust. The presence of Ediacaran-age zircon in the Freixo-Segóvia rhyolitic rocks indicates inheritance from the Cadomian arc. Inherited zircon grains with West African affinity were probably transferred into the rhyolitic magma from an older igneous source formed in the Cadomian arc. Based on their major and trace element composition, combined with isotopic and geochronological data, the Freixo-Segóvia rhyolitic rocks record recycling of arc crust during a Late Cambrian rifting event along the northern Gondwana margin. The transition from Cadomian accretion to peri-Gondwana break-up leading to the opening of the Rheic Ocean is also known in other parts of the Variscan belt.

Late Triassic magmatic rocks in the southern East Kunlun Orogenic Belt, northern Tibetan Plateau: Petrogenesis and tectonic implications

ABSTRACT The East Kunlun Orogenic Belt (E-KOB) documents successive subduction and accretion processes of the Paleo-Tethyan Ocean. However, the tectonic regime of the E-KOB at the late orogenic stage remains ambiguous. This study presents new results of whole-rock geochemistry and zircon U-Pb-Hf isotopes of the Xidatan pluton in the southern E-KOB. Detailed investigation reveals that the pluton is composed of quartz monzodiorite, host monzogranite, and igneous microgranular enclaves (IMEs). Zircon U-Pb dating results suggest that they are coeval magmatic rocks emplaced at ca. 206–203 Ma. Geochemical studies reveal that the quartz monzodiorite and IMEs share a common magma source, evidenced by consistent compositions of major elements, congruently enriched LREEs and LILEs, and consistently moderately depleted HFSEs and HREEs. Enriched to slightly juvenile zircon Hf isotopes (εHf(t) = −4.24 to +0.49) indicate that the primitive magmas of the quartz monzodiorite and IMEs involve the addition of mantle-derived magmas. The quartz monzodiorite and IMEs have identical ages, and consistent isotope features with the host monzogranite, together with the observation of ingestion of xenocrysts from host felsic rocks into the IMEs indicating that they were produced through the mixing of the mantle-derived mafic magmas and host felsic magmas. In contrast, The host monzogranite has moderately high SiO2 and Na2O+K2O, but low Fe2O3 T and MgO contents. It shows crust-like trace element patterns such as strong enrichments in LREEs and LILEs, but remarkable depletions in HREEs. Along with enriched zircon Hf isotopes (εHf(t) = −3.51 to −0.25), the host monzogranite was most likely to have originated from partial melting of lower crustal mafic rocks induced by underplated mantle-derived magmas. Considering abundant Late Triassic igneous rocks across the E-KOB, these new data provide further evidence that the Late Triassic magmatism in the E-KOB may be related to the delamination of thickened orogenic root under an extensional setting.

Detrital Zircon Records of the Banxi Group in the Western Jiangnan Orogen: Implications for Crustal Evolution of the South China Craton

AbstractThe Neoproterozoic evolution of the Jiangnan Orogen is important for understanding the tectonic history of South China. As a volcanic‐sedimentary sequence developed in the Nanhua rift, the Banxi Group preserves the records of important magmatic and tectonic events linked to the assembly and breakup of the Rodinia supercontinent. In this study, we report the results from whole‐rock major‐ and trace‐element concentrations, with zircon LA‐(MC)‐ICP‐MS U‐Pb ages, trace elements and Lu‐Hf isotopic compositions of sandstones from the Banxi Group. The rocks are characterized by high SiO2 (65.88%–82.76%, with an average of 75.50%) contents, moderate (Fe2O3T + MgO) (1.81%–7.78%, mean: 3.79%) and TiO2 (0.39%–0.54%, mean: 0.48%), low K2O/Na2O (0.03–0.40, mean: 0.10) ratios and low Al2O3/SiO2 (0.11–0.24, mean: 0.15) ratios. The sandstones have high ΣREE contents (mean: 179.1 ppm), with chondrite‐normalized REE patterns similar to the upper crust and PAAS, showing enriched LREE ((La/Yb)N mean: 14.85), sub‐horizontal HREE curves and mild Eu (Eu/Eu*: 0.75–0.89, mean: 0.81) negative anomalies. Their geochemical characteristics resemble those of passive continental margin sandstones. Most of the zircons are magmatic in origin and yield a U‐Pb age distribution with three peaks: a major age peak at 805 Ma and two subordinate age peaks at 1990 Ma and 2470 Ma, implying three major magmatic sources. The Neoproterozoic zircons have ɛHf(t) values ranging from –47.4 to 12.4 (mostly –20 to 0), suggesting a mixture of some juvenile arc‐derived material and middle Paleoproterozoic heterogeneous crustal sources. The Hf model ages of middle Paleoproterozoic zircons (∼1990 Ma) with negative ɛHf(t) values (–12.65 to –6.21, Ave. = –9.8) concentrated around the Meso‐Paleoarchean (mean TDMC = 3.3–3.1 Ga). For late Neoarchean detrital zircons (∼2470 Ma), ɛHf(t) values are divided into two groups, one with negative values (–9.16 to –0.6) with model ages of 3.5–2.9 Ga, the other featuring positive values (1.0 to 3.9) with model ages of 2.9–2.7 Ga, recording a crustal growth event at ∼2.5 Ga. Neoproterozoic zircons show volcanic arc affinities with partly intraplate magmatic features. We propose that the Banxi Group formed in a rift basin within a passive continental margin setting, which derived detritus from felsic to intermediate rocks from the Yangtze Block and a small amount of arc volcanic rocks. The middle Paleoproterozoic detrital zircon data suggest Columbia‐aged basement lies beneath the western Jiangnan orogen.

Mesozoic magmatic evolution of the Laiyuan complex: Tracing the crust-mantle and lithosphere-asthenosphere interactions in the central North China Craton

The Laiyuan complex in the central North China Craton (NCC) incorporating different magmatic suites offers an excellent opportunity to investigate the lithospheric evolution and cratonic destruction. However, the petrogenesis and tectonic implications of this magmatic suite remain debated due to lack of integrated studies. Here we evaluate the magmatism and tectonic setting assembling data from multidisciplinary investigations of the Laiyuan complex. The complex is composed of volcanic suites, granitoids, ultramafic-mafic intrusions, and dykes showing common features of enrichments in LREEs and LILEs and depletions in HFSEs. Detailed petrogenetic considerations suggest that crust-mantle and lithosphere-asthenosphere interactions contributed to the formation of various magmatic suites. The involvement of thickened lower crust and enriched lithospheric mantle in the source, and diverse magmatic processes including partial melting, fractional crystallization, and magma mixing have played a significant role in the petrogenesis of the Laiyuan complex. Furthermore, the lithosphere-asthenosphere interaction induced by thinning lithosphere and upwelling asthenosphere controlled the source variations from dolerites to lamprophyres. The complex formed in an extensional tectonic setting triggered by the subduction of the Paleo-Pacific Plate. The subduction, rollback, and stagnation of the Paleo-Pacific slab contributed to the modification of the lithospheric architecture of the North China Craton. A slow and gradual thermal-mechanical erosion occurred at the central North China Craton whereas the rapid and intense lithospheric delamination occurred at the eastern North China Craton contributing to different lithospheric evolution. Both of the mechanisms combined with the subduction of Paleo-Pacific slab played a significant role in the destruction of the North China Craton and the formation of various magmatic suites. An integrated model is proposed to describe the magmatic evolution of the Laiyuan complex. During Jurassic, the subduction of the Paleo-Pacific Plate reached beneath the central North China Craton. At 145–140 Ma, the fast slab rollback occurred and lead to hot asthenosphere upwelling and extensional setting in the central North China Craton inducing the crust-mantle interaction accounting for the petrogenesis for the formation of granitoids with MMEs (137–126 Ma), volcanic rocks (131–127 Ma), and felsic dykes (131–127 Ma). Through time, the lithosphere became substantially thin with the asthenospheric input increasing to form dolerite dykes at 125–117 Ma and lamprophyre dykes at 115–111 Ma.

Meso- to Neoproterozoic terrane accretion: Insights from juvenile mafic magmatism from the Votuverava Group and Embu Complex, southern Ribeira Belt, Brazil

Terranes of the Neoproterozoic Ribeira Belt record processes related to West Gondwana assembly. However, the evolutionary history of the belt in the preceding Rodinia supercontinent cycle and its role in the transition to West Gondwana assembly is poorly known. New geochemical and geochronological data of mafic magmatism from Meso- to early Neoproterozoic rock units (1500–900 Ma) integrated with published data provides new constraints to unravel the tectonic setting of the Ribeira Belt. Geochemical and isotopic data from the Votuverava Group and Embu Complex from the southern Ribeira Belt record three distinct periods of juvenile magmatism. Mafic rocks from the Votuverava Group define the oldest magmatism as Calymmian (1490–1475 Ma) and Ectasian (1300–1260 Ma). Both age groups display Th/Yb ratios greater than N-MORB, low Ta/Yb and Nb/La ratios, low Nb and Zr concentrations, negative Nb anomaly, whole-rock ɛNd(T) from +2.25 to +4.01, an Nd two-stage TDM model age spanning 1629–1471 Ma, and zircon εHfT mainly between +5 and +10, characteristics of juvenile mantle source arc-related magmatism. The third event occurred during early Tonian (900 Ma) and is recorded in the mafic rocks from the Embu Complex. Geochemical data indicate that these mafic rocks belong to the tholeiitic series with low LREE enrichment, negative Nb and Ta anomalies, low Nb and Zr contents and low Nb/La and Zr/Y ratios, consistent with arc-related basalts. Whole-rock Sm–Nd isotopic data yielded ɛNd(T) of +0.72 and Nd two-stage TDM model age of 1921 Ma, with zircon Hf isotopes indicating juvenile mantle sources to the mafic magmatism with ɛHfT between +4.5 and +17.2. Thus, mafic rocks from the southern Ribeira Belt record multiple periods of accretion and convergence settings associated with assembly of Rodinia and Gondwana.

Tourmaline as an indicator of ore-forming processes: Evidence from the Laodou gold deposit, Northwest China

A detailed textual and geochemical study of tourmaline is presented to trace fluid evolution and ore genesis of the Laodou gold deposit in West Qinling Orogen, Northwest China. Based on petrographic observations and relationships to gold mineralization, three generations and five types of tourmaline were identified. The pre-ore tourmaline generation (Tur A) occurs as phenocryst replacement within quartz diorite porphyry (Tur A1) and metasomatic filling in hydrothermal breccia (Tur A2). It shows intense oscillatory zoning and variable Mg/(Mg + Fe) ratios (0.30–0.67), implying rapid compositional changes controlled by host-rock composition and fluid-rock interaction. The syn-ore tourmaline generation (Tur B) forms as open-space filling (Tur B1) and peripheral overgrowth (Tur B2) of Tur B1. It occurs in quartz-tourmaline and quartz-stibnite-sphalerite-tourmaline veins. Narrow Mg/(Mg + Fe) range (0.48–0.76), poor chemical zoning and more homogeneous compositions of Tur B reflect a higher fluid flux. The post-ore tourmaline generation (Tur C) occurs as monopolar overgrowths developed at the − c pole of previous tourmaline within granitoids and hydrothermal veins. This has a composition between dravite and foitite with Mg/(Mg + Fe) from 0.65 to 0.95 and no zoning, which reflect formation under low-temperature conditions. Relatively high Sn, Nb and Ta concentrations, LREE enrichment accompanied by strong positive Eu anomalies of Tur A mostly support a magmatic-hydrothermal origin. Slight LREE enrichment and low abundances of ΣREE of Tur B are caused by progressive input of meteoric waters. Tur C shows relative enrichment in Ba, depletions in V and Ni, and a flat REE pattern with negative Eu anomalies, suggesting a metamorphic source. Boron isotope compositions suggest that boron of Tur A (δ11B average − 6.49 ‰) is sourced from an I-type melt involved with the Laodou quartz diorite porphyry, while the lighter δ11B values in paragenetically younger tourmaline (Tur C with δ11B average − 9.10 ‰) support cooling and a metamorphic input sourced from argillaceous slate of the Lower Permian Daguanshan Group. The variations in textural characteristics, chemical and boron isotope compositions from Tur A to Tur C support a fluid mixing between a magmatic-hydrothermal fluid and meteoric waters leaching metasedimentary rocks. Typical morphology, core-to-rim decrease in Mg/(Mg + Fe), Fe3+Al−1, (□XAl)(NaR2+)−1 and (AlO)(R2+OH)−1 vectors, decreasing Na, Ca concentrations and Eu anomalies of tourmaline may record cooling, oxidation and dilution during fluid evolution. This process is possibly caused by fluid mixing and results in gold precipitation. Boron sources, as well as fluid provenance and evolution interpreted by tourmaline provide insight into the Laodou deposit as an intrusion-related gold deposit.

Late Paleozoic tectonics of Southern Central Asian orogenic belt: Evidence from magmatic rocks in the northern Alxa, Northwest China

Late Paleozoic magmatic rock outcrops are common in the Northern Alxa, Southern Central Asian Orogenic Belt (CAOB), which is a key area for understanding tectonic processes and defining the final closure time of the Paleo-Asian Ocean (PAO). We present zircon U‒Pb chronology and whole-rock geochemistry data for late Paleozoic magmatic rocks from the Yagan area of northern Alxa. This paper reveals two periods of magmatism: syenogranite (374.8 Ma) and bimodal intrusive rocks, which consist of gabbro (261.4 Ma), diabase (262.9 Ma) and biotite monzogranite (263.4 Ma). The syenogranite is high-K calc-alkaline and shows enrichments in Th, Zr, Hf and LREEs; depletions in Sr, Nb, Ta, and Ti; and low Mg# values (6.9–13.2); the syenogranite was derived from partial melting of the crust and has volcanic arc characteristics. The gabbro and diabase have similar geochemical characteristics, such as enrichments in Pb, Rb, Sr, Zr, and Hf and depletions in Nb, Ta, and Ti, with positive εHf(t) values (+0.9–+2.7 and +2.6–+3.6, respectively), indicating that they originated from partial melting of depleted mantle and experienced crustal contamination during magma emplacement. The biotite monzogranite shows depletions in Nb, Ta, and Ti and εNd(t) values of -2.6 to −2.4 and resulted from partial melting of the lower crust caused by asthenospheric underplating. The bimodal intrusive rocks formed in a postcollision extensional setting. Combined with previous data, we conclude that northern Alxa was an active continental margin during the late Devonian and that the final closure of the Yagan branch ocean of the PAO occurred prior to the middle Permian.

Petrogenesis and relationship with REE mineralization of the quartz syenite from Chishan and Longbaoshan alkaline complex, southeastern North China Craton: Insights from zircon U–Pb geochronology, element, and Sr–Nd–Pb–Hf isotope geochemistry

Mesozoic alkaline complexes associated with the rare earth element (REE) mineralization are developed in southeastern North China Craton (NCC), and they recorded some important information about the lithospheric thinning and destruction of the NCC. Zircon U–Pb dating results reveal that syenitic rocks from the Chishan and Longbaoshan alkaline complexes were emplaced at ca. 126.2–123.7 Ma. These syenitic rocks have an arc-like affinity with enriched LREEs and LILEs (e.g., Ba and Th) and depleted HFSE (e.g., Nb and Ta). However, they plot within the extensional alkaline–calc-alkaline field in the lg (CaO/(K2O + Na2O)) versus SiO2 discrimination diagram and plot within the fields of the divergent plate and within-plate settings in the NbN versus ThN discrimination diagram, implying an extensional tectonic setting. Thus, we suggest that the syenitic rocks should form in an extensional tectonic settings related to rollback and retreat of the subducting Paleo-Pacific plate. These syenitic rocks have similar Sr–Nd–Pb–Hf isotope compositions ((87Sr/86Sr)i = 0.7062 to 0.7101, εNd(t) = −8.2 to −15.0, εHf(t) = −13.5 to −9.6), which are the same as the enriched subcontinental lithospheric mantle (SCLM) of the NCC. Taking into account not only our new data but also previously published data regarding the evolution of NCC in the Mesozoic, we conclude that the syenitic rocks from the Chishan and Longbaoshan complexes are derived from partial melting of the lithospheric mantle triggered by the Paleo-Pacific plate subduction. Moreover, the geochemical and Sr–Nd–Pb isotopic similarity between the Chishan REE ores, Longbaoshan altered rocks, and the syenitic rocks implied that they are homologous products of Early Cretaceous and that the mineralization shows inheritance to the magmatic hydrothermal evolution.

Geochemical and Characteristics of Mafic and Ultramafic Rocks of the Suprasubduction Zone Mawat Ophiolite, NE Iraq

New geochemical data were presented for the mafic (basalts and gabbros) and ultramafic (dunites, serpentinites and pyroxenites) rocks from the Mawat ophiolite exposed in the Zagros suture zone in northeastern Iraq. The mantle section of the ophiolite is composed of serpentinized peridotites, harzburgites, dunites, chromitites and pyroxenite dykes. The crustal section is composed of layered amphibole-rich gabbros, pegmatitic gabbros as well as basalts and minor felsic dykes. The ultramafic rocks are highly depleted in the LILEs and most of the HFSEs. The mafic rocks are tholeiitic, characterized by depletion of LREEs and display a positive trend from LREE to HREE with low Ti/V ratios. The younger gabbros are mostly calc-alkaline, characterized by enrichments of LREE with a negative REE trend and high Ti/V ratios. The basaltic and ultramafic samples plot below the N-MORB reference line suggesting that these elements have been mobile. They fall within the boninitic field in the Ti/V diagram and are related to the subduction initiation setting. The younger gabbros are related to rifting which postdated the formation of ophiolite. These geochemical data are consistent with the subduction-related suprasubduction zone tectonic setting.

Emplacement and evolution of zoned plutons: Multiproxy isotopic and geochemical evidence from the peraluminous Laojunshan leucogranite suite, southwestern China, and implications on the regional geodynamic and metallogenic history

The petrogenesis and geodynamic setting of peraluminous leucogranites are important topics for reconstructing regional geodynamic and metallogenic models. Here we present the results of an integrated study of zircon U-Pb ages and Hf-O isotopic compositions, monazite U-Th-Pb-Nd isotopes, tourmaline boron isotope, and bulk-rock elemental and Sr-Nd isotope geochemistry of an unusual suite of leucogranite, the Laojunshan leucogranite (LJSL), from the southwestern margin of the Yangtze Block in southwest China. The LJSL samples are strongly peraluminous with their zircon δ18O (8.8–10.2 ‰) and tourmaline δ11B (−14.32 to −12.75 ‰) values suggesting S-type origin. The LJSL rocks show high SiO2, Al2O3, K2O and Rb/Sr, low P2O5, CaO, FeOt, MgO, low REE abundances with moderate LREE enrichment and strong negative Eu anomalies, high (87Sr/86Sr)i (0.7127–0.7421) and low εNd(t) (−10.36 to −12.67). These features suggest protracted fractional crystallization from S-type parental granitoid magmas with Ti-in-zircon thermometry giving a mean temperature of ∼760 °C. The bulk-rock Sr-Nd, monazite Nd and zircon Hf-O isotope data suggest that the S-type magmas parental to the LJSL formed through partial melting involving muscovite-breakdown in the Paleoproterozoic middle-upper crustal metasedimentary rocks. Zircon and monazite U-(Th)-Pb dating reveals three intrusive phases for the LJSL (91–89 Ma, 87–85 Ma and 83–82 Ma), correlating with the compression-to-extension tectonic regime transition in response to the subduction polarity change from northwestward subduction of the Okhotomorsk block to northward subduction of the Neotethys oceanic lithosphere. Our findings contribute to the understanding of Mesozoic plate reconstruction associated with the India-Asia collision in the Cenozoic.