Enrichment In Large Ion Lithophile Elements Research Articles

Coeval formation and exhumation of metamorphic sole and ophiolite in the Saga ophiolitic mélange: Insights into subduction initiation of the Neo‐Tethys

AbstractSubduction initiation is recorded by upper plate magmatism and lower plate metamorphism, that is, supra‐subduction zone (SSZ) ophiolite–metamorphic sole pair. Here, we report geochemical and geochronological data as well as P–T calculations of amphibolites (metamorphic sole) and hornblende gabbros (SSZ ophiolite) from the Saga ophiolitic mélange in Tibetan Plateau. Amphibolites show trace element contents compatible with normal‐mid‐ocean ridge basalt (N‐MORB), indicating that the protolith of amphibolite formed in a MOR setting. Instead, hornblende gabbros show significant high field strength elements (HFSEs) negative anomalies, enriched large ion lithophile elements (LILEs) and high zircon εHf(t) values, suggesting they formed by fluid‐induced partial melting of a depleted mantle. Thermobarometry and phase equilibrium modelling suggest two stages of metamorphism for garnet–clinopyroxene amphibolites: (I) a peak metamorphic stage (~1.9 GPa and 1000°C) and (II) a retrograde metamorphic stage (1.1–1.6 GPa and 800–1000°C). Zircon U–Pb ages of amphibolite and hornblende gabbro are 128.8 ± 5.1 Ma and 128.1 ± 1.5 Ma, respectively, suggesting subduction initiation within the eastern Neo‐Tethys occurred no later than 128 Ma and SSZ ophiolite formed at ~128 Ma. Apatite U–Pb ages of amphibolite and hornblende gabbro are 121.8 ± 2.1 Ma and 117.5 ± 4.5 Ma, respectively. Titanite U–Pb age of amphibolite is 122.2 ± 1.5 Ma. Overall, our data suggest that the metamorphic sole and SSZ ophiolite were exhumed since 128–118 Ma, and finally exhumed into the ophiolitic mélange.

Petrogenesis of Late Triassic adakitic plutons in the East Kunlun Orogen, Northern Tibet: Geodynamic implications for the Paleo-Tethyan orogeny and crustal evolution

Adakitic rocks found within continental arcs are pivotal in unraveling the geodynamic processes and tectonic evolution of East Kunlun Paleo-Tethyan orogen. We present an integrated study on the petrography, zircon U-Pb geochronology, whole-rock geochemistry, and Sr-Nd-Hf isotopes of newly identified Late Triassic adakitic plutons in the East Kunlun Orogenic Belt (EKOB), Northern Tibet, aiming to constrain their petrogenesis and geodynamic implications. These plutons comprise by mylonitized granite, monzogranite, granodiorite and dioritic porphyrite, exhibiting consistent zircon U-Pb ages of approximately 204–212 Ma. They have high SiO2, K2O and Sr (average of 479 ppm) contents, but low contents of Y (5.6–18 ppm) and Yb (0.5–1.0 ppm) with high Sr/Y ratios (17–63), resembling adakitic rocks. These adakitic rocks can be classified into high-Mg (Mg#= ∼51) and low-Mg groups (Mg#= ∼35), although they all display enriched large ion lithophile elements and light rare earth elements but depleted high field strength elements. The studied low-Mg adakitic rocks have low MgO (∼0.68 wt%) and Cr (∼35 ppm) with enriched zircon Hf isotopes (εHf(t) = − 1.16 t 2.44), suggesting their derivation by partial melting of thickened juvenile crust. Conversely, the high-Mg adakitic rocks exhibit high MgO (∼1.94 wt%) and Cr (∼61 ppm) with relatively high zircon εHf(t) values (−0.68 to 4.95), indicating their derivation from foundered crustal root with subsequent interaction with enriched mantle-derived magma. Tectonic reconstructions suggest that the formation of these Late Triassic adakitic rocks occurred in a post-collisional extension setting and the East Kunlun Paleo-Tethyan Ocean closed no later than Norian.

The layered gabbroic intrusion in the Southern Eastern Desert of Egypt: Implications for Fe-Ti-rich magmatism at the end of the East African Orogeny

The Rahaba layered intrusion in the Southern Eastern Desert of Egypt is mainly composed of pyroxene hornblende-, and hornblende gabbros. It hosts thick FeTi oxide ore layers that are composed of titanomagnetite with subordinate ilmenite and magnetite. To understand the petrogenesis of the FeTi rich gabbroic rocks, the nature of their parent high-Fe primitive mantle melts and condition of FeTi oxide layer formation, we obtained new mineral chemistry and whole-rock major and trace element compositions. The geochemical characteristics of the gabbroic rocks resemble those of picro-basalts, suggesting generation from picritic basaltic-like melts of tholeiitic affinity. Their high-Fe-Ti-Mg nature (TiO2, ∼1.91 wt%; MgO, ∼11.0 wt%; Fe2O3, ∼14 wt%) probably indicates elevated mantle potential temperatures due to upwelling of hot asthenospheric mantle. Rare earth elements (REEs) of clinopyroxene show flat patterns with depletion in LREEs, similar to those of the host gabbros and picritic basaltic lava, suggesting a mantle origin for their parental melts. Moreover, clinopyroxene compositions show differentiation trends similar to low-pressure igneous intrusions, and reveal a rift-related origin. Chemistry of plagioclase (An45–62), clinopyroxene (Wo38.4–45.3, En39.9–43, Fs14–19.3; Mg#: 0.71–0.92) and olivine (Fo56–80) shows wide compositional ranges due to variable degrees of crystal fractionation. The calculated equilibrium temperature for the Rahaba Fe-Ti-rich gabbros is mainly 820–1000 °C, at pressure below 5 kbar, reflecting depth of rock solidification of <15 Km. These rocks show both subduction-like and rift-related geochemical signatures comparable to other post-collisional layered intrusions in the Arabian-Nubian Shield. The low average Ce/Pb (8.3) and high Ba/Nb (391) ratios together with the enriched large-ion lithophile elements (Cs, Ba, K and Sr) and depleted high-field strength elements (Th, Nb, Ta and Zr) reflect the contribution from a preceding subduction event. The lineament map, structural lineament and lineament density map reveal main WNW-ESE and NW-SE structural trends of the Najd System in the Rahaba area. The Rahaba FeTi rich gabbroic intrusion was likely formed by initial impact of a mantle plume head with Fe-rich streaks at the base of metasomatized lithospheric mantle during an extensional phase associated with activity of the Najd-related strike-slip faults, at the end of the East African Orogeny. The resultant mantle-derived melt compositions and Najd-related structures possibly controlled the distribution of FeTi ore deposits during this Orogeny. The Rahaba gabbroic intrusion was emplaced during post-collisional extension after slab break-off and lithospheric delamination following collision of accreted island-arcs with the Saharan Metacraton.

Recycled crustal components in the depleted mantle source: Evidence from Early Cretaceous mafic rocks in the southern margin of the North China Craton

Abundant subducted materials are recycled into the deep mantle during the plate subduction process and are further reflected in the postcollision mafic rocks. The Early Cretaceous Mid-Oceanic-Ridge-Basalt (MORB)-like mafic rocks in the southern margin of the North China Craton (SNCC) preserve significant information on recycled materials. These rocks provide a window for discussing the crust-mantle interaction and tracing the history of subduction. The studied rocks comprise the Jingwan metadiabase, Putaozhuang, Nianpanzhuang, and Niujiaowan metahornblende gabbro, with Early Cretaceous ages (140–129 Ma). They show large ion lithophile element enrichment in both the Enriched-MORB and Normal-MORB rare earth element patterns. They are characterized by slightly depleted isotopic compositions, high MgO, Cr, Ni and Mg# values as well as the low SiO2, suggesting a depleted mantle source affinity. The enriched NdHf (εNd(t) = −14.50 to −13.47; εHf(t) = −9.27 to −8.82) and unradiogenic Pb isotopes imply that Jingwan samples originated from a depleted mantle source metasomatized by lower continental crustal materials. Their relatively enriched zircon εHf(t) values (−15.8 to −1.6) and δ18O values (4.82‰–8.77‰) along with their Paleoproterozoic to Neoproterozoic inherited zircons also argue for the contribution of low continental crustal materials from the North China Craton. In contrast, Putaozhuang, Nianpanzhuang, and Niujiaowan samples have positive whole-rock εNd(t) values (4.08–5.20) and εHf(t) values (3.23–5.62). Combined with the existence of Paleozoic zircons, high zircon εHf(t) values, and high Nb/U ratios, these materials suggest that they originated from a depleted mantle source metasomatized by juvenile crust and terrigenous sediments. This is corroborated by variable zircon δ18O values (5.57‰–9.63‰) and mantle-like δ18O values (5.08‰–5.60‰) of Paleozoic inherited zircons from Niujiaowan samples. The distinct geochemical characteristics of Early Cretaceous mafic rocks are induced by the incorporation of different components from subducted crust and sediments into the depleted mantle source. Paleo-Pacific Plate far-field effects controlled the intense lithospheric extension and thinning of the Early Cretaceous MORB-like rocks in the SNCC, which were placed in an immature back-arc basin setting.

Petrogenesis and metallogenic implications of Eocene–Oligocene magmatism in the Yulong porphyry copper belt, eastern Tibet: a review and analysis of geochronological, geochemical, and Sr–Nd–Pb–Hf isotopic data

The Eocene Yulong porphyry copper belt (YPCB) in eastern Tibet contains more than 10 Mt of Cu metal, associated with subvolcanic to volcanic porphyry systems intruding Cenozoic sedimentary basins. Geochemical data analysis shows that all samples belonged to high-K calc-alkaline and shoshonite series with metaluminous–peraluminous characteristics, light rare earth elements and large ion lithophile elements enrichment, high field strength elements depletion, and share geochemical similarities to adakitic rocks. We have established a spatiotemporal evolution framework through geochronological data analysis, combined with calculated bulk zirconium saturation temperature, calculated crustal thickness and geophysical evidence. We identified three magmatic processes triggered by asthenospheric upwelling and thermal erosion of thickened crust: (1) ultrapotassic magma derived from metasomatized mantle; (2) coeval ultrapotassic melt that promoted the partial melting of eroded lower crust and the generation of high-K adakitic felsic melt; high-K and Mg# adakitic fertile magma was formed by interaction between the eroded sulfide-rich juvenile lower crust-derived melt and mantle peridotite; (3) high-K adakitic fertile magma was derived from sulfide-rich juvenile lower crust, triggered by injecting hydrous ultrapotassic magma into thinned lower crust. The difference between deposit size in the north/south section of Yulong belt may be caused by the temperature of magmatic source and the volume of coeval potassic–ultrapotassic magmatism. Our analysis of the YPCB suggests that magmatism after 175 Ma within Intermontane arc complex of the Canadian Cordillera has high potential for porphyry deposits.

An early Paleozoic accumulation-foundering cycle of ultramafic cumulates in the Harlik arc and its implications for continental crustal growth in the Altaids

Lower crustal ultramafic cumulates that are normally denser than the ambient mantle peridotites are prone to founder back into the convecting mantle, prompting the formation of continental Moho and andesitic crust at subduction zones. However, foundering events of ultramafic cumulates have not been widely reported in island arcs, including the Altaids, one of the largest accretionary orogens. The Altaids involved prominent continental crustal growth by the development of an archipelago in the Phanerozoic. But the scarcity of foundering events of ultramafic cumulates hinders a better understanding of the formation mechanism of andesitic crust in the Altaids. In this study, we report two dioritic plutons in the Harlik arc, southern Altaids, trying to identify the accumulation-foundering cycles of dense ultramafic cumulates. LA-ICP-MS dating revealed that the Miaoergou and Banfanggou plutons in the Harlik arc were intruded at ca. 445 ± 6 Ma and 423 ± 4 Ma, respectively. The homogeneous Miaoergou pluton is mainly composed of quartz diorites, they have a high- to medium-K calc-alkaline composition, and show enrichments in light rare earth elements (LREE) and large ion lithophile elements (LILE), but depletions in high field strength elements (HFSE, especially Nb and Ta) and heavy rare earth elements (HREE). They also have juvenile isotopic signatures (εNd(t) = +3.3 − +3.7, initial 87Sr/86Sr = 0.70399–0.70452), which together suggest that they were derived from fractional crystallization of the coeval hydrous basaltic magmas in an arc setting. The heterogenous Banfanggou pluton comprises a spectrum of lithologies from hornblende gabbro to diorite. The hornblende gabbros and diorites have a high- to medium-K calc-alkaline composition, and show enrichments in LREE and LILE, but depletions in HREE and HFSE. They all have juvenile isotopic compositions with εNd(t) ranging from +3.5 to +4.2, and (87Sr/86Sr)i from 0.70384 to 0.7047, respectively, indicating that their parental magmas were intruded in a subduction-related setting, and underwent fractional crystallization of clinopyroxene, plagioclase, amphibole and quartz. Both pulses of hydrous basaltic magmatism involved open-system conditions, with the development of mafic microgranular enclaves (MMEs) in the diorites and their mingling. In a synthesis of the published data, we found that the Harlik arc was continuously affected by subduction in the early Paleozoic, through extensive arc magmatism from ca. 452 to 440 Ma, the Harlik arc matured with the accumulation of thick ultramafic cumulates in its lower crust. The foundering of these ultramafic cumulates occurred later, which led to strong extension and related A-type granitic magmatism at ca. 436 Ma. This accumulation-foundering cycle of ultramafic cumulates facilitated the formation of andesitic composition and continental Moho in the Harlik arc, and contributed to the significant continental crustal growth in the Altaids.

Petrogenesis of Neoproterozoic high-K intrusion in the southwestern Yangtze Block, South China: Implication for the recycled subducted-sediment in the mantle source

The study of K-enriched intrusive rocks is essential for deciphering mantle metasomatism beneath active continental arcs. In this contribution, high-precision zircon U–Pb–Hf isotope, whole-rock geochemistry, Sr–Nd isotope, and mineral chemistry analyses were performed to evaluate the petrogenesis and geodynamic system of the Yunnongfeng intrusion on the southwestern margin of the Yangtze Block. The Yunnongfeng intrusion consists of a high-K to shoshonitic rock assemblage with variable lithology from gabbro-diorite to granite. Zircon U–Pb dating gives concordant crystallization ages of ca. 782.5 ± 3.8 Ma for gabbro-diorite, ca. 774 ± 4.1 and 776 ± 4.1 Ma for diorite, ca. 770 ± 4.7 Ma for quartz monzonite, ca. 763 ± 3.4 Ma for quartz syenite, and ca. 764 ± 16 Ma for granite. These samples also show similar Sr–Nd, and Lu–Hf isotopic compositions, implying a common magma source. The similar crystallization age and regular variation of major and trace element contents suggest that these rocks were formed through fractional crystallization of cogenetic primitive mantle magmas. The enriched εNd(t) (−5.7 to − 5.1) and εHf(t) (−6.7 to − 1.2) values, high Rb/Y and Th/La ratios, slight Nd–Hf decoupling, and high-K and Th contents demonstrate that their lithospheric mantle source was enriched by slab-related fluid and sediment-related melt. The samples also exhibit remarkable enrichment in large-ion lithophile elements and depletion in high-field-strength elements, indicative of subduction-related arc magmatism. Taking into account previous studies, we suggest that the western margin of the Yangtze Block experienced a long-term subduction process during the Neoproterozoic, and the Yunnongfeng intrusion formed in an extensional back-arc basin. Based on the K-enriched mafic–intermediate rocks from the western margin of the Yangtze Block commonly show high K2O/Na2O, Rb/Sr, low Ba/Rb ratios, and enriched εNd(t) values, our study, coupled with numerous previous reports, proposes that the K-enrichment resulted from the breakdown of phlogopite, owing to subduction-related sediment melt reacting with peridotite in the mantle source area.

Petrogenesis of Late Cretaceous A-type plutonic rocks from the Eastern Pontides Orogenic Belt (NE Turkey): constraints from zircon U-Pb geochronology, zircon Lu-Hf and whole-rock Sr-Nd-Pb-Hf isotopes

ABSTRACT The Late Cretaceous magmatic history of the Eastern Pontides Orogenic Belt (NE Turkey) was generally characterized by common I-type and lesser A-type plutons in varying size and composition. Of these, A-type plutons from the Şebinkarahisar (Giresun) area yield SHRIMP zircon U-Pb age of 77.36 ± 0.96 Ma. The rocks of these plutons display a wide range of SiO2 contents (53.63 to 70.96 wt.%) and Mg-numbers (6.2–32.19) with A-type affinities characterized by enrichment in K2O and Na2O, Ga, Zr, and strong negative Eu anomalies. They also show arc-type petrochemical features including enrichment of large-ion lithophile elements (e.g. Cs, K, Rb) and light rare earth elements (LaN/LuN = 6.27–15.61), but depletion in high strength field elements (e.g. Nb, Ta, Ti, and P). The (87Sr/86Sr)i ratios and ɛNd(i) values of the studied rocks are in the range of 0.705129 to 0.0705347 and −3.64 to −3.12, respectively, while bulk rock ɛHf(i) values are between 1.86 and 4.36. They are characterized by positive zircon ɛHf(i) (1.74 to 10.88) plotting between the chondrite and depleted mantle evolutionary line values. Our data combined with the previous studies suggest that the magmas, that formed the Late Cretaceous plutons, were generated in an extensional tectonic environment due to slap roll-back and trench ward migration of the subduction zone to the south. The generation of hybrid magmas forming the studied plutons is in response to the melts derived from a metasomatized lithospheric mantle source, a relatively deeper mantle source, and the juvenile crust along with the heat transfer and decompression. Multi-sourced magmas rise through the continental crust and reach the final composition by predominantly fractional crystallization processes.

Petrogenesis of Paleocene−Eocene gabbros in the Gangdese belt: Geochemical tracking of transitioning from oceanic subduction to continental collision related magmatism in southern Tibet

The composition of the sub-arc mantle and the mode and nature of geodynamic processes during the India-Asia collision that controlled the melt evolution beneath the Gangdese belt (southern Tibet) are still unclear. Here, we present new U-Pb ages and Hf isotopes of zircon, and whole-rock geochemical and Sr-Nd-Pb isotopic data of the Paleocene−Eocene Najinla gabbros from the East Gangdese magmatic belt, aiming to track the transitioning magmatism formed from oceanic subduction to continental collision in the region. Zircon U-Pb analyses of these mafic rocks yield emplacement ages of 54 ± 1 Ma and 63 ± 1 Ma. The gabbros are characterized by variable SiO2 (45.87−55.44 wt%), MgO (1.03−8.18 wt%), FeOT (3.74−12.33 wt%), and Al2O3 (13.45−25.45 wt%) contents. Most samples exhibit high Al2O3 (17.15−25.45 wt%) and relatively low MgO (1.03−6.11 wt%), similar to typical high-alumina basalts and high-alumina basaltic andesites. The Najinla gabbros show characteristic subduction-related signatures with enriched large-ion lithophile elements and depleted high field strength elements. They have depleted Sr-Nd isotopic compositions with low and relatively homogeneous initial 87Sr/86Sr isotopic ratios of 0.7045−0.7049 and positive εNd(t) ratios of +2.2 to +3.2. The Najinla gabbroic rocks also have positive zircon εHf(t) values, ranging from +5.6 to +10.9. These results collectively suggest that magmas of the gabbros formed by partial melting of the asthenosphere with negligible crustal contamination during their emplacement. We propose that the mantle source of the Najinla gabbros was strongly influenced and metasomatized by subducted Neotethyan oceanic crust-derived fluids in the mantle wedge. Rollback of the subducted Neotethyan slab in the early Eocene led to partial melting of the subduction-modified mantle and the formation of these gabbros.

Late Cretaceous bimodal volcanic rocks in Shuanghu induced by lithospheric delamination beneath the Southern Qiangtang, Tibet

The southern Qiangtang (SQT) in central Tibet is characterized by sporadic Late Cretaceous magmatic rocks whose petrogenesis and tectonic implications remain unclear. Here, new zircon UPb ages and Hf isotope compositions, together with whole-rock element and isotope data have been collected for the Duoma volcanic rocks from the SQT, central Tibet. Zircon UPb dating indicates that the Duoma volcanic rocks crystallized at ca. 98 Ma. The basalts of the Duoma volcanic rocks have low SiO2 (51.49–53.36 wt%), high total alkalis (4.65–6.25 wt%), and display OIB-like isotopic signatures (87Sr/86Sr)i = 0.7051 to 0.7058, εNd(t) = +0.04 to +1.10), overall similar to intra-plate alkali basalts. Moreover, basalts exhibit slight depletion of high-field-strength elements (HFSEs; e.g., Nb, Ta, and Ti), but enrichment of light rare earth (LaN/YbN = 6.7–7.6) with no negative Eu anomalies (Eu/Eu* = 0.91–0.99). These geochemical features, together with their high La/Nb (1.56–1.70) and Zr/Hf (44.70–45.87) but low La/Ba (0.08–0.09) and Ca/Al (0.43–0.65) ratios, suggest that the Duoma basalts originated from partial melting of garnet clinopyroxenite that was metasomatized by depleted mantle-derived melts. The coexisting rhyolites have high SiO2 (69.78–69.92 wt%) and exhibit significant enrichment in large-ion lithophile elements (LILEs) and depletion in HFSEs (Nb, Ta, and Ti). Furthermore, the Duoma rhyolites are characterized by the decoupled Nd (εNd(t) values ranging from −4.4 to −11.1) and Hf (ɛHf(t) values ranging from −0.7 to +2.7) compositions, that character indicates that they were probably derived from the partial melting of the lower continental crust. The compositional bimodality and heterogeneous source of the Late Cretaceous volcanism indicate that they were formed in an extension setting, presumably triggered by lithospheric delamination beneath the SQT.

The genetic relationship between coeval Ediacaran mafic and granitoid plutons in the Antigonish Highlands, Nova Scotia

Abstract Granitic plutons dominated by felsic-intermediate compositions are commonly spatially and temporally associated with mafic intrusions; however, the genetic relationship between the apparently coeval but compositionally dissimilar magmas is controversial. To better understand this relationship, we present new lithogeochemical and isotopic data from coeval late Neoproterozoic plutonic rocks in the Antigonish Highlands of Nova Scotia where the regional context is well documented. The predominantly mafic Greendale Complex contains lamprophyre, appinites and leucocratic dykes. The appinites are remarkably variable in their textures, and consist of hornblende pegmatites, hornblende cumulates, porphyritic hornblende gabbro and coarse-grained, equigranular hornblende gabbro. Geochemical data show enrichment in large-ion lithophile elements and depletion in the high field strength elements suggesting an arc setting. ε Nd (607) values from the Greendale Complex range from +3.2 to +5.0 and are on average slightly more juvenile than coeval granitic rocks which have petrological characteristics typical of continental arc magmas. Hydrous mafic magmas were likely contaminated by subducted sediments and their ascent was facilitated by lithospheric-scale faults. Felsic magmas were derived by anatexis of heterogeneous Avalonian crust that oscillated between fluid-saturated to fluid-deficient (dehydration) melting, consistent with the evolution from arc to intra-arc rift environment.

Petrogenesis of two Cretaceous granitoid episodes in the Luk Ulo region, Central Java, Indonesia, and its implication for the tectonic evolution of the easternmost Neo-Tethys

The central part of Java, Indonesia, is located on the southern margin of Sundaland, and records the tectonic-magmatic history of the easternmost Neo-Tethys. Elucidating the petrogenesis and its tectonic setting of the Cretaceous Luk Ulo granites in Central Java is key to reconstructing the late subduction-accretion history of the easternmost Neo-Tethys. This study reports for the first time the zircon U-Pb geochronology, Hf-O isotope, whole-rock major and trace elements, and Sr-Nd isotope geochemistry of two granitoid episodes from Central Java. Zircon U-Pb dating results show that the Luk Ulo granites formed in two Cretaceous episodes at ca. 115 Ma and ca. 70 Ma. These two Cretaceous granitoid episodes contain both calc-alkaline with enriched large-ion lithophile elements and light rare earth elements and depleted high field strength elements, exhibiting arc geochemical signatures. The older (ca. 115 Ma) granites show slightly higher (87Sr/86Sr)i values of 0.70772 to 0.70784 and weakly negative εNd(t) values of −2.37 to −1.67, indicating the contribution of ancient crust. However, the scattered zircon εHf(t) values of +0.23 to +11.39 suggest a probable mixture of ancient crustal components and juvenile crustal additions in the magma source. The high zircon δ18O values (+8.86‰ to +14.24‰) show the possible incorporation of supracrustal components (sediments and fluids) in the magma formation. In contrast, the younger (ca. 70 Ma) granites show low (87Sr/86Sr)i values of 0.70427−0.70442, strong positive εNd(t) values of +3.89 to +4.03, and zircon εHf(t) values of +14.04 to +17.38 and slightly higher zircon δ18O values of +5.01‰ to +6.49‰ than the depleted mantle, indicating that the magma was predominantly derived from the juvenile crust with negligible supracrustal involvement. In general, zircon εHf(t) values and whole-rock εNd(t) values of the Late Cretaceous (after ca. 90 Ma) granitic rocks in the southwest margin of Sundaland were higher than those of the Early Cretaceous granitoids, suggesting a change in the source region from ancient to the juvenile in response to the rollback of the Neo-Tethys ocean. Our findings from the two Cretaceous granitoid episodes in Central Java provide new insights into the tectono-magmatic evolution along the southwestern margin of Sundaland and the two different episodes of crustal growth in the easternmost Tethys during the Cretaceous.

Tonian Continental Arc Magmatism of the Porongos Complex, Dom Feliciano Belt, Southern Brazil

This study provides new information on the deformation, metamorphism, and tectonic setting of the metavolcanic rocks of the southern portion of the Porongos Complex, southern Brazil. The structural analysis enabled the identification of three deformational phases, formed under ductile to semi-ductile conditions. D1 progressed through deformation partitioning, comprising closed to isoclinal folds and an axial plane foliation. Then, simple shear prevailed, resulting in isoclinal folds, sigma-type porphyroclasts, foliation transposition, and mylonitic rocks. D2 comprises open to gentle folds and an axial plane cleavage. The formation of shear bands is ascribed to the semi-ductile D3. The mineral assemblage represented by phengite + chlorite + clinozoisite-epidote + actinolite + albite + quartz associated with the temperatures obtained through the chlorite geothermometer (316°C) indicates greenschist facies conditions. The protoliths of these metavolcanic rocks are geochemically discriminated as calc-alkaline, dacite-to-rhyodacite, with peraluminous compositions. The bulk trace element compositions show enrichment in large-ion lithophile elements and light rare-earth elements and depletion in heavy rare-earth elements. Also, negative Nb, P, Ti, and Ta anomalies are observed in the multielement diagram. All these geochemical features are typical of rocks formed in continental magmatic arcs. Geochemical comparison with other pre-collisional Tonian orthometamorphic rocks from the Dom Feliciano Belt demonstrates strong similarities, which corroborates the prior interpretation of a continental arc setting for the origin of these Tonian rocks.

Development of a back-arc basin from initial rifting to seafloor spreading: Constraints from Paleozoic basic rocks in the North Qinling accretionary orogen, central China

The progression from initial back-arc rifting to back-arc opening in ancient orogenic collages is important for reconstructing subduction histories, constraining tectonic switching, and understanding crustal growth and evolution. However, it is difficult to constrain this transition in ancient arcs. The Qinling-Dabie orogenic belt is one of the most important orogenic belts in eastern Asia, yet the history of Paleozoic accretionary processes in this belt remains equivocal, owing to a poor understanding of back-arc opening processes in the Erlangping unit. In this study, we present whole-rock geochemical analyses of mafic dikes in the Erlangping unit, along with U-Pb ages and Hf-O isotope compositions of zircon from these dikes. Zircon secondary ion mass spectrometry (SIMS) dating of these mafic dikes yielded U-Pb ages of 453 ± 3 Ma. The mafic dikes are calc-alkaline and are characterized by high K2O contents (1.16−3.16 wt%). They have enriched ([La/Yb]N = 4.3−14.9) light rare earth elements (REEs) and relatively flat heavy REE ([Dy/Yb]N = 0.9−1.5) patterns, and they exhibit enrichment in large ion lithophile elements (LILEs) but depletion in high field strength elements (HFSEs), resembling arc-like magmatism. These mafic dikes are characterized by relatively enriched initial 87Sr/86Sr (0.7049−0.7059), chondritic to slightly radiogenic εNd(t) (−0.36 to 1.33), and radiogenic whole-rock and zircon εHf(t) (+7.2−7.6 and +7.5−7.8, respectively). The zircons have δ18O values (5.0‰ ± 0.1‰) similar to those of normal mantle zircon. Accordingly, we interpret that the mafic dikes were derived from an enriched lithospheric mantle source metasomatized by subducted Proto-Tethys (Shangdan) ocean material. In contrast, previously reported ca. 440 Ma gabbros from the Erlangping unit are tholeiitic and have lower incompatible trace-element concentrations with less enrichment in LILEs and less depletion in HFSEs than the mafic dikes presented here. Whole-rock Nd and zircon Hf isotopic compositions suggest that these Silurian gabbros were derived from partial melting of a more depleted mantle with the involvement of asthenosphere, and they have a close geochemical affinity with back-arc basin basalts. In addition, trace-element ratios and geochemical modeling suggest higher melting pressures for the mafic dikes than the gabbros. The geochemical differences show systematic variations from island-arc basalt into back-arc basin basalt types, which is consistent with the magma source evolution of the Mariana Trough. Thus, we interpret that the ca. 454 Ma mafic dikes were emplaced when the infant arc split, and they record initial back-arc rifting in the Erlangping unit, whereas the Silurian gabbros subsequently formed during the opening of the back-arc basin. Our study provides a paradigm for deciphering the evolution of back-arc basins through the study of spatiotemporal geological and geochemical variations of mafic intrusions in ancient accretionary orogens.

Mantle refertilization at high-pressure conditions of continental subduction channel: Evidence from orogenic peridotites in the Sulu UHP terrane, eastern China

Refertilization of depleted peridotites widely occurs in lithospheric mantle but is poorly understood in a continental subduction channel. The Sulu orogenic belt is a typical ultra-high pressure (UHP) terrane formed by continental subduction and collision. To understand the complex metasomatic processes on peridotites during continental subduction, we combined detailed petrographic and geochemical studies for fertile garnet lherzolites (Chijiadian) from the Sulu UHP terrane, as well as refractory peridotites (Xugou and Lijiatun) for comparison. The fertile garnet lherzolites (Chijiadian) are not residues of low-degree partial melting, but they formed from refertilization processes during peak-UHP metamorphic stages (garnet-facies) in a continental subduction channel, as reflected by abundant clinopyroxene (12 vol%) and garnet (12 vol%), low bulk-rock Mg# of 91 and high Al2O3 of 2.4–3.3 wt%. Garnet, clinopyroxene, and orthopyroxene display secondary poikilitic textures and often contain early phases of olivine, spinel, and garnet inclusions, indicating later precipitation of these minerals into depleted peridotite protoliths. The clinopyroxenes show enrichment of large ion lithophile elements (LILE) and high 87Sr/86Sr of 0.7061–0.7084. The estimated pressure-temperature are 720–770 °C at 4.4–4.8 GPa, consistent with previous constraints on UHP conditions. These features suggest strong refertilization processes at UP-UHP conditions (garnet-facies) in continental subduction channels. The metasomatic agents for refertilization would be basaltic-intermediate melts which incorporated continental materials. Similarly, Xugou harzburgites and Lijiatun dunites also record the subducted continental signatures with enrichment of LILE and high Sr isotopes on clinopyroxenes (87Sr/86Sr: 0.7050 to 0.7065). However, Xugou harzburgites and Lijiatun dunites overall do not display the same typical refertilization features as for Chijiadian, as reflected by low abundances of clinopyroxene and high Mg# of 92–93. Consequently, continental subduction could lead to diverse types of metasomatism of refractory peridotites, including refertilization at UP-UHP metamorphic stages in subduction channels, as in other tectonic settings.

Petrology and geochemistry of Li-bearing pegmatites and related granitic rocks in southern Thailand: implications for petrogenesis and lithium potential in Thailand

Lithium (Li) can be found in many minerals, including lepidolite. Lepidolite is found in pegmatite-related tin deposits in the Phang Nga area in southern Thailand. According to their field occurrence, petrography, mineral chemistry, and whole-rock geochemistry, the Li-bearing pegmatites and the granitic rocks in the study area can be linked to tin deposits in southern Thailand as part of the SE Asian tin belt. The Li-bearing pegmatites are characterized by an abundance of lepidolite, K-feldspar, plagioclase, and quartz with some accessory minerals of fluorite, cassiterite, apatite, monazite, and beryl. The granitic rocks show various compositions, including porphyritic biotite–muscovite granite, biotite granite, and muscovite—tourmaline granite with different proportions of K-feldspar, plagioclase, quartz, biotite, muscovite, and tourmaline. Whole-rock geochemistry indicates that both the Li-bearing pegmatites and granitic rocks have a close relationship rooted in their peraluminous S-type granite affinity. The Li-bearing pegmatites evolved from highly fractionated S-type granitic rocks comparable with the Western Belt Granite of Thailand. The enrichment of large-ion lithophile elements (e.g., Rb and K) and the depletion of Ba, Nb, and Ti together with similar rare Earth element patterns reflect the collisional setting indicating the Sibumasu–West Burma and West Burma—Indo-Burma collisions during the Cretaceous to the Eocene. The crystallization pressure—temperature conditions of these rocks were 3.49–4.25 kbar and 622°C–675°C, respectively, with an emplacement depth of 13–15 km. The Li-bearing pegmatites had a relatively high average Li grade compared with those of other Li-bearing pegmatites in the world.

Chronolgy and Geochemistry of the Sijiaying Iron Deposit in Eastern Hebei Province, North China Craton: Implications for the Genesis of High-Grade Iron Ores

The Sijiaying iron deposit is located in the Eastern Hebei area of the southern section of the northern margin of the North China Craton (NCC) and is the largest single iron deposit in China. The deposit contains many banded iron formations (BIFs) and was proven to have more than 3 million tons of high-grade iron ore resources. This study carried out geochemistry and zircon U–Pb analysis of normal-grade iron ore, high-grade iron ore, and wall rock (biotite–leptynite, chlorite–sericite schist) in the Sijiaying deposit and discussed the genesis and metallogenic age of high-grade iron ore. BIFs have low concentrations of Al2O3 and TiO2 and high field strength element (HFSE) depletion, indicating almost no contamination via terrestrial debris. The standardized post-Archean Australian shale (PAAS) rare earth element (REE) distribution pattern indicates that the iron formation exhibits positive Eu, Y, and heavy rare earth element (HREE) anomalies and lacks negative Ce anomalies, indicating that the Sijiaying BIF was enriched with iron sources via high-temperature hydrothermal fluids from the seabed and deposited in an anoxic ancient marine environment. In addition, geological field work identified two types of high-grade iron ore in the mining area: primitive sedimentary and hydrothermally altered high-grade iron ore. Further ore geochemical research showed that the primitive sedimentary-type iron ore is similar in geochemistry to the BIF. In addition to low Eu/Eu* values, the hydrothermally altered high-grade iron ore shows geochemical characteristics similar to those of the BIF, suggesting that they share the same iron source but did not form at the same time. The total large ion lithophile element (LILE) (Sr, Ba, Pb) contents in primitive sedimentary-type high-grade iron ore are higher than those in hydrothermally altered high-grade iron ore, indicating that LILEs are carried away via fluids during the hydrothermal alteration process in normal-grade iron ore. The geochemical characteristics of biotite–leptynite and chlorite–sericite schist include high contents of SiO2 and Al2O3, light rare earth elements (LREEs), LILE enrichment (Rb, Ba, Sr, Zr), and HFSE depletion (Nb, Ta, P, Ti), characteristics that are similar to island arc volcanic rocks. The reconstruction of the original rock indicates that the wall rock is a product of volcanic sedimentary cycles in an island arc setting. Zircon cathodoluminescence images and LA–ICP–MS zircon U–Pb dating can be divided into four age groups (3283 Ma, 2547 Ma, 2500 Ma, and 2407 Ma), which correspond to the earliest volcanic activity in eastern Hebei, the main mineralization age of the Sijiaying BIF (the mineralization age of primitive sedimentary high-grade iron ore), a regional tectonic–metamorphic event, and the occurrence of migmatization (the mineralization age of hydrothermally altered high-grade iron ore), respectively. Therefore, the Sijiaying BIF and primitive sedimentary high-grade iron ores were deposited and mineralized at 2547 Ma, and the iron orebody was later altered via the hydrothermal solution at 2407 Ma, forming large-scale high-grade iron ores.

Recycling of continental crust materials: Geochemical constraints from post-collisional alkaline intrusive rocks in the Dabie orogen

Post-collisional alkaline intrusive rocks from the Dabie orogen were studied for their whole-rock major-trace elements and Sr-Nd-Hf-Pb isotopes, as well as zircon U-Pb ages and Hf-O isotopes. The results provide geochemical constraints on the nature of their mantle sources and thus insight into crust-mantle interaction in the continental collision zone. The alkaline intrusive rocks are composed of syenite and nepheline syenite. Syn-magmatic zircon U-Pb dating by LA-ICP-MS for them yielded Early Cretaceous ages of 131.3 ± 1.4 Ma to 122.6 ± 0.6 Ma, coeval with the post-collisional magmatism in the Dabie orogen. One relict zircon with U-Pb age of 211 Ma is consistent with the timing of metamorphism for the ultrahigh-pressure (UHP) metamorphic rocks in this orogen. They have arc-like trace element distribution patterns, such as enrichment in LILE (large ion lithophile element) and LREE (light rare earth element) but depletion in HFSE (high field strength element), and enriched whole-rock Sr-Nd-Hf isotope compositions with high (87Sr/86Sr)i ratios of 0.7077–0.7131 but negative εNd(t) values of −16.0 to −9.4 and εHf(t) values of −17.5 to −12.7. Their syn-magmatic zircons have three groups of Hf-O isotope compositions comparable to those of UHP metamorphic rocks in Central-South Dabie and North Dabie, which represent the upper and middle continental crust of the subducted South China Block, respectively. In this regard, slab–mantle interaction is evident during the Triassic continental collision. We suggest that the melts derived from the subducted South China Block reacted with the lithospheric mantle wedge of the North China Block, resulting in phlogopite-bearing metasomatites, whose partial melting would generate the post-collisional alkaline intrusive rocks during the Early Cretaceous.

Petrogenesis and geochemical evidence from early Jurassic intrusive rocks and their implications for subduction of the Paleo-Pacific Plate in the Yanbian area, Northeast China

Subduction of the Paleo-Pacific Plate plays a key role in the Early Jurassic magmatism evolution in the Yanbian area, Northeast China. In this paper, zircon U–Pb ages, whole-rock geochemical, and Sr–Nd–Hf isotopic compositions are presented. The Early Jurassic gabbros, diorite enclaves, granodiorites, and granites ages range from 183 to 189 Ma. They are characterized by enrichment in large-ion lithophile elements and depletion in high-field strength elements. The gabbros are classified as low- and medium-K tholeiitic, granodiorites and granites are high-K calc-alkaline I-type, and diorite enclaves are calc-alkaline transitional series. The gabbros have zircon εHf(t) values of 6.81–9.89, whole-rock 87Sr/86Sr(i) = 0.7043–0.7044, and εNd(t) from 2.72 to 2.80. The diorite enclaves have zircons with εHf(t) values of 8.26–12.80, whole-rock 87Sr/86Sr(i) = 0.7051, and εNd(t) from 0.96 to 1.09. The granodiorites and granites have zircon εHf(t) values of 7.59–12.87, whole-rock 87Sr/86Sr(i) = 0.7042–0.7066, and εNd(t) from 2.33 to 2.61. These data indicate that gabbroic magmas were derived from partial melting of a depleted mantle wedge metasomatized by subduction-related fluids. The basaltic magma underplated and heated the juvenile crust, whereas the granodiorites and granites might be the product of fractional crystallization of a mixture of basaltic and felsic magmas or derived from partial melting of the juvenile lower crust. Our data constrain the petrogenesis of these Early Jurassic intrusive rocks which are attributed to subduction of the Paleo-Pacific Plate beneath Eurasia continent in northeastern China.

Petrogenesis of late Jurassic Mufushan high-Mg diorites and late Mesozoic tectonic evolution of the eastern South China Block

Abundant Mesozoic magmatic rocks are widely distributed in southeastern China, forming several remarkable magmatic-metallogenetic belts. However, the geodynamic mechanism for the Late Jurassic-Early Cretaceous magmatic processes and their relation to the rich polymetallic deposits of the region remain highly controversial. Here, we report zircon U-Pb ages, geochemistry, and Sr-Nd-Hf-Pb isotopic data for the high-Mg diorites in the Mufushan region, northeastern Hunan Province. The high-Mg diorites were emplaced in the late Jurassic (U-Pb ages of ∼151–155 Ma), contemporaneous with the surrounding granitic rocks which have ages ranging from 140 to 150 Ma. These high-Mg diorites are characterized by the enrichment of large-ion lithophile elements (LILEs, e.g. Ba, Pb, K and Rb) and depletion of high field strength elements (HFSEs, e.g., Nb, Ta, Ti), displaying typical geochemical features of continental-arc basalts (CAB). The relatively high Mg# values (71–76, average of 72), Cr contents (546–1562 ppm, average of 944 ppm) and Ni contents (211–519 ppm, average of 335 ppm) suggest that their derivation was likely from a relatively primitive magma, accompanied with weak crustal assimilation or fractional crystallization.The high-Mg diorites display isotopic characteristics similar to those of enriched mantle-derived mafic rocks, such as low 87Sr/86Sr(i) (0.70692–0.70789), εNd(t) and εHf(t) ranging from −2.4 to −4.3 and −3.8 to + 1.1 respectively, with single-stage Nd model ages (TDM1) of 1.3–1.5 Ga. In combination with a compilation of coeval magmatic rocks in eastern South China, a significant addition of ancient lithospheric mantle materials was likely involved in the Mesozoic magma genesis. The decreasing radiogenic Nd isotope compositions and negative Hf signatures suggest an increasing contribution of enriched components to the magma source from the Late Jurassic to Early Cretaceous. Our results suggest that the Mufushan high-Mg diorites were derived from mantle-derived magma, followed by the incorporation of slab-derived materials into mantle and subsequent sediment melting. Integrated with previous work, the widespread Late Jurassic-Early Cretaceous magmatic rocks in eastern South China formed within a continental back-arc setting that was formed by subduction-induced convection in the mantle wedge. The subsequent roll-back of subducted Paleo-Pacific slab (Izanagi plate) triggered extensive mantle upwelling and melt-mantle interaction, accounting for the production of contemporaneous felsic rocks and related polymetallic deposits in regions.