Whole-rock Sr Research Articles

Petrogenesis and tectonic setting of the proto-tethyan dachaidan ophiolite, North China: insights from clinopyroxene chemistry and whole-rock Sr–Nd–Pb and zircon Hf isotopes

IntroductionThe Dachaidan ophiolites outcrop within an ultrahigh-pressure metamorphic belt along the northern margin of the Qaidam Basin. However, their age, source, and tectonic setting remain still in debate.MethodIn this study, we investigated the geochemistry and geochronology of the Dachaidan ophiolitic gabbros.Results and DiscussionZircon U–Pb dating yielded a crystallization age of 510.0 ± 2.8 Ma and 510.0 ± 2.9 Ma for the gabbro. The gabbros have low SiO2 contents (47.15–50.10 wt.%) and high MgO contents (6.35–9.04 wt.%) and Mg# values (55–74). The total rare earth element (∑REE) contents are 8.35–28.07 ppm, lower than those of normal-type mid-ocean ridge basalts (MORBs), and the gabbros exhibit light REE depletion or flat REE patterns, with small positive Eu anomalies (Eu/Eu* = 1.06–1.40). Trace element patterns are depleted to enriched in Nb and Ta, similar to island arc rocks and MORB. Clinopyroxene thermobarometry indicates the parental magma of the gabbros formed by high-temperature (1,318°C–1,363°C) and medium-pressure (1.27–1.64 GPa) partial melting in a mantle wedge. The gabbros have depleted Sr–Nd–Pb-Hf isotopic compositions, with (87Sr/86Sr)i = 0.704586–0.707441, εNd(t) = 4.7–6.6, and zircon εHf(t) = 7.6–11.4. The age-corrected Pb isotope ratios of these volcanic rocks are variable, with 206Pb/204Pb(t) = 18.085–18.253, 207Pb/204Pb(t) = 15.595–15.614, and 208Pb/204Pb(t) = 37.880–38.148, which are similar to the isotopic compositions of typical Indian MORBs. The source of the Dachaidan ophiolite is inferred to have been depleted mantle. The Dachaidan ophiolite likely formed in a forearc oceanic setting along the northern margin of the Qaidam Basin, during the initial subduction of an oceanic plate.

Two syenitic phases in the Early Paleogene Silhouette Island volcano-plutonic complex, Seychelles

The Silhouette alkaline volcano-plutonic complex is the largest exposed Early Paleogene (62–64 Ma) igneous complex of the Seychelles microcontinent. The rocks of this study were collected from Baie Cipailles, Pte Vareur and Pte Ramasse Tout and include syenite (SiO 2 = 60–63 wt%), microgranite (SiO 2 = 70–74 wt%), tuffaceous trachyte (SiO 2 = 64–65 wt%) and a basaltic xenolith (SiO 2 = 44 wt%; MgO = 6.4 wt%; Mg# = 47.6). The silicic rocks of this study are ferroan, metaluminous to weakly peralkaline and classify as A1-type granitoids. The whole-rock Sr–Nd ( 87 Sr/ 86 Sr i = 0.703894–0.706534; ε Nd ( t ) = +0.5–+1.8) isotopes of the silicic rocks are similar to those of the basalt xenolith ( 87 Sr/ 86 Sr i = 0.703576; ε Nd ( t ) = +1.9). There is limited to no geochemical evidence for crustal contamination in any of the rocks. The syenitic rocks were probably derived by fractional crystallization of an alkaline basaltic parental magma in the upper crust and under reducing conditions ( Δ FMQ = −1). The Sr–Nd isotopes and zircon Hf ( ε Hf ( t ) = +3.3–+9.1) isotopes of the silicic rocks are indicative of a moderately depleted source. We found distinct textural, mineralogical and compositional differences between the syenitic rocks, suggesting that the Silhouette complex is composed of at least two syenitic magma pulses.

Mixed sources and complicated petrogenetic processes of the Himalayan granites recorded by apatite in-situ geochemistry of the Eocene Yardoi-Lhunze complex, southeast Tibet

The Cenozoic Himalayan granites are widely accepted as the anatectic products of crustal rocks. However, previous geochemical studies on their petrogenesis are mainly based on whole-rock analyses. Here we conducted a detailed petrographic, trace elemental and Sr isotopic investigations of apatite from the Middle Eocene (44–40 Ma) two-mica granites and subvolcanic porphyritic leucogranites of the Yardoi-Lhunze complex from the Tethyan Himalayan Sequence in the southeast Tibet, with the aim to constrain their magma sources and petrogenetic processes. The results show that apatites from the Quedang and Dala two-mica granites are characterized by euhedral to subhedral crystal shape, and have a wide range of initial 87Sr/86Sr ratios (0.7016–1.0088). They show significant negative Eu anomaly and varied concentrations of Sr and Y, indicating they are typical magmatic apatites. Apatites from the Qiaga porphyritic leucogranites can be divided into two groups based on Eu anomaly. Group-I apatites with positive Eu anomaly mainly display euhedral to subhedral crystal shape but with alteration rim, suggesting from the influence of hydrothermal fluids. Group-II apatites with negative Eu anomaly also have euhedral to subhedral crystal shape but they exhibit patchy or oscillatory zones, consistent with a magmatic origin. However, both group-I and II apatites have comparable initial 87Sr/86Sr ratios, 0.7198–0.9966 and 0.7174–0.9999, respectively. Previous studies based on whole-rock petrology and geochemistry have suggested that the Quedang and Dala two-mica granites and the Qiaga porphyritic leucogranites represent cumulates and fractionated melt-rich magmas, respectively. This is evidenced by systematic variations between Sr, Y, (La/Sm)N and Eu/Eu* in magmatic apatites from Quedang and Dala, consistent with the fractionation crystallization of plagioclase and monazite.Apatite has a wide range of initial 87Sr/86Sr ratios (0.7016–1.0088), far exceeding the range of whole-rock records (0.7011–0.7204). Notably, most of the analyzed apatites show lower range of initial 87Sr/86Sr ratios consistent with locally exposed amphibolites (0.7109–0.7332). The remaining small portion of apatites have initial 87Sr/86Sr ratios falling within the data range of the locally exposed metapelites (0.8517–0.9527). Therefore, the investigated granites have mixed magma sources dominated by amphibolites with subordinate metapelites. This can be inferred by apatite Sr isotopes alone, but not by whole-rock Sr isotopes alone. Our investigations highlight that in-situ geochemistry of apatite can provide a powerful means to unravel the mixed magma sources and complicated magmatic processes for granites.

The Merensky-Bastard interval at Hackney, eastern Bushveld Complex: results of a combined Sr–Nd-isotopic investigation

The Merensky and Bastard reefs of the Bushveld Complex occur within what has been called a transitional macro-unit along the boundary of the Critical and Main zones. The transitional unit is characterised by a geochemical hiatus recording distinct inflections in mineral chemistry and isotopic compositions. Previously these inflections in mineral chemistry and changes in isotopic compositions were attributed mostly to the influx of a magma that was compositionally distinct from the resident magma and that was parental to the Main Zone of the complex. Sr-isotopic variations across this interval have been particularly well-studied, but despite this, little consensus exists regarding the petrogenesis and metallogenesis of this economically important interval. Here we report whole-rock Sr–Nd-isotopic, major- and trace element geochemical and mineral chemical data across the Merensky-Bastard interval as intersected by borehole BH8172 on the farm Hackney in the eastern Bushveld Complex. Variations in whole-rock Cr/MgO values and initial Sr isotopic ratios across the interval are consistent with the results of previous studies that argued for the co-accumulation of minerals from compositionally and isotopically distinct magmas, of Critical and Main Zone lineages, respectively. In our model, a magma of Critical Zone affinity enters the chamber causing erosion along the chamber floor. Orthopyroxene and plagioclase crystallise from the Critical Zone magma to form the Merensky Reef, as suggested by high whole-rock Cr/MgO ratios (> 80) and unradiogenic Sr-isotopic compositions (87Sr/86Sri < 0.7068). A plagioclase-laden magma of Main Zone affinity subsequently intruded the chamber as a basal flow, elevating the resident Critical Zone magma. Plagioclase within the former floated, forming a solid raft onto which the Bastard Reef was deposited, a model that is entirely consistent with density considerations and an upward increase in the An-content of plagioclase as observed in the anorthositic package between the Merensky and Bastard reefs. From a metallogenetic viewpoint, this would imply that the Main Zone could not have been the source of the PGEs within the Merensky Reef.

The spatial–temporal evolution of Neotethyan subduction in central to Southeast Iran: Constraints from geochemistry, zircon U–Pb–Hf, and Sr–Nd–Pb isotopes

Igneous rocks in central Iran preserve proof of several episodes of Late Mesozoic–Cenozoic Neotethyan subduction, and record temporal and spatial variations from subduction to collisional geodynamic regimes that remain a subject of debate. We revisit magmatic rocks from the central Urumieh–Dokhtar magmatic arc (UDMA; 32° 30′ N to 36° 00′ N) and report new major and trace element analyses, whole-rock Sr–Nd–Pb-isotopic data and zircon U–Pb–Hf ages on plutonic rocks from the Kajan region. The data reveal magmatic pulses between 32 and 22 Ma that are represented by igneous rocks with geochemical and isotopic features resulting from melting of a metasomatized, enriched mantle. Modeling indicates the samples represent a mixture of 97% batch melt (from a source with 98.5% mantle melt +1.5% terrigenous sediment) with 3% upper continental crust. The oldest yielded age (i.e., 32 Ma) rules out a lull in magmatism in the central UDMA between 37 and 26 Ma and speaks against distinct flare-up magmatic episodes at ∼54–37 Ma and ∼ 20–5 Ma in the frontal arc. We hypothesize that magmatism records down-going slab tearing into two pieces beneath the central to SE UDMA, which then changes the geometry and buoyancy of subducting Neotethyan lithosphere. The down-going slab remained involved in central UDMA (i.e., rollback) during Early Miocene whilst arc retreat beneath the SE UDMA proceeded with detachment of subducted oceanic lithosphere (i.e., break-off). We hypothesize a diachronous collision in the UDMA, first in the southeastern segments, and subsequently in the central UDMA. These findings bear important implications for the geodynamic evolution of the Zagros orogen and run counter to a model suggesting that the collision initiated in the northwest to central and propagated progressively to the southeast along the Zagros suture zone.

Constraints of alkaline magmatic evolution on the enrichment and mineralisation of rare-earth elements in the eastern part of the northern margin of the North China Craton: a case study of the Baerzhe and Saima deposits

The eastern part of the northern margin of the North China Craton is a key metal metallogenic province in China, hosting many important rare metal and rare earth element (REE) deposits and having great resource potential. This study investigated the Saima deposit, a niobium (Nb) and tantalum (Ta) deposit and the Baerzhe deposit hosting zirconium (Zr), Nb and REEs. Based on previous LA-ICP-MS zircon U–Pb dating results obtained by the authors’ team, this study compared the whole-rock geochemistry and whole-rock Sr–Nd–Pb isotopes of an orebody with its surrounding rocks. The geochemical characteristics of both deposits are used to systematically review the evolution of the alkaline magma and the differences in the physical and chemical conditions for enrichment and mineralisation of late-stage rare elements and REEs such as Nb, Ta and Zr. Comparing the mineralisation of the Saima and Baerzhe deposits, the alkaline magmatic evolution plays a crucial role in the enrichment of rare-earth elements, which are generally formed in the late stage of alkaline magmatic evolution.

First identification of late Mesozoic intraplate magmatism in the Chinese north Tianshan: implications for the orogenic architecture and crustal evolution

The formation and dynamics of granitoids in an intra-continental setting are crucial for understanding the architecture and evolution of continental crust. Here, we report geochronological, geochemical and Sr–Nd–Hf isotopic data for newly discovered late Mesozoic granitic intrusions in the Tianshan belt, northwestern China. These granitoids are I-type granites derived from an igneous precursor and were emplaced at c . 145–132 Ma. They have positive ε Nd ( t ) values and young Nd model ages, together with relatively low Sr/Y ratios, indicating that they might have originated from partial melting of the juvenile lower crust. There is a prominent decoupling between zircon Hf and bulk-rock Nd isotopes, which may have resulted from the early crystallization of Ti-rich minerals. These granitic intrusions also display subduction-related geochemical characteristics, which are probably inherited from Paleozoic crustal sources that were metasomatized by subduction-related fluids. We conclude that these late Mesozoic granitoids were emplaced in an intra-continental setting, and were probably triggered by thermal relaxation owing to crustal shortening and thickening. These data further imply that the Tianshan changed into crustal reworking during the Mesozoic from its prominent crustal growth in the Paleozoic. Supplementary material: LA-ICP-MS zircon U–Pb dating results, whole-rock major and trace element compositions, whole-rock Sr–Nd isotopic data and zircon Lu–Hf isotopic data are available at https://doi.org/10.6084/m9.figshare.c.7167890 Thematic collection: This article is part of the Mesozoic and Cenozoic tectonics, landscape and climate change collection available at: https://www.lyellcollection.org/topic/collections/mesozoic-and-cenozoic-tectonics-landscape-and-climate-change

Cretaceous magmatic arc in Hainan and the peri-South China Sea as evidenced by geochemical fingerprinting of granitoids in the region

Mesozoic magmatic rocks occur widely in the South China Block and are generally interpreted as the manifestations of the subduction of the Paleo-Pacific oceanic lithosphere beneath Asia. Subduction-driven magmatism in southeast (SE) China continued from the Late Permian through the Late Cretaceous with an inferred lull between 125 Ma and 115 Ma that is known in the literature as the Cretaceous “magmatic quiescence”. We report in-situ zircon U–Pb ages, Hf–O and whole-rock Sr–Nd isotopes, and whole-rock geochemistry of Cretaceous granitoids on Hainan Island and discuss their magmatic evolution within the framework of the Late Mesozoic geodynamics of SE China. We recognize two main stages of the emplacement of Cretaceous granitoids on Hainan, first around 120 Ma and then around 100–95 Ma, displaying high-K calc-alkaline, I-type geochemical affinities. Granites in both age groups are enriched in LILE and LREE, but depleted in Nb, Ta, Ba, Sr, and Eu. The 120 Ma granites have zircon εHf(t) values of –2.6 to 2.3 corresponding to Hf crustal model ages, ranging from 0.79 Ga to 1.03 Ga, and δ18O values ranging from 6.9‰ to 7.7‰. Zircons from 100–95 Ma granites have εHf(t) values of –4.2 to 1.1 corresponding to Hf crustal model ages of 1.08 Ga to 1.42 Ga, and δ18O values ranging from 6.7‰ to 8.4‰. Increasing εHf(t) values of the Cretaceous Hainan granites with younger crystallization ages indicate addition of more juvenile components and reworking of crustal material into their melt evolution. The εNd(t) values of the 120 Ma and 100–95 Ma granitoids range between –4.1 to –0.4 and –7.7 to –4.0, respectively. The calculated two–stage model age of the 100–95 Ma granitoids clusters between 1.25 Ga and 1.53 Ga. These isotopic data suggest that magmas of the Cretaceous granitoids were produced by partial melting of Mesoproterozoic metabasaltic rocks, which make up much of the crystalline basement of the southern Cathaysia block. The geochemical and isotopic characteristics of the Cretaceous granitoids on Hainan resemble those of magmatic arcs in the Circum–Pacific orogenic belts and identical to those of nearly coeval granitoid intrusions in the continental fragments within the South China Sea basin. We interpret these Cretaceous granitoids in the Peri–South China Sea region as the remnants of a once contiguous Late Mesozoic magmatic arc system that bounded the southern margin of the entire continental Southeast Asia. Our findings do not support the existence of an episode of magmatic quiescence in the geological record of SE China during the Aptian.

Genesis of the Devonian Habaqin ultramafic arc complex and associated low-grade Fe–Ti oxide mineralization in the northern North China Craton

In the northern North China Craton (NCC), abundant non-layered ultramafic complexes and associated low-grade Fe–Ti oxide ores (over 1000 million tons) were uncovered. However, their petrogenesis and metallogeny have remained unclear. The Habaqin complex is one of the larger intrusions of the region and hosts the largest low-grade magnetite occurrence. Here we present petrography, zircon U–Pb geochronology, mineral and whole-rock major and trace element compositions, and whole-rock Sr–Nd and zircon Lu–Hf isotopes of the complex to constrain its genesis. The complex includes early pyroxenite (∼75% clinopyroxene, ∼15% amphibole, ∼10% magnetite) and late hornblendite (∼80% amphibole, 15–20% magnetite, 0–5% apatite). Zircon grains from the hornblendite yielded a weighted mean 206Pb/238U age of 395 ± 2 Ma. The increasing whole-rock FeOtot and TiO2 contents from pyroxenite (FeOtot = 18.9 wt%, TiO2 = 1.31 wt%) to hornblendite (FeOtot = 19.1–30.4 wt%, TiO2 = 2.63–3.12 wt%) correlate with higher modal proportions of amphibole and magnetite. The pyroxenite and hornblendite display similar arc-like trace element characteristics of enrichment in LILEs and LREEs, and depletion in HFSEs. The complex formed by fractional crystallization of clinopyroxene, amphibole, magnetite, and apatite from a hydrous parent magma. Early removal of clinopyroxene caused H2O enrichment in evolved residual magmas and crystallization of magnetite and amphibole, forming low-grade mineralization with Ti-V-poor magnetite in hornblendite. The complex displays high initial 87Sr/86Sr ratios of 0.706413–0.707341, negative εNd(t) values of −16.0 to −14.2 and negative εHf(t) value of −30.6 to −12.0, suggesting a parent magma derived from an enriched lithospheric mantle, with possible existence of amphibole-bearing pyroxenite veins within the source. During the Early Devonian, the retreat of the subducted Paleo-Asian oceanic slab as well as arc-continental collision along the northern NCC induced partial melting of the veins and eventually formation of the complex.

Petrogenesis of Jurassic granitic rocks in South China Block: Implications for events related to subduction of Paleo-Pacific plate

Abstract Frequent magmatic intrusion and eruption during the early Yanshanian in South China formed a unique and diverse igneous rock assemblage, mainly comprising granite, rhyolite, and some basic rocks. Nevertheless, the tectonic regime responsible for the formation of these granitic rocks remains controversial. The comprehensive available data suggest that the Jurassic granitic rocks formed during the period from 180 to 145 Ma, with an age peak at ca. 160 Ma. Jurassic rocks are predominantly concentrated in Wuyi, southern Hunan, northeast Guangdong, and southern Jiangxi, followed by the eastern Guangxi-western Guangdong areas, mainly including I-, S-, and A-type granites, and a few I–S transformation-type granites. These granitic rocks invariably exhibit enrichment in Rb, Th, U, and Pb, while Ba, Nb, Sr, P, and Ti are depleted, and variable Eu/Eu* ratios. The whole-rock Sr–Nd isotopic and zircon Hf isotopic compositions show that the Jurassic granitic rocks are mainly recirculation products of Paleo-Mesoproterozoic rocks, as well as the mantle-derived magma, which played a major role in the formation process. Among them, the granites in eastern Guangxi-western Guangdong and Northeast Guangdong areas display higher ε Nd(t) and ε Hf(t) values, suggesting a relatively larger contribution from mantle-derived materials. Based on the comprehensive regional geological data, we interpret that these Jurassic granitic rocks as a product of lithosphere extensional-thinning, tectonic-magmatic interaction formed during the process of flat-slab subduction.

Late Cryogenian and early Ediacaran rare-metal rich granites in the Eastern Desert of Egypt: constraints from zircon ages and whole-rock Sr- and Nd- and feldspar Pb-isotopic compositions

Rare metals (Nb, Ta, Y, Zr, Sn, U, W and REE) are economically important and new supplies need to be found. In order to understand Neoproterozoic rare metal granites of the Arabian–Nubian Shield (ANS), six samples from five rare-metal mineralized alkali feldspar granites, syenogranites and granodiorite from the Central and SE Desert of Egypt were studied for zircon U–Pb ages and O-isotopic compositions as well as whole-rock Sr- and Nd- and alkali feldspar Pb-isotopic compositions. These are transitional between I-type and A-type granites, mostly high-K calc-alkaline, peraluminous granites with gullwing-shaped REE patterns and strongly negative Eu anomalies. Four granites gave mantle-like zircon δ 18 O V-SMOW between 4.2 and 5.96‰ and yielded ages of 628–633 Ma. This is about when subduction-related magmatism began to be replaced by collision-related magmatism. Igla Ahmr granites are older, formed at 691.7–678.9 Ma with δ 18 O V-SMOW c . 5.95‰. All have positive initial ε Nd values (+3.3 to +6.9) typical for mantle and juvenile crust. Pb isotopic compositions are unusually radiogenic compared with unmineralized ANS granitic rocks. The data indicate similar magmatic sources for ANS mineralized and unmineralized granites. Exploration for other rare-metal mineralized granites in the ANS should focus on bodies with similar characteristics. Supplementary material : Details of analytical details and supplementary tables and figures are available at https://doi.org/10.6084/m9.figshare.c.6949402

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.

Discovery of the ∼35 Ma granite porphyry dikes in the giant Dachang Sn ore field: Tectonic and metallogenic implications

The world’s second-largest Dachang Sn ore field in the southwest of China exhibits genetic connections to the Late Cretaceous highly evolved S-type granitoids (93–91 Ma). In this study, we report the newly identified Eocene granite porphyry dikes with zircon U-Pb ages of 35–33 Ma at the Gaofeng Sn deposit from the Dachang ore field. The petrogenesis and mineralization potential are also constrained by employing whole-rock and zircon geochemical data. These Eocene dikes coincide with the emplacements of granitoids (35–30 Ma) along the Ailaoshan–Red River fault and its neighboring regions, indicating that they could be related to a similar lithospheric extension setting after the Indo-Asian collision. However, unlike those of the latter granitoids (35–30 Ma), which are grouped into I-type granites, the high-silica granite porphyry dikes show elevated ASI values (1.32–1.38) with high P2O5 (0.41–0.44 wt%) and normative-CIPW corundum (4.15–4.71 wt%) contents and could be classified as S-type granites. The strongly negative whole-rock Sr, Ba, and Eu anomalies signify separation of plagioclase and K-feldspar during magma crystallization, which is also confirmed by the zircon trace elements compositions. The Gaofeng Eocene granite porphyry dikes have a high initial zircon Tti-zr of 855 ℃, high Ba/Pb ratios (11.5–16.8), and low Pb contents (7.56–9.98 ppm) with low zircon εHf(t) values (−7.4 to −0.6), hinting at the dehydration melting of biotite from the metasedimentary basement. Additionally, the dikes are characterized by low oxygen fugacity (zircon Ce/Ce* <70; ΔFMQ from −6.0 to +0.6 with an average of −2.0) and show evidence of magmatic-hydrothermal interaction (whole-rock Nb/Ta < 5), in line with those of the Dachang Late Cretaceous fertile granites, suggestive of Sn mineralization potential. Therefore, it is recommended that attention should be paid to the Eocene granite porphyry dikes during future prospecting.

Petrogenesis of early Paleozoic granitoids in the North Qinling Orogen, Central China: Implications for crustal evolution in an accretionary orogen

Accretionary orogens are sites of extensive continental crustal growth and modification. The mechanism by which mafic crust is transformed into silicic melts (i.e., maturation of continental crust) is important for understanding the formation of the continental crust. The North Qinling Orogen (NQO) is a composite orogenic belt and contains an early Paleozoic accretion-dominated orogenic system, which is ideal for investigating continental crustal maturation. We obtained zircon and monazite U–Pb age and O isotope data, zircon Lu–Hf isotope data, and whole-rock major- and trace-element and Sr–Nd isotope data for early Paleozoic granitoids of the NQO. The granitoids are divided into three groups. Group 1 includes the Taiping tonalite (445 ± 3 Ma), the Manziying syenogranite (445 ± 2 Ma), and the Huoshenmiao granodiorite (436 ± 2 Ma). The Taiping and Huoshenmiao plutons have relatively high SiO2 contents (68.64–71.67 wt.%) and Na2O/K2O ratios (1.15–4.19), with enrichments in Rb, Ba, Th, and U and depletions in Nb, Ta, P, and Ti, and they are geochemically similar to sodic arc magmas. The Manziying syenogranite is a peraluminous potassic granite with high K2O contents (4.59–5.27 wt.%). Grantioids from Group 1 have similarly depleted Sr–Nd–Hf–O isotopic features (εHf[t] = +5.9 to +8.8; δ18O = 3.98‰–5.41‰), indicative of derivation via partial melting of oceanic arc crust, which suggests that partial melting of oceanic arc crust in a subduction system contributes to the generation of continental crust and causes its maturation. Group 2 consists of the Wuduoshan monzogranite (418 ± 2 Ma) and the Sikeshu granodiorite (423 ± 3 Ma). These plutons have relatively high SiO2 (65.59–72.06 wt.%), K2O (3.26–4.79 wt.%), and Al2O3 (14.65–16.12 wt.%) contents and Sr/Y (33–87) and (La/Yb)N (23–48) ratios. The Wuduoshan monzogranite has positive zircon εHf(t) (+0.4 to +3.1) and uniform δ18O (6.38‰–8.07‰) values, but the Sikeshu granodiorite has more variable isotopic compositions (εHf[t] = −1.9 to +5.0; δ18O = 6.37‰–10.60‰). The Wuduoshan monzogranite and Sikeshu granodiorite have similar whole-rock Sr–Nd isotopic compositions to basement rocks of the NQO. These features indicate that the two plutons formed by partial melting of basement rocks (i.e., subducted into the lower crust) of the North Qinling unit, along with juvenile crustal material. Group 3 is represented by the Xiaguan monzogranite, which formed at 434–430 Ma, and can be subdivided into heavy rare earth element (REE)-depleted and -enriched units. The former has high Sr/Y (56–98) and (La/Yb)N (34–73) ratios and low MgO (0.13–0.24 wt.%), Cr (0.37–1.69 ppm), and Ni (0.32–1.09 ppm) contents, similar to adakites derived from metabasaltic sources. The heavy REE-enriched nature of the Xiaguan monzogranite may reflect modification of its source by melt or fluid. Our results show that partial melting of enriched oceanic arc crust contributed to crustal maturation in an accretionary orogen. The addition of evolved crustal material also facilitated this process; therefore, the basement rocks and crustal thickness should be considered when assessing crustal dynamics in an accretionary orogen.

Early Cretaceous magmatism in the northern Gyeonggi Massif and the northcentral Yeongnam Massif, Korean Peninsula: Its implications for the Cretaceous tectonic evolution of the Northeast Asia

Most Cretaceous igneous rocks in the northeastern North China Cratons (NCC) and southeastern South China Cratons (SCC) in Northeast Asia are interpreted to have formed in the extensional tectonic environment caused by subducted slab roll-back. On the other hand, the Cretaceous magmatism in the proto-Southwest Japan was confirmed to have occurred in the compressional state produced by the paleo-Pacific plate subduction. The granitoids and enclaves in the northern Gyeonggi Massif intruded at ca. 110–109 Ma and 112 Ma respectively, and the granite and pyroclastic rock in the northcentral Yeongnam Massif were formed at 106 Ma and 103 Ma respectively. The enclaves in the Ganghwa area which located in northern Gyeonggi Massif have high whole-rock Sr isotope composition (0.70748–0.71181) and low whole-rock εNd(t) (−14 to −15) value and low zircon εHf(t) (−19.2 to −14.7) values with enrichment of LILE, suggesting their origin from enriched lithospheric mantle metasomatized by subducted slab sediment. The Early Cretaceous granitoids on the northern Gyeonggi and northcentral Yeongnam massifs have adakite like geochemical composition which are derived from the lower crust instead of subducted oceanic crust derived melt. The geochemical features of the Early Cretaceous igneous rocks in the Korean Peninsula including the study area imply that the Early Cretaceous magmatism on the Korean Peninsula mainly occurred in the extension regime caused by slab roll-back of the paleo-Pacific plate as in the NCC. On the other hand, the igneous rocks formed at 80–66 Ma in the Korean Peninsula have geochemical characteristics similar to that in the Cretaceous igneous rocks in proto-Southwest Japan, indicating that the 80–66 Ma magmatism in the Korean Peninsula were produced by the subduction of the paleo-Pacific plate. The Cretaceous igneous rocks in the northeastern NCC and on the Korean Peninsula show a southeastward-younging trend from 145 Ma to 80 Ma, indicating that southeastward slab roll-back had started at 145 Ma and continued until 80 Ma. Arc-related igneous activity occurred from 116 Ma in southwestern Japan and propagated northward, resulting arc related magmatism in the northern margin of proto-southwestern Japan and the southeastern margin of the Korean Peninsula at 80–66 Ma.

Age and origin of Early Cretaceous magmatism in the southernmost Omineca Belt, northern U.S. Cordillera

We present results of a geochronological and geochemical study of the Kaniksu batholith, located in northern Idaho and Washington near the Canadian border. This batholith occurs at the southern end of the Omineca Belt and forms an integral part of the Priest River metamorphic complex. The Kaniksu batholith is compositionally diverse, but granitoids can be grouped into a volumetrically major peraluminous and metaluminous suite and a volumetrically minor alkalic suite based on lithology and major-element composition; additional compositionally similar alkalic plutons extend far to the south of the main footprint of the batholith. Laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb zircon dating revealed that most granitoids of the batholith formed in the Early Cretaceous between 120 Ma and 100 Ma, although magmatism continued until ca. 73 Ma. The dominance of Early Cretaceous ages closely matches magmatic age distributions elsewhere in the Omineca Belt. There is no clear relationship between age and suite assignment or other geochemical characteristics. Whole-rock Sr, Nd, and Pb isotope data along with zircon Hf isotope data suggest that the alkalic and metaluminous suites can largely be explained as mixtures of partial melts from the Mesoproterozoic Moyie sills and metasedimentary rocks of the Belt Supergroup, whereas a third component, perhaps Paleoproterozoic or Neoarchean meta-igneous basement, is required to explain the origin of the peraluminous suite. All of these granitoids formed well behind the Cordilleran arc front in an area characterized by multiple episodes of crustal thickening. As there is little evidence of contemporaneous mantle-derived magmatism, crustal thickening was probably the main driver of batholith formation.

Secular change in the nature of mantle and tectonic evolution of northwestern margin of the Yangtze Block during Neoproterozoic: Constraints from the mafic intrusions and associated granitoids of the Hannan and Xiaomoling complexes

To reveal the Neoproterozoic tectonic evolution of the northwestern margin of the Yangtze Block, a petrological and geochemical study was carried out for the mafic intrusions and associated granitoids from the Hannan and Xiaomoling Complexes. The ca. 900–880 Ma Xiaomoling diorite and granodiorite have high K2O contents (2.29–3.57 wt%) and belong to the high-K calc-alkaline series. They display depleted whole-rock Sr–Nd–Hf isotope compositions (0.70151–0.70398, +0.6 to +2.5 and +4.4 to +11.2, respectively) and positive zircon εHf(t) (+3.6 to +9.9) with slightly elevated bulk zircon δ18O values (6.4–6.8 ‰) and minor Paleoproterozoic (∼2.2 Ga) inherited zircons, indicative of derivation from remelting of juvenile mafic crust with crustal contamination in a continental arc setting. The ca. 866 Ma Hannan gabbro-diorites show enrichments in LILEs and LREEs and depletions in HFSEs and display depleted Sr–Nd–Hf isotope compositions (0.70267, +5.4 and +17.8, respectively), positive zircon εHf(t) values (+8.6 to +12.0) and mantle-like zircon δ18O values (5.0–6.2 ‰), suggesting that they were generated through flux melting of a depleted mantle wedge. The ca. 850–840 Ma Xiaomoling hornblende gabbro and diabase display depleted Sr–Nd–Hf isotope compositions (0.70279–0.70288, +2.7 to +3.2 and +9.8 to +10.5, respectively) and are characterized by high Nb/U ratios and less enrichment in LREEs and LILEs, indicating that they were formed by decompression melting of a depleted mantle wedge in a back-arc setting. However, the coeval high-Mg diorite displays enriched whole-rock Sr–Nd–Hf isotopes (0.70477–0.70701, −4.0 to −6.0 and −7.2 to −7.4, respectively), negative zircon εHf(t) values (−1.6 to −9.2) and elevated zircon δ18O values (5.3–7.5 ‰), reflecting that the mantle-derived melts have underwent various degrees of crustal contamination. The ca. 780 Ma Hannan gabbro is characterized by earlier crystallization of plagioclase than clinopyroxene, implying that their parental magma is anhydrous. Additionally, the relatively shallow emplacement depth and tholeiitic affinities imply that they were generated by mantle decompression melting probably associated with a continental rift environment in response to Rodinia breakup. Taken together, the secular changes in the geochemical and isotopic compositions of these mafic rocks in the northwestern margin of the Yangtze Block record the evolution from a continental arc at ca. 900–880 Ma to continental rifting at ca. 780 Ma.

Crust–mantle interaction beneath the Pamir orogenic belt: Insights from the Miocene Dunkeldik carbonatite–syenite complex in Tajikistan

Only some carbonatite–syenite complexes in the world could form carbonatite-related rare earth element (REE) deposits, which is regarded as the main REE origin. The Miocene Dunkeldik carbonatite–syenite complex is located in the Dunkeldik magmatic field in Tajikistan between the Pamir and Tibetan plateaus. So far, the origin, tectonic setting, and REE mineralization potential of the Dunkeldik carbonatite–syenite complex remain unclear due to the limited isotopic values, which inhibit the further understanding the genesis of such carbonatite complex. This study presents zircon U–Pb age, whole-rock geochemical, and Sr–Nd–Pb isotope, and zircon Hf–O isotope data for the Dunkeldik carbonatite–syenite complex on the Pamir Plateau. Zircon U–Pb dating of syenite yielded two concordant ages of 10.27 ± 0.16 and 10.48 ± 0.21 Ma, indicative of formation in a post-collisional extensional setting related to India–Asia collision. Both the carbonatite and syenite exhibit similar enrichments in large-ion lithophile elements (e.g., Sr and Ba) and light REEs, depletions in high-field-strength elements (e.g., Nb and Ta) and heavy REEs, and small Eu anomalies. Whole-rock Sr–Nd–Pb isotopic compositions are highly enriched, with high (87Sr/86Sr)i (0.70971 to 0.70979), low εNd(t) (–8.9 to –9.2), and radiogenic Pb isotope composition (206Pb/204Pb (18.322–18.358), 207Pb/204Pb (15.689–15.695), and 208Pb/204Pb (39.091–39.149)). Zircons Hf-O isotope analyses yield low εHf(t) values (–9.5 to –19.0) and high δ18O value (7.4 ‰–9.5 ‰), respectively. These geochemical and isotopic features indicate that the complex was formed by partial melting of enriched lithospheric mantle during the Miocene India–Asia collision and was assimilated by upper crustal material during magma ascent. The carbonatite and syenite in the complex formed by melt immiscibility. The involvement of crustal material in the magma and less development of fenitization may responsible for the limited REE mineralization potential as compared with other large REE deposits that formed during coeval India–Asia collision.

Petrogenesis of the alkali basalt and trachy-andesite suite in the northern Tarim Basin, NW China: Implications for crust-mantle interactions controlled by the Permian mantle plume

The Permian Tarim large igneous province (LIP), which contains both large volumes of basalt and silicic volcanic sequences, has been intensively studied in the past decades, but some issues still remain controversial, including the petrogenesis of each component in the LIP, their interrelationships, and the geodynamic processes that produced them. In the Halahatang area of the northern Tarim Basin, a well-developed eruptive suite composed of basalt and trachy-andesite has been penetrated by numerous boreholes, which provide us an opportunity to better understand these issues. In this paper, we carried out an integrated study on the eruptive suite, including petrography, geochronology and geochemistry. The results indicate that the eruptive suite formed at around ∼290 Ma, generally synchronous with the main episode of magmatism within the Permian Tarim LIP. The basalts have similar compositions to typical oceanic island basalts (OIBs), which have alkaline, high-Ti and sodic compositions and show enrichments in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs). Whole-rock Sr–Nd isotopes are slightly enriched, with εNd(t) and (87Sr/86Sr)i values from −4.5 to −3.4 and 0.7065 to 0.7086, respectively. The trachy-andesites are alkaline, metaluminous and ferroan, and display highly enriched LREEs and LILEs. High Ga/Al ratios and Zr + Nb + Ce + Y values constrain their A-type affinity and A1-type nature. They have εNd(t) values from −8.2 to −4.5 and (87Sr/86Sr)i values from 0.7083 to 0.7189, more enriched than those of the basalts. Based on these results, we proposed that the different components in the igneous suite have close petrogenetic relationships. The alkali basalts may have been formed by mantle plume-derived melts coupled with somewhat assimilation of continental crust, while the trachy-andesites were products of variable fractionation of the basaltic magmas involving more continental crustal components in the magma evolution. In combination with the existing results in Tarim, we further suggested that the alkali basalt and trachy-andesite suite may have been generated via a crust–mantle interaction process related to the Permian mantle plume activity.

Assessing crustal contamination in the 28 to 18 Ma Dulce dike swarm with Nd-Sr isotopic data, southwestern Colorado

Alkaline to subalkaline mafic dikes in the 28 to 18 Ma Dulce swarm were emplaced in a zone of incipient extension from southern Colorado into northern New Mexico on the northeastern boundary of the San Juan Basin. The 87Sr/86Sr ratios for the dikes are 0.70503 to 0.70584, akin to most post-28 Ma mafic rocks across the northern San Juan Basin. These data are consistent with melting of metasomatized subcontinental lithospheric mantle with little to no crustal contribution as revealed by the geochemical and Sr-Nd isotopic signatures of most 28–0.6 Ma mafic rocks in the region. Time-corrected εNd(t) values of −4.1 to −7.4 for rocks in the Dulce swarm, however, indicate that magma production involved the crust. A previous hypothesis for Dulce magmas was contamination of lithospheric mantle melts with up to 45% mafic lower crust ± 0.5% upper crust. In this investigation, six new whole-rock Sr and Nd isotopic analyses were combined with published data to further investigate the contamination of lithospheric mantle melts with different crustal reservoirs. The Nd isotope signatures of the Dulce swarm offer evidence for the long-term involvement (∼10 Ma) of lower crust in the production of rift-related mantle magmas. Isotopic mixing curves support previous hypotheses for the contamination of lithospheric mantle melts with 10 to 40 percent lower mafic crust. This provides further insight into regional variations in mantle magmas produced after 28 Ma in the Four Corners region that likely triggered crustal melting related to caldera complexes in the western San Juan Mountains.