Heavy Rare Earth Element Patterns Research Articles

Petrogenesis of Miocene volcanism in the Central Anatolia: Geochemical, isotopic and geochronological evidence

This paper presents the first 40Ar/39Ar age data, new bulk-rock major-trace element compositions and NdSr isotope geochemistry, combined with detailed geological mapping of Galatia Volcanic Province (GVP) in the northwest Central Anatolia (Turkey). Here, there are voluminous extrusive rocks in a wide compositional range: a basalt, andesite and trachyte suite, and their pyroclastic equivalents. 40Ar/39Ar dating of two whole-rock samples from the GVP yielded plateau ages of 21.76 ± 0.8 Ma and 20.97 ± 0.5 Ma, constrain the volcanic activity at ca. 21 Ma (Aquitanian). The samples show by sub-parallel light rare earth element (LREE)-enrichment and relatively flat heavy rare earth element (HREE) patterns and moderate fractionation [average (La/Yb)N = 13.5]. Their initial 87/86Sr values vary between 0.704619 and 0.704998, while initial 143/144Nd values lie between 0.512673 and 0.512755. Integration of the geochemical and geochronological with geological data we propose that the volcanic rocks in the northern area of GVP with calc-alkaline affinity were produced by a lithospheric mantle magma in an extensional setting.

Insights from Dikes for Multistage Granitic Magmatism in the Huayangchuan Uranium Polymetallic Deposit, Qinling Orogen

The Huayangchuan U-polymetallic deposit in the Qinling Orogen is a newly verified carbonatite-hosted deposit on the southern margin of the North China Craton (NCC) in Central China. Granitic magmatism is extensively developed in the Huayangchuan deposit area and is lacking analysis on the reasons for these situations; however, its ages, petrogenesis, and relationship with uranium mineralization are not well constrained. Zircon U–Pb ages for the hornblende-bearing granite porphyry and medium-fine-grained biotite granites in close proximity to carbonatite rocks are 229.8 ± 1.1 and 135.3 ± 0.6 Ma, respectively. High-K calc-alkaline series and weakly peraluminous Triassic hornblende-bearing granite porphyry are slightly enriched in light rare earth elements (LREE) with flat heavy rare earth element (HREE) patterns, enriched in Ba and Sr, and depleted in Nb, Ta, P, and Ti, i.e., geochemical characteristics similar to those of adakite-like rocks. The Early Cretaceous medium-fine-grained biotite granites are characterized by LREE enrichment and flat HREE patterns, which belong to high-K calc-alkaline series, and metaluminous belong to weakly peraluminous I-type granite, with U and large ion lithophile element (LILE) enrichment and high field strength element depletion. The high initial 87Sr/86Sr ratios and enriched Nd (εNd(t) = −10.7 to −9.5 and −19.9 to −18.9, respectively) and Hf (εHf(t) = −21.8 to −13.0 and −30.5 to −19.0, respectively) isotopes revealed that both granitic rocks from the Huayangchuan deposit mainly originated from lower crustal materials, generated by partial melting of the ancient basement materials of the Taihua Group. Triassic hornblende-bearing granite porphyry is significantly different from the mantle origin of the contemporaneous U-mineralization carbonatite. In combination with tectonic evolution, we argue that the Qinling Orogenic Belt was affected by the subduction of the North Mianlian Ocean during the Late Triassic. The ongoing northward subduction of the Yangtze Craton resulted in crustal thickening, forming large-scale Indosinian carbonatites, U-polymetallic mineralization, and contemporaneous intermediate-acid magmatism. Additionally, due to the tectonic system transformation caused by Paleo-Pacific Plate subduction, intracontinental lithosphere extension and lithospheric thinning occurred along the southern NCC margin in the Early Cretaceous. Intense magma underplating of the post-orogeny created a large number of magmatic rocks. The tremendous heat could have provided a thermal source and dynamic mechanism for the Yanshanian large-scale U-polymetallic mineralization events.

Arc volcanism initiated on the eastern margin of Eurasia during the Early Cretaceous: Geochemistry of the Takanokura volcanic rocks in the Abukuma Mountains, Northeast Japan

AbstractThe Early Cretaceous Takanokura Formation in the eastern part of the Abukuma Mountains consists of a lower felsic ignimbrite and upper intermediate lavas and volcaniclastic rocks, representing the initial arc‐type in northeast Japan. In this study, I analyzed the major and trace element contents and Sr‐Nd‐Pb isotopic ratios of these eruptive products; then, I discussed their magma genesis based on their geochemical properties. Although igneous rocks of the same period in other localities of northeast Japan are characterized by the occurrence of adakites, these volcanism are composed of non‐adakitic high‐ to medium‐K andesite to rhyolite that are rich in large‐ion lithophile elements and poor in high‐field‐strength elements and have low Sr/Y values and flat heavy rare earth element patterns. Furthermore, these rocks have high radiogenic Pb isotopic ratios. The rhyolite and dacite have been thought to derive from crustal melting, whereas the andesite formed by the mixing of crustal felsic melts and mafic magmas generated by melting of the lithospheric mantle. Although previous studies attributed the formation of the Early Cretaceous adakites to the hot subduction of a mid‐ocean ridge, recent global plate motion reconstructions reject this model. To generate magma from a cold slab and lithospheric mantle that does not spontaneously melt, the mantle wedge under northeast Japan must have experienced heating. During this period, the volcanic province along the eastern margin of Eurasia expanded rapidly toward the trench, forming grabens. Therefore, I concluded that the advance of the hot asthenosphere into the forearc region that led to this expansion, which caused the retreat of the subduction boundary of the paleo‐Pacific plate to retreat and ultimately converted northeast Japan from an accretionary complex into a volcanically active region.

Late Jurassic Haobugao granites from the southern Great Xing’an Range, NE China: Implications for postcollision extension of the Mongol–Okhotsk Ocean

Abstract The Mongolia-Okhotsk tectonic regime had a significant impact on the tectonic evolution of Northeastern (NE) China. However, there is no consensus on the role of this regime in the geological evolution of the Xing’an Massif during the late Mesozoic. This article presents the results of zircon U–Pb geochronology, whole-rock major and trace-element geochemistry, and zircon Hf isotopic compositions for granites in the Haobugao area of the southern Great Xing’an Range, NE China, to determine their petrogenesis, source, and tectonic setting. The zircon U–Pb ages indicate that the granites crystallized at 152.7 ± 0.5 Ma. The granites exhibit high SiO2 (70.75–73.19 wt%) and K2O + Na2O (8.00–8.65 wt%) contents and extremely low MgO (0.40–0.59 wt%) and TiO2 (0.24–0.33 wt%) contents. They belong to the metaluminous, high-K calc-alkaline, and ferroan series, with mostly right-inclined REE curves, flat heavy rare earth element patterns, high 10,000 Ga/Al ratios, and intensely negative Eu anomalies. The Zr/Hf ratios are 24.2–27.7, Nb/Ta ratios are 6.4–8.9, and Y/NbN ratios are >1.2. These characteristics suggest an A2-type granite affinity. The zircon εHf values of the rocks range from +5.62 to +9.12, corresponding to T DM2 values of 621–906 Ma, indicating that juvenile materials in the Neoproterozoic may be a source of these granites. Geochemically, these Late Jurassic A2-type granites are similar to those from post-collision extension settings. The primary magma was likely derived from the partial melting of a delaminated region of the lower crust.

Pressure–temperature–time path of ultrahigh-pressure metamorphic terranes constrained by zircon petrochronology: A case study of orthogneisses from the northern Sulu belt, China

Reconstruction of pressure–temperature–time (P–T–t) path of volumetrically-dominant orthogneisses in deeply subducted continental crusts is challenging due to their simple amphibolite-facies mineral assemblages. To provide new insights into this issue, a detailed study of mineral inclusions, trace elements and U-Pb ages within robust zircon containers, together with titanite U-Pb dating and petrology were conducted on ultrahigh-pressure (UHP) orthogneisses from the northern Sulu belt. The following P–T–t conditions of three metamorphic stages were constrained: (i) cathodoluminescence (CL)-grey zircon cores (ca. 238–237 Ma) with flat heavy rare earth element (HREE) patterns include garnet, phengite and omphacite that yield estimated P–T conditions of 2.3–2.8 GPa/730–800 °C, corresponding to prograde high-pressure (HP) metamorphism before the UHP peak. (ii) CL-dark zircon mantles (ca. 224 Ma) contain multiphase solid inclusions of plagioclase + K-feldspar + quartz and show extremely low Th/U ratios (≤0.08) and negative Eu anomalies, demonstrating their crystallization from hydrous melt at 2.0–1.8 GPa/890–820 °C during HP eclogite-facies exhumation. In addition, titanite grains are characterized by high LREE contents (1939–5239 ppm), Th/U ratios (0.37–0.81) and crystallization temperatures (830–801 °C), suggesting their peritectic origin. The lower intercept age (220 ± 4 Ma) of titanite is broadly consistent with the ages of the zircon mantles, indicating that they both crystallized from the melt at the same stage. (iii) CL-bright zircon rims (ca. 212–211 Ma) preserve low-pressure mineral inclusions of plagioclase, K-feldspar and quartz, and they show low Th/U ratios (0.03–0.12) and steep HREE patterns with negative Eu anomalies, witnessing amphibolite-facies overprinting at 1.0–0.8 GPa/610–550 °C. Combined with the metamorphic peak conditions of 3.2 GPa/840 °C at ca. 232 Ma constrained by previous studies from the same area, the P–T–t path of the northern Sulu UHP belt is reconstructed. The data reflects two-stage exhumation of this terrane, including an early-stage (5.8 km/m.y.) and a later-stage slower (3.1 km/m.y.) exhumation processes. The faster exhumation during early decompression was probably enhanced by extensive melting activity recorded by the CL-dark zircon mantles from the orthogneisses.

From divergent to convergent plate boundary: A ca. 200 Ma Wilson cycle recorded by ultrahigh-pressure eclogites in the Dora-Maira Massif, Western Alps

Sparse eclogite exposure in accretionary-to-collisional orogens cannot only reveal the sites of ancient subduction zones and plate boundaries, but also elucidate the multi-stage tectonic evolution in the Wilson cycle. The southern part of the Dora-Maira Massif in the Western Alps is well known for its ultrahigh-pressure (UHP) rocks and contains small occurrences of petrographically distinct types of eclogite. Due to the lack of detailed geochronological and geochemical data on these rocks, the nature of their protolith and related geodynamic setting has remained unknown. In the current study, 33 samples from 13 localities were studied by whole-rock major- and trace-element analysis, and a selection of the samples were studied using Sr-Nd-Hf isotopes, as well as high-resolution elemental mapping and U-Pb geochronology of zircon. According to macroscopic appearance, petrography, chemical composition, and principal-component analysis of multi-elements, two major types are distinguished. Light-colored eclogite is phengite-rich, commonly foliated, bears kyanite and quartz/coesite, shows relatively high MgO, K2O, SiO2, εNd(t) (−2.2 to +1.4), and εHf(t) (+3.5 to +7), with a protolith geochemically similar to an enriched-type mid-oceanic-ridge basalt (E-MORB). Dark eclogite generally is massive, rutile-rich, shows higher values of Ti, Fe, P, Nb, and Zr, εNd(t) values between −2.8 and −0.8, εHf(t) values between −6.1 and −3.2, with an oceanic island basalt (OIB)-type protolith composition. Magmatic zircon cores, which are characterized by steep heavy rare earth element (HREE) patterns and negative Eu anomalies, yield consistent protolith ages of ca. 253−252 Ma in both eclogite types. Metamorphic zircon domains with flat HREE patterns and insignificant Eu anomalies yield a younger mean age of ca. 34 Ma, which is the age commonly assigned to eclogite-facies UHP metamorphism. Considering the various geochemical signatures of E-MORB- to OIB-like eclogites and the regional tectonic evolution, their protolith is best explained as rocks, which crystallized from rift-related basaltic magma, associated with the break-up of the Pangea Supercontinent that eventually resulted in the birth of the Piemonte-Liguria Ocean (Alpine Tethys Ocean) between the future Eurasia and Adria plates. Therefore, the UHP eclogites in the Dora-Maira Massif likely fingerprint a multi-stage tectonic evolution from divergent (continental extension, rifting) to convergent (subduction zone) plate boundary, corresponding to the beginning and end of a Wilson cycle. Incidentally, they reveal that mafic rocks in the Alpine basement units may not be polymetamorphic, but may actually consist of post-Variscan products that underwent only Alpine metamorphism.

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.

Provenance, protolith and metamorphic ages of jadeite-bearing orthogneiss and host paragneiss at Tavagnasco, the Sesia Zone, Lower Aosta Valley, Italy

Abstract. An eclogite-facies orthogneiss and host paragneiss from a quarry near Tavagnasco in the Lower Aosta Valley were studied in order to refine the protolith, provenance and metamorphic ages of the Eclogitic Micaschist Complex of the Sesia Zone. The orthogneiss contains jadeite with quartz + phengite + K-feldspar ± garnet + rutile + zircon, whereas the paragneiss hosts garnet + jadeite + phengite ± glaucophane + epidote + rutile + quartz. Phase diagram modeling of two representative samples yields minimum equilibration conditions of 550 ± 50 ∘C and 18 ± 2 kbar. Cathodoluminescence images of zircon from the orthogneiss show oscillatory-zoned cores that are embayed and overgrown by complex, oscillatory-zoned rims. Four concordant secondary ion mass spectrometry analyses from the cores give a weighted mean 206Pb / 238U age of 457 ± 5 Ma. The cores have Th/U = 0.1 and negative Eu anomalies indicative of an igneous protolith, which we interpret to have crystallized in the Ordovician at 780 ∘C, based on Ti-in-zircon measurements. Zircon rims yield a range of 206Pb / 238U dates from 74 to 86 Ma, and four concordant analyses define a weighted mean 206Pb / 238U age of 78 ± 2 Ma. The rims are interpreted to have grown in the eclogite facies based on their lower Th/U (0.01), less negative Eu anomalies and steeper heavy rare earth element (HREE) patterns at <600 ∘C. The paragneiss yielded a detrital zircon population with major peaks at 575–600, 655 and 765 Ma; minor older components; and a maximum depositional age of approximately 570 Ma. The prominent Neoproterozoic zircon population and Ediacaran depositional age suggest derivation from the Gondwana margin. The metamorphic zircon is consistent with the oldest eclogite-facies event in the Sesia Zone; it does not show evidence of multiple periods of rim growth or any pre-Alpine (e.g., Variscan) metamorphism.

Timescales and mechanisms of felsic lower continental crust formation: Insights from U-Pb geochronology of detrital zircon (Malenco Unit, eastern Central Alps)

Burial of supracrustal sedimentary material is one of the processes that contribute to the formation of the lower continental crust. We investigate the rate of this process in Permian lower continental crust from the Malenco Unit (eastern Central Alps) by constraining the age of protolith deposition and subsequent high-grade metamorphism of felsic, garnet-rich granulites. Detrital zircon cores, many of which are small (<50 μm), were dated by SIMS and yield concordant dates ranging from 387 ± 21 Ma (2σ) to 2380 ± 16 Ma, with the vast majority of the dates falling in the range 450–1000 Ma. For the seven cores in each sample that gave the youngest concordant dates, replicate analyses were carried out. None of these cores had all measured U-Pb dates agree within error, indicating significant disturbance by the Permian granulite facies metamorphism. A 206Pb/238U age of 424 ± 6 Ma for a core with two overlapping replicate analyses is the best available constraint on the maximum depositional age of the sedimentary protolith. This maximum depositional age also coincides with the first significant population in the probability density plot of detrital core dates, which is a double peak at ∼425 Ma and ∼450 Ma.Zircon detrital cores are separated from metamorphic overgrowths by a small seam of zircon that is riddled with tiny inclusions of quartz, biotite and muscovite. The first generation of metamorphic zircon rims has a 206Pb/238U age of 272.9 ± 2.8 Ma, displays steep heavy rare earth element (HREE) patterns and a moderate negative Eu-anomaly, and returns Ti-in-zircon temperatures of 740–780 °C. The second generation of metamorphic rims has a 206Pb/238U age of 263.8 ± 2.6 Ma, a flat HREE pattern and a pronounced negative Eu-anomaly, and gave Ti-in-zircon temperatures of 780–810 °C. Collectively these observations indicate that the first zircon rim formed on the prograde path at the onset of partial melting where muscovite was present but the rocks contained little garnet and no K-feldspar. Therefore, the metapelites resided at lower amphibolite facies, subsolidus conditions up until the intrusion of a Permian gabbro, which was emplaced shortly after the formation of the first generation of metamorphic zircon and caused heating to granulite facies conditions and widespread partial melting. There is no record of high-pressure metamorphism preceding granulite facies conditions, indicating that felsic lower crust in the Malenco unit was not formed by relamination.The Malenco unit was situated at the north-eastern active margin of Gondwana 420 Ma ago. Subduction-related accretion of sediments at lower crustal levels shortly after their deposition could explain the formation of this felsic lower crust that resided at subsolidus conditions for up to 100 My prior to Permian extension, gabbro intrusion and granulite facies metamorphism.

First identification of Early Cretaceous mafic dikes in the Baingoin area, central Tibet: Implications for crust-mantle interactions and magmatic flare-up

Mafic dikes are generally emplaced in extensional tectonic settings and provide key information regarding deep mantle processes and sources. The Bangong-Nujiang suture zone was formed by the collision of the Qiangtang and Lhasa terranes and experienced intense magmatism during the Early Cretaceous. However, the deep mantle processes and mechanisms involved in this magmatic flare-up (ca. 115 Ma) in the collisional belt remain controversial because of the lack of evidence for coeval mafic magmatism. Here, we present detailed petrological, geochronological, geochemical, and Sr-Nd-Hf-O isotope data for the newly discovered hypersthene-bearing mafic dikes in the Baingoin area in the middle-eastern parts of the Bangong-Nujiang suture zone, central Tibet. Secondary ion mass spectroscopy (SIMS) zircon U-Pb dating showed that the mafic dikes were emplaced during 120−115 Ma. These mafic rocks are characterized by variable MgO contents (2.7−5.2 wt%) and Mg# values (38.5−52.8), slight enrichment in light rare earth elements (REEs; [La/Yb]N = 7.5−8.1), relatively flat heavy REE patterns ([Gd/Yb]N = 1.75−1.84), and negative Eu, Ta, Nb, and Ti anomalies. The dikes also have relatively low initial 87Sr/86Sr ratios (0.7060−0.7062) and negative εNd(t) (−2.2 to −1.6) and positive εHf(t) (+2.5 to +3.6) values, and variable zircon εHf(t) (−2.2 to +7.2) and slightly elevated zircon δ18O (5.6‰−7.0‰) values. These geochemical characteristics indicate that the mafic dikes were derived from an enriched lithospheric mantle source. However, compared with coeval magmatic rocks, the mafic dikes have relatively high εNd(t) and εHf(t) values, indicating that they contain a depleted mantle component. The mafic dikes contain clinopyroxene and orthopyroxene (i.e., hypersthene), indicative of derivation from a high-temperature magma source. Clinopyroxene-melt thermobarometry yielded a temperature range of 1167−1213 °C, further supporting the involvement of a high-temperature asthenospheric component. Therefore, we suggest that the parental magmas of the Nakoulai mafic dikes were probably generated by the interaction between the asthenospheric mantle and overlying metasomatized lithospheric mantle. Combined with data from nearby Cretaceous magmatic rocks and sedimentary rocks, we suggest that the mafic dikes were generated in a postcollisional setting caused by upwelling of asthenospheric mantle owing to slab breakoff beneath the Bangong-Nujiang suture zone. Slab breakoff played a key role in the crust-mantle interactions and the onset of the magmatic flare-up in the middle-eastern parts of the Bangong-Nujiang suture zone.

Major, trace and rare earth element geochemistry of the Permian Lucaogou oil shales, eastern Junggar Basin, NW China: implications for weathering, provenance and tectonic setting

The Permian Lucaogou oil shales in the eastern Junggar Basin have long been regarded as important source rocks and reservoirs for tight oil exploration. The oil shales can also be used to assess the provenance and tectonic setting of the East Junggar region using major, trace and rare earth element geochemistry. The low chemical index of alteration ratios (37.36–64.18, 45.54 on average), low plagioclase index of alteration ratios (26.99–62.95, 42.99 on average), and high index of compositional variability ratios (0.64–1.66, 1.10 on average) suggest that the Permian Lucaogou oil shales mostly consist of immature sediments that have experienced a weak weathering intensity. The samples have low TiO2/Al2O3 and TiO2/Zr ratios that indicate a felsic origin with minor intermediate igneous rocks. The high Th/Sc and Zr/Sc ratios suggest a felsic origin without sediment recycling. The samples show fractionated light rare earth elements and relatively flat heavy rare earth elements patterns with weak negative Eu anomalies, which implies that the main provenance is lower Carboniferous intermediate–acid volcanic rocks. In addition, multiple major- and trace-element‐based discrimination diagrams show that the parent rock of clastic sediments formed mainly in a continental island arc and active continental margin environment, consistent with previous studies on the tectonic background of the East Junggar region during the late early Carboniferous. Key Points Immature sediments are present in the Permian Lucaogou oil shales and have a weak weathering intensity. The provenance of the clastic sediments of the Permian Lucaogou oil shales was predominantly early Carboniferous intermediate–acid volcanic rocks. The clastic sediments of the Permian Lucaogou oil shales were developed in a continental island arc and active continental margin environments. The Permian Lucaogou oil shales provide crucial information for tectonic setting and evolution of the northern Xinjiang during the Carboniferous–Permian.

Exotic REE behaviors of zircon in the Koktokay No. 3 granitic pegmatite, Xinjiang, northwest China

Rare earth elements (REEs) can be incorporated into zircon at significant concentrations that are controlled by the temperature, pressure, and composition of their growth environment. However, the distribution of REEs between zircon and melt does not always conform to the currently available partitioning data. This study presents evidence of exotic REE behaviors in zircon from the marginal zone of the Koktokay No. 3 granitic pegmatite in Xinjiang, northwest China. The behavior of the Koktokay REEs represents by large negative Ce anomalies, flat heavy REE patterns, and lanthanide tetrad effects. Two generations of zircon (Zrn-I and Zrn-II) are identified, which were formed by dissolution and re-precipitation processes. Zrn-I represents relic zircon domains with dissolution textures that formed from pegmatite melt during the late stage of the magmatic–hydrothermal transition. Zrn-II occurs as overgrowths and infilling fractures around or within Zrn-I, which formed by hydrothermal re-precipitation. From Zrn-I to Zrn-II, the zircon exhibits increasingly negative Ce anomalies and greater enrichment in light REEs and other non-stochiometric elements (e.g., Ca, Al, Fe, and Mg). Given the lack of Ce depletion in the host granite, the negative Ce anomalies in zircon can be attributed to Ce being sequestered into hydrothermal minerals (i.e., cerianite and/or Fe–Mn oxyhydroxides) as tetravalent Ce. In addition, the flat heavy REE patterns of both Zrn-I and Zrn-II likely resulted from the crystallization of heavy REE-enriched minerals, such as zircon and garnet, and were further enhanced by hydrothermal alteration. The two generations of zircon display identical M-type lanthanide tetrad effects in REE patterns, with similar TE3 values. This ubiquitous M-type tetrad effect of the REEs in garnet, zircon, and the host granite can be attributed to the exsolution and subsequent interaction with magmatically derived fluids containing F–aquo-complexes. Therefore, we conclude that such anomalous REE incorporation into zircon from a hydrous pegmatitic melt is controlled by the evolution of the hydrous melt and dissolution–re-precipitation processes. Our results suggest that the reliability of zircon REE partitioning data should be revaluated, and that further experimental studies are needed to fully understand the behavior of REEs in hydrothermal systems.

Protracted eclogite‐facies metamorphism of the Dulan area, North Qaidam ultrahigh‐pressure terrane: Insights on zircon growth during continental subduction and collision

AbstractContinental subduction and collision are recorded by ultrahigh‐pressure (UHP) terranes; UHP terranes that form at early stages of an orogeny tend to be small and experience short residence at eclogite‐facies depths, whereas terranes that form at mature stages of an orogeny tend to be larger and experience longer residence at these depths, but accurately determining eclogite‐facies residence time requires a large geochronologic dataset tied to metamorphic conditions (via trace elements and/or inclusions). In the Dulan area, North Qaidam UHP terrane, China, it remains unclear whether the terrane experienced a long residence at eclogite‐facies depths, marking the mature stage of an orogeny or two distinct (ultra)high pressure ([U]HP) events (with short residence times), interpreted as the transition from oceanic subduction to continental collision, where one (U)HP event is related to the former and second (U)HP event to the latter. To address this issue, we report new zircon U–Pb ages and trace‐element data from eclogite and host paragneiss from the Dulan area and show that this terrane records ~42 Myr of eclogite‐facies metamorphism at (U)HP conditions, similar to other large UHP terranes. Zircon from 11 eclogite and 2 gneiss samples yields weighted mean ages of 463–425 Ma, flat heavy rare earth element (HREE) patterns without negative Eu anomalies, and eclogitic mineral inclusions, indicating eclogite‐facies conditions. Paragneiss metamorphic ages overlap with ages from eclogite but are generally younger, suggesting that a lack of internally generated fluids may have inhibited zircon growth and/or recrystallization until early decompression and white mica consumption in felsic gneiss generated fluids; thus, we interpret that these felsic rocks record the later stages of continental collision. Dataset patterns from all new and previously published analyses for the Dulan area (34 eclogite and 14 gneiss) suggest that metamorphic zircon in eclogite records prograde, peak and possibly early retrograde conditions, in contrast to the prediction from mass balance models that metamorphic zircon should only grow during exhumation and cooling. We reconcile our observations with these model predictions by recognizing that differential solubility can lead to grain‐scale zircon growth or recrystallization over a large segment of the pressure–temperature (P–T) path even where zircon abundance decreases at the whole‐rock scale.

Growth of the continental crust induced by slab rollback in subduction zones: Evidence from Middle Jurassic arc andesites in central Tibet

Andesitic rocks have similar compositions to average continental crust and have played a central role in improving our understanding of the formation and evolution of the continental crust. Here we present data for andesites from the Jiaqiong area of the Bangong–Nujiang suture zone (BNSZ), Central Tibet. LA–ICP–MS zircon U–Pb ages demonstrate that these andesites were generated during the Middle Jurassic (169–164 Ma). The Jiaqiong andesites have variable Mg# (molar 100 × Mg/[Mg + Fe] = 35–63) values, and Cr (3.02–175 ppm) and Ni (5.70–47.7 ppm) contents. These andesitic rocks are enriched in light rare earth elements (REEs) ([La/Yb]N = 5.2–14.6) and yield relatively flat heavy REE patterns ([Gd/Yb]N = 1.4–2.1) with negative Ta, Nb, and Ti anomalies. They also have moderately high initial 87Sr/86Sr (0.7077–0.7092) ratios and variable negative εNd(t) (−11.0 to − 3.3) and zircon εHf(t) (−13.7 to + 2.1) values. These geochemical characteristics suggest that the Jiaqiong andesites were most probably produced by partial melting of mélange rocks. Combining our data with the regional geology, particularly that of nearby ophiolites and Jurassic andesitic rocks, we conclude that the formation of these andesites was triggered by asthenospheric upwelling and hot corner flow caused by the rollback of the northward-subducting Bangong–Nujiang Tethyan oceanic lithospheric slab during the Middle Jurassic. The mélanges provided the material for the formation of andesites, and probably make a substantial contribution to the vertical growth of the continental crust in subduction zones.

Discrimination of thermodynamic and kinetic contributions to the heavy rare earth element patterns in metamorphic garnet

AbstractVariations of rare earth element (REE) concentrations in metamorphic garnet are an important source of information of geodynamic and geochemical processes in the deeper Earth. In order to extract this information, the thermodynamic equilibrium and kinetic contributions of the REE uptake in garnet must be distinguished and quantified. Utilizing high‐resolution trace element and μ‐Raman mapping together with combined thermodynamic–geochemical–diffusion models, we demonstrate that the equilibrium and kinetic aspects of the REE uptake in metamorphic garnet can be discriminated by interpreting 2D trace element mapping in a single sample. The heavy (H) REE (Tb to Lu) zoning in the investigated garnet from a high‐pressure blueschist comprises an inner part with an overall decrease from core to inner rim, followed by a concentric zone of HREE enrichment and a drastic HREE decrease towards the outermost rim. The central peak in the garnet core decreases in intensity with decreasing atomic number of the REE. The broad overall shape of this pattern resembles those often observed in metamorphic garnet from different rock types and tectonic settings. Superimposed on this trend is a concentric pattern of minor recurring fluctuations in the HREE concentrations with at least six regularly spaced sets of peaks and troughs along the entire garnet radius. Comparison of the observed inclusion suite, the trace element maps and thermodynamic–geochemical models show that the inner part with decreasing HREE concentrations results from fractional garnet growth in an unchanged mineral assemblage, whereas the REE enrichment zone is caused by the breakdown of titanite. We suggest that the width of the central peak is controlled by the bulk permeability of the interconnected transport matrix and the fraction of matrix minerals that the garnet equilibrates with. The superimposed REE fluctuations result from changing element transport properties of the host rock and mark recurring changes from equilibrium REE uptake to transport‐limited REE uptake in garnet. Such fluctuating element transport properties can be best explained by pulse‐like fluid fluxes that rhythmically change the interconnectivity of the intercrystalline transport matrix. Increasing numbers of published spatially highly resolved REE analyses show that such trace element fluctuations are common in metamorphic garnet indicating that recurring changes in rock permeabilities due to pulsed fluid fluxes are a common phenomenon during metamorphism.

Petrogenesis of the Early Jurassic Longtang and Menglong Peraluminous Granites in Tengchong Terrane, and their Tectonic Implication

AbstractA comprehensive study of zircon U‐Pb geochronology, in situ Hf isotopes, whole‐rock major and trace element geochemistry, and Nd isotopes was carried out for two Early Jurassic two‐mica granites (Longtang and Menglong) in the southern part of the Tengchong terrane, which is in the northern part of the larger Sibumasu terrane. We assess the origin of the granites and explore their possible genetic relationship to the Paleo‐Tethyan regime. LA‐ICP‐MS zircon U‐Pb dating shows that they were simultaneously emplaced in the Early Jurassic (ca. 199 Ma). They have SiO2 contents of 69.7–75.1 wt% and are mainly strongly peraluminous with alumina saturation index (ASI) values ranging from 1.06 to 1.46. They show similar Mg# (0.29–0.42) to experimental partial melts of metasedimentary rocks under continental pressure‐temperature (P‐T) conditions. They are enriched in light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs), with moderately negative Eu anomalies and flat HREEs patterns. They show negative εNd(t) values (–9.0 to –12.4) and εHf(t) values (–8.0 to –9.1). Elemental and isotopic data suggest that they most likely to formed by muscovite‐dehydration melting of a metapelitic source at lower temperatures in the range of 700°C to 750°C. The granites might represent a post‐collisional tectonic setting response to Paleo‐Tethyan regime.

Syn-orogenic tectonomagmatic evolution of the Qilian Orogen: Insights from the Lumanshan gabbro–granite association in the Qilian Block, Northwest China

Gabbro–granite associations provide critical geological clues for unraveling crust–mantle interactions, regional tectonics and geodynamic processes. Many Late Ordovician–Early Devonian granitoids occur in the Qilian Block, display high-K calc-alkaline affinities but have less well-constrained geodynamic mechanisms. Here, we report the newly discovered Late Ordovician-Early Devonian Lumanshan (LMS) gabbro–granite association in the Qilian Block that is composed mainly of syenogranites and alkali-feldspar granites, with minor gabbro. Zircon U Pb dating yields crystallization ages of ca. 440 Ma for the gabbros and ca. 450 Ma and 414 Ma for the syenogranite and alkali-feldspar granite, respectively. The gabbros are calc-alkaline series and display enriched mid-ocean ridge basalt ( E -MORB)-like rare earth element (REE) patterns; U, Cs, and Rb enrichments; Nb, Ta, Zr, and Hf depletions; and depleted Sr–Nd–Hf isotope compositions. The results indicate that the gabbros were possibly derived from partial melting of an asthenospheric mantle wedge metasomatized by aqueous fluids. Two granite types within the LMS pluton are peraluminous and have a transitional geochemical affinity between S- and I-type, low Mg# values, and similarly evolved Sr Nd isotope compositions, which overlap with those of Neoproterozoic basement rocks exposed in the study area. The syenogranites show adakitic geochemical characteristics with high La/Yb and Nb/Ta ratios, whereas the alkali-feldspar granites are non-adakitic granites with flat heavy REE patterns and significant Ba, Sr, and Eu depletions, suggesting that they formed at lower and middle crustal levels, respectively. The LMS ca. 450–440 Ma gabbro and syenogranite were emplaced during a back-arc extensional regime associated with roll-back of the northward-subducting South Qilian Ocean slab. Synthesized data from this and previous studies suggest that the final closure of the South Qilian Ocean and continental collision between the Qilian and Qaidam blocks likely occurred at ca. 435 Ma, and the ca. 414 Ma LMS alkali-feldspar granites generated in syn -collisional setting related with exhumation of the subducted continental crust. The LMS alkali-feldspar granites also show decoupled Nd Hf isotopic systems, which should result from significant disequilibrium melting of metasedimentary materials with the apparent abundance of inherited zircons in their source due to a less intense thermal regime. • A gabbro–granite association in the Qilian Block formed in early–middle Paleozoic. • The ca. 472–435 Ma magmatism formed in an extensional setting related to slab roll-back. • The ca. 435–400 Ma magmatism occurred in syn-collisional setting.

Geochronology and geochemistry of the granitoids in the Diancangshan-Ailaoshan fold belt: Implications on the Neoproterozoic subduction and crustal melting along the southwestern Yangtze Block, South China

The Neoproterozoic magmatic and tectonic history of the South China Craton (SCC) is critical for the reconstruction of Rodinia break-up and subsequent Gondwana assembly. Nonetheless, the tectonic attribution of the Neoproterozoic igneous rocks in the southwestern Yangtze is contentious, and the mechanism for crustal anatexis remains unclear. Here, we report mineralogical, whole-rock geochemical and Sr-Nd isotope data, and zircon U-Pb-Hf-O isotope data from the newly found Neoproterozoic trondhjemites and granites in the Diancangshan-Ailaoshan (DCS-ALS) fold belt along the southwestern Yangtze. Both the trondhjemites (ca. 771–762 Ma) and granites (ca. 767 Ma) have high SiO2 and low MgO contents. The trondhjemites have relatively high CaO, Na2O and Sr contents, but low K2O contents and Rb/Sr ratios, and show positive to slightly negative Eu anomalies and heavy rare earth elements (HREEs) depletions, resulting in high Sr/Y and La/Yb ratios. In contrast, the granites have lower CaO, Na2O, and Sr contents, but higher K2O and Rb/Sr, and display negative Eu anomalies and flat HREEs patterns. The trondhjemites have negative εNd(t) (-4.12 to −3.85), positive to negative εHf(t) (-2.50 to + 5.28) and low δ18O (4.37 ‰–7.27 ‰) values. In comparison, the granites have positive εNd(t) (0.09 to 0.26), εHf(t) (+1.24 to + 9.86) and higher δ18O (6.50 ‰–7.24 ‰) values. The distinct elemental and isotopic compositions between the trondhjemite and granite samples reflect two different types of crustal anataxis within different crustal levels. The trondhjemites were formed by water-fluxed melting of the juvenile mafic lower continental arc crust (with addition of ancient Yangtze basement). The granites were derived from dehydration melting of juvenile felsic middle crust. Our results imply that the trondhjemites and granites were formed in a continental arc setting. The melting processes revealed here should be applicable to decipher the widespread Neoproterozoic crustal melting along the southwestern Yangtze, which was most likely fluid-controlled via subduction-induced asthenosphere upwelling. Combined with available regional geological data, we suggest that a long-lived Neoproterozoic (∼880–730 Ma) subduction zone may have extended from the Panxi-Hanna region in the northern/western Yangtze Block to the DCS-ALS fold belt in the southwestern Yangtze. Thus, the SCC may have located on the northwestern margin of Rodinia.

Formation of the Nephrite Deposit With Five Mineral Assemblage Zones in the Central Western Kunlun Mountains, China

Abstract The Saidikulam nephrite deposit, which is famous for its high-quality white nephrite no matter in history and nowadays, is located in the eastern part of the Western Kunlun Orogen, in the northwestern margin of the Tibetan Plateau, Yutian County, Xinjiang, China. Field investigations and petrographic observations revealed the presence of five mineral assemblage zones from granodiorite to dolomitic marble in the Saidikulam nephrite deposit: slightly epidote-altered granodiorite → epidote-altered diopsidite → nephrite → calcite tremolitite → calcite-bearing dolomitic marble. However, little is known about the relationships among these five zones or the formation age of the deposit. The slightly epidote-altered granodiorite consists of plagioclase, orthoclase, quartz, biotite, and epidote, with accessory apatite, zircon, and allanite. They exhibit slightly negative Eu anomalies (0.69–0.72), with declined light rare earth element (LREE) and flat heavy rare earth element (HREE) patterns. They are also strongly enriched in Rb, Zr, U, and Sm and depleted in Hf, Ti, P, and Nb. The epidote-altered diopsidite close to the granodiorite intrusions is a relatively high-temperature metasomatic rock composed of Fe-rich diopside and actinolite, while the calcite tremolitite adjacent to the dolomitic marble is a relatively low-temperature rock composed of prismatic tremolite and calcite. The main mineral component of the nephrite is tremolite, with minor epidote, chlorite, apatite, calcite, graphite, and allanite. The nephrites have low ∑REE contents (4.38–18.53 ppm), with declined LREE and flat HREE patterns. All of the analyzed nephrite exhibits pronounced negative Eu anomalies (δEu = 0.20–0.82), and they are strongly enriched in Rb, U, and Ti and relatively depleted in Ba and HFSEs. SHRIMP U–Pb dating of zircons from the granodiorite yielded a Concordia age of 453.8 ± 3.7 Ma, which is interpreted as the upper limit of the Saidikulam nephrite formation age. The zircons in the epidotized syenite in the deposit yielded a SHRIMP Concordia U–Pb age of 489.2 ± 4.2 Ma. These two ages indicate that the dolomitic marble experienced at least two thermal events. The approximate ages of the nephrite deposits along the giant Kunlun-Altyn Tagh belt indicate that the closure of the Proto-Tethys may have been the predominant driving force for the formation of the whole nephrite deposits. The multistage metasomatism and the Fe absorption of epidote-altered diopsidite had a positive impact on the formation of high-quality nephrite.

Development of Open Transport of Aqueous Fluid from Pegmatite Revealed by Trace Elements in Garnet

We investigated the fluid flow and elemental transport from a granitic body to the middle crust by determining the trace element compositions of garnet in pegmatites related to a quartz diorite intrusion and metamorphic rocks on Kinkasan Island, northeast Japan. Garnet in the pegmatites and biotite schists is characterized by spessartine– (Sps–) almandine- (Alm-) rich compositions of Sps14–69Alm22–70Prp2–14Grs1–13 and Sps16–30Alm54–66Prp9–16Grs3–6, respectively. A garnetite pod in the metamorphic unit has grossular- (Grs-) rich compositions (Sps1–4Alm8–11Prp0.1–0.4Grs80–87Adr3–4). The peak temperature ( T ) and pressure ( P ) conditions of the biotite schist during contact metamorphism were 600–650°C and 0.27–0.41 GPa, respectively. The primary fluid inclusions in quartz crystals within the pegmatites hosted by the quartz diorite and hosted by the metamorphic rocks have a wide range of homogenization temperatures (200–380°C). These correspond to the trapping temperature of 500–700°C, assuming a salinity of 4 wt.% NaClequivalent at pressure of the crystallization of the quartz diorite. Chondrite-normalized rare earth element (REE) patterns of garnets in the pegmatites in the quartz diorite and metamorphic unit are generally characterized by enrichment of heavy REEs and negative Eu anomalies with the REE contents in the schists which are systematically lower than in the pegmatites. However, garnet in the biotite schists close to the pegmatites has similar REE contents to garnet in the adjacent pegmatites. These geochemical features suggest that garnet in the biotite schists grew in response to fluid infiltration from the pegmatites. Besides, the Grs-rich garnet in the garnetite pod and its host quartz schist have flat heavy REE patterns and no Eu anomalies, which probably reflect a metasomatic process related to plagioclase replacement that produced Ca-Al-rich fluids. Our results suggest that the infiltration of pegmatitic fluids enhances elemental transport and metamorphic reactions in the middle crust.