Zircon eHf Research Articles

Petrogenesis of the late Mesozoic Bashan complex in the Lower Yangtze River Belt, eastern China: Implications for the definition and significance of A-type granite

Many late Mesozoic granites within the Lower Yangtze River Belt (LYRB), eastern China, are defined as A-types based on their geochemical characteristics, although their petrogenesis is unclear. We present an integrated study combining geochronology, whole-rock geochemistry, Sr, Nd, Pb and zircon Hf isotopes and mineral chemistry for a lithologically-diverse suite of A-type granitoids at Bashan. The Bashan complex in the Chizhou area is composed of quartz monzonite, quartz syenite, syenogranite and alkali-feldspar granite. Zircon UPb geochronology indicates that these intrusive rocks formed at 126–123 Ma. The quartz monzonites have intermediate SiO2 (60.5–63.1 wt%) and high Na2O + K2O (8.66–9.83 wt%) contents, arc-like trace element compositions, enriched whole-rock Sr, Nd and zircon Hf isotopes ((87Sr/86Sr)i = 0.7082–0.7091; εNd(t) = −6.9 to −7.1; εHf(t) = −5.3 to −8.2) and high radiogenic Pb isotopes (206Pb/204Pb(t) = 18.581–18.792). They are inferred to have been derived from partial melting of an enriched lithospheric mantle source, followed by fractional crystallisation and limited crustal contamination. The quartz syenites show high SiO2 (65.9–69.8 wt%) and Na2O + K2O (11.3–12.3 wt%) contents, low MgO (0.14–0.23 wt%) contents, high 104 * Ga/Al (2.34–3.61) values, arc-like trace element compositions, and whole-rock Nd and Pb and zircon Hf isotopic compositions (εNd(t) = −6.8 to −7.0; εHf(t) = −7.3 to −12.3; 206Pb/204Pb(t) = 18.559–18.970) similar to the quartz monzonite, suggesting that they were derived through fractional crystallisation of the quartz monzonite, with some crustal assimilation. The quartz monzonite has higher Ti-in-zircon temperatures (TTi-in-Zrn = 696–832 °C) and lower zircon saturation temperatures (TZr = 772–818 °C) and oxygen fugacities (ΔFMQ = +1.8 to +2.8) than the quartz syenite (TTi-in-Zrn = 623–805 °C; TZr = 856–909 °C; ΔFMQ = +3.5 to +4.8), indicating that the zircons from the quartz syenite crystallised at lower temperatures, and that the oxygen fugacity significantly elevated as the magmatic temperature decreases. The syenogranites display high SiO2 (74.6–75.5 wt%) contents, high TTi-in-Zrn (671–871 °C), TZr (799–822 °C), and low oxygen fugacities (ΔFMQ = +0.9 to +3.7), and thus did not evolve from the quartz syenite. They show high 104 * Ga/Al values (2.67–2.95), low MgO (0.1–0.17 wt%) contents and (La/Yb)N (7.60–10.19) ratios, pronounced negative Eu anomalies (Eu/Eu* = 0.28–0.38) and enriched Sr, Nd and zircon Hf isotopic compositions (εNd(t) = −7.2 to −7.5; εHf(t) = −5.1 to −14.0), indicating that they were derived from fluid-absent partial melting of Neoproterozoic calc-alkaline igneous rocks under low-pressure–high-temperature conditions. The alkali-feldspar granites are characterised by high SiO2 (76.5–78.0 wt%) and Na2O + K2O (8.34–9.02 wt%) contents, but low MgO (0.03–0.08 wt%) contents. They show less enriched Nd isotopes (εNd(t) = −5.7), a wide range of zircon εHf(t) values (−1.9 to −11.2) and high oxygen fugacities (ΔFMQ = +2.9 to +4.3). They were produced by mixing of highly-fractionated alkaline basaltic magmas and melts derived from partial melting of Mesoproterozoic crust. The inferred petrogenesis of the Bashan complex magmatic rocks suggests that the A-type granites within the LYRB have multiple magmatic sources and crystallised under a wide range of temperatures, water contents and oxygen fugacities. During the Mesozoic, subduction and rollback of the Palaeo-Pacific slab induced crustal extension and intense crust–mantle interaction, which played a dominant role in the formation of the A-type granites within the LYRB.

Discovery of multi-crustal rejuvenations for the formation of the Lincang granitic batholith, Southwest China: magmatism relating to Changning–Menglian Paleo–Tethyan termination

ABSTRACT The consumption of the Changning–Menglian Palaeo-Tethys ocean in the Triassic formed the Lincang granitic batholith in the southwestern China, once the oceanic supra-subduction zone. The batholith provides us a critical window to understand the nature of the crustal basements and the process of Palaeo-Tethyan termination. Here we report zircon U-Pb ages and Hf isotopes, and whole-rock elements and Sr-Nd isotopes of granitic samples from the central part of the Lincang batholith. The geochemical results reveal two genetic types of granites indicated by low-K and high-K affinities, respectively. The Group 1 low-K granitoids dated at 237–229 Ma are low-K calc-alkaline and metaluminous to peraluminous (A/CNK = 0.75–1.29), with K2O contents of 0.13–1.33 wt%, K2O/Na2O ratios of 0.03–0.30, Rb/Sr ratios < 0.18, and initial 87Sr/86Sr = 0.71693 and εNd(t) = −10.8. In contrast, the Group 2 high-K granitoids dated at 239–200 Ma are calc-alkaline and peraluminous (A/CNK = 1.02–1.20), with higher K2O contents (3.15–4.7 wt%), K2O/Na2O ratios (1.15–2.00) and Rb/Sr ratios (0.77–2.62), and more enriched isotopic compositions (initial 87Sr/86Sr = 0.73129 to 0.74037, εNd(t) = −13.1 to −11.0) as well as highly negative zircon εHf(t) values of −16.8 to −6.7. The formation time of these granitoids is compatible with that of the post-collision of the Palaeo-Tethys, subsequently to the magmatic gap (250–240 Ma) coupling with the syn-collision stage. The magmatic source discriminations suggest that the low-K granitoids were derived from partial melting of the dehydrated low-K amphibolite source and the high-K granitoids were dominantly from the psammitic source with minor mantle-derived magma involvement. Using the geochemical variations of the granitic rocks related to the Palaeo-Tethyan closure, we reveal melting of multiple crustal sources rather than a sole source to construct the Lincang batholith.

Late Jurassic–Early Cretaceous irregular slab rollback of paleo-Pacific plate beneath southeastern China: Insights from the petrogenesis of volcanic rocks of Moshishan Group in Dazhou volcanic basin, Gan-Hang Belt

The geodynamic model of the paleo-Pacific plate subduction beneath southeastern China during the Late Mesozoic, though generally accepted, remains controversial with respect to the exact plate subduction model. The Moshishan Group volcanic strata are well developed in the Dazhou volcanic basin in the Gan-Hang Belt (GHB). In this study, detailed investigations on geochronology, elemental geochemistry, and isotopic compositions of these volcanic rocks were performed to constrain their formation ages and petrogenesis as well as the tectonomagmatic evolution of the GHB. LA-ICP-MS zircon UPb dating results show that the volcanic eruptions in the Dazhou volcanic basin began approximately 150.9 ± 2.7 Ma to 128.8 ± 1.9 Ma and lasted for ~20 Myr from the Late Jurassic to Early Cretaceous. These volcanic rocks show high Ga/Al ratio, high K2O + Na2O and Zr + Nb + Ce + Y contents, and high formation temperature (~812–1022 °C), indicating A-type granite affinity. Such types of A-type volcanic rocks formed in an extensional tectonic environment, likely corresponding to a back-arc extension due to the slab rollback of the paleo-Pacific plate. The εNd(t) values of the volcanic rocks of Moshishan Group vary from −11.60 to −5.41, and the zircon εHf(t) values vary from −18.30 to −3.84, indicating that these volcanic rocks were mainly derived from the partial melting of para-metamorphic rocks. Different proportions of mantle-derived materials were also involved in their sources, such as little or no addition of mantle-derived material in the early Dashuang Formation. However, a significantly increased proportion of mantle-derived material addition was found in the later Gaowu Formation, Xishantou Formation, and Jiuliping Formation. We found that the volcanic rocks in the southwestern GHB were formed during the Early Cretaceous (137–132 Ma), while those in the northeastern GHB formed during the Late Jurassic to Early Cretaceous (150–120 Ma). Jurassic calc-alkaline granites (181–151 Ma) that formed in a continental arc environment related to the paleo-Pacific plate subduction are also distributed along the GHB. This indicates that the shift from an arc environment dominated by the subduction of the paleo-Pacific plate to a back-arc extensional environment characterized by slab rollback began at ~150 Ma in the GHB. We propose an irregular slab rollback model beneath southeastern China to interpret the distribution pattern of the volcanic basins in the GHB. The slab rollback in the GHB did not initiate simultaneously, or with homogeneous velocity, throughout the region. Slab rollback in the southwestern part of the GHB occurred between 137 and 132 Ma, whereas in the northeastern part, rollback occurred between 150 and 120 Ma. In addition, the mantle-crust interaction that produced A-type granitic magmas along the northeastern part of the GHB gradually intensified from the early to late stage between ~150 and ~ 120 Ma.

Ore-hosting igneous rocks in the Xiahe–Hezuo district, West Qinling orogen, China, and their relationships with gold mineralization

• The Gulangdi Formation in West Qinling orogen (WQO) was formed prior to Triassic. • Gold-bearing igneous rocks in WQO were crystallized prior to gold mineralization . • Gold-bearing igneous rocks were generated by subduction of the Mian–Lue Ocean. • The Mian–Lue Ocean started the subduction process since ca. 251 Ma. • The gold metallogeny in WQO postdates regional magmatism by 40 to 20 m.y. The Xiahe–Hezuo district in the West Qinling orogen hosts a large number of gold deposits, most of which are spatially associated with igneous rocks. However, the nature and origin of these igneous rocks and their possible relation with gold mineralization remain poorly understood. In this study, the ore-hosting igneous rocks from the Zaozigou, Yidi’nan, and Gangcha gold deposits are selected to perform geochronological and geochemical analyses, aiming to clarify their petrogenesis and their role in gold mineralization. The quartz diorite porphyry from the Zaozigou gold deposit, the quartz diorite from the Yidi’nan gold deposit, and the andesite from the Gangcha gold deposit yield LA-ICP-MS zircon U-Pb ages of 250 ± 2, 251 ± 1, and 247 ± 1 Ma, respectively, representing the earliest Triassic magmatic rocks in the West Qinling orogen. The quartz diorite porphyry and quartz diorite display relatively high ( 87 Sr/ 86 Sr) i ratios (0.707188–0.710693), negative ε Nd (t) (−11.7 to −6.2) and zircon ε Hf (t) values (−13.0 to −1.6), and ancient two-stage Nd and Hf model ages (2073–1357 Ma), indicating they were generated by partial melting of ancient crustal materials. The andesite exhibits slightly positive ε Nd (t) values (0.8–1.8) and negative ε Hf (t) values (−20.6 to −2.2), indicating it was derived by partial melting of a metasomatized lithospheric mantle with significant crustal contamination. All of these igneous rocks show enrichments in large ion lithophile and light rare earth elements, depletions in high field strength elements, and high Mg # values (>50), indicating their generation in a subduction-related magmatic arc. These imply that the northward subduction of the Mian–Lue Ocean beneath the West Qinling orogen may had already started since the earliest Triassic. The geological and geochronological evidences suggest these igneous rocks were not genetically associated with the ore-forming process, and they merely provided the space for gold deposition.

Early Cretaceous crust–mantle interaction linked to rollback of the Palaeo-Pacific flat-subducting slab: constraints from the intermediate–felsic volcanic rocks of the northern Great Xing’an Range, NE China

AbstractLate Mesozoic igneous rocks are important for deciphering the Mesozoic tectonic setting of NE China. In this paper, we present whole-rock geochemical data, zircon U–Pb ages and Lu–Hf isotope data for Early Cretaceous volcanic rocks from the Tulihe area of the northern Great Xing’an Range (GXR), with the aim of evaluating the petrogenesis and genetic relationships of these rocks, inferring crust–mantle interactions and better constraining extension-related geodynamic processes in the GXR. Zircon U–Pb ages indicate that the rhyolites and trachytic volcanic rocks formed during late Early Cretaceous time (c. 130–126 Ma). Geochemically, the highly fractionated I-type rhyolites exhibit high-K calc-alkaline, metaluminous to weakly peraluminous characteristics. They are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) but depleted in high-field-strength elements (HFSEs), with their magmatic zircons ϵHf(t) values ranging from +4.1 to +9.0. These features suggest that the rhyolites were derived from the partial melting of a dominantly juvenile, K-rich basaltic lower crust. The trachytic volcanic rocks are high-K calc-alkaline series and exhibit metaluminous characteristics. They have a wide range of zircon ϵHf(t) values (−17.8 to +12.9), indicating that these trachytic volcanic rocks originated from a dominantly lithospheric-mantle source with the involvement of asthenospheric mantle materials, and subsequently underwent extensive assimilation and fractional crystallization processes. Combining our results and the spatiotemporal migration of the late Early Cretaceous magmatic events, we propose that intense Early Cretaceous crust–mantle interaction took place within the northern GXR, and possibly the whole of NE China, and that it was related to the upwelling of asthenospheric mantle induced by rollback of the Palaeo-Pacific flat-subducting slab.

Age constraints on the genesis of the Changkeng tungsten deposit, Nanling region, South China

Abstract The Changkeng quartz vein tungsten deposit is located in the eastern part of the Helong ore field, Nanling region, South China. It has a proven WO3 reserves of 9680 tons WO3, and an average grade of 2.98%. It contains 28 ore-bearing quartz veins, and the mineralization stages are divided as follows: (I) quartz-wolframite-cassiterite-scheelite stage; (II) quartz-wolframite-scheelite-molybdenite-pyrite-sphalerite-galena-flourite stage; (III) quartz-calcite-flourite stage. The intrusion within the deposit is concealed, and comprising medium- and coarse-grained biotite monzogranite. Zircon grains from the Changkeng pluton have a U-Pb age of 156.7 ± 0.54 Ma (MSWD = 2.9). Cassiterite grains from stage I yield a weighted mean 206Pb/238U age of 156.1 ± 3.4 Ma (MSWD = 3.4). Molybdenite separates from stage II have a well-defined 187Re/187Os isochron age of 156.9 ± 2.0 Ma (MSWD = 0.77), with model ages ranging from 155.6 ± 2.5 to 157.5 ± 2.2 Ma. The zircon eHf(t) values range from −14.1 to −11.1, and the Hf two-stage model ages range from 1.87 to 2.06 Ga, indicating a primary reworking of the Paleoproterozoic crust. The low rhenium contents (0.3238 to 8.763 μg/g) in molybdenite suggest that the ore-forming materials was derived from a crustal source. The age consistence between the intrusion and mineralization, together with the Hf and Re isotope signature, all indicate a close genetic link between magmatism and mineralization. The Changkeng deposit is an example of a Jurassic regional-scale tungsten-tin ore-forming event between 160 and 150 Ma in the Nanling region, South China.

Insights into OIB-like magmatism contemporaneous with oceanic subduction: Petrogenetic constraints on the Kendelong metagabbro in the North Qaidam

Compared with the statement of the mantle plume hypothesis for large-scale plus high-volume intraplate magmatism, a series of OIB-like mafic rocks which are low-volume and spatially closely related to subduction zones still remain mysterious and inapposite to be explained by using traditional mantle plume hypothesis. In the North Qaidam, most previous studies have focused on ultra-high pressure metamorphic rocks and have paid little attention to mafic magmatism as a mechanism of generating magma with a mantle signature. The Kendelong (KDL) metagabbro with consistently intrusive ages of 485–490 Ma within the Olongbuluke microblock that is overriding plate above the subduction zone is analogous to Oceanic Island Basalts (OIBs) in geochemistry and formed contemporaneously with oceanic subduction. Analyzed samples of the KDL metagabbro have sporadic (87Sr/86Sr)i ratios of 0.7008–0.7063 that have been influenced by paulopost alteration, relatively uniform positive εNd(t) values of +0.3 to +1.4, zircon εHf(t) values of +1.5 to +6.8, and coupled Nd–Hf isotopes that lie on the terrestrial array. After ruling out the possibility of contributions from subducted enriched components and mantle metasomatites, geochemical features and isotopic compositions indicate that the magmas were derived from a relatively primitive asthenosphere without prior large-volume melt extraction. Rare earth element modeling shows that a low degree of partial melting (<10%) of an asthenospheric mantle source in the garnet stability field should be responsible for the magmatic origin of the KDL OIB-like metagabbro. Considering the temporal and spatial correlations between the KDL metagabbro and other types of mafic rock in adjacent regions, we propose that decompression melting of upwelling asthenosphere triggered by rollback of subducted oceanic slab is a plausible regime to explain the petrogenesis of the early Paleozoic KDL OIB-like metagabbro and to trace a fortuitous tectonic transition event. Moreover, even if the OIB-like magmatism is spatially related to subduction zones, the participation of subduction materials could not be an exclusive prerequisite to sentence its genesis.

Early Cretaceous arc-related volcanic rocks in the northern Great Xing’an Range, NE China: records of Paleo-Pacific ocean subduction

There are widespread Late Mesozoic volcanic rocks in the Great Xing’an Range, Northeast China. However, the precise age data and tectonic evolution are still controversial. In this paper, a suite of intermediate-felsic volcanic rocks and mafic volcanic rocks have been recognized in the Daheishan of the northern Great Xing’an Range. Zircon U–Pb dating constrains two groups of the crystallization ages at 129 Ma (dacite) and 124–127 Ma (rhyolite, rhyodacite and andesite), respectively. They all have magmatic zircon geochemical characteristics, including systematic positive Ce anomalies, weak negative Eu anomalies, and high Th/U ratios (average 0.84). The intermediate-felsic volcanic rocks show crustal signatures with high K2O (1.91–7.57 wt% and low MgO (0.19–2.78 wt%) contents. The zircon eHf(t) values mostly range from + 4.7 to + 10.2, with dominant two-stage model ages (TDM2) of 530–878 Ma. Therefore, combined with previous data, the mantle-derived mafic melts may have triggered partial melting of the juvenile lower crust to produce intermediate-felsic magmas, and the source rock is considered to be an amphibolite. In addition, the trace element data of the volcanic rocks display arc-like signatures. The published and our geochronological data indicate that these Late Mesozoic volcanic rocks in the Great Xing’an Range were mainly formed in the Early Cretaceous with a peak range of 124–133 Ma. The large-scale Early Cretaceous volcanic rocks in the Great Xing’an Range were formed in an active continental margin setting related to the subduction of the Paleo-Pacific Ocean.

Reappraisal of the Mesozoic tectonic transition from the Paleo-Tethyan to Paleo-Pacific domains in South China

Abstract The southeastern (SE) South China Block was mainly influenced by the Paleo-Tethyan and Paleo-Pacific dynamic domains during the Mesozoic. The initial timing of the tectonic transition between these two domains in the SE South China Block still remains debated. The transition would affect the nature of the lithosphere and material provenance of sediments, and, therefore, igneous and sedimentary rocks in the area could record such dynamic processes. In this study, published geochronological and geochemical data of the Triassic and Jurassic igneous rocks and detrital zircon data of contemporaneous sedimentary rocks in the SE South China Block were compiled, aiming to provide constraints on the tectonic transition via tracing the spatial-temporal variations in the nature of the lithosphere and sedimentary provenance signals. The compiled results suggest that the magmatic intensity and volume decreased significantly from the Late Triassic to Early–Middle Jurassic, with an obvious magmatic quiescence between them, and increased from the Early–Middle Jurassic to Late Jurassic. The εNd(t) and zircon εHf(t) values of mafic rocks, granitoids, and shoshonitic rocks remarkably increased from the Late Triassic to Early–Middle Jurassic, indicative of variations in the lithospheric mantle and continental crust. Such variations suggest that the initial tectonic transition occurred at the earliest Early Jurassic. Based on the southward paleocurrents from Early Jurassic sandstone, E-W–trending extension of Early–Middle Jurassic mafic and shoshonitic rocks, and similar sedimentary provenances of Late Triassic and Early–Middle Jurassic sedimentary rocks, these features imply that the SE South China Block was not immediately influenced by the Paleo-Pacific domain during the Early–Middle Jurassic. However, from the Early–Middle Jurassic to Late Jurassic and Early Cretaceous, the spatial distribution, geochemical signatures, magmatic intensity, and magmatic volume of igneous rocks and provenance of sedimentary rocks exhibit obvious variations, and the regional fold hinge direction changed from E-W–trending to NE-trending, suggesting significant effects from Paleo-Pacific subduction on the SE South China Block. Thus, the Mesozoic tectonic transition from the Paleo-Tethyan to the Paleo-Pacific dynamic domain in the SE South China Block likely occurred during the Early–Middle Jurassic.

Bimodal volcanic rocks in the northeastern margin of the Yangtze Block: Response to breakup of Rodinia supercontinent

Mid-Neoproterozoic protoliths of high–ultrahigh-pressure metamorphic rocks in the northeastern margin of the Yangtze Block preserve one of the best examples of 18O-depleted magmatic provinces worldwide, which is the significant record for lithospheric extension associated with continental rifting. In this contribution, we report the depleted 18O signature of felsic meta-volcanic rocks from the Zhangbaling Uplift (ZBLU), a segment of the Tan–Lu fault zone, which is located between the Dabie and Sulu orogenic belts. The low-δ18O signature of these rocks provides crucial information for understanding their origin and tectonic setting. Zircons yield UPb protolith ages of 771 ± 5 Ma to 761 ± 5 Ma and show a narrow range in δ18O values of 1.97‰ to 3.11‰. The typical syn-magmatic characteristics of these zircons indicate that they record inheritance from low-δ18O magmas during the mid-Neoproterozoic, which suggests the occurrence of high-temperature surface-water–rock interaction in a continental rift setting. Quartz keratophyres in the ZBLU exhibit A-type granite geochemical compositions with high contents of high-field-strength elements (Zr + Nb + Ce + Y > 350 ppm) and a zircon saturation temperature of 829 °C. The zircon grains have slightly depleted εHf(t) values of −3.4 to +1.9, which suggest Mesoproterozoic juvenile crustal formation in response to the amalgamation of the Yangtze Block with the Rodinia supercontinent. A few coeval spilites (758 Ma) crop out sporadically in the ZBLU. Their geochemical characteristics include high SiO2 contents (51.06–57.03 wt%), low Mg# values (42–53), and arc-like trace-element distribution patterns. The variable zircon εHf(t) values for the spilite samples can be divided into two groups: negative εHf(761 Ma) values of −16.4 to −11.1, and positive εHf(744 Ma) values of +5.9 to +8.5, which supports source mixing between enriched mantle and residual oceanic crustal material. In conclusion, the ZBLU spilite–keratophyre sequence is composed of a typical bimodal rock association, providing new petrological evidence for the origin of low-δ18O magmas and for a continental rift setting related to the separation of the Yangtze Block from the Rodinia supercontinent at ca. 771 Ma.

Provenance and Hf isotopic variation of Precambrian detrital zircons from the Qilian Orogenic Belt, NW China: Evidence to the transition from breakup of Columbia to the assembly of Rodinia

The Qilian Orogenic Belt (QOB) in Northwest China consists of three distinct terranes: North Qilian, Central Qilian, and South Qilian, which have experienced complex rifting and converging. Here, we present new U-Pb ages and Hf isotopes of detrital zircons from the Neoproterozoic and Mesoproterozoic sedimentary rocks in the QOB to constrain the tectonic affinity of the different terranes and provide new evidence to the transition from Columbia to Rodinia. Two Mesoproterozoic zircon samples from the North and the Central Qilian display similar age spectra with a dominant population varying from 1.21 Ga to 1.78 Ga. The population show two age peaks at ~1.5 Ga and ~1.75 Ga and have same range of εHf(t) values from −9.1 to +9.8. Except for this population, the rest zircons yield Paleoproterozoic or even older ages with a peak at ~2.45 Ga. These results suggest that the three terranes were once of a united block in the late Mesoproterozoic. The two Neoproterozoic samples from the Central Qilian and the South Qilian both have a main population of ages at 0.85–1.0 Ga and a minor population peaked at ~2.45 Ga. Their zircon εHf(t) values indicated that the source rocks of the main population were dominantly composed of recycled crustal materials. Because of the comparable and homogeneous age patterns and Hf isotopic compositions of Proterozoic detrital zircons, the QOB has a close relationship to South China from Mesoproterozoic to Early Neoproterozoic. Our zircon ages and Hf isotopic data indicate that the juvenile components increased at 1.6–1.2 Ga, which response to an exterior orogen that newly-formed crust has replaced the lower crust possibly triggered by slab retreat or upwelling of asthenosphere materials related to the breakup of Columbia. In contrast, the abundant addition of the reworking materials at 1.2–0.8 Ga reacts to an interior orogen characteristic because continental collision was involved in the convergent of Rodinia. This changing, therefore, was a response to the transition from breakup of Columbia to the assembly of Rodinia.

Triassic igneous activities in the east flank of the East Kunlun orogenic belt: the Daheba complex example

ABSTRACT The Triassic igneous complexes in the east flank of the East Kunlun orogenic belt (E-KOB) provide a window through which to investigate the Mesozoic magmatism in the E-KOB. Here, we report the precise timing of Triassic magmatism and magma petrogenesis of different rock types in the east flank of the E-KOB based on the petrology, mineral chemistry, geochemistry, zircon geochronology and Lu-Hf isotopes. Zircon U-Pb dating reveals that the Daheba complex was crystallized in 242–239 Ma. The rocks display enrichment in large-ion lithophile elements (LILEs) and LREEs and depletion in high field strength elements (HFSEs) and HREEs, which are typical of subduction-related magmatic rocks. The fine-grained granites (FGs) are characterized by a high SiO2 content (72.25–74.28 wt. %), a high differentiation index (DI = 95) and marked depletion in Sr, Ba and Eu, which indicates that the FGs are highly fractionated I-type granitoids. The zircon εHf(t) values from −3.6 to −1.1 with two-stage model ages (TDM2) of 1496–1340 Ma, which suggests that they were derived from an ancient Mesoproterozoic lower crust. The similarity in the REE patterns of the mafic microgranular enclaves (MMEs) and host granites (HGs) indicates that the mixing between mafic and felsic magmas could have contributed to the chemical variation of the MMEs. The mafic melts were derived from the partial melting of the mantle that had been metasomatized by slab-derived fluids. The MMEs show typical igneous textures, acicular apatites, sieve texture of plagioclase and reverse zoning of plagioclase, which correspond to magma mixing during petrogenesis. Our geochemical and isotopic data suggest that the Daheba complex formed in an active continental margin setting closely related to the northward subduction of the Palaeo-Tethys Ocean Plate beneath the E-KOB.

Mineralization of the Daolundaba Cu–Sn–W–Ag deposit in the southern Great Xing'an Range, China: Constraints from geochronology, geochemistry, and Hf isotope

Abstract The Daolundaba deposit, located in the southern Great Xing'an Range, is a medium-sized Cu–Sn–W–Ag deposit dominated by Cu. The intrusive rocks in the ore district mainly include the Early Permian Qianjinchang medium-grained biotite monzogranite and the Early Cretaceous Zhangjiayingzi fine-grained granite and porphyritic granite. Cassiterite U–Pb dating for two Cu–W–Sn ore samples yielded ages of 134.7 Ma and 136.8 Ma, respectively. Zircon U–Pb dating for fine-grained granite and porphyritic granite of the Zhangjiayingzi intrusion yielded ages of 134–136 Ma and 135 Ma, respectively. The Zhangjiayingzi intrusion is high-K calc-alkaline and featured by enrichment of Rb, Th, U, K, Nd, Zr, Hf, and Sm and steady depletion of Ba, Sr, P, Eu, and Ti, with extremely negative Eu anomalies and noticeable tetrad effect of rare earth elements, indicating the intrusion has a highly fractionated granite affinity. The Zhangjiayingzi intrusion has high zircon eHf(t) values (4.1–10.2) and relatively young zircon Hf two-stage model ages (541–929 Ma), suggesting that it dominated originated from the partial melting of a Neoproterozoic juvenile lower crust. The geochronology and geochemistry indicate that the mineralization of the Daolundaba deposit is closely related to Early Cretaceous highly fractionated granites. Fractional crystallization of granitic magmas is the important controlling factor for the formation of the Zhangjiayingzi highly fractionated granite and the Daolundaba Cu–W–Sn–Ag deposit.

Neoproterozoic bimodal magmatism in the eastern Himalayan orogen: Tectonic implications for the Rodinia supercontinent evolution

Neoproterozoic magmatism associated with the assembly and configuration of the Rodinia supercontinent is widely distributed in the India-Himalayan terrane. However, its petrogenesis and tectonic settings remain controversial. This study provides new geochronological and geochemical data on the Neoproterozoic bimodal magmatism from the eastern Himalayan orogen. In situ zircon UPb dating revealed that the protoliths of amphibolites were emplaced at ca. 826 Ma and the granitic gneisses have crystallization ages of 825–820 Ma. The granitic gneisses exhibit geochemical features of A-type granites, with high initial (87Sr/86Sr)i ratio (0.7182–0.7394), low whole-rock εNd(t) (−8.4 to −6.6), and variable zircon εHf(t) (−7.4 to +1.0) values. They were probably generated by partial melting of the ancient lower crust with minor input of mantle components. The amphibolite samples are enriched in light rare earth elements (LREEs) and depleted in heavy rare earth elements (HREEs), suggesting an arc affinity. They have relatively high initial (87Sr/86Sr)i ratios (0.7113–0.7136), low whole-rock εNd(t) (−1.1 to 1.4) and a wide range of zircon εHf(t) (−4.1 to 8.3) values, indicating that the protoliths of amphibolites were likely generated by partial melting of an enriched subduction-modified continental lithospheric mantle. Their geochemical signatures are similar to typical back-arc basin basalts. The presence of coeval A-type granites and arc-related mafic rocks is probably due to the existence of a back-arc system. We argue that the Neoproterozoic bimodal magmatism is a product of back-arc extension initiated at an early stage, resulting from the rollback of the Mozambique Oceanic slab. Combined with previous studies on Neoproterozoic magmas from India and the Himalayas, we suggest that an extensive Neoproterozoic back-arc system may have existed along the northwestern margin of the Rodinia supercontinent. This theory supports a scenario of an Andean-type continental margin for the India-Himalayan terrane during the middle Neoproterozoic.

Timing of formation and tectonic setting of Paleozoic granitoids in the eastern Mongol–Okhotsk Belt: Constraints from geochemical, U[sbnd]Pb, and Hf isotope data

The Mongol–Okhotsk Belt is one of the major structural elements in East Asia. However, many aspects of the evolution of this structure from the oceanic domain to the suture zone remain unresolved. The most controversial of these aspects are the timing and polarity of subduction processes as well as the timing of the final closure of the Mongol–Okhotsk Ocean. Here, we present whole-rock geochemical and UPb geochronological data for granitoids in the eastern Mongol–Okhotsk Belt to constrain their timing of formation and tectonic setting. Our new data suggest three stages of granitic magmatism in the eastern part of the Mongol–Okhotsk Belt: Cambrian (ca. 511 Ma), Middle Devonian (ca. 393 Ma), and Late Permian–Early Triassic (257–250 Ma). Cambrian (511 ± 4 Ma) plagiogranites in the Galam Terrane have adakitic geochemical signatures, and εHf(t) values of zircons from these granites vary mostly from +8.8 to +14.8 with Hf model ages (tHf(DM)) ranging from 0.77 to 0.53 Ga. These plagiogranites most likely represent the roots of an island arc that formed above a NW-dipping (present-day coordinates) subduction zone beneath the edge of the Siberian Craton. Middle Devonian (393 ± 7 Ma) granites from the Selemdzha Terrane, as well as Late Permian–Early Triassic (257 ± 4, 253 ± 3, 253 ± 2, 252 ± 3, and 250 ± 4 Ma) granodiorites and granites from the Selemdzha and Tokur terranes, have supra-subduction whole-rock geochemical signatures. Slight negative and positive zircon εHf(t) values of −6.1 to +8.4, tHf(DM) ages of 1.1–0.6 Ga, and tHf(C) ages of 1.4–0.6 Ga suggest that the studied rocks contain a continental crustal component. The Middle Devonian and late Permian–Early Triassic granodiorites and granites formed either in a continental island arc or on an active continental margin. During the Middle Devonian and at the late Permian–Early Triassic boundary, the Mongol–Okhotsk oceanic crust was subducting beneath the edge of the Siberian Craton. Thus, middle Cambrian, Middle Devonian, and Permian–Triassic granitoids in the eastern Mongol–Okhotsk Belt reflect three stages of subduction, meaning that the accretionary wedge terrane assemblages started to form during the Cambrian then continued during the Devonian and at the Permian–Triassic boundary, but was not started at Jurassic as previously proposed.

Evolution of the Tethyan Bangong-Nujiang Ocean and its SE Asian connection: Perspective from the Early Cretaceous high-Mg granitoids in SW China

High-Mg diorites (HMDs) have important implications on crust-mantle interactions and subduction processes, and are widely used to delineate ancient convergent plate margins. In this study, we described a newly-identified high-Mg diorite-granodiorite suite in the Tengchong terrane (SE Tibetan Plateau), and presented its petrography, whole-rock geochemistry, SrNd isotopes, and zircon U-Pb-Hf-O isotopic compositions. High-precision SIMS zircon UPb dating yielded consistent Early Cretaceous ages (~120 Ma) for all lithologies in this high-Mg suite. The high-Mg diorite is characterized by high MgO (4.7–9.2 wt%), Cr (169–323 ppm) and Ni (32–138 ppm) contents, together with high (87Sr/86Sr)i (0.70867–0.71060) and negative εNd(t) (−4.07 to −6.80) values. Their zircon εHf(t) vary from −5.64 to +4.12, and their δ18O (6.0 to 6.9‰) are slightly higher than the average mantle value. The granodiorite has higher SiO2 and lower MgO contents, and varying zircon εHf(t) (−12.19 to −2.71) and δ18O (6.3 to 8.1‰). The rocks are characterized by enrichments in large ion lithophile elements (LILEs: e.g., Rb, Th, U, K) and light rare earth elements (LREEs), but depleted in high field strength elements (HFSEs), and have also high (87Sr/86Sr)i (0.71057–0.711860) and negative εNd(t) (−8.42 to −9.14). Mafic microgranular enclaves (MMEs) in the granodiorite have intermediate SiO2, MgO, and trace elements and Sr-Nd-Hf-O isotope compositions between the high-Mg diorite and the granodiorite. Based on these geochemical and isotopic features, we interpreted that the high-Mg diorite was generated from the interaction of subducted sediment-derived melt with the overlying mantle wedge, whilst the granodiorite was sourced from the ancient continental crust with minor mantle-derived input. Mixing between the mantle-derived high-Mg dioritic melt and the crustal-derived granodioritic melt may have formed the MMEs. Our new geochemical (incl. isotopic) data, combined with published ones across the region, suggest that these Early Cretaceous plutons were formed by asthenospheric upwelling, which was probably led by slab rollback of the west-dipping subduction of the Luxi-Ruili Ocean between the Baoshan and Tengchong terranes. This implies that the southeastern continuation of the Tethyan Bangong-Nujiang Ocean was located between the Baoshan and Tengchong terranes, instead of between the Tengchong and West Burma terranes as previously suggested.

Cambrian and Cryogenian tectonothermal events in the Amdo microcontinent, Central Tibet: Implications for paleogeographic reconstruction and tectonic evolution of northern Gondwana

The Early Paleozoic paleogeographic locations of the Tibetan terranes in Gondwana remain poorly constrained. This study reports petrology, zircon LA-ICP-MS U-Pb ages, whole-rock major and trace element compositions, and Sr-Nd-Hf isotopic data for the gneissic granites in the Amdo microcontinent, central Tibetan Plateau. LA-ICP-MS zircon U-Pb dating yielded Cambrian ages of ca. 500 Ma for the granites. A Cryogenian age of ca. 800 Ma was recognized from inherited zircons. Geochemical data indicate that these granites belong to high-K calc–alkaline I-type granitoids. They mostly exhibit negative values of zircon εHf(t) (−7.1 to +0.1) and whole-rock εNd(t) (−5.9), as well as old zircon Hf model ages (TCDM = 1718–1472 Ma) and whole-rock Nd model ages (TCDM = 1771 and 1777 Ma). These granites can be interpreted as resulting from partial melting of ancient crustal materials that have been modified by mantle-derived magmas. Integrating previous magmatic studies with the data presented in this contribution, we suggest that the Amdo microcontinent was linked to the Indian Gondwanan margin and recorded the arc-related magmatism along the proto-Tethyan margin.

Magmatic Processes at Snæfell Volcano, Iceland, Constrained by Zircon Ages, Isotopes, and Trace Elements

AbstractWe present the first zircon‐based U‐Pb geochronology, trace element concentrations, and O and Hf isotope compositions for Snæfell, an off‐rift volcano in eastern Iceland. These data provide constraints on the longevity and petrogenetic conditions of the Snæfell magmatic system. U‐Pb zircon ages range from 545 ± 59 to 266 ± 16 ka (2SE), but differences between grain core and mantle ages within each sample reveal zircon residence times of 100–200 kyr—far longer than observed at other Neovolcanic systems in Iceland. Zircon δ18O is restricted to ∼3.5–4‰, and zircon εHf ranges ∼+13 to ∼+17, which is substantially more radiogenic than Snæfell basalts. This combined O and Hf isotopic perspective suggests rhyolite petrogenesis at Snæfell can be attributed to fractional crystallization of mantle‐derived basaltic magmas with limited influence of pre‐existing crustal material. Trace element evidence further characterizes the magmatic source material: Sc/Yb &lt;∼0.01 suggests an amphibole‐free petrogenetic environment, and Ti concentrations in zircon &lt;5 ppm suggest a cool, near‐solidus, crystallization environment for the majority of the zircon's pre‐eruptive history, with elevated Ti in zircon surfaces suggesting a late thermal perturbation, perhaps a mafic input that remobilized (to the point of eruption triggering) near‐solidus magmas. These zircon‐based conclusions are broadly consistent with previous interpretations of rhyolite petrogenesis conditions at Snæfell but provide a multi‐faceted perspective with more detailed resolution of source materials, magma generating processes, system longevity, and pre‐eruptive conditions.

Middle to Late Triassic granitic magmatism in the East Kunlun Orogenic Belt, NW China: Petrogenesis and implications for a transition from subduction to post‐collision setting of the Palaeo‐Tethys Ocean

In this study, we present new geochronological and petrogenetic data for the Triassic granitoids of the East Kunlun Orogenic Belt (EKOB), in order to constrain their precise ages, petrogenesis, and tectonic settings. LA‐ICP‐MS zircon U–Pb data indicate that the Triassic granitoids were emplaced in two stages: (a) Middle Triassic (247–240 Ma), represented by a suite of porphyritic granites and granodiorites; and (b) Late Triassic (234–227 Ma), forming an intrusive rock association of K‐feldspar granites, granodiorites, and porphyritic granites. Geochemical analyses and mineral associations suggest that all the Triassic granitoids belong to I‐type granites but have different origins. The Middle Triassic granitoids have high SiO2, low to moderate Mg# values (25–37 for XSG porphyritic granite; 45–47 for DB granodiorite), low Sr/Y ratios (2.2–4.6 for XSG porphyritic granite; 17.8–20.8 for DB granodiorite), and relatively restricted zircon εHf(t) values (+2.4 − +4.6 for the ca. 247 Ma porphyritic granite, and − 8.0 to −1.5 for the ca. 240 Ma granodiorite), indicating that they were dominantly generated from partial melting of different crust sources (either juvenile or ancient) in a normal lower crust level. In contrast, the Late Triassic granitoids have high SiO2, K2O, and Y contents, low MgO and HREE contents, and variable zircon εHf(t) values (from negative to positive, −4.9 to +3.3), implying a strong crustal–mantle interaction that occurred during the Late Triassic, and this stage of granitoids were derived from a complex magma source possibly a mixture of mantle‐derived and ancient crustal‐derived materials. By combining these new data with the previous data, we conclude that the two stages of Triassic granitoids were emplaced in an active continental margin setting and a post‐collisional extension setting, respectively. Moreover, this study suggests a tectonic shift of the Palaeo‐Tethys Ocean in the EKOB from subduction during the Middle Triassic to a post‐collision during the Late Triassic.

Middle Neoproterozoic (ca. 700 Ma) tectonothermal events in the Lhasa terrane, Tibet: Implications for paleogeography

The evolution of the Mozambique Ocean spans the Rodinia and Gondwana supercontinent cycles playing a key role in Earth history. Integrated petrological, geochronological, geochemical, and isotopic data on the Neoproterozoic units from the Lhasa terrane, Tibet, constrain the magmatic and metamorphic evolution of the ocean. Our results reveal two magmatic (ca. 695 Ma and ca. 671 Ma) and two metamorphic (ca. 700 Ma and ca. 650 Ma) pulses. Gabbros define the first magmatic suite. They have within-plate affinities and experienced crustal contamination. Their positive zircon εHf(t) (+1.6 to +9.6) and whole-rock εNd(t) (+2.7 to +6.1) values are indicative of a depleted mantle source. Gabbros from the second magmatic event belong to the tholeiitic series, lack evidence for crustal contamination, and are geochemically similar to E-MORB. They have negative to positive zircon εHf(t) (−3.2 to +1.5) and low positive whole-rock εNd(t) (+1.4 to +2.0) values, and were probably derived from an E-MORB-type enriched mantle source, slightly modified by subduction components. Evidence for the metamorphic events (ca. 700 and ca. 650 Ma) was obtained from UPb dating of zircon rims of marbles and gabbros, which are characterized by low Th/U ratios (0.01–0.07) indicative of a metamorphic origin. Integrating previous studies with the data presented in this contribution, we suggest that the history of the Lhasa terrane was linked to the evolution of the Mozambique Ocean. The older mafic magmatism (ca. 695 Ma) and metamorphism (ca. 700 Ma) formed during terrane accretion processes along an active continental margin to the ocean. The relatively younger mafic magmatism (ca. 671 Ma) provides a record from an oceanic fragment that was metamorphosed (ca. 650 Ma) during closure of the Mozambique Ocean.