Single-stage Hf Model Ages Research Articles

1.38 Ga magmatism and the extension tectonics in East Kunlun, northern Tibetan Plateau

Numerous Paleo- to Mesoproterozoic magmatic rocks were emplaced during the assembly, accretion and break-up of the Columbia supercontinent, and are keys to illustrating the supercontinent circle. The geological, petrological, geochronological and geochemical data of the newly discovered meta-felsic and meta-mafic magmatic rocks from the Wulonggou area are presented in this study to shed light on the Mesoproterozoic geodynamic setting of the Central Kunlun Belt and the Qaidam Block. Zircon U-Pb geochronological results indicate that the crystallization ages of the meta-felsic samples are 1385–1376 Ma, and the meta-mafic is 1379 ± 25 Ma (MSWD=0.17). Samples from the meta-felsic unit have SiO2 contents of 68.37–73.03 wt% and belong to the high-K calc-alkaline series. They exhibit similar REE distribution patterns and display enrichment in light rare earth elements (LREES) and negative Eu anomalies. Enrichments in Rb, Ba, Th, U and K, depletion in Nb, Ta, Sr, P and Ti are seen in the primitive mantle-normalized trace element pattern diagram. Magmatic zircons from different meta-felsic samples yield variable εHf(t) values of −8.14 to + 9.37 corresponding to ca. 1.7–2.0 Ga two-stage Hf model ages. Samples from the meta-mafic unit have low SiO2 concentrations of 49.87–50.43 wt%, high contents of Fe2O3T, MgO, CaO and TiO2, and Mg# values of 52–58. They show low total REE concentrations of 19.8–30.4 µg/g, and depletion in LREES, flat HREES distribution patterns with (La/Yb)N of 0.27–0.53 and insignificant Eu anomalies. Flat distribution pattern of high field strength elements (HFSEs) is observed in the primitive mantle-normalized trace element pattern diagram. They display similar immobile elements’ concentrations and distribution patterns, low Ti/Y, Nb/Y, La/Yb, and high Nb/La ratios, comparable with the present-day normal middle ocean ridge basalt. Zircons from the meta-mafic sample have mostly positive εHf(t) values ranging from −0.10 to + 4.10 with a single stage Hf model ages of ca. 1.7–2.0 Ga. The geochemical result implies that the meta-felsic unit was generated by partial melting of the Paleoproterozoic juvenile mafic lower-crustal material with mantle attributions, and the meta-mafic unit was probably from partial melting of the lithospheric mantle. Synthesizing the above evidences, Wulonggou Mesoproterozoic meta-magmatic rocks are a bimodal suite formed in a continental extensional tectonic setting which is probably related to the break-up of the Columbia supercontinent.

Early Paleozoic Continental Arc Mafic Magmatism in the North Qaidam Tectonic Belt: Implications for Subduction of the Proto-Tethyan Oceanic Lithosphere

Abstract The North Qaidam tectonic belt is characterized by the development of high-pressure to ultrahigh-pressure eclogite formed during deep subduction of the continental lithosphere. However, the tectonic processes that occurred prior to continental collision/subduction are relatively poorly studied and this leads to controversy over the evolutionary history of the North Qaidam tectonic belt. In this contribution, we present an integrated study of field observations, petrography, geochronology, and geochemistry (whole-rock major elements, trace elements, and Sr–Nd isotopes as well as zircon Lu–Hf isotopes) of continental arc mafic rocks in the North Wulan metamorphic complex to track Proto-Tethyan oceanic subduction and the nature of metasomatism of the mantle wedge. Zircon U–Pb geochronology demonstrates that continental arc mafic rocks crystallized at ca. 483–472 Ma. Mafic intrusions are enriched in light rare earth elements and large ion lithophile elements but are depleted in high field strength elements; these are typical features of arc-related magmatic rocks. They have relatively radiogenic Sr–Nd–Hf isotope compositions, with relatively high initial 87Sr/86Sr ratios of 0.710363 to 0.719404, low εNdt values of -7.77 to -2.30, variable zircon εHft values ranging from -8 to +2.8, and the single-stage Hf model ages of ca. 1.4–1.0 Ga. These features suggest that the mafic intrusions were sourced from ancient subcontinental mantle that was modified by subducted oceanic slab-derived components. The modified mantle source was generated by the interaction between subcontinental lithospheric mantle peridotite in the overlying mantle wedge and hydrous fluid and felsic melt that were derived from a subducted oceanic slab and seafloor sediments with ancient terrestrial origin in the rutile stability field. Crust-mantle interaction transferred the subducted crustal geochemical signatures to the mantle source during subduction of the Proto-Tethyan oceanic lithosphere. Retreat and rollback of the Proto-Tethyan oceanic slab triggered asthenosphere upwelling and the partial melting of metasomatized and enriched fertile lithospheric mantle to form continental arc mafic magmas in the North Qaidam tectonic belt. Combined with the coeval LP-HT metamorphism, the North Wulan metamorphic complex records the early Paleozoic tectonic evolution of a continental arc-back-arc system, and these continental arc mafic rocks record the subduction of the Proto-Tethyan oceanic lithosphere.

New constraints on volcanism during Ordovician-Silurian transition: Insights from marine bentonites in northern Yili Block (NW China)

The Yili Block is located at the Chinese western Tianshan and is a constituent of the Kazakhstan microcontinent. It played an important role in the evolution of the SW Central Asian Orogenic Belt (CAOB). The Yili Block underwent the Paleozoic orogeny during the subduction and closure of the Junggar Ocean. Late Paleozoic arc-type magmatic rocks have been widely recorded in the Yili Block; however, it remains poorly constrained when and how the Junggar Ocean started to subduct beneath the Yili Block. In this study, we report new results of zircon LA-ICP-MS UPb dating and LuHf isotopic analysis on a series of bentonite beds in tightly folded upper Ordovician siliceous black shales of the Guozigou Section in the northern Yili Block. The occurrence of bentonite layers and associated sulfides within graptolite-bearing siliceous black shales indicates multiple marine volcanic eruptions. Magmatic zircons separated from seven representative bentonite samples yielded consistent UPb ages ranging from 449.5 ± 2.4 Ma to 444.8 ± 2.0 Ma (Katian). In situ zircon LuHf isotopic analyses show mostly positive εHf(t) values (−0.01 and 1.31 to 11.7) and Neoproterozoic single-stage Hf model ages (TDM1 = 0.6– 1.0 Ga). A linear correlation between the εHf(t) and TDM1 suggests that these zircons likely crystallized from a common magma originated from a depleted mantle source with variable involvement of Precambrian continental crust. Our new zircon ages and Hf isotopic data are comparable with those of the Middle-Late Ordovician continental arc-type magmatic rocks in the nearby Wenquan area, both indicating that the subduction of the Junggar Ocean beneath the northern Yili Block started in the Middle-Late Ordovician, and are therefore important for early Paleozoic paleogeographic and tectonic reconstruction of the Paleo-Asian Ocean domain. In addition, the Late Ordovician marine bentonites and black shales coincide with the worldwide intensive volcanic activities and related climate/environment changes during late Katian to Hirnantian. Thus, these new data also provide further arguments for possible causes of the global climate change and mass extinction during the Ordovician-Silurian transition.

Origin of Late Permian amphibole syenite from the Panxi area, SW China: high degree fractional crystallization of basaltic magma in the inner zone of the Emeishan mantle plume

ABSTRACTThe c.a. 259–260 Ma Emeishan mantle plume caused extensive flood basalts and associated peralkaline rocks; petrogenesis of these peralkaline rocks can provide detailed information about the melting process in the mantle. In this paper, we present detailed petrology, chronology, major and trace element geochemistry, Sr-Nd isotope and in situ zircon Hf isotope for an amphibole syenite from Panxi area, inner zone of the Emeishan large Igneous province (ELIP). The amphibole syenite has a zircon U-Pb ages of 260 ± 2 Ma (MSWD = 1.2, n = 22), which is synchronous with Emeishan mantle plume. It displays moderate SiO2 (64.89–65.67 wt.%), high Na2O (7.35–7.71 wt.%) and K2O (4.78–4.96 wt.%) contents, with A/CNK values of 0.86 to 0.90, with high Rittman index of 6.6 to 7.1, showing peralkaline affinity. It displays significantly negative Sr anomalies (133 to 200 ppm) and slightly negative Eu anomalies (Eu/Eu* = 0.78–1.04). The rock displays high εNd(t) values of +2.4 to +2.5. Zircons from the amphibole syenite display positive εHf(t) values from +8.9 to +11.0, with single-stage Hf model ages ranging from 0.47 to 0.55 Ga, combined with the well-developed correlation of TFe2O3, CaO/Al2O3, TiO2, Ni and Cr with MgO, suggesting the amphibole syenite was derived from the mantle source, and the fractional crystallization played a significant role in the generation of the magma. The MELTS model results show that Houzishan amphibole syenite can be formed by the high degree (~80%) fractional crystallization of ~24% clinopyroxene, ~4% orthopyroxene, ~35% plagioclase and ~5% oxide from parental magma resembling high-Ti basalts. These results indicate that Houzishan amphibole syenite represents the juvenile mantle-derived peralkaline melts addition to the continental lower crust in the inner zone of the ELIP and assimilating with minor crust material.

Petrology, geochemistry, and zircon U–Pb–Hf isotopes of Late Triassic enclaves and host granitoids at the southeastern margin of the Songnen–Zhangguangcai Range Massif, Northeast China: Evidence for magma mixing during subduction of the Mudanjiang oceanic plate

We report zircon U–Pb ages and geochemistry (major and trace elements, mineral compositions, and Hf isotopic compositions) for the Hailin pluton in the southeastern Songnen–Zhangguangcai Range Massif, northeastern China, to constrain the role of magma mixing in its petrogenesis and to reconstruct the tectonic evolution of the Mudanjiang Ocean. Zircon U–Pb dating yields ages of 215 ± 1 Ma, 217 ± 1 Ma, and 219 ± 1 Ma for the host granitoids of the Hailin pluton, and 221 ± 1 Ma for mafic microgranular enclaves (MMEs) within the granitoids, indicating they were emplaced coevally. The host granitoids are characterized by 63.18–70.44 wt% SiO2 and 0.69–1.94 wt% MgO, and are depleted in high field strength elements (HFSEs; e.g., Nb, Ta, and Ti) and heavy rare earth elements (HREEs), and enriched in large ion lithophile elements (LILEs; e.g., Rb, Sr, and Ba) and light rare earth elements (LREEs). The zircons in the host granitoids yield two-stage model ages of 1058 Ma to 1770 Ma, together with εHf(t) values of −6.6 to +3.1. These results indicate that the host granitoid magmas were generated by partial melting of Mesoproterozoic lower crust contaminated by ancient crustal material. The MMEs have a restricted SiO2 range (47.61–57.4 wt%), high contents of Ni, Co, and Cr, and high Mg# values (42–54). They have higher Fe2O3T, MgO, Cr and Co contents than the host granitoids, as well as higher total rare earth element (REE) concentrations. Their REE patterns are sub-parallel to those of the host granitoids, characterized by enrichments in LILEs and LREEs, and depletions in HFSEs and HREEs. Zircons from two samples of the MMEs yield single-stage Hf model ages of 666–1077 Ma and εHf(t) values of −5.4 to +5.2. The geochemical characteristics of the MMEs are similar to those of the host granitoids, indicating crust-mantle interaction during their formation. The mafic magmas were derived from melting of depleted lithospheric mantle that had been metasomatized by slab-derived fluids. MMEs are common in the host granitoids and are characterized by irregular spheroidal shapes, with sharp, rounded, or irregular transitional contacts, acicular apatites, and plagioclase reverse zoning, suggesting the importance of magma mixing during petrogenesis. Our geochemical and isotopic data suggest that the Late Triassic Hailin pluton formed by the mixing of felsic and mafic magmas in an active continental margin setting, closely related to the westward subduction of the Mudanjiang oceanic plate beneath the Songnen–Zhangguangcai Range Massif.

Zircon Lu-Hf isotope systematics and U–Pb geochronology, whole-rock Sr-Nd isotopes and geochemistry of the early Jurassic Gokcedere pluton, Sakarya Zone-NE Turkey: a magmatic response to roll-back of the Paleo-Tethyan oceanic lithosphere

The early Mesozoic was a critical era for the geodynamic evolution of the Sakarya Zone as transition from accretion to collision events in the region. However, its complex evolutionary history is still debated. To address this issue, we present new in situ zircon U–Pb ages and Lu-Hf isotope data, whole-rock Sr-Nd isotopes, and mineral chemistry and geochemistry data of plutonic rocks to better understand the magmatic processes. The Gokcedere pluton is mainly composed of gabbro and gabbroic diorite. LA-ICP-MS zircon U–Pb dating reveals that the pluton was emplaced in the early Jurassic (177 Ma). These gabbros and gabbroic diorites are characterized by relatively low SiO2 content of 47.09 to 57.15 wt% and high Mg# values varying from 46 to 75. The samples belong to the calc-alkaline series and exhibit a metaluminous I-type character. Moreover, they are slightly enriched in large ion lithophile elements (Rb, Ba, Th and K) and light rare earth elements and depleted in high field strength elements (Nb and Ti). Gabbroic rocks of the pluton have a depleted Sr-Nd isotopic composition, including low initial 87Sr/86Sr ranging from 0.705124 to 0.705599, relatively high e Nd (t) values varying from 0.1 to 3.5 and single-stage Nd model ages (T DM1 = 0.65–0.95 Ga). In situ zircon analyses show that the rocks have variable and positive e Hf (t) values (4.6 to 13.5) and single-stage Hf model ages (T DM1 = 0.30 to 0.65 Ga). Both the geochemical signature and Sr-Nd-Hf isotopic composition of the gabbroic rocks reveal that the magma of the studied rocks was formed by the partial melting of a depleted mantle wedge metasomatized by slab-derived fluids. The influence of slab fluids is mirrored by their trace-element characteristics. Trace-element modeling suggests that the primary magma was generated by a low and variable degree of partial melting (~5–15%) of a depleted and young lithospheric mantle wedge consisting of phlogopite- and spinel-bearing lherzolite. Heat to melt the mantle material was supplied by the ascendance of a hot asthenosphere triggered by the roll-back of the Paleo-Tethyan oceanic lithosphere. The rising melts were accompanied by fractional crystallization and encountered no or minor crustal contamination en route to the surface. Taking into account these geochemical data and integrating them with regional geological evidence, we propose a slab roll-back model; this model suggests that the Gokcedere gabbroic pluton originated in a back-arc extensional environment associated with the southward subduction of the Paleo-Tethyan oceanic lithosphere during the early Jurassic period. Such an extensional event led to the opening of the northern branch of the Neotethys as a back-arc basin. Consequently, we conclude that the gabbroic pluton was related to intensive extensional tectonic events, which peaked during the early Jurassic in response to the roll-back of Paleo-Tethyan oceanic slab in the final stage of oceanic closure.

Age and tectonic implications of Paleoproterozoic Deo Khe Granitoids within the Phan Si Pan Zone, Vietnam

We report the first U–Pb zircon ages of 1855–1873Ma for the Deo Khe Granitoids (DKG) in the Phan Si Pan Zone (PSPZ) of northern Vietnam. The DKG are medium-grained two-mica granitoids predominantly composed of quartz, K-feldspar, and muscovite. Trace element analyses indicate that the DKG are enriched in large ion lithophile elements but depleted in high field strength elements. Zircons from the granitoids have negative εHf(t) values ranging from −23.6 to −17.5. The magmatic zircons from the DKG have single-stage Hf model ages (TDM1) that range from 3.3 to 2.8Ga and their εHf(t) data all plot well below the evolution trend of 2800Ma average juvenile mantle. Observed Hf model ages are contemporaneous with the emplacement of 2.90–2.84Ga tonalite–trondhjemite–granodiorite (TTG) gneiss observed in a nearby Ca Vinh Complex, suggesting that PSPZ in northern Vietnam is a product of partial melting of Archean crust. A sequence of similar tectonic events including initial emplacement of TTG protolith at 2.8–2.9Ga, metamorphic development of TTG gneiss at 1.9–2.0Ga, and magmatic activity at 1.8–2.0Ga are now recognized both in northern Vietnam and Yangtze block which we interpret to indicate basement rocks in northern Vietnam are similar to those found along southern China.

Neoarchean subduction: A case study of arc volcanic rocks in Qinglong-Zhuzhangzi area of the Eastern Hebei Province, North China Craton

We report on volcanic rocks of the Qinglong-Zhuzhangzi area in northeastern Eastern Hebei Province, North China Craton. The supracrustal rock associations in the study area consist of amphibole/biotite plagioclase gneiss, mica (plagioclase) gneiss/schist, biotite K-feldspar gneiss, (pyroxene) plagioclase amphibolite and amphibolite. Twenty-five new mafic, intermediate to felsic samples can be geochemically classified into four groups. Group 1 is similar to fore arc low Ti tholeiite and is characterized by low SiO2 (49.7wt%) and TiO2 (0.43wt%), slightly depleted LREE and MREE pattern with (La/Yb)N ratio of 0.74, (Gd/Yb)N ratio of 0.82. It most likely derived from depleted mantle in a subduction initiation environment. Group 2 is akin to primitive arc tholeiites with low SiO2 (49.7–51.8wt%), slight enrichments of LREE ((La/Sm)N: 1.06–1.53) and negative Nb anomalies ((Nb/La)PM: 0.50–0.76). Group 3 resembles evolved arc tholeiites and displays pronounced enrichments of LREE ((La/Yb)N of 2.38–11.3, (La/Sm)N of 1.70–2.86) and negative Nb, Ta anomalies ((Nb/La)PM: 0.14–0.34, except sample 11JD28-4 with that ratio of 1.00). Group 2 and 3 formed by partial melting of sub-arc depleted mantle wedge modified by differing quantities of slab flux addition. Group 4 comprises K-rich andesite-dacites, which are distinguished by the highest SiO2 (54.0–71.8wt%), K contents (up to 5.03wt%), strong enrichments of LREE ((La/Yb)N ratios of 4.83–19) and negative Nb–Ta anomalies, analogous to shoshonitic rocks marking late stage of subduction.LA-ICPMS zircon U–Pb and Lu–Hf analyses of evolved arc basalts and K-rich andesite-dacites display crystallization ages of 2511–2604Ma and single stage Hf model ages of 2534–2927Ma (most focus on 2650–2800Ma). In contrast to ∼2510–2560Ma arc associations, ∼2604Ma shoshonitic dacites most plausibly represent an older orogenic event. Collectively, supracrustal rocks in the Qinglong-Zhuzhangzi area constitute a series of volcanic arc associations that correspond to an entire subduction cycle ranging from orogen to primitive arc–evolved arc and back to an orogenic setting, which further demonstrate that, by the late Archean, plate tectonic and subduction processes already closely resembled modern counterparts.

Late Triassic intrusive complex in the Jidong region, Jiamusi–Khanka Block, NE China: Geochemistry, zircon U–Pb ages, Lu–Hf isotopes, and implications for magma mingling and mixing

Whole-rock major and trace element geochemistry together with zircon U–Pb ages and Lu–Hf isotope compositions are reported for a Late Triassic intrusive complex in the Jidong region, Jiamusi–Khanka Block, NE China. Zircon U–Pb dating yields ages between 211 and 208Ma for enclaves of microgranular diorite and quartz diorite, and between 211 and 209Ma for the host granitoids. These ages correlate with a previously established intensive Late Triassic magmatic event along the eastern Asian margin. Field observations, together with petrographic features, geochemistry, and zircon Hf isotope data, preclude simple crystal fractionation or restite unmixing as a genetic link for the various rock types within the intrusive complex. The syenogranite suite has high SiO2 (75.5–76.3wt.%) and low MgO (0.15–0.19wt.%), and yields enriched LILE and LREE patterns. Most of the zircons in the syenogranites have two-stage model ages of 766 and 1461Ma, together with positive εHf(t) values of +0.6 to +9.1. These results indicate that the granitoid magmas were generated by partial melting of Meso- to Neoproterozoic lower crust. The gabbro suite has a restricted range of SiO2 (46.1–51.9wt.%) together with high Mg# values (49–70) and high concentrations of Ni, Co, and Cr. Zircons from two diorite samples have single-stage Hf model ages of 557–787Ma and εHf(t) values of +1.9 to +8.3 that are consistent with the coeval gabbros previously studied in the Jidong region. These features, together with the observation that all the gabbros are enriched in LREE and LILE, suggest that the mafic magmas were derived from melting of depleted Neoproterozoic lithospheric mantle that had been metasomatized by slab-derived fluids. It is concluded that the dominant igneous suites within the Late Triassic intrusive complex formed by mingling/mixing of felsic and mafic magmas. The geochemical data, combined with regional geological investigations, indicate that the Late Triassic intrusive complex formed during lithospheric extension caused by slab break-off of the Yuejinshan oceanic plate that was subducting westward beneath the Jiamusi–Khanka Block.

Magma mixing origin for high Ba–Sr granitic pluton in the Bayankhongor area, central Mongolia: Response to slab roll-back

Petrogenesis of high Ba–Sr granitoids is a matter of debate. This paper presents whole-rock geochemical, Sr–Nd isotopic and zircon U–Pb and Hf isotopic data for a suite of high Ba–Sr biotite monzogranite and associated mafic microgranular enclaves (MMEs) from the Ulaan Uul batholith in the Bayankhongor area, central Mongolia, aiming at elucidating the complex petrogenesis of the high Ba–Sr granitoids. The data manifest that magma mixing between a felsic magma derived from sedimentary rocks and a mafic magma derived from Sub-Continental lithospheric mantle (SCLM) may account for the formation of high Ba–Sr granitoids. The host granite and MMEs yielded U–Pb ages of 546±3Ma and 547±3Ma, respectively, suggesting that the mafic and felsic magmas were coeval. Rocks of the host granitoids possess high SiO2 and low MgO contents and display peraluminous high-K calc-alkaline and shoshonitic characteristics. Their relatively high Ba, Sr but low HREE (Yb<2ppm) resemble those of typical high Ba–Sr granitoids. Rocks from the granitoids have crust-like Sr–Nd isotopic compositions [(87Sr/86Sr)i=0.7059–0.7065, and εNd(t)=−1.8 to −2.1], and most zircons from the host rocks show negative εHf(t) values (−0.63 to −11.9) with two-stage Hf model ages of 1.54–2.25Ga. The MMEs have low SiO2 and high MgO concentrations, enriched Sr–Nd isotopic compositions [(87Sr86/Sr)i=0.7060–0.7066, and εNd(t)=−1.6 to −2.1], and most zircons from the MMEs have positive εHf(t) values (+0.7 to +2.6), with late Mesoproterozoic single-stage Hf model ages (1.05–1.12Ga), suggesting an origin of late Mesoproterozoic SCLM. We interpret that the Ulaan Uul batholith has a mixed origin, i.e. partial melting of sedimentary rocks triggered by upwelling of lithospheric mantle-derived magma (represented by MMEs) under an extensional scheme, and subsequent mixing of the mantle- and crust-derived melts resulted in the high Ba–Sr granitoids. We speculate that the late Neoproterozoic to early Cambrian extensional environment in the Bayankhongor area was caused by the roll-back of subducting Bayankhongor oceanic lithosphere.

Geochronology, geochemistry and Sr–Nd–Hf isotopes of mafic dikes in the Huicheng Basin: Constraints on intracontinental extension of the Qinling orogen

The diabase dikes intruded the Lower Cretaceous sandstone in the Huicheng Basin are key to understanding the Mesozoic tectonic evolution of the Qinling orogen. LA-ICP-MS zircon U–Pb dating yields a crystallization age of 107±1Ma for them. The dikes have low contents of SiO2 (42.46–50.16wt.%), MgO (3.47–5.59wt.%) with low Mg# of 49–59, and TiO2 (1.35–1.63wt.%), high TFe2O3 (8.15–9.36wt.%), Al2O3 (14.75–17.23wt.%) and K2O (0.87–3.61wt.%). Their Ni and Cr contents are in range of 16.8–111 and 45.7–315ppm, respectively. They are significantly enriched in light rare earth elements and large ion lithophile elements (e.g., Cs, Pb and Ba), and depleted in Rb, K, P, and Ti. The dikes show relatively high whole-rock initial 87Sr/86Sr ratios (0.7071–0.7079) and negative εNd (t) values (−1.5 to −4.8) with single-stage Nd model ages of 941–1186Ma. In addition, they have low zircon εHf (t) values (−8.6 to+3.5) with single-stage Hf model ages of 674–1117Ma. Both elemental and isotopic geochemistry suggests that the magma of these dikes has undergone significant crystallization fractionation of olivine and pyroxene but weak crustal contamination during magma evolution. They were derived from partial melting of an enriched lithospheric mantle source. Together with regional geological data, these results suggest that the Qinling orogen experienced an intracontinental extension during the late Early Cretaceous.

Geochemistry and geochronology of the Precambrian high-grade metamorphic complex in the Southern Central Tianshan ophiolitic mélange, NW China

The Central Tianshan Arc Terrane is one of the major constituents of the Tianshan orogen in the southwestern Altaids. Thus its Precambrian evolution is important for unravelling the geodynamic and continental evolution of the Altaids. Biotite-plagioclase-hornblende orthogneisses intruded by leucogranite dykes are exposed as exotic blocks within the Southern Central Tianshan ophiolitic mélange. Zircon U–Pb age dating of one orthogneiss sample yields an upper intercept age of 2466±51Ma and a weighted mean 207Pb/206Pb age of 1812±19Ma around the lower intercept. These ages are interpreted as protolith crystallization age and subsequent metamorphic overprint age, respectively. The orthogneisses have moderate SiO2 contents, high MgO and Na2O contents, high Sr/Y ratios (32–43) and moderately fractionated REE patterns ([La/Yb]N=7.59–13.64) with negligible Eu anomalies, enriched Sr contents and pronounced negative Th–U, Nb–Ta, Zr–Hf and Ti anomalies resembling high-SiO2 TTGs derived from subducted basaltic slab-melts. Positive ɛHf(t) values (+3.4 to +9.2) and low (176Hf/177Hf)i ratios (0.281304–0.281469) with Neoarchaean to early Paleoproterozoic single-stage Hf model ages (TDM1=2431–2652Ma) suggest that the orthogneisses probably originated from partial melting of juvenile subducted oceanic crust. The orthogneisses were subsequently also affected by collisional orogenic events associated with the assembly of the supercontinent Columbia. In contrast, the zircon U–Pb age of 785±15Ma obtained for the intrusive leucogranite dykes is consistent with the timing of rifting events associated with the breakup of Rodinia. The leucogranites have a high-Al trondhjemitic composition characterized by extremely low MgO and K2O contents as well as high SiO2, Al2O3 and Na2O contents, dramatically low ∑REE abundances and REE patterns with moderate LREE enrichments ([La/Yb]N=3.55–8.4) and pronounced positive Eu anomalies (Eu/Eu*=1.29–2.61). The high zircon initial Hf compositions (0.281670–0.281841) and negative ɛHf(t) values (−21.7 to −15.4), in conjunction with high whole-rock initial 87Sr/86Sr ratios (0.70737–0.70751) and negative ɛNd(t) values (−4.7 to −5.1) suggest that the leucogranite resulted from reworking of ancient lower crust. Based on the presented data we conclude that the Central Tianshan Arc Terrane has undergone three tectonothermal events, namely ∼2.5Ga continental crustal growth, ∼1.8Ga collision related to the Columbia assembly and ∼785Ma crustal reworking due to the Rodinia breakup. The Central Tianshan Arc Terrane, which experienced the same tectonic events as the Tarim Craton during the Precambrian, is considered to be a micro-Precambrian block that separated from the Tarim Craton during the Rodinia breakup.

Zircon U–Pb and molybdenite Re–Os geochronology and Sr–Nd–Pb–Hf isotopic constraints on the genesis of the Xuejiping porphyry copper deposit in Zhongdian, Northwest Yunnan, China

The Xuejiping porphyry copper deposit is located in northwestern Yunnan Province, China. Tectonically, it lies in the southern part of the Triassic Yidun island arc. The copper mineralization is mainly hosted in quartz-dioritic and quartz-monzonitic porphyries which intruded into clastic-volcanic rocks of the Late Triassic Tumugou Formation. There are several alteration zones including potassic, strong silicific and phyllic, argillic, and propylitic alteration zones from inner to outer of the mineralized porphyry bodies.The ages of ore-bearing quartz-monzonitic porphyry and its host andesite are obtained by using the zircon SIMS U–Pb dating method, with results of 218.3±1.6Ma (MSWD=0.31, N=15) and 218.5±1.6Ma (MSWD=0.91, N=16), respectively. Meanwhile, the molybdenite Re–Os dating yields a Re–Os isochronal age of 221.4±2.3Ma (MSWD=0.54, N=5) and a weighted mean age of 219.9±0.7Ma (MSWD=0.88). They are quite in accordance with the zircon U–Pb ages within errors. Furthermore, all of them are contemporary with the timing of the Garzê-Litang oceanic crust subduction in the Yidun arc. Therefore, the Xuejiping deposit could be formed in a continental margin setting.There are negative εNd(t) values ranging from −3.8 to −2.1 and relatively high initial 87Sr/86Sr ratios from 0.7051 to 0.7059 for the Xuejiping porphyries and host andesites. The (206Pb/204Pb)t, (207Pb/204Pb)t and (208Pb/204Pb)t values of the Xuejiping porphyries and host andesites vary from 17.899 to 18.654, from 15.529 to 15.626, and from 37.864 to 38.52, respectively, indicative of high radiogenic Pb isotopic features. In situ Hf isotopic analyses on zircons by using LA-MC-ICP-MS exhibit that there are quite uniform and slightly positive εHf(t) values ranging from −0.2 to +3.2 (mostly between 0 and +2), corresponding to relatively young single-stage Hf model ages from 735Ma to 871Ma. These isotopic features suggest that the primary magmas of the Xuejiping porphyries and their host andesites were mainly derived from a metasomatized mantle, with contamination of about 5–10% crustal rocks during ascending.Comparing with typical porphyry Cu deposits, the Xuejiping porphyry Cu deposit is distinct by strong silicific and phyllic alteration and major stockwork veining mineralization in the ore-bearing porphyries, but lack of pervasive potassic alteration and disseminated mineralization. This indicates that there could be more prospective Cu resources in the Xuejiping ore district.

Zircon U–Pb ages and geochemistry of the Qinglong volcano-sedimentary rock series in Eastern Hebei: Implication for ∼2500 Ma intra-continental rifting in the North China Craton

Voluminous Archean rocks (including orthogneisses and supracrustal rocks) are exposed in Eastern Hebei, northern part of the North China Craton (NCC), and nearly all of these Archean rocks have experienced high-grade metamorphism of granulite-facies or upper amphibolite facies from 2.54Ga to 2.52Ga. The Qinglong area of the eastern part of Hebei Province has a set of low-grade (greenschist- to lower amphibolite-facies) supracrustal rocks, e.g., the Qinglong volcano-sedimentary rock series (QVRS), unconformably overlying on the high-grade metamorphosed Qianxi Complex. Zircons yield a weighted mean 207Pb/206Pb age of 2511±12Ma meta-felsic volcanic rocks and 2503±13Ma meta-basic volcanic rocks. These ages indicate that the QVRS formed during the period from 2511Ma to 2503Ma. The meta-felsic volcanic rocks display low MgO (0.96–2.67wt.%), Cr (50.90–87.10ppm), and Ni (2.78–6.18ppm) contents, and low Nb/Ta ratios (13.56–14.54), and high light rare earth elements (LREEs) ((La/Yb)n=14.75–22.04), which are interpreted to have been produced by the partial melting of the lower crust. The meta-basic volcanic rocks (low-grade amphibolites) have abundance levels from 3.86wt.% to 7.74wt.% of MgO, and from 46.11wt.% to 51.99wt.% of SiO2. They are enriched in LREEs and depleted in high field strength elements (HFSEs) with positive ɛNd(t) values (+2.6 to +2.9). The volcanic rocks in the QVRS show bimodal compositions. The ɛHf(t) (t=2511Ma) values of amphibolite range from −4.7 to +5.7, and their single-stage Hf model ages vary from 2622Ma to 3030Ma (peak at 2835Ma). For the meta-felsic volcanic rocks, the ɛHf(t) values range from −3.2 to +5 with two-stage model ages changing from 2683Ma to 3180Ma (peak at 2872Ma). The QVRS is proposed to have formed during an intra-continent rift at 2500Ma, after the regional high-grade metamorphism from 2.52Ga to 2.54Ga. The QVRS probably formed during an intra-continental rift, as a part of the late Neoarchean cover of the NCC.

A geochemical study of syn-subduction and post-collisional granitoids at Muzhaerte River in the Southwest Tianshan UHP belt, NW China

A combined study of zircon U–Pb ages and Lu–Hf isotopes, whole-rock elements and Sr–Nd isotopes was carried out for two granitic intrusions at Changawuzi and Alasan in Muzhaerte River of Southwest Tianshan (NW China). SHRIMP zircon U–Pb dating gave two 206Pb/238U ages of 333±3Ma and 326±3Ma for the Changawuzi pluton. LA-ICP-MS zircon U–Pb dating yielded two 206Pb/238U ages of 293±3Ma and 294±2.2Ma for the Alasan pluton. The two groups of ages predate and postdate the UHP metamorphic event at 320–305Ma in this region, respectively, registering to syn-subduction and post-collisional magmatism. The Changawuzi pluton is composed of gneissic porphyritic tonalite and garnet-bearing gneissic granitoids, having SiO2 of 55.1–64.5wt.%, K2O+Na2O of 2.58–5.07wt.%, MgO of 2.07–4.32wt.%, and Al2O3 of 17.4–19.9wt.% with A/CNK ratios of 0.99 to 1.27. They show arc-like geochemical affinity, with enrichment of LILE (K, Rb, Sr and Ba) and depletion of HFSE (Nb, Ta and Ti) and P, and moderate initial 87Sr/86Sr ratios of 0.7062–0.7071 and neutral εNd(t) values of −1.5 to 1.6. Except for one zircon with a negative εHf(t) value of −3.5, the others have positive εHf(t) values of 2.5–11.4 and single-stage Hf model ages of 509 to 869Ma. The Changawuzi pluton would be formed by partial melting of possible juvenile arc-derived rocks with minor involvement of ancient continental rocks during subduction of the paleo-South Tianshan oceanic crust and subsequent arc–continent collision. The Alasan pluton consists of medium-grained and porphyritic granitoids. The medium-grained granitoids are composed of granodiorite, quartz monzodiorite and quartz monzonite, displaying SiO2 of 59.37–65.49wt.%, K2O+Na2O of 6.24–8.2wt.% and MgO of 1.45–3.06wt.%, with A/CNK ratios of 0.87–1.04. They have relatively high initial 87Sr/86Sr ratios of 0.7082–0.7103, negative εNd(t) values of −5.9 to −5.6 and negative εHf(t) values of −4.5 to −1.1. The porphyritic granitoids consist of granite, granodiorite and quartz monzonite, with relatively high SiO2 (65.94–75.27wt.%) and K2O+Na2O (7.55–9.13wt.%), low MgO (0.18–1.22wt.%), and A/CNK ratios of 0.82–1.11. They also have relatively high initial 87Sr/86Sr ratios of 0.7073–0.7103, negative εNd(t) values of −6.0 to −0.5 and neutral εHf(t) values of −1.4 to 2.8. Both of them also display arc-like patterns of REE and trace element distribution. They belong to post-collisional granitoids and were produced by partial melting of the Paleo-Mesoproterozoic crust of obducted Central Tianshan block with minor involvement of juvenile crust. The geological occurrences of both the syn-subduction and post-collisional granitoids in this region suggest differential tectonic processes during the Late Paleozoic collision between the Tarim and Yili-Central Tianshan Blocks. It is inferred that slab breakoff occurred subsequent to the UHP metamorphism, resulting in partial melting of the subducted upper crust and the overlying lower crust and thus formation of voluminous granitoids like the Alasan pluton.

Ages of the Laocheng Granitoids and Crustal Growth in the South Qinling Tectonic Domain, Central China: Zircon U‐Pb and Lu‐Hf Isotopic Constraints

Abstract:The Laocheng granitoid pluton is located in the South Qinling tectonic domain of the Qinling erogenic belt, southern Shaanxi Province, and consists chiefly of quartz diorite, granodiorite and monzogranite. A LA‐ICP‐MS zircon U‐Pb isotopic dating, in conjunction with cathodoluminescence images, reveals that the quartz diorite and granodiorite were emplaced from 220 Ma to 216 Ma, while the monzogranite was emplaced at ∼210 Ma. In‐situ zircon Hf isotopic analyses show that the εHf(t) values of the quartz diorite and granodiorite range from –8.1 to +1.3, and single‐stage Hf model ages from 809 Ma to 1171 Ma, while the εHf(t) values of the monzogranite are –14.5 to +16.7 and single‐stage Hf model ages from 189 Ma to 1424 Ma. These Hf isotopic features reveal that the quartz diorite, granodiorite and monzogranite were formed from the mixing of the magmas derived from partial melting of the depleted mantle and the lower continent crustal materials, and there were two stages of continental crust growth during the Neoproterozoic (∼800 Ma) and Indosinian (∼210 Ma) eras, respectively, in the south Qinling tectonic domain of the Qinling orogrnic belt, Central China.

Zircon U–Pb and Lu–Hf isotopic systematics of the Daping plutonic rocks: Implications for the Neoproterozoic tectonic evolution of the northeastern margin of the Indochina block, Southwest China

The Daping Neoproterozoic plutonic rocks at the northeastern margin of Indochina block in southwest China provide an ideal opportunity for studying the tectonic setting and relationship between the Indochina and Yangtze Blocks. LA-ICP-MS U–Pb dating on the zircon cores and rims of a hornblende-gabbro yield 206Pb/ 238U weighted means ages of 873 ± 9.1 Ma and 769 ± 7 Ma, respectively, and that for cores, mantles and rims of a granodiorite yield 206Pb/ 238U weighted means ages of 981–987 Ma, 829 ± 10 Ma and 761 ± 11 Ma, respectively. The zircon cores and mantles are interpreted as inherited from their source region. The zircon rims are magmatic, their ages represent the emplacement timing. The zircon cores and rims from the hornblende-gabbro have ε Hf ( t) values ranging from − 5.0 to − 5.8 and + 0.6 to + 6.4. Corresponding single-stage model ages range from 1626 to 1662 Ma and 1094 to 1311 Ma, respectively. For the granodiorite, the inherited mantles (including cores) show two groups: (1) ε Hf ( t) values of + 3.3 to + 12.3 with single-stage Hf model ages of 897 to 1235 Ma; and (2) ε Hf ( t) values of − 1.9 to − 7.8 with single stage model ages of 1470–1667 Ma. The zircon rims are characterized by positive ε Hf ( t) values (+ 5.4 to + 8.2) with single-stage model ages ranging from 977 to 1108 Ma. Whole-rock geochemical data for the hornblende-gabbro, such as enrichment of LILE and LREE, negative anomaly of Nb and Ta, and high Mg # (52.1–65.4), suggest magma generation in a subduction-related setting. An island-arc affinity is strongly supported by the features of high-alumina basalt and abundant hornblende in a large hornblende-gabbro sill. The granodiorites are characterized by high Sr contents and Sr/Y ratios, strong enrichment of LILE and LREE, and negative anomaly of Nb, Ta, P and Ti, comparable with the features of subduction-related plutonic rocks. These data show that the hornblende-gabbro was generated by the partial melting of a metasomatized mantle wedge peridotite with contribution from aqueous fluids derived from a subducted slab. The granodiorite magma is a product of the mixing of mafic magma produced by partial melting of a slab-fluid-enriched metasomatized mantle wedge peridotite and felsic magma formed by the partial melting of crustal materials. The emplacement ages and geochemical features of subduction-related Daping plutonic rocks are the same as those reported from the western margin of the Yangtze block, suggesting the presence of an oceanic crust in between, with subduction to either side generating island-arc magmatism in the Neoproterozoic.

Ca. 2.5 billion year old coeval ultramafic–mafic and syenitic dykes in Eastern Hebei: Implications for cratonization of the North China Craton

A group of extremely rare coeval ultramafic–mafic and syenitic dykes was discovered in the Eastern Hebei region of the North China Craton. These dykes intrude the 3.8–2.55 Ga old Caozhuang complex. An olivine gabbro dyke and syenite dyke yield, respectively, SHRIMP zircon U–Pb ages of 2516 ± 26 Ma and 2504 ± 11 Ma, interpreted as the magmatic crystallization age. Their zircons have single-stage Hf model ages 2677 Ma and 2705 Ma. The olivine gabbros have Mg # values of 59–63, similar to high-magnesian tholeiitic basalt. They show relatively LREE-enriched patterns without Eu anomalies (La/Yb N = 9.28–9.78, Gd/Yb N = 2.8, δEu = 0.96–1.00), with enrichment in LILEs and depletion in HFSEs. They also contain high Cr and Ni, and have Zr/Hf and Nb/Ta ratios similar to those of the primitive mantle. The syenites are alkaline in composition, with 8.67–8.88 wt.% Na 2O + K 2O, and show high total REE contents (543–854 ppm) and strong LREE-enriched patterns with minor Eu negative anomalies (La/Yb N = 50–101, Gd/Yb N = 5.2–5.3, δEu = 0.80–0.82). Trace element diagrams show strong LILE-enrichment relative to HFSE. Zr/Hf and Nb/Ta ratios are 47–49 and 26–76, respectively, which are much higher than those of primitive mantle and higher than the average Nb/Ta ratio of post-Archean continental crust. The petrological and geochemical features indicate that these dykes were derived from a deep subcontinental lithospheric mantle source, which implies that the NCC probably had continental crust of considerable thickness at this time. Combined with evidence for ∼2.5 Ga granite intrusion and metamorphism in the Eastern Hebei region and adjacent areas, we propose that the NCC has been a present scale craton at the end of Archean.

Magma mixing origin for the post-collisional adakitic monzogranite of the Triassic Yangba pluton, Northwestern margin of the South China block: geochemistry, Sr–Nd isotopic, zircon U–Pb dating and Hf isotopic evidences

Petrogenesis of high Mg# adakitic rocks in intracontinental settings is still a matter of debate. This paper reports major and trace element, whole-rock Sr–Nd isotope, zircon U–Pb and Hf isotope data for a suite of adakitic monzogranite and its mafic microgranular enclaves (MMEs) at Yangba in the northwestern margin of the South China Block. These geochemical data suggest that magma mixing between felsic adakitic magma derived from thickened lower continental crust and mafic magma derived from subcontinental lithospheric mantle (SCLM) may account for the origin of high Mg# adakitic rocks in the intracontinental setting. The host monzogranite and MMEs from the Yangba pluton have zircon U–Pb ages of 207 ± 2 and 208 ± 2 Ma, respectively. The MMEs show igneous textures and contain abundant acicular apatite that suggests quenching process. Their trace element and evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.707069–0.707138, and eNd(t) = −6.5] indicate an origin from SCLM. Some zircon grains from the MMEs have positive eHf(t) values of 2.3–8.2 with single-stage Hf model ages of 531–764 Ma. Thus, the MMEs would be derived from partial melts of the Neoproterozoic SCLM that formed during rift magmatism in response to breakup of supercontinent Rodinia, and experience subsequent fractional crystallization and magma mixing process. The host monzogranite exhibits typical geochemical characteristics of adakite, i.e., high La/Yb and Sr/Y ratios, low contents of Y (9.5–14.5 ppm) and Yb, no significant Eu anomalies (Eu/Eu* = 0.81–0.90), suggesting that garnet was stable in their source during partial melting. Its evolved Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.7041–0.7061, and eNd(t) = −3.1 to −4.3] and high contents of K2O (3.22–3.84%) and Th (13.7–19.0 ppm) clearly indicate an origin from the continental crust. In addition, its high Mg# (51–55), Cr and Ni contents may result from mixing with the SCLM-derived mafic magma. Most of the zircon grains from the adakitic monzogranite show negative eHf(t) values of −9.4 to −0.1 with two-stage Hf model ages of 1,043–1,517 Ma; some zircon grains display positive eHf(t) of 0.1–3.9 with single-stage Hf ages of 704–856 Ma. These indicate that the source region of adakitic monzogranite contains the Neoproterozoic juvenile crust that has the positive eHf(t) values in the Triassic. Thus, the high-Mg adakitic granites in the intracontinental setting would form by mixing between the crustal-derived adakitic magma and the SCLM-derived mafic magma. The mafic and adakitic magmas were generated coevally at Late Triassic, temporally consistent with the exhumation of deeply subducted continental crust in the northern margin of the South China Block. This bimodal magmatism postdates slab breakoff at mantle depths and therefore is suggested as a geodynamic response to lithospheric extension subsequent to the continental collision between the South China and North China Blocks.

Geochemical evidence for origin of magma mixing for the Triassic monzonitic granite and its enclaves at Mishuling in the Qinling orogen (central China)

Whole-rock major-trace elements and Sr–Nd isotopes, mineral chemistry, and zircon U–Pb and Lu–Hf isotope compositions are reported for I-type monzogranite and its mafic microgranular enclaves from the Triassic Mishuling granite in the Qinling orogen, central China. Zircon U–Pb dating yields an age of 212 ± 5 Ma for the mafic enclaves and 213 ± 3 Ma for the host monzogranite. Whole-rock major-trace elements, Sr–Nd isotopes and zircon Hf isotopes preclude simple crystal fractionation or restite unmixing for a genetic link between the enclaves and the monzogranite. Mixing between mafic and felsic magmas is compatible with the geochemical data. The monzogranite displays an enriched LILE and LREE pattern, high ( 87Sr/ 86Sr) i ratios of 0.7068 to 0.7071 and low ε Nd( t) values of − 9.2 to − 5.7, suggesting a crustal origin. Its high Mg# (47.6 to 50.7), Cr and Ni contents clearly indicate its derivation from mafic refractory lower crust. Most of the zircons in the monzogranite have negative ε Hf( t) values of − 12.5 to − 0.1 and two-stage Hf model ages of 1049 to 1683 Ma, and some zircons in the host monzogranite have positive ε Hf( t) values of 0.3 to 4.8 and single-stage Hf model ages of 665 to 845 Ma. This indicates that the monzogranite is most likely to be formed by partial melting of a Proterozoic lower continental crust. The mafic enclaves have fine-grained igneous textures and contain abundant acicular apatite that indicates rapid cooling. They also have a high ( 87Sr/ 86Sr) i ratio of 0.7071 and a low ε Nd( t) value of − 6.5, suggesting a derivation from ancient subcontinental lithosphere mantle (SCLM). Zircons in the mafic microgranular enclaves have ε Hf( t) values of − 1.6 to 5.1 and single-stage Hf model ages of 700 to 910 Ma, suggesting a possible origin from partial melting of the Neoproterozoic SCLM that may form in association with contemporaneously supercontinental rifting in the South China Block. Therefore, the Mishuling I-type granite may form by anatexis of Meso-Neoproterozoic lower crust, with mixing between the mafic and felsic magmas. In particular, we interpret the positive ε Hf( t) values as being derived from reworking of Neoproterozoic juvenile lithosphere in response to tectonic extension in the late stage of the Triassic continental collision between the South China Block and the North China Block.