Back-arc Extensional Environment Research Articles

Paleo-Pacific plate subduction direction change (122−118 Ma): Insight from late kinematic plutons in the Wulian metamorphic core complex, Jiaodong Peninsula, eastern China

The late Mesozoic subduction of the paleo-Pacific plate (Izanagi plate) beneath Eurasia involved complex changes in the direction and angle of subduction, which have been controversial in recent decades. This study investigated Early Cretaceous late kinematic plutons of the Wulian metamorphic core complex that formed in the southern Jiaodong Peninsula. Based on geochemical and Sr-Nd isotopic analyses, we conclude that the granites in the Heshan (122 Ma) and Kuaijishan (118 Ma) plutons are I-type granites that formed in a back-arc extensional environment, and tensile fractures played an important role in the process of magma emplacement. Gravity anomaly inversion revealed the significant role of tension fractures in the uplift of the late kinematic plutons. Analysis of magnetic fabrics showed that the dominant plunging direction of magnetic lineation in the late kinematic plutons is NW-SE (127°−307°), while the plunging direction of the mineral stretching lineation in the wall rock is nearly E-W (102°−282°). We posit that the extensional stress field in the Jiaodong Peninsula underwent a significant transformation during the Early Cretaceous period, specifically from 135 Ma to 122 Ma, when it was dominated by a regional E-W or WNW-ESE extensional stress field. Later, from 122 Ma to 118 Ma, the stress field was controlled by a NW-SE extensional stress field. This variation in the regional stress field in the Jiaodong Peninsula indirectly indicates the changing subduction and slab rollback direction of the paleo-Pacific plate. From 135 Ma to 122 Ma, the paleo-Pacific plate subducted beneath Eurasia along a WNW-ESE direction. However, back-arc extension and slab rollback led to extensive extension in the Jiaodong Peninsula. Subsequently, the subduction direction of the paleo-Pacific plate shifted to the NW, with rollback occurring eastward at a high angle under the Jiaodong Peninsula from 122 Ma to 118 Ma. This shift resulted in extensive magmatism and widespread gold mineralization. The findings of this study offer new insights into the complex geodynamic processes that occurred during the Early Cretaceous period in the region.

Petrogenesis of the Weideshan Pluton in Jiaodong and Its Implications for Gold Polymetallic Mineralization: Constraints from Zircon U-Pb-Hf Isotopes, Petrogeochemistry, and Whole-Rock Sr-Nd Isotopes

The Early Cretaceous Weideshan granites are associated with large-scale Au and polymetallic Cu-Mo-Pb-Zn mineralization. To investigate the petrogenesis of the Weideshan granite and constrain its tectonic setting during the Early Cretaceous, we conducted a zircon U-Pb-Hf isotope and whole-rock geochemical and Sr-Nd isotopic study of the granite. In situ zircon U-Pb dating of three granite samples yielded Early Cretaceous ages of 112.83 ± 0.80, 112.64 ± 0.91, and 111.82 ± 0.78 Ma. The samples had high-K calc-alkaline compositions and were enriched in the light rare earth and large-ion lithophile elements (e.g., K, Rb, Ba, Th, and U) and depleted in high-field-strength elements (e.g., Nb, Ti, and P). The samples had small negative Eu anomalies and initial 87Sr/86Sr and εNd(t) values of 0.70853–0.71029 and –18 to –14, respectively. The zircon εHf(t) values varied between −16 and −12, with corresponding two-stage model ages (tDM2) of 2180–2000 Ma. These characteristics indicated that the Weideshan pluton was formed in a back-arc extensional environment caused by subduction of the Paleo-Pacific Plate toward the Asian continent during the early Cretaceous. The magma was generated by crust–mantle interaction during lithospheric thinning. The diagenetic age of the Weideshan granites was the same as the formation age of gold and polymetallic ores in the Jiaodong area. The extensive fluid circulation induced by the magma emplacement may be the main source of ore-forming materials for the gold and polymetallic Cu-Mo-Pb-Zn deposits.

Development of a Modern-Style Trench-Arc-Backarc System in the Proto-Tethys Ocean (Qilian Orogenic Belt, NW China)

Abstract The development of trench-arc-backarc (TABA) systems is uniquely associated with modern-style plate tectonics on Earth. The Qilian orogenic belt in NW China records the evolution history of the Proto-Tethys Ocean at the transition time from the Proterozoic to Phanerozoic. This paper presents systematic studies of petrography, U–Pb chronology and geochemistry on various rocks from a middle-ocean ridge (MOR)-type ophiolite belt, active continental margin and back-arc basin in the Qilian orogenic belt to address the development of a modern-style TABA system. Arc magmas include felsic intrusions with ages of 531 to 477 Ma and felsic-mafic arc volcanic rocks with ages of 506 to 439 Ma, showing distinctive features of typical magmatic rocks formed at an Andean-type continental margin. The back-arc basin is recorded by a 490- to 448-Ma suprasubduction zone (SSZ)-type ophiolite with boninite, and Silurian turbidite flysch formation. We establish a three-stage tectonic history from the initiation of subduction to the formation of a mature Japan-Sea-type back-arc basin at the active continental margin in the Early Paleozoic era. (1) Northward subduction of Proto-Tethys Ocean initiated and the Andean-type continental arc developed at ~530 to 500 Ma with continual crustal thickening; (2) a tectonic transition occurred from an Andean-type active continental margin to a West Pacific-type active continental margin at ~500 to 490 Ma with rapid thinning of crust to ~35 km; and (3) mature ocean basins and back-arc-basin (BAB) ophiolites were formed in the back-arc extensional environment at ~490 to 450 Ma.

Mineralogical, Geochronological, and Geochemical Characteristics of Early Cretaceous Granite in South China: Implications for Tectonic Evolution and REE Mineralization

One of the most important geological features of South China are the widespread Mesozoic igneous rocks that play a key role in revealing the tectonic evolution of South China. Due to the thick covering of vegetation and Quaternary sediments, the early Cretaceous magmatism in southwestern South China is still not well constrained. In this paper, we report newly identified early Cretaceous granites in Guangxi, South China. Zircon U–Pb dating results showed that representative fine-grained and coarse-grained granites in northeastern Guangxi indicate the early Cretaceous ages of 141 ± 3 Ma and 141 ± 4 Ma, respectively. Geochemically, both fine-grained and coarse-grained granites had high 10,000 × Ga/Al ratios and belonged to A-type granite. They had undergone high degrees of magma differentiation, as evidenced by extremely negative Sr, Ba, and Eu anomalies. They had high REE (rare earth elements) contents (>451 ppm). The fine-grained granites were characterized by higher HREE (heavy rare earth elements) contents, lower LREE (light rare earth elements) contents, and lower LREE/HREE ratios than the coarse-grained granites. Integrated with regional geological data, the early Cretaceous granites were likely formed in a back-arc extensional environment in response to the increased subduction angle of the Paleo-Pacific plate. Different REE contents in the fine- and coarse-grained granites may be a result of fractional crystallization. Magma differentiation and hydrothermal alteration might have played an important role in REE mineralization of the early Cretaceous granites in Guangxi.

Genesis of the Tashan porphyry host tin deposit, eastern Guangdong, Southeast China: Constrains from geology, geochronology, and geochemistry

The medium-sized Tashan Sn deposit, mainly hosted in the Tashan granite porphyry, is located in eastern Guangdong, Southeast China. Zircon U–Pb dating of the granite porphyry yielded a concordant age of 138.0 ± 1.1 Ma, which is consistent with the cassiterite U–Pb age of 138.0 ± 2.3 Ma from the Sn ores, indicating a temporal link between the emplacement of granitic pluton and Sn mineralization. The contents of Fe3+, Fe2+, and Mg2+ in biotite, low calculated zircon Ce4+/Ce3+ of 4.4–71.6, low Eu/Eu* ratios of 0.06–0.35, and low whole-rock Fe2O3/FeO of 0.04–0.13 of the granite porphyry reveals its low oxygen fugacity, which may have facilitated the Sn enrichment during magma evolution and played a key role in the Sn mineralization. Low K/Rb (150–175, aver. = 160), Nb/Ta (7.3–8.2, aver. = 7.8), and Zr/Hf (23.2–25.1, aver. = 23.9) values were observed for the granite porphyry, indicating its high fractionation and probable interaction with an aqueous fluid phase to some extent. The reduced and highly fractionated features and high F and Sn contents of the granite porphyry thus indicate it is a typical Sn-fertile granite. The granite porphyry is weakly peraluminous and belongs to high-K calc-alkaline type. The rocks are characterized by high SiO2, K2O + Na2O, F, and Zr + Nb + Ce + Y (360–410 ppm) contents; and 10000 Ga/Al (2.31–3.06) ratios and low CaO, MgO, TiO2, and P2O5 contents; Rb, Cs, Th, and U enrichment; and Ba, Sr, Zr, Ti, and P depletion, exhibiting an A-type affinity. Moreover, the rocks display whole rock initial 87Sr/86Sr values of 0.70927–0.71218, εNd(t) of − 6.5 to − 6.3, and zircon εHf(t) values from − 5.7 to − 1.5 with Mesoproterozoic TDM2 ages for both Nd and Hf isotopes, indicating their formation by partial melting of a crustal source of Mesoproterozoic overall residence ages with minor input of mantle materials. By integrating new and published data on tectonic evolution, we suggest that the granite porphyry and associated Sn mineralization in the Tashan deposit that was formed in a back-arc extensional environment started before ∼ 141 Ma, related to the rollback of the paleo-Pacific Plate.

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.

Geochronology, Geochemistry, and Pb–Hf Isotopic Composition of Mineralization-Related Magmatic Rocks in the Erdaohezi Pb–Zn Polymetallic Deposit, Great Xing’an Range, Northeast China

Late Mesozoic intermediate–felsic volcanics and hypabyssal intrusions are common across the western slope of the Great Xing’an Range (GXAR). Spatiotemporally, these hypabyssal intrusions are closely associated with epithermal Pb–Zn polymetallic deposits. However, few studies have investigated the petrogenesis, contributions and constraints of these Pb–Zn polymetallic mineralization-related intrusions. Therefore, we examine the representative Erdaohezi deposit and show that these mineralization-related hypabyssal intrusions are composed of quartz porphyry and andesite porphyry with concordant zircon U–Pb ages of 160.3 ± 1.4 Ma and 133.9 ± 0.9 Ma, respectively. These intrusions are peraluminous and high-K calc-alkaline or shoshonitic with high Na2O + K2O contents, enrichment in large ion lithophile elements (LILEs; e.g., Rb, Th, and U), and depletion in high field strength elements (HFSEs; e.g., Nb, Ta, Zr, and Hf), similar to continental arc intrusions. The zircon εHf(t) values range from 3.1 to 8.0, and the 176Hf/177Hf values range from 0.282780 to 0.282886, with Hf-based Mesoproterozoic TDM2 ages. No differences exist in the Pb isotope ratios among the quartz porphyry, andesite porphyry and ore body sulfide minerals. Detailed elemental and isotopic data imply that the quartz porphyry originated from a mixture of lower crust and newly underplated basaltic crust, while the andesite porphyry formed from the partial melting of Mesoproterozoic lower crust with the minor input of mantle materials. Furthermore, a magmatic–hydrothermal origin is favored for the Pb–Zn polymetallic mineralization in the Erdaohezi deposit. Integrating new and published tectonic evolution data, we suggest that the polymetallic mineralization-related magmatism in the Erdaohezi deposit occurred in a back-arc extensional environment at ~133 Ma in response to the rollback of the Paleo-Pacific Plate.

A-type volcanic–intrusive complex in the Huanggangshan Basin: Implications for early cretaceous crust–mantle interaction in the Gan-Hang Belt and adjacent areas, South China

The origin and petrogenesis of Early Cretaceous A-type volcanic–intrusive complexes in the Gan-Hang Belt and adjacent areas of South China remain controversial. An A-type volcanic–intrusive complex in the Huanggangshan Basin of North Wuyi Mountain comprises syenite porphyry, granite porphyry, granite, and tuffaceous volcanic and pyroclastic rocks. These rocks were subjected to geochronological, geochemical, and isotopic analyses in order to investigate their age and petrogenesis, and crust–mantle interactions in South China. Field observations reveal that this A-type volcanic–intrusive complex contains abundant mafic microgranular enclaves. Zircon UPb dating of the various rock types yielded ages between 136 and 128 Ma. Geochemically, the rocks have A-type affinities, including high large-ion lithophile and high-field-strength element contents, Ga/Al ratios, and zircon saturation temperatures. (87Sr/86Sr)i values vary from 0.702704 to 0.711298, and εNd(t) = −10.63 to −7.28 and εHf(t) = −13.26 to −7.70. Three stages of magmatism are proposed to explain the petrogenesis of the A-type volcanic–intrusive complex in the Huanggangshan Basin: (1) syenitic magmas formed by mingling of crustally derived silicic and mantle-derived mafic magmas; (2) fractional crystallization of anorthitic plagioclase + biotite + ilmenite from the syenitic magmas generated A-type igneous rocks with SiO2 < 71 wt%; and (3) fractional crystallization of plagioclase + K-feldspar + biotite + apatite + allanite + ilmenite then generated A-type igneous rocks with SiO2 > 71 wt%. Combined with previous studies, we suggest that the Early Cretaceous A-type igneous rocks in the Gan-Hang Belt and adjacent areas might have been generated by crust–mantle-derived magma mixing, and the crust–mantle interaction was more intense from southwest to northeast. This increased crust–mantle interaction may have been a response to the Early Cretaceous back-arc extensional environment, possibly as a result of the rollback of the Paleo-Pacific Plate.

Subduction-related middle Permian to early Triassic magmatism in central Hainan Island, South China

There was extensive Permian to Triassic magmatism in Hainan Island, southern part of the South China Block (SCB), which began earlier and exhibits different rock types and geochemical characteristics compared to the Indosinian magmatism in the hinterland of the SCB. Two stages of magmatism have been identified in central Hainan Island in this study. The first-stage magmatism (253–262Ma) is characterized by strongly deformed granites. These gneissic granites exhibit SiO2 (64.0–69.8wt%), MgO (0.97–1.60wt%), Fe2O3T (2.68–6.78wt%), A/CNK (most <1.1), zircon εHf(t) values (−2.08 to −0.41), the geochemical features of I-type granites. These early granites can be subdivided into high-REE and low-REE groups. Compared to the low-REE granites, the high-REE ones have higher K2O, Ba, K2O/Na2O, Gd/Yb and Th/Ta, low U, Rb/Ba, Nb/La and U/Pb. The low-REE granites show whole-rock εNd(t) (−4.33 to −3.96) and ISr (ca 0.7074), while the high-REE granites have lower εNd(t) (−7.34 to −6.33) and higher ISr (0.7096–0.7109). The gneissic granites probably derived from the partial melting of the Baoban Complex at different crustal levels. The second-stage magmatism (245–256Ma) is characterized by weakly deformed to massive gabbroic-dioritic rocks. They exhibit relatively low SiO2 (48.9–59.7wt%), high Fe2O3T (6.57–11.6wt%), total alkalies (Na2O+K2O) (3.91–7.35wt%), and variable MgO (2.74–7.05wt%), Ni (3.84–60.1ppm) and Cr (7.22–299ppm) concentrations. They also have low whole-rock εNd(t) values (−7.92 to −5.11) and high ISr (0.7083–0.7118), and a range of zircon εHf(t) values (−5.55 to −1.11), suggesting that their parental magmas were derived from heterogeneous enriched-mantle sources, and experienced AFC processes. The rock assemblages and their geochemical features indicate that both granites and gabbroic-dioritic rocks were formed in a back-arc extensional environment, which was probably related to the subduction of Paleo-Pacific oceanic plate. Hence, the Indosinian intracontinental orogeny in the hinterland of the SCB probably was triggered by this subduction in the southeast, rather than the collision between the SCB and the Indochina Block in the southwest.

Origin of Early Cretaceous A-type granite and related Sn mineralization in the Sanjiaowo deposit, eastern Guangdong, SE China and its tectonic implication

The recently discovered Sanjiaowo Sn deposit is located in eastern Guangdong, SE China. The Sn mineralization, hosted in granite porphyry and the Lower Jurassic Jinji Formation low-grade metamorphic wall rocks, is considered to be granite-related. Zircon U–Pb dating of the granite porphyry yielded concordant ages of 140.5 ± 1.4 Ma, which is consistent with the cassiterite U–Pb age of 139.8 ± 5.0 Ma, indicating a temporal link between the emplacement of granitic plutons and Sn mineralization. The granite porphyry samples are weakly peraluminous and show an A-type geochemical signature with high Na2O + K2O (7.19–8.51 wt.%), REE (261–308 ppm), Zr + Nb + Ce + Y (353–476 ppm) contents, FeOT/MgO (0.93–0.99), and Ga/Al (2.65–3.42) ratios. They display bulk rock initial 87Sr/86Sr of 0.7110 to 0.7165, εNd(t) of −5.90 to −4.04, and zircon εHf(t) values from −8.36 to −4.16 with Mesoproterozoic TDM2 ages for both Nd and Hf isotopes. Detailed elemental and isotopic data suggest that the Sanjiaowo Granite porphyry belongs to the A2-subtype and were formed by partial melting of a crustal source of Mesoproterozoic overall residence age with minor input of mantle materials.The low calculated zircon Ce4+/Ce3+ (6.5–81.2) and EuN/EuN∗ (0.05–0.24) ratios demonstrates that the Sanjiaowo granitic magma had a low oxygen fugacity, which may have facilitated the Sn enrichment and played a key role in the Sn mineralization. Lead isotopes indicate that there is no difference for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios between the granite porphyry and the sulfide minerals from the orebodies. Moreover, the sulfur isotopic compositions for the sulfides (pyrite and pyrrhotite: δ34S = −1.6–1.0‰, average = −0.32) is similar to magmatic δ34S, but different from the δ34S in the Jinji Formation strata. Consequently, these data favor of a magmatic-hydrothermal origin for the Sn mineralization in the Sanjiaowo deposit. Integrating new and published data on the tectonic evolution, we suggest that the granite porphyry and associated Sn mineralization in the Sanjiaowo deposit formed in a back-arc extensional environment at ∼141 Ma, related to the rollback of the paleo-Pacific Plate.

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.

Fluid Inclusions, C–H–O–S Isotope and Geochronology of the Bujinhei Pb–Zn Deposit in the Southern Great Xing'an Range of Northeast China: Implication for Ore Genesis

AbstractThe Bujinhei Pb–Zn deposit is located in the southern Great Xing'an Range metallogenic belt. It is a representative medium‐ to high‐temperature hydrothermal vein type deposit controlled by fractures, and orebodies hosted in the Permian Shoushangou Formation. The hydrothermal mineralization is classified into three stages: pyrite ± arsenopyrite–quartz (Stage 1), polymetallic sulfide–quartz (Stage 2), and polymetallic sulfide–calcite (Stage 3). Fluid inclusion petrography, laser Raman analyses and microthermometry indicate that the liquid‐rich aqueous inclusions (L) and vapor‐rich CO2 ± CH4–H2O inclusions (C) occur in the Stage 1 and as medium‐ to high‐ temperature and low‐ to medium‐salinity NaCl–H2O–CO2–CH4 hydrothermal fluids. The liquid‐rich (L) and rare vapor‐rich CO2 ± CH4–H2O inclusions (C) occur in the Stage 2 with medium‐temperature and low‐salinity NaCl–H2O ± CO2 ± CH4 hydrothermal fluids. The exclusively liquid‐rich (L) fluid inclusions are observed in the Stage 3, and the hydrothermal fluid belongs to medium‐temperature and low‐salinity NaCl–H2O hydrothermal fluids. The results of hydrogen and oxygen isotope analyses indicate that ore‐forming fluids were initially derived from the magmatic water and mixed with local meteoric water in the late stage (δ18OH2O‐SMOW = 6.0 to 2.2‰, δDSMOW = −103 to −134‰). The carbon isotope compositions (−18.4‰ to −26.5‰) indicate that the carbon in the fluid was derived from the surrounding strata. The sulfur isotope compositions (5.7 to 15.2‰) indicate that the ore sulfur was also primarily derived from the strata. The ore vein No. 1 occurs in fractures and approximately parallel to the rhyolite porphyry; orebodies have a close spatial and temporal relationship with the rhyolite porphyry. The rhyolite porphyry yielded a crystallization age of 122.9 ± 2.4 Ma, indicating that the Bujinhei deposit may be related to the Early Cretaceous magmatic event. Geochemical analyses reveal that the Bujinhei rhyolite porphyry is high in K2O and peraluminous, and derived from an acidic liquid as a result of strong interaction with hydrothermal fluid during the late magmatic stage; it is similar to A2‐type granites, and formed in a backarc extensional environment. These results indicate that the Bujinhei Pb–Zn deposit was a vein type system that formed in Early Cretaceous and influenced by the Paleo‐Pacific tectonic system.Bujinhei deposit is a representative hydrothermal vein type deposit on the genetic types, and occurs on the western slope of the southern Great Xing'an Range. The ore‐forming fluids were medium‐ to high‐temperature and low‐to medium‐salinity NaCl–H2O–CO2–CH4 hydrothermal fluids, which became medium‐temperature and low‐salinity NaCl–H2O hydrothermal fluids in later stages, and came from magmatic water and mixed with meteoric water, whereas the ore‐forming materials were mainly derived from the surrounding strata. The LA–ICP–MS zircon U–Pb dating indicates that the Bujinhei deposit formed at the period of late Early Cretaceous, potentially in a backarc extensional environment influenced by the Paleo‐Pacific tectonic system.

Petrogenesis and geodynamic significance of the late Palaeozoic Dongwanzi Complex, North China Craton: constraints from petrological, geochemical, and Os-Nd-Sr isotopic data

The late Carboniferous Dongwanzi Complex in the northern North China Craton is composed of intrusive pyroxenite, hornblendite, gabbro, and syenite. The mafic-ultramafic rocks of the complex exhibit typical cumulate textures, curved-upward REE patterns, and variable contents of compatible elements, suggesting a cumulate origin. The syenite shows Sr-Nd isotopic ratios similar to the mafic-ultramafic complex and positive Eu anomalies in the chondrite-normalized REE patterns, suggesting that the syenite may represent residual melt after significant fractional crystallization of mafic melt. The mafic-ultramafic cumulates have low HREE abundance and high (Tb/Yb)N (2.5–4.2) and Dy/Yb ratios (>2), indicating that they may have originated from melting of garnet peridotite in the mantle. The Dongwanzi Complex is characterized by a large variation in Sr-Nd isotopic composition, with ISr = 0.7035 to 0.7052 and εNd(t) = −4.0 to +5.2, which may be accounted for by mixing melts of depleted asthenospheric and enriched lithospheric sources. The radiogenic Os isotopic compositions of the complex ((187Os/188Os)i = 0.1344 to 0.3090) suggest slight contamination by mafic lower crust (≤2.5% based on Os isotopic modelling). The Dongwanzi Complex exhibits arc-related whole-rock and mineral geochemical affinities, such as enrichment in LILE (e.g. Sr, Ba, K) and depletion in HFSE (e.g. Nb, Ta, Ti). The abundance of hornblende and high CaO contents (22–24 wt.%) of clinopyroxene suggest that the source was rich in H2O, probably due to the formation above a subduction zone. We conclude that the Dongwanzi Complex and the related crust–mantle interactions probably reflect formation in a back-arc extensional environment related to the subduction of the Palaeo-Asian Ocean beneath the northern margin of the North China Craton in late Palaeozoic time.

Re-Os dating of magnetite from the Shaquanzi Fe-Cu deposit, eastern Tianshan, NW China

Magnetite separates from the Shaquanzi Fe-Cu deposit in the eastern Tianshan are used for Re-Os geochronological study. Re-Os data show that magnetite separates contain ca. 0.7 to 50.9 ppb Re and ca. 16 to 63 ppt Os. Eight samples yield a model 3 isochron age of (303 ± 12) Ma (2 σ), which is within uncertainty consistent with of the Re-Os date (295±7 Ma) of associated pyrite. Tectonic evolution shows that the Late Carboniferous Aqishan-Yamansu belt was a back-arc rift. Therefore, the Re-Os age of ca. 300 Ma indicates that the Shaquanzi Fe-Cu deposit may have formed in a back-arc extensional environment and was closely related to mantle-derived magmatism. The successful application of Re-Os magnetite geochronology in the Shaquanzi Fe-Cu deposit suggests that the purity of magnetite, relatively high Re and Os contents, and the closure of Re-Os systematic are base factors for a successful Re-Os geochronology. There would be a good prospect for Re-Os geochronology for magnetite.

Geochemical Characteristics of Late Mesozoic Dikes, Jiaodong Peninsula, North China Craton: Petrogenesis and Geodynamic Setting

Widespread Cretaceous dikes in the Jiaodong Peninsula of eastern China are genetically related to gold deposits. Bulk-rock major element analyses show that they are composed of medium-K, subalkaline to shoshonitic rocks characterized by wide ranges of SiO2 (48.98-71.38%) and MgO (0.63-10.02%), and extremely high contents of Al2O3 (14.32-16.06%). They are enriched in large-ion lithophile elements (LILE, e.g., K, Rb, Sr, Ba) and light rare-earth elements (LREE), but depleted in high-field-strength elements (HFSE, e.g., Ta, Nb, Zr, Hf), and display consistent primitive mantle- and chondrite-normalized multi-element patterns on spidergrams, different from Cenozoic basalts in the Peninsula. These data suggest a magma origin by partial melting of an ancient enriched lithospheric mantle. High Th/Yb and Pb/Ce ratios and weak negative Ce anomalies reflect involvement of recycled sediments in the mantle reservoir. Strong fractionation in REE, with LREE >100 times chondrite and HREE <10 times chondrite, indicates the presence of garnet in the mantle source. Nb* values of 1.49-2.38 (>1), K* values of 0.08-0.14 (<1), and similar Zr/Hf (32.15-40.65), U/Th (0.10-0.33), and Hf/Sm (0.45-0.67) ratios in all the dikes, along with published chemical and Sr-Nd isotopic compositions of lamprophyres (Cheng et al., 1998; Sun et al., 2000; Yang et al., 2004) preclude crustal contamination during magma ascent and binary mixing between mantle and crustal magmas. The geochemical features of these dikes can be interpreted as the result of interaction of old lithospheric mantle with melt and/or fluid generated from subducted lower/middle crust of the Yangtze craton and/or Izanagi oceanic crust. The source for the Late Mesozoic dikes was completely reconstructed, enriched lithospheric mantle containing disseminated amphibole-and phlogopite-bearing pyroxenite veins surrounded by refractory peridotite wall rock. The enriched source was modified by subsequent partial melting due to sinistral transtensional strike-slip movement of the Tan-Lu fault that led to widespread dike intrusion. Trace element geochemistry and regional geology indicate that these dikes formed in a back-arc extensional environment.

Modern Seismic Observations in the Tatun Volcano Region of Northern Taiwan: Seismic/Volcanic Hazard Adjacent to the Taipei Metropolitan Area

The Tatun volcano group is located adjacent to the Taipei metropolitan area in northern Taiwan and was a result of episodic volcanisms between 2.8 and 0.2 Ma. Earthquake data collected over the last 30 years are analyzed to explore seismicity patterns and their associated mechanisms of faulting in the area. Using a Joint Hypocenter Determination (JHD) method, a few sequences of relocated earthquake hypocenters are tightly clustered; these seemed to be blurry in the original catalog locations. Numerous earthquakes, previously unnoticed and not reported in the CWB catalog, have been identified from careful examination of the continuous recordings of a nearby broadband seismic station. These newly identified earthquakes show similarities in waveforms and arrival time differences between direct P- and S-waves indicating that their hypocenter locations are very close to each other and their source mechanisms are similar. A relatively high b-value of 1.22 is obtained from the analysis of crustal earthquakes (depth < 30 km) in the region, which may suggest that clustered local seismicity in the Tatun volcanic region probably resulted from subsurface hydrothermal or volcano-related activities. Focal mechanism solutions determined in this study are dominated by normal faulting. Thus, these earthquake clusters are most probably associated with hydrothermal/magmatic activities in a back-arc extensional environment.

Neotectonic development of the Struma (Kraištid) Lineament, southwest Bulgaria and northern Greece

AbstractThe Struma (Kraištid) Lineament is a part of a fault belt of regional importance. It strikes NNW-SSE and cuts through different Alpine tectonic zones along the whole Balkan Peninsula. Normal and strike-slip faults occurred in environments of extension and graben formation during collapse after or between collision epochs in the Palaeogene and Early Neogene, and in a back-arc extensional environment during the neotectonic (end of Middle Miocene-Quaternary) stage. The last Alpine compression phase occurred in the beginning of the Miocene, and Early-Middle Miocene planation formed the initial peneplain. New intense faulting marked the beginning of the neotectonic stage (Late Badenian), and the neotectonic development, including sedimentation, proceeded in four regional macrocycles: Badenian-Sarmatian; Maeotian; Pontian-Dacian; and Eopleistocene-Pleistocene. The neotectonic development was marked by formation of the Serbo-Macedonian Swell as well as by rifting (the Vardar and Struma rifts).