Subduction Of Paleo-Pacific Plate Research Articles

From Rodinian passive margin to peri-Siberian continental arc: Evidence from the multiphase Neoproterozoic–early Paleozoic magmatic record of the Zavkhan Block in the Mongolian Collage

Geochronological and geochemical study of the Neoproterozoic–early Paleozoic magmatic record was carried out in two sections of the Zavkhan Block, one of the largest continental fragments within the Mongolian Collage of the Central Asian Orogenic Belt. Our results show that two distinct magmatic events occurred in both studied domains, the Late Tonian (c. 840–800 Ma) and the Cambro–Ordovician (c. 510–470 Ma). Based on the variable geochemical signatures and two-stage Nd model ages (TNd.2stgDM=1.4–3.0 Ga), the late Tonian magmatism included both mantle-derived magmas that suffered a crustal contamination as well as partial melts of a heterogeneous continental crust that ranged from Paleo- to Mesoproterozoic (felsic metaigneous rocks or immature metagreywackes) to Neoarchean–Paleoproterozoic (metabasalts or metatonalites). The Cambro–Ordovician basic–acidic magmatism (TNd.2stgDM=1.3–2.2 Ga) reflected interplay between two components, mafic juvenile (depleted-mantle-derived) and felsic, generated by anatexis of Tonian, mainly metabasic crust. The late Tonian (c. 840–800 Ma) magmatic activity is interpreted as result of intracontinental rifting, whereas the Cambro–Ordovician (c. 510–470 Ma) magmatism is linked to a continental magmatic-arc system.The comparison of the data from the Zavkhan Block with other microcontinents of the Mongolian Collage shows that most of them experienced an identical two-stage magmatic evolution. The first stage is interpreted as a result of the Rodinia break-up, whereas the second one was associated with inception of the Paleo-Pacific Plate subduction. Based on similarities in the magmatic record, we conclude that the Mongolian Collage microcontinents were located at the Rodinia periphery, as a part of either the Siberian or Tarim cratons. They were separated in the late Tonian, established a passive margin and drifted towards southern Siberia, where they recorded arc magmatism related to the Cambro–Ordovician subduction of the Paleo-Pacific Plate.

Fluid Evolution and Ore Genesis of the Songjianghe Au Deposit in Eastern Jilin Province, NE China: Constraints from Fluid Inclusions and H-O-S-Pb Isotope Systematics

The medium-sized Songjianghe Au deposit is located in the southeastern part of the Jiapigou-Haigou gold belt (JHGB) in central eastern Jilin Province, NE China. The gold mineralization is primarily characterized by disseminated-style ores and hosted in the low-/medium-grade metamorphic rocks of the Seluohe Group. The ore bodies are governed by NNW-striking brittle-ductile structures and spatially correlated with silicic and sericitic alterations. Four alteration/mineralization stages have been distinguished: (I) Quartz-pyrrhotite-pyrite, (II) quartz-polymetallic sulfides, (III) quartz-pyrite, and (IV) quartz-calcite. The fluid inclusion (FI) assemblage in quartz from Stage I comprises C1-type, C2-type, C3-type, and VL-type FIs, with total homogenization temperatures (Th-total) of 292.8 to 405.6 °C and salinities of 2.8 to 9.3 wt% NaCl eqv. Quartz from Stage II (main ore stage) developed C2-, C3-, and VL-type FIs, with a Th-total of 278.5 to 338.9 °C and salinities of 2.8 to 8.1 wt% NaCl eqv. Stage III is characterized by coexisting C3- and VL-type FIs in quartz, with a Th-total of 215.9 to 307.3 °C and salinities of 2.4 to 7.2 wt% NaCl eqv. Only VL-type FIs are observed in Stage IV, with a Th-total of 189.5 to 240.4 °C and salinities of 3.7 to 5.7 wt% NaCl eqv. The Laser Raman spectroscopic results demonstrated minor CH4 in the C-type FIs from Stages I and II. The results suggest that ore fluids may have evolved from a medium-high temperature, low-salinity immiscible CO2-NaCl-H2O ± CH4 system to a low temperature, low-salinity homogeneous NaCl-H2O system. Fluid immiscibility caused by the rapid drop in pressure may have been the main trigger for gold-polymetallic sulfide precipitation. The Songjianghe Au deposit may have been formed under 352–448 °C and 850–1380 bar pressure, based on the isochore intersection for Stage II fluid inclusions. The H-O isotopic compositions (Stage I: δ18Ofluid = 5.6 to 5.8‰, δD = −96.2 to −95.7‰; Stage II: δ18Ofluid = 3.7 to 4.2‰, δD = −98.7 to −89.8‰; Stage III: δ18Ofluid = 1.2 to 1.4‰, δD = −103.5 to −101.2‰) indicate that the hydrothermal fluids are dominated by magmatic water in the early stages (Stages I and II) and mixed with meteoric water since Stage III. The pyrite S-Pb isotope data (δ34S: −2.91 to 3.40‰; 206Pb/204Pb: 16.3270 to 16.4874; 207Pb/204Pb: 15.2258 to 15.3489; 208Pb/204Pb: 36.6088 to 36.7174), combined with Pb isotopic compositions of the intrusive rocks and wall rocks (the Seluohe Group) in the ore district, indicate that the ore-forming materials at Songjianghe are predominantly from a magmatic source and may have been affected by the contamination of the Seluohe Group. In accordance with the features of ore geology, ore-forming fluids and metals, and geodynamic setting, the Songjianghe Au deposit belongs to a mesothermal magmatic hydrothermal vein gold deposit, which formed in the intermittent stage of Paleo-Pacific plate subduction during the Late Jurassic.

Petrogenesis and tectonic setting of Early Cretaceous mafic dykes in the North Qinling Orogenic Belt, central China: constraints on the lithospheric lower crust delamination

Late Mesozoic mafic dykes, which are widely developed in the North Qinling Orogenic Belt (NQOB), include abundant geodynamic information. This paper describes the mafic dykes that intrude the Late Jurassic granite in the Dayu and Kuyu areas, and reports important petrological constraints for the late Mesozoic tectonic transition from compression to extension in the NQOB. Three zircon U–Pb results show that the minimum ages of the mafic dykes are 139.8 ± 1.4 Ma, 137.4 ± 1.7 Ma and 133.4 ± 0.9 Ma, indicating that the emplacement age of the Dayu and Kuyu mafic dykes is 140–133 Ma. Petrogeochemical analyses suggest that the mafic dykes belong to the high-K calc-alkaline shoshonite series with low SiO2 (46.93–56.73 wt%), MgO (1.88–9.10 wt%) and TiO2 (1.17–1.82 wt%), and high Al2O3 (13.98–17.46 wt%), TFe2O3 (7.81–10.92 wt%) and K2O (1.28–4.78 wt%). The mafic dykes are enriched in large ion lithophile elements (e.g. Rb, Ba, K, La, Sr) and depleted in high-field-strength elements (e.g. Nb, Ta, Zr, Ti). These samples have the right-sloping chondrite-normalised rare earth element patterns, which suggest light rare earth element enrichment and heavy rare earth elements depletion with no obvious Eu anomalies (δEu = 0.94–1.11). The I Sr, ε Nd(t), ε Hf (t) and T DM2(crust) values are 0.7056–0.7060, −10.60 to −5.98, −14.1 to −2.8, and 1382.4 ± 25.1 to 2081.9 ± 47.6 Ma, respectively. Both elemental and isotopic geochemistry show that the formation of Dayu and Kuyu mafic dykes is due to the partial decompression melting of previously enriched lithospheric mantle during a delamination process. The mafic dykes have undergone fractionation crystallisation of Mg–Fe phase minerals during magma ascent, accompanied by some crustal contamination. Combined with the regional tectonic setting, we suggested that the NQOB experienced intra-continental extension during the Early Cretaceous. KEY POINTS Early Cretaceous (140–133 Ma) mafic dykes have been discovered in the middle part of the North Qinling Orogenic Belt. The remote effect of the Paleo-Pacific Plate subduction has reached the middle of the North Qinling Orogenic Belt. The North Qinling Orogenic Belt entered the extensional stage in the Early Cretaceous (140–133 Ma).

Petrogenesis and relationship with REE mineralization of the quartz syenite from Chishan and Longbaoshan alkaline complex, southeastern North China Craton: Insights from zircon U–Pb geochronology, element, and Sr–Nd–Pb–Hf isotope geochemistry

Mesozoic alkaline complexes associated with the rare earth element (REE) mineralization are developed in southeastern North China Craton (NCC), and they recorded some important information about the lithospheric thinning and destruction of the NCC. Zircon U–Pb dating results reveal that syenitic rocks from the Chishan and Longbaoshan alkaline complexes were emplaced at ca. 126.2–123.7 Ma. These syenitic rocks have an arc-like affinity with enriched LREEs and LILEs (e.g., Ba and Th) and depleted HFSE (e.g., Nb and Ta). However, they plot within the extensional alkaline–calc-alkaline field in the lg (CaO/(K2O + Na2O)) versus SiO2 discrimination diagram and plot within the fields of the divergent plate and within-plate settings in the NbN versus ThN discrimination diagram, implying an extensional tectonic setting. Thus, we suggest that the syenitic rocks should form in an extensional tectonic settings related to rollback and retreat of the subducting Paleo-Pacific plate. These syenitic rocks have similar Sr–Nd–Pb–Hf isotope compositions ((87Sr/86Sr)i = 0.7062 to 0.7101, εNd(t) = −8.2 to −15.0, εHf(t) = −13.5 to −9.6), which are the same as the enriched subcontinental lithospheric mantle (SCLM) of the NCC. Taking into account not only our new data but also previously published data regarding the evolution of NCC in the Mesozoic, we conclude that the syenitic rocks from the Chishan and Longbaoshan complexes are derived from partial melting of the lithospheric mantle triggered by the Paleo-Pacific plate subduction. Moreover, the geochemical and Sr–Nd–Pb isotopic similarity between the Chishan REE ores, Longbaoshan altered rocks, and the syenitic rocks implied that they are homologous products of Early Cretaceous and that the mineralization shows inheritance to the magmatic hydrothermal evolution.

P-wave velocity structure in the crust and the uppermost mantle of Chao Lake region of the Tan-Lu Fault inferred from teleseismic arrival time tomography

Chao Lake is a Geoheritage site on the active Tan-Lu Fault between the Yangtze craton, the North China craton, and the Dabie orogenic belt in the southeast. This segment of the fault is not well constrained at depth partly due to the overprinting of the fault zone by intrusive materials and its relatively low seismic activity and sparse seismic station coverage. This study took advantage of a dense seismic array deployed around Chao Lake to delineate the P-wave velocity variations in the crust and uppermost mantle using teleseismic earthquake arrival time tomography. The station-pair double-difference with waveform cross-correlation technique was employed. We used a multiscale resolution 3-D initial model derived from the combination of high-resolution 3-D vS models within the region of interest to account for the lateral heterogeneity in the upper crust. The results revealed that the velocity of the upper crust is segmented with structures trending in the direction of the strike of the fault. Sedimentary basins are delineated on both sides of the fault with slow velocities, while the fault zone is characterized by high velocity in the crust and uppermost mantle. The high-velocity structure in the fault zone shows characteristics of magma intrusion that may be connected to the Mesozoic magmatism in and around the Middle and Lower Yangtze River Metallogenic Belt (MLYMB), implying that the Tan-Lu fault might have formed a channel for magma intrusion. Magmatic material in Chao Lake is likely connected to the partial melting, assimilation, storage, and homogenization of the uppermost mantle and the lower crustal rocks. The intrusions, however, seem to have suffered severe regional extension along the Tan-Lu fault driven by the eastward Paleo-Pacific plate subduction, thereby losing its deep trail due to extensional erosion.

Mesozoic magmatic arc in East Asian continental margin triggered by Paleo-Pacific plate subduction: Constraints from gravity and magnetic anomalies

The Mesozoic East Asian Continental Margin (EACM) is generally considered as a continental magmatic arc triggered by Paleo-Pacific Plate subduction. However, the magmatic arc was not evident for a long time. This paper tries to use the gravity and magnetic responses along the magmatic arc to deeply explore its spatio-temporal distribution. Based on the gravity and magnetic anomalies of the Mesozoic magmatic arc on the EACM and combined with the integration of previous geological and geochronological studies, it is found that the tectonic features of the magmatic arc are more obvious in the fourth-order approximation/detail gravity anomaly fields by using wavelet multi-scale analysis. Furthermore, the residual crustal gravity anomalies containing the medium-wavelength components of the complete Bouguer gravity anomalies are analyzed for the identification of the Jurassic magmatic arcs. Finally, line-drawing analysis is performed on the fourth-order detail gravity anomaly field to explore the response of the Mesozoic magmatic arcs to Paleo-Pacific subduction on the EACM. Our results support that the magmatic arcs on the EACM mainly developed in the Late Triassic-Early Cretaceous Andean Active Continental Margin and in the late Early Cretaceous-Eocene transtensional continental margin. Our study contributes to the formation and refinement of the Paleo-Pacific Plate subduction mechanism and the Mesozoic tectonic evolutionary model of the EACM, and also provides geophysical evidence for the reconstruction of dynamic processes of the EACM.

Re Variation Triggered from the Paleo-Pacific Plate Evolution: Constrains from Mo Polymetallic Deposits in Zhejiang Province, South China Mo Province

Although highly dispersed, critical Re metal has attracted lots of attention from geoscientists, the controlling factors of Re-content variation are not completely understood, especially with regards to the genetic relationship between Re-bearing Mo polymetallic deposits and plate subduction evolution. It is well documented that the South China Mo Province, in Zhejiang Province, is characterized by multi-stage Mo polymetallic mineralization associated with Paleo-Pacific plate subduction. The Xianlin Mo(Cu)–Fe deposit occurs in Western Zhejiang as porphyry mineralization or skarn mineralization between the granodiorite and limestone. Zircon U–Pb analysis of the ore-forming granodiorite yields a Concordia age of 150.8 ± 1.1 Ma. Six molybdenite samples have relatively high Re contents (128.9~155.7 ppm) and deliver a weighted mean model age of 149.6 ± 1.3 Ma. These geochronological data suggest the Xianlin polymetallic mineralization was genetically related to the granodiorite in the Late Jurassic. Moreover, a new compilation of reliable Re contents and Re–Os isotope age data in Zhejiang Province indicates a decreasing trend in Re contents of molybdenite from the Jurassic Fe-/Cu-dominated Mo mineralization stage to the Cretaceous PbZn-enriched Mo mineralization stage in the South China Mo Province. Based on previously proposed models relating tectonic, magmatic, and hydrothermal processes, it is suggested that the Jurassic Re-enriched Mo mineralization, associated with I-type granitoids, formed in a compressive setting during the low-angle subduction of the Paleo-Pacific slab, whilst the Cretaceous Re-poorer Mo/Mo–Pb–Zn mineralization, related to both I- and A-type granitoids, formed in an extensional back-arc setting triggered by the rollback of the Paleo-Pacific slab.

The long-lived transcrustal magmatic systems of Southeast China in response to paleo-Pacific plate subduction, recorded by the Cretaceous volcanic sequences in southeastern Zhejiang Province

Processes taking place in subduction zones are highly debated. The Cretaceous volcanic rocks are voluminously distributed along the coastal area of southeastern (SE) China. To elucidate their petrogenesis and relationship with subduction, we use new zircon U-Pb ages and Hf-O isotopes for the representative Cretaceous volcanic sequences in Zhejiang Province, China. According to stratigraphic investigations, zircon U-Pb ages, and Hf-O isotopes, these volcanic rocks can be divided into different groups corresponding to three stages of volcanic activity: the early (Stage 1, 136−129 Ma), middle (Stage 2, 125−115 Ma), and late (Stage 3, 110−94 Ma) stages. Diverse zircon populations (including antecrysts, autocrysts, and xenocrysts) are recognized. Hf-O isotopes of autocrysts suggest different protoliths for the identified three stages of volcanic activity. Xenocrysts show obvious different compositions from autocrysts. Antecrysts share similar compositions with autocrysts, which favor similarities in the magmas from which they were generated. Our observations (very small age intervals between antecrysts and autocrysts, sources for volcanic rocks throughout the entire crust, and volcanic magmatism with long-term and discontinuous characteristics), were inconsistent with the traditional melt-dominated magma chamber model. Consequently, we propose that those volcanic rocks were derived from long-lived transcrustal magmatic systems (TCMS), dominated by crystal mush, instead of melt-dominated magma chambers, maintained and recharged by a discontinuous contribution of contemporaneous underplated mantle-derived magmas, triggered by paleo-Pacific plate subduction. We suggest the different stages of volcanic activity and corresponding long-lived TCMS were produced by the change of Pacific plate motion beneath SE China during the Cretaceous period.

Petrogenesis of early cretaceous intermediate to felsic rocks in Shanghai, South China: Magmatic response to Paleo-Pacific plate subduction

Voluminous magmatism and extensional deformation are the prominent late Mesozoic tectonic features in East Asia, which recorded the evolutionary history of the continental marginal lithosphere. Due to the thick covering of Quaternary and Pliocene sediments in the region, the Early Cretaceous magmatism in northeastern South China is not well constrained. In this study, we collected samples from the boreholes in Shanghai, East China and provided geochronological, elemental and isotopic analyses of andesite and granodiorite. Zircon UPb age results indicate that andesites and granodiorites were formed at ca. 135 to 102 Ma. The andesites exhibit moderate MgO contents, slightly radiogenic Sr (87Sr/86Sr(t) = 0.706149 to 0.706170) and variable unradiogenic Nd and Hf isotopic compositions (εNd(t) = −5.32 to −5.12; εHf(t) = −10.8 to −0.6). The granodiorites exhibit arc-like trace-element patterns with high Sr/Y ratios, as well as slightly radiogenic initial Sr (87Sr/86Sr(t) = 0.706754 to 0.706790) and moderately unradiogenic Nd and Hf isotopic compositions (εNd(t) = −6.54 to −6.27; εHf(t) = − 4.0 to −6.7). Both andesites and granodiorites exhibit similar initial Pb isotopic compositions (andesites: 206Pb/204Pb = 18.232–18.259, 207Pb/204Pb = 15.547–15.548, and 208Pb/204Pb = 38.338–38.350; granodiorites: 206Pb/204Pb = 17.930–17.946, 207Pb/204Pb = 15.542–15.543, and 208Pb/204Pb (38.256–38.293). The results recommend that the ca.130 Ma andesites were produced by the partial melting of an enriched lithospheric mantle, which was previously metasomatized with sediments in the source region in a continental arc setting of the Paleo-Pacific subduction. The ca.102 Ma adakitic granodiorites probably originated from partial melting of the lower crust with the addition of mantle-derived magmas. With the consolidation of previous studies, we argue that the flat slab subduction of the paleo-Pacific Plate led to the formation of these intermediate to felsic magmatism.

Jurassic contractional deformation in the central–western North China Craton in response to multi-plate convergence in the East Asia

The Jurassic multi-plate convergence in East Asia led to the intra-continental deformation in the North China Craton (NCC). However, much controversy is highlighted in the intra-continental deformation style and timing, and unanimous kinematics. The central-western North China Craton of the East Asia, typical of the Jurassic structural belts around the Ordos Block, is a key region to delineate the intra-continental deformation. Here, using new structural analysis, the definition of Jurassic growth strata, and paleo-stress inversion, grouping with our previous work on the NCC, we reconstructed a multi-directional shortening pattern in the central-western NCC during the Middle–Late Jurassic (170–160 Ma), which is characterized by N-S shortening along the northern margin of the NCC, by NW-SE shortening in the western margin and along the central zone of the NCC. In combination with regional structural analysis, we propose that the central-western North China Craton is roughly situated at the center of the multi-plate convergence during the Middle-Late Jurassic, and the southward compression was caused by the closing of the Mongolia-Okhotsk Ocean in the north, the northwestward compression by the subduction of paleo-Pacific plate beneath the East Asia continent in the east, and southeastward compression by the closure of the Meso-Tethys in the west.

Subduction of the paleo-Pacific plate recorded by arc volcanism in the South China Sea margin

Accretionary prisms along the East Asia continental margin have been interpreted as having formed in response to subduction of the paleo-Pacific plate in the Paleozoic–Mesozoic. However, early arc magmatic records in the East Asia margin have been partly destroyed owing to episodic rifting. Widespread arc magmatism and fold and thrust belts in the South China block have recorded Jurassic and Cretaceous paleo-Pacific plate subduction. In addition, the exterior of the South China continental margin was rifted during the Cenozoic to form the South China Sea, and it is presumed that a relic arc is preserved in the northern and southern South China Sea. In this study, we obtained samples of volcanic rocks through drilling >2000 m in the southern South China Sea. The drill core consisted of interbedded basalts, trachytes, and minor trachy-andesites, with arc-type trace-element patterns. Zircon U–Pb dating of trachyte samples from the drill core yielded clustered ages with an overall weighted mean age of 211.2 ± 1.6 Ma, providing robust evidence for the existence of a volcanic arc in the East Asia margin during the Late Triassic. Primitive basalts with MgO > 8.5 wt% have the highest values of εNd(i) and εHf(i) and indicate derivation from Indian-type sub-arc mantle wedge. The increasing Th/Nb with decreasing Nd−Hf isotope values of the basalt samples indicates the incorporation of sediment into the melts. The trachyte samples have enriched K2O and isotopic compositions and are consistent with a sediment-dominated source. On the basis of a comparison with regional magmatic rocks related to plate subduction during Triassic–Jurassic time, the arc volcanic rocks in the southern South China Sea are interpreted to mark a temporal compositional transition in the mantle source, representing steepening subduction of the paleo-Pacific plate during the Late Triassic.

Geochemistry and Isotopic Characteristics of Apatite and Zircon From Late Jurassic Granites in the Jiaobei Terrane, East China: Implications for Petrogenesis and Geodynamic Setting

The North China Craton (NCC) was stable for more than 2.0 Gyr before a Jurassic–Cretaceous large-scale lithospheric thinning event, but the geodynamic setting during the early phases (Late Jurassic) of NCC reworking remains controversial. We present new petrological and whole-rock geochemical data, zircon and apatite geochemistry, U–Pb ages, O isotopic data, and Sr–Nd isotopic data for two phases of Late Jurassic granite (Linglong and Luanjiahe) from the Jiaobei terrane, southeastern NCC. LA-ICP-MS zircon U-Pb dating suggests that the Linglong granite formed about 6 Myr earlier than Luanjiahe granite (158 Ma vs 152 Ma), after the inception of the paleo-Pacific plate subduction. High zircon U/Yb ratios, high δ18O values [7.89 ± 0.10‰ to 7.67 ± 0.14‰ (2σ)], and inherited zircon age spectra, as well as high apatite F/Cl ratios and Sr–Nd isotopic compositions, suggest that the Linglong and Luanjiahe granites formed by partial melting of ancient thickened lower continental crust of the NCC and Yangtze Craton. Magma evolution modelling based on Rb and Rb/Nb data suggests a similar decoupled assimilation-fractional crystallization process for the generation of Linglong and Luanjiahe granite but with different assimilation degrees. The water contents of parental magma evaluated by using whole-rock Ba, Sr and apatite F, Cl data indicate that the Linglong granite was formed in a relatively water-rich environment than Luanjiahe. This is consistent with the presence of amphibole and minor negative Eu anomalies in the Linglong granite, as water input can promote amphibole fractionation and suppresses plagioclase crystallization. Considering the similar magma sources but distinct water contents of the granites, and the oblique Paleo-Pacific plate subduction setting in the Late Jurassic, the fluids were likely released from the ocean plate beneath a stacked thickened crust. Since the earliest mafic dikes (OIB-type) in the NCC are coeval with the Luanjiahe granite, we suggest that the lower water contents of the Luanjiahe granite were associated with roll-back that resulted in an increasing distance from slab to continental crust. Such a tectonic transition from subduction compression (158 Ma) to initial extension (152 Ma) in the Late Jurassic perhaps possibly marks the beginning of the reworking of the NCC.

Migration of Middle-Late Jurassic volcanism across the northern North China Craton in response to subduction of Paleo-Pacific Plate

The primary driver of magmatism, crustal deformation and metallogeny of the North China Craton in the Jurassic remains actively debated, either attributed to the subduction of the Paleo-Pacific (i.e., Izanagi) plate in the east or to the closure of the Mongol-Okhotsk Ocean in the north. This issue is addressed here by examining temporal and spatial variation of the contemporary volcanic rocks (i.e., Tiaojishan Formation) of the Yanshan belt in the northern part of the craton. These rocks are mainly calc-alkaline and intermediate-felsic in composition and show a highly coherent geochemical composition and unradiogenic zircon Hf isotope, indicating that they have similar origins and formed in a subduction-related setting. The eruption age of the Jurassic volcanic rocks decreases westward (i.e., inland-ward) from Western Liaoning (166–153 Ma), through Pingquan-Chengde-Luanping (162–153 Ma) and Jingxi-Xuanhua (158–149 Ma), to Yuxian (157–142 Ma). This temporal and spatial variation is consistent with the motion of the subducted Paleo-Pacific plate over that time. In addition to the westward migration, the earliest volcanic rocks in the Yanshan belt formed earlier than those in the adjacent southern Great Xing'an Range in the north. These observations suggest that the southward subduction and closure of the Mongol-Okhotsk plate played a limited role in generation of these volcanic rocks. Two episodes of Middle-Late Jurassic volcanism (166–153 Ma and 150–139 Ma) are observed in the northern Great Xing'an Range. They are interpreted as results of closure of the Mongol-Okhotsk Ocean and subduction of the Paleo-Pacific plate, respectively. Temporal-spatial variation of volcanic activity in the northern North China Craton and adjacent region demonstrates that the Paleo-Pacific subduction was the principal cause of the evolution of Eastern China during the late Mesozoic, whereas the Mongol-Okhotsk tectonic regime is largely confined to the northern Great Xing'an Range and the area north to it.

Discovery of Late Mesozoic volcanic seamounts at the ocean-continent transition zone in the Northeastern margin of South China Sea and its tectonic implication

The Northern South China Sea (SCS) margin experienced the switch from Andean-type active subduction to passive extension during late Mesozoic era. However, the mechanism of cessation of paleo-Pacific subduction in the SCS region is unclear, and the location of suture zone between paleo-Pacific plate and South China block is obscure. In this study, we report new 40Ar/39Ar ages and whole-rock Hf isotopes of basalts from two poorly studied volcanic seamounts (namely, Puyuan and Beipo) at the ocean-continent transition zone in the Northeastern SCS margin, with an emphasis on the existence of exotic, buoyant oceanic plateau and its geodynamic effect on subduction cessation. Results indicate that the OIB-type Puyuan and E-MORB-type Beipo volcanic seamounts were erupted at 154.1 Ma and 93.2 Ma, respectively. Whole-rock trace element, Sr-Nd-Pb-Hf isotopic compositions, and subduction polarity point out that these samples were oceanic intraplate basalts formed within the paleo-Pacific plate, independent of the coeval, Late Mesozoic paleo-Pacific subduction system in the region. The ages and geochemical data of the two volcanic seamounts, combined with geochemical data of Late Mesozoic Pacific oceanic plateaus and seismic data of the Northeastern SCS margin, show that there may exist a Late Mesozoic exotic, buoyant oceanic plateau in the region. The collision between the oceanic plateau and the South China block could lead to the cessation of paleo-Pacific plate subduction, and changes in tectonic regime during the Late Cretaceous era (ca. 100–65 Ma). The pre-existing, Late Mesozoic suture zone has been well constrained by the remnant oceanic plateau and continental arc, and may play a key role in the formation of Cenozoic rifting tectonic units in the region.

Crustal Structure and Anisotropy in the Lower Yangtze Region and its Metallogenic Implications

In this study, we performed receiver function profiling and fitted harmonic functions to the arrival time variations of Pms phases to calculate the crustal seismic anisotropy with delay time and fast polarization direction, using broadband seismic data obtained from 55 temporary stations in two linear profiles and 39 stations in the Lower Yangtze and adjacent region. Moreover, we determined the crustal thickness and Poisson’s ratio using a novel H-κ-c stacking method. Our results revealed that the Middle-Lower Yangtze Metallogenic Belt and the north east section of the Qinzhou-Hangzhou Metallogenic Belt are characterized by Moho upliftment (<32 km), a relatively high Poisson’s ratio (>0.26), local lithospheric thinning (<70 km), and a pattern of deep faults that connect the crust and asthenosphere and serve as conduits for magma upwelling. The NE-SW fast polarization direction was consistent with the SKS splitting results, and the average delay time was 0.45 s. Moreover, underplating of deep magma and upwelling along the weak zone caused local Moho uplift and ductile shear of the lower crust, resulting in the directional arrangement of amphibole and other minerals, which may be the controlling mechanism for the crustal anisotropy in the study area. The variations in crustal structure and anisotropy characteristics indicated that in the context of the northeastern Paleo-Pacific plate subduction, the existence of weak lithospheric zones and the northeastern asthenospheric flow are important conditions for metal supernormal enrichment in the Lower Yangtze region.

Paleo–Pacific plate subduction on the eastern Asian margin: Insights from the Jurassic foreland system of the overriding plate

Abstract The subduction of the paleo–Pacific plate beneath the North China block has been well documented in terms of magmatic activity, geophysical investigations, and numerical modeling, but the timing of its onset along the eastern Asian margin and the tectonic processes involved remain poorly understood. We have now reconstructed the structural evolution and sedimentation of the Shihuiyao–Gangzidian–Yuantai region of the southern Liaodong Peninsula, on the overriding plate at the eastern Asian margin, using geologic mapping, borehole data sets, field structural studies, and zircon geochronology. In the Shihuiyao area, based on geologic mapping and drilling, top-to-the-NW thrust faults truncated earlier top-to-the-NE reverse faults and then were subsequently overprinted by normal faults. In the Gangzidian area, geologic mapping and 40 boreholes allowed us to construct eight cross sections revealing the top-to-the-WNW/W thrust faults from the surface to a depth of at least 0.5 km; the sections show that the thrust faults extend to the basement and that subsequent normal faults dip subvertically. In the Yuantai thrust system, the top-to-the-NW thrusts, expressed as four tectonic windows and a duplex on the map scale, were intruded by late porphyry and diabase sills. Integration of the data from the three study areas allowed us to identify one angular unconformity and at least two phases of later deformation (D1 and D2). The pre-D1 unconformity is marked by a foreland basin that was filled with Middle Jurassic clastic rocks that unconformably overlie the Neoproterozoic and Cambrian basement. The D1 deformation is represented by NE-SW–striking thrust faults that displaced Neoproterozoic or Cambrian strata onto the Middle Jurassic strata. The D2 deformation is defined by kilometer-scale, high-angle normal faults with variable dips and strikes. Although three samples from the Middle Jurassic clastic rocks did not yield ideal maximum deposition ages (MDAs; ca. 246 Ma), a porphyry dike and a diabase sill that intruded the Yuantai thrust system and remain undeformed yielded zircon U-Pb ages of ca. 124 and 117 Ma, respectively. Detrital zircon geochronology yielded a MDA of ca. 129 Ma for Cretaceous deposits in graben structures with hanging walls formed by D2 normal faults. Thus, the D1 and D2 deformation events can be constrained to the Late Jurassic and Early Cretaceous, respectively (D1 between ca. 174 and 124 Ma, and D2 after ca. 129 Ma). The provenance of the detritus in the Middle Jurassic Wafangdian Formation suggests that the deposits in the retroarc foreland basin had a source in a thrust sheet of the Paleoproterozoic basement, but the source of deposits in a minigraben (D2) was possibly the nearby Neoproterozoic rocks. Therefore, we reconstructed the retroarc foreland basin and fold-and-thrust belt of the southern Liaodong Peninsula in terms of a subduction margin and constrained a Toarcian–Aptian (ca. 174–124 Ma) age for the onset of paleo–Pacific plate subduction. We interpret the foreland basin system and the subsequent synconvergent extension to have been the result of slab flattening and rollback during episodic subduction of the paleo–Pacific plate.

Mesozoic Magmatic Arc in East Asian Continental Margin Triggered by Paleo-Pacific Plate Subduction: Constraints from Gravity and Magnetic Anomalies

Mesozoic Magmatic Arc in East Asian Continental Margin Triggered by Paleo-Pacific Plate Subduction: Constraints from Gravity and Magnetic Anomalies

Permian to Cretaceous tectonic evolution of the Jiamusi and Songliao blocks in NE China: Transition from the closure of the Paleo-Asian Ocean to the subduction of the Paleo-Pacific Ocean

Zhangguangcailing Orogen constitutes a significant part of the Eastern Asian Continental margin. Its evolution was constrained by the oceanic consumption processes between the Jiamusi and Songliao blocks during late Paleozoic and Mesozoic time, resulted from the closure of the Paleo-Asian Ocean and subsequent Paleo-Pacific Ocean subduction. Considerable and continuing controversy has surrounded when, where, and how the ocean was subducted and the two blocks were eventually amalgamated, which plays a crucial role in understanding the tectonic transition from the closure of the Paleo-Asian ocean to the subduction of the Paleo-Pacific Plate, especially the initial formation of the Mesozoic foreland basin of the Songliao Basin. This review synthesizes the most recent data and geologic evidence that place critical constraints on the evolution of this oceanic domain.Available data and geological evidence indicate that there exists a stable micro-continent consisting of several microblocks of NE China and adjacent area within the Paleo-Asian Ocean regime during the Late Paleozoic. In the early Permian, voluminous magmatism occurred in an extensional setting induced by the subduction of the Paleo-Asian Ocean to the south and the Mongol-Okhotsk Ocean to the northwest. As the crustal extension continued, a continental rifting was initiated and further developed into an oceanic domain (the Heilongjiang Ocean or the Mudanjiang Ocean) separating the Jiamusi and Songliao blocks during Permian and Early Triassic time. Later, an active continental margin was situated along the Zhangguangcailing Orogen due to the subduction of the Heilongjiang oceanic plate in the Late Triassic, followed by a change in the regional stress regime controlled by Paleo-Pacific plate subduction. The high-pressure metamorphism occurred synchronously with the subduction/collision between the Jiamusi and Songliao blocks, forming the Heilongjiang Complex. The oceanic closure and collision events were sequentially followed by the A-type granitic magmatism, which commenced in the earliest Cretaceous.

Petrogenesis of the Early Cretaceous Hongshan Complex in the Southern Taihang Mountains: Constraints from Element Geochemistry, Zircon U-Pb Geochronology and Hf Isotopes

The Hongshan complex, located in the southern part of the Taihang Mountains in the central part of the North China Craton, consists of syenite stocks (including fine-grained biotite aegirine syenite, medium-grained aegirine gabbro syenite, coarse-grained aegirine gabbro syenite, syenite pegmatite, and biotite syenite porphyry), with monzo-diorite and monzo-gabbro dikes. This paper presents zircon U-Pb ages and Hf isotope data and whole-rock geochemical data from the Hongshan complex. LA–ICP-MS zircon U–Pb age from the fine-grained biotite aegirine syenite, monzo-diorite, and monzo-gabbro are 129.3 ± 2.0 Ma, 124.8 ± 1.3 Ma, and 124.1 ± 0.9 Ma, respectively, indicating their emplacement in the Early Cretaceous when the North China Craton was extensively reactivated. The monzo-diorite and monzo-gabbro have low SiO2 contents (48.94–57.75 wt%), total alkali contents (5.2–9.4 wt%), and εHf (t) values of −22.3 to −18.4 and are enriched in MgO (4.0–8.2 wt%), Al2O3 (14.3–15.8 wt%), light rare earth elements (LREEs) and large ion lithophile elements (LILEs). Interpretation of elemental and isotopic data suggests that the magma of monzo-diorite and monzo-gabbro were derived from partial melting of the enriched lithospheric mantle metasomatized by slab-derived hydrous fluids. Syenites with high alkali (K2O + Na2O = 9.4–13.0 wt%) and Sr contents (356–1737 ppm) and low Yb contents (0.94–2.65 ppm) are enriched in Al (Al2O3 = 16.4–19.1 wt%), but depleted in MgO (0.09–2.56 w%), Cr (Avg = 7.16 ppm), Co (Avg = 6.85 ppm) and Ni (Avg = 9.79 ppm), showing the geochemical features of adakitic rocks associated with thickened lower crust. Combining zircon 176Hf/177Hf ratios of 0.282176 to 0.282359, εHf(t) values of −18.3 to −11.8 and εNd (t) values of −11.1 to −8.2, we conclude that the syenite magma was derived from the mixing of the thickened lower crust and the enriched lithospheric mantle magma. These magma processes were controlled by Paleo-Pacific plate subduction and resulted in the destruction and thinning of the central North China Craton.

Jurassic to Early cretaceous sedimentary record: indications of Paleo-Pacific Plate subduction in Southeast China

ABSTRACT Plate subduction plays a decisive role in continental tectonics, and subduction history is closely connected to deformation and sedimentation in the overriding plate. When and how the Paleo-Pacific Plate subducted beneath Southeast China in the Mesozoic is debated. Here, we show that provenance data, including detrital composition point counts, palaeocurrent data, and detrital zircon U-Pb geochronology from basins in Southeast China, record the timing and process of Paleo-Pacific Plate subduction during the Jurassic and Earliest Cretaceous. The U-Pb dating of detrital zircons from Jurassic samples indicates that the sediments were transported from multiple sources, including intracratonic mountains, tectonic belts and the inferred active continental margin arc that resulted from Paleo-Pacific Plate subduction. The clastic composition and U-Pb dating of Early Cretaceous tuffs suggest that they were sourced from nearby magmatic rocks and the eastern inferred magmatic arc. Based on detrital zircon U-Pb dating, igneous rock isotope, sedimentary, and structural data, we infer that an active continental magmatic arc developed along the coast of Southeast China in the Early Jurassic to Early Cretaceous (~200 Ma-135 Ma). The westward subduction of the Paleo-Pacific Plate initiated in the Early Jurassic (~200 Ma) and continued to the Early Cretaceous (~135 Ma). Paleo-Pacific Plate subduction resulted in the uplift of the northern Wuyi Mountain region and eastern South China and controlled the provenance characteristics of the sedimentary basins in Southeast China during the Jurassic to Early Cretaceous.