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Onset of the North-South Gravity Lineament, NE China: Constraints of Late Jurassic bimodal volcanic rocks
Abstract
The North-South Gravity Lineament (NSGL) is an important geophysical boundary within NE China. Gravity anomaly, heat flow, crustal and lithospheric thickness, terrain and altitude all change dramatically across the NSGL. However, the timing of onset of the NSGL remains enigmatic, and the Mesozoic lithospheric thinning mechanism to the east of the NSGL is still unclear in NE China. Here we study a Late Jurassic bimodal volcanic suite located on the NSGL including N-MORB-like basalts, which constrains the initial formation of the NSGL. The bimodal volcanic rocks consist mainly of basalts and basaltic andesites with subordinate rhyolites. Our results show that the bimodal volcanic rocks are products of crust-asthenosphere interaction. Calculations indicate that lithospheric thicknesses in NE China have significantly changed from ~90-100 km in the west to 49-62 km in the east since the Late Mesozoic. The variation of lithospheric thicknesses on both sides of the NSGL could be ascribed to lithospheric delamination. Zircons from the bimodal volcanic rocks yield Late Jurassic ages of 147 Ma, nearly synchronous with the onset of the Songliao Basin and marking the start of extensional tectonics in NE China. Combined with previous studies, we confirm that the delamination induced by Paleo-Pacific subduction gave rise to the formation of the NSGL, which initiated at the Late Jurassic in NE China.
... At the end of the orogeny, the thermal disturbance within the asthenosphere led to the partial melting of the previous metasomatic mantle and the resulting arc-type mafic rocks . According to the modeled partial melting of pyrolites (Dong et al., 2019;Jourdan et al., 2007;Tang et al., 2014), the ratios of key trace elements indicate that there are subtle lithological differences in the mantle sources of the post-collisional mafic rocks. Although the plots straddle the transition zone from garnet-phase lherzolite to spinelphase lherzolite and present a large degree of overlap, the mantle source of ANMT has a close affinity with the former, especially the samples from the Lenghu area (Figure 7). ...
... Compared with AMT, ANMT is more prone to metasomatic mantle with garnet-phase lherzolite. Modeling and mantle melting curves are fromJourdan et al. (2007),Tang et al. (2014), andDong et al. (2019). ...
Post‐collisional mafic rocks not only record geodynamic processes at the end of the orogenic cycle but also retain various clues regarding preceding interactions between subducted slabs and mantle wedges. However, the latter and related indicators have often been overlooked; in particular, how the continental crust interacts with the mantle in subduction zones and modifies its nature remains ambiguous. This study of post‐collisional mafic rocks in the North Qaidam orogen provides new insights into sophisticated crust–mantle interactions via variable continental subduction. These post‐collisional mafic rocks are consistent with the geochemical nature of arc lava and originated from partial melting of the antecedent metasomatic mantle. Although they possess relatively uniform whole‐rock Sr–Nd isotopes, the mafic rocks from the ultrahigh‐pressure metamorphic terranes present more enriched zircon Hf isotopes and remarkable signals of melt‐driven mantle metasomatism than the other rocks within the non‐ultrahigh‐pressure metamorphic zone. This is attributed to spatially variable continental subduction and the consequent differentiated crust–mantle interactions. Trace element modeling also reveals that a greater proportion of continental component‐derived metasomatic melts are needed to form mafic rocks in ultrahigh‐pressure metamorphic terranes. The whole‐rock magnesium isotopes are between −0.15‰ and −0.38‰, which are negatively correlated with (⁸⁷Sr/⁸⁶Sr)i and positively correlated with ɛNd(t) and ɛHf(t). These findings indicate that these mafic rocks exhibit lighter magnesium isotopes when their mantle source is metasomatized by more continental component‐derived melts enriched with radiogenic isotopes. The geochemical distinctions of the post‐collisional mafic rocks verify the contributions from continental subduction to heterogeneous mantle metasomatism and magmatic diversity.
... This variation was interpreted as lithospheric thinning affected by the Paleo-Pacific subduction during the late Mesozoic, and the location of depth mutation (also referred to as north-south gravity lineaments in China; Fig. 1a) east of the GXR might represent the western boundary of the Paleo-Pacific subduction (He et al. 2022). (3) The occurrence of late Jurassic bimodal volcanic suite (147 Ma) in the Songliao Basin marked the beginning of the extensional setting (Dong et al. 2019). The calculated lithospheric thickness and the formation age of the Songliao Basin indicate that the Paleo-Pacific subduction-induced delamination started in the late Jurassic (Dong et al. 2019). ...
... (3) The occurrence of late Jurassic bimodal volcanic suite (147 Ma) in the Songliao Basin marked the beginning of the extensional setting (Dong et al. 2019). The calculated lithospheric thickness and the formation age of the Songliao Basin indicate that the Paleo-Pacific subduction-induced delamination started in the late Jurassic (Dong et al. 2019). This evidence indicates that the influence of the Paleo-Pacific rollback extended to the GXR during the late Jurassic-early Cretaceous. ...
To determine the emplacement age, petrogenesis, and geodynamic setting of the Xizhelimu diorite in Keyouzhongqi, Inner Mongolia of northeastern China, a detailed study of the petrography, geochronology, and whole-rock geochemistry has been conducted. Geological and petrographic studies show that the Xizhelimu diorite is zoned: the central lithofacies zone is composed of medium-fine-grained monzodiorite and quartz diorite, and the marginal lithofacies zone is fine-grained diorite. The zircon U–Pb dating results show that the ages of the central and marginal facies are 133.5 ± 1.9 and 133.4 ± 1.4 Ma, respectively. The whole-rock rare earth and trace element characteristics of the Xizhelimu diorite show an O-type adakite affinity. Combining the analysis of zircon Hf isotope composition (εHf(t) values of +7.7 to +10.0), the geochemical features of whole rock, and the results of partial melting modeling, we suggest that the parental magma of the Xizhelimu diorite was derived from the partial melting of altered oceanic crust mixing with subducting sediments at shallow depths. In the early stage of early Cretaceous, the Xizhelimu diorite originated in an extensional setting, mainly related to the closure of the western part of the Mongol–Okhotsk Ocean. The upwelling asthenospheric flow in this extensional setting induced partial melting of the paleo-oceanic crust to form the parental magma of the Xizhelimu diorite.
... In a Sr-Nd isotopic diagram, Nd isotopes are relatively uniform, with ε Nd (t) values ranging from +0.3 to +1.4, whereas Sr isotopes vary greatly and all plots are nearly parallel to the horizontal axis (Fig. 7). This trend reflects the alteration of rocks by hydrothermal fluids (Dong et al., 2019;Rollinson, 1993), and a consistent conclusion is also obtained from the determination of elemental stability (Fig. S3), meaning that Sr isotopes could reflect illusory information for mantle source. On the basis of the stability of the whole-rock Nd and zircon Hf isotopic systems, the KDL metagabbro has relatively uniform and coupled Nd-Hf isotopic compositions. ...
... In fact, previous researches have proven the occurrence of different stages of back-arc extension above the subduction zone in response to the northward subduction of the North Qaidam oceanic slab during the early Paleozoic. Evidence for back-arc extension has been found in Tomorite, south of the Olongbuluke microblock, as well as in the Lvliangshan and Xietieshan Jourdan et al. (2007) and Dong et al. (2019). Partition coefficients are based on McKenzie and O'Nions (1991). ...
Compared with the statement of the mantle plume hypothesis for large-scale plus high-volume intraplate magmatism, a series of OIB-like mafic rocks which are low-volume and spatially closely related to subduction zones still remain mysterious and inapposite to be explained by using traditional mantle plume hypothesis. In the North Qaidam, most previous studies have focused on ultra-high pressure metamorphic rocks and have paid little attention to mafic magmatism as a mechanism of generating magma with a mantle signature. The Kendelong (KDL) metagabbro with consistently intrusive ages of 485–490 Ma within the Olongbuluke microblock that is overriding plate above the subduction zone is analogous to Oceanic Island Basalts (OIBs) in geochemistry and formed contemporaneously with oceanic subduction. Analyzed samples of the KDL metagabbro have sporadic (⁸⁷Sr/⁸⁶Sr)i ratios of 0.7008–0.7063 that have been influenced by paulopost alteration, relatively uniform positive εNd(t) values of +0.3 to +1.4, zircon εHf(t) values of +1.5 to +6.8, and coupled NdHf isotopes that lie on the terrestrial array. After ruling out the possibility of contributions from subducted enriched components and mantle metasomatites, geochemical features and isotopic compositions indicate that the magmas were derived from a relatively primitive asthenosphere without prior large-volume melt extraction. Rare earth element modeling shows that a low degree of partial melting (<10%) of an asthenospheric mantle source in the garnet stability field should be responsible for the magmatic origin of the KDL OIB-like metagabbro. Considering the temporal and spatial correlations between the KDL metagabbro and other types of mafic rock in adjacent regions, we propose that decompression melting of upwelling asthenosphere triggered by rollback of subducted oceanic slab is a plausible regime to explain the petrogenesis of the early Paleozoic KDL OIB-like metagabbro and to trace a fortuitous tectonic transition event. Moreover, even if the OIB-like magmatism is spatially related to subduction zones, the participation of subduction materials could not be an exclusive prerequisite to sentence its genesis.
... Lithos 376-377 (2020) 105771 6.1.3. Mantle source Mafic igneous rocks were generally derived from either lithospheric mantle or asthenospheric mantle (Dong et al., 2019;George and Rogers, 2002). Rocks derived from the lithospheric mantle are characterized by low ε Nd (t) values and high initial 87 Sr/ 86 Sr ratios, whereas those from the asthenospheric mantle have depleted signatures in isotopes (e.g., Dong et al., 2019). ...
... Mantle source Mafic igneous rocks were generally derived from either lithospheric mantle or asthenospheric mantle (Dong et al., 2019;George and Rogers, 2002). Rocks derived from the lithospheric mantle are characterized by low ε Nd (t) values and high initial 87 Sr/ 86 Sr ratios, whereas those from the asthenospheric mantle have depleted signatures in isotopes (e.g., Dong et al., 2019). In our study, although some basaltic samples 05,39 and 41) show relatively high initial 87 Sr/ 86 Sr ratios due to the subsequent alteration (Fig. 8A), all the basaltic lavas are characterized by high ε Nd (t) (+1.27 to +4.65) and slightly enriched in LREEs and HFSEs with insignificant crustal contamination (Figs. ...
The tectonic evolution of the Proto- and Paleo-Tethys oceans had a significant influence on ocean-continent distributions in East Asia in the Phanerozoic, and major implications for continental growth in the region. However, it remains ambiguous when and how the Proto-Tethys Ocean transformed into the Paleo-Tethys Ocean. Here we present petrologic, mineralogical, chronological and geochemical data for Early Devonian mafic igneous rocks, located in the East Kunlun Orogen. The mafic igneous rocks include basaltic lavas and diabase dykes, and are tholeiitic in composition. Geochemical and Sr-Nd isotopic data indicate that the basaltic lavas were derived from melting of a spinel-bearing asthenospheric mantle (E-MORB) at normal mantle potential temperatures (1384-1400 °C) with negligible (1-4%) crustal contamination. The diabase dykes probably originated from melting of a spinel-bearing lithospheric mantle metasomatized by subduction-related fluids, with 5-20% crustal contamination, and crystallized at 1100-1135 °C. Both basaltic lavas and diabase dykes have the geochemical characteristics of within-plate basalts. Magmatic zircons from the mafic rocks yield Early Devonian ages (407-403 Ma), postdating the East Kunlun ultrahigh-pressure metamorphism by 19-25 Myr. Comparing our results with the location and timing of the high- and ultrahigh-pressure metamorphic belt, we conclude that the mafic igneous rocks formed in a post-collisional extensional setting. Their generation was associated with both the terminal stages of the Proto-Tethys orogenic belt, (with orogenic collapse promoted by repeated and localized delamination of lithospheric mantle), and early continental rifting related to the evolution of the Paleo-Tethys Ocean to the south. The period of ~426-390 Ma is important for the transition from Proto- to Paleo-Tethys oceans in the East Kunlun Orogen.
... There is ample evidence that this evolution in East China involves a transition from compressional to extensional tectonics, which is usually explained using plate-driven models Davis et al., 2009;Zheng et al., 2018;Wu et al., 2019;Zhu and Xu, 2019;Meng and Lin, 2021;Ma and Xu, 2021). Despite abundant research, the East China tectonic framework from the Jurassic to the Early Cretaceous remains controversial (Zhai et al., 2004;Jiang et al., 2010;Deng et al., 2017;Dong et al., 2019;Li et al., 2019). This uncertainty encompasses changes in the structure and direction of the subduction of the Paleo-Pacific Plate, changes in the nature of the mantle source for magmatic rocks, and the exact ways by which changes at the plate scale were translated into the patterns of deformation, magmatism and sedimentation recorded within East China. ...
Tectonic transitions from compression to extension at an active continental margin provide clues for a better understanding of the geodynamics of plate subduction. Here, we present detailed investigations of Jurassic to Cretaceous dike swarms in the east margin of the North China Craton (NCC). It is demonstrated that the Jurassic dikes are distinctly different from the Cretaceous dikes, in terms of both occurrence and geochemistry. The Jurassic dike swarms, which include both mafic and felsic types, all strike in NW-dominated directions. Most Jurassic felsic dike swarms display adakitic geochemical characteristics, interpreted to be formed by melting of the ancient lower crust of the NCC. Jurassic mafic dike swarms originated from the metasomatic enriched lithospheric mantle. Thus, the Jurassic dike swarms are indicative of a compressional environment. However, the Cretaceous dike swarms strike in NE-dominated directions. The Cretaceous felsic dike swarms are characterized by geochemical features of A-type granites. The Cretaceous mafic dike swarms have originated from the asthenospheric mantle during back-arc extension. We propose that the Jurassic crustal thickening, lower crustal reworking, and the melting of ancient lithospheric mantle are the result of the Paleo-Pacific subduction and related compression. In contrast, crustal thinning indicated by the orientation of Cretaceous dike swarms and their high-temperature features, as well as the reworking of the upper and middle crust and the appearance of depleted mantle, are the result of extension caused by the rollback of the Paleo-Pacific Plate. Variations of composition and orientation of dike swarms effectively reflect the tectonic transition from compression in the Jurassic to extension in the Cretaceous along the east margin of the NCC.
... With the subduction of the Jurassic Paleo-Pacific plate, the Table S1. structure and composition of the mantle beneath the NCC were significantly impacted, resulting in the complete destruction of the NCC (Zhu et al., 2012;Zhang et al., 2014;Dong et al., 2019;Zhu and Xu, 2019;Fang et al. (2021); Chen et al., 2023). Concurrently, two giant igneous sub-provinces have been identified within the NCC during the Jurassic (175-155 Ma) and the Early Cretaceous (135-115 Ma) (Wu et al., 2019). ...
... In summary, we speculate that the volcanic events in GXAR and Mongol-Okhotsk suture region are mainly controlled by Mongol-Okhotsk tectonic domain. 35 , the purple dashed line denotes high-velocity body, and the number within lines denotes P-wave velocity increment taken from Ma et al. 36 , and the black dashed line represents the Songliao Basin; ?0 is the conductivity value of the corresponding layer in the initial model. This map was generated using the GMT software version 5.0 (https://www.generic-mapping-tools.org). ...
In the eastern segment of the Central Asian Orogenic Belt (CAOB), there is widespread volcanic magma activity. However, there is still considerable controversy over the formation mechanisms and material sources of these volcanoes. The mantle transition zone (MTZ), as a necessary channel for the upward and downward movement of mantle material and energy exchange may provide crucial constraints on the dynamic mechanisms of volcanic activity. This paper intends to obtain the deep structure beneath the eastern CAOB based on the geomagnetic depth sounding (GDS) method. First, the data of geomagnetic observatories in the study region are collected and processed, and the C-response curves are obtained by the bounded influence remote reference processing method (BIRRP). Then, the staggered grid finite difference method is used for forward modeling, and the finite memory quasi-Newton method based on L1-norm is used for three-dimensional (3-D) inversion. After that, 3-D inversion is carried out in spherical coordinates. Finally, the electrical conductivity model is obtained. The inversion model shows that there are two high conductivity anomalies in the MTZ beneath the Mongol-Okhotsk suture. Combined with the geological background of the structural domain, and constrained by the spatiotemporal variations in magmatism, we speculate that the high conductivity anomaly bodies are the stagnant oceanic crust material of the Okhotsk Ocean or the delaminated island arc accretionary wedge. The sinking slab or the detached lithosphere residual descending into the lower MTZ causes the upwelling of hot mantle material, forming widely distributed volcanic rocks on both sides of the Mongol-Okhotsk suture.
... Our observed NE-SW FPDs can be attributed to large-scale mantle flow in the BMW encountering the thick lithosphere beneath the GXOB (Fig. 7). Although the west of the NSGL exhibits significant changes in the gravity anomaly, thickness of the crust and lithosphere, and geothermal anomaly, the thick lithosphere there suggests that it was not strongly affected by the extensional processes (Xu, 2007;Xu et al., 2013;Zhang et al., 2014;Dong et al., 2019). The Erlian Basin area exhibits intricate anisotropic characteristics, and the FPD distribution is not uniform. ...
We deployed a linear seismic array consisting of 50 broadband stations in the southern part of the Great Xing’an orogenic belt to investigate seismic anisotropy in the Songliao Basin, the orogenic belt and the Erlian Basin by making SKS wave splitting measurements. Different features of anisotropy are revealed on two sides of the north-south gravity lineament. The anisotropy in the east of the gravity lineament primarily results from mantle flows in the big mantle wedge above the flat Pacific slab in the mantle transition zone, and the SKS fast polarization direction is generally NW-SE, being consistent with the subduction direction of the western Pacific plate. In the west of the gravity lineament, the SKS splitting is attributed to freezing anisotropy in the stable lithosphere under the Erlian Basin. The SKS splitting delay time shows some changes with the teleseismic events in different azimuths, but the fast polarization directions are generally the same, indicating that the anisotropies below these seismic stations have the same origin in the continental lithosphere and active flows in the big mantle wedge.
... Notable differences in crustal thickness caused by subduction of the Pacific Plate have been revealed using seismology methods (Zhu et al., 2011). The crustal thickness shows considerable spatial variation ranging from west to east, i.e., the thickness in the east is no >35 km, whereas that in the western region ranges from 40 to 60 km (Tang et al., 2013;Chen et al., 2009Chen et al., , 2014Dong et al., 2019). ...
North China has suffered numerous devastating historical and recent earthquakes, causing huge loss of life and property. Therefore, it is vital to identify the regions with the greatest potential to generate large earthquakes in the future. Previous studies mainly evaluated the seismic hazard in North China using earthquake records or geodetic methods separately. Here, we evaluated the seismic records and geodetic observations in combination to comprehensively assess the earthquake potential. We first evaluated the earthquake catalog and determined that the completeness range of magnitude is 1.9 to 2.8 based on the b-values estimated by magnitude-frequency distribution using catalogs with different minimum-magnitude cutoffs. We then obtained the spatial distributions of three strain rate parameters, including the maximum shear, the principal, and second invariant of the strain rates, using interseismic Global Navigation Satellite System (GNSS) observations. The robustness of the strain rate model was validated by comparing previous studies. We then analyzed the seismic hazard in North China based on the strain rates and b-values and determined the regions with relatively high seismic hazard potentials. The results suggested that large strain rates are mainly concentrated on the western border of the Ordos Block and the Tangshan fault zone. Different strain rate parameters yielded similar spatial distributions of recurrence intervals with different magnitudes. We further revealed that the western border of the Ordos Block and Tangshan fault regions exhibit relatively short (∼7000–15,000 year) ML 7.5 earthquake recurrence times, compared with those of other regions in North China. Although the results obtained in this study were limited by the completeness of earthquake records, the spatial distribution of the GNSS observation, the assumption of elastic strain accumulation, and the complexity of the mechanisms of intraplate earthquakes, our findings will contribute to the assessment of seismic hazards in North China.
... Fan et al., 2021;Guo et al., 2016;Tang et al., 2014;Tao et al., 2014;Yang et al., 2022;Ye et al., 2022) and petrological and geochemical studies (e.g. Dong et al., 2019;Guo et al., 2019Guo et al., , 2020Ji et al., 2019;Xu et al., 2019) to investigate the deep structures in NE China, which have great significance for further understanding the tectonic evolution in NE China. Nevertheless, these investigations are typically derived from independent geophysical or petrological methods. ...
The North-South Gravity Lineament (NSGL), characterized by an abrupt variation in both the Bouguer gravity anomaly and topography, acts as a significant geological boundary in eastern China. The formation of such a lateral feature is believed to be associated with lithospheric thinning. The Songliao Basin (SLB) is considered to mark the centre of the lithospheric thinning in Northeast China, whereas its thinning mechanism remains unclear. Here we perform integrated geophysical-petrological modelling of the crust and upper mantle structure along a passive-source seismic profile crossing the NSGL, to understand the thinning mechanism beneath the SLB based on the constructed lithospheric extension and delamination models. Our delamination model identifies the compositional variation across the NSGL and the S-wave velocity signature dominated by high velocity beneath the SLB, which are compatible with previous geochemical and geophysical results. The multidisciplinary analyses suggest that the lithospheric delamination is a plausible mechanism to account for the thinning to the east of the NSGL. The resulting model also shows that the NSGL is a lithospheric-scale geophysical and geochemical boundary separating two distinct structural domains. In combination with previous evidence, structural discrepancies across the NSGL can be attributed to the lithospheric delamination driven by the subduction of the Palaeo-Pacific plate.
... The crust is <35 km thick in the east and 40-60 km thick in the west, with the lithospheric thickness of the Ordos Block reaching ~200 km (Chen et al., 2009(Chen et al., , 2014Tang et al., 2013). These dramatic variations in crustal thickness in North China were strongly associated with the subduction of the Pacific Plate (Zhu et al., 2011;Dong et al., 2019). ...
North China is characterized by numerous faults, where many earthquakes were recorded. However, the fault slip and crustal deformation mechanism underlying the effects of neighboring block motions remain poorly understood. In this study, we first constructed a strain model using global navigation satellite system (GNSS) velocities to analyze the relationship between seismicity and strain. We then constructed a block model based on the fault distribution and GNSS observations to obtain fault slip. The strain and fault slip rates were then used to determine the crustal deformation mechanism in North China. The strain model suggests the western border of the Ordos Block and the northern North China Plain had larger maximum shear strain rates. These areas are located in a transition between positive and negative dilatation rates, which indicate intense crustal deformation. Based on the block model, we found the NNE-trending faults were dominated by dextral slip. The northern and southern borders of the Ordos Block indicate left-lateral motion (0.4–2.5 mm/a), while its western and eastern margins exhibited right-lateral slip (1.0–3.0 mm/a). The periphery of the Ordos Block exhibited normal slip (0.2–3.2 mm/a), except at the southwestern tip. These results suggest that the eastward extrusion of the Tibetan Plateau caused left-lateral shear in North China, which led to sinistral slip of NNE-trending faults. The northward expansion of the northeastern Tibetan Plateau also contributes to sinistral slip along the northern and southern margins of the Ordos Block, as well as dextral slip along its western and eastern margins.
... These plate remnants would have produced an extensional tectonic setting in eastern Asia as a result of asthenospheric upwelling and lithospheric thinning. Different degrees of lithospheric thinning have been observed in NE China and the NCC (Figures 8d and 8e), and the bimodal volcanic rocks of the western SB (~147 Ma) may signify the initiation of lithospheric thinning in NE China (Dong et al. 2019). ...
Tracing coupled ocean closure and intra-continental orogenic processes using tectonomagmatic thermal events is important for performing plate reconstructions. In this study, we collected geochronological data obtained from Jurassic–Early Cretaceous volcanic sequences in the Great Xing apos; an Range (GXR) and compared and merged the data to obtain a better understanding of the timing of volcanism in the GXR. Jurassic–Early Cretaceous volcanism, which is mainly represented by the Tamulangou, Manketouebo, and Manitu formations, are ubiquitous in the GXR and do not contain a magmatic hiatus. Thus, ‘diachronous’ volcanism did not occur in the GXR. The volcanic sequences in these formations are also slightly ambiguous and therefore cannot be used to determine the timing of volcanism. However, absolute ages and rock assemblages can provide better constraints on this timing. Combined with data from Jurassic–Early Cretaceous tectonomagmatic thermal events in Mongolia, Siberia, and NE China, we re-evaluated the influences of the Jurassic–Early Cretaceous closure of the Mongol–Okhotsk Ocean (MOO), as well as the subduction and rollback of the Izanagi Plate, at different times in the GXR. The GXR likely underwent flat-slab subduction during the Late Jurassic–early Cretaceous as the MOO closed in an irregular ‘scissor-like’ fashion (i.e. not in a progressive west to east manner). Large-scale lithospheric delamination likely led to a large pulse of magmatism and mineralisation in the GXR during the late Early Cretaceous.
... This model records the Mesozoic shallower and steeper process of the Izanagi plate, and retreated eastward to the trench (i.e., slab rollback), which has been interpreted as causing removal and regeneration of the lithospheric mantle prior to Early Cretaceous (~100 Ma) . The occurrence of mantle-derived basalt at 147 Ma (Dong et al., 2019) suggests that the change of slab dip and the onset of rollback (or slab sinking) may have occurred between 145 Ma and 150 Ma, resulting in Table 4 Zircon Hf isotopic compositions of the Liyang granitoids. Fig. 9. Zircon ε Hf (t) vs. U-Pb age (Ma) diagram of the Liyang granitoids. ...
Late Mesozoic magmatism in eastern China was extensive and closely related to the subduction of the Paleo-Pacific plate, but genetic relationship between magmatism and subduction remains unclear. Cretaceous magmatic rocks distributed along the eastern coast of China can provide an opportunity to evaluate the influence of subduction on the evolution of the magmatism. Here, a combined study of whole-rock major-trace elements and Sr-Nd-Pb-Hf isotopes as well as zircon U-Pb ages was carried out for the Cretaceous granitoids in Liyang volcanic basin, eastern China. Zircon U-Pb dating results indicate that the Liyang granitoids were emplaced in the Early Cretaceous (ca. 130–125 Ma). These samples (SiO2 = 70.8–75.3 wt%) are mainly composed of high-K calc-alkaline granite and granite porphyry. Their geochemical characteristics are similar to I-type granites, with enrichment in LREEs, Rb, Th and U, and negative Eu (δEu = 0.04–0.51), Ba, Sr, P and Ti anomalies. All samples show negative whole-rock εNd(t) (−9.8 to −5.7) and zircon εHf(t) (−15.7 to −3.60) values (corresponding crustal Hf model ages of 1.40–2.17 Ga), and high radiogenic Pb isotopic compositions. The integrated interpretation of geochemical data reveal that rollback of the Paleo-Pacific subducted slab resulted in lithospheric extension and thinning, and the upwelling of hot asthenosphere in eastern China, which induced more intensive underplating of the mantle-derived mafic magma. The ascent of heat and mafic magma, intruded in Meso-Neoproterozoic crust, and triggered partial melting of the Meso-Neoproterozoic crust. The Liyang I-type granitoids were the result of the Meso-Neoproterozoic crustal components mixing with a small number of mantle-derived magmas in the extensional tectonic setting, which was response to slab rollback of the Paleo-Pacific plate.
... Tectonically, it is divided into two parts by the nearly north-south trending NSGL, which is a prominent lithospheric-scale feature within NE China. Both geologic and geophysical investigations see a dramatic change occurring across the NSGL in topography, gravity anomaly, crustal and lithospheric thickness, and geothermal anomaly as well (Dong et al., 2019;Xu, 2007;Xu et al., 2013;Zhang et al., 2014). Large-scale seismic tomography images seem to show that the location of the NSGL corresponds to the front of the subducting Pacific slab (Huang & Zhao, 2006;Tao et al., 2018). ...
Plain Language Summary
Seismic anisotropy, which is used to describe the directional dependence of seismic wave velocity, can illuminate preferred orientations or fabrics in the Earth aligned by mantle deformation. The tectonic evolution of Northeast China is largely affected by the “Big Mantle Wedge”, which is formed by the stagnation of the subducting Northwest Pacific slab. Based on the SKS shear‐wave splitting measurements of a dense seismic array, we identified a contrast in anisotropy characteristics on both sides of the North‐South Gravity Lineament (NSGL) region. Westward of the NSGL, the fast axis direction is generally consistent with small delay times, indicating a limited fossil deformation pattern that originated in the stable lithosphere; eastward of the NSGL, the anisotropy pattern is more complicated with a NNW‐dominated fast axis direction, implying contributions from other regional tectonic mechanisms other than solely asthenospheric flow model. More interestingly, we reveal a clockwise rotation in the fast axis direction within a narrow width of 200 km across the NSGL. We argue that the sharp change in the thickness of the lithosphere across the NSGL caused a deflection in flow around the edge of a thick continental lithosphere keel, leading to the observed rotation in the fast axis direction.
... This pattern tracks the Mesozoic shallowing and steepening, respectively, of the Izanagi slab and eastward trench retreat (i.e., slab rollback), which has been interpreted to have caused removal and regeneration of the lithospheric mantle until Early Cretaceous (~100 Ma) . Evidenced by the occurrence of 147 Ma-aged mantle-derived basalts (Dong et al., 2019), the change in slab dip and initiation of rollback (or slab sinking) probably occurred between 145 and 150 Ma, causing asthenosphere upwelling. Subsequent mantle convective flow caused the cratonic lithospheric mantle to be eroded, marked by the formation of arc-like basalts, and thinned and then destructed to the peak time at 130-120 Ma . ...
The intracontinental response to plate-boundary and mantle processes, such as subduction or continental collisions, is an important topic in the Earth Sciences. Cretaceous to Cenozoic extension across East China is related to the interaction between the Asian continental lithosphere and western Pacific oceanic subduction system, and the associated intra-plate structures and basins provide robust temporal and spatial records of regional continental deformation. Although extension may primarily be driven by slab rollback and/or back-arc opening, mid-ocean-ridge subduction may have played a significant role in this process. To explore how the western Pacific subduction system impacts intracontinental deformation across East China, we use onshore-offshore geological records, geophysical survey observations, and time-dependent Pacific plate models to document the temporal and spatial variation of continental deformation in the Lower Yangtze region adjacent to the Yellow Sea. Based on our compilation, we argue that mantle-wedge convection, with coupled subduction of the trench-parallel Izanagi (Paleo-Pacific)-Pacific mid-ocean ridge, exerts a first-order control on extensional deformation in East China. Beneath eastern China, the mantle wedge history consists of two phases: (1) initial development of the mantle wedge as the Cretaceous Izanagi slab transitioned from flat-slab to steep subduction; and (2) mature mantle wedge convection during Cenozoic subduction of the Pacific plate. Relatively steep subduction of the progressively younging Izanagi plate during the Late Cretaceous-Paleocene resulted in strong mantle convection and coupled upper plate extension in eastern China. The collision of the Izanagi-Pacific plate ridge system with the trench resulted in a magmatic gap and short-lived uplift of eastern China. Following this, subduction of the rapidly aging Pacific plate resulted in a larger, but relatively weaker, mantle convection cell that drove weaker upper plate extension. The vigor of wedge mantle convection was modulated by the age of the Pacific subducting slab, which directly influenced the formation and evolution of intracontinental extension and sedimentary basins across eastern China. The pre-existing Triassic Tan-Lu fault zone has reactivated since the Late Eocene (ca. 40 Ma) due to westward subduction of the Pacific slab to partition strain between extensional dextral faulting in the Bohai Bay and Lower Yangtze basins, thereby diversifying Cenozoic lithospheric extensional deformation in eastern China. Several major Cenozoic extensional basins spatially correlate with the Triassic Sulu orogenic belt, which we interpret to suggest that these basins are underlain by the older and fragmented orogenic basement that acted as mechanically weak zones for localized high-magnitude extension. We present a comprehensive tectonic model that relates intracontinental deformation to subduction zone dynamics, particularly emphasizing the importance of the age of the subducting slab, thus providing a coherent geologic process to explain the deformation kinematics in eastern China and the diversity of observed Mesozoic-Cenozoic basin formation.
... There are two main hypotheses for the mechanisms associated with orogenic unrooting and lithospheric thinning. One is physical lithospheric delamination or detachment (Deng et al., 2004;Dong et al., 2019;Gao et al., 2004;Houseman et al., 1981;Wu et al., 2003), and the other is thermo-mechanical-chemical erosion (Bird, 1979;Griffin et al., 1998;Menzies et al., 1993;Niu, 2021;Wang et al., 2021;Zheng et al., 2006;Zheng et al., 2007aZheng et al., , 2007b. The former is a relatively rapid process in which the thickened lithosphere detaches and sinks into the subjacent asthenosphere; the latter is a slower thermal process in which the lower lithosphere is eroded by rising asthenosphere. ...
Post-collisional magmatism contains important clues for understanding processes of orogenic belts, potentially including unrooting and collapse. Here we report new geochronological and geochemical data for a suite of post-collisional mafic dykes in the North Qaidam ultrahigh-pressure metamorphic (UHPM) belt. Two groups of magmatic rocks can be distinguished: (1) Middle Devonian (392–375 Ma) basic-intermediate dykes; (2) Upper Devonian (~360 Ma) ultrabasic dykes. Whole-rock geochemistry and zircon Hf isotopes reveal that the intermediate-basic dykes are derived from a lithospheric mantle source, whereas the ultrabasic dykes are melting of asthenosphere mantle. We suggest that such mantle-derived mafic magmatism is the critical indicator for the start of post-collisional magmatism and orogen unrooting and collapse. We propose a geodynamic model explaining the activities of the lithospheric and asthenospheric mantle during the post-collisional stage, which reveals a ~ 35 million year unrooting process, from slow lithospheric mantle erosion between ~395–375 Ma, to final collapse by lithosphere delamination and asthenosphere upwelling at ~360 Ma. Addition of juvenile mantle materials to the crust by the post-collisional mafic magmatism suggests that the post-collisional stage is an important period for continental growth in Earth's history.
... In contrast, WNC possesses thicker crust, i.e., greater than 40 km, especially the Ordos Block, whose lithosphere layer can reach 200 km (Chen et al., 2009(Chen et al., , 2014Sun and Kennett, 2017;Tang et al., 2013;Xia et al., 2017). These significant changes in the thickness of the crust and lithosphere are closely related to the formation and activity of the NSGL under the subduction of the western Pacific Plate (Dong et al., 2019;Zhu et al., 2011). ...
North China is characterized by significant lithosphere heterogeneity and numerous faults, with the occurrence of many historical and ongoing devastating earthquakes. To extend our understanding of the mechanisms of seismicity initiation and fault activity due to lithospheric rheology and lateral difference, we first established a three-dimensional viscoelastic finite element model based on the lithospheric lateral heterogeneity of physical properties across the north–south gravitational lineament (NSGL), spatial distribution of faults, and interseismic global navigation satellite system (GNSS) velocities (1999–2018). We then analyzed the stress accumulation characteristics across the NSGL and its relationship with seismicity. Finally, we explored the temporal and spatial variations of stress along major faults and further analyzed potential relationships between the stress components and rupture mechanisms of typical faults. The results showed that high maximum shear stress, mainly distributed in eastern North China (ENC) and western North China (WNC), corresponding to the focal depth, which suggests that the different brittle crustal thicknesses across the NSGL may be one of the major factors that dominates earthquake depth in North China. Significant maximum shear stress is mainly accommodated on faults around the Ordos Block in WNC and northern faults in ENC, which is consistent with frequent seismic activity in these regions. The relationship between the calculated stress components and rupture mechanisms of typical faults imply that the differential tectonic loading from neighboring blocks may be one of the major dynamic factors for seismogenic processes in North China.
... Furthermore, the migration pattern of Mesozoic magmatism in NE Asia shows mantle-derived magmatism migrated from east to west between 200 and 150 Ma, and subsequently migrated from west to east between 150 and 100 Ma Tang et al., 2018;Li et al., 2019). Recent studies of the Haolaoluchang bimodal volcanic rocks show the occurrence of N-MORB like basalts at 147 Ma (Dong et al., 2019). From this observation, we propose that the slab had substantially rolled back by 145 Ma causing asthenosphere upwelling. ...
The arc-type characteristics of subducted crustal material incorporated into the mantle are closely related to metasomatism by melts and fluids derived from subducted oceanic crust. However, the specific compositions and proportions of melts and fluids from different end members of oceanic crust that metasomatize the mantle, as well as the subsequent processes of magma ascent, emplacement, and the evolution of mineral crystallization, remain inadequately understood. We conducted a detailed investigation of the major and trace element compositions of clinopyroxene, amphibole, and plagioclase in gabbro and gabbroic diorite from the North Qilian orogen in Northwest China to quantify the mechanisms of melt-fluid metasomatism, reveal the nature of the mafic magmatic arc source, and elucidate the processes of recharge and mixing of cognate magmas. Zircon U-Pb dating results indicate that the mafic intrusions in Zhamashi have emplacement ages of 505−481 Ma. The Sr-Nd isotopic compositions are relatively depleted, with (87Sr/86Sr)i ratios ranging from 0.7047 to 0.7086, and εNd(t) values ranging from 0.62 to 2.63. Trace elements display arc-related characteristics that are closely associated with the subduction of oceanic slab, which suggests that the Zhamashi mafic magmatic rocks originated from a mantle source metasomatized by aqueous fluids and hydrous melts derived from subducted North Qilian oceanic basaltic crust and sediments. Amphibole thermobarometer results indicate that the temperature for magma crystallization ranged from 940 °C to 994 °C, with pressure ranging from 417 MPa to 774 MPa, which corresponds to depths of 17.4 km to 29.5 km. The oxygen fugacity ranged from +1.3 to +1.9. The water content for melt varied from 5.3 wt% to 8.0 wt%, which is indicative of the typical heating and hydrous melting processes associated with contributions from subducting slab-derived fluids and melts. Complex mineral zoning structures and compositional variations record multiple episodes of cognate magma recharge and mixing during magma evolution, providing new insights into the dynamic processes that occur in subduction zone environments. Simulation results suggest that 5%−12% of oceanic crust-derived fluid and 0.5%−1.3% of sediment-derived melt reacted with garnet-phase lherzolite in the mantle wedge to form the metasomatized mantle, followed by 21% partial melting of the mantle source. These findings underscore the significance of melt-fluid metasomatism and multiple episodes of cognate magma recharge for understanding the genesis and diversity of arc magmatic rocks in subduction zones.
The Zhangjiakou-Bohai Seismic Belt (ZBSB) is a crucial intraplate seismic belt in Eastern China, covering densely populated and economically developed regions with significant seismic hazards. A refined model is essential to investigate the intraplate seismic mechanism here. We obtained the three-dimensional resistivity model based on magnetotelluric array data covering the central segment of the ZBSB. The model shows a thick high-resistivity body within the Sanhe-Pinggu seismogenic zone and a widespread low-resistivity anomaly in the mid-lower crust east of the Xiadian fault. Moreover, the Tangshan seismogenic zone exhibits a highly resistive layer in the mid-upper crust and a high-conductivity anomaly from the lower crust to the upper mantle west of the Tangshan fault. The ZBSB is primarily affected by mantle thermal material migration from the west and mantle material upwelling in the central segment of the ZBSB. In the central-western sections, the horizontal eastward push, caused by the migration of mantle thermal material from the west, accumulates stress within the rigid blocks in the Sanhe-Pinggu seismogenic zone, leading to strike-slip and rupture along the Xiadian fault. The ductile mid-lower crust on the east side of the Xiadian fault unloads stress from the rigid body, hindering eastward stress transmission. In the central-eastern segments, seismic events are mainly triggered by mantle-derived material upwelling. The NE horizontal stress exerted on the rigid body of the Tangshan seismogenic zone, resulting in strike-slip and rupture along the brittle Tangshan fault, leading to earthquakes.
Graphical Abstract
Convergent continental margins are the major sites for the formation, differentiation, preservation, and destruction of continental crust. This article focuses on the Mesozoic crustal modification history of northeastern China from a magmatic perspective. During Mesozoic times, NE China was influenced by three convergent systems, namely the Paleo-Asian Ocean (PAO) regime to the south, the Mongol-Okhotsk Ocean (MOO) regime to the northwest, and the Paleo-Pacific Ocean (PPO) regime to the east. This study comprehensively synthesizes information on Early Triassic to Early Cretaceous magmatic rocks. We unravel the spatiotemporal effects of the above-mentioned convergent regimes by evaluating the migration of major magmatic belts and other geological and geophysical evidence. The PAO regime is confined to the southernmost part of NE China and exerted influence during pre-late Late Triassic times. The MOO regime-related magmatism lasted until the early Early Cretaceous and occurred throughout the Great Xing'an Range and adjacent regions. The spatial effect of the PPO did not exceed the eastern margin of the Songliao Basin until the Early Jurassic; low-angle to flat subduction of the PPO slab led to the westward migration of continental arc front in the Middle Jurassic and the waning of PPO regime-related magmatism in the Late Jurassic. Since the earliest Cretaceous, the rollback and retreat of the PPO slab became the predominant geodynamic control in NE China, but the superposition of the MOO regime played a role during the early Early Cretaceous. Employing whole-rock Nd and zircon Hf isotope spatial imaging, this study elucidates that, although the pre-Mesozoic lithospheric heterogeneity provides first-order control, the Mesozoic crustal architecture of NE China was further carved by Mesozoic tectonics. Retreating subduction (slab rollback) and post-collisional lithospheric delamination resulted in the prolonged extensional background and crustal growth (rejuvenation); on the contrary, low-angle subduction and syn-collisional compression could cause transient periods of ancient crust reworking. Our results also estimate the high altitude of the Great Xing'an Range and adjacent regions in the Early Cretaceous. This study opens new possibilities to explicitly document crustal modification processes in fossil orogens from a magmatic perspective.
Abstract: The post-collision magmatic activity is of great significance to understand the orogenic collapse, delamination and continental crust growth. In this paper it summarizes the geochronological and geochemical characteristics of post-collisional magmatism at 400-360 Ma in the North Qaidam ultrahigh-pressure metamorphic belt. The granite intrusions have I-type characteristics and were formed by the crust-mantle magmatic mixing. Mafic dykes can be divided into two groups: (1) Intermediate-basic dykes at 392-375 Ma; (2) Ultrabasic dykes about 360 Ma. The geochemical characteristics show that the trace elements and isotopes of mafic dyke are gradually depleted with time, and the mantle source changes from lithospheric mantle to asthenosphere mantle. The mafic magmatic activity is the key of post-collision magmatic activity and orogen collapse. Combining the feature of magmatism after the collision, it proposes a geodynamic model to explain the activities of lithosphere and asthenosphere mantle in post-collision stage about 35 million years (Ma), the unrooting process of orogeny began at slow lithospheric mantle erosion in 395-375 Ma, and ended up with lithosphere delamination and asthenosphere upwelling at 360 Ma. The addition of mantle magma indicates that the post-collision stage is an important period for continental growth in earth history.
Early Cretaceous intraplate volcanic rocks are widespread in NE Asia, but their origin remains controversial. This work presents zircon U-Pb ages, whole-rock element and Sr-Nd isotope data for mafic volcanic rocks from the Erlian Basin, a wide rift basin in NE Asia. There were two episodes of Early Cretaceous mafic volcanism in the Erlian Basin, and the eruptions show contrasting geochemical compositions. The early mafic volcanic rocks, with U-Pb ages of ca. 140–135 Ma, show slightly depleted Sr-Nd isotope compositions (ISr(t) = 0.7042–0.7052; εNd(t) = + 0.82 to +3.0) and arc-like trace-element compositions, which are derived from subductionrelated fluid/melt metasomatized lithosphere mantle. The late mafic volcanic rocks (dated at ca. 125 Ma) have enriched Sr-Nd isotopes (ISr(t) = 0.7055–0.7077; εNd(t) = −0.50 to −2.67) and oceanic-island basalt (OIB)-like trace-element compositions, revealing the metasomatism of melts from crustal materials and asthenosphere mantle. The two types of mafic volcanic rocks may record the interactions of the mantle and melts from the subducted paleo-Pacific oceanic slab at different depths. The landward-then-oceanward migration pattern of the Mesozoic volcanismfrom NE Asia can be explained by the flat subduction and subsequent slab roll-back of the Paleo-Pacific Ocean, consistent with migration patterns from the North China Craton and South China Block, implying similar Jurassic–Cretaceous subduction evolution along the entire East Asia margin. Some Late Jurassic to Early Cretaceous dates from east Mongolia and the southern margin of the Erlian Basin diverge from this trajectory. In combination with previous studies, we suggest that the Early Cretaceous pervasive intraplate volcanism in the Erlian Basin and adjacent areas of NE Asia mainly resulted from the slab rollback of the Paleo-Pacific Ocean with a combined effect from the post-collision extension of the Mongol-Okhotsk orogen.
The chemical composition of eastern North China Craton (NCC)'s alkaline basaltes likely influenced by subducting Pacific Plate. Nonetheless, the influence of these processes on the western craton remains uncertain due to the lack of geochemical evidence. The recent discovery of Ulanqab maar volcanic cluster (UMVC) in the western NCC has become significant research windows. However, their petrogenesis and magmatic processes remain poorly understood. In the present study, 40Ar/39Ar, whole rock and mineral geochemistry, and Sr–Nd–Mg isotope data for the UMVC in the western NCC are reported. These data reveal that the UMVC was formed during the Neogene (7.60 ± 0.04 Ma), diverging from Quaternary basalts or as part of the Hannuoba basalts (eastern Ulanqab). The UMVC rocks, featuring typical oceanic island basalts traits with moderate (87Sr/86Sr)i values (0.70487–0.70524) and εNd(t) values from -4.95 to +0.82, likely originated from an enriched EMI-type mantle source during the mid-Proterozoic. This involved mixing melts from 30–50% partial melting of garnet lherzolite and 5–15% partial melting of garnet pyroxenite in a deep magma chamber. Crystallization of clinopyroxene and garnet in this chamber created high-Ti alkaline lavas, with limited presence in erupted lavas due to sluggish magma ascent in the profound lithospheric mantle. In shallower lithospheric mantle regions, interactions between alkaline magma and orthopyroxenite improved transport kinetics, enabling clinopyroxene and olivine crystallization under lower pressure. These magmas integrated mafic crystal mushes from accumulation chambers. The study also identified a low Mg isotope composition (δ26Mg = −0.56‰ to −0.42‰) in samples, suggesting a hybrid source influenced by early decarbonation in Precambrian subduction zones. This finding indicates the incorporation of Mg-rich carbonate minerals from marine sediments into the magma source, contributing to the observed variations in eastern China's alkaline basalts.
Mesozoic mantle-derived potassic magmatic rocks in the Trans-North China Orogen (TNCO) are important for constraining tectonic evolution. However, potassic intermediate plutons in this orogen are rarely reported in literature, despite the significance of such a potassic magmatic event being well understood. Here, we describe the petrography of the Longmen potassic diorites and present zircon U-Pb ages and elemental and Sr-Nd-Mg isotopic compositions for this pluton. The Longmen potassic diorites were emplaced during the Early Cretaceous (ca. 140 Ma), showing shoshonite features with marked depletion of high-field strength elements relative to large ion lithophile elements with a slightly negative Eu anomaly. It possesses restricted ranges of initial 87 Sr/ 86 Sr ratios (0.70592 to 0.70667), ε Nd (t) values (− 15.86 to − 13.26), and low δ 26 Mg (− 0.37 to − 0.26‰). Low-degree partial melting of the ancient and enriched mantle, characterised by amphibole-bearing lherzolite in the spinel-garnet transition zone, was the likely mantle source of the evaluated Longmen potassic diorites, resulting from fluid-related metasomatism and upwelling mantle (carbonated) peridotite. Minor crustal materials were present in the lithospheric mantle because of Eastern Block and Western Block subduction/collision during the Paleoproterozoic. Based on our data and previous studies, slab sinking and rollback of the Izanagi plate (Paleo-Pacific plate) may have created an ancient, enriched mantle beneath the East Asian continent, inducing litho-spheric thinning and heterogeneity of the lithospheric mantle in the North China Craton and lithospheric extension via thermo-mechanical erosion at 145-110 Ma in the TNCO.
The mineral composition, whole-rock geochemistry, Sr-Nd isotopes, and in situ zircon U-Pb and Lu-Hf isotopes for mafic and intermediate-felsic volcanic rocks in the Sonid Youqi area were determined to unravel their petrogenesis and the Early Permian tectonic setting of the Solonker Zone, Central Asian Orogenic Belt. Zircon U-Pb ages suggested that these rocks formed in the Early Permian. The mafic volcanic rocks were depleted in Ba, Th, and Nb, enriched in Rb, U, and Sr, and had variable zircon ε Hf (t) values, with constant Th/Yb and variable Ba/La ratios, indicating that the mafic magma originated from the fluid-metasomatized mantle. The intermediate-felsic volcanic rocks were depleted in Ba, Th, Nb, Ta, and Ti and enriched in Sr, Zr, and Hf. Geochemical modelling and mineralogical observations suggested that magma mixing dominated the magmatic process. These features, along with variable SiO 2 and MgO contents and zircon ε Hf (t) values of +9.40 to +13.47, suggested that these rocks originated from the mixing of melts from the mantle and juvenile crust. Early Permian volcanic rocks from the Solonker Zone had depleted Sr-Nd isotopic compositions, similar to those from the Japan Sea back-arc basin basalts. Their geochemical characteristics differed from those rocks formed during the initial subduction. In combination with previous studies, we suggest that the generation of these Early Permian rocks from the Solonker Zone occurred in an extensional tectonic setting that induced by the slab break-off of Hegenshan Ocean. ARTICLE HISTORY
The zircon LA-ICP-MS U-Pb dating and geochemistry are reported for the volcanic rocks in Manitu Formation in Chaihe, central Great Xing'an Range in order to discover its formation time, petrogenesis and the regional tectonic setting Petrographically. The volcanic rocks in Manitu Formation are composed of andesite and dacite, but they are trachyte, trachydacite, and dacite based on its chemical composition. Zircon from these volcanic rocks are euhedral-subhedral in shape, with fine-scale oscillatory growing ring. The Th/U ratio of (0.33-2.08) indicates a magmatic origin. The dating results show that the volcanic rocks in Manitu Formation are formed in Late Jurassic from 139 to 148 Ma. The geochemical study shows that the SiO2 of the andesite is between 64.87%-68.65%, Al2O3 is between 15.53%-16.72%, Na2O+K2O is between 6.75%-9.22%, K2O/Na2O is between 0.63-1.57, falling in the series of high-potassium calc-alkaline. All samples have a similar REE pattern with a significant fractionation of LREE from HREE((La/Yb)N=11.50-14.88)and negative Eu anomalies(δEu=0.45-1.17). The total rare earth element is from 124.95×10-6 to 208.57×10-6. The trace element geochemistry is characterized evidently by enrichment of Rb, Ba, K, depletion of Nb, P, Ti. Its original magma is derived from the partial melting of the crust, during an extension after the closure of the Mongol-Okhotsk Ocean.
The Zhangguangcai Range in the Xing'an Mongolian Orogenic Belt, NE China, contains Early Jurassic (c. 188 Ma) Dabaizigou (DBZG) porphyritic dolerite. Compared with other island-arc mafic rocks, the DBZG dolerite is characterized by high trace-element contents, relatively weak Nb and Ta enrichments, and no Zr, Hf or Ti depletions, similar to OIB-type rocks. Analysed rocks have (87 Sr/ 86 Sr) i ratios of 0.7033-0.7044, relatively uniform positive ε Nd (t) values of 2.3-3.2 and positive ε Hf (t) values of 8.5-17.1. Trace-element and isotopic modelling indicates that the DBZG mafic rocks were generated by partial melting of asthenospheric mantle under gar-net-to spinel-facies conditions. The occurrence of OIB-like mafic intrusion suggests significant upwelling of the asthenosphere in response to lithospheric attenuation caused by continental rifting. These processes occurred in an incipient continental back-arc environment in the upper plate of a palaeo-Pacific slab subducting W-NW beneath East Asia.
Basaltic volcanism 'samples' the Earth's mantle to great depths, because solid-state convection transports deep material into the (shallow) melting region. The isotopic and trace-element chemistry of these basalts shows that the mantle contains several isotopically and chemically distinct components, which reflect its global evolution. This evolution is characterized by upper- mantle depletion of many trace elements, possible replenishment from the deeper, less depleted mantle, and the recycling of oceanic crust and lithosphere, but of only small amounts of continental material.
SUMMARY: Trace-element data for mid-ocean ridge basalts (MORBs) and ocean island basalts (OIB) are used to formulate chemical systematics for oceanic basalts. The data suggest that the order of trace-element incompatibility in oceanic basalts is Cs ≈ Rb ≈ (≈Tl) ≈ Ba(≈ W) > Th > U ≈ Nb = Ta ≈ K > La > Ce ≈ Pb > Pr (≈ Mo) ≈ Sr > P ≈ Nd (> F) > Zr = Hf ≈ Sm > Eu ≈ Sn (≈ Sb) ≈ Ti > Dy ≈ (Li) > Ho = Y > Yb. This rule works in general and suggests that the overall fractionation processes operating during magma generation and evolution are relatively simple, involving no significant change in the environment of formation for MORBs and OIBs.
In detail, minor differences in element ratios correlate with the isotopic characteristics of different types of OIB components (HIMU, EM, MORB). These systematics are interpreted in terms of partial-melting conditions, variations in residual mineralogy, involvement of subducted sediment, recycling of oceanic lithosphere and processes within the low velocity zone. Niobium data indicate that the mantle sources of MORB and OIB are not exact complementary reservoirs to the continental crust. Subduction of oceanic crust or separation of refractory eclogite material from the former oceanic crust into the lower mantle appears to be required. The negative europium anomalies observed in some EM-type OIBs and the systematics of their key element ratios suggest the addition of a small amount (≤1% or less) of subducted sediment to their mantle sources. However, a general lack of a crustal signature in OIBs indicates that sediment recycling has not been an important process in the convecting mantle, at least not in more recent times (≤2 Ga). Upward migration of silica-undersaturated melts from the low velocity zone can generate an enriched reservoir in the continental and oceanic lithospheric mantle. We propose that the HIMU type (eg St Helena) OIB component can be generated in this way. This enriched mantle can be re-introduced into the convective mantle by thermal erosion of the continental lithosphere and by
The effect of paleo-Pacific subduction on the geological evolution of the western Pacific and continental China is likely complex. Nevertheless, our analysis of the distribution of Mesozoic granitoids in the eastern continental China in space and time has led us to an interesting conclusion: The basement of the continental shelf beneath East and South China Seas may actually be of exotic origin geologically unrelated to the continental lithosphere of eastern China. By accepting the notion that the Jurassic– Cretaceous granitoids in the region are genetically associated with western Pacific subduction and the concept that subduction may cease to continue only if the trench is being jammed, then the termination of the granitoid magmatism throughout the vast region at *88 ± 2 Ma manifests the likelihood of ''sudden'', or shortly beforehand (*100 Ma), trench jam of the Mesozoic western Pacific subduction. Trench jam happens if the incoming ''plate'' or portion of the plate contains a sizeable mass that is too buoyant to subduct. The best candidate for such a buoyant and unsubductable mass is either an oceanic plateau or a micro-continent. We hypothesize that the basement of the Chinese continental shelf represents such an exotic, buoyant and unsubductable mass, rather than sea-ward extension of the continental lithosphere of eastern China. The locus of the jammed trench (i.e., the suture) is predictably located on the shelf in the vicinity of, and parallel to, the arc-curved coastal line of the southeast continental China. It is not straightforward to locate the locus in the northern section of the East China Sea shelf because of the more recent (\20 Ma) tectonic re-organization associated with the opening of the Sea of Japan. We predict that the trench jam at *100 Ma led to the re-orientation of the Pacific plate motion in the course of NNW direction as inferred from the age-progressive Emperor Seamount Chain of Hawaiian hotspot origin (its oldest unsubdued Meiji and Detroit seamounts are *82 Ma), making the boundary between the Pacific plate and the newly accreted plate of eastern Asia a transform fault at the location east of the continental shelf of exotic origin. This explains the apparent *40 Myr magmatic gap from *88 to *50 Ma prior to present-day western Pacific subduction initiation. We propose that basement penetration drilling on well-chosen sites is needed to test the hypothesis in order to reveal the true nature of the Chinese continental shelf basement. This testing becomes critical and cannot longer be neglected in order to genuinely understand the tectonic evolution of the western Pacific and its effect on the geology of eastern China since the Mesozoic, including the cratonic lithosphere thinning, related magmatism/min-eralization, and the mechanism of the subsequent South China Sea opening, while also offering novel perspectives on aspects of the plate tectonics theory. We also suggest the importance of future plate tectonic reconstruction of the western Pacific to consider the nature and histories of the Chinese continental shelf of exotic origin as well as the probable transform plate boundary from *100 to *50 Ma. Effort is needed to reveal the true nature and origin of the *88 ± 2 Ma granitic gneisses in Taiwan and the 110–88 Ma granitoids on the Hainan Island.
Volcanism that occurs far from plate margins is difficult to explain with the current paradigm of plate tectonics. The Changbaishan volcanic complex, located on the border between China and North Korea, lies approximately 1,300 km away from the Japan Trench subduction zone and is unlikely to result from a mantle plume rising from a thermal boundary layer at the base of the mantle. Here we use seismic images and three-dimensional waveform modelling results obtained from the NECESSArray experiment to identify a slow, continuous seismic anomaly in the mantle beneath Changbaishan. The anomaly extends from just below 660 km depth to the surface beneath Changbaishan and occurs within a gap in the stagnant subducted Pacific Plate. We propose that the anomaly represents hot and buoyant sub-lithospheric mantle that has been entrained beneath the sinking lithosphere of the Pacific Plate and is now escaping through a gap in the subducting slab. We suggest that this subduction-induced upwelling process produces decompression melting that feeds the Changbaishan volcanoes. Subduction-induced upwelling may also explain back-arc volcanism observed at other subduction zones.
Subduction of the Paleo-Pacific Oceanic Plate is widely considered to have caused extensive Mesozoic magmatism, lithospheric deformation and mineralization in East Asia. However, it is still unclear when this subduction began. Here we report an Early Jurassic (~ 187 Ma) mafic intrusive complex (including olivine norite, gabbro, and diorite) from the Tumen area in NE China. The olivine norite contains a mineral assemblage of olivine, pyroxene, Ca-plagioclase, and hornblende that crystallized in a water-saturated parental magma. The rocks in the complex show variable degrees of plagioclase and ferromagnesian mineral accumulation as reflected by positive Sr and Eu anomalies in primitive mantle-normalized incompatible element patterns. Mass-balance calculations indicate that the parental magma was calc-alkaline with arc-type trace element features (i.e., large ion incompatible and light rare earth element enrichment and Nb-Ta depletion). It also had Sr–Nd–Hf isotopic compositions (87Sr/86Sr(i) = 0.7042 to 0.7044, εNd(t) = + 2.5 to + 3.5 and εHf(t) = + 8.4 to + 10.5) similar to those of modern arc basalts. The parental magma was likely derived from 5–20% melting of a mantle wedge metasomatized by an addition of 3–4% hydrous sediment melt from the subducting Paleo-Pacific Oceanic slab. The Tumen mafic intrusive complex, together with other contemporaneous mafic intrusions, I-type granitoids, and felsic lavas, constitutes an Early Jurassic N–S-trending arc magmatic belt that was formed by westward subduction of the Paleo-Pacific Ocean.
As a paradigm of greenhouse climate in Earth's history, the Cretaceous provides significant rock records of global climate changes under conditions of greenhouse climate. The Songliao Basin, among the longest duration (85-90 m.y.) of continental sedimentary basins, provides an excellent opportunity to recover a nearly complete Cretaceous terrestrial sedimentary record. Extensive lake deposits, ten-kilometers deep and covering an area of 260,000 km(2) of the Songliao Basin, provide unique, detailed records that can be tied to the global stratigraphic time scale, thereby improving our understanding of the continental paleoclimate and ecological system. The two coreholes at SKIs and SKIn sites were drilled into this basin and completed with a total length of 2485.89 m of recovered core that spanned the complete middle-to-Upper Cretaceous strata in the basin. The unique geological setting of long-term continuous subsidence within the largest Cretaceous landmass in the world makes the Cretaceous Songliao Basin of northeastern China an ideal place to study Cretaceous climate change on the continent. This paper reviews the literature on the paleogeography and paleoclimate of the northern East Asia and the Songliao Basin during the Cretaceous. Based on the climatologically sensitive deposits, oxygen isotope studies, and paleontology, the climate during the Cretaceous in the Songliao Basin was temperate and humid with relatively abundant rainfall. During the period, significant changes - four cooling, three warming, and three semiarid events - are generally consistent with the oxygen isotope data from East Asia, and the four cooling events, in Berriasian-Valanginian, Aptian-Albian, early Santonian, and Campanian-Maastrichtian, may be related to potential glaciations in Cretaceous.
Lithospheric thickness is closely associated with lithospheric response to tectonic forces or thermal processes. Analysis of S-wave receiver functions (SRFs) from a dense seismic array has revealed lithospheric thickness variations along a 1200 km long profile that spans all major geological terranes of Northeast China. The SRF images identify the shallowest Moho and lithosphere-asthenosphere boundary (LAB) depths beneath the rifted Songliao Basin, thus spatially correlating well with the thick sediment accumulation. These features can be explained by a pure shear regime operating in the lower crust and upper mantle, suggesting the predominance of roughly symmetric lithosphere stretching from continental rifting. In contrast, thicker lithosphere in western and eastern terranes of NE China indicates the lithosphere in these regions was not strongly affected by the extensional processes during the Late Mesozoic. Flat LAB structure beneath the Erguna and Xing'an terranes along with the overlying relatively deeper Moho beneath the Great Xing'an Range, suggests the North-South Gravity Lineament may not be a trans-lithospheric structure.
We present a new shear velocity model of the upper mantle beneath the
East Asia region derived by inverting Rayleigh wave group velocity
measurements between 10 and 145 s combined with previously published
Rayleigh wave phase velocity measurements between 150 and 250 s.
Rayleigh wave group velocity dispersion curves along more than 9500
paths were measured and combined to produce 2D dispersion maps for
10–145 s periods. The group velocity maps benefit from the
inclusion of new data recorded by the China National Seismic Network and
surrounding global stations. The increase in available data has resulted
in enhanced resolution compared with previously published group velocity
maps; the horizontal resolution across the region is about 3° for
the periods used in this study. The new shear-wave velocity models
indicate varying velocity structure beneath eastern China, which yields
estimates of a lithosphere–asthenosphere boundary depth from
around 160 km beneath the Yangtze block to approximately 140 km beneath
the western part of the North China Craton (NCC), up to depths of
70–100 km beneath the eastern NCC, Northeast China, and the
Cathaysia block. The models reveal the subduction of two opposite-facing
continental plates under the southern and northern margin of Tibet. An
obvious low-velocity anomaly appears in the top 200 km of the upper
mantle beneath northern Tibet, which is inconsistent with the presence
of subducted Asian or Indian mantle lithosphere beneath northern Tibet.
The Cenozoic volcanism fields in the Mongolian plateau are characterized
by an obvious upper mantle negative anomaly, but no signature of
deep-seated plume was observed.
We imaged the crust and uppermost mantle structure beneath northeastern
(NE) China with fundamental mode Rayleigh waves recorded by 125
broad-band stations deployed in the region. Rayleigh wave phase and
group velocities along more than 700 interstation paths were estimated
using the wavelet transformation method and then these data were
utilized to construct 2-D phase and group velocity maps in the period
range of 15-60 s. Owing to the dense ray coverage and short ray path,
our results provide better lateral resolution in the NE China region
compared with previous phase and group velocity studies. The regularized
Rayleigh wave phase and group velocity dispersions at each cell were
jointly inverted to determine 1-D shear wave velocity structure using
the linear inversion method and then assembled into a 3-D model. Our
results show that obvious low velocities exist in the uppermost mantle
beneath the Changbaishan volcanic region, which may be due to
asthenospheric upwelling. The thin lithosphere with fast S-wave velocity
in the lower crust of the Songliao Basin implies that the lithospheric
mantle beneath NE China is partly removed.
LA-ICP-MS zircon U–Pb dating and geochemical data have been obtained from five representative mafic–ultramafic intrusions in the Lesser Xing'an–Zhangguangcai Range, NE China, with the aim of improving our understanding of the Mesozoic tectonic evolution in the region, and in particular, determining the time of initiation of the circum-Pacific tectonic system. The selected zircons exhibit striped absorption in cathodoluminescence (CL) images and have high Th/U ratios (0.20–3.16), indicating a magmatic origin. The zircon U–Pb dates indicate that most of these magmatic zircons (other than a few relics that were captured and entrained in the magma) formed in the late Early Jurassic (186–182 Ma), and not as previously supposed in the Middle Caledonian. The five mafic–ultramafic plutons are composed of olivine-gabbro, hornblendite, gabbro, hornblende-gabbro, and gabbro–diorite. The olivine-gabbro and hornblendite display cumulate textures, implying that fractional crystallization of olivine and plagioclase took place during magma evolution. These mafic–ultramafic igneous rocks have SiO2 = 37.3%–55.7%, MgO = 3.05%–13.3%, Al2O3 = 11.8%–23.8%, Mg# = 42–69 [Mg# = 100Mg / (Mg + Fe2 +total)], and δEu = 0.88–1.32, and they display three types of rare earth element (REE) distribution patterns: right-slipped, flat patterns, or dome-like. The trace element spider diagrams show that the rocks are enriched in large ion lithophile elements (LILEs) such as Ba, K, and Sr, and depleted in high field strength elements (HFSEs) such as Nb, Ta, Zr, and Hf. The zircons have εHf (186–182 Ma) = + 2.7 to + 12.0, and TDM1 = 366–732 Ma. The geochemical data indicate that the Early Jurassic mafic magma originated in an extensional environment from the partial melting of a depleted mantle wedge that had been metasomatized by fluids released from a fossil subducted slab. These data, combined with information on the spatial variation of coeval igneous rocks, indicate that the formation of the Early Jurassic mafic–ultramafic rocks in the Lesser Xing'an–Zhangguangcai Range was related to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent, and this event would mark the beginning of the circum-Pacific tectonic system.
In an attempt to assess the relative contribution of asthenosphere and mantle lithosphere to magmas across the western United States we have analyzed, for major and trace elements, a suite of 750 basic (MgO>4%) lava samples from all the major volcanic fields in the region. It is concluded that many of the magmas have inherited their chemical and isotopic characteristics from a lithospheric mantle source enriched by fluids expelled from a subducted slab. It appears that subduction-enriched lithospheric mantle was involved in the generation of all extension related basic magmas across the western United States until relatively recently. -from Authors
Zircon has long been recognized as the best geochronometer and the most
important timekeeper in geosciences. Modern microbeam techniques such as
SIMS and LA-ICPMS have been successfully applied to in situ U-Pb zircon
age determinations, at spatial resolutions of 20-30 μm or less.
Matrix-matched calibration by external standardization of
well-characterized natural zircon references is a principal requirement
for precise microbeam U-Pb zircon age determination due to fractionation
effects between Pb and U, which usually result in an external age error
exceeding 1%. Alternatively, zircons with a closed U-Pb system can be
directly dated by measurement of 207Pb/206Pb
isotopic ratio without external standardization, which has been a common
practice for zircons older than 1.0 Ga, but not for relatively young
(<1.0 Ga and particularly Phanerozoic) ones because of limitations of
analytical precision. We describe in this paper a method of
207Pb/206Pb measurement on Phanerozoic zircons
using a new generation of large radius magnetic sector multicollector
Cameca IMS-1280 SIMS. In combination with multicollector mode, a Nuclear
Magnetic Resonance (NMR) magnet controller and oxygen flooding
techniques, we achieve precisions of 207Pb/206Pb
ratio of <0.1% and 0.1 ˜ 0.2%, propagating to Pb/Pb age errors
<0.4% and 1-3% (excluding U decay constant uncertainties), for
zircons of latest Neoproterozoic and late Paleozoic to Mesozoic age,
respectively. Therefore, the multicollector SIMS is capable of direct
determination of zircon Pb/Pb ages as young as Mesozoic age with
uncertainties of geological significance. This technique is useful for
direct dating of zircons in thin sections. Moreover, it has significance
for dating of some other U-rich minerals (i.e., baddeleyite and
zirconolite) that are not suitable for SIMS U-Pb dating by external
standardization.
A high-resolution P wave tomographic model of the crust and mantle down to 1100 km depth under China and surrounding regions is determined by using about one million arrival times of P, pP, PP, and PcP waves from 19,361 earthquakes recorded by 1012 seismic stations. The subducting Pacific slab is imaged clearly as a high-velocity zone from the oceanic trenches down to about 600 km depth, and intermediate-depth and deep earthquakes are located within the slab. The Pacific slab becomes stagnant in the mantle transition zone under east China. The western edge of the stagnant slab is roughly coincident with a surface topographic boundary in east China. The active Changbai and Wudalianchi intraplate volcanoes in northeast China are underlain by significant slow anomalies in the upper mantle, above the stagnant Pacific slab. These results suggest that the active intraplate volcanoes in NE China are not hot spots but a kind of back-arc volcano associated with the deep subduction of the Pacific slab and its stagnancy in the transition zone. Under the Mariana arc, however, the Pacific slab penetrates directly down to the lower mantle. The active Tengchong volcano in southwest China is related to the eastward subduction of the Burma microplate. The subducting Indian and Philippine Sea plates are also imaged clearly. The Indian plate has subducted down to 200-300 km depth under the Tibetan Plateau with a horizontal moving distance of about 500 km. High-velocity anomalies are revealed in the upper mantle under the Tarim basin, Ordos, and Sichuan basin, which are three stable blocks in China.
Plutonic igneous rocks of the Sierra Nevada batholith and from the Peninsular Ranges show regular geographic variations of Nd isotopic composition. These and other isotope data can be explained if the batholiths are mixtures of island arc and metasedimentary components. About 50% of the crustal component is from juvenile Mesozoic crust; the rest appears to be derived from 1.5Ga crust. If this result applies to large portions of the Cordillera, the average age of the North American continent may be greater than previously estimated.-from Author
Some basic concepts of basaltic magma generation and evolution are discussed in the context of global tectonics. These concepts need better understanding before invoking elusive possibilities in igneous petrogenesis on all scales and in all tectonic environments.
A simplified model highlights the physical and chemical processes that have been invoked as being important in controlling the composition of volcanic arc magmas. Magma compositions can help gain further understanding of these physical and chemical processes. This review first summarizes knowledge of the behavior of elements in the subduction system, then focuses on compositional evidence for the processes which are grouped as follows: 1) deviation of the subduction component, 2) transport of the subduction component to the melting column, 3) depletion and enrichment of the mantle wedge, and 4) processes in the melting column. -from Authors
We report here the results of a study to develop natural zircon geochemical standards for calibrating the U-(Th)-Pb geochronometer and Hf isotopic analyses. Additional data were also collected for the major, minor and trace element contents of the three selected sample sets. A total of five large zircon grains (masses between 0.5 and 238 g) were selected for this study, representing three different suites of zircons with ages of 1065 Ma, 2.5 Ma and 0.9 Ma. Geochemical laboratories can obtain these materials by contacting Geostandards Newsletter.
A Paleozoic ultrahigh-pressure metamorphic (UHPM) belt extends along the northern margin of the Qaidam Basin, North Tibetan Plateau. Eclogites in the Yuka eclogite terrane, northwest part of this UHPM belt, occur as blocks or layers of varying size intercalated with granitic and pelitic gneisses. These eclogites have protoliths geochemically similar to enriched-type mid-ocean ridge basalts (E-MORB) and oceanic island basalts (OIB). On the basis of Ti/Y ratios, they can be divided into low-Ti and high-Ti groups. The low-Ti group (LTG) eclogites exhibit relatively low TiO2 (most <2.5 wt%) and Ti/Y (<500) but comparatively high Mg# (48–55), whereas the high-Ti group (HTG) eclogites have high TiO2 (most >2.5 wt%) and Ti/Y (>500) but lower Mg# (46–52). Zircons from two eclogite samples gave a magmatic crystallization (protolith) age of ∼850 Ma and a UHPM age of ∼433 Ma. The occurrence, geochemical features and age data of the Yuka eclogites suggest that their protoliths are segments of continental flood basalts (CFBs) with a mantle plume origin, similar to most typical CFBs. Our observation, together with the tectonic history and regional geologic context, lend support for the large scale onset of mantle plume within the Rodinia supercontinent at ∼850 Ma. The Qaidam block is probably one of the fragments of the Rodinia supercontinent with a volcanic-rifted passive margin. The latter may have been dragged to mantle depths by its subducting leading edge of the oceanic lithosphere in the Early Paleozoic.
Editor: R.L. Rudnick Keywords: Zircon U–Pb and 40 Ar– 39 Ar geochronology Mesozoic volcanic rocks Great Xing'an Range Delamination Northeastern China Mesozoic volcanic rocks and granitoids are widespread in the Great Xing'an Range, which is part of a large igneous province in the eastern China. However, the ages of the volcanic rocks, especially those in the southern segment of the range, are poorly constrained. Here we present zircon U–Pb and whole rock Ar–Ar ages of 43 volcanic rocks from the four recognized formations (Manketouebo, Manitu, Baiyingaolao and Meiletu) in the southern Great Xing'an Range. The volcanic rocks of the Manketouebo Formation have a large span of ages ranging from 174 to 122 Ma, while those of the Manitu Formation exhibit a smaller age range from 156 to 125 Ma. The Baiyingaolao and Meiletu volcanic rocks both have Early Cretaceous ages between 139 and 124 Ma. These data indicate that the mapped units are not strictly 'formations' and further studies are required to resolve this issue. However, when taken together, these new data define two episodes of magmatism (Late Jurassic and Early Cretaceous) with the Early Cretaceous volcanic rocks being dominant. Combined with previously published data from the northern Great Xing'an Range, and available age data from other parts of northeastern China and surrounding regions, two stages of magmatism, i.e., Jurassic and Early Cretaceous, can be identified throughout this part of Asia. The Jurassic rocks mainly comprise granites, while volcanic rocks are dominant in the Early Cretaceous. These two stages of magmatism form opposite spatial trends, that is, the Jurassic rocks become younger to the west, whereas the Cretaceous rocks become younger to the east. Between the two stages of magmatism, the 'magma gap' increases eastward in duration from less than 10 Ma in the Great Xing'an Range to more than 40 Ma in Japan. These trends can be explained by westward subduction of the Paleo-Pacific oceanic Plate and its control on subsequent geodynamic processes. Jurassic subduction of the oceanic slab caused crustal shortening and thickening, and formed the westward decrease in age of the granites with characteristics of an active continental margin, while volcanism was rare. By the end of the Jurassic, westward flat-slab subduction of the Paleo-Pacific Oceanic plate changed its direction to the north or northwest. This subsequently caused a transformation in tectonic regime from compression to extension in the Cretaceous and induced large-scale delamination of the thickened lower crust and lithospheric mantle. Delamination was initiated at the western margin of the subducting slab, and migrated eastward. Delamination and consequent upwelling of the asthenosphere triggered extensive volcanic eruption, with only minor granite emplacement. Similar age trends are also observed for other parts of eastern China, suggesting this model can also be applied to explain the geodynamic setting of the Mesozoic large igneous events in China and adjacent regions.
Mesozoic volcanic rocks exposed in the Songliao basin form a major part of the basin fill and are the important pertroleum reservoir rocks. Petrogenesis of these volcanic rocks and their tectonic setting, however, are poorly understood due to lack of sufficient chemical and isotopic data. These volcanic rocks cover a wide compositional spectrum ranging from basaltic trachyandesite to high-silica rhyolite. They can be divided into two formations, the upper Jurassic Huoshiling formation (157-146 Ma) with dominant dark gray andesite and the lower Cretaceous Yingcheng formation (136-113 Ma) with dominant yellowish rhyolite. Compositionally, these rocks are peraluminous to metaluminous, high-K to medium-K, calc-alkaline. They are enriched in large-ion lithophile and light rare earth elements, depleted in high field strength and heavy rare earth elements, and commonly display negative anomalies of Nb, Ti and P. Initial 87Sr/86Sr ratios and initial εNd-values of these volcanic rocks range from 0.7034 to 0.7106 and from -3.4 to 5.3, respectively, and their δ18O values vary from 3.2‰ to 14.8‰. Initial 206Pb/204Pb and 207Pb/204Pb ratios have ranges of 17.65 to 18.22 and 15.52 to 15.57, respectively. The characteristic in Nd and Sr isotopic composition probably indicates a mixing of enriched MORB-like material and crustal component. Primary magmas for both the Jurassic and Cretaceous volcanic rocks could be derived from metasomatized enriched MORB-like sources, but the Cretaceous rhyolites commonly show crustal assimilation indicated by high initial 87Sr/86Sr ratios and low initial εNd values. Whether the volcanic rocks in the Songliao basin formed related either to the post-collisional setting of the Mongolia-Okhotsk suture or the subduction of the western Pacific plate remains to be discussed.
Cretaceous magmatism in the Sverdrup Basin of Arctic Canada is widely considered to be part of the circum-Arctic High Arctic Large Igneous Province (HALIP). Recent studies have questioned: (i) plume involvement in the HALIP, and (ii) whether the younger magmatic events constitute the same large igneous province. We present an integrated geochemical and geochronological study to better constrain the initiation and evolution of magma genesis in the Canadian HALIP. Six new U–Pb and four ⁴⁰Ar/³⁹Ar ages of mafic lavas and intrusive sheets range from 120.9 ± 0.9 Ma to 78.4 ± 0.1 Ma, which is within the published timespan of the HALIP. The U–Pb ages are the first analyzed from the mafic intrusions of Axel Heiberg and Ellesmere Islands. The new geochronology, combined with all recently (post-2000) published ages, detail a >50 Myr duration of magmatism (128 to 77 Ma) with three main pulses. Tholeiites dominate the first 25 Myr while the latter 25 Myr consisted of coeval emplacement of alkali and tholeiitic basalts. Rare-earth-element inversion models reveal that the alkalic and tholeiitic magmas were generated beneath a bimodal lithospheric ‘lid’ thickness of 65 ± 5 and 45 ± 4 km, respectively. Whole-rock Sr–Nd isotope ratios indicate that both magma types are derived from a similar source dominated by convecting mantle. Further, these two magma types were spatially segregated by the tectonic domains of the Sverdrup Basin. We suggest that the early 128–120 Ma tholeiitic event is primarily plume-generated and correlates across the circum-Arctic with the other HALIP tholeiites. The younger magmatism, with coeval alkalic and tholeiitic magmas erupting over 25 Myr, is likely not plume-generated and may be explained by alternating modes of edge-driven mantle convection as the primary control on magma genesis. A distal plume would intensify magma production by edge-driven convection, but its influence would be secondary.
Magmatism in eastern China in response to paleo-Pacific plate subduction during the Mesozoic was complex, and it is unclear how and when exactly the magmas formed via thinning and partial destruction of the continental lithosphere. To better understand this magmatism, we report the results of a geochronological and geochemical study of Early Cretaceous adakitic rhyolite (erupted at 125.4 ± 2.2 Ma) in the Xintaimen area within the eastern North China Craton (NCC). In situ zircon U-Pb dating shows that this adakitic rhyolite records a long (~ 70 Myrs) and complicated period of magmatism with concordant ²⁰⁶Pb/²³⁸U ages from 193 Ma to 117 Ma. The enriched bulk rock Sr-Nd isotopic compositions of the Xintaimen adakitic rhyolite, as well as the enriched zircon Hf and O isotopic compositions, indicate that the magmas parental to the adakitic rhyolite were derived from partial melting of the Paleoproterozoic mafic lower crust, heated by mafic melts derived from the mantle during the paleo-Pacific plate subduction. A minor older basement component is indicated by the presence of captured Neoarchean to Early Paleoproterozoic zircons. The Mesozoic zircons have restricted Hf and O isotopic compositions irrespective of their ages, suggesting that they formed from similar sources at similar melting conditions. The Xintaimen adakitic rhyolite offers an independent line of evidence that the ancient lower crust of eastern China underwent a long period (~ 70 Myrs) of destruction, melting or remelting, from ~ 193 to ~ 120 Ma, related to the subduction of the paleo-Pacific plate beneath eastern China.
We measured shear wave splitting from SKS data recorded by the transcontinental NECESSArray in NE China to constrain lithosphere deformation and sublithospheric flows beneath the area. We selected several hundreds of high quality SKS/SKKS waveforms from 32 teleseismic earthquakes occurring between 09/01/2009 and 08/31/2011 recorded by 125 broadband stations. These stations cover a variety of tectonic terranes, including the Songliao basin, the Changbaishan mountain range and Zhangguancai range in the east, the Great Xing'an range in the west and the Yanshan orogenic belt in the southwest. We assumed each station is underlaid by a single anisotropic layer and employed a signal-to-noise ratio (SNR) weighted multi-event stacking method to estimate the two splitting parameters (the fast polarization direction φ, and delay time, δt) that gives the best fit to all the SKS/SKKS waveforms recorded at each station. Overall, the measured fast polarization direction lies more or less along the NW–SE direction, which significantly differs from the absolute plate motion direction, but is roughly consistent with the regional extension direction. This suggests that lithosphere deformation is likely the general cause of the observed seismic anisotropy. The most complicated anisotropic structure is observed beneath the southern Great Xing'an range and southwest Songliao basin. The observed large variations in splitting parameters and the seismic tomographic images of the area are consistent with ongoing lithospheric delamination beneath this region.
In this paper we present new petrological and whole-rock geochemical data for the Paleoproterozoic metavolcanic rocks in the lower part of the Ji’an Group within the northern segment of the Jiao–Liao–Ji Belt, China, as well as U–Pb dates and Lu–Hf isotope data for the zircons in these rocks. The new data help in understanding the original nature of these volcanic rocks, as well as their tectonic setting and the formation and evolution of the belt. The zircons can be divided into two groups, of which one is of a typical magmatic origin, whereas the othe shows a metamorphic origin. Zircon U–Pb dating and Lu–Hf isotopic results show that the magmatic zircons have peak ages at 2184, 2365, 2503, and 2710 Ma with εHf(t) values ranging from +1.40 to +8.95, –10.86 to +1.48, –14.68 to +7.53, and –1.05, and with corresponding model ages of 2.18–2.69, 2.80–3.57, 2.54–3.83, and 3.22 Ga, respectively. The metamorphic zircons show two peak ages at 1857 and 1914 Ma. Geochemical data show that the protolith of the metavolcanic rocks consists mainly of basaltic andesite, andesite, and dacite, along with minor basalt and rhyolite, and that these rocks formed a continuous calc-alkaline magmatic sequence. In this sequence, the intermediate–basic rocks have relatively low SiO2 contents, are enriched in MgO, FeOT, the light rare-earth elements (LREEs), Cr, Co, and Ni, and depleted in heavy rare-earth elements (HREEs) and high-field strength elements (HFSEs, such as Nb, Ta, and Ti), indicating derivation by partial melting of a depleted lithospheric mantle that had been metasomatized by fluids or melts derived from a subducted slab. On the other hand, the acidic rocks have high contents of SiO2 and low contents of MgO and FeOT, are enriched in alkalis, Ba, Th, U, and K, and depleted in Nb, Ta, P, and Ti. These geochemical characteristics, together with their Hf isotope data, indicate the acidic rocks could have been derived from the partial melting of juvenile crust and a small amount of ancient crustal material, possibly as old as late Archean. We consider, therefore, that the metavolcanic rocks in the lower part of the Ji’an Group were formed at ca. 2.18 Ga as an assemblage of volcanic rocks typical of an active continental margin, and that they were affected by two-stage metamorphic events at 1.90 and 1.85 Ga. Taking into account the regional geological data, we consider that the evolution of the northern segment of the Jiao–Liao–Ji Belt was related to an arc–continent collision before the middle Paleoproterozoic.
The Raohe accretionary complex, located at the border between the Russian Far East and Northeast China, is a significant part of the western Pacific Oceanic tectonic regime. Due to lack of precise age and geochemical constraints, the tectonic setting and petrogenesis of the magmatic rocks in this area remain undefined, resulting in debate about crustal growth mechanisms and subduction-related accretionary processes in Northeast China. Here, we report whole-rock major and trace element and Sr-Nd isotope data, together with zircon U-Pb ages and in situ zircon Hf isotope data for calc-alkaline andesites, dacites, rhyolites, rhyolitic crystal tuffs, Nb-enriched andesites and basaltic andesites, and high-Mg andesites of the Raohe accretionary complex in NE China. Samples were collected from Late Triassic to Early Jurassic strata. However, geochronological results in this study indicated that the studied magmatism occurred in the Early Jurassic (187–174 Ma). The calc-alkaline volcanic rocks possess geochemical characteristics typical of arc magmas that form at active continental margins, such as moderate enrichments in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depletions in high field strength elements (HFSEs). They have positive εHf(t) values of +3.4 to +10.6 and relatively high (⁸⁷Sr/⁸⁶Sr)i values of 0.7047 to 0.7102. While the Nb-enriched andesites and basaltic andesites have higher TiO2, Hf, Nb, and Zr contents and higher Nb/Ta (24.03–87.60), Nb/U (11.89–75.94), (Nb/Th)PM (0.67–2.70), and (Nb/La)PM (1.95–5.00) ratios than typical arc basalts. They have negative εNd(t) values (–5.47 to –6.04) and relatively variable (⁸⁷Sr/⁸⁶Sr)i values of 0.7047 to 0.7114, suggesting an origin via the partial melting of mantle wedge peridotite that had been metasomatized by slab-derived melt. The occurrence of calc-alkaline, Nb-enriched, and high-Mg volcanic rocks at modern arcs where oceanic slabs are subducting has been well documented. Therefore, we propose that a similar mechanism may explain the generation of the Early Jurassic volcanic rocks of the Raohe accretionary complex in Northeast China.
Neoproterozoic intraplate magmatism is widely distributed in NW China and generally thought to be related to the breakup of the Rodinia supercontinent. Here we report a fragmented Large Igneous Province (LIP) formed at 850–820 Ma in the northern margin of the Qaidam block, northern Tibetan Plateau (named herein as the “North Qaidam LIP”). The associated rocks have undergone various grades of metamorphism from greenschist to ultrahigh-pressure (UHP) eclogite facies, including the greenschist-facies Yingfeng dolerite dikes and basalts (846–821 Ma), the amphibolite- to HP granulite-facies Aolaoshan meta-volcanic sequence (protolith age of 832 Ma and metamorphic age of 439 Ma), and the North Qaidam UHP eclogites (protolith age of 847–828 Ma and metamorphic age of 440–420 Ma). Geochemical data reveal that they resemble present-day E-MORB/OIB and typical continental flood basalts. These features, together with high potential temperatures (Tp = 1434–1524 °C) for “primary” magmas, suggest that these basaltic rocks were most likely derived from a mantle plume source and were emplaced in a continental extensional environment.
We presented the preferential filtering method for denoising and anomaly separation of gravity data based on Green's equivalent-layer concept and Wiener filter. Compared to the conventional upward continuation and the preferential continuation, the preferential filtering method has the advantage of no requirement of continuation height. The data test on the synthetic gravity data showed that the preferential filtering method produced better separation of gravity anomaly than both the band-pass filtering and the conventional upward continuation. We then used the preferential filtering method to suppress the noise in the Bouguer gravity anomaly data of Chinese continent and separate the regional gravity anomaly. Then with constraints of the scattered Moho depth information deduced from the deep seismic detection, the regional gravity anomaly was inversed to obtain the Moho depth distribution of the whole Chinese continent. The preferential filtering method provides technique support for the deep detection and tectonic study of Chinese continent.
This chapter reviews the present-day composition of the continental crust, the methods employed to derive these estimates, and the implications of the continental crust composition for the formation of the continents, Earth differentiation, and its geochemical inventories. We review the composition of the upper, middle, and lower continental crust. We then examine the bulk crust composition and the implications of this composition for crust generation and modification processes. Finally, we compare the Earth's crust with those of the other terrestrial planets in our solar system and speculate about what unique processes on Earth have given rise to this unusual crustal distribution.
Variations in the thickness of the lithosphere are likely to influence the trace element compositions of basalts by controlling the distribution of melt and the minerals in the melting assemblage in the asthenosphere. Examples of systematic trace element variations that appear to be related to the thickness of the lithosphere are found in both oceanic and continental basalts. In the oceans, lithospheric thickness is closely related to age, and its control is manifested by a relation between trace element composition and age of basement ocean crust in Atlantic ocean-island basalts. Beneath continents, lithospheric thickness is less age sensitive, but it varies widely in response to extensional tectonics. Contemporary hot spots show variations in the rare earth element content of their associated basalts which apparently reflect changes in lithospheric thickness between the time of flood-basalt eruption and the present. -from Author
The Tertiary to Recent basalts of Victoria and Tasmania have mineralogical and major element characteristics of magmas encompassing the range from quartz tholeiites to olivine melilitites. Abundances of trace elements such as incompatible elements, including the rare earth elements (REE), and the compatible elements Ni, Co and Sc, vary systematically through this compositional spectrum. On the basis of included mantle xenoliths, appropriate 100 Mg/Mg + Fe+2 (68-72) and high Ni contents many of these basalts represent primary magmas (i.e., unmodified partial melts of mantle peridotite). For fractionated basalts we have derived model primary magma compositions by estimating the compositional changes caused by fractional crystallization of olivine and pyroxene at low or moderate pressure. A pyrolite model mantle composition has been used to establish and evaluate partial melting models for these primary magmas. By definition and experimental testing the specific pyrolite composition yields parental olivine tholeiite magma similar to that of KilaeauIki, Hawaii (1959-60) and residual harzburgite by 33 per cent melting. It is shown that a source pyrolite composition differing only in having 0.3-0.4 per cent TiO2 rather than 0.7 per cent TiO2, is able to yield the spectrum of primary basalts for the Victorian-Tasmanian province by ∼4 per cent to ∼25 per cent partial melting. The mineralogies of residual peridotites are consistent with known liquidus phase relationships of the primary magmas at high pressures and the chemical compositions of residual peridotite are similar to natural depleted or refractory lherzolites and harzburgites. For low degrees of melting the nature of the liquid and of the residual peridotite are sensitively dependent on the content of H2O, CO2 and the CO2/H2O in the source pyrolite.The melting models have been tested for their ability to account for the minor and trace element, particularly the distinctively fractionated REE, contents of the primary magmas. A single source pyrolite composition can yield the observed minor and trace element abundances (within at most a factor of 2 and commonly much closer) for olivine melilitite (4-6 per cent melt), olivine nephelinite, basanite (5-7 per cent melt), alkali olivine basalt (11-15 per cent melt), olivine basalt and olivine tholeiite (20-25 per cent melt) provided that the source pyrolite was already enriched in strongly incompatible elements (Ba, Sr, Th, U, LREE) at 6-9 x chondritic abundances and less enriched (2.5-3 x chondrites) in moderately incompatible (Ti, Zr, Hf, Y, HREE) prior to the partial melting event. The sources regions for S.E. Australian basalts are similar to those for oceanic island basalts (Hawaii, Comores, Iceland, Azores) or for continental and rift-valley basaltic provinces and very different in trace element abundances from the model source regions for most mid-ocean ridge basalts. We infer that this mantle heterogeneity has resulted from migration within the upper mantle (LVZ or below the LVZ) of a melt or fluid (H2O, CO2-enriched) with incompatible element concentrations similar to those of olivine melilitite, kimberlite or carbonatite. As a result of this migration, some mantle regions are enriched in incompatible elements and other areas are depleted.Although it is possible, within the general framework of a lherzolite source composition, to derive the basanites, olivine nephelinites and olivine melilitites from a source rock with chondritic relative REE abundances at 2-5 x chondritic levels, these models require extremely small degrees of melting (0.4 per cent for olivine melilitite to 1 per cent for basanite). Furthermore, it is not possible to derive the olivine tholeiite magmas from source regions with chondritic relative REE abundances without conflicting with major element and experimental petrology arguments requiring high degrees (≧15 per cent) of melting and the absence of residual garnet. If these arguments are disregarded, and partial melting models are constrained to source regions with chondritic relative REE abundances, then magmas from olivine melilitites to olivine tholeiites can be modelled if degrees of melting are sufficiently small, e.g., 7 per cent melting for olivine tholeiite. However, the source regions must be heterogenous from ∼1 to ∼5 x chondritic in absolute REE abundances and heterogerieous in other trace elements as well. This model is rejected in favor of the model requiring variation in degree of melting from ∼4 per cent to ∼25 per cent and mantle source regions ranging from LREE-enriched to LREE-depleted relative to chondritic REE abundances.
Major reservoirs in the Songliao Basin (SB) are composed of volcanic rocks below 3000 m (9843 ft) of buried depth. Gas accumulations are mostly found in the buried volcanic highs, which in general correspond to paleovolcanic centers. Porosity in the volcanic rocks depends on both primary and secondary processes. The best porosity is preferentially developed in a proximal facies near the central part of each volcanic edifice. Porosity and permeability decrease with depth of burial for both volcanic and non-volcanic sections, but their porosity-depth trends differ. Lava and welded ignimbrite slowly lose porosity with burial depth because they solidified by cooling, and their groundmasses (>= 75%) are poor in quartz and calcite precipitation, thus preserving porous space. In contrast, the associated sandstone, conglomerate, tuff, and tuffite are more sensitive to overburden pressure, suffering more intense compaction and cementation. As a result, porosity and permeability of lava and ignimbrite exceed that of the other rocks, and thus, they are the best reservoir rocks below burial depths of ca. 3000 m (9843 ft) in the SB. The paleovolcanic domes are rich in both lava rocks and fractures of diverse origin, and the topographic highs provide favorable locations for gas migration and accumulation.
The Nadanhada Terrane, located along the eastern margin of Eurasia, contains a typical accretionary complex related to paleo-Pacific plate subduction-accretion. The Yuejinshan Complex is the first stage accretion complex that consists of meta-clastic rocks and metamafic-ultramafic rocks, whereas the Raohe Complex forms the main parts of the terrane and consists of limestone, bedded chert and mafic-ultramafic rocks embedded as olistolith blocks in a weakly-sheared matrix of clastic meta-sedimentary rocks. Geochemical data indicate that the Yuejinshan metabasalts have N-MORB affinity, whereas the Raohe basaltic pillow lavas have an affinity to OIB. SHRIMP U-Pb zircon analyses of gabbro in the Raohe Complex yielded weighted mean 206Pb/238U zircon age of 216 ± 5 Ma, whereas two samples of granite intruded into the complex yield weighted mean 206Pb/238U zircon ages of 128 ± 2 Ma and 139 ± 2 Ma. LA-ICPMS U-Pb zircon analyses of basaltic pillow lava in the Raohe Complex define a weighted mean age of 167 ± 1 Ma. Two sandstone samples in the Raohe Complex record the younger concordant zircon weighted mean ages of 167 ± 17 Ma and 137 ± 3 Ma. These new data support the view that accretion of the Raohe Complex was between 170 and 137 Ma, and that final emplacement of the Raohe Complex took place at 137-130 Ma. The accretion of the Yuejinshan Complex probably occurred between the 210 Ma and 180 Ma, suggesting that paleo-Pacific plate subduction was initiated in the Late Triassic to Early Jurassic.
Seafloor alteration of the basaltic upper oceanic crust provides one of
the major geochemical pathways between the mantle, the ocean/atmosphere
and subduction zone regimes. Yet, no reliable mass balances are
available, in large part because of the extremely heterogeneous
distribution of altered materials in the oceanic crust but also because
of the limited availability of high recovery drill cores. In this paper,
we document the feasibility of determining the bulk altered and fresh
composition of the oceanic crust on a 10-500 m length scale, from a
region in the western Atlantic Ocean (DSDP/ODP Sites 417-418). Unaltered
compositions were obtained from glass and phenocryst data and altered
compositions were determined through analysis of composite samples. Most
of the alteration-related chemical inventory resides preferentially in
the upper oceanic crust and in highly permeable volcaniclastics. Most
major elements (Si, Al, Mg, Ca, and Na) and many trace elements (Sr, Ba,
LREE's) experience substantial large scale redistribution, but fluxes
are relatively low. Overall, 12 wt % are added to the crust, mostly
H2O, CO2, and K, but the distribution varies
widely. High field strength elements, Th, Ti and Fe remain essentially
immobile during low temperature alteration, while most other elements
are affected to some degree. While the total fluxes are relatively
small, the re-distribution of alteration-sensitive elements in the ocean
crust is much larger, even on length scales exceeding 100m. The bulk
composition of the upper 500m at Sites 417/418 can be used to constrain
the impact of ocean crust subduction on element recycling to volcanic
arcs. Flux balances indicate that the altered domains within the upper
basaltic crust may contribute a very large proportion of some element
fluxes recycled to the arc (H2O, CO2, K, Rb, U),
while other element fluxes require additional contributions from
sediments and deeper oceanic crust.
A new global P-wave tomography model is determined using a flexible-grid parameterization. This new model better reveals the mantle structure under the polar regions than the previous tomographic models. The subducting slabs are generally imaged clearly as high-velocity (high-V) zones. The young slabs are still subducting in the upper mantle and the mantle transition zone (MTZ), whereas the old and ancient slabs are either stagnant in the MTZ or have subducted down to the lower mantle, even reaching the core-mantle boundary. Low-velocity (low-V) anomalies are generally revealed in the mantle under the hotspot regions. It seems that a variety of mantle upwelling (plumes) exist. Some strong plumes are visible in the whole mantle under the long-living hotspots, such as those in south-central Pacific, Africa, Hawaii and Iceland, whereas weak plumes are visible in only some depth range under the minor hotspots. Under the intraplate volcanoes in East Eurasia, Bering Sea and West Alaska, significant low-V anomalies are revealed in the upper mantle, which may reflect hot and wet upwelling associated with corner flows in the big mantle wedge (BMW) above the stagnant Pacific slab in the MTZ and perhaps deep slab dehydration as well. The subduction-triggered magmatism in the BMW may be a new class of mantle plumes. We also used the new global model to investigate the influence of whole-mantle heterogeneity on the determination of upper-mantle tomography under Japan with a teleseismic inversion method. The results show that the mantle heterogeneities outside the target volume of regional tomography can cause significant changes (~ 0.2-0.4 s) to the observed relative travel-time residuals of a distant earthquake. The pattern of regional tomography remains the same even after correcting for the whole-mantle heterogeneity, but there are some changes in the amplitude of velocity anomalies in the regional tomography. Hence it is necessary to correct for the mantle heterogeneities outside the target volume so as to obtain a better regional tomography.
Precise 40Ar/39Ar and K/Ar ages were determined on sanidine and whole-rock separates of volcanic rocks in the Songliao basin (SB). The volcanism is characterized by a series of continual and episodic eruptions in the late Jurassic (147–157 Ma) and early Cretaceous (113–136 Ma). A fairly accurate chronostratigraphic column was first established based on those data. The volcanic activities happened synchronously with closure of the Mongolia–Okhotsk ocean north to the SB. Structures of the volcanogenic successions are S-dipping or SE-dipping fault blocks coupled with N-dipping or NW-dipping major detachment systems. Distribution of the volcanics, revealed by high-resolution seismic reflections, is neither restricted within nor in agreement with the framework of the overlying sedimentary sequence striking NNE. This new evidence fits the overlap model, proposed in the present paper, that formation of the volcanic successions in the SB is related to the subduction of the Mongolia–Okhotsk plate under the Mongolia–North China block during the Jurassic and early Cretaceous, whereas the overlying sedimentary sequence unconformably on the volcanics is tectonically controlled by the oblique subduction of the Pacific plate under the Eurasian plate since late Aptian. In addition, the coal-bearing epiclastic sediments intercalating the volcanics were deposited at the subduction–quiescence episodes when extensional collapse occurred.
The Kamchatka arc (Russia) is located in the northwestern Pacific Ocean and is divided into three segments by major sub-latitudinal fault zones (crustal discontinuities). The southern (SS) and central (CS) segments are associated with the subduction of old Pacific lithosphere, whereas the northern, inactive segment (NS) was formed during westward subduction of young (< 15 Ma) Komandorsky Basin oceanic crust. Further segmentation of the arc is outlined by the development of the Central Kamchatka Depression (CKD) intra-arc rift, which is oriented parallel to the arc and is splitting the CS into the active Eastern Volcanic Front (EVF) and the largely inactive, rear-arc Sredinny Range. The NS volcanics (15-5 Ma) include calc-alkaline lavas, shoshonites, adakites, and Nb-enriched arc basalts. Isotopically all magma types share high 143Nd/144Nd ratios of 0.512976-0.513173 coupled with variable 87Sr/86Sr (0.702610-0.70356). NS lavas plot within or slightly above the Pacific MORB field on the Pb isotopic diagrams. The EVF volcanoes have more radiogenic 143Nd/144Nd (0.51282-0.513139) and 208Pb/204Pb (38.011–38.1310) than the NS lavas. CKD lavas display MORB-like Nd isotope ratios at slightly elevated 87Sr/86Sr values accompanied by a slightly less radiogenic Pb composition. Kamchatka lavas are thought to be derived from a MORB-like depleted source modified by slab-derived siliceous melts (adakites) and fluids (NS), or fluids alone (CS and SS). The NS and EVF lavas may have been contaminated by small fractions of a sedimentary component that isotopically resembles North Pacific sediment. Petrogenesis in the Kamchatka arc is best explained by a three-component model with depleted mantle wedge component modified by two slab components. Slab-derived hydrous melts produced incompatible element characteristics associated with northern segment lavas, while hydrous slab fluids caused melting in the depleted mantle below the southern and central segments of the Kamchatka arc. Trace element characteristics of Kamchatka lavas appear to be controlled by slab fluids or melts, while radiogenic isotope ratios which are uniform throughout the arc reflect depleted composition of sub-arc mantle wedge.
Splitting or thinning of lithosphere above a mantle plume can result in voluminous melt generation, leading to the formation of large igneous provinces, or LIPs. Examples of LIPs include continental flood basalt provinces and oceanic plateaus. Basaltic samples from the Ontong Java Plateau, Nauru Basin and Manihiki Plateau, which are among the largest of the LIPS, have isotopic compositions within the range of ocean island basalts. The majority of continental basalts, however, record a trace element and isotopic contribution from the lithosphere through which they have erupted. A combination of mantle sources is indicated, with the thermal energy being supplied by voluminous melts from a plume, and the lithospheric components in continental flood basalts being inherited by contamination of plume-derived melts by low melting point hydrous and carbonated fractions in the lithosphere. The authors favour a model whereby the thermal anomaly builds gradually, incubating beneath a steady-state lithospheric cap. -from Authors
Zircon and baddeleyite U–Pb geochronological dating is widely used in solid Earth sciences and the advent of rapid in-situ methods of analysis, such SIMS and ICP-MS, has re-emphasized the importance of having uniform standards. Recently, it has been shown that Hf isotopic data can provide important information on these minerals since they contain high concentrations of Hf, but have low Lu/Hf ratios, which results in negligible age correction. However, the complex internal structures that result from multiple thermal events, such as inherited cores and metamorphic overgrowths, require that the Hf isotopic data be measured with high spatial resolution. However, the isobaric interferences of 176Yb and 176Lu on 176Hf hamper the precise determination of the 176Hf/177Hf ratio during in-situ laser ablation MC-ICPMS analysis. It is shown here that mass biases of Yb (βYb) and Hf (βHf) change with time during analyses and behave differently for solutions and solid material. Therefore, it is suggested that the mean βYb value of the individual spot be used to obtain the precise isotopic composition for in-situ zircon and baddeleyite Hf isotopic analyses. For low Yb/Hf (176Yb/177Hf0.001) zircons and baddeleyites, since the interference of 176Yb on 176Hf is significant. Using the mean βYb value of the individual spot and newly published Yb isotopic abundance data, six standard zircons and two standard baddeleyites, have been investigated using a Neptune MC-ICPMS, with 193 nm laser. For zircons, the obtained 176Hf/177Hf ratios are 0.282307±31 (2SD) for 91500, 0.282680±31 (2SD) for TEMORA, 0.281729±21 (2SD) for CZ3, 282177±17 (2SD) for CN92-1, 0.282983±17 (2SD) for FM0411, and 281234±11 (2SD) for Phalaborwa. The baddeleyites from Phalaborwa and SK10-2 have 176Hf/177Hf ratios of 0.281238±12 and 0.282738±13 (2SD). These results agree well with the values obtained by the solution method and indicate that these standards have different Hf isotopic compositions, in which the extremely low 176Lu/177Hf and 176Yb/177Hf values of CZ3 zircon and Phalaborwa baddeleyite make them excellent standards for machine calibration during in-situ zircon Hf isotopic measurement, with the other standards being more suitable for the development of the correction method.
The Mesozoic Liaonan metamorphic core complex (mcc) of the southeastern Liaoning province, North China, is an asymmetric Cordilleran-style complex with a west-rooting master detachment fault, the Jinzhou fault. A thick sequence of lower plate, fault-related mylonitic and gneissic rocks derived from Archean and Early Cretaceous crystalline protoliths has been transported ESE-ward from mid-crustal depths. U–Pb ages of lower plate syntectonic plutons (ca. 130–120 Ma), 40Ar–39Ar cooling ages in the mylonitic and gneissic sequence (ca. 120–110 Ma), and a Cretaceous supradetachment basin attest to the Early Cretaceous age of this extensional complex. The recent discovery of the coeval and similarly west-rooting Waziyu mcc in western Liaoning [Darby, B.J., Davis, G.A., Zhang, X., Wu, F., Wilde, S., Yang, J., 2004. The newly discovered Waziyu metamorphic core complex, Yiwulushan, western Liaoning Province, North China. Earth Science Frontiers 11, 145–155] indicates that the Gulf of Liaoning, which lies between the two complexes, was the center of a region of major crustal extension.
The Gangbian alkaline complex in the southeastern Yangtze Block (South China) is composed of Si-undersaturated pyroxene syenites and Si-saturated to -oversaturated syenites and quartz monzonites. SIMS zircon U–Pb analyses indicate that the complex was emplaced at 848 ± 4 Ma, during a previously-recognized interval of magmatic quiescence between the ca 1.0–0.89 Ga Sibaoan orogenic magmatism and the ca 0.83–0.78 Ga magmatic flare-up. The Gangbian rocks are characterized by wide, coherent variations in major and trace elements (SiO2 = 47.6–68.4%, K2O + Na2O = 4.5–10.5%, K2O/Na2O = 0.4–1.2, MgO = 1.2–8.5%, Cr = 4.5–239 ppm, and Ni = 4.5–143 ppm) and by enrichment in LIL and LREE and depletion in Nb, Ta and P in trace element spidergrams. Their whole-rock εNd(T) (− 6.5 to − 0.4) and εHf(T) (− 10.7 to 0.4) are positively correlated, suggesting involvement of both metasomatized mantle and continental crust materials in their genesis. In situ zircon Hf–O isotopic measurements for the most evolved quartz monzonite sample yield a binary mixing trend between the mantle- and supracrustal-derived melts. It is suggested that the pyroxene syenites were derived by partial melting of metasomatized, phlogopite-bearing lithospheric mantle, and the parental magma experienced extensive fractionation of pyroxene and olivine associated with varying degrees of crustal contamination. Subsequent fractional crystallization of hornblende and minor amounts of plagioclase from the alkali basaltic magmas, accompanied by crustal contamination, produced the Si-saturated to -oversaturated syenites and quartz monzonites. These ca. 0.85 Ga alkaline rocks and neighboring contemporaneous dolerite dykes are the products of the anorogenic magmatism after the Sibao Orogeny. They post-date the final amalgamation between the Yangtze and Cathaysia Blocks, most likely manifesting the initial rifting of South China within the Rodinia supercontinent.
Active volcanism in Galápagos is far more widespread (> 40,000 km2) than in other hotspot-related archipelagos, such as Hawaii (~ 20,000 km2). Here we employ both geochemical and geophysical models to constrain the causes of this large spatial extent of melt generation and the diverse compositions of erupted basalts. Insight in to the physical as well as the chemical nature of the melting regime beneath Galápagos –– and the cause of the relatively widespread, non-linear age-progressive distribution of volcanism – is provided by incompatible-trace-element ratios of basaltic magmas. Whilst variations in these (and isotopic) ratios of basalts from individual Galápagos volcanoes are limited, considerable differences have been observed in basalts erupted across the archipelago. We have used rare-earth-element inversion modelling for basalts dominated by “plume” and depleted MORB mantle components to constrain the depth to the top of the melt column beneath different Galápagos volcanoes. By converting S-wave data from a recently published tomographic experiment [Villagomez, D.R. et al., 2007. Upper mantle structure beneath the Galápagos Archipelago from surface wave tomography. J. Geophys. Res. 112] to temperature we have been able to map the base of the Galápagos lithosphere, i.e. where the geotherm, with a mantle potential temperature of 1315 °C, intersects the anhydrous peridotite solidus.
New Sr–Nd–Pb isotope dataset of Mesozoic igneous rocks shows that the NE China can be subdivided into four Sr–Nd–Pb isotope provinces: (1) the northern Hinggan Mountains (NHM) region; (2) the southern Hinggan Mountain (SHM)–Yanji-Liaoyuan (YL) region; (3) the Zhangguangcai Range–Jiamusi (ZGJ) Block; and (4) the exotic Wandashan massif (WDM). The Wandashan massif contains Mesozoic (high-μ)-type oceanic island basalts (HIMU-OIBs) with highly radiogenic Pb isotopic compositions (e.g., ²⁰⁶Pb/²⁰⁴Pb(i) = 18.9–22.7), quite different from other regions that have moderately radiogenic Pb (i.e., ²⁰⁶Pb/²⁰⁴Pb(i)) is generally less than 18.6. These HIMU-OIBs also show negative Δ7/4 and Δ8/4 values, signatures of Southern Gondwanaland Continent. By contrast, the majority of Mesozoic igneous rocks in other areas of NE China have positive Δ7/4 and Δ8/4 values, akin to the Northern Laurasian Continent. Such isotopic variations were probably due to the enrichment processes mainly caused by Paleozoic Paleo-Asian ocean subduction and to some extent by subduction of the Paleo-Pacific Ocean since early Mesozoic.
In the Brévenne Series (NE Massif Central), a low-grade bimodal association of metabasalts and metarhyolites is exposed,
together with intrusive trondhjemite bodies. Zircon U-Pb dating constrains their magmatic emplacement at 366 ± 5 Ma and 358 ± 1 Ma,
respectively. The metabasalts are characterized by a distinct enrichment in incompatible elements (e.g. Th and LREE) and positive
ɛNdi (from +5 to +8). Combined isotope and trace element systematics rule out crustal contamination of mafic melts as a suitable
cause of the LILE (large ion lithophile element)-enrichment. Rather, a mixing process between a component similar to mid ocean
ridge basalts and an enriched end-member with ɛNdi > +5 is suggested. An enriched-mantle source of ocean island basalt affinity is precluded by the relative depletion of high
field strength elements, especially Nb which shows negative anomalies in chondrite-normalized patterns. On the contrary, a
subduction-related origin for the LILE enrichment would be more consistent. It may be inferred that arc-like melts [enriched
in Th and LREE (light rare earth elements) and depleted in Nb, with ɛNdi > +5] were produced through partial melting of a depleted-mantle source, to which a small amount of crustally derived component
had been added. The metarhyolites are enriched in LILE, and have a close genetic relationship with the metabasalts, as evidenced
by their high ɛNdi (from +4.7 to +6.8). Although the chemical evidence remains ambiguous, it is suggested that fractional crystallization, accompanied
by subordinate assimilation, is the petrogenetic process most consistent with the data. The trondhjemites are isotopically
distinct from the metarhyolites. Their ɛNdi values (from −1.0 to +2.2) reflect an important contribution of continental crust to their genesis, and disprove their inferred
cogenetism with the felsic volcanics. A review of modern environments in which such bimodal suites are exposed, shows that
settings involving incipient rifting of a volcanic arc fringing a continental margin, or built upon young, thin continental
crust might provide suitable analogues. Geodynamic reconstructions are complicated by subsequent tectonic events which disrupted
the initial patterns, and by Mesozoic-Cenozoic sedimentary cover. However, this subduction-related magmatism enlarges the
growing body of evidence for southward subduction processes until the Late Devonian during the evolution of the northern flank
of the European Variscides. As a general implication, it is suggested that the combined use of the Sm-Nd system with incompatible
elements relatively resistant during alteration and low-grade metamorphism (REE, Th, Zr, Nb) may provide diagnostic criteria
for recognizing the tectonic setting of bimodal metaigneous suites in ancient orogenic belts.