Lushan Area Research Articles

Interaction between magmatism and deformation in the Mesozoic of the Lushan area, eastern China

Interaction between magmatism and deformation in the Mesozoic of the Lushan area, eastern China

Redox-stratified seawater during the GOE: Evidences from rare earth elemental and C-O isotopic compositions of Paleoproterozoic BIF and carbonate rocks from the Taihua Group, North China Craton

The Paleoproterozoic Great Oxidation Event (GOE) is known for the dramatic change in atmospheric composition that led to oxygen becoming a persistent component of global surface processes. Drastic environmental changes greatly affected ocean and atmosphere chemistry. However, the distribution of dissolved oxygen in the ocean during the GOE is still unclear as rare contemporary sediment-records have been found. In order to identify the redox state of both shallow and deep water, and reveal the ocean redox structure during this time, we present new major and trace elemental and C-O isotopic data of bulk samples of the banded iron formation (BIF), dolomitic marble, and graphite-bearing marble of the early Paleoproterozoic upper Taihua Group in the Lushan area, southern margin of the North China Craton (NCC). The BIF samples exhibit typical major and trace element characteristics akin to very pure marine chemical sediments, such as the shale-normalized positive La, Ga, and Y anomalies, indicative of a marine depositional setting. In these BIF samples, both negative Ce anomalies and the lack of Ce anomalies reflect the presence of both oxic and suboxic/anoxic water in the ocean. Samples of the dolomitic marble are characterized by extremely low amounts of crustal material (such as Al, Ti, Zr, Hf) and high Y/Ho ratios (46–66), also implying a pure marine chemical origin. Excluding some contaminated samples, the rest of the graphite-bearing marble samples display similar features of the dolomitic marble. Significant positive Ce anomalies and high light to heavy REE (Pr/Yb(SN)) ratios in both dolomitic marble and graphite-bearing marble are essentially consistent with the 2.2–2.1 Ga carbonates from the Guanmenshan Formation of the Liaohe Group in the NE Sino-Korean Craton. Combined with the Ce anomalies in the BIF samples, we consider that the seawater when the upper Taihua Group deposited was redox-stratified from oxic shallow water to deeper anoxic water. Besides, the graphite-bearing marble record coeval surface water in the column that was enriched in nutritive materials and beneficial to microbiology and high productivity. Moreover, the early Paleoproterozoic IFs in the Songshan Group and the Lüliang Group in the NCC also exhibit significant both positive and negative Ce anomalies and enrichment of redox sensitive elements. These evidences, therefore, suggest that redox-stratified seawater occurred extensively during the GOE.

The Mesozoic Tectonic Transition from Compression to Extension in the South China Block: Insight from Structural Deformation of the Lushan Massif, SE China

The Lushan Massif has been considered an extensional dome which represents a typical extensional structure in South China. However, the composition and structure of the Lushan Massif are still unclear. In this study, we identified the eastern detachment fault (EDF) for the first time. In addition, many sinistral strike-slip structures have also been recognized in the Lushan area, such as the Xingzi shear zone (XZSZ) and Lianhua shear zone (LHSZ). Detailed field observation and structural analysis revealed that the former sinistral faults are tectonic boundaries of the later Lushan extensional dome (LSED). The tectonic evolution sequence was established after the structural analysis combined with zircon U-Pb dating and mica 40Ar-39Ar dating of metamorphic rocks, veins, and intrusive rocks from the strike-slip fault and detachment fault. The Lushan Massif has undergone sinistral ductile shearing within 162–150 Ma. The LSED was then formed in an extensional tectonic setting from 140 to 114 Ma. Together with the regional geological setting, we believe that the sinistral strike-slip structures, represented by the XZSZ and LHSZ, are coeval with the Tanlu fault system and could be controlled by a transpressional stress field resulting from the subduction of the Pacific Plate. The LSED was formed in a back-arc extension setting resulting from the rollback of a subducted slab. The tectonic transition from compression to extension in the South China Block took place at 150–140 Ma.

Highly heterogeneous Pb isotope composition in the Archean continental lower crust: Insights from the high-grade metamorphic suite of the Taihua Group, Southern North China Craton

The petrological and geochemical compositions of Archean crust hold important information about the interaction between continental crust and lithospheric mantle. Here we apply in-situ microanalysis of Pb isotopes of the feldspars, which were collected from the high-grade terranes of the Taihua Group in Lushan area along the Southern North China Craton (S-NCC), by femtosecond laser ablation (fLA)-MC-ICPMS technique. The results show large variation in Pb isotopic compositions in plagioclase grains in different rock types. The feldspars in the granitoid gneisses show lower and variable radiogenic Pb isotopic ratios (206Pb/204Pb = 13.8–16.8, 207Pb/204Pb = 14.6–15.1 and 208Pb/204Pb = 33.9–36.6) than that in the amphibolites (206Pb/204Pb = 14.1–17.7, 207Pb/204Pb = 14.6–15.6, 208Pb/204Pb = 33.6–38.1) and lenticular garnet granulites (206Pb/204Pb = 14.4–17.7, 207Pb/204Pb = 14.7–15.7, 208Pb/204Pb = 33.7–37.7). Moreover, based on the single-stage Pb evolution model (common Pb in plagioclase), the age of the granitoid gneisses extracted from the mantle was estimated at 2.74 ± 0.33 Ga, corresponding to the age of important period of crustal growth (~2.7 Ga), and suggesting that the low-Pb granitoid gneisses are representative of the direct samples of the Archean crust. In addition, we also newly reported what was the Pb isotope characteristics of garnet granulites and pyroxene granulites in the Lower Taihua Group once located at the lower crustal level. The ubiquitous Pb isotope disequilibrium among different feldspar grains, especially in the overgrowth rims possessing more radiogenic Pb isotopes, suggests input of exotic melt/fluid with high μ (238U/204Pb) during the process of retrograde metamorphism. The large present-day Pb isotopic heterogeneity is probably due to various geological conditions experienced by different rocks.

Paleoproterozoic tectono-metamorphic evolution of the southernmost North China Craton: New insights from the metamorphic evolution and geochronology of the Taihua complex at Lushan area

The Taihua metamorphic complex along the southern margin of the North China Craton (NCC) mainly consists of tonalite–trondhjemite–granodiorite (TTG) gneisses, amphibolites, metapelites, marbles, quartzites, and banded iron formations (BIFs). The region has been subjected to upper amphibolite–granulite facies metamorphism and intense deformation, with NWW–SEE striking gneissosity. Garnet porphyroblasts from metapelites of the Lushan area are chemically zoned. Based on petrological and geochemical analyses, quantitative phase equilibria modeling, and traditional geothermobarometry, a clockwise pressure–temperature (P–T) path with a nearly isothermal decompression (ITD) segment was retrieved from the Lushan metapelites. The peak metamorphic conditions of ~770 °C and 11.7 kbar were recorded in sample L93, which reached to the upper amphibolite–granulite facies. High-resolution secondary ion mass spectrometry (SIMS) zircon U–Pb dating demonstrated that the metapelites record a peak or near-peak metamorphic age of ~1.93 Ga. Hornblende 40Ar/39Ar dates from amphibolite illustrated that the Taihua metamorphic complex have cooled to 500–600 °C by ~1.84 Ga. Accordingly, a prolonged Paleoproterozoic continental collision event is supported.

Establishment of a seismic topographic effect prediction model in the Lushan Ms 7.0 earthquake area

SUMMARYThe seismic topographic effect is one of the debated research topics in seismology and earthquake engineering. This debate is due to the discrepancy between the observed amplification and the amplification underestimation in numerical simulations. Although the numerical simulation of ground motion, which began in the 1970s, has been an important and effective way to study topographic effects, the quantitative mathematical model of topographic amplification is urgent. The actual influences on ground motion due to the topography depends on multiple topographic features, such as the topographic slope, topographic geometrical scale. To date, no definite conclusions regarding the main influencing factors and how to express the influencing factors have been made. In this paper, by introducing the back-propagation (BP) neural network technique, a set of mathematical parameters are determined to establish a quantitative topographic effect prediction model. These parameters are the elevation, the first gradient of the elevation and the higher order gradient in two orthogonal directions. Theoretically, the set of mathematical parameters is directly related to the simple topographic features, such as the elevation, topographic slope and height-to-width ratio. Furthermore, their combinations indirectly denote the complex topographic geometrical features, such as the different topographic geometrical scales, designated by the elevation (large-scale variable), the first gradient (middle-scale variable), the second-order gradient (small-scale variable) and so on (smaller scale variable), and the hill ridges that correspond to the sites with the first gradient of the elevation equal to zero and an elevation larger than its surrounding. In 2013, an earthquake of Ms 7.0 occurred in the Lushan area of Sichuan Province in Western China, where the topography sharply fluctuates. At station 51BXD, an acceleration was recorded close to 1.0 g, while at station 51BXM (14 km away from station 51BXD), the acceleration was recorded at only 0.2–0.3 g. In this paper, the spectral element method (SEM) is used to simulate the ground motion in the Lushan Ms 7.0 earthquake area. Then, the topographic amplification ratio of the simulated ground motion is calculated. Furthermore, a BP topographic amplification prediction model is established and compared based on different parameters. A rms of less than or close to 10 per cent between the BP model prediction results and topographic amplification ratio calculated using the simulated ground motion suggests that the parameters of the topographic elevation, the first gradient of the elevation and the second-order gradient in two orthogonal directions are enough to provide the acceptable topographic effect model in the Lushan area. Finally, using the prediction model, the topographic spectral ratio at stations 51BXD and 51BXM is predicted, and the topography amplification due to the scattering of seismic waves by the irregular topography at 51BXD is found to be 1.5–2 times that of 51BXM. The most important highlights of this paper identify the main factors of the topographic effect for the first time and provide an effective method for establishing a quantitative topographic effect prediction model.

Deep-seated gravitational slope deformation in Lushan, Taiwan: Transformation from cleavage-controlled to weakened rockmass-controlled deformation

The northern slope in Lushan (NSL) in central Taiwan is recognized as a large deep-seated gravitational slope deformation (DSGSD) in anisotropic slate and is often activated by typhoons and heavy rainfall. However, a recent southward movement of the DSGSD was apparently not caused by translational sliding along the eastward-dipping cleavage in the NSL. Because the regional attitudes of the cleavages in the NSL moderately vary and no major fault has been observed in the field, the deformed cleavages with their random dips indicate that the deformation of the rockmass was caused by older mass movements. In addition, ancient progressive erosion of the NSL toe by the Talowan River, which passes through the eastern side of the slope, induced deep, large-scale mass movement toward the east. Based on detailed core recognition measurements including the cleavage dips, log Q values, and inclinometer measurements, we identified five geometrical styles and geotechnical zones of the deformed rockmasses for characterization of the evolution of the NSL DSGSD with regard to the spatial distributions of old mass movements and recent transformations. Curves obtained from recent inclinometer measurements suggested two deformation behaviors, namely, specific sliding and progressive deformation. The underlying rockmass with overturned and flexural folding in cleavages at the slope toe have baffled a recent southward advancement of the DSGSD. Consequently, the deep specific sliding surface close to the basal disintegrated rockmass at the center of the NSL was transformed into progressive deformation, which mostly occurred in the overturned rockmass at the toe. Based on our observations, we propose the following relationship between the P-wave velocity (Vp, km/s) and log Qc (where Qc is a normalized Q) as an empirical model for slate in Taiwan: Vp=0.6 × log Qc + 4.0. This empirical model might be suitable for the low-uniaxial compressive strength slate in the Lushan area and could be used for the quantitative evaluation of the geotechnical zone of the DSGSD of the NSL.

Revisiting the Lushan-Taihua Complex: New perspectives on the Late Mesoarchean-Early Neoarchean crustal evolution of the southern North China Craton

The Taihua Complex in the Lushan area was divided into gneisses series and supracrustal rocks. The former is mainly composed of TTG gneisses and amphibolites, and records the Late Mesoarchean-Early Neoarchean crustal evolution of the southern North China Craton (NCC). Late Mesoarchean amphibolites (>2.84 Ga) are enriched in light rare-earth elements (LREEs) and large-ion-lithophile elements (LILEs) with negative Nb-Ta and Ti anomalies, derived from partial melting of a metasomatic mantle source. Late Mesoarchean TTG gneisses (>2.81 Ga) have high La/Yb and Sr/Y ratios and low MgO, Cr and Ni contents, and are interpreted as resulting from partial melting of a thickened lower crust with residual garnet and amphibole in the source. Early Neoarchean amphibolites (2.80–2.73 Ga) show flat REE patterns but slightly negative Nb-Ta and Ti anomalies due to the fractionation of Ti-rich minerals and have depleted Hf-Nd isotopes, and were derived from large degree partial melting of a depleted mantle source. Most of the Early Neoarchean TTG gneisses show pronounced positive Eu anomalies and have relatively lower trace element concentrations and more depleted Nd-Hf isotopes than the Late Mesoarchean TTG gneisses, resulted from the partial melting of thickened juvenile lower crust with residual garnet and amphibole in the source and subsequent plagioclase accumulation. In addition, some of the Early Neoarchean TTG gneisses have high MgO, Cr and Ni contents, which were likely derived from partial melting of delaminated lower crust or a thickened lower crust interacting with mafic melt.Late Mesoarchean amphibolites and TTG gneisses are restricted in the center of Lushan area, denoting an ancient continental nucleus. In contrast, Early Neoarchean amphibolites and TTG gneisses outcrop in peripheral of the ancient nucleus, exhibiting as an elongated belt along the Dangzehe River, which is evidence for the subduction. Overall, the Lushan amphibolites and TTG gneisses show slightly negative to positive whole-rock εNd(t) and εHf(t) values and zircon εHf(t) values, indicating a dominant crustal accretion with subordinate crustal reworking. Therefore, the continental growth recorded in the Lushan-Taihua Complex was caused by the subduction-accretion process, suggesting a convergent margin in the southern NCC during the Late Mesoarchean and Early Neoarchean.

Low-frequency Topographic Amplification Effect of Lushan Earthquake Based on Three-dimensional Numerical Simulation

The study of the influence of terrain on ground motion is quantified as the amplification effect of terrain on ground motion. In this paper, we use the spectral element method to simulate the three-dimensional real terrain model, and select the aftershocks of the station ground motion observation records in the Lushan area as the source, and analyze the amplification effect of the three-dimensional topography on the ground motion of the Lushan earthquake in the low frequency range (1-5hz). In this paper, the Fourier model of the real terrain of Lushan and the half-space free model are used to calculate the Fourier amplitude spectrum. The results show that in the low frequency range, the ground fluctuations have a large variation, which will have complex effects on the propagation of seismic waves. Especially in the bulging mountain and depression terrain, the time course peak is large. In addition, this paper selects the mountain near the BXD station from the calculation area, and analyzes the terrain magnifying effect of the convex mountain. Under 2-3hz, the mountain slope near the source side has a magnification effect of 3-4 times.

Formation and evolution of the Paleoproterozoic meta-mafic and associated supracrustal rocks from the Lushan Taihua Complex, southern North China Craton: Insights from zircon U-Pb geochronology and whole-rock geochemistry

The Upper Taihua Complex in the Lushan area is one of the best preserved Paleoproterozoic metamorphic basements exposed in the southern North China Craton, which is mainly composed of Khondalite-dominated supracrustal rocks and associated meta-mafic intrusions (amphibolites) and granitic plutons. Detrital zircon U-Pb ages of four paragneisses from the supracrustal rocks can be divided into two groups: 2.39–2.10Ga and 1.97–1.83Ga. Concordant 207Pb/206Pb ages from the older group show typical magmatic features with a distinct peak around 2.31–2.23Ga, whereas the metamorphic zircons define a remarkable age peak at ∼1.93Ga. Magmatic zircons from the amphibolite sample have a weighted mean 207Pb/206Pb age of 2128±29Ma, representing the crystallization age of the protolith. Therefore, the depositional age of the supracrustal rocks in the Upper Taihua Complex can be well constrained at ∼2.23–2.13Ga. Geochemically, the amphibolites are classified as subalkaline high-Fe tholeiitic basalts in compositions and characterized by slightly enrichment in LREE, flat HREE and weakly negative Eu anomalies, with depleted high field strength elements (e.g., Nb, Ta, and Ti), inconsistent with typical rocks derived from the asthenospheric mantle. The high FeOT (>13.59wt%), CaO (>6.67wt%), low Al2O3 (<13.09wt%), Th (<3.09ppm), and lack of correlation between εNd (t) values and crustal contamination sensitive elements suggest that crustal assimilation was limited during magma evolution. The varied εNd (t) values (−2.99 to +0.44) and Nd model ages (2.65–2.98Ga) suggest that they were likely originated from Archean subcontinental lithospheric mantle (SCLM) which have been refertilized by metasomatic processes. These geochronological and geochemical signatures indicate the meta-mafic rocks and associated supracrustal rocks of the Upper Taihua Complex in the Lushan area were most likely formed in an intra-continental rift setting, implying the extensional tectonic scenario rather than subduction-related regime occurred between ∼2.2 and 2.0Ga at least in the southern margin of the NCC. Metamorphic ages suggest that the Taihua Complex in the Lushan area was involved in subduction-collision processes from 1.97Ga to 1.83Ga.

Depositional age and protoliths of the Paleoproterozoic upper Taihua Group in the Wuyang area in the southern margin of the North China Craton: New insights into stratigraphic subdivision and tectonic setting

The Paleoproterozoic upper Taihua Group is widely exposed in the southern margin of the North China Craton (NCC). Stratigraphic correlation of the upper Taihua Group in the region remains unclear due to the inconsistent variation of its depositional ages, lithology assemblages and sedimentary characteristics at different locations. Here we present systematic whole-rock major and trace element, Nd isotope and zircon U-Pb age data for the upper Taihua Group from the poorly studied Wuyang area. Protoliths of the upper Taihua Group in this area are a suite of sequences containing mafic volcanic rocks, chemical sedimentary rocks, pelites and arenites. The depositional age of the upper Taihua Group in the Wuyang area is well constrained from 2470 to 2158Ma. Comprehensive lithological comparison of protoliths of the upper Taihua Group in the Wuyang, Lushan, Xionger’ershan and Xiaoqinling areas shows striking lithologic similarity among them. Two widely exposed strata, the iron-rich and carbonaceous strata in these areas, can be considered as the lithological markers. In combination with the previously dated 2.3Ga maximum depositional age of the carbonaceous stratum in the Lushan area, we propose that the upper Taihua Group in the whole region can be subdivided into the 2.47–2.3Ga Iron-rich Formation and the 2.3–2.15Ga overlying Carbonaceous Formation. Additionally, two types of meta-mafic volcanic rocks in the Iron-rich Formation from the Wuyang area are identified. One is characterized by depletion in Nb, Ta and Ti, enrichment in LREE and positive εNd(t) values (+0.9 to +1.3), similar to that of arc-type basalt. The other have nearly flat REE patterns and higher εNd(t) values (+2.1 to +3.3), depleted in Th instead of Nb and Ta, suggesting geochemical affinity similar to N-MORB components. Taking all the data together, we infer that the upper Taihua Group deposited in a back-arc basin at 2.47–2.3Ga as oceanic subduction coupled with contemporary mantle upwelling related to back-arc basin extension can account for the typical interaction between MORB and arc-like components, and then at stable continental margin at 2.3–2.15Ga based on the carbonaceous pelitic series.

Extrusion deformation process of ground surface during the Lushan earthquake in China

Based on finite element method, the extrusion deformation process of ground surface during the Lushan earthquake (April 20, 2013) is investigated in this work. In order to construct the finite element model of Lushan earthquake structure, the geophysical layer model of Lushan area, the frictional characteristic of slip-weaken along the fault surface, and the Coulomb failure criterion are considered. Through the computation and the comparison with achievement on the Lushan focal dynamics, our researches indicate that: (1) The most extrusion deformation of ground surface occurred in the initial phase of earthquake procession, i.e., between the fourth and sixth seconds after the earthquake occurred. (2) Between the first and sixth seconds after the earthquake, the extrusion deformation concentrates on the surface projection of earthquake fault. (3) Between the first and third seconds after the earthquake, the extrusion deformation of ground surface is very tiny. Meanwhile, the extrusion deformation reaches maximum at the sixth second after earthquake. (4) After 6 s of Lushan earthquake, the extrusion deformation spread out of earthquake structure projection. (5) During the earthquake, the maximum of extrusion deformation on ground surface is larger than the final deformation of the post-earthquake, in other words, the ground extrusion deformation will lastly reach a relatively small value after the Lushan earthquake occurred.

Medical Efforts and Injury Patterns of Military Hospital Patients Following the 2013 Lushan Earthquake in China: A Retrospective Study.

The aim of this paper is to investigate medical efforts and injury profiles of victims of the Lushan earthquake admitted to three military hospitals. This study retrospectively investigated the clinical records of 266 admitted patients evacuated from the Lushan earthquake area. The 2005 version of the Abbreviated Injury Scale (AIS-2005) was used to identify the severity of each injury. Patient demographic data, complaints, diagnoses, injury types, prognosis, means of transportation, and cause of injury were all reviewed individually. The statistical analysis of the study was conducted primarily using descriptive statistics. Of the 266 patients, 213 (80.1%) were admitted in the first two days. A total of 521 injury diagnoses were recorded in 266 patients. Earthquake-related injuries were primarily caused by buildings collapsing (38.4%) and victims being struck by objects (33.8%); the most frequently injured anatomic sites were the lower extremities and pelvis (34.2%) and surface area of the body (17.9%). Fracture (41.5%) was the most frequent injury, followed by soft tissue injury (27.5%), but crush syndrome was relatively low (1.2%) due to the special housing structures in the Lushan area. The most commonly used procedure was suture and dressings (33.7%), followed by open reduction and internal fixation (21.9%).The results of this study help formulate recommendations to improve future disaster relief and emergency planning in remote, isolated, and rural regions of developing countries.

Post-earthquake rainfall-triggered slope stability analysis in the Lushan area

The “4.20” Lushan earthquake in Sichuan province, China has induced a large amount of geological hazards and produced abundant loose materials which are prone to post-earthquake rainfall-triggered landslides. A detailed landslide inventory was acquired through post-earthquake emergent field investigation and high resolution remote sensing interpretation. The rainfall analysis was conducted using historical rainfall records during the period from 1951 to 2010. Results indicate that the average annual rainfall distribution is heterogeneous and the largest average annual rainfall occurs in Yucheng district. The Stability Index MAPping (SINMAP) model was adopted to assess and analyze the post-earthquake slope stability under different rainfall scenarios (light rainfall, moderate rainfall, heavy rainfall, and rainstorm). The model parameters were calibrated to reflect the significant influence of strong earthquakes on geological settings. The slope stability maps triggered by different rainfall scenarios were produced at a regional scale. The effect of different rainfall conditions on the slope stability is discussed. The expanding trend of the unstable area was quantitatively assessed with the different critical rainfall intensity. They provide a new insight into the spatial distribution and characteristics of post-earthquake rainfall-triggered landslides in the Lushan seismic area. An increase of rainfall intensity results in a significant increase of unstable area. The heterogeneous distribution of slope instability is strongly correlated with the distribution of earthquake intensity in spite of different rainfall conditions. The results suggest that the both seismic intensity and rainfall are two crucial factors for post-earthquake slope stability. This study provides important references for landslide prevention and mitigation in the Lushan area after earthquake.

Early Paleoproterozoic (2.45–2.20 Ga) magmatic activity during the period of global magmatic shutdown: Implications for the crustal evolution of the southern North China Craton

A global database of zircon ages from both granitoids and detrital sediments shows an exceptional and robust age gap between 2.45 and 2.20Ga. The early Paleoproterozoic magmatism dramatically decreased on the Earth, which was proposed to relate to the global plate tectonic shutdown. However, the available data indicate that 2.45–2.20Ga magmatic rocks are widespread in the Taihua Complex during the quiet interval. The Taihua Complex is located in both the Lushan and Xiaoqinling areas along the southern segment of North China Craton (NCC). The Complex is composed mainly of a gneiss series and the Khondalite-dominated supracrustal rocks. There was an original unconformable relationship between the two rock associations. The latter occurs as a linear structural belt and was named the Khondalite Belt. The gneiss series in the Lushan area has an age of 2.85–2.72Ga, whereas the Xiaoqinling region contains widespread tonalitic–trondhjemitic–granodioritic gneisses with ages of 2.45–2.20Ga. Significant variations of Hf isotopes in zircons and whole-rock ɛNd(t) values suggest that these rocks were produced by variable mixing of a juvenile materials with older crust in an Andean-type continental margin arc or island arc setting. The depositional age of the sedimentary protoliths of the khondalite series can be roughly constrained in the period between 2300 and 1970Ma. It has been considered that they were deposited in a stable continental margin environment. The U–Pb dating of metamorphic zircons from the Taihua Complex suggest that the southern NCC underwent a polyphase tectonic evolution during the period 1.97–1.82Ga, and the peak metamorphism coeval with crustal thickening has occurred at ∼1.94Ga.

Petrogenesis of the Archean tonalite–trondhjemite–granodiorite (TTG) and granites in the Lushan area, southern margin of the North China Craton: Implications for crustal accretion and transformation

Tonalite–trondhjemite–granodiorite (TTG) assemblages and abundant high-K calc-alkaline magmatism are considered to be closely related to craton stabilization, and usually record a tectonic transition. Three episodes of granitic magmatism are identified in the Lushan area, at the southern margin of the North China Craton (NCC) on the basis of new LA-ICP-MS zircon U–Pb ages and whole-rock geochemical data. They are TTG gneisses, sodic–potassic granite (SPG) suite and potassic granite dated at ∼2840Ma, ∼2760Ma and ∼2570Ma, respectively. The TTG gneisses have high SiO2, variable Mg#, Cr and Ni contents with high (La/Yb)N and Sr/Y ratios, suggesting that the parental magmas were derived by partial melting of subducted oceanic crust. They show no obvious correlations between Sr/Y and Dy/Yb or (La/Yb)N and Dy/Yb, and all have negative Nb and Ta but positive Zr and Hf anomalies and variable Dy/Yb, Nb/Ta, Zr/Hf and Zr/Sm ratios, suggesting melting of a deep source (>45km) with rutile eclogite and garnet amphibolite in the residue. The TTG samples have positive ɛHf(t) values (+2.6 to +7.0) with the youngest TDMC (2896Ma) close to the crystallization age, indicating that they were produced by partial melting of juvenile crustal materials in an arc setting. Rocks of the SPG suite have high SiO2 contents, variable Na2O/K2O ratios (0.41–10.91), and low Y and Yb contents with high Sr/Y ratios, indicating an affinity to I-type granites. They display depleted and concave-upward REE patterns between middle and heavy REEs and higher Zr/Sm than TTG gneisses, possibly resulting from amphibole removal by fractional crystallization or as a residual phase during melting. Meanwhile, the rocks have a large variation of Dy/Yb without obvious correlations between Sr/Y and Dy/Yb, and (La/Yb)N and Dy/Yb, in combination with variable but uniformly low Y contents. Therefore, we suggest that the SPG samples were derived from partial melting with garnet in the residue. Magmatic zircons have ɛHf(t) values from −4.3 to +4.2 with TDMC ages from 3200Ma to 2877Ma, indicative of older mafic lower crustal sources. The rocks formed within a short time interval with the TTG, and their emplacement was accompanied by regional granulite and amphibolite metamorphism, implying that they may also have formed in an arc setting. The potassic granite has high SiO2 (74.79wt.%) and K2O+Na2O (9.22wt.%) contents with a low 10,000Ga/Al (1.89) ratio and TZr (670°C). Magmatic zircons have a narrow range of ɛHf(t) values (−2.0 to +4.1) with TDMC ages (2942–2692Ma) older than the formation age, but consistent with the formation age (ca. 2760Ma) of the Lushan SPG suite. The potassic granite has a Hf isotope composition identical to that of the SPG suite and is enriched in LREE and depleted in Y with a positive Eu anomaly, suggesting that the SPG suite is the main source. One zircon yielded an inherited age of 2837±36Ma consistent with the age of the TTG gneisses, indicating that the gneisses could also be the source of the potassic granite. The two types of source rocks indicate a shallow crust depth. Hence the heat required for partial melting of granodioritic rocks to form the potassic granite could have been provided by the upwelling of mantle or mantle-derived melts in an extensional setting. Therefore, the TTG gneisses, SPG suite and potassic granite in the Lushan area record a tectonic transition from a compressive to an extensional setting in the southern margin of the NCC between 2.85 and 2.50Ga.

Geochronological and geochemical constraints on the petrogenesis of the early Paleoproterozoic potassic granite in the Lushan area, southern margin of the North China Craton

We conducted a geochronological and geochemical study on the Paleoproterozoic potassic granites in the Lushan area, southern margin of the North China Craton (NCC) to understand the tectonic regime of the NCC at 2.2–2.1Ga. This rock suite formed at 2194±29Ma. The rocks are rich in SiO2 (76.10–77.73wt.%), and K2O (5.94–6.90wt.%) with high K2O+Na2O contents from 7.56wt.% to 8.48wt.%, but poor in CaO (0.10–0.28wt.%), P2O5 (0.02–0.05wt.%) and MgO (0.01–0.30wt.%, Mg#=1.08–27.3), indicating they experienced fractional crystallization. Major element compositions suggest the potassic granites share an affinity with high K calc-alkaline granite. Even though the Lushan potassic granitic rocks have high A/CNK ratios (1.11–1.25), which can reach peraluminous feature, the very low P2O5 contents and negative correlation of P2O5 and SiO2 ruling out they are S-type granites. Different from peralkaline A-type granites, the Lushan potassic granites have variable Zr concentrations (160–344ppm, 226ppm on average) and 10,000Ga/Al ratios (1.76–3.00), together with high zircon saturation temperatures (TZr=826–885°C), indicating they are fractionated aluminous A-type granites. Enriched LREE ((La/Yb)N=9.72–81.8), negative Eu anomalies, and low Sr/Y with no correlations in Sr/Y and Sr/Zr versus CaO suggest the possible presence of Ca-rich plagioclase and absence of garnet in the residual. Magmatic zircon grains have variable εHf(t) values (−2.4 to +7.3) with zircon two-stage Hf model ages (TDMC) varying from 2848Ma to 2306Ma (mostly around ca. 2.5Ga), and are plotted in the evolution line of crustal felsic rock. We propose that the rocks mainly formed by partial melting of ca. 2.50Ga tonalitic–granodioritic crust as a result of upwelling mantle-derived magmas which provided thermal flux and source materials in an intra-continent rifting. The ca. 2.2Ga magmatism suggests that intra-continental rifting occurred at 2.35–1.97Ga at least in the southern margin of the NCC after its final cratonization in the late Neoarchean.

Geochronology and geochemistry of volcanic rocks from the Shaojiwa Formation and Xingzi Group, Lushan area, SE China: Implications for Neoproterozoic back-arc basin in the Yangtze Block

We present a systematic SHRIMP zircon U–Pb geochronological, geochemical and Nd isotopic study on the Shaojiwa basaltic–rhyolitic suite and Xingzi amphibolites in Lushan area, Yangtze Block. The protoliths of the amphibolites are low potassium tholeiitic basalts. Rhyolite and amphibolite yield SHRIMP zircon U–Pb ages of 828±6Ma and 811±12Ma respectively. These ages suggest that the Xingzi Group formed in the Neoproterozoic rather than Paleoproterozoic as previously suggested. The basalts from the Shaojiwa Formation have slightly positive ɛNd(t) values (0 to +0.7) and exhibit significant enrichment in LILE and LREE and strong depletion in HFSE, similar to arc-like volcanics. These geochemical and isotopic signatures suggest that the basalts might have been derived from a slightly depleted mantle source metasomatized by fluid/melt released from a newly subducted slab with high degree of mantle melting. The rhyolites exhibit similar Nd isotopic ratios as the associated basalts and show subparallel REE patterns which can be explained by fractional crystallization of the basalt magma. In contrast, the amphibolites from the Xingzi Group have relatively high ɛNd(t) values (+3.4 to +4.0) and are characterized by nearly flat REE distributions with slight LREE depletion and generally similar contents of moderately incompatible elements to those of the average N-MORB, but with relative enrichment in LILE, similar to the Mariana back-arc basin basalt (BABB). The precursor magma of the amphibolites might have been originated from an N-MORB source modified by slab-derived fluids with relatively low degree of mantle melting. The geochemical affinity to arc (or intra-continental BABB) of the Shaojiwa basalts and intra-oceanic BABB of the Xingzi amphibolites appears to support the development of a Neoproterozoic incipient to mature back-arc basin in the southeastern Yangtze Block during the 828±6 to 811±12Ma period in response to the rollback of subducted slab. Widespread Neoproterozoic igneous rocks in the Jiangnan Orogen with ages older than 800Ma most likely occurred in an arc-back-arc environment

The Chinese national emergency medical rescue team response to the Sichuan Lushan earthquake

At 8:02 on 20 April 2013, an earthquake of magnitude 7.0 struck the Lushan area of Sichuan province, China. The affected population was 1.99 million, spread over 19 cities involving 115 countries. Over 458,000 people were resettled urgently (http://zh.wikipedia.org/wiki/ 2013Yaanearthquake). The number of those injured and killed is reported to be 13,000 and 196 by the time 12:00, 26 April (http://news.sina.com.cn/c/2013-04-27/090126968052.shtml). The decision to set out an emergency medical rescue team was taken at a national level the Ministry of Health, and the immediate response was funded by The General Hospital of the Chinese People’s Armed Police Forces. The request to the medical team came through 2 h after the earthquake, and the team arrived at Yaan People’s Hospital with medicine and equipment 14 h later. This paper describes the team’s experience during the immediate aftermath of the earthquake and the following 8 days.

Episodic crustal growth in the southern segment of the Trans-North China Orogen across the Archean-Proterozoic boundary

The Dengfeng and Taihua Complexes are well-exposed Neoarchean to Paleoproterozoic units in the southern segment of the Trans-North China Orogen (TNCO). Zircon U–Pb dating shows that the Dengfeng Complex records two episodes (2568±11Ma and 2306±6Ma) of tonalite–trondhjemite–granodiorite (TTG) magmatism. All of the TTG rocks are characterized by high SiO2 (66.7–75.4wt%), Na2O (3.20–5.06wt%) and relatively low MgO (0.40–1.78wt%). The Late Neoarchean TTG gneisses have very low contents of HREE (YbN=0.69–2.75) and Y (1.73–7.07ppm), with moderate [La/Yb]N (24.1–53.8) and high Sr/Y (65.1–291.3) ratios. The Early Paleoproterozoic TTG gneisses have low contents of HREE (YbN=2.93–6.37) and Y (6.7–11.0ppm), with moderate [La/Yb]N (10.1–27.0) and Sr/Y (10.6–52.1) ratios. Both suites show pronounced negative Nb–Ta, P and Ti anomalies but positive Sr and Pb anomalies. The Late Neoarchean TTG gneisses all have similar bulk-rock Nd and zircon Hf model ages with mainly positive ɛNd(t), and are interpreted as resulting from the melting of dominantly juvenile thickened lower crust with residual garnet and amphibole. The Early Paleoproterozoic TTG gneisses have extremely variable ɛNd(t) (−6.23 to +4.23) and heterogeneous zircon ɛHf(t) (−3.3 to +3.1), which are also best interpreted as resulting from the partial melting of thickened lower crust with residual amphibole and garnet. The Taihua Complex in the Xiaoqinling area records three episodes of Early Paleoproterozoic TTG magmatism (2.48Ga at Caotan, 2.31Ga at Houjiacun and 2.16Ga at Bayuan), younger than the Taihua Complex in the Lushan area (2.85–2.72Ga). All rocks have relatively low contents of HREE (YbN=1.03–8.32) and Y (2.84–24ppm), with moderate [La/Yb]N (8.7–88.4) and Sr/Y (19.8–125.8) ratios, and show negative Ta–Nb and Ti anomalies and positive Sr and Pb anomalies. The Caotan gneisses at 2.48Ga and the Houjiacun TTG gneisses at 2.31Ga have low Mg# (0.14–0.45), low Cr (<42ppm) and Ni contents (1–21ppm), with variable but overall positive ɛNd(t) and ɛHf(t) values, and were derived from the partial melting of thickened lower crust with residual garnet and amphibole. The younger Bayuan TTG gneisses at 2.16Ga have low SiO2 (57.11–64.89wt%), high MgO (2.64–4.62wt%), Cr (100–247ppm) and Ni (32–80ppm), with negative whole rock ɛNd(t) and zircon ɛHf(t) values, resulted from the partial melting of delaminated lower crust that interacted with peridotitic mantle.The geochronology of the Dengfeng Complex (in the Dengfeng area) and the Taihua Complex (in the Lushan, Xiong’er and Xiaoqinling areas) reveals at least four magmatic episodes in the southern segment of the TNCO from the Late Mesoarchean to Early Paleoproterozoic (2.85–2.72Ga, 2.57–2.48Ga, 2.34–2.30Ga and 2.20–2.07Ga). The rocks of the two early episodes are dominantly of juvenile compositions with mostly positive whole rock ɛNd(t) and zircon ɛHf(t) values, suggesting two episodes of crustal growth formed in a subduction tectonic setting. The magmatic rocks of the third episode consist of both the juvenile and pre-existing crustal materials with variable whole rock ɛNd(t) and zircon ɛHf(t) values, which were generated in a subduction zone during the initial assembly of the NCC within the Columbia supercontinent cycle. The final episode of magmatism lacks juvenile materials with whole rock ɛNd(t) and zircon ɛHf(t) values being consistently negative. These may have resulted from the orogenic collapse. The episodic continental growth recorded in the southern segment of the TNCO was caused by subduction and consequent orogeny, consistent with global supercontinent cycles within the Late Archean and Early Paleoproterozoic.