Early‐Cretaceous Syenites and Granites in the Northeastern Tengchong Block,SW China: Petrogenesis and Tectonic Implications

Whole-rock major and trace element and Sr-Nd isotopic data, together with zircon LA ICPMS in-situ U-Pb and Hf isotopic data of the syenites and granites in the Tengchong Block are reported in order to understand their petrogenesis and tectonic implications. Zircon U-Pb data gives the emplacement ages of ca. 115.3±0.9 Ma for syenites and 115.7±0.8 Ma for granites, respectively. The syenites are characterized by low SiO_2 content(62.01–63.03 wt%) and notably high Na_2O content(7.04–7.24 wt%) and Na_2O/K_2O ratios(2.02–2.10), low MgO, Fe_2O_3 T and TiO_2, enrichment of LILEs(large-ion lithophile element) such as Rb, Th, U, K, and Pb) and obvious depletion HFSE(high field strength element; e.g. Nb, Ta, P, and Ti) with clearly negative Eu anomalies(d Eu=0.53–0.56). They also display significant negative whole-rock εNd(t) values of-6.8 and zircon εHf(t) values(-9.11 to-0.27, but one is +5.30) and high initial ~(87) Sr/~(86) Sr=0.713013. Based on the data obtained in this study, we suggest that the ca. 115.3 Ma syenites were possibly derived from a sodium-rich continental crustal source, and the fractionation of some ferro-magnesian mineral and plagioclase might occur during the evolution of magma. The granites have high SiO_2 content(71.35–74.47 wt%), metaluminous to peraluminous, low Rb/Ba, Rb/Sr, and Al_2O_3/(MgO+FeOT+TiO_2) ratios and moderate(Al_2O_3+MgO+FeOT+TiO_2) content. They show low initial ~(87) Sr/~(86) Sr(0.703408 to 0.704241) and εNd(t) values(-3.8 to-3.5), plotted into the evolutionary trend between basalts and lower crust. Hence, we suggest that the granites were derived from the melting of mixing sources in the ancient continental crust involving some metabasaltic materials and predominated metasedimentary greywackes. Together with data in the literatures, we infer that the Early Cretaceous magmatism in the Tengchong block was dominated by magmas generated by the partial melting of ancient crustal material, which represent the products that associated to the closure of Bangong-Nujiang Meso-Tethys.     

Comparison of the Ordovician-Carboniferous Boundary Between Korea and NE China: Implications for Correlation and Tectonic Evolution

There is a great hiatus between Ordovician and Carboniferous strata in the Northeast China and Korean Peninsula. In order to understand geology and tectonic evolution, and to find out the similarities and differences in both regions, two sections in the Western Hill near Beijing in NE China and several sections in the Korean Peninsula were selected to examine their geologic boundaries between Lower and Upper Paleozoic strata to compare their characteristic features. At four sites in the two sections in the Western Hill near Beijing were examined their contact relations. The Hui Yu section is the same horizon where one site is top of a quarry hill and the other of down hill. Mid-Carboniferous Qingshuijian Formation rests on the Ordovician Majiagou Formation. Limestone beds are more commonly intercalated with shale and sandstone at site 2 of the Hui Yu section, while at site 1, conglomerate beds are dominant. Site 1 of the Se Shu Fen section shows eroded and concealed karst topography and conglomerate beds are intercalated within shale beds. Silurian and Devonian strata are absent in these areas. In the Korean Peninsula, most O-C contacts occur between Ordovician limestone formation and Carboniferous strata, although Silurian strata occur beneath the Carboniferous strata in the Jeongseon area and Pyeongnam Basin. Most contact relations are parallel unconformity and angular unconformity is rarely seen. The O-C relations in both regions are similar to each other, and these indicate that the Korean Peninsula was located near or belonged to the Sino-Korean paraplatform during Paleozoic time.     

Tectonic Evolution and Key Geological Issues of the Proto‐South China Sea

There are numerous controversies surrounding the tectonic properties and evolution of the Proto-South China Sea(PSCS).By combining data from previously published works with our geological and paleontological observations of the South China Sea(SCS),we propose that the PSCS should be analyzed within two separate contexts:its paleogeographic location and the history of its oceanic crust.With respect to its paleogeographic location,the tectonic properties of the PSCS vary widely from the Triassic to the mid-Late Cretaceous.In the Triassic,the Paleo-Tethys and the Paleo-Pacific Oceans were the major causes of tectonic changes in the SCS,while the PCSC may have been a remnant sea residing upon Tethys or Paleo-Pacific oceanic crust.In the Jurassic,the Meso-Tethys and the Paleo-Pacific oceans joined,creating a PSCS back-arc basin consisting of Meso-Tethys and/or Paleo-Pacific oceanic crust.From the Early Cretaceous to the midLate Cretaceous,the Paleo-Pacific Ocean was the main tectonic body affecting the SCS;the PSCS may have been a marginal sea or a back-arc basin with Paleo-Pacific oceanic crust.With respect to its oceanic crust,due to the subduction and retreat of the Paleo-Pacific plate in Southeast Asia at the end of the Late Cretaceous,the SCS probably produced new oceanic crust,which allowed the PSCS to formally emerge.At this time,the PSCS was most likely a combination of a new marginal sea and a remnant sea;its oceanic crust,which eventually subducted and became extinct,consisted of both new oceanic crust and remnant oceanic crust from the Paleo-Pacific Ocean.In the present day,the remnant PSCS oceanic crust is located in the southwestern Nansha Trough.     

北淮阳构造带的属性及其对板块缝合线位置的启示

北淮阳构造带为一条沿大别造山带北麓展布的狭长构造带,是华北板块与扬子板块碰撞造山的产物,其物源组成、构造属性等研究内容是揭示南北板块在中国东部的大地构造单元格局以及时空演化的关键信息。结合构造变形、岩石学、岩石地球化学、锆石U-Pb年代学等方法对北淮阳前中生代岩石地层单元苏家河群/卢镇关群和信阳群/佛子岭群进行对比分析,就其构造属性形成这样的基本认识:北淮阳构造带是杨子板块向华北板块俯冲-碰撞的结合带,其物质组成具有双源性-主体部分来源于杨子板块,由代表了杨子板块北缘基底的晚元古代苏家河群(784±19Ma)/卢镇关群(756±12Ma)和杨子板块被动陆缘复理石沉积建造-中泥盆世至晚泥盆世早期的信阳群南湾组组成,后者相当于商丹缝合带以南的刘岭群;信阳群龟山组亲缘于华北板块并构成华北板块南缘的弧前楔形沉积体,是沿商丹缝合带分布的桃花铺弧前楔形沉积体的东延。佛子岭群总体可与信阳群相连,全岩成分显示,佛子岭群沉积构造环境既有活动大陆边缘,又有被动大陆边缘。由于中生代以来多期构造变形叠加(至少三期),佛子岭群岩石地层单元的叠置关系被改造或重置,使其缺少"构造解体"的证据,而无法在构造单元上细分。对于两大板块之间的缝合带位置,笔者认为:商丹缝合带进入北淮阳构造带后,应沿着北淮阳的北界,与龟山-梅山-六安-磨墩水库一线延伸的断裂带相连。     

Petrogenesis of the Early Cretaceous Lianyang Granitic Complex, South China: Tectonic and Metallogenic Implications

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华南陆块岩石圈有效弹性厚度及其构造意义

华南陆块受多阶段超大陆聚合、裂解,碰撞、陆内造山,及伸展等作用影响,造成其深部结构和构造极其复杂。岩石圈有效弹性厚度(Te)是表征地质时间尺度上岩石圈力学强度的定量指标,可为深入认识岩石圈力学结构及演化提供有效约束。本文基于导纳和相关函数联合方法对地壳布格重力异常和地形数据进行计算,获得华南陆块Te的空间分布。Te高值(>20 km)区域主要分布于扬子地块的四川盆地周边区域,而Te低值(<20 km)区域集中于华夏地块和江南造山带区域。由于Te分布特征与地热场、地震关系密切。通过分析研究区Te与地热场(地表热流、居里面深度)、地震之间的关系,本文得到如下认识:(1)Te与地热场参数具有较好的相关性,但受浅部地壳被破坏,深部仍为克拉通地壳影响,导致龙门山断裂带和江南造山带区域的Te与地表热流或居里面深度之间的部分对应关系相反。(2)Te与地震关系复杂,Te较薄区域并不代表着地震频发区域,地震活动性与其所处的深部环境相关。龙门山断裂带强震频发的原因是受周边两块体中上地壳刚性地层长期相互作用,致使应力和能量积累较强;华夏地块区域地震较少是因为深部热物质上涌对华夏地块的壳幔进行强烈...     

Obduction-type granites within the NE Jiangxi Ophiolite: Implications for the final amalgamation between the Yangtze and Cathaysia Blocks

Leucogranitic lenses are found within the Xiwan ophiolitic mélange in northeastern Jiangxi Province, South China. The leucogranites occur exclusively within the serpentinized peridotite unit of the ophiolite suite. SHRIMP U–Pb zircon dating results indicate that these granites were formed at 880 ± 19 Ma, and were overprinted by an Indosinian tectono-thermal event at ~ 230 Ma. The leucogranites are peraluminous (A/CNK = 1.0–1.24), characterized by high Al₂O₃ (14–18.33%) and Na₂O (6.5–10%) and clearly low εNd(T) values of 0.8 to − 3.9 compared with the other rock units of the ophiolite suite. On the basis of their REE characters, the leucogranites can be divided into three groups. Group I leucogranites show the most fractionated LREE-enrichment patterns (with La_N/Yb_N and La_N/Sm_N ratios of 30.1–75.0 and 2.3–3.9, respectively). Group II leucogranites have moderately fractionated LREE-enrichment patterns (with La_N/Yb_N and La_N/Sm_N ratios of 13.1–26.5 and 0.8–1.9, respectively). Group III leucogranites are characterized by obviously low total REE contents and flat REE patterns with significant positive Eu anomalies, probably due to small degrees of partial melting. All these leucogranites were likely formed by partial melting of sedimentary rocks from a marginal basin at the Yangtze side of the orogen, beneath a major thrust fault during the obduction of the ophiolite onto the continental crust. They are broadly similar to obduction-related granites within ophiolites identified in many places worldwide. Identification of the ca. 880 Ma obduction-type granites in the NE Jiangxi ophiolite provides a petrological constraint on the timing of the ophiolite obduction onto the continental crust. In combination with the termination of the Shuangxiwu arc magmatism at ca. 890 Ma, we interpret that the close of the Neoproterozoic back-arc basin and the termination of the continental amalgamation between the Yangtze and Cathaysia Blocks occurred at ca. 880 Ma.     

Nature and Evolution of Pre-Neoproterozoic Continental Crust in South China: A Review and Tectonic Implications

South China as an amalgamation of the Yangtze and Cathaysia blocks is composed of Archean to Mesoproterozoic basement overlain by Neoproterozoic and younger cover. Both the constituent Yangtze and Cathaysia blocks contain well-preserved Neoproterozoic rocks that have been extensively studied in terms of the age and tectonic nature, but less is known about their earlier crustal history due to the incomplete rock record. Recent efforts in investigating the yet survived crustal nature based on isotopic and elemental signatures preserved in igneous and sedimentary rocks have steadily improved our knowledge about the pre-Neoproterozoic continental crustal evolution in South China. In this paper, we summarize the up-to-date pre-Neoproterozoic records, including petrological, geochronological, geochemical and geophysical data, across South China, and discuss its spatiotemporal patterns of the pre-Neoproterozoic crust and the relevant tectonic events. While the xenocrystic/inherited and detrital zircon records suggest widespread Archean (mainly ca. 2.5 Ga) crustal components within both the Yangtze and Cathaysia blocks, exposed Archean rocks are only limited to isolated crustal provinces in the Yangtze Block. These Archean rocks are dominated by TTGs (tonalite-trondhjemite-granodiorite) with varied ages (3.3–2.5 Ga) and zircon Hf isotopes, indicating a compositionally heterogeneous nature of the Archean Yangtze Block and, by inference, the development of multiple ancient terranes. The early Paleoproterozoic (2.4–2.2 Ga) tectonomagmatic events characterize the western Yangtze Block and are supportive of an east-west subdivision of the Yangtze basement, whereas the late Paleoproterozoic (2.1–1.7 Ga) orogeneses may have affected a larger area covering both the western and eastern parts of the Yangtze Block, and also the Cathaysia Block. The eastern Yangtze Block with generally northeastward-younging late Paleoproterozoic magmatism and metamorphism likely experienced a prolonged 2.05–1.75 Ga orogenic process welding the various Archean proto-continents, consistent with the documentation of a buried late Paleoproterozoic orogenic belt imaged by deep seismic profiling from its central part and of a slightly older ophiolitic mélange in the northern part. The Cathaysia Block was probably involved in a short-lived 1.9–1.8 Ga orogenic event. The two orogeneses overlapped in time and may have contributed to the cratonization of a possible unified South China, and are referred to be linked with the assembly of the Nuna Supercontinent. The subsequent late Paleoproterozoic to early Mesoproterozoic rift successions and intrusions (1.7–1.5 Ga) in the southwestern Yangtze Block, and the ca. 1.43 Ga rifting in Hainan Island of the Cathaysia Block could be responses to the Nuna break-up. Late Mesoproterozoic (1.2–1.0 Ga) magmatism of varied age and nature in different localities of the Yangtze Block is reflective of a complex tectonic process in the context of the assembly of the Rodinia Supercontinent. Similar-aged metamorphism (1.3–1.0 Ga) is recorded in Hainan Island, reflecting the Grenvillian continental collision during the Rodinia assembly, but further studies are necessary to better constrain the late Mesoproterozoic tectonic framework of South China.     

华南地块震旦纪晚期—早寒武世古大陆位置暨灯影组蒸发岩成钾条件分析

海相钾盐的形成需要构造、气候和成钾物源三者的耦合作用, 而构造是首要控制因素。华南地块发育震旦晚期—早寒武世灯影组海相蒸发岩, 本文通过古大陆位置、古纬度和古气候、古地理以及古海水成分等文献综合分析, 提出灯影组沉积时期, 华南地块位于印度大陆西北缘, 是冈瓦纳大陆的一部分。华南地块灯影组沉积与印度、巴基斯坦、阿曼、伊朗等地的同时期沉积相似, 并形成一个巨型的碳酸盐台地, 其中发育众多的局限盐盆地蒸发岩, 古气温最高可达40℃。同时, 通过与巴基斯坦盐岭地区含钾蒸发岩盐岭组的构造-沉积背景对比, 表明两者均为冈瓦纳大陆北缘被动大陆边缘盆地沉积, 具有相似的蒸发岩沉积序列和形成条件。因此, 综合来看, 认为华南地块灯影组具有必要的成钾条件。     

南海构造变形分区及成盆过程

为了系统认识新生代南海沉积盆地形成演化过程和成盆机制, 在对南海及其周缘区域构造和沉积研究进展调研的基础上, 利用覆盖南海主要盆地新近采集和重处理的地震剖面开展详细的构造-地层分析.基于盆地结构构造、演化特征和成盆动力学的差异性, 以红河-越东-Lupar线大型走滑构造带为界, 将南海及其周缘沉积盆地划分为古南海俯冲拖拽构造区沉积盆地群和挤出-逃逸构造区沉积盆地群, 前者主要是古南海俯冲及其所引起的区域构造变形形成的盆地, 包括北部湾、琼东南、珠江口、曾母、北康、文莱-沙巴和礼乐等盆地, 后者是印度-欧亚大陆碰撞导致印支地块挤出和旋转形成的盆地, 如莺歌海、湄公、中建南、万安等盆地.最后, 结合周缘板块动力学事件和本次对盆地构造研究的成果, 特别是盆地中重要界面属性的重新厘定, 建立了南海及其周缘沉积盆地演化过程.      

Detrital Zircon U‐Pb Dating of Nanshuangyashan Formation in the Jiamusi Massif,NE China and its Tectonic Implications

A suite of the fossil-rich marine-land interbedded strata(Nanshuangyashan Formation) is distributed at the eastern margin of the Jiamusi massif in the eastern Heilongjiang Province, NE China. The authors had recently discovered a suite of arkose beneath the marine-land interbedded strata, which overlays unconformably on the Permain granite in the eastern margin of the Jiamusi massif. The LA-ICP-MS zircon U-Pb dating indicate that all detrital zircons from the analysed four arkose samples show the four population ages of 800 Ma, 538–481 Ma, 269–250 Ma and 223–215 Ma. The former three population ages are widely recorded in the Jiamusi-Khanka massif and the Songnen massif. The later group is the minimal age population in the analyzed samples, limiting the sedimentation time of the arkoses occurred after the Late Triassic. At present, the minimal age population is not recorded in the Jiamusi massif, but the granites with the ages of 228–210 Ma are widely distributed in the Songnen-Zhangguangcai Range massif and the Khanka massif. The predominantly Permian zircons are characterized by oscillatory zoning and euhedral shapes, with variable zircon ε_(Hf)(t) values(-5.5 to +11.2), indicating that they were derived from mixture sources, possibly mixed with components of the Songnen-Zhangguangcai Range massif and the Jiamusi-Khanka massif. These results, combined with regional analyses, indicate that the closing of Mudanjiang ocean and Panthalassa ocean possibly existed from Early Permian to Late Triassic.     

Two‐Stage Sulfide Mineral Assemblages in the Mineralized Ultramafic Rocks of the Laowangzhai Gold Deposit (Yunnan,SW China): Implications for Metallogenic Evolution

The Laowangzhai gold deposit, located in the Ailaoshan gold belt (SW China), is hosted in various types of rocks, including in quartz porphyry, carbonaceous slate, meta‐sandstone, lamprophyre, and altered ultramafic rocks. In contrast to other wall rocks, the orebodies in altered ultramafic rocks are characterized by the occurrence of a large amount of Ni‐bearing minerals. The ore‐forming process of the orebodies hosted by altered ultramafic rocks can be divided into two stages: pyrite‐vaesite‐native gold and gersdorffite‐violarite stages. The contents of As and Sb increased during the evolution of ore‐forming fluid based on the mineral assemblages. Thermodynamic modeling of the Ni‐Cu‐As‐Fe‐S system using the SUPCRT92 software package with the updated database of slop16.dat indicates the fS₂ in ore‐forming fluid decreases significantly from stage I to stage II. The decreases of fS₂ due to crystallization of sulfides and fO₂ due to fluid–rock reaction were responsible for ore formation in altered ultramafic rocks of the Laowangzhai gold deposit. Geological evidence, the in situ sulfur isotope values of pyrite, and the other published isotopic data suggest that the ore‐forming fluid for ultramafic rock ores was dominantly composed of evolved magmatic fluid with the important input of sediments.     

High-Pressure Granulites (Retrograded Eclogites) from the Hengshan Complex, North China Craton: Petrology and Tectonic Implications

Both high- and medium-pressure granulites have been found asenclaves and boudins in tonalitic–trondhjemitic–granodioriticgneisses in the Hengshan Complex. Petrological evidence fromthese rocks indicates four distinct metamorphic assemblages.The early prograde assemblage (M₁) is preserved only in thehigh-pressure granulites and represented by quartz and rutileinclusions within the cores of garnet porphyroblasts, and omphacitepseudomorphs that are indicated by clinopyroxene + sodic plagioclasesymplectic intergrowths. The peak assemblage (M₂) consists ofclinopyroxene + garnet + sodic plagioclase + quartz ±hornblende in the high-pressure granulites and orthopyroxene+ clinopyroxene + garnet + plagioclase + quartz in the medium-pressuregranulites. Peak metamorphism was followed by near-isothermaldecompression (M₃), which resulted in the development of orthopyroxene+ clinopyroxene + plagioclase symplectites and coronas surroundingembayed garnet grains, and decompression-cooling (M₄), representedby hornblende + plagioclase symplectites on garnet. The THERMOCALCprogram yielded peak (M₂) P–T conditions of 13·4–15·5kbar and 770–840°C for the high-pressure granulitesand 9–11 kbar and 820–870°C for the medium-pressuregranulites, based on the core compositions of garnet, matrixpyroxene and plagioclase. The P–T conditions of pyroxene+ plagioclase symplectite and corona (M₃) were estimated at     

Petrogenesis of Early Carboniferous Alkaline Basalt from the Wusun Mountain: Implications for Tectonic Evolution of the Western Yining Block,NW China

Late Paleozoic volcanic rocks are well exposed in the Yining Block, NW China, and are predominately composed of andesites, rhyolites and volcaniclastics as well as minor basalts. Study of the petrology, whole-rock geochemistry and zircon U-Pb dating for the Early Carboniferous alkaline basalts from Wusun Mountain, western Yining Block, constrains their petrogenesis and tectonic evolution. The alkaline basalts consist mainly of plagioclases, mostly albite and labradorite, as well as clinopyroxenes and olivines; zircon U-Pb dating indicates their formation at ca. 350 Ma. Geochemically, the basaltic samples have low SiO2 contents, and high TiO2, Al2O3 and alkaline contents, coupled with high Na2O/K2O ratios, displaying an alkaline basalt affinity. They show remarkable LILE enrichment and HFSE depletion. Meantime, these samples have relatively high TFe2O3, MgO, and Mg# values as well as Ni and Cr, relatively high Sm/Yb and U/Th, suggesting origination from a mantle source metasomatized by slab fluids. They formed in a transitional tectonic setting from arc to intraplate, showing a typical affinity of back-arc basin basalts. The alkaline basalts were likely generated in a nascent back-arc extension setting resulting from slab rollback of the southern Tianshan oceanic lithosphere. A bi-directional subduction model seems more reasonable for the evolution of the southern Tianshan Ocean. These new data will provide a new tectonic model for Late Paleozoic tectonic evolution of the western Yining Block.     

Sedimentary Facies,Provenance and Geochronology of the Heshangzhen Group: Implications for the Tectonic Evolution of the Eastern Jiangnan Orogen,South China

The Neoproterozoic Jiangnan orogen plays an important role in the study of the Precambrian tectonic evolution of South China. The tectonic nature of the Neoproterozoic sedimentary basins is still controversial, due to poor understanding of the sedimentary sequences and the lack of geochronological data. Here, we present sedimentological, provenance and geochronological data from the Heshangzhen Group in the eastern Jiangnan orogen. Sedimentological analysis shows that the Luojiamen Formation was deposited in a submarine fan, and the overlying Hongchicun Formation was deposited in front of a fan delta. The youngest detrital zircons constrain the lower Luojiamen and Hongchicun formations with ages of 827.3 ± 8.4 Ma and 825 ± 12 Ma, respectively. The sandstones of the Luojiamen Formation are characterized by a large number of intermediate to felsic volcanic grains, suggesting a volcanic arc source. In contrast, quartz and sedimentary lithic grains increase in the Hongchicun Formation, showing a new input from a collisional orogenic source. Detrital zircon from six sandstone samples in the Luojiamen and Hongchicun formations yield similar age spectra of 930–820 Ma with a peak at ca. 845–860 Ma, with one main cluster at 930–820 Ma. Detrital zircons of 930–845 Ma show a positive value of εHf(t)(+2.4 to +11, mean +7.6), which is similar to the volcanic arc of the nearby Shuangxiwu Group. There are a minor group of zircons with U-Pb ages ranging from 820 Ma to 845 Ma from the middle part of the Luojiamen Formation and Hongchicun Formation, with εHf(t) values between-20 to +2.4, which are consistent with the characteristics of the Shuangqiaoshan Group. within light of the bidirectional paleocurrents in the Luojiamen Formation, it is speculated that the zircons of 820–845 Ma were recycled from the Shuangqiaoshan Group, which is derived from a continental arc to the northwest. Our data suggests that the Luojiamen Formation was formed in an inter-arc basin, while the Hongchicun Formation was formed in an accretionary wedge-top basin. When juxtaposed with the conglomeratic characteristics at the bottom of the Luojiamen Formation, it is believed that the unconformity represented by the ‘Shen Gong Movement' reflects the rapid erosion and accumulation process of island arc volcanic material. The disconformity between the Luojiamen and Hongchicun formations is the imprint of transition from inter-arc basin to accretionary wedge-top basin,which represents the collision between the Shuangxiwu arc and the Yangtze Plate.