鲁西晚中生代基性脉岩的成因和源区性质：岩石学和地球化学

本文从岩石学和地球化学方面对鲁西地区淄博盆地几个岩区的基性脉岩的成因和源区性质进行了探讨。脉岩的K-Ar年龄(72．2～116．3 Ma)表明其为晚中生代(白垩纪)岩浆作用的产物。主量元素显示该脉岩总体属钙碱性系列。微量元素特征表明脉岩为交代富集地幔部分熔融作用的产物，成岩过程可能同时经历了橄榄石、单斜辉石、Ti-Fe氧化物以及少量斜长石的分离结晶作用。Pb同位素组成：^208Pb／^204Pb=36．308～38．329；^207Pb／^204Pb=15．170～15．632；^206Pb／^204Pb=16．658～18．470，可以和下地壳组成相比，暗示成岩过程中存在大量下地壳物质的参与。岩浆在构造上受控于燕山造山带坍塌和沂沭断裂带(郯庐断裂山东段)的活动(左行平移和伸展)，但在上升侵位过程中没有遭受地壳物质的混染，且具有大陆边缘弧玄武岩的特性。这暗示岩浆早期鲁西地区存在古大洋板块(苏-鲁洋)的俯冲作用(即古俯冲作用)。     

Dating Magmatic Hornblende and Biotite and Hydrothermal Sericite by Laser Probe Technique：Constraints on Genesis of Wangershan Gold Deposit, Eastern Shandong Province, China

INTRODUCTIONEasternShandongProvincerepresentsthelargestgoldconcentrationregioninChina ,withannualproduc tion >5 5 - 6 0tgoldandpresentreserveofmorethan90 0tgold ,accountingforaquarterofthetotalgoldproductioninthecountry .Amajorityofthegoldde positsishostedintheJurassic Cretaceousgranitoidin trusions .TimingofgoldmineralizationineasternShan donghasbeenamajorfocusofmanypreviousstudies .K Ar,Rb SrandconventionalU Pbisotopicdatinginthelasttwodecadeshaveyieldedawiderangeofminer alizationag…     

Geochemical and isotopic constraints on the age and origin of mafic dikes from eastern Shandong Province,eastern North China Craton

Post-orogenic mafic dikes are widespread across eastern Shandong Province, North China Craton, eastern China. We here report new U–Pb zircon ages and bulk-rock geochemical and Sr–Nd–Pb isotopic data for representative samples of these rocks. LA-ICP-MS U–Pb zircon analysis of two mafic dike samples yields consistent ages of 118.7 ± 0.25 million years and 122.4 ± 0.21 million years. These Mesozoic mafic dikes are characterized by high (⁸⁷Sr/⁸⁶Sr) _i ranging from 0.7082 to 0.7087, low ?_Nd(t) values from??17.0 to??17.5, ²⁰⁶Pb/²⁰⁴Pb from 17.14 to 17.18, ²⁰⁷Pb/²⁰⁴Pb from 15.44 to 15.55, and ²⁰⁸Pb/²⁰⁴Pb from 37.47 to 38.20. Our results suggest that the parental magmas of these dikes were derived from an ancient, enriched lithospheric mantle source that was metasomatized by foundered lower crustal eclogitic materials prior to magma generation. The mafic dikes underwent minor fractionation during ascent and negligible crustal contamination. Combined with previous studies, these findings provide additional evidence that intense lithospheric thinning beneath eastern Shandong occurred at ～120 Ma, and that this condition was caused by the removal/foundering of the lithospheric mantle and lower crust.     

Arc plutonism in a transtensional regime: the late Palaeozoic Totoltepec pluton,Acatlán Complex,southern Mexico

The ENE-trending, ca. 306–287 Ma, Totoltepec pluton is part of a Carboniferous–Permian continental magmatic arc on the western Pangaean margin. The 15 km?×?5 km pluton is bounded by two N–S Permian dextral faults, an E–W thrust to the south, and an E–W normal fault to the north. Thermobarometric data indicate that the main, ca. 289–287 Ma, part of the pluton was emplaced at ≤20 km depth and ≥700°C and was exhumed to 11 km and 400°C in 4 ± 2 million years. We have documented the following intrusive sequence: (1) the 306 Ma northern marginal mafic phase; (2) the 287 Ma main trondhjemitic phase; and (3) ca. 289–283 Ma sub-vertical dikes that vary from (a) N39E, undeformed with crystal growth perpendicular to the margins, through (b) ca. N50–73E, foliated and folded with sinistral shear indicators, to (c) N73–140E and boudinaged. The obliquity of the boundary between the folded and stretched dikes relative to the N–S dextral faults suggests sequential emplacement in a transtensional regime (with 20% E–W extension), followed by different degrees of clockwise rotation passing through a shortening field accompanied by sinistral shear into an extensional field. The ca. 289–287 Ma intrusion also contains a steep ENE-striking foliation and hornblende lineations varying from sub-horizontal to steeply plunging, probably the result of emplacement in a triclinic strain regime. We infer that magmatism ceased when some of the dextral motion was transferred from the western to the eastern bounding fault, causing thrusting to take place along the southern boundary of the pluton. This mechanism is also invoked for the rapid uplift and exhumation of the pluton between ca. 287 Ma and 283 Ma. The distinctive characteristics of the Totoltepec pluton should prove useful in identifying similar tectonic settings within continental arcs.     

U-Pb Zircon Age,Geochemical, and Sr-Nd-Pb Isotopic Constraints on the Age and Origin of Mafic Dykes from Eastern Shandong Province,Eastern China

U-Pb zircon age, geochemical, and Sr-Nd-Pb isotopic data of mafic dykes from eastern Shandong Province, eastern China is reported herein. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb zircon analyses of two samples from the investigated mafic dykes yield consistent ages ranging from 121.9 Ma ± 0.47 Ma to 122.9 Ma ± 0.61 Ma. The mafic dykes are characterized by high (87Sr/86Sr) i ranging from 0.7087 to 0.7089, low εNd(t) values ranging from -16.9 to -17.8, 206Pb/204Pb = 17.15 to 17.17, 207Pb/204Pb = 15.45 to 15.47, and 208Pb/204Pb = 37.59 to 37.68. Results from the current study suggest that the mafic dykes are derived from partial melting of ancient lithospheric mantle that was variably hybridized by melts derived from foundered lower crustal eclogite. The mafic dykes may have been generated through subsequent insignificant crystal fractionation and very minor crustal contamination during magma ascent. Combined with previous studies, the current findings provide new evidence that the intense lithospheric thinning beneath the eastern Shandong Province of eastern China occurred at ~120 Ma, and that this condition was caused by the removal of the lower lithosphere (mantle and lower crust).     

Metallogenesis related to Mesozoic Granitoids in the Nanling Range, South China and Their Geodynamic Settings

Affected by the compressive stress from the South-Central （Indo-China） Peninsula, the Indosinian orogenesis, characterized by collision, thrust and uplifting, took place inside the South China Plate during 250-230 Ma. The ages of the Indosinian granitoids in the Nanling Range and vicinity areas are mostly 240-205 Ma, indicating that they were emplaced in both late collision and post-collision geodynamic environments. No important granite-related metallogenesis occurred in this duration. A post-orogenic setting started at the beginning of the Yanshanian Period, which controlled large-scale granitic magmatism and related metallogenesis. This paper makes the first attempt to divide the Yanshanian Period into three sub-periods, i.e. the early, middle and late Yanshanian Periods, based mainly on the features of magmatism, especially granitoids and related metallogenesis and their geodynamic environments. The magmatic association of the Early Yanshanian （about 185-170 Ma） comprises four categories of magmatism, i.e. basalt, bimodal volcanics, A-type granite and intraplate high-K calc-alkaline （HKCA） magmatism, which indicates an extension-thinning of lithosphere and upwelling of mantle material to a relative small and local extent. Pb-Zn, Cu and Au mineralizations associated with HKCA magmatism represents the first high tide of Mesozoic metallogenesis in the Nanling Range area. During the middle Yanshanian, the lithosphere was subjected to more extensive and intensive extending and thinning, and hence mantle upwelling and basaltic magma underplating caused a great amount of crust remelting granitoids. This period can be further divided into two stages. The first stage （170-150 Ma） is represented by large-scale emplacement of crust remelting granites with local tungsten mineralization at its end. The second stage （150-140 Ma） is the most important time of large-scale mineralizations of non-ferrous and rare metals, e.g. W, Sn, Nb-Ta, Bi, Mo, Be, in the Nanling Range area. The late Yanshanian （140-65 Ma） was generally characterized by full extension and breakup of the lithosphere of South China. However, owing to the influence of the Pacific Plate movement, the eastern part of South China was predominated by subduction-related compression, which resulted in magmatism of calc-alkaline and shoshonite series and related metallogeneses of Au, Ag, Pb-Zn, Cu and （Mo, Sn）, followed by extension in its late stage. In the Nanling Range area, the late Yanshanian magmatism was represented by granitic volcanic-intrusive complexes and mafic dikes, which are genetically related to volcanic-type uranium and porphyry tin deposits, and the mobilization-mineralization of uranium from pre-existing Indosinian granites.     

Mesozoic Bimodal Volcanic Suite in Zhalantun of the Da Hinggan Range and Its Geological Significance： Zircon U-Pb Age and Hf Isotopic Constraints

1 Introduction Mesozoic volcano-intrusive rocks are widely distributed in the Da Hinggan Range of northeastern China, and are considered as one of the most spectacular geological sights in eastern Asia. Recently, studies on granites with high εNd(t) values and Phanerozoic crustal growth in the Centra Asian Orogenic Belt have greatly promoted fundamental research into the geology of this area (Jahn et al., 2000, 2001, 2004; Wu et al., 2000, 2002, 2003). However, work on the eruption time,…     

Late Mesozoic magmatism in the Jiaodong Peninsula,East China:Implications for crust-mantle interactions and lithospheric thinning of the eastern North China Craton

A section from the Linglong gold deposit on the northwestern Jiaodong Peninsula,East China,containing Late Mesozoic magmatic rocks from mafic and intermediate dikes and felsic intrusions,was chosen to investigate the lithospheric evolution of the eastern North China Craton(NCC).Zircon U-Pb data showed that low-Mg adakitic monzogranites and granodiorite intrusions were emplaced during the Late Jurassic(～145 Ma) and late Early Cretaceous(112-107 Ma),respectively;high-Mg adakitic diorite and mafic dikes were also emplaced during the Early Cretaceous at～139 Ma and ～118 Ma,and 125-145 Ma and 115-120 Ma,respectively.The geochemical data,including whole-rock major and trace element compositions and Sr-Nd-Pb isotopes,imply that the mafic dikes originated from the partial melting of a lithospheric mantle metasomatised through hydrous fluids from a subducted oceanic slab.Low-Mg adakitic monzogranites and granodiorite intrusions originated from the partial melting of the thickened lower crust of the NCC,while high-Mg adakitic diorite dikes originated from the mixing of mafic and felsic melts.Late Mesozoic magmatism showed that lithosphere-derived melts showed a similar source depth and that crust-derived felsic melts originated from the continuously thickened lower crust of the Jiaodong Peninsula from the Late Jurassic to Early Cretaceous.We infer that the lower crust of the eastern NCC was thickened through compression and subduction of the Palaeo-Pacific plate beneath the NCC during the Middle Jurassic.Slab rollback of the plate from ～160 Ma resulted in lithospheric thinning and accompanied Late Mesozoic magmatism.     

鲁西早白垩世基性侵入岩的锆石Hf同位素组成变化及其成因

鲁西北部地区的淄博和临朐辉长闪长岩的锆石原位U-Pb定年分析显示,它们分别形成于128±2Ma和132±1Ma,与鲁西早白垩世大规模岩浆活动时间一致,是华北克拉通岩石圈减薄过程的岩浆活动产物。与鲁西北部同时期的济南、邹平辉长岩相比,淄博和临朐辉长闪长岩的锆石εHf(t)值(分别为-5.1～-2.4和-2.4～+3.2)明显较高,并具有相对年轻的亏损地幔Hf模式年龄TDM(分别为876～984Ma和639～867Ma),对应于相对高的全岩εNd(t)值,指示其岩浆源区有更大比例的软流圈地幔物质贡献。鲁西北部地区的早白垩世基性侵入岩的岩浆源区以华北克拉通内部的古老富集岩石圈地幔为主,从华北克拉通内部往郯庐断裂带锆石εHf(t)值显示出升高趋势,表明软流圈地幔物质的参与比例增大。郯庐断裂带是导致鲁西地区早白垩世基性岩浆活动时空不均一性的重要岩石圈薄弱带。     

Isotopic evidence of middle proterozoic magmatism from bombay high field: Implications to crustal evolution of western offshore of India

Precambrian granitic basement rocks obtained from well BH-36 of Bombay High Field, western offshore of India has been studied both by Rb-Sr and K-Ar dating methods. Seven basement samples chosen from two cores have yielded whole rock Rb-Sr isochron age of 1446 ± 67 Ma with an initial⁸⁷Sr/⁸⁶Sr ratio of 0.7062 ± 0.0012. This age has been interpreted as the formation/emplacement time of the granite. Two biotite fractions of different grain size separated from a sample CC6B2T have yielded Rb-Sr mineral isochron age of 1385 ± 21 Ma. However, these fractions when studied by K-Ar dating method have yielded slightly higher but mutually consistent ages of 1458 ± 43 Ma and 1465 ± 43 Ma, respectively. Further, two biotites separated from additional samples CC5B9T and CC6B3B have yielded K-Ar ages of 1452 ± 42 Ma and 1425 ± 40 Ma with an overall mean age of 1438 ± 19 Ma. This mean K-Ar age is indistinguishable from whole rock Rb-Sr isochron as well as mineral isochron age within experimental error. The similarity in the whole rock and biotite ages obtained by different isotopic methods suggests that no thermal disturbance has occurred in these rocks after their emplacement/formation around 1450 Ma ago. The present study provides the evidence for the existence of an important Middle Proterozoic magmatic event around 1400-1450 Ma on the western offshore of India which, hitherto, was thought to be mainly confined to the eastern Ghats, Satpura and Delhi fold belt of India. This finding may have an important bearing on the reconstruction of Proterozoic crustal evolution of western Indian shield.     

藏北阿索地区早白垩世基性岩脉地球化学及年代学特征:对班公湖－怒江洋闭合时限的约束

班公湖－怒江洋闭合时限是中特提斯洋演化争论的焦点之一,而基性岩脉对于识别构造环境转变具有重要意义.本文对阿索地区侵入构造混杂岩带中的基性岩脉进行了1件LA-ICP-MS锆石U-Pb年龄测定和10件全岩地球化学元素分析.辉长岩脉获得了102.90±0.86 Ma（MSWD=1.4）的谐和年龄,全岩地球化学分析显示基性岩脉属于亚碱性系列,钙碱性辉长闪长岩类,样品轻微富集轻稀土元素,富集大离子亲石元素Th、U、Pb等,亏损高场强元素Nb、Ta等.基性岩脉来源于被俯冲沉积物和板片流体改造过的亏损地幔源区,由尖晶石二辉橄榄岩高程度部分熔融形成,未经历明显的地壳混染过程.结合构造环境判别图解和前人研究结果,认为阿索地区的基性岩脉可能形成于早白垩世晚期伸展环境之中.系统收集研究区及附近120~95 Ma之间的岩浆岩数据,发现在110~100 Ma之间班公湖－怒江缝合带及两侧处于伸展环境之中,但不同区域伸展事件的时代有所不同,原因可能与班公湖－怒江洋的不均一闭合相关.班公湖－怒江洋最后的闭合时限可能在103 Ma左右.      

Geochronologic Constraints on the Magmatic Underplating of the Gangdise^ Belt in the India-Eurasia Collision： Evidence of SHRIMP II Zircon U-Pb Dating

Abstract  Abundant small mafic intrusions occur associated with granitoids along the Gangdisê magmatic belt. In addition to many discrete gabbro bodies within the granitoid plutons, a gabbro‐pyroxenite zone occurs along the southern margin of the Gangdisê belt to the north of the Yarlung Zangbo suture. The mafic intrusion zone spatially corresponds to a strong aeromagnetic anomaly, which extends ～1400 km. The mafic intrusions consist of intermittently distributed small bodies and dikes of gabbro and dolerite with accumulates of pyroxenite, olivine pyroxenite, pegmatitic pyroxenite and amphibolite. Much evidence indicates that the Gangdisê gabbro‐pyroxenite assemblage is most likely a result of underplating of mantle‐derived magma. Detailed field investigation and systematic sampling of the mafic rocks was conducted at six locations along the Lhasa‐Xigazê segment of the mafic intrusive zone, and was followed by zircon SHRIMP II U‐Pb dating. In addition to the ages of two samples previously published (47.0±1 Ma and 48.9±1.1 Ma), the isotopic ages of the remaining four gabbro samples are 51.6±1.3 Ma, 52.5±3.0 Ma, 50.2±4.2 Ma and 49.9±1.1 Ma. The range of these ages (47–52.5 Ma) provide geochronologic constraints on the Eocene timing of magma underplating beneath the Gangdisê belt at ca. 50 Ma. This underplating event post‐dated the initiation of the India‐Eurasia continental collision by 15 million years and was contemporaneous with a process of magma mixing. The SHRIMP II U‐Pb isotopic analysis also found several old ages from a few zircon grains, mostly in a range of 479–526 Ma (weighted average age 503±10 Ma), thus yielding information about the pre‐existing lower crust when underplating of mafic magma took place. It is believed that magma underplating was one of the major mechanisms for crustal growth during the Indian‐Eurasia collision, possibly corresponding in time to the formation of the 14–16 km‐thick “crust‐mantle transitional zone” characterized by Vp = 6.85–6.9 km/s.     

Re–Os Geochronology of Molybdenite from Yinyan Porphyry Sn Deposit in South China

The Yinyan Sn deposit, one of the three typical porphyry Sn deposits in China, is located in the western Guangdong province of the Cathaysia Block. Rhenium and osmium isotopes of molybdenites from the Yinyan deposit were first used to constrain the age of mineralization. Rhenium concentrations in molybdenite samples range from 0.13 to 1.3 µg g^?1, indicating a crustal source for the ore‐forming materials. The Re–Os dating yield model ages ranging from 78.1 to 79.52 Ma, with an average of 78.65 ± 0.98 Ma, and give an isochron age of 78.8 ± 2.6 Ma. Evidently, isochron age is consistent with model ages in the error within the allowable range, so we can constrain the precise age of Yinyan Sn deposit at the Late Cretaceous. Based on the geological history and spatial‐temporal distribution of the Sn deposits, it is proposed that the formation of Sn deposits in the Cathaysia Block were related to lithospheric extension that are associated with a change in the polarity of the subduction of the Paleo‐Pacific Plate from oblique subduction to parallel the eastern margin of the Eurasian Plate after 135 Ma.     

闽东南基性岩脉形成的构造应力场地质意义

闽东南沿海地处长乐-南澳构造带内,发育晚中生代以来的基性岩脉(岩群),其几何形态和产状记录着区域应力场活动。通过对漳浦赤湖和东山海边基性岩脉的几何形态和产状的分析,推测长乐-南澳带在白垩纪为左旋韧性剪切带,古新世转变成左行脆性剪切带。长乐-南澳带的左旋走滑活动,是由古太平洋板块朝NW斜向俯冲引起的。晚白垩世以来,闽东南应力场性质为剪切与拉张相互变换,指示地壳拉张不是连续的,而是分期次;晚白垩世以来大洋板块属于低应力型俯冲,拉张是东南部主构造。     

Geochemistry of middle-late Mesozoic mafic intrusions in the eastern North China Craton: New insights on lithospheric thinning and decratonization

We present detailed geochronological, geochemical and Sr-Nd-Pb isotopic data for late Mesozoic mafic intrusions in the Taili region (western Liaodong Province) of the eastern North China Craton (NCC). We obtained laser-ablation inductively-coupled plasma mass spectrometry U-Pb zircon ages from lamprophyres with ages ranging from 139 to 162 Ma and diorites with clusters of ages at 226 ± 11 Ma, 165 ± 5.8 Ma and 140 ± 4.8 Ma. We interpret the Triassic zircons in diorites to be inherited from the Paleo-Asian Ocean slab. Both the lamprophyres and diorites contain abundant inherited grains (2644–2456 Ma) that were likely derived from the ancient NCC basement, reflecting a contribution from old lower crustal material. Like contemporaneous late Mesozoic mafic rocks in the Jiaodong and Liaodong Peninsula areas of the NCC, the Taili lamprophyres reveal a strong subduction signature in their normalized trace element patterns, including depletion of high field strength elements and enrichment of large ion lithophile elements. The rare-earth element patterns of the Taili intermediate-mafic intrusions are best explained if they were principally derived from partial melting of amphibole-bearing lherzolite in the spinel-garnet transition zone. Slab-derived melts likely contributed to the formation of late Mesozoic mafic rocks along three margins of the craton: due to accretion of the Yangtze Block along the southern margin of the craton, subduction of the Paleo-Asian Ocean along the northern margin, and subduction of the Paleo-Pacific oceanic plate along the eastern margin of NCC. We present a synthesis of the geochemical, spatial, and temporal patterns of magmatic rocks and periods of deformation that contributed to decratonization of the NCC in response to the Mesozoic tectonic evolution of adjacent plates along its northern, southern, and eastern margins.     

Zircon U–Pb,molybdenite Re–Os and muscovite Ar–Ar isotopic dating of the Xitian W–Sn polymetallic deposit,eastern Hunan Province,South China and its geological significance

The Xitian tungsten–tin (W–Sn) polymetallic deposit, located in eastern Hunan Province, South China, is a recently explored region containing one of the largest W–Sn deposits in the Nanling W–Sn metallogenic province. The mineral zones in this deposit comprise skarn, greisen, structurally altered rock and quartz-vein types. The deposit is mainly hosted by Devonian dolomitic limestone at the contact with the Xitian granite complex. The Xitian granite complex consists of Indosinian (Late Triassic, 230–215 Ma) and Yanshanian (Late Jurassic–Early Cretaceous, 165–141 Ma) granites. Zircons from two samples of the Xitian granite dated using laser ablation-inductively coupled mass spectrometer (LA-ICPMS) U–Pb analysis yielded two ages of 225.6 ± 1.3 Ma and 151.8 ± 1.4 Ma, representing the emplacement ages of two episodic intrusions of the Xitian granite complex. Molybdenites separated from ore-bearing quartz-veins yielded a Re–Os isochron age of 149.7 ± 0.9 Ma, in excellent agreement with a weighted mean age of 150.3 ± 0.5 Ma. Two samples of muscovites from ore-bearing greisens yielded ⁴⁰Ar/³⁹Ar plateau ages of 149.5 ± 1.5 Ma and 149.4 ± 1.5 Ma, respectively. These isotopic ages obtained from hydrothermal minerals are slightly younger than the zircon U–Pb age of 151.8 ± 1.4 Ma of the Yanshanian granite in the Xitian area, indicating that the W–Sn mineralization is genetically related to the Late Jurassic magmatism. The Xitian deposit is a good example of the Early Yanshanian regional W–Sn ore-forming event (160–150 Ma) in the Nanling region. The relatively high Re contents (8.7 to 44.0 ppm, average of 30.5 ppm) in molybdenites suggest a mixture of mantle and crustal sources in the genesis of the ore-forming fluids and melts. Based upon previous geochemical studies of Early Yanshanian granite and regional geology, we argue that the Xitian W–Sn polymetallic deposit can be attributed to back-arc lithosphere extension in the region, which was probably triggered by the break-off of the flat-slab of the Palae-Pacific plate beneath the lithosphere.     

Structure and geochronology of the Tongbai complex and their implications for the evolution of the Tongbai orogenic belt,central China

The Tongbai orogenic belt has an overall antiformal geometry and the hinge of the antiform is sub-horizontal and trends NW–SE. The Tongbai complex (TBC) in the core of the antiform is bounded by the S-dipping Yindian–Malong shear zone in the south, the sub-horizontal Taibaiding shear zone at the top and the N-dipping Hongyihe–Tongbai shear zone in the north. The three shear zones have dextral, top-to-NW and sinistral movement, respectively. They are parts of a single shear zone, termed the Tongbai shear zone, that has a uniform top-to-NW sense of shear. Three samples of deformed granitoid (mylonite or protomylonite) from the shear zone have U–Pb zircon ages of 145 ± 6 Ma, 142 ± 2 Ma and 131 ± 6 Ma, respectively. An L-tectonite in the TBC yielded a metamorphic age of 137 ± 8 Ma and a migmatite an age of 137 ± 1 Ma. The Tongbai shear zone is intruded by undeformed Early Cretaceous granite and dykes and deformation in the shear zone is constrained to ca. 140–135 Ma, synchronous with metamorphism and migmatization in the TBC. Early Cretaceous magma emplacement and the associated uplift modified the TBC into a gentle antiform and the uplift may have continued to ca. 102–85 Ma. Similar geometry and kinematics have been documented in the Dabie orogenic belt to the east, which suggests that the Central Orogenic Belt in China probably experienced a uniform orogen-parallel extension and top-to-NW shearing in the ductile lithosphere in the Early Cretaceous.     

Geochemical, zircon U�CPb dating and Sr�CNd�CHf isotopic constraints on the age and petrogenesis of an Early Cretaceous volcanic-intrusive complex at Xiangshan, Southeast China

The Late Mesozoic geology of Southeast China is characterized by extensive Jurassic to Cretaceous magmatism consisting predominantly of granites and rhyolites and subordinate mafic rocks, forming a belt of volcanic-intrusive complexes. The Xiangshan volcanic-intrusive complex is located in the NW region of the belt and mainly contains the following lithologies: rhyodacite and rhyodacitic porphyry, porphyritic lava, granite porphyry with mafic microgranular enclaves, quartz monzonitic porphyry, and lamprophyre dyke. Major and trace-element compositions, zircon U?CPb dating, and Sr?CNd?CHf isotopic compositions have been investigated for these rocks. The precise SHRIMP and LA?CICP?CMS zircon U?CPb dating shows that the emplacement of various magmatic units at Xiangshan took place within a short time period of less than 2?Myrs. The stratigraphically oldest rhyodacite yielded a zircon U?CPb age of 135?±?1?Ma and the overlying rhyodacitic porphyry has an age of 135?±?1?Ma. Three porphyritic lava samples yielded zircon U?CPb ages of 136?±?1?Ma, 132?±?1?Ma, and 135?±?1?Ma, respectively. Two subvolcanic rocks (granite porphyry) yielded zircon U?CPb ages of 137?±?1?Ma and 137?±?1?Ma. A quartz monzonitic porphyry dyke, which represented the final stage of magmatism at Xiangshan, also yielded a zircon U?CPb age of 136?±?1?Ma. All these newly obtained precise U?CPb ages demonstrate that the entire magmatic activity at Xiangshan was rapid and possibly took place at the peak of extensional tectonics in SE China. The geochemical data indicate that all these samples from the volcanic-intrusive complex have an A-type affinity. Sr?CNd?CHf isotopic data suggest that the Xiangshan volcanic-intrusive complex derived mainly from remelting of Paleo-Mesoproterozoic crust without significant additions of mantle-derived magma. However, the quartz monzonitic porphyry, which has zircon Hf model ages older than the whole-rock Nd model ages, and which has ??_Nd(T) value higher than the other rocks, may indicate involvement of a subordinate younger mantle-derived magma in its origin. Geochemical data indicate that the various rocks show variable REE patterns and negative anomalies of Ba, Nb, Sr, P, Eu and Ti in the trace element spidergrams, suggesting that these rocks may have undergone advanced fractional crystallization with separation of plagioclase, K-feldspar and accessory minerals such as allanite. We suggest that this Cretaceous volcanic-intrusive complex formed in an extensional environment, and the formation of the Xiangshan mafic microgranular enclaves can be explained by the injection of mafic magma from a deeper seated mantle magma chamber into a hypabyssal felsic magma chamber at the crustal emplacement levels.     

Geology and age of mafic magmatism on Jeannette Island (De Long archipelago)—Implications for paleotectonic reconstructions for the Arctic

The paper presents the first data on the geology of a series of mafic dikes emplaced in the volcanogenic sedimentary sequence composing the structure of Jeannette Island. We show that the island hosts two generations of dikes consisting of fine-grained dolerites. Both magmatic stages were followed by fold deformations. The youngest and weakly deformed dikes are none the less chloritized and serpentinized. However, we were able to obtain the first ever precise geochronological ⁴⁰Ar/³⁹Ar data for them. According to this analysis, the emplacement age of the intrusive complex of Jeannette Island is close to the Ediacaran (553.6 ± 10.3 Ma). The ages of several superimposed tectonothermal events were determined: Middle Ordovician (463.3 ± 11.7 Ma), Late Silurian (425.3 ± 8.7 Ma), Late Devonian (360.7 ± 8.3 Ma) and Early Carboniferous (341.3 ± 6.8 Ma).     

桂中都安钾镁煌斑岩金云母~(40)Ar/~(39)Ar年龄及其构造意义

右江褶皱带地处扬子陆块和华夏陆块的结合部位,区内广泛发育以基性-超基性岩墙群和超酸性岩脉为主的燕山晚期双峰式岩浆活动。对桂中都安地区的金云钾镁煌斑岩脉进行了金云母~(40)Ar/~(39)Ar法测年,斑晶和基质金云母样品均给出了较一致的坪年龄,分别为93.6±0.6Ma和92.9±0.6Ma,相应的等时线年龄分别为93.4±0.9Ma和92.7±0.8Ma,上述年龄代表了岩脉的侵位年龄。结合区内报道的同期年代学资料,进一步证明右江褶皱带及其周缘晚白垩世双峰式岩浆活动响应了华南地区燕山晚期的构造转换事件及随后大规模的岩石圈伸展减薄。