Zircon U-Pb geochronology of the volcanic rocks from Fanchang-Ningwu volcanic basins in the Lower Yangtze region and its geological implications

The latest eruptions in two important Mesozoic volcanic basins of Fanchang and Ningwu located in the middle-lower reaches of the Yangtze River formed the bimodal volcanic rocks of the Kedoushan Formation and ultrapotassic volcanic rocks of the Niangniangshan Formation, respectively. The representative volcanic rocks of the two Formations were selected for LA-ICPMS zircon U-Pb dating. The results indicate that there exist a large amount of magmatic zircons as indicated by high Th/U ratios in these volcanic rocks. The weighted mean age of 21 analyses is 130.7±1.1 Ma for the Kedoushan Formation, and that of 20 analyses is 130.6±1.1 Ma for the Niangniangshan Formation. These U-Pb ages are interpreted to represent the formation times of the volcanic rocks. In combination with other known geochronological data for Mesozoic volcanic rocks from the Lower Yangtze region, it is proposed that the latest volcanic activations in the Jinniu, Luzong, Fanchang and Ningwu volcanic basins probably came to end prior to ca. 128 Ma. There is no significant time interval between the early and later volcanic activities in the Luzong and Ningwu basins, suggesting a short duration of volcanic activities and thus implying the onset of an extensional tectonic setting at about 130 Ma in the Lower Yangtze region. Integrated studies reveal that the Early Cretaceous magmatic activities and their geochronological framework in the Lower Yangtze region are a response to progressively dynamic deep processes that started with the transformation of tectonic setting from compression to extension, followed by delaminating of the lower part of the thickened lithosphere, lithospheric thinning, asthenosphere upwelling, and crust-mantle interaction.     

^40Ar-^39Ar geochronology of Cenozoic Linzizong volcanic rocks from Linzhou Basin, Tibet, China, and their geological implications

Whole-rock and mineral separate Ar-Ar dating was carried out for the Linzizong volcanic rocks at Linzhou Basin in Tibet to constrain the time span of volcanism and the corresponding stratigraphic sequence. Sampling was based on detailed geologic mapping and stratigraphic sequence of Dianzhong, Nianbo, Pana Formations, systematically from the bottom to near the top. The results indicate that the Linzizong volcanic rocks erupted from Paleocene to middle of Eocene (64.43--43.93 Ma). Among them, the Pana Formation formed from ca. 48.73 to 43.9 Ma, the Nianbo Formation around 54 Ma and the Dianzhong Formation from 64.4 to 60.6 Ma. In combination with evidence from the geochemical characteristics of the volcanic rocks, and from stratigraphy in southern Tibet, it is postulated that the age of the lowest member in the Dianzhong Formation of the Linzizong volcanic rock, which overlies unconformably the Late Cretaceous Shexing Formation, likely corresponds to the inception of the collision between Indian and Asian continents in southern Tibet.     

Zircon U-Pb SHRIMP dating for the volcanic rocks of the Xiong’er Group: Constraints on the initial formation age of the cover of the North China Craton

The volcanic rocks of the Xiong‘er Group occur widely in the southern part of the North China Craton, which mark the beginning of the cover in the southern part of the North China Craton. The age of the volcanic rocks is thus crucial to understand the tectonic regime and evolutionary history of the North China Craton in the Proterozoic age. Zircons from five volcanic rocks and intrusions were dated by U-Pb SHRIMP method. The results indicate that the Xiong‘er Group formed in 1.80--1.75 Ga of Paleo-Proterozoic. Since the Xiong‘er Group formed earlier than the Changcheng System, the earliest rocks in the Changcheng System is therefore assumed to be formed in 1.75 Ga. A thermal-tectonic event of ca. 1.84 Ga is indicated by new zircon U-Pb SHRIMP ages in the southern part of the North China Craton. The volcanic rocks of the Xiong‘er Group thus represent the initial magmatism of the Paleo-Proterozoic breakup of the North China Craton. Numerous inherited zircons in the volcanic rocks mainly formed in ～2.20 Ga, indicating that the source magma of the volcanic rocks may be derived from the ～2.20 Ga crust, or from a mantle magma with significant contamination of the ～2.20 Ga crust.     

Zircon U-Pb dating and Lu-Hf isotope study of intermediate-mafic sub-volcanic and intrusive rocks in the Lishui Basin in the middle and lower reaches of Yangtze River

Eight Mesozoic sub-volcanic and intrusive rocks in the Lishui Basin have been selected for zircon U-Pb dating and Lu-Hf isotopic analysis. The Laohutou, Datongshan,Daweizhuang, Yeshanao, Yanwaqiao, Xibeishan, Changshantou and Jianshan porphyrites yield concordant early Cretaceous ages of 130.5 ± 1.6 Ma, 136.0 ± 3.4 Ma, 132.7 ±2.7 Ma, 127.0 ± 1.9 Ma, 129.4 ± 1.7 Ma, 133.2 ± 2.1 Ma,131.1 ± 2.3 Ma and 127.4 ± 1.8 Ma, respectively. Zircon eHf(t) values for these rocks range from-3.54 to-9.11, mostly between-3.54 and-5.93, suggesting sources similar to those for coeval igneous rocks in other volcanic basins along the middle and lower reaches of Yangtze River. Considering the geochemical characteristics of coeval magmatic rocks in the middle and lower reaches of Yangtze River and its adjacent areas, this paper proposes a model of slab rollback to explain the Mesozoic magmatism in the east-central part of China continent. In the middle-late Jurassic to early Cretaceous(170–135 Ma), the Pacific plate started to subduct beneath the Eurasian continent, producing a compressive tectonic setting,and magmatism progressed from coast to the inland. The magmas of this period were derived mostly from partial melting of ancient crust. After *135 Ma, the subduction weakened, and the rollback of the subducted Pacific plate produced an extensional environment. This led to the formation of volcanic basins, the partial melting of enriched mantle sources and less ancient crust, and the intrusion of voluminous intermediate-mafic igneous rocks.     

Petrogenesis of the Neoproterozoic bimodal volcanic rocks along the western margin of the Yangtze Block： New constraints from Hf isotopes and Fe/Mn ratios

High-precision major element and Hf isotope data are reported for the Neoproterozoic Suxiong volcanic rocks along the western margin of the Yangtze Block. These volcanic rocks have variable εHf（T） values and Fe/Mn ratios. The relatively primitive basalts have high Fe/Mn ratios and high Hf-Nd isotopic compositions, indicating that they were generated by partial melting of garnet clinopyroxene in mantle plume at high pressure. Thus, the Suxiong basalts are genetically related to the proposed Neoproterozoic superplume. On the contrary, a few differentiated basalts have low Fe/Mn ratios and low Hf-Nd isotopic compositions. They are likely to experience assimilation-fractional crystallization process. The Suxiong rhyolites have consistent Hf and Nd model ages of 1.3-1.4 Ga. They are likely generated by shallow dehydration melting of pre-existing young arc igneous rocks associated with the basaltic underplating/intrusion in a continental rift.     

Fundamental geological elements for the occurrence of Chinese marine oil and gas accumulations

The Paleozoic strata in the Tarim Basin, Sichuan Basin and Ordos Basin are the major targets for marine petroleum exploration, with developed high quality hydrocarbon, mainly argillite. The deep burial of these source rocks suggests that they mainly develop gas instead of oil. But different maturities of organic matter may lead to different hydrocarbon facies. Through thermochemical sulfate reduction （TSR）, the hydrocarbon in the carbonate rocks may undergo a process of pyrolytic cracking and be catalyzed into gases. The marine reservoirs mainly consist of carbonate and clastic rocks, and the former is controlled by sedimentary facies, dolomitization, solution, TSR and cracking. The multiphase tectonic cycling develops multiple source-reservoir-cap combinations and diversified types of traps and reservoirs, featuring multiphase reservoir formation, mainly late-phase formation or consolidation. Palaeo-uplifts play a controlling role in hydrocarbon accumulation. Differences in major source rocks in the three basins lead to different locations of oil-gas accumulation layers, different types and patterns of reservoirs and different features of reservoir formation.     

The zircon SHRIMP chronology and trace element geochemistry of the Carboniferous volcanic rocks in western Tianshan Mountains

The genesis of the Carboniferous volcanic rocks in the Dahalajunshan group, mainly consisting of trachytes and trachy-andesites and distributing widely in western Tianshan Mountains, remains to be controversial. It has been proposed to be relevant to “rift” or “plume”. Detailed petrology and geochemical data presented in this paper show that these volcanic rocks represent typical continental arc magmatism. The volcanic rocks are mainly trachy-andesitic, and the magma source is enriched in LILE, Th and Pb, and depleted in HFSE and Ce. Trace element geochemical study suggests that the basalts could be modeled by 7% -11% partial melt of garnet Iherzolite. The volcanic rocks in the Dahalajunshan group are neither the products of “rift” nor so-called “plume”but represent the continental island arc of the Paleo-Southern Tianshan Ocean. The mantle wedge had been modified by the melt generating in subduction zone during a long evolution history of this island arc. The continental crust materials （i.e. mainly sediment on ocean floor） had been added into island arc through melt in subduction zone. Volcanic rocks occurring in different regions might represent magma eruption in different time. The zircon SHRIMP dating indicates that the ages of the basalt varies between 334.0 Ma and 394.9 Ma. The 13 analyses give an average age of 353.7±4.5 Ma （MSWD = 1.7）. The apparent ages of zircons in trachy-andesite vary between 293.0 Ma and 465.4 Ma. All analyses fall on the U-Pb concordant line and are divided into two groups. 8 analyses produce an average age of 312.8±4.2 Ma （MSWD = 1.7）, which represents the crystallizing age of zircon rims in trachy-andesite. The acquired two ages （i.e. 354 and 313 Ma） belong to the Early Carboniferous and Late Carboniferous epochs, respectively. Thus, the Dahalajunshan group would be separated into several groups with the accumulation of high-quality age dating and data of trace element and isotopic geochemistry, in our opinion.     

U-Pb zircon age for the Daohugou Biota at Ningcheng of Inner Mongolia and comments on related issues

SHRIMP U-Pb zircon dating was carried out for the Daohugou Biota near Ningcheng of Inner Mongolia and for lavas overlying or underlying sala-mander-bearing strata at Reshuitang in Lingyuan of West Liaoning. The results suggest that the Dao- hugou Biota occurred at an interval from 168 Ma to 164―152 Ma. Both the Daohugou Biota and the salamander-bearing fossil assemblage are the same biota and thus developed from 168 to 152 Ma, i.e. from late Middle Jurassic to the early Late Jurassic. The Daohugou Biota-bearing rocks, resting on the Jiulongshan Formation in disconformity and being overlain in unconformity by Late Jurassic Tuchengzi Formation and Early Cretaceous rocks containing the Jehol Biota, are mainly composed of volcanic-sedi- mentary rocks in a normal sequence. It is recom- mended that the Daohugou Biota and the related stratigraphy should be correlated with the Tiaojishan Formation (Lanqi Formation in West Liaoning) or its synchronous rocks. It is suggested that the Dao- hugou Biota and the Jehol Biota would be neither taken into one biota nor considered as the earliest elements of the Jehol Biota. The Daohugou Biota and the related rocks and the Yixian Formation were respectively formed in different periods of volcanic-sedimentary tectonics.     

Permian volcanisms in eastern and southeastern margins of the Jiamusi Massif, northeastern China: zircon U-Pb chronology, geochemistry and its tectonic implications

LA-ICP-MS zircon U-Pb dating and geochemical data for the Late Paleozoic volcanic rocks from eastern and southeastern margins of the Jiamusi Massif are presented to understand the regional tectonic evolution. Zircons from eight representative volcanic rocks are euhedral-subhedral in shape and display striped absorption and fine-scale oscillatory growth zoning as well as high Th/U ratios （0.33-2.37）, implying a magmatic origin. The dating results show that the Late Paleozoic volcanic rocks in the study area can be divided into two stages, i.e., the Early Permian （a weighted mean 205^Pb/238^U age of 288 Ma） and the Middle Permian volcanisms （a weighted mean 205^Pb/238^U age of 268 Ma）. The former is composed mainly of basalt, basaltic-andesite, andesite and minor dacite. They are characterized by low SiO2 contents, high Mg^# （0.40-0.59）, enrichment in Na （Na2O/K2O = 1.26-4.25） and light rare earth elements （LREEs）, relative depletion in heavy rare earth elements （HREEs） and high field strength elements （HFSEs）, indicating that an active continental margin setting could exist in the eastern margin of the Jiamusi Massif in the Early Permian. The latter consists mainly of rhyolite and minor dacite with high SiO2 （77.23%-77.52%）, low MgO （0.11%-0.14%）, enrichment in 1（20 （Na2O/K2O ratios 〈 0.80） and Rb, Th, U and depletion in Eu, Sr, P and Ti, implying a crust-derived origin. Therefore, it is proposed that the Middle Permian volcanic rocks could have formed under the collision of the Jiamusi and the Khanka Massifs.     

青藏措勤盆地下白垩统则弄群火山岩岩石地球化学特征

青藏高原措勤盆地早白垩世火山活动十分强烈,则弄群火山岩主要分布在措勤盆地中央隆起带的冈底斯构造岩浆岩带中,则弄群以安山岩—英安岩—流纹岩钙碱性系列火山熔岩及火山碎屑岩为主,盆地北部坳陷带中火山活动相对较弱,见有少量玄武安山岩、安山岩。则弄群火山岩岩石化学、地球化学特征分析表明,该火山岩形成于岛弧区构造环境中,这与古特提斯洋壳沿雅鲁藏布江结合带向北俯冲作用有关     

大兴安岭北段漠河地区塔木兰沟组火山岩地球化学特征

 大兴安岭北段漠河地区分布有大面积的晚中生代火山岩,塔木兰沟组是其中喷发较早的一期。塔木兰沟组是一套中基性火山岩,包括熔岩和火山碎屑岩,研究其成因对于理解大兴安岭北段的构造演化具有重要意义。对塔木兰沟组火山岩样品进行了系统的主量元素和微量元素地球化学分析。结果表明：样品大部分呈碱性,SiO₂的质量分数为52.7%～57.8%,为中基性岩石,岩石类型为玄武质粗面安山岩和粗面安山岩;相容的过渡族金属元素如Cr、Co、Ni等含量低,微量元素比值如Nb/La、Ba/Nb、Nb/U、Ce/Pb等及稀土元素总量的变化范围大,具有右倾的轻稀土元素富集的稀土元素分布模式。岩石成因分析表明,富集的大陆岩石圈地幔先经历了部分熔融,然后熔浆受到了轻度的地壳物质的混染以及一定程度的分离结晶作用,最终形成了塔木兰沟组火山岩。     

SHRIMP U-Pb zircon dating for the dacite of the Sangxiu Formation in the central segment of Tethyan Himalaya and its implications

The Tethyan Himalaya is located between the In-dus-Yarlung Zangbo suture zone to the north and HigherHimalayan crystallines to the south, paleogeographicallybelonging to the northern margin of “Greater India”. TheSangxiu Formation is used to define a series of specialstrata with rich volcanic interbeds, and is only distributedto the southeast of Yangzuoyong Co within 200 km×50 km along the eastern part of central Tethyan Hima-laya[1]. Based on overlapped relationships of strata andfos…     

内蒙古中部宝力高庙组长英质火山岩U-Pb-Hf同位素特征及其地质意义

锆石SHRIMP U-Pb定年结果显示: 内蒙古中部白音乌拉地区原宝力高庙组的流纹岩形成时代为300.0±2.9 Ma, 属晚石炭世; 青格勒宝拉格地区原宝力高庙组的凝灰岩结晶年龄为159.6±1.4 Ma, 并获得 3颗捕获锆石的年龄分别为291.8±3.4, 304.0±3.3和734.7±9.2 Ma, 应属于晚侏罗世满克头鄂博组。锆石LA-MC-ICP-MS Hf同位素分析显示: 流纹岩锆石ε_Hf(t) 值为+10.5~+12.9, T_DM^C值为493~645 Ma; 凝灰岩岩浆锆石ε_Hf(t)值为+10.1~+13.1, T_DM^C值为369~563 Ma。研究结果表明, 流纹岩源于晚古生代新生地壳的重熔并混入少量老地壳物质, 凝灰岩源于晚古生代地壳的熔融。Hf同位素特征显示晚古生代流纹岩和中生代凝灰岩源于相似的源区, 揭示了晚古生代的一次重要的增生事件, 并且在约160 Ma时期发生过地壳的再造。结合前人的研究成果表明, 兴蒙造山带在约300 Ma时处于古亚洲洋演化过程中岛弧向碰撞后伸展环境的转换时期, 在约160 Ma受到蒙古?鄂霍茨克构造域的影响。     

长江流域岩体的时空分布与碎屑锆石物源示踪

结合区域大地构造演化,系统总结并讨论了长江流域火成岩的时空分布特征及其对长江中下游沉积物物源的指示意义.结果表明,长江流域岩体的时空分布特征与区域构造演化密切相关;青藏高原新生代酸性侵入岩和火山岩以及云贵川高原晚二叠世峨眉山玄武岩及伴生酸性岩是长江上游地区的特征岩体;汉江和湘江—赣江流域早古生代中酸性侵入岩是长江中游地区的特征岩体;白垩纪中酸性侵入岩和火山岩是长江下游地区的特征岩体.这一成果可直接用于指导长江中下游地区利用碎屑锆石U-Pb年代学和Hf同位素进行物源示踪的工作.在钻孔沉积物中准确识别出这些特征年龄信息,并结合沉积物古地磁年代的标定对研究长江的地质演化历史具有重要指示意义.     

江西南部版石盆地火山岩SHRIMP锆石U-Pb年龄及其地质意义

应用SHRIMP锆石U-Pb定年方法,对江西南部三南-寻乌火山岩带版石盆地原鸡笼嶂组凝灰岩和原版石组流纹岩开展年代学研究.结果表明,凝灰岩SHRIMP锆石U-Pb年龄为(142.5±1.3)Ma,流纹岩SHRIMP锆石U-Pb年龄为(131.4±1.3)Ma.根据新的国际地层表,晚侏罗世与早白垩世的界线划在(145.5...     

鄂东南大冶地区中生代火山岩时代及意义

文中通过应用SHRIMP锆石微区U-Pb测年技术对马架山组凝灰岩进行了精确测年,判定其形成时代为早白垩世（123.6 Ma）,与长江中下游地区其他火山岩盆地形成时代类似.通过邻区对比,认为白垩纪长江中下游地区处于伸展构造背景.     

高松山金矿床控矿特征、成因及找矿前景

高松山金矿是与火山作用关系较为密切的浅成低温热液型金矿床。矿石类型主要为含金石英脉—构造角砾岩蚀变岩型。矿石硫同位素组成具有深源硫的特点。流体包裹体中H、O同位素反映在成矿的过程中有大气降水加入。矿床产于中生代上白垩统火山岩中,与英安岩、安山岩、安山玢岩、流纹岩关系密切。岩石化学分析反映了活动大陆边缘环境。金矿体与赋矿围岩存在着明显的物性差异,与Au、Ag、As、Sb、Cu有密切的相关关系,并以零磁场、高电阻为特征,从而本区具备了物化探找矿前提。该区为黑龙江东北部的一个重要的找矿远景区。     

Chronological record of the Early Mesozoic underplating in the northern margin of North China─U-Pb chronometry of zircons in the Late Mesozoic andesite from Western Liaoning

Cathodoluminescence spectroscopic analysis of zircons in the uppermost sanukites of the early Cretaceous Yixian Formation in Western Liaoning reveals zoning structures of the zircons in the volcanic rocks. LA-ICP-MS chronometry shows that the central part of the zircons is remnant zircon with a U-Pb dating of 254 Ma, which is consistent with the age of the Early Mesozoic underplating granulites, and that the periphery is zircon crystallizing out of the host magma with a U-Pb dating of 116 Ma, which is highly consistent with the Ar-Ar dating of the host volcanic rocks. Compositions of the clinopyroxenes xenocrystals in the sanukites are similar to those of the clinopyroxenes in the underplating granulites, indicating that the granulites accreted to the lower crust in the Early Mesozoic have been reformed and disrupted by the upwelling diapers of the asthenosphere and taken part in formation of the sanukites in the Late Mesozoic.