LA-ICP-MS dating of Zhangjiakou Formation volcanic rocks in the Zhangjiakou region and its geological signi?cance

The geological cross section of the Zhangjiakou Formation is originally established in the Zhangjiakou region. From the dating of four samples from typical cross sections of the Zhangjiakou Formation in the Zhangjiakou region, the age range is found to be mainly from 143.0 ± 3.7 Myr to 136.1 ± 1.4 Myr, which almost equals the sum of the age range of the Zhangjiakou Formation (136–135 Myr) and that of the Tuchengzi Formation (143–136 Myr) in the Luanping region, and the Zhangjiakou and Tuchengzi Formations in Zhangjiakou-Luanping of North Hebei are found to be conformable. The evidence above indicates that the Zhangjiakou and Tuchengzi Formations, which are distributed in Zhangjiakou-Luanping, were formed under the same geological setting. Volcanic activity was earlier and longer (143–136 Myr) in the western region (Zhangjiakou) than that in the eastern region (Luanping). The ‘‘Zhangjiakou Formation” in Chengde and Pingquan formed in the period in which the Yixian Formation in West Liaoning formed, so should be renamed the Yixian Formation.     

Constraints on the upper boundary age of the Tiaojishan Formation volcanic rocks in West Liaoning-North Hebei by LA-ICP-MS dating

In West Liaoning-North Hebei, the Tiaojishan Formation volcanic rocks are developed and many isotopic ages with a very wide age range were obtained in them as well. These different ages have direct influence on confirming and interpreting the related geological issues in West Liaoning-North Hebei and have restricted the study of geological problems related. In the field, the Tiaojishan Formation and the overlying Tuchengzi Formation are in parallel unconformable contact. By systematical sampling and dating of LA-ICP-MS in the Tiaojishan Formation （especially in the upper part） in Luanping, Chengde basins of Northern Hebei Province and in Lingyuan, Jinlingsi-Yangshan basins of Western Liaoning Province, combined with the dating data obtained by previous researchers, results indicate that the upper boundary age of the Tiaojishan Formation is from 156 to 153 Ma, and the lower boundary age of the Tiaojishan Formation is about 165 Ma, the Tiaojishan Formation was formed from the late of middle Jurassic to late Jurassic Epoch.     

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.     

乌伦古地区石炭纪火山岩地球化学特征分析

乌伦古地区是准噶尔盆地重要的探区之一,目前已获工业油气流,但仍存在石炭系资源潜力不清、富集规律不明、勘探开发风险大等问题。采用元素地球化学方法,探讨区内火山岩地球化学特征、源区以及构造环境特征。结果表明:姜巴斯套组火山岩具有低TiO_2含量,且Nb、Ta等元素负异常的岛弧火山岩特征;而巴山组具有高TiO_2含量,轻稀土元素和高场强元素较为富集等特征,为大陆板内裂谷火山岩,但受到火山岛弧的混染;同时揭示上下石炭统火山岩均来自于壳源,在物质来源判别图和构造判别图解中也得到很好的印证。这也表明研究区石炭系经历了从岛弧环境向陆内裂谷环境演变的过程,该研究成果对本区及邻区构造演化及成藏条件研究均具有指导作用。     

SHRIMP zircon U-Pb dating for volcanic rocks of the Dasi Formation in southeast Hubei Province, middle-lower reaches of the Yangtze River and its implications

The Jinniu Basin in southeast Hubei,located at the westernmost part of middle-lower val-ley of the Yangtze River,is one of the important vol-canic basins in East China. Volcanic rocks in the Jin-niu Basin are distributed mainly in the Majiashan Formation,the Lingxiang Formation and the Dasi Formation,consisting of rhyolite,basalt and basaltic andesite,(trachy)-basalt and basaltic trachy-andesite and (trachy)-andesite and (trachy)-dacite and rhyolite respectively,in which the Dasi volcanism is volumet-rically dominant and widespread. The Dasi volcanic rocks were selected for SHRIMP zircon U-Pb dating to confirm the timing of volcanism. The results indi-cate that there exist a large amount of magmatic zir-cons characterized by high U and Th contents in the volcanic rocks. The concordia ages for 13 points are 128 ± 1Ma (MSWD = 3.0). On account of the shape of zircons and Th/U ratios,this age is considered to represent the crystallization time of the Dasi volcan-ism. The volcanic rocks in the Dasi,Majiashan and Lingxiang Formations share similar trace elementand REE partition patterns as well as Sr-Nd isotopic compositions. In combination with the regional geol-ogy,it is proposed that the southeast Hubei volcanic rocks were formed mainly during the Early Creta-ceous,just like other volcanic basins in middle-lower Yangtze valley. A lithospheric extension is also sug-gested for tectonic regime in this region in the Cre-taceous Period.     

K-Ar age of the Cenozoic volcanic rocks in the Nangqen Basin, Qinghai Province and its geological significance

The Nangqen Basin is one of the typical shearing-extensional basins situated in the north part of the Hengduan Mts. Nine ages of the Cenozoic volcanic rocks from the basin by K-Ar isotope-dilution technique have been reported in the note. The apparent ages of whole rock and separated biotite range from 32.04 to 36.50 Ma. The₄₀Ar-K (%) correlation isochronal line shows that there existed neither argon excessing, nor radiogenetic argon loosing, i.e. the age represents the true diagenetic time. Volcanic activity in this area occurred in the post-collision environment and is controlled by lithosphere substantial thickening and violent intercontinental shearing-strike slip faults. There is age difference of the magmatic activity in the east, the west and the north Tibet. The multi-periodic eruption-intrusion of the magmatic melt since Cenozoic is a tracer showing the pulsating uplift of the Tibetan Plateau.     

浙西北钟吕群流纹岩锆石U-Pb年龄及其年代地层学意义

 钟吕群出露于浙西北地区,为一套强变形、浅变质的碎屑岩-火山岩系,其下未见底,上被南华纪休宁组不整合覆盖。传统上将其对比于赣东北双桥山群。本文主要研究钟吕群流纹岩的锆石U-Pb 年龄。样品A120408-4 采自苏庄断层以北,层位上属钟吕群的中上部,岩性为灰白色致密块状流纹岩,强烈片理化。样品A120408-4 中获得大量锆石,完成LA-ICP-MS 锆石UPb定年测点38 个,获得年龄加权平均值为(777.1±3.6)Ma,结合地层特点及变质变形特征,得出钟吕群主体相当于河上镇群上墅组,应属扬子古陆新元古代晚期大陆裂谷的地质记录。     

Characteristics and geological significance of olivine xenocrysts in Cenozoic volcanic rocks from western Qinling

Cenozoic volcanic rocks from the Haoti, Dangchang County of the western Qinling Mountains, contain a few clearly-zoned olivines. These olivines are relatively big in grain sizes and usually have cracks or broken features. Their cores have similar compositions (Mg#=90.4-91.0) to those for the peridotitic xenoliths entrained in host volcanic rocks and their rims are close to the compositions of olivine phenocrysts (Mg#=85.5-81.9). The CaO contents in these zoned olivines are lower than 0.1%. These features demonstrate that the clearly-zoned olivines are xenocrysts and disaggregated from mantle peridotites. The zoned texture was the result of the interaction between the olivine and host magma. Available data show that the volcanic rocks would have been derived from the mantle source metasomatized by subducted hydrathermally-altered oceanic crust. The formation of these Cenozoic volcanic rocks was perhaps related to the rapid uplift of the Tibetan Plateau.     

Constraints on the late Mesozoic regional angular unconformity in West Liaoning-North Hebei by LA-ICP-MS dating

The angular unconformity between the Tuchengzi Formation and the Zhangjiakou Formation in the Chengde area is commonly regarded as the direct evidence of boundary between Jurassic and Cretaceous systems, and as the evidence of tectonic transformation interfaces in Mesozoic Era in the West Liaoning-North Hebei. Moreover, the lower boundary age of the Zhangjiakou Formation in the Chengde was to be thought about 135 Myr in the past. Based on the detailed study and systematic sampling in Yixian-Beipiao, Lingyuan, Chengde and Luanping areas in the field, the authors obtained the following isotopic ages by means of LA-ICP-MS dating: In the Lingyuan area, the Zhangjiakou Formation and the Yixian Formation are in angular unconformable contact; the age range of the Zhangjiakou Formation volcanic rock is from 133 to 130 Myr (seven samples), and that of the Yixian Formation volcanic rock is from 126 to 121 Myr (eleven samples). In the Luotuoshan of Chengde, the Tuchengzi Formation and the Zhangjiakou Formation are in angular unconformable contact; the age at the lower Zhangiiakou Formation is about 124 Myr (one sample), which reflects that the age range of the lower Zhangjiakou Formation is from 129 to 124 Myr combined with the previous study. In the Luanping basin, there is the largest sediment gap in upper Mesozoic, whose lower boundary age is confined in 131 to 130 Myr. In the Yixian-Beipiao area, the second Member of the Yixian Formation, making up more than 3/4 thickness of the Yixian Formation, was formed in 126 to 119 Myr, and the age of the Yixian Formation at the bottom is about 132 Myr. Moreover, the upper boundary age of the Tuchengzi Formation is about 136 Myr in the West Liaoning-North Hebei region. Based on the above isotopic data combined with the geological characteristics in the field in the West Liaoning-North Hebei region, it reflects that there was a widespread tectonic event in the period of 136 to 126 Myr (mainly in 130-126 Myr) in the West Liaoning-North Hebei, and it formed the late Mesozoic regional angular unconformity. Moreover, the lower boundary ages of the late Mesozoic regional angular unconformity in different areas are different, and the tectonics and the creature assemblage are different in the strata at the top and bottom of the regional angular unconformity. It implies that the late Mesozoic regional angular unconformity is a regional interface of tectonic transformation in Mesozoic Era in the West Liaoning-North Hebei.     

Constraint on the late Mesozoic regional angular unconformity in West Liaoning--North Hebei by LA-ICP-MS dating

The angular unconformity between the Tuchengzi Formation and the Zhangjiakou Formation in the Chengde area is commonly regarded as the direct evidence of boundary between Jurassic and Cretaceous systems, and as the evidence of tectonic transformation interfaces in Mesozoic Era in the West Liaoning-North Hebei. Moreover, the lower boundary age of the Zhangjiakou Formation in the Chengde was to be thought about 135 Myr in the past. Based on the detailed study and systematic sampling in Yixian-Beipiao, Lingy- uan, Chengde and Luanping areas in the field, the authors obtained the following isotopic ages by means of LA-ICP-MS dating： In the Lingyuan area, the Zhangjiakou Formation and the Yixian Formation are in angular unconformable contact; the age range of the Zhangjiakou Formation volcanic rock is from 133 to 130 Myr （seven samples）, and that of the Yixian Formation volcanic rock is from 126 to 121 Myr （eleven samples）. In the Luotuoshan of Chengde, the Tuchengzi Formation and the Zhangjiakou Formation are in angu- lar unconformable contact; the age at the lower Zhangjiakou Formation is about 124 Myr （one sample）, which reflects that the age range of the lower Zhangjiakou Formation is from 129 to 124 Myr combined with the previous study. In the Luanping basin, there is the larg- est sediment gap in upper Mesozoic, whose lower boundary age is confined in 131 to 130 Myr. In the Yixian-Beipiao area, the second Member of the Yixian Formation, making up more than 3/4 thickness of the Yixian Formation, was formed in 126 to 119 Myr, and the age of the Yixian Formation at the bottom is about 132 Myr. Moreover, the upper boundary age of the Tuchengzi Formation is about 136 Myr in the West Liaoning-North Hebei region. Based on the above isotopic data combined with the geological characteristics in the field in the West Liaoning-North Hebei region, it reflects that there was a widespread tectonic event in the period of 136 to 126 Myr （mainly in 130-126 Myr） in the West Liaoning-North Hebei, and it formed th     

冈底斯西段第四纪钾质火山岩的发现及赛利普组的建立

冈底斯西段赛利普一带分布有大面积的钾质火山岩,以前曾被划归上新世-早更新世赛利普群.作者对这套火山岩地层进行了详细野外调查并重新测制了火山岩地层剖面,在该套火山岩下伏及层间湖相碎屑沉积层中获得ESR年龄0.268 Ma与0.349 Ma.岩石学、岩石化学研究表明,这套火山岩是以安粗质熔岩为主的钾玄岩系列-钾质碱性玄武岩系列火山岩,岩石类型及岩石组合稳定,岩层产状平缓,岩石新鲜,层位相对清楚,故将其命名为赛利普组,属第四纪早-中更新世.这一发现对研究冈底斯构造带新生代以来岩浆活动、构造演化,进而探讨青藏高原隆升机制有着重要的地质意义.     

冲绳海槽火山岩同位素特征及铀系组分法测年

借助多种分析方法,对采自冲绳海槽北段和中段的火山岩进行了常量元素、稀土元素、Sr-Nd同位素组成分析及铀系组分法年龄测定,在此基础上对此地区的海槽岩浆的起源、演化及岩石类型进行了研究。结果表明:(1)海槽晚更新世(大约10万年)以来有一期广泛的酸性火山喷发活动;(2)受板块俯冲的控制,此期火山活动年龄按照与琉球岛弧距离的由近到远而由新到老;(3)海槽北、中段岩浆演化过程相似,其下很可能存在有巨大的、统一的岩浆房,但与海槽南段岩浆作用差异明显,岩浆物质很可能来源于PREMA地幔源区;(4)此地区的黑色浮岩为钙碱性系列流纹岩,岩石化学组成与海槽产出的白色浮岩相似,形成年代为17.7±2.1 ka。     

西藏措勤布嘎寺组中基性火山岩特征及成因

西藏措勤地区布嘎寺组火山岩是一套钾质到超钾质的岩石,根据化学组成可以分为中酸性和中基性火山岩.在前人研究的基础上对布嘎寺组中基性火山岩进行野外地质调查及全岩地球化学分析,并对该构造背景、源区性质及岩石成因做了进一步的研究和探讨.该火山岩具有轻稀土元素强烈富集、大离子亲石元素和高场强元素不同程度富集、弱负铕异常等地球化学...     

LA-ICP-MS zircon U-Pb dating for high-pressure basic granulite from North Qinling and its geological significance

Inrecentyears,discoveriesofhightoultrahighpressuremetamorphicrocksatthenorthandsouthsidesoftheQinlingGroupinthenorthernpartoftheQinlingMountains(hereafterNorthQinling)haveattractedfocusattentionofgeologistsworldwide.Thenorthhigh-pressure(HP)metamorphicbelt,distributedintheareafromGuanpotoShuanghuaishunorthwardtoShizipin,LushCountyinHenanProvince,consistsmainlyofeclogiteoutcroppedaslenticularmassesofdifferentsizesinthegneissesoftheQinlingGroupclosetothesouthsideofZhuyangguan-Xiaguanfault…     

湘西南-桂北交界地区加里东期基性岩的发现及其地质意义

湘西南-桂北交界地区属于江南造山带(或称之为钦杭结合带)西南段的重要组成部分,该区与Rodinia超大陆聚合与裂解有关的岩浆活动十分强烈,但至今还没有加里东期基性岩存在的报道。采用LA-ICP-MS锆石U-Pb法对湘西南—桂北交界地区首次发现的加里东期变辉长辉绿岩进行了年代学研究。结果表明,变辉长辉绿岩形成于(421.9±7.8)Ma,为加里东期岩浆活动的产物。该基性岩的发现及其年龄的确定;并结合区域上的研究结果表明,扬子板块和华夏板块结合带(钦杭结合带)西南段可能存在有从新元古代至早古生代的双向洋陆俯冲-碰撞过程。     

桂西北泥盆系罗富组泥页岩烃源岩有机地球化学特征及意义

通过野外剖面勘测、室内烃源岩地球化学分析以及碳同位素测试的方法和手段,对桂西泥盆系罗富组(D_2l)的黑色泥页岩烃源岩进行了研究。结果表明:研究区烃源岩有机质丰度普遍较高,TOC值为0.62%~2.74%,平均值为2.12%,均超过生烃门限(TOC=0.2%);氯仿沥青"A"含量为0.003 5%~0.143 8%,平均值为0.0439%,总烃含量(HC)为9.144~1177.5×10~(-6)(ppm),平均值为302.14×10~(-6)(ppm),生烃条件好。河池-南丹剖面烃源岩有机质类型主要为Ⅰ_1型,其他绝大部分为Ⅰ_2型在河池西南剖面有机质出现Ⅲ_1型。R_0的值为3.47%~3.73%,均处于过成熟阶段。有机质正构烷烃的分布既有单峰型,又有双峰型;Pr/Ph的比值均小于1,三环萜烷以C_(23)和C_(21)为主峰,藿烷以C_(30)藿烷含量最高,其次为C_(29)藿烷,伽马蜡烷/C_(30)。藿烷的比值为0.15~0.16,伽马蜡烷含量较高;C_(28)甾烷含量最高,其次是C_(29)甾烷,C_(27)甾烷含量最低,甾烷呈反"L"型分布。以上生物标志化合物的参数表明:罗富组烃源岩的有机质来源以低等水生生物为主,并有陆源高等植物的输入,沉积环境为还原的微咸水海相沉积环境。     

1965年以来张家口地区季节干旱变化特征分析

 干旱对河北省张家口地区农业生产具有重大不利影响,典型干旱年份导致了严重的经济损失。基于张家口地区14个气象站1965-2018年的气象观测资料,采用标准化降水指数（SPI）和数理统计方法,分析了张家口地区季节干旱的时空变化特征。结果表明：影响农业生产显著的夏季干旱呈显著增加趋势,特别是20世纪90年代以来,干旱持续时间较长,空间范围较大;春秋季干旱出现减轻趋势,21世纪以来干旱相对偏轻;冬季干旱没有显著的变化趋势。因此,应重点加强对夏季干旱的防御管理。     

吉林西林河地区糜棱岩显微构造及其地质意义研究

介绍了吉林西林河地区糜棱岩的显微构造特征 ,指出韧性变形是该区太古宙断裂变形的主导变形机制 ,同时阐述了在该区研究糜棱岩显微构造的重要意义     

焦石坝五峰组-龙马溪组页岩硅质生物成因的证据及其地质意义

以页岩露头样品和钻井岩屑样品元素测量结果为依据,结合岩心和薄片观察,分析焦石坝五峰组-龙马溪组页岩的硅质生物成因的证据,并探讨其地质意义。结果表明:焦石坝五峰组—龙马溪组页岩中存在大量的笔石、角石、珊瑚等古生物化石和放射虫微体生物化石;SiO_2含量介于40%~80%,平均达63%;过量硅含量为2.6%~55.31%,其中五峰组和龙马溪组页岩下段过量硅含量较高,平均分别为41.3%和25.5%;Al/(Al+Fe+Mn)值介于0.62~0.87,与纯生物成因硅质岩相近,Al-Fe-Mn三角图上,研究区页岩样品绝大部分落在生物成因区,表明页岩硅质成分属于生物成因;深水陆棚贫氧、缺氧沉积环境有利于页岩有机质的富集和有机孔隙的发育,对于页岩天然裂缝的形成和后期人工压裂都具有重要意义。