广西大明山地块寒武系碎屑锆石U Pb年龄及其构造意义

大明山地区位处扬子地块与华夏地块结合带的西南段,对该区寒武系碎屑锆石开展U-Pb年龄谱研究可为新元古代—早古生代扬子地块与华夏地块结合带构造属性的进一步确定和对华南大地构造演化的深入研究提供新的依据。本次研究对该区2件寒武纪砂岩样品分选出的碎屑锆石进行了LA-ICPMS U-Pb年代测定和分析。锆石的透射光及阴极发光图像、Th/U比值、稀土元素特征说明其主要为岩浆岩锆石。222个谐和年龄数据显示出5个主要年龄区间:550~560Ma、750~780 Ma、950~1020 Ma、1560~1740Ma和2390~2450Ma,其中以950~1020Ma区间表现为最突出的峰值。对比大明山与大瑶山地区的寒武系碎屑锆石年龄谱系,两者表现出相似的分布特征。最明显的年龄峰值(980Ma)揭示物源区曾是Grenville期造山带的一部分。与扬子地块和华夏地块碎屑锆石年龄谱系进行比较,结合古水流及相关地质证据,认为所研究样品的碎屑锆石主要来自华夏地块。根据现有的资料,我们更倾向于认为寒武纪时扬子—华夏之间可能没有大洋的分割,而是一陆间海(intercontinental sea)格局。本次研究还在大明山寒武系中测得8颗具有太古宙年龄的古老锆石(≥2500Ma,其中1颗≥3200Ma)。这些锆石具振荡环带结构,Th/U比值均≥0.4,稀土分布多具有明显的Ce正异常与Eu负异常,指示其来自岩浆岩,反映物源区在太古宙发生过岩浆作用。结合前人研究获得的华夏地块太古宙锆石年龄信息,认为华夏地块可能存在太古宙地壳基底或接受过古老陆块的物源供给。     

锦丰（烂泥沟）金矿地质特征及找矿方向浅析

锦丰（烂泥沟）金矿由磺厂沟复式向斜控制了矿体的空间分布,发育于磺厂沟复式向斜中的一系列断裂控制了矿床的产出位置,不同的岩性组合控制了矿体的具体产出层位.在矿床的形成过程中F3断裂不仅是矿区的主要导矿构造也是矿区的主要容矿构造,在矿床的形成过程中由于不同断块的差异运动导致了磺厂沟向斜不同的变形部位出露在同一水平的现象,为今后的矿床预测指明了方向.     

A multidisciplinary study on the emplacement mechanism of the Qingyang–Jiuhua Massif in Southeast China and its tectonic bearings. Part I: Structural geology,AMS and paleomagnetism

During the Cretaceous, the South China Block (SCB) experienced a widely distributed extensional event including numerous plutons emplacement and basin opening. Investigations on the tectonic regime coeval with pluton emplacement, and emplacement mechanism of the pluton remain relatively rare in the SCB. In order to address these questions, a multidisciplinary approach, including field structural and petrographic observations, anisotropy magnetic susceptibility (AMS) and paleomagnetic analyses, was carried out on the Qingyang–Jiuhua granitic massif which intrudes into the Lower Yangtze fold belt in the northeastern part of the SCB. The Qingyang–Jiuhua massif is composed of the granodioritic Qingyang and monzogranitic Jiuhua plutons dated by zircon U–Pb method at ca. 142 Ma, and ca. 131 Ma, respectively. Our structural observations show that the intrusion of the Qingyang–Jiuhua massif does not modify the fold strike. A weak ductile deformation of the country rocks and granitoid can be only observed in the boundary zone with limited contact metamorphism. In the contact aureole of the massif, the foliation follows the pluton contour, and the mineral lineation is rare. When present, it exhibits a down-dip attitude. Field and microstructural observations indicate isotropic magmatic textures in most parts of the massif. The AMS analysis of 93 sites reveals weak values for the anisotropy degree (P_J < 1.2) and oblate magnetic fabric dominance (T > 0) for most of the measured samples. Two principal foliation patterns are identified: horizontal foliations in the center of the plutons, and vertical foliations on the boundaries. Magnetic lineation strike is largely scattered, and weakly inclined at the scale of the entire massif. The paleomagnetic investigations indicate that (a) the younger Jiuhua pluton did not produce a remagnetization in the older Qingyang pluton, (b) no relative movement can be observed between these two plutons, (c) the entire massif did not experience any important relative movement with respect to South China, considering the paleomagnetic uncertainties. Integrating the newly obtained results with previous observations, our study favors a permissive emplacement mechanism for the two plutons, i.e. vertical magma intrusion into an opening space controlled by the NW–SE brittle stretching of the upper crust, which is in agreement with a weak extensional regional tectonic framework of the SCB.     

Tectonic evaluation of the Indochina Block during Jurassic-Cretaceous from palaeomagnetic results of Mesozoic redbeds in central and southern Lao PDR

Rock magnetic and palaeomagnetic studies were performed on Mesozoic redbeds collected from the central and southern Laos, the northeastern and the eastern parts of the Khorat Plateau on the Indochina Block. Totally 606 samples from 56 sites were sampled and standard palaeomagnetic experiments were made on them. Positive fold tests are demonstrated for redbeds of Lower and Upper Cretaceous, while insignificant fold test is resulted for Lower Jurassic redbeds. The remanence carrying minerals defined from thermomagnetic measurement, AF and Thermal demagnetizations and back-field IRM measurements are both magnetite and hematite. The positive fold test argues that the remanent magnetization of magnetite or titanomagnetite and hematite in the redbeds is the primary and occurred before folding. The mean palaeomagnetic poles for Lower Jurassic, Lower Cretaceous, and Upper Cretaceous are defined at Plat./Plon. = 56.0°N/178.5°E (A₉₅ = 2.6°), 63. 3°N/170.2°E (A₉₅ = 6.9°), and 67.0°N/180.8°E (A₉₅ = 4.9°), respectively. Our palaeomagnetic results indicate a latitudinal translations (clockwise rotations) of the Indochina Block with respect to the South China Block of −10.8 ± 8.8° (16.4 ± 9.0°); −11.1 ± 6.2° (17.8 ± 6.8°); and −5.3 ± 4.7° (13.3 ± 5.0°), for Lower Jurassic, Lower Cretaceous, and Upper Cretaceous, respectively. These results indicate a latitudinal movement of the Indochina Block of about 5–11° (translation of about 750–1700 km in the southeastward direction along the Red River Fault) and clockwise rotation of 13–18° with respect to the South China Block. The estimated palaeoposition of the Khorat Plateau at ca. 21–26°N during Jurassic to Cretaceous argues for a close relation to the Sichuan Basin in the southwest of South China Block. These results confirm that the central part of the Indochina Block has acted like a rigid plate since Jurassic time and the results also support an earlier extrusion model for Indochina.     

大庆油田肇35区块葡萄花油层沉积微相分布演化规律及有利相带预测

肇35区块是大庆宋芳屯油田新开发的区块,葡萄花油层为主力勘探层位。综合各类地质数据及沉积相分析,认为研究区主要发育三角洲前缘和浅湖两种沉积相类型,进一步划分为水下分流河道、河口坝、席状砂、水下分流间湾和浅湖泥岩等微相类型,建立了相应的测井相模板。垂向上,河口坝在葡上段较发育,砂岩分选较好,PI2小层为主要含油层位;水下分流河道在葡下段发育良好,PI51、PI52和PI7小层为主要含油层位。平面上,研究区主要以西北部和北部物源为主,砂体呈现西北厚、东南薄的分布特点。水下分流河道呈树枝状交叉合并,规模较大,沿河道延伸方向连续性好;河道末端及两侧发育片状、席状砂,厚度较薄,连续性好;河口坝砂体主要分布在河道末端,呈透镜状展布,规模较小。结合各沉积微相含油性、物性特征分析,在研究区预测了3类有利相带,其中水下分流河道砂体为Ⅰ类有利相带区,在研究区分布面积较大;河口坝砂体为Ⅱ类有利相带区,在研究区分布面积较小且分散;席状砂为Ⅲ类有利相带区,仅在研究区南部发育。     

扬子地块东南缘沉积岩的Nd同位素研究

扬子地块东南缘上溪群分布区及其周边沉积岩的Ｎｄ同位素研究结果,支持存在一条苏浙皖古生代裂陷槽（或江南深断裂）的观点。上溪群以北直至长江边所分布的震旦系－古生代的盖层沉积岩,其Ｎｄ模式年龄有两组,表明物源区不同。裂陷槽以北,沉积岩的物源区为Ｎｄ模式年龄约２.０～２.１Ｇａ的扬子物源区;以南的沉积岩表现出明显的幔源物质混染,显示出元古代岩浆活动的影响,而上溪群分布区以南直到江绍断裂附近主要表现上溪物源区的影响,华夏地块古老基底岩石则无显著贡献。     

华北陆块东南缘徐淮地区侵入岩岩石组合及时空分布

徐淮地区是安徽北部重要的铁多金属矿成矿区,区内晋宁期和燕山期岩浆活动频繁,形成众多基性、中性和酸性岩体,其中早白垩世岩浆作用与成矿作用关系密切。分析该区侵入岩岩石组合和分布特点,晋宁—燕山期可以划分出3个阶段3个岩石组合：晋宁期辉绿岩-石英正长斑岩-闪斜煌斑岩系列,成岩年龄为距今890.0～913.0 Ma;燕山期早侏罗世石英二长闪长岩-花岗岩系列,成岩年龄为距今180.0～191.3 Ma;燕山期早白垩世闪长斑岩+闪长（玢）岩+石英闪长玢岩+二长花岗岩组合系列,成岩年龄为距今102.1～132.2 Ma。燕山期早白垩世形成的岩体最多,活动最为强烈,与该区矽卡岩型矿床的形成密切相关,形成于中国东部构造应力由挤压转变为伸展的转折期。     

印度支那地块第三纪构造滑移与青藏高原岩石圈构造演化

印度支那地块于早第三纪至中新世发生大规模地向东南方向走滑,同时伴随着15°顺时针旋转。青藏高原及邻区晚自垩世以来的古地磁古构造及地质年代学研究新成果,说明青藏高原岩石圈的构造演化过程,即古新世初印度板块与欧亚大陆的南缘拉萨地块碰撞,至49 Ma左右印度与拉萨地块发生全面拼合,随着印度板块进一步向北挤压,从碰撞期至16 Ma,印度支那地块沿红河大型走滑断裂发生侧向滑移;印度与“欧亚大陆”之间的构造缩短通过岩右圈板块沿着大型走滑断裂系的挤出以及板块间的消减得到调整。青藏高原大规模的岩石圈构造缩短很可能始于中新世27 Ma左右沿着早期陆块间的接合带发生。一些事实还说明青藏高原的总体隆起很可能是通过10 Ma前后和3 Ma以后多期非均匀隆升形成的。     

基于断层活动资料的鄂尔多斯块体周缘未来30年大地震危险性研究

鄂尔多斯块体周缘历史上多次发生大地震,同时该地区人口稠密,城市群集中,有必要分析该地区未来30年的大地震危险性,为该地区的应急备灾工作提供参考.本文利用块体周缘活动断层的滑动速率、历史地震古地震等资料,建立了考虑大地震发生率的时间非平稳性质和大地震危险性近断层分布特征的地震危险性模型.计算了块体周缘未来30年Ⅷ度地震动的超越概率分布图,并同《中国地震动参数区划图》的地震危险性模型进行了比较.本文研究认为,由断层滑动速率估算的断层地震活动性与基于地震目录统计的地震活动性总体差别不大,但断层源的地震危险性在沿断层破裂面地区增大明显.结果认为,鄂尔多斯块体周缘未来30年地震危险性最高的地区在块体西南缘,六盘山东麓断裂和会宁—义岗断裂及周边地区是未来30年地震危险性较高的地区.