Structures of Syn-deformational Granites in the Longquanguan Shear Zone and Their Monazite Electronic Microprobe Dating

The Longquanguan shear zone is an important structural belt in the North China Craton, separating the underlying Fuping complex from the overlying Wutai complex. This shear zone has experienced three episodes of deformation： the first and main episode is a ductile top-to-ESE shear along the gently northwest-west dipping foliations, while the other two episodes are later collapse sliding. Prolonged granites parallel to the shear foliations make one of the main compositions of the Longquanguan shear zone. These granites experienced deformation to form mylonitic rocks when they emplaced during the first episode of deformation. Structural characters of the granites and their contacts to the country rocks indicate that these granites possibly resulted from in-situ partial remelting by shearing, i.e., they are syn-deformational granites. Monazites in these mylonitic granites are magmatic minerals and their crystallization ages may represent ages of the magmatic events, and also the ages for the main deformation of the Longquanguan shear zone. Monazite electronic microprobe dating were carried on two samples of granite, which gives multiple peak ages, among which 1,846 Ma and 1,877 Ma are the main peak ages for the two samples. These ages represent the main deformation of the Longquanguan shear zone, which is consistent with the main regional geological event at about 1,850 Ma caused by the collision between the Eastern and Western Blocks in North China. The good match between the monazite ages and the corresponding regional tectono-thermal events shows the feasibility and reliability of monazite electronic microprobe dating.     

中国西部新生代岩石圈汇聚和东部岩石圈离散的耦合关系与古环境格局演变的探讨

本文从印度大陆与亚洲大陆新生代碰撞前缘的多阶段构造变形与隆升过程,对比了我国南海北部大陆边缘盆地,东部渤海湾盆地的构造-沉积-岩浆事件,它们在时间上具准同时性,表现在两大陆碰撞构造变形和抬升的高峰时期正好与盆地伸展、拉张,快速构造沉降时期相对应;当构造转入相对稳定(松弛)阶段,表现为高原剥蚀夷平,岩浆活动频繁,盆地构造沉降速率减缓阶段.青藏高原多阶段构造-岩浆事件还与我国季风形成和发展以及全球新生代3次变冷事件具某种对应联系,认为深部地幔脉动上涌的热力效应可能是诱发高原季风,行星西风增强,高纬度降温的驱动力之一.它和高原地形增高引发大气热机效率增大起着互补作用,使青藏高原成为诱发我国大气环流变化的启动区.     

灵丘盆地主要活动断裂和1626灵丘地震发震构造

灵丘盆地位于山西地堑系的东北部,曾于1626年发生7级地震.文中通过解译SPOT5影像、分析SRTM3数据和野外调查,对该盆地进行宏观的构造地貌分析和对比,结合探槽以及地质剖面确定主要活动断层的几何分布和最新活动特征,进而探讨灵丘地震的发震构造.结果显示:太白维山山前断裂大部分段落最新活动时代限于晚更新世晚期.NE向的水涧-落水河断裂灵丘县城以西段造成同级地貌面高差约6m,其中最新活动在地表残留高约1m的断层陡坎,县城及以东段无明显地貌表现;NW向的华山河断裂具有枢纽断层特征,在盆地北部断裂向西倾并造成华山河Ⅰ级阶地两侧约10m的高差,在盆地南部断裂向东倾,剖面和相关地貌揭示该断裂在南段为一条高角度的活动正走滑断裂;据此认为,1626年灵丘地震为水涧-落水河断裂西段和NW向的华山河断裂共轭作用的结果.     

三论洞穴石笋沉积速率——以衙门洞一号石笋为例

综合观测我国南方洞穴30多根大石笋纵剖面。其中,精选近20根结构、构造清晰的剖面,进行高精度系统测年及沉积特征的对比研究,认定沉积速率是石笋生长过程时空轨迹的数学描述,记录石笋或石笋段沉积生长过程的时间和物质组分、结构构造及其空间组合。确认石笋沉积（生长）速率是沉积特征的缩影,具有多旋回、多级次特征,可作重建古气候环境的替代指标之一,具有时空双重含意。在石笋沉积速率研究积累和Y 1测年系列数据的基础上,提出沉积速率以10 mm/100 a作为气候冷、暖转变的分界值和冷、暖彼此趋近的临界值,建议以此作为南、北方甚至是地带性气候冷暖转变的分界或趋近的临界值。在重建古气候环境研究实践中,探索、完善、深化石笋沉积特征的综合对比;同时提出影响石笋沉积速率的自然条件是主控因素。其中,气候、地质、环境等是关键。而获得准确沉积速率,采样、测试、研究方法、剖面选择等,人为因素重大作用。在实践方面以结语的3点建议供同行参考。     

塔西南叶城地区新生代沉积物源分析:来自碎屑锆石U-Pb年代学证据

新生代以来,受印度与欧亚板块碰撞和持续汇聚作用的影响,西昆仑造山带重新活化,并向塔里木盆地强烈挤压冲断,形成塔西南新生代再生前陆盆地。本文以塔西南叶城地区柯克亚剖面新生代砂岩样品为研究对象,进行碎屑锆石LA-MC-ICP-MS U-Pb测年,探讨其主要物源方向,以期对西昆仑隆升历史进行约束。结果表明,碎屑锆石的年龄存在70 Ma,200~500 Ma,800~1000 Ma,1600~2000 Ma和2400~2800 Ma五个年龄段,反映了该区物源的复杂性。综合分析潜在的物源区岩石属性、年龄构成及古流向等研究资料,认为物源区主要包括铁克里克、奥依塔克及喀喇昆仑地区,来自西南天山的可能性比较低。其中70 Ma的碎屑锆石主要来自于南部的帕米尔和喀喇昆仑地区;而200~500 Ma年龄段的锆石来自南部的铁克里克和西南部奥依塔克地区,这两个地区是主要的物源区;800~1000 Ma年龄段的锆石主要来自库地缝合带;1600~2000 Ma和2400~2800 Ma年龄段的锆石主要来自于铁克里克地区的前寒武纪结晶基底。研究区与奥依塔克地区位于同一弧形带,二者具有相似的物源区。上新世阿图什组地层沉积前后锆石形态具有明显突变,代表了西昆仑一次快速隆升过程。     

Reservoir Characteristics and Preservation Conditions of Longmaxi Shale in the Upper Yangtze Block, South China

The Upper Ordovician-Lower Silurian Longmaxi Shale in the Upper Yangtze block represents one of the most important shale gas plays in China. The shale composition, porosity, organic thermal maturity, and methane sorption were investigated at the Qilongcun section in the Dingshan area, southeastern Sichuan Basin. The results show that the Upper Ordovician-Lower Silurian Longmaxi Shale contains: (1) sapropelic I organic matter; (2) a 40-m thick bedded sequence where total organic carbon (TOC) content is > 2%; (3) a 30-m thick layer at the base of the Longmaxi Shale with a brittle mineral content higher than 50%; and (4) a mean methane adsorption capacity of 1.80 cm3/g (7 MPa pressure). A positive correlation between TOC and sorbed gas indicates that organic matter content exerts an important control on methane storage capacity. Based on the analysis of the shale reservoir characteristics, the lower member of the Longmaxi Shale can thus be considered a favorable stratum for shale gas exploration and exploitation. It has similar reservoir characteristics with the Longmaxi Shale in the Jiaoshiba area tested with a high-yield industrial gas flow. However, based on tectonic analysis, differences in the level of industrial gas flow between the low-yield study area and the high-yield Jiaoshiba area may result from different tectonic preservation conditions. Evidence from these studies indicates the shale gas potential of the Longmaxi Shale is constrained by the reservoir and preservation conditions.     

查干凹陷中、新生代热史及烃源岩热演化

查干凹陷是内蒙古银-额盆地最具勘探潜力的凹陷, 为了揭示查干凹陷中、新生代热史及烃源岩热演化历史, 首先利用9口井的镜质体反射率数据恢复了查干凹陷中、新生代热史, 结果显示查干凹陷在早白垩世巴音戈壁组沉积开始至银根组沉积末期, 地温梯度逐渐增加, 且到银根组沉积末期达到最大, 为50~58 ℃/km; 自晚白垩世乌兰苏海组沉积开始至今, 地温梯度逐渐下降, 现今地温梯度仅为31~34 ℃/km.再以热史为基础, 结合沉积和构造发育史及烃源岩地球化学资料, 模拟了查干凹陷9口井3套烃源岩的成熟度演化历史, 模拟结果显示查干凹陷烃源岩成熟度演化受古地温控制, 3套烃源岩成熟度都在早白垩世晚期达到最大.      

Uniform Strike-Slip Rate along the Xianshuihe-Xiaojiang Fault System and Its Implications for Active Tectonics in Southeastern Tibet

Recent studies on the Xianshuihe-Xiaojiang fault system suggest that the Late Quaternary strike-slip rate is approximately uniform along the entire length of the fault zone, about 15±2 mm/a. This approximately uniform strike slip rate strongly supports the clockwise rotation model of the southeastern Tibetan crust. By approximating the geometry of the arc-shaped Xianshuihe-Xiaojiang fault system as a portion of a small circle on a spherical Earth, the 15±2 mm/a strike slip rate corresponds to clockwise rotation of the Southeastern Tibetan Block at the (5.2±0.7)×10-7 deg/a angular velocity around the pole (21°N, 88°E) relative to the Northeast Tibetan Block. The approximately uniform strike slip rate along the Xianshuihe-Xiaojiang fault system also implies that the Longmenshan thrust zone is not active, or at least its activity has been very weak since the Late Quaternary. Moreover, the total offset along the Xianshuihe-Xiaojiang fault system suggests that the lateral extrusion of the Southeastern Tibetan Block relative to Northeastern Tibetan Block is about 160 km and 200-240 km relative to the Tarim-North China block. This amount of lateral extrusion of the Tibetan crust should have accommodated about 13-24% convergence between India and Eurasia based on mass balance calculations. Assuming that the slip rate of 15±2 mm/a is constant throughout the entire history of the Xianshuihe-Xiaojiang fault system, 11±1.5 Ma is needed for the Xianshuihe-Xiaojiang fault system to attain the 160 km of total offset. This implies that left-slip faulting on the Xianshuihe-Xiaojiang fault system might start at 11±1.5 Ma.     

四川红格层状侵入体中角闪石和金云母的矿物学特征及其成因意义

红格层状岩体是峨眉山大火成岩省内带最大的赋存钒钛磁铁矿矿床的层状岩体,从底部到顶部可分为下部岩相带、中部岩相带和上部岩相带。红格岩体下部岩相带角闪(磁铁)辉石岩和角闪(磁铁)橄辉岩中角闪石含量高达5%~15%,远远高于区内其他含超大型钒钛磁铁矿矿床的层状岩体。岩体中角闪石呈嵌晶状结构,且具有均一干涉色,暗示其为岩浆成因,而非热液蚀变的产物。此外,角闪石的矿物化学特征表现为高Al2O3含量(10.5%~12.0%)、高Al/Si(0.30~0.37)和Mg/(Fe3++Fe2++VIAl)比值(1.69~2.63)以及低Si/(Si+Ti+Al)比值(0.69~0.74),进一步表明其是直接从幔源基性岩浆中结晶形成的。金云母高MgO含量(18.7%~22.9%)的特征也说明其与幔源岩浆作用有关。角闪石和金云母的成因与红格层状侵入体的地质背景相吻合,为探讨红格岩体形成过程中的物理化学条件提供了重要的矿物学依据。根据矿物电子探针成分及其化学式计算得到岩体角闪石的结晶温度为1000~1100℃,结晶压力小于2.2kbar,结晶时的氧逸度范围在NNO-0.55到NNO+0.73之间。矿物结构关系指示岩体的磁铁矿结晶早于角闪石,因此,结合MELTs模拟计算,认为红格岩体钒钛磁铁矿矿层的形成温度为1100~1165℃,氧逸度高于NNO+0.73。红格岩体下部岩相带和中部岩相带每个旋回自下而上,角闪石的Fe3+/(Fe3+/Fe2+)比值以及全岩Fe3+/Fe2+和Mt/(Mt+Ilm)比值有规律地逐渐降低,而磁铁矿V2O3含量逐渐升高,这些特征说明Fe-Ti氧化物的分离结晶导致氧逸度逐渐降低。而上部岩相带IX旋回全岩Fe3+/Fe2+和Mt/(Mt+Ilm)比值自底部到顶部随着磁铁矿V2O3含量的降低而升高,显示出与下部岩相带和中部岩相带相反的变化趋势,表明IX旋回在分离结晶过程中氧逸度是逐渐升高的,可能是受上部岩相带富P2O5母岩浆的制约。     

Structural and Chronological Evidence for the India-Eurasia Collision of the Early Paleocene in the Eastern Himalayan Syntaxis, Namjagbarwa

The eastern Himalayan syntaxis in Namjagbarwa is a high-grade metamorphic terrain formed by the India-Eurasia collision and northward indentation of the Indian continent into Asia. Right- and left-lateral slip zones were formed by the indentation on the eastern and western boundaries of the syntaxis respectively. The Dongjug-Mainling fault zone is the main shear zone on the western boundary. This fault zone is a left-lateral slip belt with a large component of thrusting. The kinematics of the fault is consistent with the shortening within the syntaxis, and the slipping history along it represents the indenting process of the syntaxis. The Ar-Ar chronological study shows that the age of the early deformation in the Dongjug-Mainling fault zone ranges from 62 to 59 Ma. This evidences that the India-Eurasia collision occurred in the early Paleocene in the eastern Himalayan syntaxis.