内蒙古莫力达瓦旗哈达阳绿泥石白云母构造片岩LA-ICP-MS锆石U-Pb年龄及其地质意义

嫩江—黑河构造带地处松嫩地块与兴安地块的结合部位,一直以来是业内学者关注的焦点。在构造带内内蒙古莫力达瓦旗哈达阳一带野外实地调研过程中,发现一套变质变形程度与周围地质体差异很大的地层。岩石组合为绿泥石白云母构造片岩、二云母微晶片岩、黝帘白云糜棱片岩等。采用LA-ICP-MS技术获得的绿泥石白云母构造片岩中锆石U-Pb年龄为413.9Ma±2.7Ma。通过分析区域地质演化历史及不同时代地质体特征,认为沿红山梁—哈达阳—依克特一带存在一条构造混杂堆积岩带,这套强变形岩石应为构造混杂岩带的一部分,其原岩形成时代为早泥盆世,后期构造定位应与嫩江—黑河构造带最后于早石炭世末拼合作用有关。     

135~130 Ma: 大别山第二次“去根”时间？

大别杂岩主要由早白垩世侵入岩和三叠纪变质岩组成。它的四周是四条区域性韧性剪切带:郯城—庐江断裂,商城—麻城断裂,襄樊—广济断裂和晓天—磨子潭断裂。其中,晓天—磨子潭断裂和襄樊—广济断裂在早白垩世具有相反的走滑剪切方向:北侧的边界断裂(晓天—磨子潭断裂)是一个左行剪切断裂,而南侧的边界断裂(襄樊—广济断裂)是一个右行剪切断裂。在大别杂岩内部,早白垩世低角度剪切面理的倾伏向以SE向或NW向为主。这些晚期剪切面理上的拉伸线理的倾伏向同样为SE或NW向。大别杂岩总体具有朝SE向挤出和顶部相对朝NW向剪切的构造特征。这些表明晚中生代是该杂岩演化的重要阶段。该杂岩的边界断裂和内部构造特征指示其晚期抬升是沿造山带方向(SE—NW)以低角度方式进行的。这一过程直接导致高压-超高压变质岩和同构造岩浆岩被抬升至近地表。同时,年代学研究表明:大别杂岩(扬子板块东北缘地壳)在晚侏罗世—早白垩世经历大规模混合岩化的时间为145~135 Ma,同造山岩浆作用的时间为145~135 Ma,后造山火山-岩浆活动的时间为135~120 Ma。因此,该杂岩中三叠纪高压-超高压变质岩所记录的早白垩世抬升过程不是印支事件的后续,而是燕山期陆内造山及随后发生的伸展过程有关。尽管这一陆内造山事件的起始时间至今仍不确定,但大别山未变形岩体(130~120 Ma)的年代学研究结果和我们新测得的同构造伟晶岩脉的锆石U-Pb年龄(130 Ma)为早白垩地壳变形提供了良好的上限制约。这样,大别山经历了三叠纪碰撞造山和伸展,晚侏罗世—早白垩世陆内造山-伸展二次过程。     

四川盆地下寒武统膏盐岩发育特征与封盖有效性分析

四川盆地下寒武统广泛发育一套膏盐岩,主要沉积于早寒武世龙王庙期。基于四川盆地深井及实测剖面中膏盐岩的识别和统计,发现下寒武统有川南和鄂西两大聚盐区,最厚达690.40m;川南聚盐区西部以含泥含云质石膏岩类和含膏砂泥岩类为主,东部及颚西聚盐区主要为含膏白云岩类。结合前人研究,认为膏盐岩发育于浅水缓坡沉积环境,有潮上萨布哈和缓坡蒸发两类成因。通过对川南聚盐区储盖匹配关系及后期构造与膏盐岩封盖性分析,表明下寒武统膏盐岩与盐下震旦系优质的白云岩储层在空间上具有良好的匹配关系;四川盆地边界大断裂及边界外寒武系的抬升暴露让膏盐岩在盆地外封盖性失效,而在盆地内膏盐岩的封盖性较好。威远气田解剖表明有膏盐岩分布的威远地区成藏,而无膏盐岩分布的资阳地区气藏被破坏,显示了下寒武统膏盐岩在四川盆地油气成藏中具有重要的作用。     

A New Enantiornithes (Aves) from the Early Cretaceous of China

A new bird from the early Cretaceous Yixian Formation of Liaoning, China is described. This new species, Grabauornis lingyuanensis, shares several synapomorphies with the Enantiornithes. The specimen is relatively well preserved. The skeletal morphology of Grabauornis bears close resemblance to that of other Chinese members of this clade. The brachial index （the ratio between the lengths of humerus and ulna） is 0.95, which is close to the average for enantiornithine birds. It indicates that Grabauornis was a rather good flyer, and the presence of an alula in the wing further testifies to this.     

Early Cretaceous trachybasalt-trachyte-trachyrhyolitic volcanism in the Nyalga basin (Central Mongolia): sources and evolution of continental rift magmas

As shown by geological, mineralogical, and isotope geochemical data, trachybasaltic-trachytic-trachyrhyolitic (TTT) rocks from the Nyalga basin in Central Mongolia result from several eruptions of fractionated magmas within a short time span at about 120 Ma. Their parental basaltic melts formed by partial melting of mantle peridotite which was metasomatized and hydrated during previous subduction events. Basaltic trachyandesites have high TiO₂ and K₂O, relatively high P₂O₅, and low MgO contents, medium ⁸⁷Sr/⁸⁶Sr(₀) ratios (0.70526-0.70567), and almost zero or slightly negative ε_Nd(T) values. The isotope geochemical signatures of TTT rocks are typical of Late Mesozoic basaltic rocks from rift zones of Mongolia and Transbaikalia. The sources of basaltic magma at volcanic centers of Northern and Central Asia apparently moved from a shallower and more hydrous region to deeper and less hydrated lithospheric mantle (from spinel to garnet-bearing peridotite) between the Late Paleozoic and the latest Mesozoic. The geochemistry and mineralogy of TTT rocks fit the best models implying fractional crystallization of basaltic trachyandesitic, trachytic, and trachyrhyodacitic magmas. Mass balance calculations indicate that trachytic and trachydacitic magmas formed after crystallization of labradorite-andesine, Ti-augite, Sr-apatite, Ti-magnetite, and ilmenite from basaltic trachyandesitic melts. The melts evolved from trachytic to trachyrhyodacitic and trachyrhyolitic compositions as a result of prevalent crystallization of K-Na feldspar, with zircon, chevkinite-Ce, and LREE-enriched apatite involved in fractionation. Trachytic, trachyrhyodacitic, and trachyrhyolitic residual melts were produced by the evolution of compositionally different parental melts (basaltic trachyandesitic, trachytic, and trachyrhyodacitic, respectively), which moved to shallower continental crust and accumulated in isolated chambers. Judging by their isotopic signatures, the melts assimilated some crustal material, according to the assimilation and fractional crystallization (AFC) model.     

Facies distribution of the Lower Cambrian cryptic microbial and epibenthic archaeocyathan-microbial communities, western Anti-Atlas, Morocco

Marine microbial communities recorded in the Moroccan Anti‐Atlas were unaffected across the Neoproterozoic–Cambrian transition. A stromatolite‐dominated consortium was replaced at the beginning of the Atdabanian (ca 20 Myr after the Neoproterozoic–Cambrian boundary) by shelly metazoan and thromboid consortia, which contain the oldest biostratigraphically significant fossils of the Moroccan Cambrian. The associated collapse of microbial mat (stromatolitic) growth appears to coincide with a change from pre‐Atdabanian shallow‐water restricted conditions into Atdabanian deeper, open‐sea conditions. It is postulated that this environmental change led to an episode of improved water circulation over carbonate platform interiors, promoting shelly metazoan immigration into the region. The Tiout/Amouslek lithostratigraphic contact in the early Atdabanian marks the end of an episodically unstable seafloor as suggested by the abundance of slumping and sliding structures, and synsedimentary microfaults and cracks recorded in the underlying Tiout Member. Concurrent with the transition is the occurrence of a network of cryptic fissures and cavities that provided habitats for a coelobiontic chemosynthetic–heterotrophic microbial community composed of stromatolitic crusts, Renalcis–Epiphyton–Girvanella intergrowths, and Kundatia thalli. In the overlying Amouslek Formation, archaeocyathan–thromboid reefs were constrained by substrate stability, water depth and subsidence rate. Four reef geometries are distinguished: (i) patch reefs surrounded by shales, (ii) bioherms in which flank beds intercalate laterally with carbonate and shale inter‐reef sediments, (iii) biostromes or low‐relief structures formed as a result of lateral accretion of patch reefs, and (iv) kalyptrate complexes that nucleated because of a marked tendency for aggregation, and in which patch reefs and bioherms occur stacked together bounded by clay–marl–silt seams.     

寒武系烃源古油藏油裂解气特征及成藏条件

在寒武系为母源的气藏中,普遍存在的固体沥青、25-降藿烷、氮气等三种物质,实际是古油藏残余油裂解的产物。25-降藿烷一般与氧化—降解油有关,氮气来自油藏水洗时溶入的大气,二者共同反映古油藏经历的抬升破坏作用。寒武系烃源一般经历过两次成藏:加里东—海西期形成“油藏”,燕山—喜马拉雅期裂解为“气藏”。寒武系油裂解气成藏的地质条件是:古构造聚油、后期改造适度、叠合盆地增熟、断层作为油气运移通道等。围绕加里东—海西期古油藏寻找原生、次生油裂解气是今后寻找寒武系来源油气的重要途径。     

华北克拉通西部临县紫金山碱性岩体地球化学特征及地质意义

山西吕梁地区出露的紫金山岩体为我国著名的碱性岩体之一.为了探讨紫金山碱性岩体的物质来源及地质意义,对该岩体主要岩石进行了岩相学及岩石地球化学研究.结果表明:紫金山岩体各期次岩石具有低硅、富碱、高钾的特点,属钾质碱性岩石;岩石轻重稀土元素分馏较强烈,具有轻稀土相对富集、重稀土相对亏损、无明显Eu异常的特征;微量元素总体表现为大离子亲石元素(Rb、Ba、K、Sr等)明显富集、高场强元素(Th、Nb、Ta、La、Ce、Nd、P、Ti等)亏损的特征.紫金山岩体岩浆来源较深,与富集地幔关系密切,岩浆演化机制以部分熔融作用为主且存在陆壳物质混染.结合山西境内多处同时期的碱性、偏碱性岩体特征,推断华北中部早白垩世碱性岩浆活动与古太平洋板块俯冲背景下华北克拉通破坏作用关系密切.     

江西萍乐坳陷带西段早三叠世地层划分与对比及沉积环境分析

萍乡—宜春一带地处萍乐坳陷带西段,早三叠世地层广泛发育,长期以来对其地层单元划分、时代归属、分布范围、名称使用均不统一,给萍乐坳陷西段的研究工作带来诸多问题。笔者通过萍乐坳陷带西段萍乡—宜春一带1∶5万区域地质调查,首次系统地从岩石地层、生物地层方面对早三叠世地层进行了划分与对比,将该地区早三叠世地层自下而上划分为:殷坑组、青龙组、周冲村组,又将周冲村组进一步划分为四个段,并依据牙形石成果资料,将殷坑组、青龙组、周冲村组一段定为印度阶,周冲村组二段至四段定为奥伦尼克阶。同时根据岩性组合及沉积相特征对其沉积环境进行了分析,认为该地区早三叠世沉积环境为"台-坡-盆"古地貌结构的"陆缘海"。