K-Ar ages of intrusive rocks in the Oban-Obudu massif and their significance for the tectonic and plutonic history of southeastern Nigeria

Abstract K-Ar ages have been obtained for mineral separates: plagioclases from two dolerites and biotites from one granite and four granodiorites in the Oban-Obudu massif of southeastern Nigeria. Dolerites in the Oban area give K-Ar plagioclase ages of 204.0 ± 9.9 Ma and 219.9 ± 4.7 Ma. The granite of the Obudu area yields a K-Ar biotite age of 507.6 ± 10.1 Ma whilst the granodiorites in the Oban area yield K-Ar biotite ages ranging from 474.6 ± 9.4 Ma to 511.8 ± 10.0 Ma. The dolerites are related to the tholeiitic basaltic magmatism at the early opening of the central Atlantic, and are compatible in age with the formation of the Ring Complexes and the rifting of the Benue Trough of Nigeria. The granites and granodiorites belong to the syntectonic Older Granite series in the Pan-African orogeny. The emplacement timing deduced from the reported Pb-Pb zircon age of 617 ± 2 Ma and the newly obtained biotite ages suggest that these older granites in southeastern Nigeria had a prolonged cooling history of ∼110 Ma.     

澜沧江中下游斜坡稳定性分区与综合评价

本文根据澜沧江中下游斜坡稳定性模糊综合评判结果，结果斜坡变形破坏特征，将澜沧江中下游地区划分出二个大区和十个亚区，并综合评述了各区动力环境特征和斜坡稳定状况。     

The role of deformation partitioning in the deformation and recrystallization of plagioclase and K-feldspar in the Woodroffe Thrust mylonite zone, central Australia

ABSTRACT In the Woodroffe Thrust mylonite zone, central Australia, recrystallization in plagioclase and K-feldspar involved subgrain rotation, assisted by grain-boundary or kink band boundary bulging, without contribution from a change in the chemical composition from host grains to new grains. The size of subgrains and new grains changes across the mylonite zone, apparently as a function of the strain rate and the H2O content of the rock. The partitioning of deformation into zones of progressive shearing and progressive shortening controls the sites of recovery and recrystallization in feldspar during mylonitization. The size of feldspar porphyroclasts in well developed mylonites is governed by the scale of deformation partitioning reached in the earlier stages of mylonitization, before the formation of a large proportion of fine-grained matrix that can accommodate the progressive shearing component of the deformation. Recrystallization occurs in microcline, apparently without involving a translation to a monoclinic structure, as microcline-twinned new grains are common adjacent to microcline-twinned host grains. K-feldspar triclinicity values calculated from XRD traces increase from the margins to the interior of the mylonite zone, in conjunction with deformation intensity. K-feldspar host grains locally have cores of orthoclase or untwinned microcline, surrounded by mantles of twinned microcline, suggesting a relationship between the presence of microcline twinning and the degree of K-feldspar triclinicity.     

菱切面角温度计作为形变温度计在五台山区的应用

肖钠双晶主要在形变过程中产生,它的结合面是菱形切面。菱形切面的位置既取决于斜长石的化学成分,也取决于它的结构状态。后者本身就是温度的函数。因此,菱切面的位置取决于形变时的温度。在五台山区峨口花岗岩内获得了49个菱切面角温度值。从直方图上可以看出存在有520℃和480℃的两个峰。这温度与围岩的石榴石—黑云母对494—508℃的温度对应。它意味着形变温度与绿片岩相变质温度一致。在五台山区绿片岩相变质时观察到两期形变作用,这与两个温度峰相符。因此,利用菱切面角温度计可以研究形变与变质的关系。     

Seismic Ground Motion Zoning Maps of the Pangxi Region

The seismotectonic environment and seismic activity in Southwest China region were studied based on new data and new results obtained during the Eighth and Ninth Five-Year Plans,the seismic areas and zones and potential seismic source zones were determined.and the relation between seismic activity parameters and ground motion attenuation was determined.Finally the seismic gound motion zoning maps of the Pangxi region was compiled by using the multi-parameter and multi-scheme method.     

科科斯脊玄武岩斜长石矿物化学及地质意义

综合大洋钻探计划(IODP) 334和344航次在U1381站位处的两个钻孔(A孔和C孔)获得了中美洲西海岸外科科斯脊基底拉斑玄武岩,对其岩浆过程开展研究可为理解其岩石成因提供重要依据。本文对科科斯脊玄武岩中斜长石斑晶和微晶进行了详细的原位主微量元素分析,结果表明,斜长石种属为培长石、拉长石及少量中长石。部分斜长石斑晶具有正环带结构;但多数斜长石斑晶不具有明显环带,仅从核部到边部存在微弱的成分变化。斜长石斑晶与微晶的微量元素差别较大:斜长石斑晶富集轻稀土和大离子亲石元素、亏损高场强元素,且具有明显的Eu正异常;斜长石微晶不相容元素含量通常高于斜长石斑晶。根据斜长石温度计计算获得斜长石斑晶结晶温度为1 050～1 253℃,斜长石微晶结晶温度为866～1 033℃。基于以上特征,推测斜长石斑晶核部是相对原始岩浆的产物,而斑晶边部以及微晶是演化岩浆的结晶产物。斜长石斑晶的成分变化及熔蚀麻点结构是由于岩浆补给及岩浆减压上升造成的。最后,本研究推测科科斯脊基底玄武岩来自于开放的岩浆房,且岩浆房内可能存在原始岩浆的不断注入及岩浆对流。     

攀枝花市大田地区古元古代康定岩群斜长角闪岩LA-ICP-MS锆石U-Pb年龄及岩石地球化学特征

康定岩群出露在四川康定—西昌—攀枝花—云南元谋一带,前人曾测得过较多的康定岩群太古代—古元古代同位素年龄,随着工作的深入,亦获得大量的青白口纪同位素年龄.本文对出露于攀枝花大田地区的康定岩群咱里岩组斜长角闪岩进行岩石学、岩石地球化学特征研究,同时进行了 LA-ICP-MS锆石U-Pb年龄分析.结果表明,康定岩群咱里岩组...     

Empirical calibration of the silica–Ca-tschermak's–anorthite (SCAn) geobarometer

The pressure-sensitive equilibrium among anorthite, quartz and the Ca-tschermak component in clinopyroxene (CaAl₂SiO₆; CaTs), CaAl₂SiOCpx6+SiOQtz2=CaAl₂Si₂OPl8 (SCAn) ,can be used as a geobarometer in granulites with the proper assemblage, and has been calibrated using mineral composition data from partial melting experiments of natural assemblages and from phase equilibrium experiments on the end-member CMAS system. The experimental data cover the P – T  range 4–32  kbar and 900–1400  °C. Linear least-squares regression analysis of the experimental data resulted in the following empirical expressions for pressure in terms of composition and temperature: P = 5.066 [±0.760]+ 1300 [±800] T  −ln K 276 [±16] · T  [±2.5  kbar]or P = 6.330 [±0.116]−ln K 301 [±9]· T  [±1.0  kbar] ,where K = a PlAn a CpxCaTs  .The first equation incorporates an enthalpy term, but is less accurate than the second equation, in which the enthalpy of reaction is ignored. Application of these expressions to natural and experimental equilibrium mineral assemblages demonstrates that the empirical barometers are applicable over a wide range of pressures (≥4  kbar), temperatures (≥700  °C) and bulk compositions (Mg#≥32.5).     

Deformation-enhanced metamorphic reactions and the rheology of high-pressure shear zones, Western Gneiss Region, Norway

Microstructural and petrological analysis of samples with increasing strain in high‐pressure (HP) shear zones from the Haram garnet corona gabbro give insights into the deformation mechanisms of minerals, rheological properties of the shear zone and the role of deformation in enhancing metamorphic reactions. Scanning electron microscopy with electron backscattering diffraction (SEM–EBSD), compositional mapping and petrographic analysis were used to evaluate the nature of deformation in both reactants and products associated with eclogitization. Plagioclase with a shape‐preferred orientation that occurs in the interior part of layers in the mylonitic sample deformed by intracrystalline glide on the (0 0 1)[1 0 0] slip system. In omphacite, crystallographic preferred orientations indicate slip on (1 0 0)[0 0 1] and (1 1 0)[0 0 1] during deformation. Fine‐grained garnet deformed by diffusion creep and grain‐boundary sliding. Ilmenite deformed by dislocation glide on the basal and, at higher strains, prism planes in the a direction. Relationships among the minerals present and petrological analysis indicate that deformation and metamorphism in the shear zones began at 500–650 °C and 0.5–1.4 GPa and continued during prograde metamorphism to ultra‐high‐pressure (UHP) conditions. Both products and reactants show evidence of syn‐ and post‐kinematic growth indicating that prograde reactions continued after strain was partitioned away. The restriction of post‐kinematic growth to narrow regions at the interface of garnet and plagioclase and preservation of earlier syn‐kinematic microstructures in older parts layers that were involved in reactions during deformation show that diffusion distances were significantly shortened when strain was partitioned away, demonstrating that deformation played an important role in enhancing metamorphic reactions. Two important consequences of deformation observed in these shear zones are: (i) the homogenization of chemical composition gradients occurred by mixing and grain‐boundary migration and (ii) composition changes in zoned metamorphic garnet by lengthening diffusion distances. The application of experimental flow laws to the main phases present in nearly monomineralic layers yield upper limits for stresses of 100–150 MPa and lower limits for strain rates of 10^?12 to 10^?13 s^?1 as deformation conditions for the shear zones in the Haram gabbro that were produced during subduction of the Baltica craton and resulted in the production of HP and UHP metamorphic rocks.     

气候变化对中国农业生产的影响

根据１９５８￣１９９７年中国３１０个气象站的月平均最高、最低气温,月平均降水量,日照时数等气象参数分析了中国气候的背影特征。用３个大气环流模型,即ＨａｄＣＭ２、ＣＧＣＭ１、ＥＣＨＡＭ４来模拟未来中国气候变化的情景。同时,建立未来气候变化的６种方案。最后,根据３个ＧＣＭｓ模拟的结果,利用国际应用系统分析研究所（ＩＩＡＳＡ）发展并改进的农业生态地带模型（ＡＥＺ）评估气候变化对中国农业生产的影响。评估主