Orthopyroxene minettes and lamproites: their status and genetic significance

Orthopyroxene is a normal if rare, constituent of some minettes and lamproites. It is sometimes partly xenocrystic, but mostly authigenic, i.e. separated directly from lamprophyric magmas, as can be shown when orthopyroxene lamprophyres are compared with the experimental phase diagrams of appropriate systems. The orthopyroxene-bearing lamprophyres represent modified mantle-derived magmas characterized by a high MgO, high SiO₂ speciation which was acquired in different ways in individual regions. The crystallization of orthopyroxene lamprophyres was not controlled by particular pressure — temperature conditions. Compared with clinopyroxene potassic lamprophyres the orthopyroxene lamprophyres represent more primitive magmas. Orthopyroxene minettes can also be altered by a post-magmatic hydrothermal inflow, which occasionally increases their CaO content to the level normal in clinopyroxene minettes. If this had happened before magma consolidation, clinopyroxene instead of orthopyroxene would have crystallized. Orthopyroxene minettes are well defined mineralogically and chemically and may be related to other types in the minette series.     

Extreme Nd isotope homogeneity in a large rhyolitic province: the Estérel massif, southeast France

 Previous detailed studies of large rhyolite bodies propose that their elemental and isotopic characteristics were largely acquired in shallow crustal magma chambers. This model explains the common chemical and isotopic zonations of large volumes of rhyolites as well as the less common chemical and isotopic homogeneity of such bodies. We report an intermediate situation (the Estérel massif, southeast France) in which chemical variations contrast with Nd-isotope homogeneity. We thus infer that, in this case, large volumes of rhyolite resided for enough time in shallow magma chambers to develop chemical zonations through differentiation, but this process was not accompanied by crustal assimilation. The subordinate amount of mafic rocks cropping out in the Estérel probably evolved from basalt to trachyte through assimilation and fractional crystallization. The relatively radiogenic Nd-isotope signatures of the rhyolite compared with the Hercynian crust show that it cannot have been generated by partial melting of exposed basement rocks. Several geological similarities with large rhyolitic provinces could suggest that the rhyolite was purely mantle derived or, alternatively, generated by partial melting of an ad hoc crustal component. However, mineralogical, geochemical, and geodynamic connections between the Estérel rhyolite and the hypersolvus anorogenic granites of Corsica, as well as the extreme Nd-isotope homogeneity of the rhyolite, lead us to propose that the rhyolite was generated by mixing between mantle-derived magmas and a mafic lower crust. This scenario accounts for the relatively radiogenic Nd-isotope signatures of the rhyolite compared with the Hercynian crust. The good Nd-isotope homogeneity observed in the rhyolite implies that the mixing process, which occurred in the deep crust, was complete and provided a shallow magma chamber with isotopically and probably chemically homogeneous magmas. Received: 5 December 1997 / Accepted: 16 June 1998     

K–Ar geochronology of a Quaternary monogenetic volcano group in Ojika Jima District, Southwest Japan

 An unspiked K–Ar dating method using a mass-fractionation correction procedure was applied to a Quaternary independent group of monogenetic volcanoes, Ojikajima Volcano Group, located in northwestern Kyushu in Southwest Japan, in order to clarify in detail secular variations in eruptive volume, locations of eruptive vents, and magma compositions in a single monogenetic volcano group. The major results were as follows: (a) K–Ar ages of monogenetic volcanoes distribute from 1.08 to 0.30 Ma, with voluminous peaks at approximately 1.0 and 0.6 Ma. (b) The volcanic activity commenced in the central part of the field, expanded to the whole field at approximately 0.6 Ma, and then shrank to the central area. (3) Concentrations of incompatible elements, such as Ba, K, and Nb, increase with decreasing age, whereas P, Y, and Zr concentrations remain constant. These concentrations suggest gradual decrease in the degree of partial melting from an identical mantle source with residual garnet. Received: 15 December 1997 / Accepted: 23 May 1998     

火山与地震的共性特征及有关问题讨论

对火山与地震及其伴随现象进行了对比分析,发现二者除外在形式不同外,在分布范围、深部构造及其它方面具有明显的共性特征．讨论了火山与地震具有共性特征的原因,认为它们都与岩浆活动有关．因此,内陆地震的发生有可能是地壳内岩浆上涌的直接结果     

Temporal variations in gravity at Mt. Etna (Italy) associated with the 1989 and 1991 eruptions

 Results are presented from 11 microgravity surveys on Mt. Etna between 1987 and 1993, a period including the major 1989 and 1991–1993 flank eruptions and subordinate 1990 activity. Measurements were made with LaCoste and Romberg D-62 and D-157 gravity meters along a network around the volcano between 1000 and 1900 m a.s.l. and, since 1992, a N–S summit profile. Gravity changes of as much as 200 μGal were observed at scales from the size of the summit region to that of the volcano. None was associated with significant changes in ground elevation. The data show an increase in gravity for 2 years before the 1989 eruption. The increase is attributed to the accumulation of magma (0.25–1.7×10⁹m³) in an elongate zone, oriented NNW–SSE, between 2.5 and 6 km below sea level. Part of this magma was injected into the volcanic pile to supply the 1989 and 1990 eruptions. It also probably fed the start of the 1991–1993 eruption, since this event was not preceded by significant gravity changes. A large gravity increase (up to 140 μGal) detected across the volcano between June and September 1992 is consistent with the arrival in the accumulation zone of 0.32–2.2×10⁹m³ of new magma, thus favoring continued flank effusion until 1993. A large gravity decrease (200 μGal) in the summit region marked the closing stages of the 1991–1993 event and is associated with magma drainage from the upper levels of Etna's central feeding system. Received: 15 July 1995 / Accepted: 27 October 1997     

中国铜镍硫化物矿床类型及控矿条件

中国铜镍硫化物矿床控制的关键因素是岩浆岩岩石组合和构造环境,并以此作为划分矿床类型的依据。划分为过渡带内、地槽区内和与火山活动有关的侵入体类矿床三大类和１６种岩石组合类型。并从大地构造环境、深（大）断裂、成岩时代、复式岩体、岩石组合、岩体规模产状、岩石化学、稀土元素、岩体分异程度、造岩矿物和岩石蚀变等１０个方面,论述对铜镍硫化物矿床的控制作用。     

Numerical modelling of dyke interaction and its influence on oceanic crust formation

Dykes are an essential element in building oceanic crust, most prominent in sheeted dyke complexes in the upper crust. Since dykes alter the magnitude and orientation of the local stress field, they cannot be treated as passive infillings of extensional fractures.We use a quasi-static, iterative 2-D boundary element method allowing for a wholesale movement of fluid-filled fractures. Effects of stress and pressure gradients, buoyancy and enclosed fluid mass are considered. The implications of the dyke-induced stress field are analysed combining the simulation of fracture propagation with computation of dyke interaction. Dyke interaction occurs by the adaptation of ascending dykes to the stress field caused by previous fractures arrested in the crust and leads to focussing and crossing of dykes. Examples for applications are introduced, concerning e.g. the generation of a magma chamber and the formation of the sheeted dyke complex. Our main results are that the interaction between dykes can be considerable and that the most important controlling factor is stress. The interaction is small when the horizontal tensional stress is large compared to the pressure in the dyke head. Otherwise, dykes tend to attract each other and to form centres of high dyke density or sill layers.     

辽西义县组火山岩:拆沉作用还是岩浆混合作用的产物?

通过对所有已发表的辽西义县组火山岩数据进行系统的主、微量元素和同位素分析,发现义县组火山岩以中基性为主,其上部层位中酸性组分增多。其高的Mg~#值和埃达克质特征并不是原始岩浆本身的性质,不能作为下地壳拆沉作用的有力证据。义县组火山岩的基性部分主要来自于受洋壳俯冲作用改造过的岩石圈地幔,并存在与古老下地壳来源的熔体不同程度的混合作用。因此,义县组火山岩是岩浆混合作用的产物而非下地壳拆沉作用的产物。底侵的下地壳也参与了义县组火山岩的形成,但软流圈组分的参与有限。     

再论兴隆沟组火山岩成因

本文通过对辽西北票地区兴隆沟组安山质火山岩的组分和岩相学研究,发现作为捕虏晶的斜方辉石存在反环带和韵律环带.根据具反环带结构的斜方辉石的微量元素丰度研究,提出兴隆沟组火山岩是岩浆混合作用的产物而非下地壳拆沉作用的产物.对比华北克拉通北缘不同时代,不同成因的单斜辉石,推测辉石有可能来源于早中生代下地壳的堆晶岩.     

Social mix revisited: within- and across-neighborhood ties between ethnic minorities of differing socioeconomic backgrounds

Creating social mix in neighborhoods has been an important urban policy during the past years. The aim is the development of category-crossing ties between the middle and lower classes. However, studies found little evidence of such ties. This paper identifies two reasons why this might be the case. First, studies usually analyze the resource flow from native middle classes to disadvantaged residents. Second, the resource flow is analyzed within one neighborhood only. Based on interviews and network analysis with second generation, upwardly mobile Turkish-Germans in Berlin, I show that they act as cultural and language brokers, benefiting lower class co-ethnics. They provide exactly the information and resources to people with a lower class background that are envisioned by social mixing programs. Moreover, the resource flow occurs within but also across neighborhoods, whereby ‘second neighborhoods’ such as the previous neighborhood or the neighborhood where the workplace is located are particularly important.