An experimental study on the shallow-level migmatization of ferrogabbros from the Fuerteventura Basal Complex, Canary Islands

Spectacular shallow-level migmatization of ferrogabbroic rocks occurs in a metamorphic contact aureole of a gabbroic pluton of the Tierra Mala massif (TM) on Fuerteventura (Canary Islands). In order to improve our knowledge of the low pressure melting behavior of gabbroic rocks and to constrain the conditions of migmatization of the TM gabbros, we performed partial melting experiments on a natural ferrogabbro, which is assumed as protolith of the migmatites. The experiments were performed in an internally heated pressure vessel (IHPV) at 200 MPa, 930–1150 °C at relatively oxidizing conditions. Distinct amounts of water were added to the charge.

From 930 to 1000 °C, the observed experimental phases are plagioclase (An_60–70), clinopyroxene, amphibole (titanian magnesiohastingsites), two Fe–Ti oxides, and a basaltic, K-poor melt. Above 1000 °C, amphibole is no longer stable. The first melts are very rich in normative plagioclase (>70 wt.%). This indicates that at the beginning of partial melting plagioclase is the major phase which is consumed to produce melt. In the experiments, plagioclase is stable up to high temperatures (1060 °C) showing increasing An content with temperature. This is not compatible with the natural migmatites, in which An-rich plagioclase is absent in the melanosomes, while amphibole is stable. Our results show that the partial melting of the natural rocks cannot be regarded as an “in-situ” process that occurred in a closed system. Considerable amounts of alkalis probably transported by water-rich fluids, derived from the mafic pluton underplating the TM gabbro, were necessary to drive the melting reaction out of the stability range of plagioclase. A partial melting experiment with a migmatite gabbro showing typical “in-situ” textures as starting material supports this assumption.

Crystallization experiments performed at 1000 °C on a glass of the fused ferrogabbro with different water contents added to the charge show that generally high water activities could be achieved (crystallization of amphibole), independently of the bulk water content, even in a system with very low initial bulk water content (0.3 wt.%). Increasing water contents produce plagioclase richer in An, reduces the modal proportion of plagioclase in the crystallizing assemblage and extends the melt fraction. High melt fractions of >30 wt.% could only be observed in systems with high bulk water contents (>2 wt.%). This indicates that the migmatites were generated under water-rich conditions (probably water-saturated), since those migmatites, which are characterized as “in-situ” formations, show generally high amounts of leucosomes (>30 wt.%).     

Geomechanical Properties of Shear Zones in the Eastern Aar Massif,Switzerland and their Implication on Tunnelling

Summary ¶Rock zones containing a high fracture density and/or soft, low cohesion materials can be highly problematic when encountered during tunnel excavation. For example in the eastern Aar massif of central Switzerland, experiences during the construction of the Gotthard highway tunnel showed that heavily fractured areas within shear zones were responsible for overbreaks in the form of chimneys several metres in height. To understand and estimate the impact of the shear zones on rock mass behaviour, knowledge concerning the rock mass strength and deformation characteristics is fundamental. A series of laboratory triaxial tests, performed on samples from granite- and gneiss-hosted shear zones revealed that with increasing degree of tectonic overprint, sample strength decreases and rock behaviour shows a transition from brittle to ductile deformation. These trends may be explained by increasing fracture densities, increasing foliation intensity, increasing thickness of fine-grained, low cohesion fracture infill, and increasing mica content associated with the increasing degree of tectonic overprint. As fracture density increases and the influence of discrete, persistent discontinuities on rock mass strength decreases, behaviour of the test samples becomes more and more representative of rock mass behaviour, i.e. that of a densely fractured continuum. For the purpose of numerical modeling calculations, the shear zones may be subdivided with respect to an increasing fracture density, foliation intensity and mica content into a strongly foliated zone, a fractured zone and a cohesionless zone, which in turn exhibit brittle, brittle-ductile and ductile rock mass constitutive behaviour, respectively.Received December 17, 2001; accepted January 9, 2003 Published online April 29, 2003     

动物疾病发病规律及其与气象条件关系的初步分析

通过对1975～1998年北京动物园动物发病率的统计分析，揭示了不同动物发病率的年变化特征和长期变化趋势，并分季节进行了发病率与气象因子的相关分析，发现冬季兽类发病率、秋季禽类发病率与同期气象因子相关较好，总体看春、夏季动物发病率与气象因子的关系不明显。     

封丘县夏蝗发生与气候条件的关系

利用1980～200l午封丘县夏蝗发生资料和同期该地区气候资料，采用合成分析法，分析了气候条件对夏蝗发生的影响，并找出了一些预测夏蝗发生的定性预报因子。     

洋脊和年轻洋壳俯冲与埃达克岩生成的热模拟

新生代以来 ,环太平洋周边分布的埃达克岩 (Adakite)主要与年轻洋壳俯冲时在 70～ 90km深处的部分熔融有关。利用数值方法 ,模拟了洋壳俯冲的热演化过程并讨论了脱水、熔融对埃达克岩浆活动的影响。结果表明 :仅在活动海岭俯冲前后约 10Ma内 ,年轻的、热的俯冲海洋板片在 75～85km深度范围内 ,温度升高至 82 5～ 10 0 0℃脱水 ,导致年轻洋壳中角闪岩部分熔融 ,形成埃达克岩(Adakite)。而一般洋壳俯冲在 10 0km以下深度才脱水 ,由于脱水区压力较高洋壳自身不能熔融 ,水进入上覆地幔楔状体导致部分熔融 ,形成安山岩 (Andesite     

对波达波夫和Pride震电波方程组的对比分析

用Biot介质参数说明了波达波夫震电波方程组中弹性动力学 参数的含义，解释了第一类和第二类震电效应的意义，在忽略第一类震电效应条件下将该方 程组与Pride方程组进行比较，说明了二者在描述第二类震电效应方面的异同点. 同时指出 ：波达波夫方程组忽略了流体与固体的耦合质量；方程中的黏性耗散项丢掉了一个孔隙度因 子，依据该方程组计算出的弹性波和转换电场的幅度将偏大；边界条件之一存在错误，会影 响对波在界面上的反射透射规律的描述.     

ω循环型边界条件

通过利用预条件共轭梯度法对对称正定Toeplitz 矩阵系统进行分析，重点介绍了一种新的嵌入式预条件矩阵构造方法，证明了以前的预条件矩阵构造方法大都是这种方法的特例.提出了ω循环型边界条件，并将其与普通循环型及螺旋型边界条件作了分析、比较后得到了一种新的边界条件即混合型边界条件.     

一类热流问题解的存在性

热敏半导体器件中电位与温度满足的方程组，是一个椭圆－抛物耦合组，并带有混合边界条件。利用不动点定理可证明该问题解的存在性。     

Partial melting during tectonic exhumation of a granulite terrane: an example from the Larsemann Hills, East Antarctica

Anatectic migmatites in medium- to low-pressure granulite facies metasediments exposed in the Larsemann Hills, East Antarctica, contain leucosomes with abundant quartz and plagioclase and minor interstitial K-feldspar, and assemblages of garnet–cordierite–spinel–ilmenite–sillimanite. Qualitative modelling in the system K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O₂, in conjunction with various P–T calculations indicate that the high-grade retrograde evolution of the terrane was dominated by decompression from peak conditions of c. 7 kbar at c. 800 °C to 4–5 kbar at c. 750 °C. Extensive partial melting during decompression involved the replacement of biotite by the assemblage cordierite–garnet–spinel within the leucosomes. These leucosomes represent the site of partial melt generation, the cordierite–garnet–spinel–ilmenite assemblage representing the solid products and excess reactants from the melting reaction. The extraction and accumulation of this decompression-generated melt led to the formation of syntectonic pegmatites and extensive granitic plutons. Leucosome development and terrane decompression proceeded during crustal transpression, synchronous with upper crustal extension, during a progressive Early Palaeozoic collisional event. Subsequent retrograde evolution was characterized by cooling, as indicated by the growth of biotite replacing spinel and garnet, thin mantles of cordierite replacing spinel and quartz within metapelites, and garnet replacing orthopyroxene and hornblende within metabasites. P–T calculations on late mylonites indicate lower grade conditions of formation of c. 3.5 kbar at c. 650 °C, consistent with the development of late cooling textures.     

我国黄铁矿矿床类型、分布规律及前景评估

中国黄铁矿资源丰富，按元素和矿物组合的不同，其矿床类型可分为：黄铁矿矿床；黄铁矿型铜矿床；含锌、铜黄铁矿型矿床；黄铁矿型多金属矿床；含钨黄铁矿型铜矿床；含锡黄铁矿型矿床；含金黄铁矿型矿床。最主要的成因类型是海底喷气沉积矿床，它广泛分布于不同时期的造山带及早前寒武纪克拉通。黄铁矿形成的构造环境可分为５种，即：边缘浅海或优地槽环境；上叠的断裂坳陷带；洋中脊；岛弧地带；陆相火山盆地。