The role of syn-eruptive vesiculation on explosive basaltic activity at Mt. Etna,Italy

We investigated the dynamics of explosive activity at Mt. Etna between 31 August and 15 December 2006 by combining vesicle studies in the erupted products with measurements of the gas composition at the active, summit crater. The analysed scoria clasts present large, connected vesicles with complex shapes and smaller, isolated, spherical vesicles, the content of which increases in scoriae from the most explosive events. Gas geochemistry reports CO₂/SO₂ and SO₂/HCl ratios supporting a deep-derived gas phase for fire-fountain activity. By integrating results from scoria vesiculation and gas analysis we find that the highest energy episodes of Mt. Etna activity in 2006 were driven by a previously accumulated CO₂-rich gas phase but we highlight the lesser role of syn-eruptive vesicle nucleation driven by water exsolution during ascent. We conclude that syn-eruptive vesiculation is a common process in Etnean magmas that may promote a deeper conduit magma fragmentation and increase ash formation.     

Numerical comparison of different convergent plate contacts: subduction channel and subduction fault

At convergent plate boundaries, the properties of the actual plate contact are important for the overall dynamics. Convergent plate boundaries both mechanically decouple and link tectonic plates and accommodate large amounts of strain. We investigate two fundamental physical states of the subduction contact: one based on a fault and the other based on a subduction channel. Using a finite element method, we determine the specific signatures of both states of the subduction contact. We pay particular attention to the overriding plate. In a tectonic setting of converging plates, where the subducting plate is freely moving, the subduction channel reduces compression relative to the fault model. In a land-locked basin setting, where the relative motion between the far field of the plates is zero, the subduction channel model produces tensile stress regime in the overriding plate, even though the amount of slab roll-back is small. The fault model shows a stronger development of slab roll-back and a compressive stress regime in the upper plate. Based on a consistent comparison of fault and channel numerical models, we find that the nature of the plate contact is one of the controlling factors in developing or not of backarc extension. We conclude that, the type of plate contact plays a decisive role in controlling the backarc state of stress. To obtain backarc extension, roll-back is required as an underling geodynamic process, but it is not always a sufficient condition.     

不同类型热液金矿系统的流体包裹体特征

为使流体包裹体研究结果得到较好的解释,避免矿床地质描述与流体包裹体研究结果发生矛盾,本文试图以金矿床为例,建立科学而简便易行的矿床地质与包裹体特征之间的链接。为此,本文简单评述了现有金矿床成因分类方案,建议以主导成矿系统发育的地质作用特征划分5种类型:①浆控高温热液型,包括斑岩型、爆破角砾岩型、铁氧化物型、夕卡岩型等岩浆热液型矿床;②造山型,即变质热液型;③浅成低温热液型——陆相火山岩-次火山岩中的改造热液型;④微细粒浸染型(卡林型或/类卡林型)——沉积岩容矿的改造热液型;⑤热水沉积型(VMS 型和 SEDEX 型)——水下喷出地表的改造热液型。然后,分别介绍了5类成矿系统的标志性地质和流体包裹体特征,找出了它们之间具有成因标志意义的关键性差异;将成矿流体分为改造、变质和岩浆3个端元性成分,发现多数热液矿床具有多阶段多因复成的特点,晚阶段流体均为改造热液或有大量改造热液注入,因此指出,晚阶段的流体、蚀变和矿化特征不能用于判别矿床成因和类型,只有早阶段的特征才能准确指示矿床成因和类型。改造热液以低温、低盐度、低 CO_2含量为特征,主要来自大气降水和/或海水;变质热液以中温、低盐度、高 CO_2含量为特征,而岩浆热液则以高温、高盐度、高 CO_2含量为特征;岩浆热液矿床发育含多种子晶包裹体和高盐度富 CO_2的包襄体,变质热液矿床发育低盐度富 CO_2包裹体,改造热液矿床总体缺乏含子晶包裹体和富/含 CO_2包襄体,大量发育水溶液包裹体。最后,讨论了各类成矿系统发育的岩石圈构造背景,如造山型矿床形成于地壳挤压造山-变质-隆升过程,热水沉积型矿床形成于地壳拉张成盆过程,古生代或更早的浅成低温热液型矿床只能保存在增生型造山带等,提出矿床及其包裹体是研究大陆动力学的理想探针。     

Geodynamic settings and tectonic model of skarn gold deposits in China: An overview

Seventy skarn-type gold deposits, including 1 super-large, 19 large and 24 medium-sized, are known from different geotectonic units of China. They contain a total resource of approximately 1000 t of gold (625 t in South China), and account for 20% of China's gold reserves. These skarn deposits are sited in collisional orogenic belts, fault-controlled magmatic belts and reactivated cratonic margins. All of the Chinese skarn gold provinces were affected by Phanerozoic collisional orogenesis. The timing of the metallogenic events and the spatial–temporal distribution of the Chinese skarn gold deposits indicates that they were formed during ore-forming processes linked to the transition from shortening to extension in the geodynamic evolution of a collision orogen, and not to subduction systems as is commonly advocated for porphyry copper systems around the Pacific Rim.     

Geological and geochemical characteristics of the Baimazhai Ni-Cu-（PGE） sulphide deposit in Yunnan, China

The sulphide ores of the Baimazhai deposit, although typically orthomagmatic, locally exhibit peculiar textural features and are intimately associated with hydrothermal minerals, such as biotite, amphibole and chlorite. This association suggests that the magmatic sulphide ores were subjected to hydrothermal alteration and subsequent redistribution, resulting in the observed textural features. Geochemically, the Baimazhai sulphide ores are enriched in Cu, Pd and Au, which,according to previous studies, reflects the action of hydrothermal fluids. Interestingly, Ar-Ar dating yielded the plateau ages of about 160–170 Ma, which are at odds with the established Permian age of the Emeishan large igneous province. We interpreted these younger ages as due to thermal resetting during post-Permian tectonothermal events. We have proposed a model in which tectonic movements and hydrothermal fluids related to these events modified the pre-existing magmatic sulphides. Given the degree of overprint, we suggested two possible scenarios: 1) the sulphide disseminations that surround the massive magmatic ores are the result of deformation and hydrothermal alteration; and 2) there were both magmatic massive and disseminated sulphides, in which case the scale and relocation of remobilization would have been smaller, but still detectable.     

Thermal springs,fumaroles and gas vents of continental Yemen: Their relation with active tectonics,regional hydrology and the country’s geothermal potential

Most thermal springs of continental Yemen (about 65 emergences at 48 sampling sites) and a couple of fumaroles and boiling water pools have been sampled and analyzed for chemical and isotopic composition in the liquid phase and the associated free-gas phase. Whatever the emergence, all the water discharges have an isotopic signature of meteoric origin. Springs seeping out from high altitudes in the central volcanic plateau show a prevalent Na–HCO₃-composition, clearly affected by an anomalous flux of deep CO₂ deriving from active hydrothermal systems located in the Jurassic Amran Group limestone sequence and/or the Cretaceous Tawilah Group, likely underlying the 2000–3000 m thick volcanic suite. At lower elevations, CO₂ also affects the composition of some springs emerging at the borders of the central volcanic plateau.     

An earthquake scenario for the microzonation of Sofia and the vulnerability of structures designed by use of the Eurocodes

The study of the site effects and the microzonation of a part of the metropolitan Sofia, based on the modelling of seismic ground motion along three cross-sections are performed. Realistic synthetic strong motion waveforms are computed for scenario earthquakes (M=7) applying a hybrid modelling method, based on the modal summation technique and finite differences scheme. The synthesized ground motion time histories are source and site specific. The site amplification is determined in terms of response spectra ratio (RSR). A suite of time histories and quantities of earthquake engineering interest are provided. The results of this study constitute a “database” that describes the ground shaking of the urban area. A case study of experiment-based assessment of vulnerability of a cast-in-situ single storey, industrial, reinforced concrete frame, designed according to Eurocodes 2 and 8 is presented. The main characteristics of damage index and storey drift are discussed for the purposes of microzonation.