福建火山岩型铀矿化特征及其形成机制

福建 地处我国东南沿海，火山岩分布面积广，火山岩型铀矿化类型多，包括火山沉积型、远温地下热水淋滤型、火山期后热液型、富铀次火山岩体热液交代型等。铀矿化表现为多形态、多产状、多部位和多期、 多阶段，矿物 组合简单等特征。赋铀火山岩普遍发育有水解作用，岩石水解后，铀含量增加5－8倍，而分散铀量则减少50－80％。表明火山岩系中铀的活化与岩石的水解作用关切密切。另外，区内火山岩型铀矿化均赋存于火山杂岩系中的反差效应场中，显示出火山杂岩系中的反差效应场与铀的矿化也有密切关系。从总体上看，水解活化、渗析对流和半封闭强反差是福建火山岩型铀矿化的重要成矿机制。     

赣东南地区火山地质、矿产特征及找矿方向

赣东南火山岩区主要发育侏罗纪中、晚世中酸性火山碎屑岩－火山熔岩，其中以爆溢相产出的流纹质晶屑凝灰溶岩分布最广；火山构造主要发育破火山口、岩穹、隐爆角砾岩筒等构造，且与成矿关系；与成矿关系密切的次火山岩广泛发育。区内主要矿化类型为斑岩型、（次火山）隐爆层间裂隙带型、蚀变花岗岩型、隐爆角砾岩型，矿床成因均为与火山活动期后次火山（隐爆）活动有关的岩浆期后高－中温热液交代－充填型矿床。同时，矿床类型上具相互穿插性，隐爆层间裂隙带型、隐爆角砾型等矿床类型往往可以看作是斑岩型矿床的延伸；矿床空间分布上往往具有“一体多型”的特征。新矿化类型是赣南中生代火山岩找矿的重要方向。     

澜沧老厂银多金属矿床火山岩地球化学特征及环境识别

本文通过对矿区火山岩的主元素、微量元素和稀土元素的地球化学特征分析，以及利用TiO2-K2O-P2O5、Th-Hf-Ta、Zr-Nb-Y三角图和不相容元素配分模式的环境识别图解，得出与前人不同的结论。认为澜沧老厂银多金属矿床火山岩为大陆板内碱性玄武岩系列，其产出的构造环境可能是东冈瓦纳大陆东北部边缘的碎片。     

走滑拉分作用与相山产铀火山盆地的就位

相山火山盆地是我国著名的产铀火山盆地。本文依据相山盆地深部地质研究成果以及大陆动力学理论，探讨了相山产铀火山盆地的就位机制。研究表明，相山火山盆地火山活动划分为两个旋回，其就位受制于区域深断裂的走滑拉分作用。即第一旋回的火山机构就位于NE向深断裂右旋走滑复活产生的EW向拉分构造，形成了东西向展布的裂隙式火山喷溢带；第二旋回的火山机构就位于NE向深断裂左旋走滑复活产生的SN向拉分构造与EW向基底断裂的结点，产生了中心式火山岩浆喷溢侵出。此外，还探讨了富大铀矿的形成机制。     

The Llullaillaco volcano, northwest Argentina: construction by Pleistocene volcanism and destruction by sector collapse

Llullaillaco is one of a chain of Quaternary stratovolcanoes that defines the present Andean Central Volcanic Zone (CVZ), and marks the border between Chile and Argentina/Bolivia. The current edifice is constructed from a series of thick dacitic lava flows, forming the second tallest active volcano in the world (6739 m). K–Ar and new biotite laser ⁴⁰Ar/³⁹Ar step-heating dates indicate that the volcano was constructed during the Pleistocene (≤1.5 Ma), with a youngest date of 0.048±0.012 Ma being recorded for a fresh dacite flow that descends the southern flank. Additional ⁴⁰Ar/³⁹Ar measurements for andesitic and dacitic lava flows from the surrounding volcanic terrain yield dates of between 11.94±0.13 Ma and 5.48±0.07 Ma, corresponding to an extended period of Miocene volcanism which defines much of the landscape in this region. Major- and trace-element compositions of lavas from Llullaillaco are typical of Miocene–Pleistocene volcanic rocks from the western margin of the CVZ, and are related to relatively shallow-dipping subduction of the Nazca plate beneath northern Chile and Argentina.Oversteepening of the edifice by stacking of thick, viscous, dacitic lava flows resulted in collapse of its southeastern flank to form a large volcanic debris avalanche. Biotite ⁴⁰Ar/³⁹Ar dating of lava blocks from the avalanche deposit indicate that collapse occurred at or after 0.15 Ma, and may have been triggered by extrusion of a dacitic flow similar to the one dated at 0.048±0.012 Ma. The avalanche deposits are exceptionally well preserved due to the arid climate, and prominent levées, longitudinal ridges, and megablocks up to 20-m diameter are observed.The avalanche descended 2.8 km vertically, and bifurcated around an older volcano, Cerro Rosado, before debouching onto the salt flats of Salina de Llullaillaco. The north and south limbs of the avalanche traveled 25 and 23 km, respectively, and together cover an area of approximately 165 km². Estimates of deposit volume are hampered by a lack of thickness information except at the edges, but it is likely to be between 1 and 2 km³. Equivalent coefficients of friction of 0.11 and 0.12, and excess travel distances of 20.5 and 18.5 km, are calculated for the north and south limbs, respectively. The avalanche ascended 400 m where it broke against the western flank of Cerro Rosado, and a minimum flow velocity of 90 m s⁻¹ can be calculated at this point; lower velocities of 45 m s⁻¹ are calculated where distal toes ascend 200 m slopes.It is suggested that the remaining precipitous edifice has a high probability for further avalanche collapse in the event of renewed volcanism.     

An extremely large magnitude eruption close to the Plio-Pleistocene boundary: reconstruction of eruptive style and history of the Ebisutoge-Fukuda tephra, central Japan

An extremely large magnitude eruption of the Ebisutoge-Fukuda tephra, close to the Plio-Pleistocene boundary, central Japan, spread volcanic materials widely more than 290,000 km² reaching more than 300 km from the probable source. Characteristics of the distal air-fall ash (>150 km away from the vent) and proximal pyroclastic deposits are clarified to constrain the eruptive style, history, and magnitude of the Ebisutoge-Fukuda eruption.Eruptive history had five phases. Phase 1 is phreatoplinian eruption producing >105 km³ of volcanic materials. Phases 2 and 3 are plinian eruption and transition to pyroclastic flow. Plinian activity also occurred in phase 4, which ejected conspicuous obsidian fragments to the distal locations. In phase 5, collapse of eruption column triggered by phase 4, generated large pyroclastic flow in all directions and resulted in more than 250–350 km³ of deposits. Thus, the total volume of this tephra amounts over 380–490 km³. This indicates that the Volcanic Explosivity Index (VEI) of the Ebisutoge-Fukuda tephra is greater than 7. The huge thickness of reworked volcaniclastic deposits overlying the fall units also attests to the tremendous volume of eruptive materials of this tephra.Numerous ancient tephra layers with large volume have been reported worldwide, but sources and eruptive history are often unknown and difficult to determine. Comparison of distal air-fall ashes with proximal pyroclastic deposits revealed eruption style, history and magnitude of the Ebisutoge-Fukuda tephra. Hence, recognition of the Ebisutoge-Fukuda tephra, is useful for understanding the volcanic activity during the Pliocene to Pleistocene, is important as a boundary marker bed, and can be used to interpret the global environmental and climatic impact of large magnitude eruptions in the past.     

Magmatic gas scrubbing: implications for volcano monitoring

Despite the abundance of SO_2(g) in magmatic gases, precursory increases in magmatic SO_2(g) are not always observed prior to volcanic eruption, probably because many terrestrial volcanoes contain abundant groundwater or surface water that scrubs magmatic gases until a dry pathway to the atmosphere is established. To better understand scrubbing and its implications for volcano monitoring, we model thermochemically the reaction of magmatic gases with water. First, we inject a 915°C magmatic gas from Merapi volcano into 25°C air-saturated water (ASW) over a wide range of gas/water mass ratios from 0.0002 to 100 and at a total pressure of 0.1 MPa. Then we model closed-system cooling of the magmatic gas, magmatic gas-ASW mixing at 5.0 MPa, runs with varied temperature and composition of the ASW, a case with a wide range of magmatic–gas compositions, and a reaction of a magmatic gas–ASW mixture with rock. The modeling predicts gas and water compositions, and, in one case, alteration assemblages for a wide range of scrubbing conditions; these results can be compared directly with samples from degassing volcanoes. The modeling suggests that CO_2(g) is the main species to monitor when scrubbing exists; another candidate is H₂S_(g), but it can be affected by reactions with aqueous ferrous iron. In contrast, scrubbing by water will prevent significant SO_2(g) and most HCl_(g) emissions until dry pathways are established, except for moderate HCl_(g) degassing from pH<0.5 hydrothermal waters. Furthermore, it appears that scrubbing will prevent much, if any, SO_2(g) degassing from long-resident boiling hydrothermal systems. Several processes can also decrease or increase H_2(g) emissions during scrubbing making H_2(g) a poor choice to detect changes in magma degassing.We applied the model results to interpret field observations and emission rate data from four eruptions: (1) Crater Peak on Mount Spurr (1992) where, except for a short post-eruptive period, scrubbing appears to have drastically diminished pre-, inter-, and post-eruptive SO_2(g) emissions, but had much less impact on CO_2(g) emissions. (2) Mount St. Helens where scrubbing of SO_2(g) was important prior to and three weeks after the 18 May 1980 eruption. Scrubbing was also active during a period of unrest in the summer of 1998. (3) Mount Pinatubo where early drying out prevented SO_2(g) scrubbing before the climactic 15 June 1991 eruption. (4) The ongoing eruption at Popocatépetl in an arid region of Mexico where there is little evidence of scrubbing.In most eruptive cycles, the impact of scrubbing will be greater during pre- and post-eruptive periods than during the main eruptive and intense passive degassing stages. Therefore, we recommend monitoring the following gases: CO_2(g) and H₂S_(g) in precursory stages; CO_2(g), H₂S_(g), SO_2(g), HCl_(g), and HF_(g) in eruptive and intense passive degassing stages; and CO_2(g) and H₂S_(g) again in the declining stages. CO_2(g) is clearly the main candidate for early emission rate monitoring, although significant early increases in the intensity and geographic distribution of H₂S_(g) emissions should be taken as an important sign of volcanic unrest and a potential precursor. Owing to the difficulty of extracting SO_2(g) from hydrothermal waters, the emergence of >100 t/d (tons per day) of SO_2(g) in addition to CO_2(g) and H₂S_(g) should be taken as a criterion of magma intrusion. Finally, the modeling suggests that the interpretation of gas-ratio data requires a case-by-case evaluation since ratio changes can often be produced by several mechanisms; nevertheless, several gas ratios may provide useful indices for monitoring the drying out of gas pathways.     

Assessment of Debris-Flow Hazards of Alluvial Fans

A strategy is presented for the assessment ofdebris-flow hazards on alluvial fans on the basis ofa case study carried out on the southern foot of astratovolcano named Mt. Yatsugatake. Transformation ofcommercial forests into a golf course was planned ata corner of the Kikkakezawa fan. The case studyinvolves an assessment of hazards due to debris flowsof different magnitudes and their recurrenceintervals. The environmental conditions for therecurrence of these debris flows are discussed as wellas the extent of the areas affected by them. In orderto generalize the case study, concepts of hazardpotential, hazard and risk for debris flows onalluvial fans are established. Accordingly, the hazardpotential is the possible hazards at any location onan alluvial fan in an indefinitely long time period,which can be assessed on the basis of hydrological andother geoscientific investigations. Hazards associatedwith a particular land use can be evaluated on thebasis of the hazard potential considering the locationand the time period associated with the land use. Riskcan be further assessed considering the life styles inthis land and the social conditions.     

Special Issue Devoted to Petroleum Geology in Marginal Sea Deepwater Settings—Take the South China Sea as an Example Preface

High‐resolution tomographic images of the belt crossing the Japan Trench‐Changbai Mountains‐Dong Ujimqin Qi are represented in this paper, revealing the shape of a subducted slab in the western Pacific region and characteristics of the lithospheric structures under the Changbai Mountains and the Da Hinggan Mountains. Studies of the spatial distribution, subduction time and the time‐lag between the subduction and magmatism, combined with petrology and isotope geochemistry of the Late Mesozoic volcano‐plutonic rocks from the Da Hinggan Mountains‐Yanshan Mountains have further proved the independence of magmatic activities from the subduction of the Pacific plate. The Mesozoic tectono‐thermal evolutionary history and structural characteristics of the lithosphere in the Da Hinggan Mountains and North China suggest that the formation and evolution of magma have probably a close relationship with the delamination and thinning of the continental lithosphere and the underplating resulting from the consequent upwelling of the asthenosphere. On the other hand, the large‐scale strike‐slip fault system, resulting from sinistral shearing of the Pacific plate relative to the Asian continent in the Mesozoic, is responsible for the formation and emplacement of magma on the continental margin. It was the intense crust‐mantle interaction, together with structural deformation at the shallower levels that led to the large tectono‐magmatic belt in the East Asian continental margin.     

Pyroclastic flows and surges over water: an example from the 1883 Krakatau eruption

Pyroclastic deposits from the 1883 eruption of Krakatau are described from areas northeast of the volcano on the islands of Sebesi, Sebuku, and Lagoendi, and the southeast coast of Sumatra. Massive and poorly stratified units formed predominantly from pyroclastic flows and surges that traveled over the sea for distances up to 80 km. Granulometric and lithologic characteristics of the deposits indicate that they represent the complement of proximal subaerial and submarine pyroclastic flow deposits laid down on and close to the Krakatau islands. The distal deposits exhibit a decrease in sorting coefficient, median grain size, and thickness with increasing distance from Krakatau. Crystal fractionation is consistent with the distal facies being derived from the upper part of gravitationally segregated pyroclastic flows in which the relative amount of crystal enrichment and abundance of dense lithic clasts diminished upwards. The deposits are correlated to a major pyroclastic flow phase that occurred on the morning of 27 August at approximately 10 a.m. Energetic flows spread out away from the volcano at speeds in excess of 100 km/h and traveled up to 80 km from source. The flows retained temperatures high enough to burn victims on the SW coast of Sumatra. Historical accounts from ships in the Sunda Straits constrain the area affected by the flows to a minimum of 4x10³ km². At the distal edge of this area the flows were relatively dilute and turbulent, yet carried enough material to deposit several tens of centimeters of tephra. The great mobility of the Krakatau flows from the 10 a.m. activity may be the result of enhanced runout over the sea. It is proposed that the generation of steam at the flow/seawater interface may have led to a reduction in the sedimentation of particles and consequently a delay in the time before the flows ceased lateral motion and became buoyantly convective. The buoyant distal edge of these ash-and steam-laden clouds lifted off into the atmosphere, leading to cooling, condensation, and mud rain.