Effect of weathering on the geomechanical properties of andesite, Ankara – Turkey

Andesite exposed in different parts of Ankara has different weathering categories varying from fresh to residual soil. The geomechanical properties of the andesite are significantly affected by weathering. Buildings constructed especially in completely weathered and residual soil levels of the andesite have some geotechnical problems. In this study, the variation of the geomechanical properties of the andesite due to weathering is investigated in three selected sites of Ankara, through optical microscope, X-ray diffraction analyses, major element analyses, pressuremeter tests, physicomechanical tests, and seismic refraction method. The data gathered from the field and laboratory studies were used to assess the characteristics of the weathering zones. Based on the data obtained from this study, an idealized weathering profile is assessed. No bearing capacity and consolidation settlement problems exist in the area. However, care should be taken for immediate settlement of the buildings. Slopes with heights less than 8 m are not expected to cause any significant problems for buildings. There exists a ground amplification problem in the study area. The natural period of the building should not match that of the weathered rock. The ground response analyses reveal that the buildings with four to seven stories may be adversely affected from earthquakes due to resonance phenomenon. Therefore, this has to be taken into account when designing new buildings.     

盛源盆地中的安山岩

盛源盆地中的安山岩有斑状安山岩和隐晶质安山岩,沿盆地边缘断续分布。安山岩厚数厘米到百余米,里特曼指数（δ）多在１．８－３．３,少数＞３．３;ＳｉＯ２含量与中国安山岩平均值相近,Ｋ２Ｏ偏高,ＴｉＯ２含量０．８９％,Ｒｂ／Ｓｒ为１．１２８,δＥｕ为１．３３１８,表现出安山岩与上地幔的成因联系。     

The Moel-y-Golfa Andesite: An Ordovician (Caradoc) intrusion into unconsolidated conglomeratic sediments,Breidden Hills Inlier,Welsh borderland

The Moel-y-Golfa Andesite is a discordant intrusive body of porphyritic pyroxene andesite, 1 km across and at least 200 m thick. Textural variation defines a central facies and a marginal facies, the latter displaying well-formed lobes, tubes, and pillows. The vertically stacked lobes, brecciated tubes, and elongate pillows of andesite intrude and locally enclose enclaves of altered host sediment and are cut by steeply dipping slide scars. The host sediments are locally strongly silicified and cut by numerous, anastomosed vertical and subvertical fissures. Fissures are filled by disrupted host sediment and fragments of brecciated andesite. The formation and filling of each fissure system was probably concomitant with the brecciation of an intrusive magma lobe. The complex relationship of igneous and sedimentary material indicates that the Moel-y-Golfa Andesite was emplaced into the host sediments whilst they were unconsolidated or only partly lithified.     

Chemical Weathering of New Pyroclastic Deposits from Mt. Merapi （Java）, Indonesia

The Java Island, Indonesia with abundant amount of pyroclastic deposits is located in the very active and dynamic Pacific Ring of Fires. Studying the geochemical weathering indices of these pyroclastic deposits is important to get a clear picture about weathering profiles on deposits resulting from the eruption of Mt. Merapi. Immediately after the first phase of the eruption （March to June 2006）, moist and leached pyroclastic deposits were collected. These pyroclastic deposits were found to be composed of volcanic glass, plagioclase feldspar in various propoitions, orthopyroxene, clinopyroxene, olivine, amphibole and titanomagnetite. The total elemental composition of the bulk samples （including trace elements and heavy metals） was determined by wet chemical methods and X-ray fluorescence （XRF） analyses. Weathering of the pyroelastic deposits was studied using various weathering indices. The Ruxton ratio, weathering index of Parker, Vought resudual index and chemical index of weathering of moist pyroclastic deposits were lower than those of the leached samples, but the alteration indices （chemical and plagioclase） were slightly higher in the moist compared to the leached pyroelastic deposits.     

辉长岩的高压部分熔融实验研究

利用天然含水辉长岩在１．５～３．０ＧＰａ,９００℃～１４４０℃条件下进行了高温高压部分熔融实验,系统地分析了辉长岩部分熔融后实验产物的岩石学、矿物学、地球化学特征。获得了１）辉长岩的高压熔融曲线,２）辉长岩在不同压力下熔融时,残留相矿物组合及期间矿物相转变与熔融液相成分（相当于安山岩浆）特征。认识了辉长岩与榴辉岩相间的相变关系;辉长岩－榴辉岩－安山岩之间的内在联系以及高温榴辉岩的可能成因。     

Constraints on the evolution of the Taranaki volcanoes,New Zealand,based on aeromagnetic data

Aeromagnetic anomalies measured over the three relict andesite volcanoes (Paritutu, Kaitake and Pouakai) in Taranaki are largely dominated by topographic effects. Three-dimensional magnetic modelling, well constrained by both contrasting levels of exposure and previous gravity models, shows that the bulk magnetization of the andesite edifice and edifice core of both Kaitake and Pouakai is 2.5–3.5 A m^-1 in a direction close to that of the present earth's field. However, the large andesite dyke/stock complexes below all three edifices and a localized area of the Kaitake edifice directly above the centre of the subedifice complex have anomalously low bulk magnetizations of 0.3–1.0 A m^-1 in the same direction. These subsurface complexes represent dyke injection from magma chambers situated in the basement, probably below 6 km depth. Here, we deduce that heat from these magma chambers drove hydrothermal convection systems causing widespread demagnetization, especially in the subedifice complexes, but also locally within the edifices themselves. A lesser degree of demagnetization at Pouakai, the youngest of the three volcanoes, may indicate a shorter and consequently more intense period of activity at this centre.     

Zircon U–Pb Geochronology,Elemental Geochemistry and Geological Implications of the Andesite of the Yixian Formation in the Tiejiangyingzi Basin in Southeastern Chifeng,China

The Early Cretaceous andesite of the Yixian Formation is an ore‐host rock in the Tiejiangyingzi basin in southeastern Chifeng, China, and bears several meso‐epithermal Au deposits, such as the Nalingou and Yaogou Au deposits. The geochronology and elemental geochemistry of the Yixian Formation were investigated to reveal its metallogenic context, petrogenesis, and mineralization contribution. The ²⁰⁶Pb/²³⁸U ages of zircons reveals that the andesite formed at 125.5 ± 0.9 Ma. This rock belongs to calc‐alkali rock series according to its elemental geochemical analysis results (i.e., SiO₂ = 63.63%–67.27%, MgO = 1.17%–1.43%, K₂O = 4.07%–4.67%, Mg^# = 35–41, Al₂O₃ = 17.16%–18.63%, Na₂O = 3.35%–4.08%, Sr = (240.1–585.6) × 10^‐6, Yb = (1.83–2.22) × 10^‐6, Y = (13.9–23.36) × 10^‐6, Na₂O/K₂O = 0.7–0.9, Eu/Eu* = (0.75–0.79) × 10^‐6, Sr/Y = 13–27, La_N/Yb_N = 11–19 ). The chondrite‐normalized trace element plot shows that the andesite is enriched in light rare earth elements (LREEs) and Yb. Thus, the residual phase of andesite volcanic rocks is a granulite suite that contains amphibole and little remaining garnet in the Tiejiangyingzi basin. It can be concluded that andesite volcanic rocks distributed in the Tiejiangyingzi basin were formed in an extensional tectonic environment during the Mesozoic orogen, and the lower crust had experienced extensive high‐temperature metamorphism and melting. This gold‐bearing lower crust is closely related to mineralization. Therefore, the conclusions of this research have an important implication for further prospecting in southeastern Chifeng, China.     

大别造山带宿松球颗结构安山岩的地球化学特征

大别造山带南部宿松球颗结构安山岩的球颗结构指示此安山岩是喷发于水下环境。球颗由放射状斜长石（蚀变为钠长石和更长石）及少量位于斜长石间隙的单斜辉石、普通角闪石和蚀变矿物石英组成，基质由玻璃质、绿泥石、黝帘石、角闪石组成，SiO2=56.17%-58.31%,AI2O3=14.08% -14.67%,TiO2=0.91%-0.94%,NaO=4.19%-4.58%,K2O=0.93%-1.42%,MgO含量高达5.66%-6.98%,100Mg/(Mg Fe)=64.1-66.2。球颗结构安山岩和闪长斑岩、英安岩均为亚碱性的钙碱性系列岩石。球颗结构安山岩富集Si(1259-1588μg/g）、Ba(880-1134μg/g）、Th、Rb等大离子亲石元素。大离子亲石元素相对高场强元素富集，例如La/Nb=5.56-6.07。高场强元素的比值具有类似MORB特性，例如Zr/Nb=22.08-26.14,Hf/Ta=5.62-9.62.亏损Ta、Nb、P、Ti等高场强元素，自Ce元素向右，各元素的含量下降很快，表现出活动大陆边缘的与俯冲作用有关的火山岩的特征。轻稀土元素强烈富集，稀土分馏显著（La/Yb)N=32.84，没有Eu异常。具有岛孤安山岩的特性，但有异常高的(La-Yb)N=28.63-26.74,(La/Y)N=70.33-81.04。并且Y和Yb元素有显著的亏损，Y＜20μg/g，YN=2.74-2.86,YbN=2.18-2.35.从La-(La/Sm)图解看，本安山岩为部分熔融作用的产物。La/Nb的平均值为5．83，这在岛弧火山岩是很广泛的特点。Nd的εNd(0)为－18．71～－19．16，物源是大陆物质或被大陆物质所混染。Nd模式年龄为1．9Ga，可能反映了Nd模式年龄为1．7Ga的俯冲扬子板引起Nd模式年龄为2．5Ga的华北大陆岩石圈地幔楔物质部分熔融，产生这种玄武质岩浆。因而具有大陆基底的活动大陆边缘的岛弧火山岩的地球化学特征。     

Assessment of rockfall hazard around Afyon Castle,Turkey

The Afyon Castle is a tourist destination and a historical site in the City of Afyon in Turkey. The Castle is located on a steep hill, with a height of 226 m. In close proximity to the Castle there are settlements. The hill consists of trachitic andesite. The rock contains columnar joints and flow layering. Owing to these discontinuities, blocks of varying sizes had fallen down. The settlement areas near the Castle are now in danger because of the rockfall risk. In this study, rockfall analysis was carried out along nine profiles of the hill. Fall-out distance, bounce height, kinetic energy and velocity of the rocks along each profile were investigated. The results of the analyses were evaluated, and the areas delineated as susceptible to rockfall risk are highlighted. Remedial measures including rock bolts and protective fences were suggested on the basis of the field observations and the rockfall risk evaluation.     

Weathering damage evaluation of rock properties in the Bunhwangsa temple stone pagoda,Gyeongju, Republic of Korea

The stone pagoda of the Bunhwangsa temple in Republic of Korea was made of piling small brick-shaped stones. The majority of stone bricks are andesitic rocks with variable geneses. Rock properties of the pagoda roof suffer partial significant deterioration, such as multiple peel-offs, exfoliation, onion-peel-like decomposition, cracks forming round lines and falling-off stone pieces. The stylobates and tabernacles at the four corners are composed of granitic rocks, which are heavily contaminated by lichens and mosses. Some of these contamination marks show dark black or yellowish brown colors by inorganic secondary hydrates. The four tabernacles and northern face of the pagoda body have been exposed to relatively high humidity, which causes light gray efflorescence as stalactites between the northern and western sides of the body. The efflorescences are composed of calcite, gypsum and clay minerals. The stone lion statues at the southeast and northeast corners are made of alkali granite, while the others are lithic tuff. Total rock properties of the pagoda consist of 9,708 stone bricks. Among them, 11.0% are fractured, 6.7% are fallen off, and 7.0% show considerable surface efflorescence, which shows that the pagoda has been highly deteriorated by physical, chemical and biological weathering. The authors strongly suggest long-term monitoring and comprehensive conservation researches.