Petrogeochemical features of Oligocene-Early Miocene volcaniclastic rocks of the Sea of Japan

Petrographic and geochemical studies showed that the Oligocene-Early Miocene volcaniclastic rocks from the southern part of the Sea of Japan are ascribed to the high-potassium aluminous rocks of the subalkaline volcanic series of active continental margins. A comparative analysis revealed the spatiotemporal relation of Oligocene-Early Miocene subaerial volcanism of the Sea of Japan with Late Cretaceous and Eocene-Early Miocene ignimbrite volcanism of the East Eurasian margin. This allows us to refer the volcaniclastic rocks of the Sea of Japan to a stage of ignimbrite volcanism that occurred during relative quiescence against a general extension in the continental margin setting.     

Specific features of the mineral composition and petromagnetic properties of rocks from the Minami-Khiosi submarine volcano (Mariana island arc)

Complex studies of the mineral composition and petromagnetic properties of the rocks which compose an edifice of the Minami–Khiosi submarine volcano located in the Mariana island arc are carried out for the first time. The Minami–Khiosi Volcano is a part of the Khiosi volcanic complex within the alkaline province of the Idzu–Bonin and Mariana island arcs. All of the rocks analyzed are enriched in K₂O (1.34–3.30%), Ba (370–806 ppm), and Sr (204–748 ppm). The basalt has a porhyric texture and contains mosTy olivine phenocrysts as individual crystals and growths with a size up to 2 cm; the groundmass is finecrystalline. The samples studied contain at least three Fe-bearing oxide minerals. These are predominant magnetite and less abundant ilmenite and Fe hydroxides. It is established that the samples studied are magnetically isotropic and have high values of natural remanent magnetization and Königsberger ratio. Similarly to the other island-arc Late Cenozoic submarine volcanoes in the western part of the Pacific Ocean, the samples studied are strongly differentiated by the value of natural remanent magnetization and magnetic susceptibility. The low-coercivity magnetic minerals (titanomagnetite and magnetite) of the pseudo-single-domain structure, as well as high-coercivity minerals (hematite) are the main carriers of magnetization. The high values of natural remanent magnetization are explained by the pseudo-single-domain structure of the titanomagnetite grains, whereas the high values of magnetic susceptibility result from the high concentration of ferromagnetic grains.     

海拉尔盆地火山碎屑岩的成岩作用

对来自海拉尔盆地40口井的白垩系火山碎屑岩样品显微镜观察与分析显示,火山碎屑岩的成岩作用具有特殊性,表现在熔结作用、脱玻化与重结晶作用、粘土矿物形成与转变和交代作用等方面。熔结作用可以区分出熔浆/熔岩熔结、浆屑熔结和热灰熔结三种类型。脱玻化与重结晶作用常常是交织在一起,流纹质玻璃物质经脱玻化,并进一步重结晶,常具有特殊的结构,呈现一种复杂的硅质胶结样式。粘土矿物形成与转变在成岩作用早期就开始了,如蒙脱石化。一些特征性矿物如浊沸石、绿泥石的出现也不代表变质作用的开始,而是特定成岩环境的产物。碳酸盐的交代-胶结和溶解在火山碎屑岩中普遍发育,使得原岩面貌发生了严重的改变。成岩作用的特征揭示海拉尔盆地火山碎屑岩的成分和结构演变,这在测井岩性解释和孔隙度预测上具有意义。     

Specific features and composition of the Akkermanov manganese deposit

New data on the Akkermanov deposit characterized by specific structure and composition of primary (carbonate) and secondary (manganese oxide) ores are presented. Distribution of mineralization in host rocks and weathering crusts is considered. It is shown that manganiferous carbonate rocks, which host orebodies, formed in a marine basin with well-aerated bottom waters. Oxide ores are mainly composed of crystalline pyrolusite produced by multiple processes of the oxidation of manganese compounds. In this respect, the Akkermanov deposit differs drastically from all manganese deposits developed in Russia and Ukraine.     

Specific features of the geochemistry of Eocene-Miocene rocks in the Perovskii Gulf,northern Aral region

The major and trace element distribution was studied in the Eocene-Miocene stratotype section of the northern Aral region, northern coast of the Perovskii Gulf. The geochemical specific of ore-bearing rocks is shown. Qualitative trends in the distribution of element in genetically diverse rocks are established.     

一个与前寒武纪火山碎屑沉积岩及超镁铁岩有成因联系的锡矿床

经详细研究九毛锡矿各类地质体的含锡量,确定了锡在火山-沉积变质岩及变超镁铁岩中的初始富集。根据不同变质程度岩石含锡量变化趋势、微量元素含量的继承性关系、成因矿物等方面的研究,确定锡矿床与超镁铁岩及火山-沉积岩间有成因联系。本文强调了锡的多重性及锡在超镁铁岩中的初始富集,指出变质作用是古陆区锡成矿的重要地质营力。     

Specific features of the mineral composition of titanium-zirconium placers in Russia

Specific features of the mineral composition of economically promising titanium-zirconium placers in Russia are analyzed. Generalization of the data made it possible to classify titanium-zirconium deposits with respect to different parameters of mineral composition, which can govern technological properties, and to identify specific characteristics with insignificant differences. Ore sands of titanium-zirconium placers are characterized by the following qualitative characteristics: TiO₂ content; mineral form of this dioxide (zircon); mineral-carriers of elements that degrade the quality of sands (primarily, Cr and P); physical properties of minerals; grain size composition of sands; and mass and composition of clayey fraction in the sands. Characteristics of the currently exploited Malyshev deposit are given.     

陆东凹陷九佛堂组火山碎屑岩地层对比与划分

针对陆东凹陷稳定标志层不发育、地层对比难度大的问题,在岩电特征分析的基础上,将火山作用与沉积作用相联系,确定火山喷发期次。应用井震联合层位标定技术,精细制作合成记录,建立地震标志层与井上典型岩电特征的关系,完成地层对比与划分,有效指导了储层预测和井位部署。     

Zeolitization of Oligo-Miocene volcaniclastic rocks from Logudoro (northern Sardinia, Italy)

The present study reports the results of research on volcaniclastic products from Logudoro (northern Sardinia), a reconstructed stratigraphic succession some hundreds of meters thick, comprising two different ignimbritic units separated by an epiclastic layer (generally ˜10 m thick). Clinoptilolite is the most widely distributed authigenic phase in epiclastite and unwelded units, always associated with opal-CT and sometimes with smectite. An adularia-like feldspar, cristobalite and epigenetic quartz are typical phases of welded units. Within the zeolitized units (unwelded ignimbrite and epiclastic units) clinoptilolite and opal-CT constitute the cement deriving from the transformation of the precursor rhyolitic glass, in agreement with a single zeolitization process that developed after the deposition of the entire succession. Silica increases in zeolitized rocks with respect to the precursor material, leading to hypothesize that the secondary mineralization process was favored by mineralized hydrothermal fluids preferentially circulating through the fault system of the area. Quartz veins in welded subunit and K-feldspar (adularia-like) preferentially located near the faults are in agreement with this hypothesis. The interaction of these fluids with the glassy fraction favored its dissolution and the consequent crystallization of clinoptilolite. Furthermore, the pH increase and the silica supersaturation enhanced the contemporary or subsequent precipitation of opal-CT. Received: 5 June 2000 / Accepted: 14 August 2000     

Alteration mineralogy of eocene volcanic and volcaniclastic rocks from Sivas basin in Turkey

The study area covers volcanic-volcanosedimentary units of Eocene age in the Sivas-Ula? area from Turkey. The pyroclastic (tuffaceous claystone/siltsone/sandstone, crystal ash tuff) and volcanic (basalt, basaltic andesite, andesite) rocks of the Karacalar member from the Kaleköy Formation include volcanogenic (plagioclase, augite, hornblende, biotite), diagenetic (K-feldspar, mixed-layered chlorite-smectite/C-S, chlorite, analcime) and post-volcanic (calcite, dolomite, quartz) minerals. The volcanogenic (plagioclase), diagenetic (K-feldspar, C-S, chlorite), postvolcanic (quartz, calcite, dolomite) and detrital (illite) minerals were observed in the epiclastic (shale, siltstone, calcareous siltstone, sandstone, calcareous sandstone) and chemical (limestone, gypsum) rocks of the Yapali member from this formation. C-S + K-feldspar zoning is widely developed by due to the interaction between sea-water and volcanic glass in basic-intermediate composition, on the basis of optic and electron microscopes and also X-rays data. This zone corresponds to the deeper parts of the Sivas basin in the Eocene period and show vertically a transition into zeolite zone in approximately northern parts of the basin (Yavu area).     

海拉尔盆地贝尔凹陷南屯组火山碎屑岩成岩作用的讨论

利用偏光显微镜、扫描电镜、染色法以及X-衍射分析等方法,查明南屯组火山碎屑岩成岩作用模式.研究表明火山碎屑岩成岩作用方式包括：机械压实作用、胶结作用与自生作用、交代作用和溶蚀溶解作用.成岩共生序列为：黏土矿物包壳-泥晶碳酸盐矿物、自生蒙皂石、自生伊利石-长石溶解-自生石英-自生高岭石-绿泥石、白云石-铁方解石-铁白云石.成岩阶段划分为早成岩阶段B期和中成岩阶段A期,并且以中成岩阶段A期为主.     

南海盆海山火山碎屑岩的发现及其地质意义

海山火山碎屑岩是水下爆发性火山作用的产物.南海盆两座海山上发现火山碎屑岩表明海山顶部曾经一度高于PCL(压力补偿深度).岩相学特征显示这些火山碎屑岩的胶结物主要为一混合相,包括粘土矿物和黑色铁质矿物等,这从一个侧面反映海山的正地形限制了粗粒外生碎屑到达海山顶部.岩石的单矿物组分、主量元素化学特征与同海山玄武质熔岩具有可比性,属碱性岩浆系列.计算获得海山平均最小下沉速率为0.06mm/年,最大可能达0.30mm/年.海山岩石的机械风化产物对周围海盆的沉积作用作出重要贡献.     

Specific features in distribution of natural radioactivity in rocks of the Bazhenovo Formation on the western slope of the Surgut Dome,West Siberia

This paper presents an analysis of the radioactivity distribution in the Bazhenovo Formation rocks on the western slope of the Surgut Dome. The maximum uranium concentrations are confined to accumulations of organic matter and are proportional to its concentration in the rocks. The lowermost part of the section has lower radioactivity values than the upper one. The value of radioactivity varies throughout the area of the Surgut Dome. Coeval analogs of the Bazhenovo Formation show lower radioactivity values.     

Mineralogical composition of the Nyiragongo rocks

The author reviews briefly the application of the AMS calculation of rock analyses developed byAlfred Rittmann to the lavas of the Nyiragongo volcano in North Kivu, Republic of Congo (Kinsasha). In that volcanic field illustrated in Fig. 1, two petrographic provinces can be distinguished: the Nyamuragira area with volcanics of the basanite group (moderate degree of undersaturation) and the Nyiragongo area with those of the leucitite-nephelinite-melilitite group (high degree of undersaturation).In contrast to the Al-rich titanian clinopyroxene of the leucitites and nephelinites, that of the typical nepheline melilitites is poor in Al and Ti with relatively small amount of the acmite component. The melilitites show a significant excess of the alkalies over aluminum. At least a part of this excess is contained in the glassy material found in the rocks. It is suggested that the genesis of the Nyiragongo magma, mainly melilititic in composition, is possibly connected with carbonatic fractions in the magma basin and enrichment of the alkalies by gaseous transfer in the form of carbonates.

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Zusammenfassung Verf. überblickt kurz die Anwendung der vonAlfred Rittmann ausgearbeiteten AMS-Berechnungsmethode von Gesteinsanalysen auf die Laven des Nyiragongo-Vulkans in Nord Kivu, Republik Congo (Kinsasha). In dem betr. vulkanischen Gebiet (Fig. 1) können zwei petrographische Provinzen unterschieden werden: das Nyamuragira-Gebiet mit Vulkaniten der Basanitgruppe (mäßig untersättigt) und das Nyiragongo-Gebiet mit Gesteinen der Leuzitit-Nephelinit-Melilitithgruppe (stark untersättigt).Im Gegensatz zu dem Al-reichen Titanklinopyroxen der Leuzitite und Nephelinite, ist derjenige der typischen Nephelin-Melilitithen arm an Al und Ti mit einem relativ niedrigen Gehalt an Akmitkomponente. Die Melilitithe zeigen einen bedeutenden Überschuß von Alkalien über Aluminium. Wenigstens ein Teil dieses Überschusses ist in dem glasigen Material enthalten, das in den Gesteinen vorkommt. Es wird darauf hingewiesen, daß die Entstehung des hauptsächlich melilitithischen Nyiragongo-Magmas möglicherweise mit karbonatischen Anteilen der Magmaherde und mit Anreicherung von Alkalien durch Gastransport in Form von Karbonaten verknüpft ist.

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Résumé L'auteur étudie brièvement la méthode AMS deAlfred Rittmann pour la calculation d'analyses de roches, appliquée aux laves du Nyiragongo volcan en Kivu nord, République Congo (Kinsasha). Dans ce terrain volcanique (voir table 1) deux provinces pétrographiques peuvent etre distinguées: la région Nyamuragira avec des volcans du groupe basanite (peu sous-saturés) et la région Nyiragongo avec des volcans du groupe leucitite-nephelinite-melilitite (fort sous-saturés).Au contraire du clinopyroxène titanien, riche en Al, des leucitites et des nephelinites, celui des typiques nephelines-melilitites est pauvre en Al et Ti avec un degré peu considérable d'acmite. Les melilitites montrent un excès considérable d'alcalis sur aluminium. Au moins une partie de cet excès se trouve dans la matière vitreuse qui existe dans les roches. Il est indiqué que la genèse du Nyiragongo magma, qui est principalement melilitique, est peut-être reliée avec des fraction carbonatiques des centres magmatiques et avec l'enrichissement des alcalis par le transport de gaz.

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. . Nyiragongo . : 1. Nyamuragira ; 2. Nyiragongo , . , Nyiragongo, , , .

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Dedicated to Professor Dr. A.Rittmann on the occasion of his 75. birthday