Pressure solution as a metamorphic process in deformed terrigenous sedimentary rocks

The most widespread record that terrigenous sedimentary rocks have deformed by a pressure solution mechanism is seen in the development of spaced cleavages and transposition structures under conditions of low grade metamorphism. Such structures are most common in immature sandstones and siltstones. Mineral reactions, involving modification of detrital grains and diagenetic minerals, and forming a logical extension to diagenetic processes, are an integral part of the deformation mechanism, and the cleavage stripes represent accumulations not just of inert particles, but mostly of newly crystallized micas, the products of these reactions. The mechanism of deformation by pressure solution is now seen to involve mmetamorphic reactions, change in volume of solid phases during reaction, removal of some silica from the rock, rearrangement of reaction products to produce fabrics, solution of some detrital grains in cleavage stripes.Formal reactions have been written to describe the alteration of detrital felspar and epidote to white mica, the modification of greywacke matrix to white mica, and the transformation of diagenetic chlorite to white mica, all of which are observed to occur during formation of the pressure solution cleavages. These reactions emphasize the importance of metamorphic processes during pressure solution deformation, suggest that pressure solution may involve removal of silica released as a product of the reactions, indicate that the pH of the aqueous phase may be buffered to a level that silica solubility is increased, involve a volume reduction that contributes to the overall shortening during deformation, and also involve dehydration, the large scale circulation of released water possibly being important to the removal and redistribution of silica during pressure solution.     

Cordierite as an indicator of thermodynamic conditions of petrogenesis

A refined thermodynamic model of H₂O and CO₂ bearing cordierite based on recent data on volatile incorporation into cordierite (Thompson et al. in Contrib Mineral Petrol 142:107–118, 2001; Harley and Carrington in J Petrol 42:1595–1620, 2001) reflects non-ideality of channel H₂O and CO₂ mixing. The dependence of cordierite H₂O and CO₂ contents on P, T and equilibrium fluid composition has been calculated for the range 600–800°C and 200–800 MPa. It has been used for establishing thermodynamic conditions of cordierite formation and the following retrograde P–T paths of cordierite rocks from many localities. Estimates of the H₂O and CO₂ activities have shown that cordierites in granites, pegmatites and high-pressure granulites were formed in fluid-saturated conditions and wide range of H₂O/CO₂ relations. Very low cordierite H₂O contents in many migmatites may be caused not only by fluid-undersaturated conditions at rock formation and H₂O leakage on retrograde P–T paths but also by the presence of additional volatile components like CH₄ and N₂. The pressure dependence of cordierite-bearing mineral equilibria on fluid H₂O/CO₂ relations has been evaluated.     

Lithogeochemical features of Riphean fine-grained terrigenous rocks in the Kama-Belaya aulacogen and their formation conditions

Lithogeochemical features of Riphean fine-grained terrigenous rocks of the Kama-Belaya aulacogen are discussed. It is shown that aluminosiliciclastic material delivered to the aulacogen during the Riphean was characterized by a low maturity degree. The successively increasing K₂O/Al₂O₃ values in the Riphean summary section correlate negatively with the CIA index values, indicating a gradually strengthening tendency for climate aridization in erosion zones. Data on some indicator ratios of trace elements and REE systematics in Riphean silty mudstones and shales of the Kama-Belaya aulacogen imply the involvement of mafic and ultramafic rocks, in addition to acid igneous and metamorphic varieties, in erosion during accumulation of the Nadezhdino, Tukaevo, Ol’khovka, Usinsk, and Priyutovo formations. Comparison of data on the composition of rocks in provenances based on the mineralogical-petrographic study of sandstones and investigation of geochemical features of silty mudstones and shales revealed their sufficiently high similarity. The geochemical data made it possible to specify the composition of rocks in provenances. Low Ce/Cr values in the fine-grained terrigenous rocks of the Lower Riphean Kyrpy Group suggest their formation with a significant contribution of erosion products of the Archean substrate, which is atypical for higher levels of the section. Thus, the Early-Middle Riphean transition period was likely marked by substantial changes in the mineral composition of material delivered to the Kama-Belaya aulacogen. The lack of exhalative components in the examined specimens of silty mudstones and shales points to a relatively low permeability of the Earth’s crust in the eastern East European Platform through the entire Riphean.     

Geochemistry of paleozoic terrigenous sediments from the Oldoi terrane, eastern Central Asian orogenic belt, as an indicator of geodynamic conditions during deposition

The analysis of the major and trace element geochemistry of Paleozoic terrigenous sediments from the Oldoi terrane, eastern Central Asian orogenic belt, supports a predominantly felsic source consisting of granites and recycled sediments. The geological data suggest that detrital material could be derived from Early Paleozoic granitoids, which were identified within tectonic blocks in the Silurian and Devonian terrigenous successions. The analysis of conventional discrimination diagrams indicates that the initial stage was marked by deposition along a passive continental margin, which continued in island-arc or active continental margin environments. This interpretation is consistent with geological evidence, such as the presence of volcanic interlayers and lenses in the Middle-Late Devonian successions and granitoids with ages of 386 +/- 10 and 371+/- 5.5 Ma. The timing of changes in geodynamic settings constrained by tectonic reconstructions corresponds to the age of the inception of the Norovlya continental-margin magmatic arc.     

Clay minerals as an index of the degree of metamorphism of the carbonate and terrigenous rocks in the South Wales coalfield

Illite, illite-montmorillonite (I-M) mixed layer, kaolinite and chlorite are the principal clay minerals identified in the carbonate and terrigenous carboniferous rocks in the South Wales coalfield. Mineralogic changes and illite crystallinity are the main modifications in the studied rocks. Progressive illitization of I-M mixed layers (decrease in their expandabilities) towards the western part of the coalfield (i.e. in the direction of increase in coal-rank and grade of metamorphism), and the development of vermicular kaolinite, dickite, allevardite and pyrophyllite in the anthracite area are the main mineralogic changes. Illite crystallinity was determined by Weaver's sharpness ratio (S.R.) and isosharpness ratio maps of illite in the carbonate and terrigenous rocks are produced showing a gradual increase in illite crystallinity towards the west. Illites in the terrigenous rocks tend to be aluminous (evidenced by the increase in their intensity ratios, ‘I.R.’) with increase in coal-rank and grade of metamorphism towards the west. These changes are controlled to some extent by lithology. Correlation of illite S.R. with either its I.R. or expandability of I-M mixed layer, has outlined two diagenetic zones in the coalfield: zone I, the Diagenetic zone, in the eastern and southern regions; and zone II, the Metadi-agenetic zone, covering the central and northern parts of the central region. The Anchimetamorphic zone (zone III), which corresponds to the anthracite area in the northwestern region, was differentiated by correlating the illite S.R. with coal-rank. The occurrence of the above mentioned diagenetic zones and modifications of the clay minerals in the carboniferous rocks of the South Wales coalfield are mainly lateral without variation with increase in depth.     

Composition and geodynamic conditions of formation of the intrusive rocks of the Gromadnen-Vurguveem peridotite-gabbro massif,western Chukotka

The Gromadnen-Vurguveem peridotite-gabbro massif is confined to one of the largest ophiolite complex of western Chukotka and composed mainly of intrusive rocks. This paper reports the first comprehensive compositional data for its plutonic rocks (petrochemistry, geochemistry, and compositions of minerals). In terms of petrography, two groups of rocks can be distinguished in the Gromadnen-Vurguveem peridotite-gabbro massif. The first group includes leucocratic gabbroids (mostly gabbronorites), composing most of the massif. The second group includes olivine-bearing cumulate rocks: olivine gabbros, troctolites, plagioclase-bearing dunites, and amphibolized wehrlites. The major element variations in these rocks suggest their affiliation to low-titanium, low-potassium, and high-alumina plutonic derivatives of island-arc magmatism. According to geochemical characteristics (distribution of REEs and indicator incompatible elements), the gabbroids of the first group are akin to both island-arc tholeiites and boninites. The olivine-bearing rocks of the second groups show boninitic affinity. Based on these observations, it was concluded that the intrusive complex of the Gromadnen-Vurguveem massif was formed during an early stage of the development of an ensimatic island arc.     

Relict sulfides in terrigenous rocks of the Upper Kolyma Region

In terrigenous flysch strata of the Upper Kolyma Region in northeastern Russia, microaggregates of sulfides with spherical form are widespread. During dynamic metamorphism, spherical sulfide units were segregated into lenticular aggregates along the cleavage planes, were as seed, or were recrystallized into faceted metacrystals. Further stress on the rock sometimes led to a morphologically expressed particular dissolution of pyrite crystals along the cleavage directions, and to their depletion with admixture elements. A more substantial effect led to transformation of pyrite into pyrrotine with inclusions of chalcopyrite and Fe-Ni-Co-sulfoarsenides. Relict sulfides reflect, to a certain degree, the primary geochemical condition of the sedimentation period and its further evolution. When studying the numerous sulfidization zones, the composition of relict sulfides allows us to predict the geochemical specialization and the degree of inheritance of post-sedimentation mineralization.     

陆源碎屑岩中的古土壤在地层分析中的应用

对各种古土壤的分类标准进行比较之后,重点介绍了Mack的古土壤分类及其土壤的演变序列。Mack的古土壤分类主要依据稳定矿物和保存在土壤中的古地貌属性,将古土壤划分为9种。在地层序列中古土壤的识别标志主要包括颜色、粒度、生物成因构造（植物生长痕迹）、原生沉积构造退化特征、层次化和边界特征、宏观结构特征、矿物及地球化学特征等。可以通过野外观察、室内显微镜观察、实验室化验及地震、测井资料来进行鉴别。陆相盆地具有很强的分隔性,盆地相变又非常复杂,寻找区域性较稳定的层序界面比较困难。越来越多的研究证实用古土壤划分陆相成因地层层序、解决地区及全球对比具有重要的实用价值。     

Relations between volcanism and granitic intrusions in West Chukotka

Granite massifs closely associated with effusives constitute a long series, from diorites and syenite-granites to potassic granites. Each variety has its petrographic and petrochemical analogues among effusive rocks. This is proof positive of their magmatic origin. In each specific instance, differences in the original composition and in means by which a connection between the intrusive and effusive magmas is achieved controlled the trend of chemical differentiation. The minor effusive fields, closely related to large granitic massifs,are characterized by their complex structure, the presence of contemporaneous rocks of different composition, irregular stratification, abundance of coarse tuffs, and fairly common homogeneous inclusions of almost completely crystallized rocks. Such rocks vary in composition from highly acidic (ignimbrites) to fairly basic, with a somewhat higher alkalinity (trachybasalts and melanocratic trachyandesites). On this basis and also structure the late Mesozoic volcano-plutonic rock association of Chukotka may be assigned, as a whole, to the diorite-andesite group and perhaps to a special monzonite type within it. — R. M. Hutchinson.     

Lignin as an indicator of the sedimentation conditions on the Arctic shelf

It is shown for the first time that the proportion of lignin in shelf deposits may range from 1/3 to the total concentration of organic carbon. The distribution of wood residues and lignin in the studied sedimentary formation is very uneven, which reflects the dynamic flux environment of sedimentation and OM supply. The calculated molecular indexes showed that most of the OM in deposits underwent insignificant diagenetic alterations.     

Computer modeling of folding in rocks

The results of numerical mathematical modeling of rock deformations under compression are given. The numerical solutions are obtained using discretization of the equations of the solid mechanics with the finite element method. The model of an ideal elastic-plastic material with a Huber-Mises yield surface was used in the calculations. The layered medium structure is taken into account in modeling of the compression of layers of the lower/middle crust on a local scale. The natural folds in strongly deformed metamorphic sequences were reproduced by the mathematical deformation models. It is shown that folding in the lower part of the Earth’s crust is possible, when the yield stresses of the host rocks are approximately two orders of magnitude less than those of the hard layers. The effect of the boundary conditions and thickness of the compressed rocks on the folding is shown.     

Oxygen isotope composition of kaolinite rocks as an indicator of different stages of their lithogenesis

Doklady Earth Sciences -     

Wall-rock metasomatism of carbonaceous terrigenous rocks in the Lena gold district

The Lena gold district is situated in the fold-and-shear belt of the southern framework of the Siberian Platform. The gold deposits are hosted in the Riphean-Vendian Khomolkho and Aunakit formations, revealing the strict control of ore mineralization by folding and shearing. The microstructure of metasomatically altered ore-bearing carbonaceous sedimentary rocks at the Sukhoi Log, Golets Vysochaishy, and Verninsky deposits (the latter includes the Pervenets vein zone) testifies to parallelism in the development of shearing, foliation, and ore-forming metasomatism. The local pressure gradients are marked by removal of silica from pressured zones into opened cleavage fractures and pockets. Two metasomatic stages are recognized: (1) early sodic metasomatism, which is characterized by the assemblage of magnesian siderite and paragonite, and (2) late potassic metasomatism, with formation of muscovite in association with sideroplesite and ankerite. The rocks altered at the early stage are distinguished by elevated Ni, Cr, and probably PGE contents. The second stage, close in age to the emplacement of Hercynian granitic plutons, was accompanied by the gain of chalcophile metals and deposition of the bulk of gold. In mineral composition, the metasomatic rocks are close to beresites, but the alteration differed in somewhat elevated alkalinity, so that microveinlets of albite and potassium feldspar occur in the ore zone together with muscovite. The ratio of modal muscovite to paragonite contents in orebodies is substantially higher than in the surrounding metasomatized rocks. This ratio directly depends on the degree of rock permeability and the intensity of the flow of ore-forming solutions. Carbonaceous matter (CM) in the ore zone underwent reworking and redeposition. CM is graphitized to a lesser extent than in the rocks affected by regional metamorphism. The spatial distribution of CM containing nitro and amino groups indicates more oxidizing conditions in the zone of ore deposition than at a distance from this zone. The temperature of metasomatic processes estimated from the muscovite, muscovite-paragonite, and chlorite mineral thermometers and fluid inclusions in quartz was 300–350°C at a pressure of about 1 kbar. The S, O, and C isotopic compositions of ore-forming fluids that pertain to the second stage of metasomatism (δ³⁴S= +8.5‰, δ¹⁸O = +10‰, and δ¹³C= ?11 to ?18‰) indicate their crustal origin. The generally similar conditions and products of the ore-forming metasomatic process at the giant Sukhoi Log deposit and at the small Golets Vysochaishy deposit are combined with some differences. The formation of the described deposits was related to the deep convection of fluids along shear zones followed by more local flows of postmagmatic solutions derived from the emplaced granitic magma.     

Stochastic structure of grain sequences in granitic rocks of Newfoundland as an indicator of initial stages of metasomatism

Granitic rocks of Newfoundland are described on the basis of field observations and analysis of grain sequences of essential minerals and are compared with "ideal granite," a concept developed in investigation of Eurasian intrusives. Samples included ideal granites and granites affected by the earliest stages of metasomatism. All non-ideal granites have been albitized. Albitization is most intensive in Precambrian adamellites of the Avalon Platform, fine-grained granites of the Gander massif are albitized and silicified, and alkali granites of the Buchens Mine region are K-feldspathized and silicified. — Author.     

Spring flux as an indicator of source geomorphology,substrata, and temperature conditions in springs

Hydrogeology Journal - Spring flux (m/s), defined as spring flow (m3/s) divided by the area over which groundwater emerges (m2) at a spring, provides a meaningful way to distinguish between...     

Microbial respiratory quinones as indicator of ecophysiological redox conditions

The bacterial respiratory quinones and membrane phospholipid fatty acids (PLFA) were measured to test the biochemical responses to the redox conditions after the respiration of diverse electron acceptors by microorganisms. Shewanella putrefaciens strain CN32 was examined for its growth with O₂, nitrate, ferrihydrite, ferric citrate, and sulfite as electron acceptors. The same parameters were also measured for Desulfovibrio desulfuricans strain G-20, Geobacter metallireducens strain GS-15, Thioploca spp., two strains of magnetotactic bacteria (Magneteospirilum magnetotactium marine vibrioid strain MV-1 and M. sp. strain AMB-1), and environmental sediments. Microorganisms with aerobic respiratory of oxygen (MV-1 and AMB-1) have high ratios of monounsaturated to saturated straight chain PLFA and ubiquinone to menaquinone ratios; while those that conduct strict anaerobic respirations (G-20 with sulfate and GS-15 with ferric iron) have low ratios of monounsaturated to saturated straight chain PLFA and uniquinone to menaquinone ratios. The facultative respiratory of nitrate (Thioploca) has these parameters in the middle. The ratios of menaquinones to ubiquinones in CN32 cells systematically increase according to the increase of redox potential and bioavalibility of electron acceptors. The correlation between σUQ-n/σMK-n ratios and redox conditions indicates the structure of respiratory quinone responses sensitively to the microbial ecophysiological conditions.     

Correlation of Magmatic and Tectonic Events in the Evolution of West Chukotka Ophiolites

Doklady Earth Sciences - The article is an overview of the magmatic and geodynamic processes that formed Aluchin and Gromadnen-Vurguveem ophiolites in the territory of Western Chukotka. The...