Postsedimentation transformation of triassic terrigenous rocks in West Chukotka as an indicator of folding conditions

Postsedimentation alteration and structural assemblies of the Triassic sedimentary complexes of West Chukotka are discussed. Zoning of the alteration is based on examination of newly formed structural and mineral assemblages, the chemical composition, and the polytypes of clay minerals. Three zones of postsedimentation transformation of sandstones are distinguished: (1) the zone of chlorite, illite, and mixed-layer disordered chlorite-smectite; (2) the zone of illite and chlorite; and (3) the zone of phengite and ferroan chlorite. The grade of postsedimentation transformation and the composition of the newly formed micas are correlated with the cleavage type. The development of two-three types of cleavage leads to the highest degree of rock transformation. The assemblages of clay minerals and the crystal chemistry of the authigenic phengite show that the grade of postsedimentation transformation of the Triassic rocks attains the stage of greenschistfacies metamorphism in the zone of development of two cleavage types. Where the second cleavage is not documented or poorly developed, the rocks remain unmetamorphosed. Evidence is given that postsedimentation transformation of terrigenous rocks in the foldbelt is controlled largely by deformation.     

Sorption behaviour of pentachlorophenol in sub-Saharan tropical soils: soil types sorption dynamics

In order to predict exposure risks as well as appropriate remediation strategies for pesticides in soils, knowledge of pesticides sorption processes onto various representative soils is vital. Hence, laboratory batch experiments were carried out to study sorption of a pesticide, pentachlorophenol (PCP), on five soils obtained from different sub-Saharan agro-ecological zones (AEZs) in order to understand sorption equilibrium, kinetics, and thermodynamics. Experimental data showed that sorption equilibrium was attained within 24 h. The fitting of kinetic results and equilibrium data to different models suggested partly surface adsorption and partly partitioning of PCP within voids of the various soil components. Sorption was mainly attributed to sharing or exchange of valence electrons between negatively charged PCP molecules and positively charged soil sorption sites. The sorption process was spontaneous and accompanied by decreased entropy, but was pH and temperature dependent, reducing with increase in pH and temperature. The various soils’ PCP sorption capacities were directly proportional to their cation exchange capacities. The low PCP sorption observed in these soils suggested high risk of PCP being present in soil water solution, especially at higher temperatures, which can lead to contamination of the aquifer. This risk may be higher for soils obtained from AEZs with warmer natural temperatures.     

Raman spectroscopic study of K-lingunite at various pressures and temperatures

K-lingunite is a high-pressure modification of K-feldspar that possesses the tetragonal hollandite structure. Variations of the Raman spectra of K-lingunite were studied up to ~31.5 GPa at room temperature, and in the range 79–823 K at atmospheric pressure. The Raman frequencies of all bands were observed to increase with increasing pressure, and decrease with increasing temperature for K-lingunite. This behavior is in line with those observed for most of other materials. New sharp Raman bands appear at pressures greater than 13–15 GPa, suggesting a phase transition in K-lingunite with increasing pressure. The transition is reversible when pressure was released. The appearance of these new Raman bands may correspond to the phase transition revealed earlier at around 20 GPa by X-ray diffraction studies. Instead of transforming back to its stable minerals, such as orthoclase, microcline or sanidine, K-lingunite became amorphous in the temperature range 803–823 K at atmospheric pressure.     

Metamorphic evolution of garnet-bearing ultramafic rocks from the Gongen area, Sanbagawa belt, Japan

Garnet‐bearing ultramafic rocks including clinopyroxenite, wehrlite and websterite locally crop out in the Higashi‐akaishi peridotite of the Besshi region in the Cretaceous Sanbagawa metamorphic belt. These rock types occur within dunite as lenses, boudins or layers with a thickness ranging from a few centimetres to 1 metre. The wide and systematic variation of bulk‐rock composition and the overall layered structure imply that the ultramafic complex originated as a cumulate sequence. Garnet and other major silicates contain rare inclusions of edenitic amphibole, chlorite and magnetite, implying equilibrium at relatively low P–T conditions during prograde metamorphism. Orthopyroxene coexisting with garnet shows bell‐shaped Al zoning with a continuous decrease of Al from the core towards the rim, consistent with rims recording peak metamorphic conditions. Estimated P–T conditions using core and rim compositions of orthopyroxene are 1.5–2.4 GPa/700–800 °C and 2.9–3.8 GPa/700–810 °C, respectively, implying a high P/T gradient (> 3.1 GPa/100 °C) during prograde metamorphism. The presence of relatively low P–T conditions at an early stage of metamorphism and the steep P/T gradient together trace a concave upwards P–T path that shows increasing P/T with higher T, similar to P–T paths reported from other UHP metamorphic terranes. These results suggest either (1) down dragging of hydrated mantle cumulate parallel to the slab–wedge interface in the subduction zone by mechanical coupling with the subducting slab or (2) ocean floor metamorphism and/or serpentinization at early stage of subduction of oceanic lithosphere and ensuing HP–UHP prograde metamorphism.     

Petrology,geochemistry and geochronology of the Chilka Lake igneous complex,Orissa state,India

The Chilka Lake igneous complex of Orissa, the largest known anortosite massif of the Indian Shield, occurs in a catazonal environment of high-grade metamorphics of the Eastern Ghats Precambrian Orogenic Province. The syntectonic massif consists of the anorthositic Balugaon dome, leuconoritic Rambha lobe and quartz-mangeritic Kallikota cover. A completely gradational suite comprising anorthosite-leuconorite-norite-minor jotunite (the anorthositic suite) constitutes most of the complex. The subordinate of suite of acid rocks spatially associated with this is of a broad quartz-mangeritic lithology with minor granitic rocks (the acidic suite). Geochemical evolution of the complex in the sequence anorthosite-leuconorite-norite-jotunite-acidic rocks shows moderate iron enrichment in the noritic-jotunitic stage and is marked by an overall decrease in Al₂O₃, CaO, MgO, Ni/Co, Sr/Ba, K/Rb and increase in SiO₂, K₂O, V/Ni, K/Ba and Rb/Sr. Such progressive variation in geochemical parameters appears (i) essentially gradual and frequently overlapping in rock members of the intergradational anorthositic suite and (ii) rather abrupt across transition zones between the anorthositic suite and the acidic suite due to near absence of intervening intermediate lithologies. RbSr whole rock isochron studies indicate that the complex was emplaced ca. 1400 Ma ago. The initial ⁸⁷Sr/⁶⁸Sr (0.70661) implies limited hybridisation of the parent magma prior to emplacement. A critical appraisal of all the available evidence suggests that (i) the anorthositic suite of rocks form a perfectly consanguinous and comagmatic assemblage and (ii) the spatially associated acidic suite emerged through a convergence of magmatic and metasomatic processes (the latter brought about by contact anatexis of the host rocks). The complex as well as the host metamorphics are intruded by an atectonic suite of noritic dykes emplaced ca 850 Ma ago.     

A modeling of the structure and favorable H-docking sites and defects for the high-pressure silica polymorph stishovite

Employing first-principles methods, the docking sites for H were determined and H, Al, and vacancy defects were modeled with an infinite periodic array of super unit cells each consisting of 27 contiguous symmetry nonequivalent unit cells of the crystal structure of stishovite. A geometry optimization of the super-cell structure reproduces the observed bulk structure within the experimental error when P1 translational symmetry was assumed and an array of infinite extent was generated. A mapping of the valence electrons for the structure displays mushroom-shaped isosurfaces on the O atom, one on each side of the plane of the OSi₃ triangle in the nonbonded region. An H atom, placed in a cell near the center of the super cell, was found to dock upon geometry optimization at a distance of 1.69 Å from the O atom with the OH vector oriented nearly perpendicular to the plane of the triangle such that the OH vector makes a angle of 91° with respect to [001]. However, an optimization of a super cell with an Al atom replacing Si and an H atom placed nearby in a centrally located cell resulted in an OH distance of 1.02 Å with the OH vector oriented perpendicular to [001] as observed in infrared studies. The geometry-optimized position of the H atom was found to be in close agreement with that (0.44, 0.12, 0.0) determined in an earlier study of the theoretical electron density distribution. The docking of the H atom at this site was found to be 330 kJ mol^–1 more stable than a docking of the atom just off the shared OO edge of the octahedra as determined for rutile. A geometry optimization of a super cell with a missing Si generated a vacant octahedra that is 20% larger than that of the SiO₆ octahedra. The valence electron density distribution displayed by the two-coordinate O atoms that coordinate the vacant octahedral site is very similar to those displayed by the bent SiOSi angles in coesite. The internal distortions induced by the defect were found to diminish rather rapidly with distance, with the structure annealing to that observed in the bulk crystal to within about three coordination spheres.     

A reappraisal of episodic burial metamorphism in the Andes of central Chile

The Jurassic to Miocene sequences of the central Andes, east of Santiago, reputedly show repeated cycles of episodic sub-greenschist facies, burial metamorphism that are identified by sharp breaks in metamorphic grade at major stratigraphic boundaries. This paper presents the first detailed petrochemical analysis of these low-grade metamorphic sequences by examining the progressive development of secondary minerals, reaction progress in mafic phyllosilicates, and topological variations in the low-grade assemblages as a means of testing this model. The results indicate a progressive increase from zeolite facies through to close to the onset of greenschist facies from Miocene to Jurassic rocks. Combined analysis of reaction progress in mafic phyllosilicates and petrochemical relationships of chlorite–pumpellyite–actinolite in metabasites provides no evidence for sharp metamorphic breaks at major stratigraphic boundaries. Integrating the results presented here with the most recent models of stratigraphic/tectonic development of the central Andes shows that the metamorphism took place in two episodes, and was not episodic on a 40-million-year cycle. An absence of sharp breaks in metamorphic grade in any part of the succession, as demonstrated here, shows that the original petrographic establishment of low-grade facies provided insufficient resolution of changes in metamorphic conditions to establish definitive evidence of such breaks. Accordingly, this study suggests that re-assessment of metamorphic breaks reputedly developed in other areas of the Andean Cordillera is imperative in order to resolve the questions raised here about the origin of the low-grade metamorphism.Editorial responsibility: B. Collins     

Platinum group minerals in podiform chromitites of the Bou Azzer ophiolite, Anti Atlas, Central Morocco

Summary The Neoproterozoic Bou Azzer ophiolite complex hosts numerous, small lenticular bodies of massive and disseminated chromite. Metallurgical-grade high-Mg and high-Cr spinels (cores with 48–62 wt% Cr₂O₃) reveal complex alteration patterns of successive Cr and Mn enrichment and loss of Al towards the rims, while the Mg# ratios [(Mg/(Mg + Fe²⁺)] remain almost constant. Concentration patterns of platinum-group elements are typical for ophiolitic chromitite poor in sulfides, with predominance of the IPGE, variable Rh, and low Pt and Pd. The most abundant platinum-group mineral is Rh-bearing laurite that occurs either included in spinel or in silicate matrix, whereas Os-Ir-Ru alloy is always included in spinel. Laurite inclusions reveal complex intergrowth textures with Rh-Ru-Pt rich alloy, and with Rh-rich sulfide. Most laurites display trends to sulfur-poor compositions leading to local formation of very fine-grained Ru-Os-Ir alloy phases. Ni-Co-Fe sulfides, arsenides and sulfarsenides devoid of PGE are associated with the alteration of chromite. Textural position and chemical composition of the base metal inclusions, as well as comparison of alteration features between chromite and accessory chromian spinel in the Co-Ni-As ores of the Bou Azzer ophiolite indicate a close connection. It is suggested that hydrothermal fluids percolated through the marginal zones of the ophiolite belt during greenschist facies metamorphism and deposited Ni-Co-Fe arsenides, sulfarsenides and minor sulfides as accessories within altered chromitites, and also in structurally favourable zones as Ni-Co-As ores. Author’s address: Dr. Frank Melcher, Federal Institute for Geosciences and Natural Resources, Stilleweg 2, 30655 Hannover, Germany     

The effect of anatase, rutile and sodium titanate on the dissolution of boehmite and gibbsite at 90 °C

Industrial plants that refine bauxite to alumina using the high temperature process have always held the belief that anatase was detrimental to the extraction of boehmite while rutile was not. This study shows that this effect is real and that it is observable at temperatures as low as 90 °C. The extraction of gibbsite is shown to be unaffected which leads us to believe that the kinetics of both the Ti-bearing mineral and the Al-bearing mineral is important in this phenomenon. In addition, it is shown that not only is the presence of anatase an issue in boehmite extraction but so too is the presence of sodium titanate. Rutile was found to have the least impact of the three mineral phases.     

Adjustments by the Charwell River, New Zealand, to uplift and climatic changes

Major climatic changes and rapid local and regional tectonic movements were common in New Zealand during the late Quaternary and caused a diversity of adjustments in the drainage-basin and piedmont reaches of the Charwell River, which are separated by the Hope Fault. The onset of semi-arid, frigid climates during the latest Pleistocene probably greatly increased hillslope sediment yields in a periglacial environment, and the piedmont reach aggraded as much as 42 m on top of a broad strath. With the return of humid, mesic climates in the Holocene sediment yields decreased as dense forests again mantled the slopes, and the piedmont reach degraded as mush as 81 m. Dating of eleven cut-and-strath terraces by radiocarbon-calibrated weathering rind measurements on greyawake cobbles shows the degradation rates varied greatly during the last 14 ka (1 ka = 1000 yr). Initial degradation rates of < 4 m ka⁻¹ increased to 30 m ka ⁻¹ by 6 ka ago during a mid-Holocene climatic optimum. Since 4 ka ago degradation rates have been only 1.2 m ka⁻¹, comparable to uplift rates in the piedmont reach inferred from marine-terrace studies, and the river is again cutting a broad strath. Each broad strath represents equilibrium conditions attained by this powerful stream during interglacial times despite episodes of being overwhelmed by climatically induced sediment-yield increases during full-glacial climates and having to maintain a long-term degradation rate equal to the uplift rate.The 75–81 m of degradation since formation of the latest Pleistocene fill-terrace tread is the sum of the amount of late Pleistocene valley-floor aggradation and the amount of regional uplift that occurred between the estimated times of major strath formation at about 30 and 0 ka. The 39 m of tectonically induced degradation below the pre-aggradation strath is sufficiently large that post-30 ka uplift may have doubled Holocene degradation rates.Each of the eleven degradation terraces represents pauses of a few centuries in Holocene downcutting. Brief equilibrium conditions were attained by streambed armoring and concurrent growth of riparian plants; both processes progressively increased hydraulic roughness and the shear stresses needed to entrain streambed materials. Occasional floods, possibly from rare cyclones derived from tropical moisture sources, destroyed streambed armor and channel downcutting was renewed. Thus the formation of eleven equilibrium terraces can be accounted for without postulating additional tectonic perturbations or secular climatic changes.