The petrology and tectonic setting of Quaternary—Recent volcanic centres of Lombok and Sumbawa,Sunda arc

In the Sunda arc, only the Bali—Lombok—Sumbawa sector is apparently flanked both north and south by oceanic crust. South of Lombok Island the oceanic crust is probably of Early Cretaceous or Late Jurassic age, whereas the oldest rocks known from Lombok and Sumbawa islands are the Lower Miocene to Pliocene sediments and volcanics of the basement beneath the Quaternary—Recent volcanic centres.Three large active volcanoes form the northern parts of Lombok and Sumbawa. The volcanic rocks of Rindjani on Lombok belong to a basalt—andesite—dacite association, rich in plagioclase and hy- and Q-normative. East of Lombok, the volcanic rocks of Tambora and Sangeang Api on Sumbawa belong to a potassic ne—trachybasalt—trachy-andesite association. All three volcanoes occur only 150–190 km above the active north-dipping Benioff zone.Extinct Quaternary centres occur south of the active volcanoes on Sumbawa. Two of these centres, Soromundi and Sangenges, erupted markedly ne- and lc-normative leucitites together with andesites, dacites and trachybasalts.The volcanic composition—space—time relations in the Lombok—Sumbawa sector of the Sunda arc are not in accordance with the generalized island-arc schemata. Conventionally, potassic ne-mnormative island-arc associations are supposed to occur over the deep part of the Benioff zones, far from the trenches of mature island arcs. The SiO₂|K₂O relations of the Rindjani association are reasonably appropriate for a volcano overlying intermediate Benioff-zone depths, but both the Tambora and the Sangeang Api associations are far more potassic than would be predicted by generalized schemata, and also occur in a relatively young arc sector that apparently has developed only since Miocene time.Basalts, trachybasalts and leucitites from the Lombok—Sumbawa sector have been compared: at similar MgO contents and Mg/(Mg+Fe), the progression from hy- and Q-normative to ne- and lc-normative magmas is not marked by significant enrichment in TiO₂, Na₂O, Zr, Nb and P, but is accompanied by a substantial increase in K₂O, Rb, Sr and LREE, by increasing $\text{K}{}_2\text{O}\text{Na}{}_2\text{O}$ and by decreasing K/Rb.${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios from Rindjani (0.70386–0.70402) and Tambora (0.70385–0.70389) are very similar and among the lowest for the Sunda arc, but from Sangeang Api (0.70460–0.70500) are significantly higher and more variable in spite of the similar tectonic setting and petrological affinities. ${}^{87}\text{Sr}{}^{86}\text{Sr}$ ratios of leucities tend to be higher (0.70488–0.70529).The petrogenesis of the volcanic associations of Lombok and Sumbawa cannot be readily explained. Although even the leucitites display the poverty in TiO₂ that generally characterizes volcanics from simple island-arc tectonic settings, there is very obvious uncoupling within the “incompatible elements”: enrichment in the LIL group (K, Rb, Sr but not Na) is not accompanied by similar behaviour in the group of small highly-charged ions (Ti, Zr, Nb, P). It has proved impossible to model this behaviour without invoking inhomogeneities in the source regions, both in mineralogy and in chemical composition. Similar uncoupling within the incompatible elements has also been reported from basalt groups from the Mid-Atlantic Ridge, may also occur in the Birunga province, and might not arise from processes unique to the island-arc environment.We suggest that a LIL-rich component is being progressively added to the source regions. This component could be incorporated by the crystallization of additional phases such as phlogopite or paragasite. If this component occurs deep within the mantle, it might gain passage to shallower regions either by percolating up the downgoing slab to yield the familiar arc magma zonation, or up substantial cross-arc fractures.     

Authigenie gypsum in deep-sea manganese nodules

Gypsum crystals of authigenic origin have been found in manganese nodules of the deep-sea sediment surface (4500-5500 m water depth) from the Central Pacific Ocean.     

On the variability of the position of the equatorial electrojet axis in India

An examination of day-to-day and monthly mean positions of the electrojet axis in relation to the changes in the apparent solar declination, in the Indian equatorial region shows marked association between the two. For relatively quiet days, significant correlations are observed between the solar declination and each of the parameters, the northernSq focal latitude, the jet axis and the line of maximumSq(H). From the significant mutual association of these parameters, it has been suggested that the equatorial electrojet could be a part of the world-wideSq current system.     

Lower riphean low-carbonaceous shales in the southern urals: Raw material for the manufacture of porous rubble

A property of swelling on heating to yield firm lightweight granules has first been revealed for lowcarbonaceous shales (LCS) from the Riphean stratotype in the Bashkirian Megaanticlinorium, Southern Urals. These granules possess all technological properties of “keramzite” or claydite (an artificial, expanded clayey material) and can successfully substitute this material. Moreover, the procedure of their manufacture, analogous to the manufacture of “shungizite” (an artificial carbonaceous-clayey material) from Lower Proterozoic clayey rocks of Karelia, is more economical than that for keramzit. Productive beds of the homogeneous LCS make up extended bodies tens of meters thick. At the present time, they can be used as cover rocks during the exploitation of the Bakal siderite deposits. The LCS represent fine-grained quartz-chlorite-sericite aggregates (with variable relationships between the major components), bearing fine-dispersed shungite in amounts of 0.5–1.0%. High-quality raw material of this kind is characterized by a fine-dispersed admixture of a mixed-layer clayey material bearing montmorillonite and vermiculite. Geological, lithological, mineralogical, and chemical prognostic criteria have been developed for the shungizite raw material. It has been concluded that this construction material, new for the Urals, is promising for industrial use     

Beobachtungen an der Küste von Suriname

Ohne Zusammenfassung     

Strontium isotope studies on ultramafic inclusions from Dreiser Weiher,Eifel, Germany

The isotopic composition of strontium, and rubidium and strontium concentrations in eight ultramafic inclusions and two host basalts from Dreiser Weiher, Eifel, Germany are presented. Comparison of the ⁸⁷Sr/⁸⁶Sr ratios of the inclusions with the basaltic hosts indicates that the lherzolite inclusions are exotic in nature but the wehrlite-clino-pyroxenites are probably cognate with the host rocks. An apparent age of 550 m.y. (±a large uncertainty) shown by the lherzolites is believed to reflect a mantle event.     

Plastic compaction of cemented granular materials

We analytically relate hydrostatic stress to strain in a random dense pack of identical spheres cemented at their contacts. The spheres are elastic and the cement is perfectly plastic. This solution for the sphere pack is based on a solution for the normal interaction of two cemented spheres. Initially, the two spheres touch each other at a point. We show that, as loading increases and cement becomes plastic, a finite (Hertzian) direct-contact area between the spheres necessarily has to develop and progress. The stress-strain behavior of the pack depends on the cement's yield limit and on the amount of cement. At the same hydrostatic stress, the deformation of the cemented aggregate is smaller than that of the uncemented one. This difference becomes large as the yield limit increases. We calculate the bulk modulus of an aggregate from the stress-strain curve. In the plasticity domain, the bulk modulus of the cemented aggregate is smaller than that of the uncemented one. The difference between the two may easily reach 50%. Of course, as the cement's yield limit decreases, the aggregate's stress-strain curve and the bulk modulus approach those of the uncemented sphere pack. This theoretical conclusion is qualitatively supported by experiments on epoxy-cemented glass beads. The maximum contact stress in the cemented aggregate may be less than a half of that in the uncemented one. This result explains an experiment where an uncemented glass bead sample failed at a hydrostatic stress of 50 MPa, whereas an epoxy-cemented sample stayed intact.