Fe- and Al-Rich metapelites of the Teiskaya Group, Yenisei Range: Geochemistry, protoliths, and the behavior of their material during metamorphism

The Fe- and Al-rich metapelite of the Teiskaya Group in the trans-Angara part of the Yenisei Range are characterized by variable P-T parameters of their metamorphism. Geochemical data on these rocks were used to reproduce the nature and composition of their protolith. The metapelites were determined to be redeposited and metamorphosed Precambrian weathering crusts of the kaolinite type, which were produced by the erosion of Archean rocks of predominantly acid (granitoid) composition in shallow-water continental-margin basins in a humid climate. These results are consistent with the results of the lithological-facies analysis and geodynamic reconstruction of the Precambrian geological evolution of complexes in the Yenisei range. Differences were revealed between the REE patterns in metapelites in various metamorphic zones caused by both the compositional heterogeneity of the protoliths and the prograde (in terms of pressure) mineral reactions of collision metamorphism with the predominant effects of various processes during different evolutionary stages.     

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The effects of liquid immiscibility and thermal diffusion on oxygen isotopes in silicate liquids

Differences between the δ¹⁸O values of Si- and Fe-rich immiscible liquids in the system Fe₂SiO₄-KAlSi₂O₆-SiO₂ (Fa-Lc-Q) in isothermal experiments at 0.1 MPa have been determined experimentally to be 0.6 permil. The observed partition of ¹⁸O into the Si-rich liquid is consistent with previous experience with the preferential partition of ¹⁸O into Si-rich minerals in isothermal equilibrium with minerals of less polymerized structure. Crystallochemical principles affect the distribution of oxygen isotopes in coexisting isothermal liquids in the same way as they apply to isothermally coexisting crystals. The effects of Soret (thermal) diffusion on the distribution of oxygen isotopes in silicate liquids above the solvus in the system Fa-Lc-Q under conditions of an imposed temperature gradient of ca. 250 °C over 4 mm and at 2 GPa have also been investigated experimentally. Both the magnitude and the direction of separation of oxygen isotopes as a result of Soret diffusion are unexpected. For each of the silicate liquids, the cold end of the charge is enriched in ¹⁸O by up to 4.7 permil, and the highest δ¹⁸O values are associated with the most silica-poor compositions. The distribution of oxygen isotopes appears to be similar in each liquid, regardless of their chemical compositions, which is in contrast to the behaviour of cations whose distributions are compositionally dependent and characterized by strong crystallochemical effects wherein network-forming species such as Si and Al separate to the hot end and Mg, Fe and Ca are segregated preferentially to the cold end. Structural units in the melts are evidently less selective between oxygen isotopes than between cations, because oxygen redistribution over all possible sites in these units proceeds according to mass. Self-diffusion coefficients of oxygen in basaltic liquids estimated from the Soret experiments are in accord with those from other isotope tracer experiments, and comparable to those of Si. The possible effects of Soret diffusion on the oxygen isotopic composition of metasomatic veins in the mantle are examined in light of these data, and indicate that decay of the thermal gradients in the veins exceeds that of the diffusion of oxygen needed to produce variations in the δ¹⁸O values of mantle minerals. Variations in oxygen isotope ratios in most natural systems as a result of Soret effects are unlikely. Received: 6 January 1997 /  Accepted: 28 June 1998     

Radiating 1.0 Ga Mafic Dyke Swarms of Eastern Brazil and Western Africa: Evidence of Post-Assembly Extension in the Rodinia Supercontinent?

Several mafic dyke swarms of similar composition and age (tholeiite- ca.1.0 Ga) occur on both sides of the Atlantic Ocean in eastern South America and western Africa. When assembled to their pre-drift position in the Mesozoic, the Brazilian coastal dyke province of Bahia, and the African dykes in Cameroun (Ebolowa suite) and Congo (Comba and Sembe-Ouesso provinces) define a giant radiating pattern (1200 km × 800 km) similar to other dyke swarms elsewhere associated with large-scale continental rifting. Magma flow indicators of the Brazilian dykes and branching propagation styles of their African counterparts indicate that the dyke conduits were fed with magmas diverging from a source beneath the long axis of the Meso-Neoproterozoic West-Congolian Basin in Africa. There, MORB-like metabasalts have been described in the La Bikossi Group of the Mayombian Supergroup. Whether the rifting event and intrusion of dyke swarms were triggered or not by a mantle plume beneath part of the Rodinia subcontinental lithosphere remain to be confirmed.     

Anatomy and origin of toplap in a mixed carbonate-clastic system, Seven Rivers Formation (Permian, Guadalupian), Guadalupe Mountains, New Mexico, USA

The Seven Rivers Formation exposed in Slaughter Canyon, Guadalupe Mountains, New Mexico, reveals complex relations between long- and short-term relative changes in sea-level, shelf configuration and sedimentation, which interacted to create a distinct toplap geometry. At least five sandstones diverge basinward from a prominent boundary unit marking the surface of toplap at the top of the Seven Rivers Formation and create a series of prograding, shingled clinoforms. The boundary unit is a horizontal, well-sorted, quartz arenite underlain across the shelf by peritidal carbonate or by other merging sandstones. Preserved palaeotopography is indicated by facies changes downdip and the presence of horizontal geopetal indicators in inclined beds. Near the boundary unit (updip), merging sandstones contain rare sedimentary structures including evaporite moulds and irregular fenestrae and are bounded above and below by peritidal carbonate with microbial laminae, fenestral fabrics and mudcracks. Laterally (downdip), the sandstone-bounding peritidal carbonate facies pass into subtidal carbonate facies (ooid-peloid-fusulinid-dasyclad-mollusc pack- and grainstone) and interbedded sandstones contain sedimentary structures such as ripple marks and trough to planar cross-stratification, as well as ooids, fusulinids and other carbonate grains. Toplap is interpreted to have developed by sediment bypass across a subaerially exposed shelf while sedimentation continued in still-submerged areas downdip from the shelf crest, and hence represents depositional toplap. Physical tracing of subaerial exposure surfaces suggests that the shoreline migrated up and down palaeoslope several times. The vertical component of five short-term shoreline migrations decreased during formation of the toplap geometry. Sea-level rose to approximately the same position following each fall to create the toplap geometry. This depositional toplap is the stratigraphic result of high-‘frequency’ relative changes of sea-level that combined to produce the larger-scale geometry. We suggest that changing amplitudes of relative sea-level may play a significant role in the stratigraphic evolution of platforms and that separating ‘short-term’and ‘long-term’relative sea-level may be ambiguous in such instances.     

Microzoning of agates as a reflection of their formation dynamics

Agates are natural hierarchicalal structural assemblies composed largely of silica minerals: chalcedony, quartz, and quartzine. Microstructural heterogeneities, most typical of those of agate-forming minerals, are characterized by variations in periods of helicoidal twisting of chalcedony and by micrometric rhythmic alternation of zones with a low refractive index within the matrix of normal chalcedony, quartz, and quartzine. A study of the degree of crystallization of chalcedony using the Barsanova-Yakovleva method showed that neither helicoidal twisting nor refractive index zoning brought about any significant disturbance in the chalcedony structure. Helicoidal twisting is a result of the stresses caused by substitution of Si⁴⁺ for Al³⁺ asymmetric with respect to chalcedony crystal. It is suggested that layering by the refractive index developed due to accumulation of small portions of gel consisting of high silica polymers at the front of crystal growth.     

AbnormalSq behaviour

It is well-known that the amplitude and phase of theSq(H) variation show considerable variability from day to day. In this paper we consider one aspect of the phase variability—that associated with AQDs. AQDs (or abnormal quiet days) are defined as magnetically quiet days where the maximum excursion ofH at a mid-latitude station on the poleward side of the focus occurs outside the normal time range 0830–1330 LST. Such days exhibit properties, many of which appear quite distinct from the properties of the normalSq(H) variation. The properties of AQDs, and the proposals that have been made to explain them, are considered in detail. The consequences of these proposals and some problems which need to be addressed in order to obtain a fuller understanding of the dynamics of the ionosphere on AQDs are also discussed.     

Temporal variations in the position of the heliospheric equator

It is shown that the centroid of the heliospheric equator undergoes quasi-periodic oscillations. During the minimum of the 11-year cycle, the centroid shifts southwards (the so-called bashful-ballerina effect). The direction of the shift reverses during the solar maximum. The solar quadrupole is responsible for this effect. The shift is compared with the tilt of the heliospheric current sheet.