Geochemistry and petrogenesis of the tertiary alkaline volcanic suite of the Labe tectonovolcanic zone,Czech Republic

Summary The Tertiary alkaline, nepheline-normative olivine melanephelinite-basanite-phonolite suite in the Labe tectono-volcanic zone, Czech Republic, displays mineralogical and geochemical features indicative of the presence of products of more than one fractionation series and of more than one parental magma. The Mg- and Ni-rich olivine melanephelinites (ankaratrites), which contain disintegrated lherzolite xenoliths, represent low degree (c. 4%) partial melts of a highly enriched mande source. Many of the abundant basanitic rocks also represent primary or near-primary basaltic melts subject to a small but variable degree Fof olivine fractionation. Other basanites with lower mg-numbers and Mg and Ni contents had a crystallization history involving hornblende and phlogopite.Melaphonolites (tephriphonolites) provide a link between basaltic rocks and phonolites with plagioclase fractionation progressively playing a more significant role. The phonolites represent products of crystallization of highly evolved melts but poor correlation between SiO₂, MgO, CaO, Na₂O and degree of undersaturation as well as between major and trace element contents indicate that a single line of descent cannot have been in operation: the products of different parental magmas showing a wide compositional range that evolved under different conditions of fractionation are represented. A very advanced stage of magma evolution, characterized by the dominance of alkali feldspar fractionation is represented by phonolites that contain the highest Hf, Nb, Rb, Ta, Th, Y, Zr and REE in the whole Tertiary volcanic suite.

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Geochemie und Petrogenese der tertiären alkalischen Vulkanit-Serie der Labe tektonovulkanischen Zone, Tschechische Republik

Zusammenfassung Die tertiäre alkalische Nephelin-normative Olivin Melanephelinit-Basanit-Phonolit Serie in der Labe tektono-vulkanischen Zone, Tschechische Republik, zeigt mineralogische und geochemische Eigenschaften, die auf die Anwesenheit von Produkten von mehr als einer Fraktionierungsserie und von mehr als einem Ausgangsmagma hinweisen. Die Mg- und Ni-reichen Olivin Melanephelinite (Ankaratrite), die umgewandelte LherzolitXenolite enthalten, stellen einen niedrigen Grad (ca. 4%) teilweiser Aufschmelzung einer stark angereicherten Mantelquelle dar. Viele der verbreiteten basanitischen Gesteine stellen auch primäre oder fast-primäre basaltische Schmelzen dar, die in kleinem, aber schwankendem Ausmaß von Olivinfraktionierung betroffen wurden. Andere Basanite mit niedrigen Mg-Zahlen und Mg- und Ni-Gehalten hatten eine Kristallisationsgeschichte in der Hornblende und Phlogopit eine Rolle spielte.Melaphonolite (Tephriphonolite) stellen ein Verbindungsglied zwischen basaltischen Gesteinen und Phonoliten dar, wobei Plagioklas-Fraktionierung eine zunehmend größere Rolle spielt. Phonolite sind Produkte der Kristallisation von Schmelzen, aber die schlechte Korrelation zwischen SiO₂, MgO, CaO, Na₂O und dem Grad der Untersättigung, wie auch zwischen Haupt- und Spurenelementen weist darauf hin, daß eine einzelne Entwicklungslinie nicht in Frage kommt. Die Produkte verschiedener Ausgangsmagmen zeigen ein weites Spektrum verschiedener Zusammensetzung, das sich unter verschiedenen Bedingungen der Fraktionierung entwickelt hat. Ein sehr fortgeschrittenes Stadium der Magmaentwicklung, charakterisiert durch die Vorherrschaft von Alkali-Feldspat-Fraktionierung wird durch Phonolite dargestellt, die die höchsten Gehalte an Hf, Nb, Rb, Ta, Th, Y, Zr und REE in der gesamten tertiären vulkanischen Abfolge führen.

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With 7 Figures     

Ray amplitudes in a three-dimensional inhomogeneous medium

Summary A system of five ordinary differential equations of the first order to compute the geometrical spreading of the wave front of seismic body waves in a three-dimensional medium is suggested.     

Maximum carbon isotope fractionation in photosynthesis by blue-green algae and a green alga

The maximum carbon isotope fractionation occurring in photosynthetic fixation of carbon dioxide in pure cultures of blue-green algae was ?23.9%. and for a green alga was ?22.6%., Maximum fractionations were obtained where cell densities were low and carbon dioxide concentrations were greater than 0.5%. Fractionation was reduced at higher temperatures using a thermophilic blue-green alga. For filamentous blue-green algae wherein clumping occurs and localized cell concentrations were high, fractionation was also lower. Fractionations reported in literature for Precambrian organic materials are comparable to the maximum fractionations reported here. This suggests that the early photosynthetic organisms developed under conditions of high carbon dioxide availability, i.e. slow growth rates and/or low population densities.     

Primary and secondary synsedimentary structures in the lagoonal Almere member (Groningen Formation,Holocene, The Netherlands)

Two main types of synsedimentary structures are distinguished: those produced by the action of a transporting agent and those that have another origin. They are classified as primary synsedimentary and secondary synsedimentary, respectively. This distinction is thought to be useful, since only the first group may give information about the process of sedimentation itself and about paleocurrent directions. The second group might be indicative of sedimentation velocity and predominant grain size.In the sediments described here, the first group comprises parallel and irregular lamination, wave ripples, current ripples and orientated particles. These latter two structures allow the reconstruction of paleocurrent directions, which are compared with those found by the analysis of channel elongation and grain-size distribution. This leads to the conclusion that weak tidal currents towards the north and the south prevailed.The secondary synsedimentary structures are represented by graded bedding and shallow depressions. These latter were synsedimentarily deepened by the burden of the accumulating sediment. Their sizes vary from a few centimetres to several metres.     

Abrasion as an agent for sand supply in a holocene lagoon (Almere and Zuiderzee members,Groningen formation) in The Netherlands

Deposition of sand in a lagoonal environment is mainly restricted to the inlets and to the borders. An irregular bottom topography, however, may give rise to abrasion due to wave action. When the subsoil is sandy, this abrasion may result in a considerable supply of sand, which is redistributed throughout the lagoon. This phenomenon is observed in the sediments of the former Zuiderzee, where various types of sandy Pleistocene deposits (cover sands, river dunes and glacial till) have been abraded during the Holocene, when a lagoon covered that area.     

Late Pleistocene piñon pines in the Chihuahuan Desert

Examination of late Pleistocene packrat middens from the northern and central Chihuahuan Desert disclosed macrofossils of Colorado piñon (Pinus edulis) and Texas piñon (P. remota). Radiocarbon dating indicates that Texas piñon was widespread in Trans-Pecos Texas and northeastern Mexico between 30,000 and 11,000 yr B.P. Today it is found in small refugia east of its former range. In the late Pleistocene Colorado piñon occurred at lower elevations on the northern edge of the Chihuahuan Desert. Both species occurred in the Hueco Mountains, near El Paso, Texas. No clear evidence was found of the presence of Mexican piñon (P. cembroides), though today it is abundant in the Davis and Chisos Mountains. A paleoclimate is postulated that had the following characteristics: increased winter precipitation from Pacific frontal sources, reduced summer temperatures and precipitation, and milder winter temperatures due to a reduced frequency of Arctic airmass incursion. Winter precipitation appears to have decreased from north to south, while winter temperatures, and, possibly, summer precipitation, increased from north to south. During the late Pleistocene, the northern Chihuahuan Desert was dominated by woodlands of piñon pines, junipers, and oaks. The desert-scrub communities that characterize the area today are a Holocene phenomenon.     

Pb and Sr systematics of ultrapotassic and basaltic rocks from the central Sierra Nevada,California

This study presents Sr and Pb isotopic ratios and Rb, Sr, U, Th, and Pb concentrations of an ultrapotassic basaltic suite and related rocks from the central Sierra Nevada, California. The ultrapotassic suite yields a narrow range of Sr and Pb isotopic compositions (⁸⁷Sr/⁸⁶Sr=0.70597–0.70653; ²⁰⁶Pb/ ²⁰⁴Pb=18.862–19.018; ²⁰⁷Pb/²⁰⁴Pb=15.640–15.686; ²⁰⁸Pb/ ²⁰⁴Pb=38.833–38.950). Associated basalts containing ultramafic nodules have less radiogenic Sr (⁸⁷Sr/⁸⁶=0.70430–0.70521) and generally higher Rb/Sr ratios than the ultrapotassic suite. Leucitites from Deep Springs Valley, California, contain high ⁸⁷Sr/⁸⁶Sr (71141–0.71240) and low ²⁰⁶Pb/²⁰⁴Pb (17.169–17.234) ratios, reflecting contamination by crustal granulite.The isotopic relationships support an origin of the ultrapotassic basaltic suite by partial melting of an enriched upper mantle source. Dehydration of a gently inclined oceanic slab beneath the Sierra Nevada may have provided Ba, K, Rb, Sr, and H₂O, which migrated into the overlying upper mantle lithosphere. The end of subduction 10 m.y. ago allowed increased asthenospheric heat flow into the upper mantle lithosphere. The increased heat flow enhanced fluid movement in the upper mantle and contributed towards isotopic homogenization of the upper mantle source areas. Continued heating of the enriched upper mantle caused partial melting and subsequent eruption of the ultrapotassic lavas.     

The comparability of carbonatitic fluid inclusions in ijolites with natrocarbonatite lava

The composition of carbonate, sulphate, halide and other phases trapped as fluid inclusions in apatite crystals in ijolite and urtite from sub-volcanic complexes in East Africa have been determined using the electron micro-probe. The bulk composition of these inclusions is alkali-rich and closely comparable to the natrocarbonatite lava from the active volcano of Oldoinyo Lengai in N. Tanzania. The inclusions are interpreted as having been originally immiscible alkali carbonate-rich droplets within a nephelinitic melt. The evidence suggests that magma of the composition of natrocarbonatite lava is that most likely for the parent magma during carbonatite petrogenesis.     

Axial depth anomalies from 10 to 50° north along the Mid-Atlantic Ridge: correlation with other mantle properties

We present an analysis of intermediate- to long-wavelength (a few hundred to a few thousand kilometers) axial depth anomalies along the Mid-Atlantic Ridge between 10 and 50°N. The maximum depth of the rift valley is chosen as the elevation datum for oceanic crust of zero-age. The large depth anomalies are correlated, for short wavelengths (less than a few hundred kilometers), with some fracture zones irrespective of their offset and, for intermediate to long wavelengths, with mantle properties expressed in (1) excess elevation near triple junctions, (2) trace element and isotope geochemistry of the basaltic liquids emplaced at the ridge axis, and (3) anomalies in the Earth's gravity field. We suggest that the correlations may be explained in two ways: the depth anomalies of intermediate to long wavelength may represent the sites of upwelling and downwelling of the deep mantle; alternatively, the depth anomalies could be due to a regionalization in temperature or mineralogy in the asthenosphere.