Generation of a crust-mantle magma mixture: magma sources and contamination at Cerro Panizos,central Andes

 Cerro Panizos, a large caldera in the central Andes Mountains, produced two large dacitic ignimbrites at 7.9 Ma and 6.7 Ma and many andesitic and dacitic lava flows and domes. The older rhyodacitic Cienago Ignimbrite represents the most silicic magma erupted by the system. The younger, much larger volume dacitic Cerro Panizos Ignimbrite is very crystal-rich, containing up to 50% biotite, plagioclase, and quartz crystals in the pumice. It is weakly zoned, with most of the zoning apparent between two main cooling units. Major and most trace elements show little variation through the Cerro Panizos Ignimbrite, but the small range of composition is consistent with typical fractionation trends. Sr, Nd, and Pb isotopic ratios are very “crustal”, with initial ⁸⁷Sr/⁸⁶Sr values of 0.711 to 0.715, ɛ_Nd values of –7.5 to –10.2, and nearly invariant Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb=18.85, ²⁰⁷Pb/²⁰⁴Pb=15.67, and ²⁰⁸Pb/²⁰⁴Pb=38.80). The limited zonation observed in the Cerro Panizos Ignimbrite is explained by impeded crystal settling due to high crystal content. The magma body was a crystal-liquid mush before ascent to the pre-eruption crustal levels. Crystals formed, but did not separate easily from the magma. Limited fractionation of plagioclase and biotite may have occurred, but the composition was largely controlled by lower crustal MASH processes. AFC modeling shows that the Cerro Panizos magmas resulted from a mixture of roughly equal proportions of late Miocene mantle-derived basalts and melts from ∼1.0 Ga (Grenville age) lower crust. This occurred in a MASH zone in the lower crust, and set the crustal isotopic ratios observed in the Cerro Panizos magmas. The great thickening of the crust beneath the central Andes Mountains sent upper and middle crustal rock types to lower crustal (and deeper) depths, and this explains the “upper crustal” isotopic signatures of the Cerro Panizos rocks. Minor upper crustal assimilation of early Miocene volcanic or subvolcanic rocks produced much of the isotopic variation seen in the system. The nearly invariant high Pb isotopic values and high Pb concentrations indicate that Pb came almost entirely from the crustal source, and was little altered by any subsequent upper crustal assimilation. This Pb signature is isotopically similar to that of the southern Bolivian Tin Belt, suggesting a widely distributed Pb source. The great difference between compositions of Miocene and Quaternary central Andean volcanic rocks is explained by crustal thickening in early Miocene time leading to abundant lower crustal water and associated fluxed melting during the time of the earlier eruptions. The lower crust dried out considerably by Quaternary time, so less crustal component is present. Received: 22 December 1994 / Accepted: 13 September 1995     

Stratigraphy of the upper cretaceous and lower tertiary strata in the Tethyan Himalayas of Tibet (Tingri area,China)

The 1500-m-thick marine strata of the Tethys Himalaya of the Zhepure Mountain (Tingri, Tibet) comprise the Upper Albian to Eocene and represent the sedimentary development of the passive northern continental margin of the Indian plate. Investigations of foraminifera have led to a detailed biozonation which is compared with the west Tethyan record. Five stratigraphic units can be distinguished: The Gamba group (Upper Albian - Lower Santonian) represents the development from a basin and slope to an outer-shelf environment. In the following Zhepure Shanbei formation (Lower Santonian - Middle Maastrichtian), outer-shelf deposits continue. Pebbles in the top layers point to beginning redeposition on a continental slope. Intensified redeposition continues within the Zhepure Shanpo formation (Middle Maastrichtian - Lower Paleocene). The series is capped by sandstones of the Jidula formation (Danian) deposited from a seaward prograding delta plain. The overall succession of these units represents a sea-level high at the Cenomanian/Turonian boundary followed, from the Turonian to Danian, by an overall shallowing-upward megasequence. This is followed by a final transgression — regression cycle during the Paleocene and Eocene, documented in the Zhepure Shan formation (?Upper Danian - Lutetian) and by Upper Eocene continental deposits. The section represents the narrowing and closure of the Tethys as a result of the convergence between northward-drifting India and Eurasia. The plate collision started in the Lower Maastrichtian and caused rapid changes in sedimentation patterns affected by tectonic subsidence and uplift. Stronger subsidence and deposition took place from the Middle Maastrichtian to the Lower Paleocene. The final closure of remnant Tethys in the Tingri area took place in the Lutetian.     

A physical statistical approach to erosion

Erosion is a complex process consisting of many components such as surface runoff, impact of raindrops, wind forces, soil and rock mechanics, etc. Trying to integrate all these processes into a physical model seems to be hopeless. In order to understand the variety of natural shapes and patterns produced by erosion we present an integrated statistical approach. Our model is based on simple physical constraints for the separation of amalgamated particles (abrasion) and for the movement of loose particles (denudation) and on the laws of statistics. After some simplifications, we obtain a nonlinear system of partial differential equations which is solved using finite volume techniques. The model is suitable for the formation of different types of rill systems and the episodic behaviour of erosion processes, a kind of self-organized criticality. Besides effects of inhomogeneities, e.g. the formation of terraces can be investigated.     

Pore-water response on seasonal environmental changes in intertidal sediments of the Weser Estuary,Germany

In order to determine time-dependent changes in estuarine pore-water chemistry and flux variations across the sediment-water interface, sediment cores of an intertidal mud flat in the Weser Estuary were taken monthly over a one-year period. Sediment temperature, pH, Eh, Cl^–, O₂, NO ₃ ^– , and SO ₄ ^2– pore-water concentrations were measured and showed variations that relate to the changes of surface temperature and estuarine water composition. Fick's first law was applied to quantify diffusive fluxes from concentration gradients in the diffusive boundary layer and in the pore water. Total nitrate fluxes were calculated from flux chamber experiments. Diffusive oxygen fluxes increased from 5 mmol m^–2 d^–1 in winter to 18 mmol m^–2 d^–1 in early summer, while nitrate fluxes into the sediment increased from 3 mmol m^–2 d^–1 in winter to 60 mmol m^–2 d^–1 in early summer. Oxygen and nitrate fluxes into the sediment correlated linearly to sediment temperature. Sulfate fluxes increased from 0.5 mmol m^–2 d^–1 in winter to 10 mmol m^–2 d^–1 in August and September. Converted into carbon fluxes, the sum of these oxidants ranged from 10 mmol m^–2 d^–1 in winter to 80 mmol m^–2 d^–1 in summer. An estimation of the upper limit of the annual nitrate flux into the sediment showed that about 10% of the 250,000 t of nitrate discharged annually by the river may be decomposed within the inner Weser Estuary.     

Geochemistry of the Rare Earth Elements in Natural Terrestrial Waters：A Review of What Is Currently Knows

The range of observed chemical compositions of natural terrestrial waters varies greatly especially when compared to the essentially constant global composition of the oceans.The concentrations of the REEs in natural terrestrial waters also exhibit more variation than what was reported in seawater,In terrestrial waters ,pH values span the range from acid up to alkaline,In addition,terrestrial waters can range from very dilute waters through to highly concentrated brines.The REE concentrations and their behavior in natural terrestrial waters reflect these compositional ranges,Chemical weathering of rocks represents the source of the REEs to natural terrestrial waters and ,consequently,the REE signature of rocks can impart their REE signature to associated waters,In addition,Because of the typical low solubilities of the REEs both surface and solution complexation can be important in fractionating REEs in aqueous solution.Both of these processes are important in all natural terrestrial waters,however,their relative importance varies as a function of the overall solution composition,In alkaline waters,for example,Solution complexation of the REEs with carbonate ions appears to control their aqueous distributions whereas in acid waters,the REE signature of the labile fraction of the REEs is readily leached from the rocks.In circumneutral pH waters,both processes appear to be important and their relative significance has not yet been determined.     

Late Cretaceous rift-related upwelling and melting of the Trindade starting mantle plume head beneath western Brazil

High mantle potential temperatures and local extension, associated with the Late-Cretaceous impact of the Trindade mantle plume, produced substantial widespread and voluminous magmatism around the northern half of the Paraná sedimentary basin. Our previous studies have shown that, above the central and eastern portions of the postulated impact zone where lithosphere extension is minimal, heat conducted by the plume caused large-scale melting of the more fusible parts of the subcontinental lithospheric mantle beneath the margin of the São Francisco craton and the surrounding Brasilía mobile belt. Here we combine geochemical data and field evidence from the Poxoreu Igneous Province, western Brazil to show how more intense lithospheric extension above the western margin of the postulated impact zone permitted greater upwelling and melting of the Trindade plume than further east. Laser ⁴⁰Ar/³⁹Ar age determinations indicate that rift-related basaltic magmas of the Poxoreu Igneous Province were emplaced at ? 84 Ma. Our detailed geochemical study of the mafic magmas shows that the parental melts underwent polybaric crystal fractionation within the crust prior to final emplacement. Furthermore, some magmas (quartz-normative) appear to have assimilated upper crust whereas others (nepheline- and hypersthene-normative) appear to have been unaffected by open-system crustal magma chamber processes. Incompatible trace element ratios (e.g. chondrite-normalised La/Nb?=?1) and isotopic ratios (⁸⁷Sr/⁸⁶Sr?=?0.704 and ¹⁴³Nd/¹⁴⁴Nd?=?0.51274) of the Hy-normative basalts resemble those of oceanic islands (OIB). We therefore propose that these “OIB-like” magmas were predominantly derived from convecting-mantle-source melts (i.e. Trindade mantle plume). Inverse modelling of rare-earth element (REE) abundances suggests that the initial melts were predominantly generated within the depth range of ?80–100 km, in mantle with a potential temperature of ?1500 °C.     

Environmental and legal aspects of karst areas

 Environmental impacts on karst settings are common as they are more sensitive than those of other rock terrains. Regulatory procedures that are effective in other rock terrains are not necessarily applicable to karst settings. Development and exploitation by man that affect the karst hydrology regime can trigger catastrophic events and result in numerous legal actions where the effects of changes go beyond property boundaries. A great variety of regulations and examples of litigation exist for karst areas. Received: 25 October 1994 · Accepted: 17 October 1995     

Geochemistry and petrogenesis of Sikait leucogranite, Egypt: an example of S-type granite in a metapelitic sequence

 The Sikait leucogranite (SLG) is a body of porphyritic garnet granite intruded a metapelitic sequence and interlayered orthogneisses. Multiple deformation and low- to medium-grade metamorphism of the sequence was closely associated with pluton emplacement. Textural features of the SLG indicate that subsolidus plastic deformation was induced during the extensive thrusting. The granite is strongly peraluminous, alkali-rich and HFS (high field-strength) elements-depleted with low contents of REE, all facts that substantiate the geochemical characteristics of S-type granites. The geologic and geochemical features are consistent with a dehydration melting model of the hosted metapelites to generate the peraluminous SLG. However, geochemical modelling of trace elements and REEs suggest that the anatectic partial melt was subsequently affected by fractional crystallization of feldspars. This could explain much of the chemical attributes of SLG. Received: 20 September 1994 / Accepted: 2 August 1996     

Ophiolite remnants at the eastern margin of the Bohemian Massif and their bearing on the tectonic evolution

Summary Amphibolites are widespread in the eastern part of the Bohemian Massif, east of the South Bohemian Pluton. Based on their geological situation, their metamorphic evolution and their geochemistry, they were separated into three genetically different units: the Rehberg ophiolite, the Buschandlwand amphibolite and the Raabs group. The metamorphic Rehberg ophiolite consists of a ultramafic to mafic plutonic sequence overlain by a gabbro/dike complex and a volcanic section with basaltic, andesitic and rhyolitic volcanics associated with pelitic to psammitic sediments. The entire ophiolite underwent amphibolite facies metamorphism. Rock/MORB normalisation plots and other element ratio plots, such as Ti vs. V or Ta/Yb vs. Th/Yb argue for a supra-subduction zone environment as site of the origin of the Rehberg ophiolite. The Letovice ophiolite in Moravia is structured in a similar way. Geochemically it shows a more MORB like composition but also a distinct tholeiitic island arc group. Although there is no age data for the protolith of both ophiolites, it is believed that they have formed in an oceanic basin separating the Brunovistulian-Moravian block to the east and the Moldanubian block to the west. Possible traces of the continuation of the ophiolites towards the north can be found in the Stare Mesto ophiolites and the ophiolites surrounding the Sowie Gory block in the Sudetes.

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Ophiolitrelikte am ostrand der Böhmischen Masse und ihre bedeutung für die tektonische entwicklung

Zusammenfassung Amphibolite sind in der Böhmischen Masse, östlich des Südböhmischen Plutons weit verbreitet. Ausgehend von ihrer geologischen Position, ihrer metamorphen Entwick lung und ihrer Geochemie lassen sie sich in drei genetisch unterschiedliche Einheiten teilen: den Rehberg Ophiolith, den Buschandlwand Amphibolit und die Raabser Gruppe. Der metamorphe Rehberg Ophiolith besteht aus einer ultramatischen bis mafischen plutonischen Abfolge, die von einem Gabbro/Gangkomplex überlagert wird und einer vulkanischen Folge mit Basalten, Andesiten and Rhyoliten, die mit tonigen und sandigen Sedimenten verknüpft sind. Der gesamte Ophiolithkörper wurde unter amphibolitfaziellen Bedingungen metamorph überprägt. Rock/MORB Diagramme und andere Elementdarstellungen, wie z.B. das Ti vs. V, oder das Ta/Yb vs. Th/Yb Diagramm sprechen für eine Entstehung des Rehberg Ophioliths in einer Suprasubduction Zone. Der Letovice Ophiolith in Mähren weist eine ähnliche Struktur auf. Geochemisch zeigt er stärkere Ähnlichkeiten mit MORB, enthält aber auch Elemente von tholeiitischen Inselbogenbasalten. Auch wenn es für beide Ophiolithe noch keine Protolith-Altersdaten gibt, scheint es dennoch wahrscheinlich, daß sie in einem Ozeanbecken, das den Brunovistulisch-Moravischen Kontinentalblock im Osten vom Moldanubischen Kontinent im Westen trennt, gebildet wurden. Die mögliche Fortsetzung beider Ophiolithe könnten im Norden der Stare Mesto Ophiolith und die Ophiolithe rund um den Sowie Gory Block in den Sudeten darstellen.

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With 7 Figures