Stratigraphic,sedimentological and palaeoenvironmental constraints on the rise of the Urgonian platform in the western Swiss Jura

Urgonian‐type carbonates are a characteristic feature of many late Early Cretaceous shallow‐marine, tropical and subtropical environments. The presence of typical photozoan carbonate‐producing communities including corals and rudists indicates the prevalence of warm, transparent and presumably oligotrophic conditions in a period otherwise characterized by the high density of globally occurring anoxic episodes. Of particular interest, therefore, is the exploration of relationships between Urgonian platform growth and palaeoceanographic change. In the French and Swiss Jura Mountains, the onset and evolution of the Urgonian platform have been controversially dated, and a correlation with other, better dated, successions is correspondingly difficult. It is for this reason that the stratigraphy and sedimentology of a series of recently exposed sections (Eclépens, Vaumarcus and Neuchâtel) and, in addition, the section of the Gorges de l’Areuse were analysed. Calcareous nannofossil biostratigraphy, the evolution of phosphorus contents of bulk rock, a sequence‐stratigraphic interpretation and a correlation of drowning unconformities with better dated sections in the Helvetic Alps were used to constrain the age of the Urgonian platform. The sum of the data and field observations suggests the following evolution: during the Hauterivian, important outward and upward growth of a bioclastic and oolitic carbonate platform is documented in two sequences, separated by a phase of platform drowning during the late Early Hauterivian. Following these two phases of platform growth, a second drowning phase occurred during the latest Hauterivian and Early Barremian, which was accompanied by significant platform erosion and sediment reworking. The Late Barremian witnessed the renewed installation of a carbonate platform, which initiated with a phase of oolite production, and which progressively evolved into a typical Urgonian carbonate platform colonized by corals and rudists. This phase terminated at the latest in the middle Early Aptian, due to a further drowning event. The evolution of this particular platform segment is compatible with that of more distal and well‐dated segments of the same northern Tethyan platform preserved in the Helvetic zone of the Alps and in the northern subalpine chains (Chartreuse and Vercors).     

Modelling the Recent Geochemical Evolution of the Piton de la Fournaise Volcano, Reunion Island, 1931-1986

The Piton de la Fournaise is the only active volcano on theisland of R?union. Samples of the 1931–1986 eruptionshave been analysed for their major and trace element contentswith emphasis on high accuracy measurement of REE, Th, and Zrby isotope dilution. The lavas include transitional basalts and picrites with 40–50%olivine. Examination of the dislocation structures in the picriticolivines indicates a xenocrystic origin which is confirmed bysome mineral-melt Fe/Mg disequilibria and anomalous incompatibleelement ratios. The picrites are therefore interpreted as dueto pulses of basalt magmas which disrupted preexisting, geneticallyunrelated, olivine-rich cumulates. The combination of whole rock, mineral and groundmass analysesenabled precise calculation of the modal proportions. The phenocrystassemblages obtained thereby are consistent with the chemicalevolution of the lava series suggested by the mass balance equations,which is interpreted as reflecting little mineral sorting duringfractionation. The whole range of basaltic compositions canbe generated by up to 15% fractional crystallization of clinopyroxene-rich(20 Ol-80 Cpx) to plagioclase-bearing (20 Ol-50 Cpx-30 Plag)cumulates. By cqmparison with experimental liquid lines of descent,the order of crystallization (olivine-clinopyroxene-plagioclase)suggests a pressure of crystallization in excess of 500 MPa,i.e. much deeper than the c. 2km deep magma chamber inferredfrom magnetic measurements and seismic activity. The Ni-MgOsystem indicates that the Fe-rich primitive magma (9% MgO, 225ppm Ni) crystallized 12 ? 5% cumulate to yield the least differentiatedbasalt of the series. The historic basalts erupted prior to1931 and analyzed in this work were extracted from a more Mg-richsource. The inversion of batch melting equations for incompatible elementsis improved relative to previous attempts and a complete errorhandling is incorporated. It is suggested that the Fournaiselavas were derived by melting of a source enriched in compatibleelements leaving a harzburgitic residue without significantfractionation of the incompatible elements. Coupled variations of Ce/Yb and major element ratios over c.17-y cycles suggest that lower degrees of melting produce smallerbatches of magma which, upon ascent and cooling, undergo moresevere fractionation.