Frasnian rugose and tabulate corals from the eastern Taurus (Kozan region,Turkey)

A section near the Kilgen Lake (Adana Province) has yielded a fauna of rugose and tabulate corals including Disphyllum cf. rugosum (Wedekind, 1922), D. cf. curtum Hill, 1954, Wapitiphyllum sp., Thamnopora sp., and Alveolites sp. This coral assemblage is consistent with the previous Frasnian age assigned to the limestones of the Gümü?ali Formation. The rather well-preserved material provides new data on the structure and microstructure of Disphyllum and allows to describe in Thamnopora unusual calicinal morphology (septal ridges, median teeth, and pseudopercula) as well as new structures linked to the lateral increase (basal low wall, apical cul-de-sac).     

Paléoenvironnements et caractérisation des roches mères pétrolières des séries pré-salifères du bassin intérieur du Gabon

The Interior Basin of Gabon, created during the break-up between South America and Africa, displays thick Neoproterozoic to Aptian p.p. fluvio-lacustrine deposits overlain by Aptian to Albian marine facies. Rock–Eval analyses from outcrop and drillhole samples show high content in organic matter (up to 25%) related to types I and II. These intervals are encountered within Permian, Neocomian–Barremian as well as Aptian siliciclastic succession. They constitute fairly good to excellent potential petroleum source rocks, which are most probably at the origin of oil indices recognized both in drillholes and in surface.     

Trace element geochemistry of CR chondrite metal

We report trace element analyses by laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) of metal grains from nine different CR chondrites, distinguishing grains from chondrule interior (“interior grains”), chondrule surficial shells (“margin grains”), and the matrix (“isolated grains”). Save for a few anomalous grains, Ni‐normalized trace element patterns are similar for all three petrographic settings, with largely unfractionated refractory siderophile elements and depleted volatile Au, Cu, Ag, S. All three types of grains are interpreted to derive from a common precursor approximated by the least‐melted, fine‐grained objects in CR chondrites. This also excludes recondensation of metal vapor as the origin of the bulk of margin grains. The metal precursors were presumably formed by incomplete condensation, with evidence for high‐temperature isolation of refractory platinum‐group‐element (PGE)‐rich condensates before mixing with lower temperature PGE‐depleted condensates. The rounded shape of the Ni‐rich, interior grains shows that they were molten and that they equilibrated with silicates upon slow cooling (1–100 K h^?1), largely by oxidation/evaporation of Fe, hence their high Pd content, for example. We propose that Ni‐poorer, amoeboid margin grains, often included in the pyroxene‐rich periphery common to type I chondrules, result from less intense processing of a rim accreted onto the chondrule subsequent to the melting event recorded by the interior grains. This means either that there were two separate heating events, which formed olivine/interior grains and pyroxene/margin grains, respectively, between which dust was accreted around the chondrule, or that there was a single high‐temperature event, of which the chondrule margin records a late “quenching phase,” in which case dust accreted onto chondrules while they were molten. In the latter case, high dust concentrations in the chondrule‐forming region (at least three orders of magnitude above minimum mass solar nebula models) are indicated.     

Zircon geochronology and geochemistry to constrain the youngest eruption events and magma evolution of the Mid-Miocene ignimbrite flare-up in the Pannonian Basin,eastern central Europe

A silicic ignimbrite flare-up episode occurred in the Pannonian Basin during the Miocene, coeval with the syn-extensional period in the region. It produced important correlation horizons in the regional stratigraphy; however, they lacked precise and accurate geochronology. Here, we used U–Pb (LA-ICP-MS and ID-TIMS) and (U–Th)/He dating of zircons to determine the eruption ages of the youngest stage of this volcanic activity and constrain the longevity of the magma storage in crustal reservoirs. Reliability of the U–Pb data is supported by (U–Th)/He zircon dating and magnetostratigraphic constraints. We distinguish four eruptive phases from 15.9 ± 0.3 to 14.1 ± 0.3 Ma, each of which possibly includes multiple eruptive events. Among these, at least two large volume eruptions (>10 km³) occurred at 14.8 ± 0.3 Ma (Demjén ignimbrite) and 14.1 ± 0.3 Ma (Harsány ignimbrite). The in situ U–Pb zircon dating shows wide age ranges (up to 700 kyr) in most of the crystal-poor pyroclastic units, containing few to no xenocrysts, which implies efficient recycling of antecrysts. We propose that long-lived silicic magma reservoirs, mostly kept as high-crystallinity mushes, have existed in the Pannonian Basin during the 16–14 Ma period. Small but significant differences in zircon, bulk rock and glass shard composition among units suggest the presence of spatially separated reservoirs, sometimes existing contemporaneously. Our results also better constrain the time frame of the main tectonic events that occurred in the Northern Pannonian Basin: We refined the upper temporal boundary (15 Ma) of the youngest counterclockwise block rotation and the beginning of a new deformation phase, which structurally characterized the onset of the youngest volcanic and sedimentary phase.     

Hydrothermal calderas

The standard model of caldera formation is related to the emptying of a magma chamber and ensuing roof collapse during large eruptions or subsurface withdrawal. Although this model works well for numerous volcanoes, it is inappropriate for many basaltic volcanoes (with the notable exception of Hawaii), as these have eruptions that involve volumes of magma that are small compared to the collapse. Many arc volcanoes also have similar oversized depressions, such as Poas (Costa Rica) and Aoba (Vanuatu). In this article, we propose an alternative caldera model based on deep hydrothermal alteration of volcanic rocks in the central part of the edifice. Under certain conditions, the clay-rich altered and pressurized core may flow under its own weight, spread laterally, and trigger very large caldera-like collapse. Several specific mechanisms can generate the formation of such hydrothermal calderas. Among them, we identify two principal modes: mode 1: ripening with summit loading and flank spreading and mode II: unbuttressing with flank subsidence and flank sliding. Processes such as summit loading or flank subsidence may act simultaneously in hybrid mechanisms. Natural examples are shown to illustrate the different modes of formation. For ripening, we give Aoba (Vanuatu) as an example of probable summit loading, while Casita (Nicaragua) is the type example of flank spreading. For unbuttressing, Nuku Hiva Island (Marquesas) is our example for flank subsidence and Piton de la Fournaise (La Réunion) is our example of flank sliding. The whole process is slow and probably needs (a) at least a few tens of thousands of years to deeply alter the edifice and reach conditions suitable for ductile flow and (b) a few hundred years to achieve the caldera collapse. The size and the shape of the caldera strictly mimic that of the underlying weak core. Thus, the size of the caldera is not controlled by the dimensions of the underlying magma reservoir. A collapsing hydrothermal caldera could generate significant phreatic activity and trigger major eruptions from a coexisting magmatic complex. As the buildup to collapse is slow, such caldera-forming events could be detected long before their onset.