Antidune and chute and pool structures in the base surge deposits of the Laacher See area, Germany

Grey tuffs of late Pleistocene age form broad fans radiating from the Laacher See basin. They were derived from phreatomagmatic outbursts, and transported in turbulent pyroclastic flows, in contrast with the underlying white pumice tuffs of air fall origin. Flow origin of the grey tuffs is inferred from the well-bedded plane parallel to cross-bedded tephra characteristic of base surge deposits, and a variety of other sedimentary structures, as well as grain size distributions. We recognize a tentative sequence of five main kinds of dune structures or cross-bedded strata. With some reservations these may be compared with the high flow-regime alluvial bedforms produced experimentally in flumes. Most of the cross-bedded structures in the Laacher See deposits resemble antidunes, with steep stoss sides and very low-dipping lee sides. Upcurrent migration of antidune crests is dominant close to the source, but changes to downcurrent migration at greater distances, presumably because of decay in flow energy. The most spectacular cross-bedding is somewhat similar to chute and pool structures formed under experimental condition in alluvial flumes, but not recognized in ancient sedimentary rocks. We suggest that these structures of the Laacher See tuffs formed during deposition from phreatic pyroclastic flows of very high flow energy and high sediment concentration. The antidunes apparently formed at lesser flow velocity than chute and pool structures, although interpretation of velocity conditions by examination of the deposits is difficult because of other factors such as the cohesiveness of wet material erupted by explosive phreatic volcanic activity. The large wave lengths of the dune-like structures, however, suggest unusually high velocities. The Laacher See magmas were of phonolitic to tephritic composition, and may have erupted with greater explosive energy and in greater volume than comparable basaltic eruptions.     

Quartz-sandy, grazing-contoured stromatolites from coastal embayments of Mauritania, West Africa

A new occurrence of Recent stromatolites different from those known up to now has been discovered on tidal flats of the Bay of Saint-Jean (near Cape Timiris). Their most remarkable features are predominance of quartz sand instead of carbonate, characteristic surface-contouring by grazing fish, absence of cementation, intensive reworking by crabs, and connection with saline sabkha deposits. Entrapment of sand grains and great resistance of the algal sheaths are most important for stromatolitic growth, although today destruction appears to prevail over accretion. Quartz sand of aeolian dunes and carbonatic pellets of aragonitic ooze feeders are the main constituents of the stromatolitic sediments. According to the tidal range the following zonation of the algal mat exists: (A) knoll and cuspate zone, (B) flat zone, (C) gas-domed zone, and (D) crinkle zone.     

Magma Mixing and Crustal Recycling Recorded in Ternary Feldspar from Compositionally Zoned Peralkaline Ignimbrite A', Gran Canaria, Canary Islands

Miocene Ignimbrite ‘A’ on Gran Canaria containsthree compositional endmember fiamme types(two rhyolites andone trachyte) each of which crystallized distinct feldspar.Various textural and compositional criteria are interpretedas reflecting a complex scenario within the magma chamber inwhich the crystals formed. About 25–30% of the feldsparphenocrysts contain evidence for magma mixing in the form of(1) partial to severe dissolution–resorption rims, (2)distinct zones of drastically different compositions and (3)overgrowth textures on formerly resorbed crystals. Four majortypes of zoning in the oligoclase to anorthoclase feldsparsof ignimbrite ‘A’ include a normal and a reverselyzoned type and two complexly zoned types. The feldspars withnormal and reverse zonation show only minor compositional amplitudesbetween individual zones (     

Petrological and Chemical Evolution of the Northeastern Troodos Extrusive Series, Cyprus

Four magma series are distinguished in the northeastern TroodosExtrusive Series: (A) a Lower Low-Ti Series (Lo-LTS) of basalticandesites, (B) a High-Ti Series (HTS) of basaltic andesitesto rhyodacites, (C) a Low-Ti Series(DLTS), the last two beingof basaltic andesite. Trace-element characteristics vary systematicallyfrom Series A to D and are interpreted in terms of a variablecontribution of three major source components (SCs). LILE-enrichedwater-rich fluids (SCI) derived from dehydration of a subductedlithosphere slab were continuously added to the overlying mantlewedge. Increasing LILE/HFSE and LILE/REE ratios and decreasingabsolute HFSE and REE concentrations from Series A to Dindicateprogressive depletion of the actual mantle source (SCII). Anegative Ta anomaly in the lavas decreases from Series A toD and is interpreted to have resulted from partial melting ofthe lower crust (SCII) where Ta-Nb-Ti may be fractionated byTi-rich accessory phases. The contribution of SCIII decreaseswhen the eruptive sites successively move away from the centralaxial zone and the temperature of the lower crust decreases,preventing partial melting of the lower crust. Chemical compositionsof fresh glass separates and phenocrysts indicate a change ofmajor petrogenetic processes from series A to D. Lo-LTS andIITS lavas are intrepreted to be directly related by open-systemfractional crystallization in crustal magma chambers. Removalof observed phenocryst phases clinopyroxene, orthopyroxene,plagioclase, and magnetite, and repeated subsequent mixing ofdacitc to rhyodacitic magmas with batches of replenshing basalticandesites are the major processes, possibly induced by vesicleformation in the mafic layer after a period of some crystallization.LTS and DLTS magmas were directly fed to the surface withoutstagnating at crustal levels, with feeder dykes positioned marginalto the central rift zone and thus by-passing the central magmachambers. These magmas apparently experienced only limited fractionalcrystallization of 10–15 wt.% olivine+clinopyroxene+chromite,probably at the mantle-crust boundary.     

The Role of Partial Melting in the 15-Ma Geochemical Evolution of Gran Canaria: A Blob Model for the Canary Hotspot

The subaerial portion of Gran Canaria, Canary Islands, was builtby three cycles of volcanism: a Miocene Cycle (8•5–15Ma), a Pliocene Cycle (1•8–6•0 Ma), and a QuaternaryCycle (1•8–0 Ma). Only the Pliocene Cycle is completelyexposed on Gran Canaria; the early stages of the Miocene Cycleare submarine and the Quaternary Cycle is still in its initialstages. During the Miocene, SiO₂ saturation of the mafic volcanicsdecreased systematically from tholeiite to nephelinite. Forthe Pliocene Cycle, SiO₂ saturation increased and then decreasedwith decreasing age from nephelinite to tholeiite to nephelinite.SiO₂ saturation increased from nephelinite to basanite and alkalibasalt during the Quaternary. In each of these cycles, increasingmelt production rates, SiO₂ saturation, and concentrations ofcompatible elements, and decreasing concentrations of some incompatibleelements are consistent with increasing degrees of partial meltingin the sequence melilite nephelinite to tholeiite. The maficvolcanics from all three cycles were derived from CO₂-rich garnetlherzolite sources. Phlogopite, ilmenite, sulfide, and a phasewith high partition coefficients for the light rare earth elements(LREE), U, Th, Pb, Nb, and Zr, possibly zircon, were residualduring melting to form the Miocene nephelinites through tholeiites;phlogopite, ilmenite, and sulfide were residual in the sourceof the Pliocene–Quaternary nephelinites through alkalibasalts. Highly incompatible element ratios (e.g., Nb/U, Pb/Ce,K/U, Nb/Pb, Ba/Rb, Zr/Hf, La/Nb, Ba/Th, Rb/Nb, K/Nb, Zr/Nb,Th/Nb, Th/La, and Ba/La) exhibit extreme variations (in manycases larger than those reported for all other ocean islandbasalts), but these ratios correlate well with degree of melting.Survival of residual phases at higher degrees of melting duringthe Miocene Cycle and differences between major and trace elementconcentrations and melt production rates between the Mioceneand Pliocene tholeiites suggest that the Miocene source wasmore fertile than the Pliocene–Quaternary source(s). We propose a blob model to explain the multi-cycle evolutionof Canary volcanoes and the temporal variations in chemistryand melt production within cycles. Each cycle of volcanism representsdecompression melting of a discrete blob of plume material.Small-degree nephelinitic and basanitic melts are derived fromthe cooler margins of the blobs, whereas the larger-degree tholeiiticand alkali basaltic melts are derived from the hotter centersof the blobs. The symmetrical sequence of mafic volcanism fora cycle, from highly undersaturated to saturated to highly undersaturatedcompositions, reflects melting of the blob during its ascentbeneath an island in the sequence upper margin-corelower margin.Volcanic hiatuses between cycles and within cycles representperiods when residual blob or cooler entrained shallow mantlematerial fill the melting zone beneath an island.     

The Petrology of the Tholeiites through Melilite Nephelinites on Gran Canaria, Canary Islands: Crystal Fractionation, Accumulation, and Depths of Melting

We report major and trace element X-ray fluorescence (XRF) datafor mafic volcanics covering the 15-Ma evolution of Gran Canaria,Canary Islands. The Miocene (12–15 Ma) and Pliocene-Quaternary(0–6 Ma) mafic volcanics on Gran Canaria include picrites,tholeiites, alkali basalts, basanites, nephelinites, and melilitenephelinites. Olivineclinopyroxene are the major fractionatingor accumulating phases in the basalts. Plagioclase, Fe–Tioxide, and apatite fractionation or accumulation may play aminor role in the derivation of the most evolved mafic volcanics.The crystallization of clinopyroxene after olivine and the absenceof phenocrystic plagioclase in the Miocene tholeiites and inthe Pliocene and Quaternary alkali basalts and basanites withMgO>6 suggests that fractionation occurred at moderate pressure,probably within the upper mantle. The presence of plagioclasephenocrysts and chemical evidence for plagioclase fractionationin the Miocene basalts with MgO<6 and in the Pliocene tholeiitesis consistent with cooling and fractionation at shallow depth,probably during storage in lower-crustal reservoirs. Magma generationat pressures in excess of 3•0–3•5 GPa is suggestedby (a) the inferred presence of residual garnet and phlogopiteand (b) comparison of FeO¹ cation mole percentages and the CIPWnormative compositions of the mafic volcanics with results fromhigh-pressure melting experiments. The Gran Canaria mafic magmaswere probably formed by decompression melting in an upwellingcolumn of asthenospheric material, which encountered a mechanicalboundary layer at {small tilde}100-km depth.