Palaeoenvironmental significance of Late Permian palaeosols in the South‐Eastern Iberian Ranges,Spain

The Late Permian (Wuchiapingian) Alcotas Formation in the SE Iberian Ranges consists of one red alluvial succession where abundant soil profiles developed. Detailed petrographical and sedimentological studies in seven sections of the Alcotas Formation allow six different types of palaeosols, with distinctive characteristics and different palaeogeographical distribution, to be distinguished throughout the South‐eastern Iberian Basin. These characteristics are, in turn, related to topographic, climatic and tectonic controls. The vertical distribution of the palaeosols is used to differentiate the formation in three parts from bottom to top showing both drastic and gradual vertical upwards palaeoenvironmental changes in the sections. Reconstruction of palaeoenvironmental conditions based on palaeosols provides evidence for understanding the events that occurred during the Late Permian, some few millions of years before the well‐known Permian‐Triassic global crisis.     

Disequilibrium Induced by Fast Decompression in St-Bt-Grt-Ky-Sil-And Metapelites from the Betic Belt (Southern Spain)

Medium-grade metapelites from the Torrox unit (Western Alpujarrides,Betic Cordilleras) provide an example of the phase relationshipsto be expected from extensional collapse of thickened crust.The rocks contain assemblages formed by combinations of St–Bt–GrtKySil–And,and are characterized by a marked state of textural and compositionaldisequilibrium that originated during post-peak, near-isothermaldecompression at moderately high temperatures (from >10 to2–3 kbar, at 550–650C). Major disequilibrium featuresinclude abundant reaction textures, metastable coexistence ofthe three aluminium silicate (Als) polymorphs, wide heterogeneitiesin the composition of phases within single samples and unsystematicor abnormal Mg-Fe partitioning among biolite, garnet and staurolite.Reaction relations and compositional trends of phases can beshown to be consistent with expectations from model systems,however, indicating that disequilibrium was a consequence ofreaction overstepping along the isothermal decompression path.Ensuing fast, near-isobaric cooling abruptly terminated metamorphism,and allowed for the preservation of disequilibrium featuresafter decompression. Given that the rocks decompressed at relativelyhigh temperature, the occurrence of reaction overstepping suggeststhat decompression proceeded at a very fast rate, consistentwith a bulk exhumation velocity in the range of 5–10 km/Ma,as estimated from available radio-metric data for this stage. KEY WORDS: decompression; disequilibrium; extensional collapse; medium-grade metapelites; reaction overstepping ^*Corresponding author. Telephone: + 34 58 243355. Fax: + 34 58 243368. e-mail: agcasco{at}goliat.ugr.es     

Sedimentary dynamics and evolutionary history of a Late Carboniferous Gondwanic lake in north-western Argentina

Lacustrine deposits are well represented in the lower part of the Late Carboniferous Agua Colorada Formation in the north-west Sierra de Narváez, Catamarca Province, Argentina. Lake Narváez was one of the several water bodies formed in the region immediately after the Gondwana glaciation. The lacustrine transport system has been divided into three distinct zones: delta, shallow lake and deep lake. Delta progradation proceeded from the ESE. Coarse-grained delta plain and turbidite delta front deposits suggest that the delta was formed close to the headwaters (‘short-headed stream delta’type). During periods of high discharge, river mouths acted as bypass zones and fine and very fine sands were transported further into the lake by underflow currents. The clastic material supplied by the deltaic system was partially reworked by wave action. Sands accumulated in unstable conditions at the upper delta front as a consequence of delta progradation. As a result of the addition of clastics in the steep delta front, turbidity currents were formed, spreading their load along the lower delta slope. Deep lacustrine deposits are typically stacked, forming two different kinds of progradational turbidite lobe sequences. Type I lobes were formed in a basinal setting and were probably detached from their feeder systems as a result of sediment-bypassing in a shallow lake during periods of low lake level. These turbidite lobes are replaced upwards by type II lobes, which were formed on the delta slope during periods of lake level rise that allowed the onset of delta progradation. The presence of highly deformed sandstone bodies suggests rapid depositional rates in a high slope setting, whereas the occurrence of hummocky cross-stratified sandstones indicates wave reworking of the sands initially emplaced by turbidity currents. Therefore, the inner part of type II lobes was formed above storm wave base. The depositional history of Lake Narváez can be traced through four evolutionary stages: lake transgression, formation of type I lobes, formation of type II lobes and delta progradation. Tectonic activity was probably important at the early stage of lake evolution, but the subsequent depositional history was mainly controlled by fluctuations of lake level.     

Sedimentological and ichnological signatures of changes in wave,river and tidal influence along a Neogene tropical deltaic shoreline

Analysis of Neogene cores from the Eastern Venezuela Basin along 65 km of a west–east trending shoreline allows characterization of the sedimentological and ichnological signatures of wave, river and tidal processes. The area displays deltas prograding northward from the Guyana Shield. Twenty‐three facies are defined and grouped into four categories (wave‐influenced, river‐influenced, tide‐influenced and basinal). Wave‐dominated deltaic deposits occur mostly in the Tácata Field. The delta plain was characterized by tide‐influenced distributary channels separated by interdistributary bays. Fluvial discharge in the delta front and prodelta was repeatedly interrupted by storm‐wave reworking and suspended sediment fallout. Delta‐front and prodelta deposits contain some ichnotaxa that typically do not occur in brackish water (for example, Chondrites and Phycosiphon). Amalgamated storm deposits are unburrowed or contain vertical Ophiomorpha. Lateral (especially on the updrift side) to the river mouths, waves caused nearly continuous accretion of the associated strandplains. These deposits are the most intensely bioturbated, and are dominated by the estenohaline echinoid‐generated ichnogenus Scolicia. River‐dominated deltaic deposits are present in the Santa Bárbara, Mulata, Carito and El Furrial Fields. Low‐sinuosity rivers characterized the alluvial plain, whereas the subaerial delta plain was occupied by higher‐sinuosity rivers. The subaqueous delta plain includes distributary channels and tide‐influenced interdistributary bays. Further seaward, successions are characterized by terminal distributary‐channel and distributary mouth‐bar deposits, as well as by delta‐front and prodelta deposits showing evidence of sediment gravity‐flow and fluid‐mud emplacement. Delta‐front and prodelta deposits are unbioturbated to sparsely bioturbated, suggesting extreme stress, mostly as a result of high fluvial discharge and generation of sediment gravity flows. Tidal influence is restricted to interdistributary bays, lagoons and some distributary channels. From an ichnological perspective, and in order of decreasing stress levels, four main depositional settings are identified: river‐dominated deltas, tide‐influenced delta plains, wave‐dominated deltas and wave‐dominated strandplain–offshore complexes.     

Experimental Melting of Cordierite Gneiss and the Petrogenesis of Syntranscurrent Peraluminous Granites in Southern Brazil

To understand the petrogenesis of peraluminous granites syntectonicto the Dorsal de Canguçu Transcurrent Shear Zone in theSul-rio-grandense Shield, Brazil, melting experiments were performedon one of the potential protoliths, a cordierite-bearing semi-peliticmetasedimentary gneiss (PE-1). Experiments were conducted atpressures of 5, 10 and 15 kbar, at temperatures of 700–900°C,and under fluid-absent and 5% H₂O-present conditions. The experimentsshow that fluid-absent melting begins at near-solidus conditions,around 700°C, promoted by participation of retrogressivephengitic muscovite in the reaction Mus + Kf ± Qz = melt± Fe–Ti oxide ± Als, producing a very smallamount of melt (<9%) with widely ranging composition. Allhypersolidus experiments (>800°C) produced S-type graniticmelts promoted by participation of biotite or cordierite inthe reactions Bio + Pl + Crd + Qz = Px + Fe–Ti oxide +melt at 5 kbar, and Bio + Pl + Crd ± Qz = Grt + Als ±Kf + melt at 10 and 15 kbar, both producing a high amount ofmelt (10–63% by volume). The melt compositions obtainedat 900°C and 15 kbar under fluid-absent conditions, promotedby biotite or cordierite breakdown, are similar to the syntectonicgranites. However, it is unlikely that the granites were formedat this pressure (corresponding to a depth of melting of     

Anticlockwise P-T Path of Granulites from the Monte Castelo Gabbro (Ordenes Complex, NW Spain)

The study of mafic and aluminous granulites from the Monte CasteloGabbro (Órdenes Complex, NW Spain) reveals an anticlockwiseP–T path that we interpret as related to the tectonothermalactivity in a magmatic arc, probably an island arc. The P–Tpath was obtained after a detailed study of the textural relationshipsand mineral assemblage succession in the aluminous granulites,and comparing these with an appropriate petrogenetic grid. Additionalthermobarometry was also performed. The granulites are highlyheterogeneous, with distinct compositional domains that mayalternate even at thin-section scale. Garnets are generallyidiomorphic to subidiomorphic, and in certain domains of thealuminous granulites they show overgrowths forming xenomorphiccoronas around a more or less idiomorphic core. Both types ofgarnets show significant Ca enrichment at the crystal rims,which, together with the other mineralogical and textural characteristics,is compatible with a pressure increase with low T variation.P–T estimations indicate a peak of T > 800°C andP