Effusive Research Articles

Extensional magmatism in a continental collision zone,Tafresh area, western central Iran: structural, geochemical and mineralogical considerations

The Tafresh area, located on the middle part of Urumieh-Dokhtar magmatic arc, is a suitable area for considering the effects of tectonic structures on development of the volcanic facies in the magmatic provinces. The main structure in the area is a dextral fault (so called Rahjerd Fault) that has produced the local extensional basins and also the parallel dyke swarms (or feeder dykes) on its both sides. So, the pyroclastic deposits, basaltic-andesitic lava flows, parallel dyke swarms were appeared by explosive subaqueous and then effusive subaerial eruptions and eventually a dioritic stock intruded in the volcanic pile. Petrologically, the magmatic rocks are belonged to the calcalkaline suite that had been changed and evolved in the magma chambers of local extensional basins in a collisional tectonic setting. The mineral chemistry and geochemical modeling as well as the coexistence of different mineral paragenesis of plagioclases (An47-72 and An17-37), pyroxenes (diopside and pigeonite) and amphiboles (magnesiohastingsite and magnesiohornblende) reveal that the AFC process is a dominant process in the magma chamber. Also, according to geothermobarometric calculations, the investigated volcanic rocks of the area could be grouped into two types: one type with higher P-T (>6Kbar and about 900 °C) including Eocene pigeonite-magnesiohastingsite bearing andesites (PHA) and Miocene andesites and other with lower P-T (1-4 Kbar and 600- 800 °C) including the Eocene diopside-magnesiohornblende bearing andesites (DHA) and the dioritic stock.

Gasoline-air mixture Explosion in a Semi-Confined Space with a 90° Bend

In this work, experiments data from vented explosions tests using gasoline-air mixtures in a small, open square duct with a 90° bend were performed using high-speed camera. It is found that pressure history in the semi-confined space can be divided into four phases, i.e. constant volume explosion, effusive flow overpressure, disturbance explosion, vibration attenuation. The bend structure has an impact on the flame structure during the explosion. In the confined space, the flame transforms from sphere to brush-like shape. The transformation of flame shape in the pipe can be classified into six stages. When flame propagates to the bend structure, it bypasses the bend along the inner wall of the pipe. Flame velocity and flame front position can be divided into four stages: explosion initial stage, accelerated effusion stage, disturbance explosion stage, and energy attenuation stage. Both changes are consistent with changes in flame morphology.

OPHIOLITES FROM THE GRAMMOS-ARRENES AREA, NORTHERN GREECE: GEOLOGICAL, PALEONTOLOGICAL AND GEOCHEMICAL DATA

The Grammos-Arrenes ophiolites crop out between the Pindos Flysch to the East and the Meso-Hellenic basin to the West and represent the northernmost exposure of the Pindos Ophiolite belt in Greece. Field work and sampling on key outcrops of the Sub-ophiolitic Melange provided new data on the oceanization age and on the geochemistry of the effusive products. The basalts samples show a N-MOR affinity and are associated with latest Bajocian-early Bathonian radiolarian cherts. Emplacement of the ophiolites onto the continental margin is preceded by mass flow deposition of ophiolitic material in the basins facing the advancing ophiolitic nappe. Ophiolite-bearing debris flow deposits and slide blocks are recognized in the Early Cretaceous deep water sediments of the Beotian Unit and in the upper portion of the Tertiary Pindos Flysch. The intercalations of ophiolitic material in the Beotian Flysch first, and in the Pindos Flysch, later, are interpreted as the forerunners of the Ophiolitic Nappe which derived from the Vardar Ocean to the east, and was emplaced westwards onto the Adria continental margin. The collected data allow consolidating the constraints for the timing and mechanism of ophiolite emplacement in the Pindos-Grammos area. In addition, taking into consideration the geometry of the tectonic stack in the Grammos-Arrenes area and the ages of the involved sedimentary deposits it is possible to reconstruct a geodynamic history comparable to that of the other zones studied along the Dinaric-Hellenic Chain.

VOLCANIC CONTRIBUTIONS TO SEDIMENTATION IN UPPER BURDIGALIAN-LOWER LANGHIAN SEDIMENTS OF THE VENETIAN MOLASSIC BASIN

In the Upper Burdigalian-Lower Langhian sediments of the Venetian molassic basin two vol­caniclastic layers are clearly recognizable, respectively within the Globigerinoides trilobus and Praeorbulina glomerosa zones of Bizon & Bizon (1972). The chemical composition of well-preserved glass shards, tested by electron microprobe, reveals rhy­olitic composition and provides information about contemporaneous acidic explosive volcanism. Due to the probably depositional mechanism (fallout), the location of the effusive centres remains unknown. Although many ash layers are quite well-known in the same stratigraphic interval throughout the Apennine chain, a ten­tative correlation still seems very difficult, due to different methods of analysis and dating criteria.