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  • Author: Shane Cronin x
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Abstract

The Pannonian Basin (Central Europe) hosts numerous alkali basaltic volcanic fields in an area similar to 200 000 km2. These volcanic fields were formed in an approximate time span of 8 million years producing smallvolume volcanoes typically considered to be monogenetic. Polycyclic monogenetic volcanic complexes are also common in each field however. The original morphology of volcanic landforms, especially phreatomagmatic volcanoes, is commonly modified. by erosion, commonly aided by tectonic uplift. The phreatomagmatic volcanoes eroded to the level of their sub-surface architecture expose crater to conduit filling as well as diatreme facies of pyroclastic rock assemblages. Uncertainties due to the strong erosion influenced by tectonic uplifts, fast and broad climatic changes, vegetation cover variations, and rapidly changing fluvio-lacustrine events in the past 8 million years in the Pannonian Basin have created a need to reconstruct and visualise the paleoenvironment into which the monogenetic volcanoes erupted. Here phreatomagmatic volcanic fields of the Miocene to Pleistocene western Hungarian alkali basaltic province have been selected and compared with modern phreatomagmatic fields. It has been concluded that the Auckland Volcanic Field (AVF) in New Zealand could be viewed as a prime modern analogue for the western Hungarian phreatomagmatic fields by sharing similarities in their pyroclastic successions textures such as pyroclast morphology, type, juvenile particle ratio to accidental lithics. Beside the AVF two other, morphologically more modified volcanic fields (Pali Aike, Argentina and Jeju, Korea) show similar features to the western Hungarian examples, highlighting issues such as preservation potential of pyroclastic successions of phreatomagmatic volcanoes.

Abstract

The Efate Pumice Formation (EPF) is a trachydacitic volcaniclastic succession widespread in the central part of Efate Island and also present on Hat and Lelepa islands to the north. The volcanic succession has been inferred to result from a major, entirely subaqueous explosive event north of Efate Island. The accumulated pumice-rich units were previously interpreted to be subaqueous pyroclastic density current deposits on the basis of their bedding, componentry and stratigraphic characteristics. Here we suggest an alternative eruptive scenario for this widespread succession. The major part of the EPF is distributed in central Efate, where pumiceous pyroclastic rock units several hundred meters thick are found within fault scarp cliffs elevated about 800 m above sea level. The basal 200 m of the pumiceous succession is composed of massive to weakly bedded pumiceous lapilli units, each 2-3 m thick. This succession is interbedded with wavy, undulatory and dune bedded pumiceous ash and fine lapilli units with characteristics of co-ignimbrite surges and ground surges. The presence of the surge beds implies that the intervening units comprise a subaerial ignimbrite-dominated succession. There are no sedimentary indicators in the basal units examined that are consistent with water-supported transportation and/or deposition. The subaerial ignimbrite sequence of the EPF is overlain by a shallow marine volcaniclastic Rentanbau Tuffs. The EPF is topped by reef limestone, which presumably preserved the underlying EPF from erosion. We here propose that the EPF was formed by a combination of initial subaerial ignimbrite-forming eruptions, followed by caldera subsidence. The upper volcaniclastic successions in our model represent intra-caldera pumiceous volcaniclastic deposits accumulated in a shallow marine environment in the resultant caldera. The present day elevated position of the succession is a result of a combination of possible caldera resurgence and ongoing arc-related uplift in the region.

Abstract

The generation of silica undersaturated phonolite from silica saturated trachytes is uncommon, as it implies the crossing of the thermal barrier and critical plane of silica undersaturation. Nevertheless, a co-genetic suite displaying compositional transition from benmoreite-trachyte to phonolite has been observed within the Al Shaatha pyroclastic sequence in the Harrat Rahat Volcanic Field (Kingdom of Saudi Arabia). We performed crystallization experiments on benmoreite and trachyte starting compositions to simulate the pressure-temperature-volatile conditions that generated the observed liquid line of descent. The experimental conditions were 200–500 MPa, 850–1150 °C, 0–10 wt% H2O, 0.0–0.5 wt% CO2, and NNO+2 oxygen buffer. The experimental mineral assemblage consists of clinopyroxene, feldspar, and titanomagnetite, as well as glass in variable proportions. The degree of crystallinity of hydrous runs is lower than that of anhydrous ones at analogous pressure and temperature conditions. Clinopyroxene crystallizes with compositions diopside-augite and augite-hedenbergite, respectively, at 500 and 200 MPa. The saturation of feldspar is primarily controlled by temperature and volatile content, with the more potassic composition equilibrating at low temperature (850–900 °C) and anhydrous (for benmoreite) or hydrous (for trachyte) conditions. At low pressure (200 MPa), temperatures below 850 °C, and anhydrous conditions, the degree of crystallization is extremely high (>90%), and the residual glass obtained from trachyte experiments is characterized by peralkaline and sodic affinity. This finding is consistent with natural eruptive products containing interstitial phonolitic glass within an anorthoclase framework. The shift from trachyte to phonolite is therefore interpreted as the result of open system interaction between trachytic magma and intercumulus phonolitic melt, as well as of dissolution of anorthoclase from a crystal mush.