Sector-zoned augite megacrysts in Aleutian high alumina basalts: implications for the conditions of basalt crystallization and the generation of calc-alkaline series magmas

Several high alumina basalts from the Aleutian volcanic centers of Cold Bay and Kanaga Island contain large (up to 1.5 cm diameter) megacrysts of sector-zoned augite. The megacrysts are invariably euhedral with well developed {001}, {010} and {111} forms. All crystals display concentric bands that are rich in mineral and glass inclusions. The sector zonation typically occurs as well developed (010), (100), (111) and (110) sectors which grew at different rates. A comparison of the width of synchronous growth bands indicates that following relative growth rates: (111) ≫ (100) ∼ (110) > (010). Compositionally, SiO₂ and MgO abundances decrease, and TiO₂, Al₂O₃, FeO and Na₂O abundances increase in the different sectors in the order (111), (100) ∼ (110), (010). This order is identical to that deduced for the relative growth rates, implying that growth rate clearly had a role in the development of the sector zonation. Calculated pre-eruption H₂O contents of the basalts range from 1 to 3 wt% but actual (measured) post-eruption H₂O contents range from 0.01 to 0.3 wt%. Deteurium isotopic values are heavily depleted and range from −110 to −141‰ . Together these indicate significant vapor (H₂O) exsolution prior to eruption. Maximum H₂O abundances in primitive glass inclusions, thought to be most representative of the host liquid reservoir at the time of melt entrapment, systematically decrease from the core to the rim of one augite megacryst studied in detail. We conclude that the presence of sector-zoned augite is due to augite supersaturation and rapid crystallization brought about by magma decompression and volatile (H₂O) exsolution. The calculated pre-eruption H₂O contents of 1–3 wt% limit vapor exsolution and basalt crystallization to depths of less than 3 and more likely 1.5 km. Very rapid crystallization at very shallow depths makes it unlikely that the time scales between initial crystallization and final eruption are sufficient to permit appreciable amounts of fractional crystallization. Given that high alumina basalt fractionation is the dominant process for generating more evolved andesite, dacite and rhyolite magmas of the calc-alkaline suite, the inability of parental high alumina basalt to yield such derivative magmas in the low pressure environment places the likely site of fractionation in the high pressure environment, at or near the base of the crust. Received: 1 December 1997 / Accepted: 23 December 1998     

Baguio Mineral District: An oceanic arc witness to the geological evolution of northern Luzon,Philippines

The Baguio Mineral District exposes rock formations that evince the geological and tectonic evolution of this district from a subduction‐related marginal basin to an island arc setting. Available onshore and offshore data are consistent with an Early (onset phase) to Middle (developed phase) Miocene arc polarity reversal from the east (termination of subduction along the proto‐East Luzon Trough) to the west (initiation of subduction along the Manila Trench). Geophysical modeling and geochemical data calculation showed a 30 ± 5 km crustal thickness for the mineral district. Subduction‐related multiple arc magmatism and ophiolite accretion contributed to crustal thickening. Recent information on the Oligo–Miocene Zigzag and Klondyke formations in the mineral district reveal that the marginal basin, where these rocks were deposited, has received eroded materials from adjacent terrains characterized by siliceous lithologies. Furthermore, adakitic rocks, high permeable zones and extensional zones which are exploration markers applied to identify possible mineralization targets, are prevalent in the mineral district. The geological evolution that the district had undergone mimics the evolution of island arcs worldwide in general and northern Luzon in particular.     

Probabilistic modelling of macroseismic attenuation and forecast of damage scenarios

According to the idea now widespread that macroseismic intensity should be expressed in probabilistic terms, a beta-binomial model has been proposed in the literature to estimate the probability of the intensity at site in the Bayesian framework and a clustering procedure has been adopted to define learning sets of macroseismic fields required to assign prior distributions of the model parameters. This article presents the results concerning the learning sets obtained by exploiting the large Italian macroseismic database DBM1I11 (Locati et al. in DBMI11, the 2011 version of the Italian Macroseismic Database, 2011. http://emidius.mi.ingv.it/DBMI11/) and discusses the problems related to their use in probabilistic modelling of the attenuation in seismic regions of the European countries partners of the UPStrat-MAFA project (2012), namely South Iceland, Portugal, SE Spain and Mt Etna volcano area (Italy). Anisotropy and the presence of offshore earthquakes are some of the problems faced. All the work has been carried out in the framework of the Task B of the project.     

The influence of coastal dynamics on the upper limit of the Posidonia oceanica meadow

The recognized ecological importance of Posidonia oceanica, the most important seagrass of the Mediterranean Sea, makes it crucial to assess the state of health of its meadows, discriminating natural from anthropogenic impacts. In this paper, the hydrodynamic conditions at the upper limit of P. oceanica meadows along the Ligurian coast (NW Mediterranean Sea) were investigated. A relationship between the distance of the upper limit of the meadow from the shoreline and the morphodynamic domain of the beach (i.e. distinctive types of beach produced by the topography, wave climate and sediment composition) was found. A zonation of the state of the shallow portions of the meadows down the submerged beach profile was identified. Zone a, from the shoreline to the breaking limit, is naturally critical for the development of the meadow. Zone b, from the breaking limit to the closure depth, is subjected to natural and human impacts. Zone c, below the closure depth, is little influenced by coastal dynamics. This study quantifies for the first time how much the status of the shallow portions of P. oceanica meadows is dependent on coastal dynamics, which is important for their proper management.     

Prey versus substrate as determinants of habitat choice in a feeding shorebird

Many shorebirds on their non-breeding grounds feed on macrobenthic fauna which become available at low tide in coastal intertidal flats. The Eastern Curlew Numenius madagascariensis in Moreton Bay Australia, varies greatly in density among different tidal flats. This study asks: how important is the abundance of intertidal prey as a predictor of this variation? We quantified feeding curlews’ diet across 12 sites (different tidal flats, each re-visited at least eight times), through 970 focal observations. We also estimated the abundance of total macrobenthic fauna, potential prey taxa and crustacean prey on each tidal flat; measured as the number of individuals and a relative biomass index per unit substrate surface area obtained from substrate core samples. We estimated curlew density at each site using low-tide surveys from every site visit.     

Geology and geochemistry of the Shuanggou ophiolite (Ailao Shan ophiolitic belt), Yunnan Province, SW China: Evidence for a slow-spreading oceanic basin origin

The early Carboniferous Shuanggou ophiolite lies in the middle segment of the Ailao Shan orogenic belt between the South China Block to the north and the Indochina Block to the south. The ophiolite consists of meta-peridotite, gabbro, diabase and basalt, capped by radiolarian-bearing siliceous rocks. No layered gabbros or sheeted dikes have been observed. The meta-peridotite underwent low degrees of partial melting, consistent with the low magma budget of this oceanic lithosphere. Whole-rock rare earth element analyses of gabbro indicate a geochemical affinity with normal mid-ocean ridge basalts, consistent with the crystallization order of plagioclase followed by clinopyroxene recognized in the gabbros. The ophiolite is believed to have formed in a small, slow-spreading oceanic basin. Collision of the Indochina Block with the South China Block in the late Paleozoic was responsible for the closure of the oceanic basin and emplacement of the ophiolite in the Ailao Shan orogenic belt.