1,721,146 research outputs found
Il magmatismo dell'avampaese Ibleo (Sicilia orientale) tra il Trias e il Quaternario: dati stratigrafici e petrologici di sottosuolo
No full textTerrestrial record of post-Eocene climate history in Marie Byrd Land, West Antarctica
No full textHydrovolcanic deposits, interbedded tills and recycled microfossils, together with erosion anomalies in the Marie Byrd Land (MBL) landscape, each provide a portion of the record of glaciation and deglaciation events from late Oligocene to the present. We have attempted to synthesize these data sources to provide a more complete record, and to reconcile them with climatic events recorded elsewhere in Antarctica and the deep sea. The MBL data suggest that the late Oligocene was marked by the development of an ice cap at Mount Petras, where the MBL dome was beginning to rise from a near-sea-level position. Furthermore, unusually advanced cirque development in the dome crest area is difficult to explain unless there was a period of effective cirque erosion in that area between c. 25 and 15 Ma Bp. These inferences are consistent with evidence from the Ross Sea for an expansion of the West Antarctic Ice Sheet (WAIS) in mid-Miocene (15-17 Ma BP) time.
The deep sea oxygen isotope proxy record has been interpreted to show the inception of West Antarctic glaciation around 6 Ma BP. This can perhaps be reconciled with the terrestrial record if one considers (1) the observation that large volume changes in the WAIS cannot produce a delta(18)O signal that is significantly outside limits of error, and (2) that the landscape in West Antarctica has evolved from very low regional relief in the Oligocene, to more than 2 km of local relief in the present day, as a result of dome uplift since c. 27 Ma Bp, and the growth of large volcanoes since c. 19 Ma BP
Origin and interaction of mafic and felsic magmas in an evolving late orogenic setting: the early Paleozoic Terra Nova Intrusive Complex, Antarctica
No full textThe Abbott Unit (similar to 508 Ma) and the Vegetation Unit (similar to 475 Ma) of the Terra Nova Intrusive Complex (northern Victoria Land, Antarctica) represent the latest magmatic events related to the Early Paleozoic Ross Orogeny. They show different emplacement styles and depths, ranging from forcible at 0.4-0.5 GPa for the Abbott Unit to passive at similar to 0.2 GPa for the Vegetation Unit. Both units consist of mafic, felsic and intermediate facies which collectively define continuous chemical trends. The most mafic rocks from both units show different enrichment in trace element and Sr-Nd isotopic signatures. Once the possible effects of upper crustal assimilation-fractional crystallisation (AFC) and lower crustal coupled AFC and magma refilling processes have been taken into account the following features are recognised: (1) the modelled primary Abbott Unit magma shows a slightly enriched incompatible element distribution, similar to common continental are basalts and (2) the modelled primary Vegetation Unit magma displays highly enriched isotope ratios and incompatible element patterns. We interpreted these major changes in magmatic affinity and emplacement style as linked to a major change in the tectonic setting affecting melt generation, rise and emplacement of the magmas. The Abbott Unit mafic melts were derived from a mantle wedge above a subduction zone, with subcontinental lithospheric mantle marginally involved in the melting column. The Vegetation Unit mafic melts are regarded as products of a different source involving an old layer of subcontinental lithospheric mantle. The crustal evolution of both types of mafic melts is marked by significant compositional contrasts in Sr and Nd isotopes between mafic and associated felsic rocks. The crustal isotope signature showed an increase with felsic character. Geochemical variations for both units can be accounted for by a similar two-stage hybridisation process. In the first stage, the most mafic magma evolved mainly by fractional crystallisation coupled with assimilation of metasedimentary rocks having crustal time-integrated Sr and Nd compositions similar to those of locally exposed metamorphic basement. The second stage involves contaminated products mixing with independently generated crustal melts. Petrographic, geochemical and isotope data also provide evidence of significant compositional differences in the felsic end-members, pointing to the involvement of metaigneous and metasedimentary source rocks for the Abbott granite and Vegetation leucogranite, respectively
JOINT ITALIAN-BRITISH PETROLOGICAL-PALAEOENVIRONMENTAL INVESTIGATIONS OF NEOGENE VOLCANIC SEQUENCES IN NORTHERN VICTORIA LAND
No full textA joint investigation between the Italian Programma Nazionale di Ricerche
in Antartide (PNRA) and the British Antarctic Survey (BAS) was undertaken in
northern Victoria Land in December 2005 and January 2006 (Fig. 1). The study is
focused on volcanic sequences in the western coastal areas of the Ross Sea. The
sequences are in a pivotal position to record palaeoenvironmental and magmatic
evolution along about 400 km of the Ross Sea margin of East Antarctica over the
past 10-12 million years. It is a critical yet poorly known environmental period
of Earth history for which almost the entire regional record has been removed
by erosion in the adjacent Ross Sea. The volcanic sequences can thus fill in this
important gap in our knowledge
‘A‘ā lava-fed deltas: A new reference tool in paleoenvironmental studies
No full textLava-fed deltas are extraordinarily useful indicators of fossil water (and ice) levels in glacial,
marine, and lacustrine environments. Deltas fed by ‘a‘ā lava should be at least as common as
those sourced in pāhoehoe, yet they have been rarely described. Although facies models for
pāhoehoe lava-fed deltas are well established, the architecture and lithofacies of ‘a‘ā-fed equivalents
are substantially different and have thus far largely been unrecognized. This can have
profound consequences for paleoenvironmental investigations, particularly those attempting
to reconstruct past ice sheets. Essential features of ‘a‘ā lava-fed deltas include (1) a subaerial
‘a‘ā lava capping unit comprising massive internal sheet lava overlain by clinkers; (2) a crudely
developed subaerial to subaqueous transition (passage zone); (3) a chaotic subaqueous association
of abundant lava lobes and hyaloclastite with admixed vesicular, often reddened (oxidized)
lava clinkers; and (4) rare subaqueous stratifi cation with predominantly lower dips (~10°–20°)
than in deltas fed by pāhoehoe lava (~25°–40°). We develop a generic facies model and investigate
the emplacement conditions of ‘a‘ā lava-fed deltas in order to facilitate the recognition and
environmental interpretation of these important sequence types in ancient successions
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