18 research outputs found
The Early Cretaceous San Juan Plutonic Suite, Ecuador: A magma chamber in an oceanic plateau ?
Sections through an oceanic plateau are preserved in tectonic slices in
the Western Cordillera of Ecuador (South America). The San Juan section
is a sequence of mafic-ultramafic cumulates. To establish that these
plutonic rocks formed in an oceanic plateau setting, we have developed
criteria that discriminate intrusions of oceanic plateaus from those of
other tectonic settings. The mineralogy and crystallization sequence of
the cumulates are similar to those of intra-plate magmas. Clinopyroxene
predominates throughout, and orthopyroxene is only a minor component.
Rocks of intermediate composition are absent, and hornblende is
restricted to the uppermost massive gabbros within the sequence. The
ultramafic cumulates are very depleted in light rare-earth elements
(LREE), whereas the gabbros have flat or slightly enriched LREE
patterns. The composition of the basaltic liquid in equilibrium with the
peridotite, calculated using olivine compositions and REE contents of
clinopyroxene, contains between 16% and 8% MgO and has a flat REE
pattern. This melt is geochemically similar to other accreted oceanic
plateau basalts, isotropic gabbros, and differentiated sills in western
Ecuador. The Ecuadorian intrusive and extrusive rocks have a narrow
range of epsilonNd(i) (+8 to +5) and have a rather large range of Pb
isotopic ratios. Pb isotope systematics of the San Juan plutonic rocks
and mineral separates lie along a mixing line between the depleted
mantle (DMM) and the enriched-plume end members. This suggests that the
Ecuadorian plutonic rocks generated from the mixing of two mantle
sources, a depleted mid-oceanic ridge basalt (MORB) source and an
enriched one. The latter is characterized by high (Pb-207/Pb-204)(i)
ratios and could reflect a contamination by recycled either lower
continental crust or oceanic pelagic sediments and (or) altered oceanic
crust (enriched mantle type I, EMI). These data suggest that the San
Juan sequence represents the plutonic components of an Early Cretaceous
oceanic plateau, which accreted in the Late Cretaceous to the Ecuadorian
margin
Accreted fragments of the Late Cretaceous Caribbean-Colombian Plateau in Ecuador
The eastern part of the Western Cordillera of Ecuador includes fragments
of an Early Cretaceous ( approximate to 123 Ma) oceanic plateau accreted
around 85-80 Ma (San Juan unit). West of this unit and in fault contact
with it, another oceanic plateau sequence (Guaranda unit) is marked by
the occurrence of picrites, ankaramites, basalts, dolerites and shallow
level gabbros. A comparable unit is also exposed in northwestern coastal
Ecuador (Pedernales unit).
Picrites have LREE-depleted patterns, high epsilonNd(i) and very low Pb
isotopic ratios, suggesting that they were derived from an extremely
depleted source. In contrast, the ankaramites and Mg-rich basalts are
LREE-enriched and have radiogenic Pb isotopic compositions similar to
the Galapagos HIMU component; their epsilonNd(i) are slightly lower than
those of the picrites. Basalts, dolerites and gabbros differ from the
picrites and ankaramites by flat rare earth element (REE) patterns and
lower epsilonNd; their Pb isotopic compositions are intermediate between
those of the picrites and ankaramites. The ankaramites, Mg-rich basalts,
and picrites differ from the lavas from the San Juan-Multitud Unit by
higher Pb ratios and lower epsilonNd(i).
The Ecuadorian and Gorgona 88-86 Ma picrites are geochemically similar.
The Ecuadorian ankaramites and Mg-rich basalts share with the 92-86 Ma
Mg-rich basalts of the Caribbean-Colombian Oceanic Plateau (CCOP)
similar trace element and Nd and Pb isotopic chemistry. This suggests
that the Pedernales and Guaranda units belong to the Late Cretaceous
CCOR The geochemical diversity of the Guaranda and Pedernales rocks
illustrates the heterogeneity of the CCOP plume source and suggests a
multi-stage model for the emplacement of these rocks. Stratigraphic and
geological relations strongly suggest that the Guaranda unit was
accreted in the late Maastrichtian (approximate to 68-65 Ma). (C) 2002
Elsevier Science B.V. All rights reserved
A new collector for sampling volcanic aerosols
A new apparatus, Venturi Effect System (VES), designed for sampling volcanic plumes is described and tested at Vulcano (Italy). This device, together with purified basic NH4OH solutions, supplies optimal conditions to obtain reliable Stotal/Cl/F ratios and enrichment factors for metallic trace elements (MTE). Good concordance for acid gas ratios and metal enrichment factors in both the gas phase and the related plume allows the procedure to be validated.
The VES appears in Vulcano conditions as a simple, robust and easily portable apparatus that allows reliable collection of both acid gases and MTE within a single sample and the analysis with current chemical methods (High Pressure Liquid Chromatography, Inductively Coupled Plasma-Mass Spectrometry. This apparatus may be suitable
for more difficult volcanoes where only the plume can be sampled
Subduction of an Active Spreading Ridge Beneath Southern South America: A Review of the Cenozoic Geological Records from the Andean Foreland, Central Patagonia (46–47°S)
The Chile-Argentina Patagonian Cordillera is a natural laboratory to study the interactions between oceanic and continental lithosphere during the subduction of an active spreading ridge beneath a continent. Subduction of the South Chile spreading ridge, which separates the Nazca plate from the Antarctic plate, started around 15–14 Ma at the southern tip of Patagonia. Presently, the southernmost segment of the Chile Ridge enters the Peru-Chile trench at 46°S, at the site of the Chile Triple Junction (CTJ). We review the main events which occurred on land in the CTJ region (46–47°S), related with processes of ridge subduction. We summarize tectonic, sedimentary, and magmatic features in a 30 Ma-to Present chronological table. A preridge subduction stage, from 30 to 15 Ma, is characterized by the onset and growth of Patagonian relief and by a shift from marine to continental detrital sedimentation in the foreland at 20–22 Ma. The change from pre-ridge subduction to ridge subduction is marked on land by a transition from calc-alkaline to alkaline volcanism, at 14–12 Ma, and by the onset of eruption of very large fl ood basalt provinces (future volcanic plateaus) following rapid erosion of the eastern foreland belt. Post-plateau basaltic volcanism (<4 Ma) is coeval with a period of tectonic and morphological rejuvenation during which the eastern foreland of the Cordillera has been affected by extensional/transtensional tectonics. We place these events in the framework of a tectonomagmatic model involving the opening of slab windows due to both slab tear and ridge axis subduction
Influence of source distribution and geochemical composition of aerosols on children exposure in the large polymetallic mining region of the Bolivian Altiplano
The Bolivian Altiplano (Highlands) region is subject to intense mining, tailing and smelting activities since centuries because of the presence of large and unique polymetallic ore deposits (Ag, Au, Cu, Pb, Sn, Sb, Zn). A large scale PM(10), Pm(2.5) aerosol monitoring survey was conducted during the dry season in one of the largest mining cities of this region (Oruro, 200,000 inhabitants). Aerosol fractions, source distribution and transport were investigated for 23 elements at approximately 1 km(2) scale resolution, and compared to children exposure data obtained within the same geographical space. As, Cd, Pb, Sb, W and Zn in aerosols are present at relatively high concentrations when compared to studies from other mining regions. Arsenic exceeds the European council PM(10) guide value (6 ng/m(3)) for 90% of the samples, topping 200 ng/m(3). Ag, As, Cd, Cu, Pb and Sb are present at significantly higher levels in the district located in the vicinity of the smelter zone. At the city level, principal component analysis combined with the mapping of factor scores allowed the identification and deconvolution of four individual sources: i) a natural magmatic source (Co, Cs, Fe, K. Mn, Na, Rb and U) originating from soil dust, resuspended by the traffic activity; ii) a natural sedimentary source (Mg, Ca, Sr, Ba and Th) resulting from the suspension of evaporative salt deposits located South; iii) an anthropogenic source specifically enriched in mined elements (As, Cd, Cu, Pb, Sb and Zn) mainly in the smelting district of the city; and iv) a Ni-Cr source homogenously distributed between the different city districts. Enrichment factors for As, Cd and Sb clearly show the impact of smelting activities, particularly in the finest PM(2.5) fraction. Comparison to children's hair metal contents collected in five schools from different districts shows a direct exposure to smelting activity fingerprinted by a unique trace elements pattern (Ag, As, Cu, Pb, Sb)