Cravero F. and Domínguez E. Origin of sedimentary kaolin in the Neuquén basin, Argentina as determined by oxygen isotopes 213-222
ABSTRACT
- Two Gondwana granite-rhyolite
provinces occur in southern Argentina. These are, Choiyoi, late Carboniferous-Early
Permian to Triassic in age, in the Neuquén basin and Chon-Aike, Middle
Jurassic, in Patagonia. In both areas, the clay facies of a fluvial sequence
that overlay the complexes is composed of transported kaolinites. In Patagonia,
a paleoweathered surface was formed on Chon-Aike rocks under humid temperature
conditions indicated by the oxygen isotope composition of the kaolinites (d18O
= 16-18). In this work it
is demonstrated that isotopic composition of the kaolinites eroded from that
paleosurface and deposited in a fluvial sequence has not changed. Taking this
into consideration, the d18O values obtained
in the sedimentary kaolinites of the Neuquén basin (15 to 17),
can be considered as an indication of humid, temperature conditions as the primary
origin of the kaolinite. The clasts composing the coarser facies (rhyolitic)
indicate that the kaolinite has been eroded from a paleoweathered surface developed
on Choiyoi rocks.
This work demonstrates
that following a huge volcanic event in the southern hemisphere, paleoweathered
surfaces developed in a humid and temperate climate, conditions already established
for Chon-Aike kaolinites. Moreover all the continental sedimentary sequences
that overlay Choiyoi and Chon-Aike are promising areas for prospecting for sedimentary
kaolin deposits.
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ABSTRACT
- The capacity of
the demosponge Chondrosia reniformis to selectively incorporate and elaborate
inorganic materials is described. The sponge actively engulfs sandy grains of
various silica-bearing minerals, such as quartz, opale, chalcedony, feldspars,
mica, chlorite, amphiboles, etc. Conversely, carbonates are rejected, at different
stages during the sponge biological cycle. Different ectosome activity towards
quartz and opale is evidenced by the sponge capacity to differentially react
to different crystallographic forms of silica: while opale and chalcedony are
stored in the collagenous fibres, quartz is dissolved and residual rounded micrograins
(3-5 mm) are ejected. The ascorbic acid produced
by the sponge is considered to be the agent for quartz dissolution. Finally,
the production of calcium oxalates (whewellite and weddellite) and gypsum by
the sponge, and their release in the surrounding sediment is evidenced.
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ABSTRACT
- Late-Hercynian dyke
swarms of Sarrabus (southern Sardinia) consists of metaluminous up to peraluminous
felsic products, prevalently rhyolitic in composition (granite porphyries, microgranites
and aplites) and basic and intermediate dykes comprising few tholeiitic basalts
and a calc-alkaline suite dominated by basaltic andesites and andeites. New
geochronological data contribute to define the time-span of dyke magmatism.
Rb-Sr mica ages for rhyolitic rocks cluster around 290Ma and 270 Ma. Field data
and one Rb-Sr biotite age (259±3 Ma) obtained on a basaltic andesite sample
indicate that mafic dykes prevalently intruded later than felsic ones.
Mafic dyke rocks are
mantle-derived, but both among tholeiitic dykes and calc-alkaline one, rocks
representative of primary mantle melts are lacking. The least evolved calc-alkaline
rocks are represented by rare basalts and by high-Mg basaltic andesites. Petrography,
mineral and whole-rock chemistry, Sr and Nd isotopic features and mass balance
calculations suggest that fractional crystallization associated with minor amount
of crustal contamination controlled the evolution of the calc-alkaline suite.
A model involving fractional crystallization and assimilation of metasedimentary
materials (AFC) is proposed to account for the main geochemical and Sr and Nd
isotopic variations from the high-Mg basaltic andesites to dacites. Tholeiitic
basalts and the least evolved calc-alkaline rocks show an arc-type incompatible
element signature (LILE and LREE enrichments and Ti, Nb, P depletions), positive
etSr end negative etNd
values suggesting derivation from a mantle source possibly enriched during a
previous subduction event.
Differences in the
Sr and Nd isotopic composition, in agreement with mineralogy and geochemistry,
point to the occurrence of two different types of felsic dykes, i.e. the
peraluminous (PR) and metaluminous to mildly peraluminous (MmPR) groups. It
is proposed that PR dykes originated by partial melting from pelitic metasedimentary
sources. The high (87Sr/86Sr)t ratios
(0.7154÷0.7173) and the negative etNd
value (-7.4) show by this group of rhyolites match those of the metasediments
from the Hercynian basement of Sardinia. Mass balance calculations indicate
that only the least silicic MmPR rhyolites could be genetically related to the
basic-intermediate dykes by fractional crystallization relationships. On the
contrary, for most MmPR rocks, an origin by partial melting of crustal sources
seems more likely. Their relatively low (87Sr/86Sr)t
and high etNd values (0.7076÷0.7089
and –5.7÷-6.3, respectively), partly overlapping those of the calc-alkaline
basic-intermediate dykes, require source materials characterized by a less evolved
Nd and Sr isotopic composition than those observed for the Sardinia metamorphic
basement and imply the involvement of both crustal and juvenile mantle components.
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Ghiara
M.R., Petti C. and Morbidelli P. Analcimization processes in the pyroclastic
rocks from Phlegraen Field (southern Italy): compositional variations and geochemical
balances 261-273
ABSTRACT
- A series of scoriaceous
fragments collected from pyroclastic fall deposits belonging to different volcanoes
of the Phlegraean Fields (Neaples, Italy)shows progressive stages of analcimization.
Particularly, major and trace elements and 87Sr/86Sr ratio
in non-altered and altered samples from the Fondo Riccio volcano have been determined.
With increasing analcimization the following geochemical balance appears: Si
and K decrease, Al, Na and Fe increase, Ca and Mg remain constant. Pb is depleted
whereas Rb is slightly enriched in the more analcimized samples. All rere-earth
elements do not show any significant difference between non-altered and altered
samples. 87Sr/86Sr ratio strongly decreases in the more
altered samples. Chemical variations observed in the scoriaceous fragments from
the Fondo Riccio volcano, suggest that the analcimization process occurs in
an open system and that a fluid component enriched in Na is involved. Therefore
in a petrogenetic modelling, Pb, Rb and 87Sr/86Sr must
be used with very caution.
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Fumagalli P. and Poli S. Phase relationships in hydrous peridotites at high pressure: preliminary results of multianvil experiments 275-286
ABSTRACT
- Phase relationships
in hydrous peridotite at high pressure are investigated running experiments
in a multianvil apparatus at 4.6-5.2 GPa pressures, 680-750°C temperatures and
fluid saturated conditions in the peridotite model system Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O
(NCFMASH). Pressure calibration was carried out both al room temperature using
the transitions Bi I-II and Bi III-V (occurring at 2.55 GPa and 7.7 GPa, respectively),
and at 1000°C using the phase transitions coesite-stishovite and garnet-perovskite
structure in Ca-germanates (occurring at 8.7 and 6.1 GPa, respectively). Run
duration were from 110 to 170 hours. Run products were identified by X-ray powder
diffraction, back-scattered and secondary electron images and microprobe analysis.
In an Al-enriched
peridotite, the assemblage olivine, clinopyroxene, garnet 10
phase was found at 5.2 GPa 680°C. Since the crystals structure of the 10
phase is broadly similar to micas, this phase can incorporate more than 0.8
atoms per formula unit (a.p.f.u.) of aluminium. However diffraction patterns
do not exclude the occurrence of hydrous mixed-layered structures at these conditions.
Although in the simple system MgO- SiO2-H2O the assemblage
10 phase + forsterite is incompatible
with the join enstatite + H2O, all these phases may coexist in Fe-
and Al-bearing systems (lherzolites and pyroxenites). At 4.6 GPa 750°C the anhydrous
assemblage olivine, clinopyroxene, orthopyroxene, garnet is stable. Phase abundances
obtained by Rietveld refinement are in agreement with those obtained by mass
balance and present evidence for the reaction 10
phase = enstatite + pyrope + H2O.
The occurrences of
the 10 phase at pressures above
the chlorite stability field promotes H2O transfer to Dense Hydrous
Magnesium Silicates (DHMS) and H2O transport into the deep mantle
(> 200 km depth).
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Morbidelli P., Ghiara M.R., Lonis R. and Sau A. Zeolitic occurrences from Tertiary pyroclastic flows and related epiclastic deposits outcropping in northern Sardinia (Italy) 287-313
ABSTRACT -
This study is part of research
work aimed at identifying zeolitic occurrences of economic interest in Sardinia.
The aim of the present
work is to define genetic models for zeolite occurring in Tertiary pyroclastic
flows outcropping in northern Sardinia (Logudoro region). The sample area, covering
about 1000 km2, includes the villages of Ozieri, Ploaghe, Ittireddu,
Chilivani, Mores and Bonorva.
The most important
volcanic formations, outcropping here, include the oldest manifestations (24-15
Ma) of Oligo-Miocene orogenic magmatism in Sardinia. Tertiary volcanics include
poorly welded (ILP and IC) and welded (IS) ash and pumice flows; epiclastic
(EV and EN) and base surge (BS) deposits are locally interbedded. Field research,
devoted to lithostratigraphic reconstruction of outcropping suites, also showed
numerous degassing pipes inside ignimbritic rocks.
Laboratory research
identified various rock components (matrix, phenocrysts, pumice and xenoliths)
and modal analysis were also carried out near Bonorva. The distributive and
textural features of secondary minerals (zeolite, smectite, silica minerals
and glauconite) were also examinated. Microscopic (optical and SEM) data defined
the following crystallization order:
smectite => clinoptilolite
=> opal-CT => mordenite.
The most common zeolitic
variety is clinoptilolite, which: 1)fills vugs and degassing pipe structures;
2) lines both tubular and sub-spherical vesicles in pumice; 3) grows on cuspate
glassy fragments; 4) intergrows with silica minerals in patches randomly distributed
in the matrix; 5) constitutes the felt-form intergrowths of well-developed crystals
growing on the matrix; 6) lines veins which, in the central part, are filled
with anhedral calcitic aggregates.
The overall data set
suggests the following three genetic models for this zeolitic material: a)
crystallization from fluids entrapped inside sub-spherical vesicles hosted in
pumiceous fragments; b) crystallization linked to polyphase transformation
of glassy components by juvenile fluids and c) crystallization linked
to interaction processes on glassy components by external fluids.
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