Rosangela Bocchio*, Luca Benciolini, Silvana Martin and Paola Tartarotti Geochemistry of eclogitised Fe-Ti-gabbros from various lithological settings (Aosta Valley ophiolites, Italian Western Alps). Protolith composition and eclogitic paragenesis
ABSTRACT
- Eclogitised Fe-Ti metagabbros were collected from the Piemonte ophiolites
of the Aosta valley (Italian Western Alps). These rocks are either associated
with Mg-metagabbros or serpentinites, and show the same homogeneous metamorphic
imprint. Petrographic observations on pseudomorphic replacement in undeformed
pods and on mineral assemblages in tectonitic portions of the Fe-Ti metagabbros
reveal the presence of three different high-pressure mineral parageneses. The
first includes garnet, omphacite and rutile; the second consists of garnet,
omphacite and glaucophane; and the third contains garnet, omphacite, tremolite
and clinozoisite. Bulk rock chemical analyses show that all the studied samples
derive from similar magmatic protoliths with tholeiitic affinity. However, differences
in the amounts of major elements, namely TiO2, Na2O, CaO
and REE (particularly LREE) were found between the Fe-Ti metagabbros associated
with Mg-metagabbros and those occurring with serpentinites. Different chemical
compositions account for the three distinct eclogitic parageneses which developed
in the two groups of metagabbros. We infer that both magmatic processes and
rock alteration in an oceanic environment may have contributed towards producing
these chemical differences. We suggest that Fe-Ti gabbros crystallizing from
similar magmatic protoliths, as a consequence of the different extent of alteration
in the oceanic environment, developed distinct mineral assemblages during subduction-related
Alpine high-pressure metamorphism. This interpretation indicates that the early
pre-Alpine history of the oceanic lithosphere in the Piemonte basin played an
important role in determining the petrographic and geochemical features of its
metamorphic products.
* Corresponding author, E-mail: bocchio@r10.terra.unimi.it
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Nicoletta Buraglini and Gianbosco Traversa* Petrology and mineral chemistry of late-Hercynian dykes from NW Corsica (France)
ABSTRACT
- Intense late- to post-collisional dyke magmatism characterizes NW Corsica,
starting from the last phases of Hercynian orogeny, intruding prevalently the
Carboniferous granitoids and subordinately the rhyolites belonging to the calc-alkaline
Lower Permian volcanism. Both acidic and basic dykes are represented, whereas
there is an evident gap in the intermediate compositions; lack of crosscutting
evidence do not allow us to constrain the timing of emplacement of the various
magmatic units. Basic dykes comprise calcalkaline and mildly alkaline litotypes,
ranging in composition respectively from basaltic andesite to andesite and from
alkali basalt (dolerite) to basaltic-trachyandesite; among the latter, a fractionation
link is supported by mass balance calculations. The acidic dykes are mainly
composed of metaluminous granitic porphyries and subordinate peraluminous microgranites,
both preferentially NE-SW oriented, and by peralkaline aplites. REE and trace
elements modeling indicate derivation from a possibly subduction-modified litospheric
mantle source for both calcalkaline and mildly alkaline basaltic dykes, together
with crustal contamination probably undergone by magmas as they ascend to the
surface, as testified by Rb and Pb positive spikes relative to primitive mantle
composition. However, higher Nb and Ta contents for mildly alkaline dykes relate
them to a less depleted mantle source compared to calcalkaline ones, to be referred
to a post-collisional domain predating a true intracontinental rift-related
phase. Instead, the calcalkaline dykes were emplaced in a preceding phase still
characterized by an orogenic imprint, also shown by peraluminous microgranites.
These are probably anatectic melts deriving from a metasedimentary or meta-igneous
protolith. Preliminary 40Ar/39Ar geochronological data
from the biotite and muscovite of a few samples indicate an age around 320 Ma,
suggesting that these peraluminous microgranites are related to a magmatic event
connected with the Mg-K calcalkaline association dated at 322±12 Ma (Cocherie
et al., 1992). Granitic porphyries and peralkaline aplites show geochemical
characters typical of A-type granites and are referred to an anorogenic tectonic
setting. Strong petrographic and geochemical similarities with Upper Permian
peralkaline granite from Evisa indicate a mantle signature (Bonin, 1978). Moreover,
crustal involvement in the genesis of the peralkaline dykes is highlighted by
their high Th enrichments.
* Corresponding author, E-mail: traversa@uniroma1.it
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Rosa Anna Corsaro, Renato Cristofolini, Antonino Pezzino* and Anna Sergi Evidence for the provenance of building stone of igneous origin in the Roman Theatre in Catania
ABSTRACT
- The Roman Theatre in Catania was built using basically both local lithotypes
such as lava blocks from quarries in Etnean flows, and granite and marble from
various regions of the Mediterranean basin. In the cavea, the tiers were
made of lava blocks, originally covered by large limestone slabs, and of calcareous
blocks, probably derived from an earlier Greek theatre which had existed on
the same site. The tiers were subdivided into sectors by steps in lava, a material
resistant to the constant passage of the public and whose colour, contrasting
with that of the white limestone seats, created a pleasing chromatic effect.
Granite slabs were used to pave part of the orchestra and for the columns. Some
of these columns still remain within the theatre, where they lie, fallen in
the orchestra; however, most of them were removed from the area and used to
ornament the facade of the Cathedral and other monuments in Catania. At present,
one of them may also be admired at Largo della Marina (the Marine Parade) where
it supports the statue of St. Agatha, the patron saint of Catania. This paper
examines the petrographic and chemical characters of some types of lava used
for the construction of the tiers, and of the granitoid rocks used in the flooring
and columns of the Theatre. The resulting data made it possible to define the
probable areas of origin of the building stone, by comparing it with reference
sources concerning ancient Roman quarries. It is hypothesized that the lava
blocks forming the tiers of the cavea in the Roman Theatre, despite their heterogeneous
appearance, were all quarried from prehistoric lava flows of the Recent Mongibello,
near the monument itself, outside the Roman city walls. Petrographic and petrochemical
studies on the granitoid rocks used to pave the orchestra and for the columns
reveal their mineralogical and compositional heterogeneity. It is assumed that
all but three of these samples came from Western Turkey ("Mysian marble" from
Kozak). It is not yet possible to define the provenance of these three samples
on the grounds of their petrographic and geochemical characters.
* Corresponding author, E-mail: pezzino@mbox.unict.it
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Emma Cantisani, Roberta Canova, Fabio Fratini, Carlo Manganelli Del Fa', Roberto Mazzuoli* and Giancarlo Molli Relationships between microstructures and physical properties of white Apuan marbles: inferences on weathering durability
ABSTRACT
- The different types of Apuan white marbles show varying degrees of durability
when exposed to external weathering. Their durability has been related to different
microstructures, linked to distinct tectonometamorphic histories within the
Apuan Metamorphic Complex. This paper examines samples collected from several
sectors of the Apuan Alps, the differing geological histories of which cause
different marble microstructures. The samples are dominated by either granoblastic
or xenoblastic microstructures. Chemical, mineralogical, petrographical and
physical analyses were carried out. Microstructural characteristics were analysed
by image analysis in order to quantify parameters such as grain diameter, diameter
of equivalent circle, axial ratio, and roundness. These data are related to
petrophysical parameters such as porosity, mesopore distribution, and saturation
index. All the collected data indicate that the Apuan white marbles with low
roundness (indicating granoblastic structure) are more susceptible to weathering
than those with xenoblastic structure.
Furthermore, the different microstructural characteristics can be used to study
the provenance of the white marbles used in the past in constructing and decorating
historical buildings and monuments.
* Corresponding author, E-mail: mazzuoli@dst.unipi.it
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Gianni Cortecci*, Enrico Dinelli and Vincenzo Civitavecchia Isotopic and geochemical features of rocks from Punta delle Pietre Nere, Gargano Peninsula, southern Italy
ABSTRACT
- The locality called Punta delle Pietre Nere is a relatively small geological
system, where Upper Triassic black limestones and gypsum outcrops occur together
with Paleocene melasyenite and melagabbro and Upper Pleistocene biocalcarenites.
In the present work, sedimentary lithologies were investigated for major and
selected minor (Sr and Mn) elements and for stable isotope compositions (13C,
18O, 34S
and 87Sr/86Sr).
The results suggest that the intrusion of the igneous bodies did not substantially
alter the chemical and isotopic compositions of limestone and gypsum. Strontium
and Mn distributions in limestones reflect post-depositional recrystallization
of sediments with the intervention of seawater; the 18O
values yield an isotopic temperature of about 40°C and indicate pervasive water-rock
interaction. The 13C
values indicate variable contributions of bicarbonate in pore water due to oxidation
of organic matter, this reaction probably being associated with bacterial reduction
of sulfate ions. The 34S
of pyrite from limestones appears to be biogenic far from the contact with melasyenite
and magmatic close to the contact. The 13C
and 18O
values of biocalcarenites are normal marine.
The 87Sr/86Sr ratio in limestones matches their Upper
Triassic age. The 87Sr/86Sr and 34S
of gypsum are in good agreement with its age.
* Corresponding author, E-mail: cortecci@geomin.unibo.it
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Umberto Masi*, Ettore Azzaro, Konstanstinos Kyriakopoulos and Andreas Magganas Geochemical features of the "Plattenkalk" serie from the Hordaki Area (Western Crete, Greece)
ABSTRACT
- The paper deals with a geochemical study conducted on 28 samples of the "Plattenkalk"
Series from the Hordaki area, western Crete, Greece. In the lower and middle
portions of the sequence, the samples have a granoblastic fabric and an isotropic
texture. Instead, in the upper part of the sequence, the texture becomes oriented,
probably as a consequence of the overthrust of the Omalos (Trypali) unit over
the "Plattenkalk" Series. In the rocks, calcite accounts for over 98 vol.% on
average. The insoluble residue is represented by quartz, clinochlore, albite,
muscovite, hematite, magnetite, ilmenite, apatite, clinozoisite, and very scarce
biotite and pyrite. The rocks record greenschist facies metamorphism.
The insoluble residue of the rocks is low (x¯ = 1.5 wt%) and consists
of SiO2 (50 wt%), Al2O3 (16.5 wt%) and MnO (10.6 wt%), with subordinate Fe2O3tot.
(5.3 wt%), MgO and P2O5 (about 3 wt% each), and little
TiO2, Na2O and K2O (< 2.0 wt% each). Sr is
the most abundant trace element (x¯ = 341 ppm). Ba, Zn and Y range between
35 and 22 ppm. Cu, Rb and Zr are scarce (5-13 ppm). The insoluble residue displays
rhythmic oscillations with stratigraphy. Its fraction decreases in the upper
part of the sequence, indicating diminished detrital input into the sedimentary
basin.
Comparisons with the analogous marble from Mani, southern Peloponnesus, show
similar petrographic features, but generally different chemical features, indicating
that the two groups of marbles are not equivalent.
* Corresponding author, E-mail: umberto.masi@uniroma1.it
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