Gaeta M., Mochi L., Invernizzi C., Conte A.M. and Misiti V. Emplacement pressure conditions of Gennargentu Igneous Complex two mica granites, central Sardinia (Italy) 1-18
ABSTRACT
- Scarcity of peraluminous granites in the Sardinia-Corsica batholith is a distinctive
petrographic features of this sector of the Hercynian orogenic chain. The granites
of the Gennargentu Igneous Complex (hereafter GIC), completely isolated from
the more common calcalkaline granites, are one of the few case of pure crustal
origin, peraluminous granite suite in Sardinia. GIC includes, in order of emplacement,
three main peraluminous units: a) (G2M) two-mica, biotite-dominant monzogranite
and granodiorite (normative corundum=2.3); b) (L2M) two-mica leucogranite
(normative corundum=2.6);
c) (LGM) muscovite-dominant leucogranite with andalusite and garnet (normative
corundum=2.9). These granitoids were emplaced into the nappe zone at the point
of maximum crustal thickening within the Sardinia segment of the Hercynian chain.
Thus, study of emplacement pressure conditions of two mica granites of the GIC
allowed an indirect evaluation of minimum crustal thickening.
The pressure range under which these rocks crystallised has been first inferred
by microthermometric data, collected from quartz-hosted primary fluid inclusions
(two main populations: 1) Th=300°C and NaCl=5 wt%; 2)
Th=330°C and NaCl=20 wt%) from each two-mica peraluminous
granites. Then, petrological constraints based on chemical-petrographic characteristics
of peraluminous GIC granites (minimum melt composition, occurrence of magmatic
muscovite and andalusite), allowes to propose a more refined determination of
emplacement pressure conditions. The whole data suggest that peraluminous GIC
granites crystallised at a pressure of 300-400 MPa under conditions that varied
from Pfluid<P (G2M) to Pfluid=P
(LGM) and aH2O=1 in the fluid.
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ABSTRACT
- Mineralogical and chemical data on eight pelites and two metapelites from
the Upper Triassic to Lower Jurassic continental sedimentary sequences of the
Peloritani Mountains, Sicily, are reported. In both sediments and metasediments
studied, the mineralogical assemblage is dominated by illite that prevails over
kaolinite and chlorite. Chemical data suggest that a significant diagenetic
control on the distribution of clay minerals can be excluded. The overall mineralogical
features suggest derivation from a source area mostly composed by low- and medium-grade
metamorphic terrains. The higher kaolinite and hematite abundance in the metasediments
with respect to pelites, suggests derivation from a more weathered source. Metapelites
underwent condition of P-T typical of low-grade metamorphism (anchizone) as
indicated by the occurrence of pyrophyllite, likely deriving from the reaction
involving kaolinite and quartz.
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ABSTRACT
- Polarised optical absorption spectra of six Cr-bearing vesuvianite single
crystals have been measured at room temperature with the aim to define the Cr
site occupancy and to quantify the molar extinction coefficients of spin-allowed
electronic d-d bands in Cr3+.
The crystals under study come from Nizjne Tagilsk, Russia. They were characterised
by means of electron microprobe techniques, nuclear reaction analyses and Mössbauer
spectroscopy. Cr-contents vary from 0.05-0.74 apfu and Fe3+-concentrations
are in the range 1.3-1.9 apfu. Boron as well as fluorine contents are low.
The optical absorption spectra display two dichroic spin allowed Cr3+-bands
at 16,500 (E>O) and 23,000 cm-1 (O>E). Observed band polarisation
and extinction coefficients in combination with previously published spectra
obtained at elevated temperatures indicate that Cr3+ enters the Y3-site
in vesuvianite. The absorption bands due to spin-allowed electron transitions
in Cr3+ show a slightly skewed shape, which is attributed to splitting
of the excited 4F-states due to the low symmetry of the ligand field
at the Y3-site. The crystal field splitting, 10Dq, and Racah B- parameters are
determined to 16,500 (200) and 667 (13) cm-1, respectively. These
values are comparable to those reported for garnet group minerals, although
the B-value is slightly higher, which may be due to the presence of OH--ligands
in addition to oxygen ligands at the Y3-site of vesuvianite.
Determined molar extinction coefficients ()
for the two spin-allowed Cr3+-bands at 16,500 and 23,000 cm-1
are 15 (EIIO), 26 (EIIE) and 24 (EIIO), 16 (EIIE) l*mol-1*cm-1,
respectively. Linear regression analyses of intensity data in combination with
detailed spectrum analysis show that a substantial overlap of narrow spin-forbidden
absorption bands due to electronic transitions in Fe3+ at the Y3-site
makes intensity data obtained from the Cr3+-band at 23,000 cm-1
less suitable for analytical purposes. However, the relatively high -
value (15 l*mol-1*cm-1) calculated for the Cr3+-band
at 16,500 cm-1 in EIIO allows detection of Cr3+ in unoriented
vesuvianite crystals in, e.g., petrographic thin sections at concentration levels
of 0.25 wt% Cr2O3 or higher. As the detection limit is
inversely proportional to the sample thickness it may be considerably improved
provided larger (>30 m)
crystals are available.
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Lazzarini L. and Cancelliere S. Characterisation of the white marble of two unpublished ancient Roman quarries on the Islands of Fourni and Skyros (Greece) 49-62
ABSTRACT
- Most outcrops of good quality crystalline marble of the Mediterranean countries
were exploited in Roman times for the production of blocks to be used for statuary
and architectural elements. Hence, a wide variety of marble was employed and
is now found in archaeological excavations. The precise determination of the
provenance of a marble object is of great archaeological importance, and is
obtained in most cases by the petrographic study of a thin section with analysis
of the C and 0 stable isotopes. Despite of the fact that it is highly developed,
the already existing petrographic and isotopic data base of the most important
marbles used in antiquity is still insufficient because of the incomplete knowledge
and study of all ancient quarries. The contribution of this research is to add
new data on two unpublished small Roman quarries discovered on the Greek islands
of Fourni and Skyros. The marbles quarried there are very pure and characterised
by a low-grade metamorphism which produced a limited recrystallization on the
marine limestone protoliths. The fabrics are quite distinctive; the marble from
Fourni shows a strong lineation and traces of metamorphic stress while that
of Skyros is characterised by a polygonal mosaic equilibrium- fabric. The isotopic
data for the Fourni marbles vary from
= 3.8 to 4.9, and
= -2.7 to -3.9. The Skyros marble vary from
= 0.6 to 1.9, and
= -4.2 to 7.1. These data have been compared with those of similar marbles used
in antiquity.
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ABSTRACT
- A detailed mineralogical and geochemical study of the shales from the "Flysch
Rosso". sediments cropping out along the front of the southern Apennines
chain was conducted. Three stratigraphic sections, ranging in age from Albian
to Oligocene, were investigated in order to derive information about the provenance
and the depositional environment of the mudrock sediments, which show a characteristic
alternation of greenish and reddish colours. Sparse rhodochrosite layers, black
shales and radiolarian cherts alternations with platform-derived calcarenites
and calcirudites are also present. The investigated sections include fossiliferous
sediments similar to those of the Bonarelli Horizon in the Central Apennines
and Southern Alps (Italy), which documents the OAE2 Cretaceous worldwide Oceanic
Anoxic Event. The mineralogy of the shales is dominated by I/S mixed layers,
kaolinite, and illite; hematite is generally absent in the green shales but
is ubiquitous in the reddish ones. The mineralogical composition, as well as
the chemical characteristics (Al/Ti, Mg/Ni, Cr/Th, La/Sc, Th/Sc ratios and REE
patterns) of the studied shales point to a Sc-poor felsic upper crust provenance.
V/Cr, Ni/Co, and U/Th ratios are indicative of overall oxic condition at the
bottom-water interface. Furthermore, the absence of calcite in the shales, their
very fine grain mize, and the occurrence of radiolarian chert all point to a
pelagic sedimentation.
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Fiore S., Piccarreta G., Tateo F. and Santaloia F. The "Flysh Rosso" shales from the southern Apennines, Italy. 2. The origin of the colour 79-87
ABSTRACT
- Mineralogical and geochemical studies were carried out on red and green shales
from the "Flysch Rosso" sediments (southern Apennines, Italy) in order
to investigate on the cause(s) of the colour variation. The two chromatic types
show compositional analogies and few differences are relevant, the main consisting
in Fe content. Although the statistical analyses evidenced a slightly significant
difference in some chemical parameters indicative of the redox conditions at
the water-sediment interface (U, Corg, FeO, V/Cr, U/Th), they, nevertheless,
still indicate oxic environments both in red and in green shales. The difference
in the organic carbon content is small and cannot be seen as the cause of the
chromatic variation. The origin of the red colour is ascribed mainly to the
presence of hematite, which is ubiquitous in the red shales but in a very little
amount in a few of the green ones. Different hematite, feldspars, and kaolinite
contents of red and green shales as well as geochemical considerations suggest
a detrital hypothesis for Fe-hydroxides and a diagenetic one for the hematite.
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