ABSTRACT LXIX Aprile, 2000

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.


Di Leo P., Mongelli G., Critelli S., Laviano R., Le Pera E. and Perrone V. Chemistry and mineralogy of early Mesozoic pelite layers from continental redbeds "Verrucano", Peloritani Mountains, Sicily, Italy 19-34

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.

Hålenius U. Optical absorptions spectra of Cr-bearing vesuvianite: detection and measurement of Cr3+ at the octahedral Y3-site 35-48

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.


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.

Fiore S., Piccarreta G., Santaloia F., Santarcangelo F. and Tateo F. The "Flysh Rosso" shales from the southern Apennines, Italy. 1. Mineralogy and geochemistry 63-78

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.

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.