H.M. BAIOUMY* - Carbon and oxygen isotopic composition of lattice-bound carbonate of phosphorites ore in Egypt

ABSTRACT - Phosphorite deposits in Egypt, known as the Duwi Formation, are a part of the Middle East to North Africa phosphogenic province of late Cretaceous to Paleogene age. Based on the petrographical observation, the phosphatic grains in the Phosphorites are classified into phosphatic mudclast and phosphatic bioclast. Both of them are composed of francolite. The structural CO₂ contents in the francolite range from 3.32 to 7.21 % with an average of 5.3 %. The d¹³C_PDB and d¹⁸O_PDB values of the studied samples are highly variable but are, nevertheless, all large negative numbers. The d¹³C_PDB values range from -4.04 to -8.7 ‰ while the d18OPDB values range from -4.3 to -10.3 ‰.
Negative d¹³C values and low d¹⁸O values of structural CO₂ suggest that the CO₂ was derived from degradation of organic matter. The linear relationship between d¹³C and CO₂ % indicates that as the medium became more negative with shallow burial there was a progressive increase in the substitution of CO₃ for PO₄ in the carbonate fluorapatite. These data along with the higher CO₂ contents in the apatite from Duwi Formation in Egypt compared to the recent authigenic phosphorites indicate that the chemical composition of apatite has been changed after the formation and during diagenesis by taking CO₂ into the structure from the surrounding medium and the studied apatite was francolitized during diagenesis.

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KONSTANTIN HATZIPANAGIOTOU and BASILIOS TSIKOURAS1* - Origin and evolution of sub-ophiolitic amphibolites from Angelona (SE Peloponneus, Greece), and their significance within the Pindos Ocean

ABSTRACT - Two geochemically distinct groups of amphibolites have been found both as fragments in an ophiolite mélange and as a remnant subophiolitic metamorphic sole at the base of the Angelona ophiolite sequence. Group-1 shows T-MORB and group-2 WPB geochemical signatures. Basaltic fragments, considered to represent the amphibolitic protoliths in the ophiolite mélange, also show twofold chemistry and are divided in two groups with affinities similar to the amphibolites. The WPB rocks are similar to rift-related Triassic lavas from Pindos, Koziakas and Argolis, thus suggesting the operation of Triassic rifting in Angelona. The T-MORB rocks suggest that the ophiolite was generated in a typical oceanic environment, which was slightly influenced by the contribution of a mantle plume component. Geochemical and age constraints point to the hypothesis that the Angelona ophiolite is the link between the continental Hellenides to the North, and the Crete ophiolite, to the South within the Pindos ocean.
The metamorphic evolution of the two amphibolite groups occurred at around 450-500°C and 500-550°C, respectively, at estimated pressures at approximately 6-8 MPa. A reaction series, which is possibly related to the amphibolite formation, suggests that metamorphism occurred during pressure increase and cooling, conditions analogous to other amphibolite subophiolitic metamorphic soles.

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GIANNI CORTECCI*, ENRICO DINELLI, GIANCARLO MOLLI and GIUSEPPE OTTRIA - Geochemical evidence for fluid-rock interaction along high angle faults in the Alpi Apuane, NW Tuscany, Italy

ABSTRACT - Trace element (Sr, Mn, Rb, Cu, Zn, and Pb) and stable isotope (¹³C/¹²C, ¹⁸O/¹⁶O and ⁸⁷Sr/⁸⁶Sr) analyses were performed on calcite from profiles across high angle brittle faults developed in the Carrara marble of Alpi Apuane, in order to investigate the role and origin of fluids possibly involved during deformation. The tectonic structures studied are characterized by an inner cataclasite core grading outwards through cataclastic marble to fractured or undeformed host marble.
The d¹⁸O values indicate that large amounts of water were involved during formation of the cataclasite calcite at the inner core of the structure. With respect to the host marble, this calcite is depleted in both ¹⁸O and ¹³C, is enriched in base metals and in radiogenic ⁸⁷Sr. These features concur to suggest that the Palaeozoic siliciclastic basement of Alpi Apuane may have been the source of metals, ¹³C-depleted carbon and radiogenic Sr.

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D. PERUGINI - The Ordovician gneisses of north-eastern Sardinia (Italy): hypotheses for the petrological evolution of their protoliths

ABSTRACT - In this paper new whole rock data on Ordovician ortho- and augen-gneisses cropping out in North-Eastern Sardinia are presented. It is evidenced that the core of the outcrop (ortho-gneisses) has a less evolved geochemical composition than the peripheral portions (augen-gneisses). Inter-digitate contacts between the different portions are observed. Within ortho-gneisses are recognised Mafic Microgranular Enclaves, indicating the possible role played by magmatic interaction processes in their genesis, whereas magmatic enclaves are lacking in augen-gneisses.
Discriminant diagrams are used in order to understand the type and extent of metasomatic/ alteration processes suffered by these rocks. Analysis reveals that the effect of these processes can be considered negligible and that the geochemical characteristics of studied samples are the result of magmatic evolution.
On the basis of field and geochemical data, a two-stage petrogenetic evolution is suggested to explain the compositional spectrum of studied rocks. Ortho-gneisses are considered as produced by Mixing plus Fractional Crystallization processes whose evolution in time produced, in the core region of the magma chamber, magma batches with different degrees of hybridisation bearing Mafic Microgranular Enclaves. Augen-gneisses are interpreted as the result of Fractional Crystallization processes acting in the peripheral portions of the magma chamber.

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DAVIDE LENAZ*, HENRIK SKOGBY - Flux growth of synthetic single crystal spinels in the (Mg, Fe²⁺)(Cr, Fe³⁺)₂O₄ system

ABSTRACT - Single crystals of spinel with compositions corresponding to (Mg, Fe²⁺)Cr₂O₄ and Mg(Cr, Fe³⁺)₂O₄ end members and their solid solutions were synthesised by flux-growth method. Low cooling rates (4°C h^-1) and linear temperatures profiles were applied in the thermal interval 1200-900°C. Thermal runs were performed in air and in controlled atmosphere. The resulting crystals underwent preliminary structural and spectroscopic characterisation for future thermodynamic and kinetics studies of spinel order-disorder with temperatures.

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GIOVANNI NAPPI and MICHELE MATTIOLI - Evolution of the Sabatinian Volcanic District (central Italy) as inferred by stratigraphic successions of its northern sector and geochronological data

ABSTRACT - The Sabatinian Volcanic District (central Italy) consists of a system of several vents clustered around the lake-filled Bracciano volcano-tectonic depression. Its activity spans between 0.8 My and 0.04 My and overlaps that of the neighbouring Vico and Alban Hills Volcanoes in time and space. A field survey of the northern sector together with new geochronological and geochemical data allows us to reconstruct the stratigraphy and volcanic evolution of the Sabatinian Volcanic District. Six main phases of activity have been recognised on the basis of geological data, following the UBSU criteria. The presence of three unconformities of regional significance also enables us to distinguish three different synthems, representing the main stages in the volcanic evolution of the Sabatinian District. The initial phases of activity (from I to IV) are within the Barca di Parma Synthem (800-200 ky) and are dominated by pyroclastic fall and flow deposits and subordinate phonolitic tephritic lava flows. The phase V is related to the Archi di Pontecchio Synthem (200-155 ky) and characterised by prevailing strombolian deposits with a composition ranging from phonolitic tephrite to tephrite. During the phase VI (Casalone Synthem) both magmatic and phreatomagmatic eruptive cycles occurred in the northern sector. The magmatic cycle (154-134 ky) is dominated by pyroclastic fall and lava flows with a variable composition from phonolitic tephrite to potassic trachibasalt. The phreatomagmatic cycle marked the end of the volcanic activity (<134 ky) in the northern sector of the Sabatinian District and produced small, single eruptive centres (maars) or coalescent craters which are aligned in a NW-SE regional fracture system at a right angle to the tectonic orientation of the initial Sabatinian Volcanic activity. This fracture system could be connected to the main regional tectonic line that crosses the Vico Volcano and the eastern Sabatinian District.

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ROSALDA PUNTURO*, PAOLO CENSI and ANTONINO LO GIUDICE - Trace element measurements on selected reference materials using HR-ICP-MS after microwave digestion: an application to archaeometric studies

ABSTRACT - In order to enhance a fast and reliable routine method of analysis which could be useful in Earth Sciences and Archaeometry purposes, a number of certified international geo-standards available for laboratory measurements of trace element concentrations were investigated by HR-ICP-MS.
In particular, the study has been focused on Rare Earth Elements and other trace elements as Sc, V, Cr, Co, Cu, Th, U, Rb, Sr, Y, Zr, Nb, Ba, Hf, Ta, Pb. The reference materials used on the basis of different rock matrices was andesite (AGV-1), basalt (BHVO-1), basalt (BCR-1), marine mud (MAG-1) and diorite gneiss (SY-4). Elements have been determined at ng*g-1 levels by High Resolution Inductively-Coupled Plasma Mass Spectrometry (HR-ICP-MS) after acid decomposition at controlled pressures and temperatures with a microwave device (Pmax=400psi; Tmax=230°C). The proposal method of sample preparation and multi-elemental measurements revealed its usefulness because: i) it is applicable to different geological matrices; ii) it requires small quantities of powder and iii) provides the reproducibility of measurements which are fast and precise in a wide range of elements by combining a single internal standard (e.g. rhodium) and an external calibration.
Moreover, the high dilution factors applied on the samples (= 20000) and an accurate instrumental optimization permitted to overcome several problems due to the occurrence of both spectroscopic and not-spectroscopic interferences which negatively affect the quality of measurements.
The proposed methology finds noteworthy applications when the samples to be analysed are archaeological artefacts. For this reason, we tested its applicability in the geochemical characterization and determination of provenance of some granitoid rocks from the Roman Theatre in Catania (Italy).

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