ABSTRACT LXIX December, 2000 n.3

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 32212 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 40C 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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