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Petrographic, Physical–Mechanical and Radiological Characterisation of the Rosa Beta Granite (Corsica-Sardinia Batholith)
No full textSardinian granites are widely used as tiles, flooring, in columns, or within other architectural elements. The Rosa Beta granite is the most commonly traded variety of these granites and is used worldwide. The aim of this paper is to provide a thorough characterisation of the mineralogy, petrology, physical-mechanical properties and radiological hazards associated with this granite. The samples analysed here were obtained from the Bassacutena area, which is the largest currently active Rosa Beta quarrying district. Although the Rosa Beta granite does not contain any evident strain fabrics, it does have a magmatic flow linear-planar anisotropy and one or more networks of micro-fractures are also present around major faults. Experiments were undertaken to examine the influence of these structures on the physical-mechanical properties of the granite, and involved positioning a fracture network either parallel or perpendicular to the direction used to obtain a directional property. The results are summarised in a high readability spreadsheet and possible uses for the Rosa Beta granite are suggested
Post-collisional late Variscan granites of southern Sardinia: evidences of contrasting suites
No full textThe Sardinia portion of the Sardinia-Corsica batholith emplaced mostly during the post collisional
evolution of the Variscan chain. The majority of plutons emplaced in about 40 Ma during two phases, the
older of which clustered at about 310 Ma, whereas the younger, clustered in the range of 290 Ma.The former is dominated by monzogranitic and granodioritic calc-alkaline plutons, while the latter mostly consists of leucogranites; minor mafic intrusions are more spread in northern Sardinia and preferentially associated to the latter phase. In southern Sardinia, in the frontal part of the orogenic wedge (nappe zone), the younger phase consists of two main rock-suites. The prevalent suite dissects porphyritic granodiorites, is dominated by syderophyllite monzogranites and leucogranites (SGS, syderophyllite granite suite) and, in turn, is crosscut with sharp vertical contacts by Fe-hastingsite granites (HGS, Fe-hastingsite granite suite). Both series are classified as ferroan, F-bearing granites, showing an alkali-calcic character; near liquidus temperatures indicate values in excess of 850°C. Siderophyllitic mica contains ilmenite inclusions and shows in the whole SGS a continuous trend in the Mg-Al plot. Conversely, in the HGS Fe-hastingsite amphibole occurs as early phase hosting magnetite and allanite inclusions.
Magnetic susceptivity data indicate that SGS shows values typical of an ilmenite series in the range 20 to
60.10-6 SI unit, whereas HGS plots on the ilmenite/magnetite series boundary with values in the range 1.7-
2.8.10-3 SI unit. Different trends displayed by variation diagrams confirm the contrasting behavior between the HGS leucogranites and the SGS monzogranites-leucogranites, the chemical variations of which can be
accounted for with crystal/liquid fractionation processes dominated by plagioclase + biotite ± minor amounts of accessory phases. Field occurrences and petrochemistry indicate a dominant crustal origin for both suites, according to εNd290 (–7.47) from Conte et al. (2015) and δ18OS.M.O.W. values (10.5±0.2 – 12.1±0.3) from Boni et al. (1992). A Proterozoic age of the involved crustal source may be inferred from the distribution of studied suites in the frontal part of the chain. Constraints on source materials are offered by chemical compositions of our rocks, which meet the liquids experimentally obtained by low degrees of partial melting of a meta-igneous source (Conrad et al., 1988). Mass balance calculations indicate for SGS a low degree of partial melting of about 25%. Conversely, the whole data set indicate a different path for the HGS, possibly due to inhomogeneities in the crustal source. The younger phase of magmatism marks the transition from late- to post-collisional Variscan events in southern Sardinia. Further questions arise from the thermal regimes, which triggered the partial melting, as far modelled by adiabatic decompression during the exhumation of the chain and shear heating (Casini et al.,
2015). Boni, M., Iannace, A., Köppel, V., Früh-Green, G., Hansmann, W. (1992): Late to post-hercynian hydrothermal activity and mineralization in south-west Sardinia (Italy). Econ. Geol., 87, 2113-2137.
Casini, L., Cuccuru, S., Puccini, A., Oggiano, G., Rossi, P. (2015): Evolution of the Corsica–Sardinia Batholith and lateorogenic shearing of the Variscides. Tectonophys., 646, 65-78.
Conrad, W.K., Nichols, I.A., Wall, V.J. (1988): Water saturated and undersaturated melting of metaluminous and peraluminous crustal compositions at 10 kbar: evidence for the origin of silicic magmas in the Taupo Volcanic Zone, New Zealand and other occurrences. J. Petrol., 29, 765-803. Conte, A.M., Cuccuru, S., Oggiano, G., Naitza, S., Secchi, F., Tecce, F. (2015): Cassiterite veins deposits related to latevariscan
ilmenite series in south western Sardinia (Italy): insights for a new tin province. Proc.13th SGA Meeting, 1,
69-72
Into the depth of the Arburèse vein system: arsenide-sulfide evolution in the Ni-Co ores
No full textThe Arburese region (SW Sardinia) is well-known for a large swarm of mineralized veins hosted in low-grade metamorphic rocks, extending for about 20 km around the Late Variscan (304±1 Ma) Arbus pluton, made up of a core of cordierite-bearing biotite leucogranites and an external shell of granodiorites with subordinate mafic rocks (Cuccuru et al., 2015). The largest veins occur in the northern and western sectors (past Montevecchio district), with a primary mineral association of Pb-Zn (Cu, Ag) sulfides in quartz and siderite. Recent works on the southern branch of the vein system (Naitza et al., 2015) evidenced a more complex metallogeny, with Ni-Co-As-Sb-Bi-Pb-Zn-Cu-Ag ores. New microscopy, SEM-EDS and EPMA data allow to highlight the depositional history of these ores, defining two stages of mineralization. In the Arsenide stage, early precipitation of quartz with abundant Ni monoarsenide (nickeline) was followed by growth of Ni antimonide (breithauptite) on nickeline, by Ni, Ni-Co, Co, Fe diarsenides, triarsenides, sulfoarsenides (rammelsbergite, skutterudite, safflorite/löllingite, cobaltite, gersdorffite/ullmannite) and Bi minerals (bismuthinite and native Bi), replacing nickeline and forming rims and inclusions on previously crystallized phases. The Sulfide stage was preceded by cataclasis of the ores; initial abundant precipitation of quartz, siderite and sphalerite, cementing the fragments of the arsenide ores, was followed by Pb-Cu-Ag sulfides (galena, chalcopyrite, tetrahedrite/freibergite, bournonite, proustite/pyrargirite, stephanite), and, at last, by ankerite, calcite and pyrite. These sequences of mineralization suggest a relationship with distinct hydrothermal fluids: 1) an earlier fluid, carrying Ni, Co, As and Sb, deposited as arsenides and sulfoarsenides into structurally-controlled traps in response to variations in physicochemical conditions; 2) a late, carbonate and Pb-Zn-Cu-Ag -rich fluid, permeating fractures re-opened after a tectonic phase that involved the previous mineralization, and originating a “Montevecchio-type” mineral association. Depositional histories similar to the Arsenide stage have been reported in other hydrothermal Ni-Co deposits worldwide, as in the Bou Azzer district, Morocco (Ahmed et al., 2009). The geochemistry of these ores requires the presence in the geological sequence of a Ni-Co-rich source (e.g. mafic rocks) from which these elements could be leached by acidic and moderately oxidizing hot fluids. Analogous conditions may have been attained in the Arbus pluton, which includes an olivine-bearing end member (Secchi et al., 1991), possible Ni-Co source for fluids of magmatic origin. During the emplacement of the igneous complex, widespread uralitization of pyroxenes in granodiorites point to high fluid/rock interactions; moreover, quartz- Fe-cordierite graphic intergrowths in leucogranites support a late dehydration of magmas.
References
Ahmed A.H., Shoji A., Ikenne M. (2009). Mineralogy and Paragenesis of the Co-Ni Arsenide Ores of Bou Azzer, Anti-Atlas, Morocco. Econ. Geol. 104, 249-266
Cuccuru S., Naitza S., Secchi F., Puccini A., Casini L., Pavanetto P., Linnemann U., Hofmann M., Oggiano G. (2015): Structural and metallogenic map of late Variscan Arbus Pluton (SW Sardinia, Italy), Journal of Maps, DOI: 10.1080/17445647.2015.1091750.
Naitza S., Cuccuru S., Oggiano G., Secchi F. (2015). New observations on the Ni-Co ores of the southern Arburese Variscan district (SW Sardinia, Italy). Geoph. Res. Abs. 17, EGU2015-12659.
Secchi F. A., Brotzu P., Callegari E. (1991). The Arburèse igneous body (SW Sardinia, Italy) - An example of dominant igneous fractionation leading to peraluminous cordierite-bearing leucogranites as residual melts. Chem. Geol., 92, 213-249
Preliminary minero-petrographical analyses of stones and mortars in a roman bridge (the “Pont’Ezzu” of Illorai-Sardinia)
No full textIl Giallo San Giacomo: un granito alterato come lapideo ornamentale
No full textThe “Giallo San Giacomo” represents a granite variety of dimension stone quarried
in northern Sardinia, which recently acquired large appreciation because of its rare yellowish
colour. Field survey and characterization data-set documented in the pluton a diffuse microcracks
network which decreases near the contacts and disappear in the host granitic rocks.
This micro-crack network favours a pervasive water infiltration and promote weathering
effect such as diffusion of iron oxi-hydroxides which, locally, can precipitate and resealed the
micro-fractures. This mechanism account for yellowish colour and the unexpectedly high
values of measured uniaxial compressive strength (UCS) in the analysed samples
RADIOLOGICAL CHARACTERIZAZION OF THE GRANITOID SARDINIAN DIMENSION STONES
No full textA large part of Sardinia, about 6,000 Km2 in total area, consists of Sardinia-Corsica Batholith
plutonic rocks. Particularly in northern Sardinia, many granitoids are exploited as dimension
stones and traded worldwide. This study focuses on the radiological characterization using in
situ gamma ray spectrometry of seven main plutonic dimension stones (Rosa Beta,
Ghiandone, Giallo San Giacomo, Rosa Cinzia, Grigio Malaga, Bianco Sardo and Grigio
Perla) in order to characterize their radiological hazard for ornamental/superficial or bulk
utilises as recommended by the EU standards
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