Application of various geochemical proximity indicators to the tin favorability of south-Sardinian granites (original) (raw)
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Periodico di Mineralogia, 2012
A metamorphosed volcanoclastic-sedimentary succession has been discovered near the village of Lula, in NE Sardinia with the following main lithotypes from bottom to top: 1) metavolcanics; 2) yellowish metasandstones; 3) greenish to grey metapsammopelites. The metavolcanics, of dacitic/andesitic composition, and with variable modal amounts (10-40%) of albite phenocrysts, include two albitite layers. The yellowish metasandstones form sequences with basal microconglomerates, passing upwards with a wavy erosional surface to the overlying metasiltite-metapelite sequence. The greenish to grey metapsammopelites mainly consist of alternating albite-rich phyllites and dark phyllites. The Lula metavolcanics show REE patterns similar to those of the Ordovician metavolcanics from Gerrei, Sarcidano and Sarrabus. NMORB normalised trace element patterns with negative Nb and Ta anomalies demonstrate the calcalkaline affinity of the Lula metavolcanics. The albitite layers consist of up to 90-95 modal percent albite and up to 9 wt.% Na2O. Rocks with this unusual composition could have been generated only by hydrothermal fluids associated with the Ordovician volcanism or to the final cooling stages of a calcalkaline pre-Variscan intrusive body. The K2O, Na2O, SiO2 and Al2O3 contents reveal that the yellowish metasandstones plot in the fields of greywackes or pelitic greywackes, the greenish to grey metapsammopelites in the fields of pelites or pelitic greywackes. Compared to the metapsammopelites, the metasandstones reveal slightly higher SiO2, Zr and Hf contents, significant Na2O enrichment, slightly lower Al2O3, Fe2O3, Ba, Rb and Cs contents and strong K2O depletion. The greenish to grey metapsammopelites show chondrite normalised REE patterns almost identical to those of the Ordovician metavolcanics and similar to those of North American Shale Composite and Post Archaean Australian Shale. The shallow metavolcanic succession is probably made up of primary volcanic products and/or deposits of their reworked detrital materials. Thin, discontinuous dark metapelitic layers in the metavolcanic succession are actually sedimentary layers marking short quiescence periods in volcanic activity. The protoliths of the metasedimentary rocks were shallow marine sediments. The protoliths of the yellowish metasandstones may be attributed to mid- to high-energy environments while those of the metapsammopelites indicate alternating mid- to low-energy environments. In the yellowish metasandstones 45 microsequences have been identified. They show thickening-to-thinning-upwards sedimentary trends that may be interpreted as a backward to forward migration of the sequences with respect to the shoreline resulting in an increase or decrease of depositional energy, respectively. All the rocks were metamorphosed and multideformed during the Variscan orogeny. The P-T conditions of a biotite-bearing metasandstone have been estimated at T = 430-470 °C, P = 0.65-0.95 GPa by P-T pseudosection modelling.
GEOCHEMISTRY OF VOLCANIC ROCKS FROM THE MONTE ARCI (WEST SARDINIA, ITALY)
The Plio-Pleistocene volcanic complex of Monte Arci (central western Sardinia, Italy) is predominantly composed of rocks which belong to an intraplate basalt--rhyolite association, with basalts having predominantly subalkaline character and more rarely transitional and alkaline affinities. Trachytes and high silica rhyolites also occur in Monte Arci but do not show obvious genetic relations with the basalt-rhyolite suite. The basalts were probably formed by variable degrees of partial melting of upper-mantle garnet peridotite. The differentiated rocks of the suite (andesites, dacites and some rhyolites) were produced by fractional crystallization of the basalt magma. Trachytes were probably affected by volatile transfer which led to their enrichment in alkalies, Zr and Hf, whereas high-SiO2 rhyolites seem to be derived by partial melting of deep continental crust
GEOCHEMISTRY AND ORIGIN OF ANDESITIC ROCKS FROM NORTHWESTERN SARDINIA
The chemical variations in the continental-margin andesitic suite from northwestern Sardinia are compatible with a mechanism involving both fractional crystallization and contamination. Contamination is probably related to interaction with ignimbrites formed by crustal anatexis, which are spatially and temporally associated with andesitic rocks. A similar process may have also affected other calc-alkaline andesitic suites such as those of New Zealand and southern Peru where the contents of lithophile elements and the Sr isotope ratios appear to be related to the composition of associated ignimbrites.
European Journal of Mineralogy
Granulite-facies rocks, of metasedimentary and metaigneous type are exposed in the Serre (Southern Calabria). They are representative of the intermediate - lower crust remaining after the Hercynian orogeny and exhumed during the Alpine orogeny. In this paper, we deal with the composition of the lower crust and its relationship with granitoids along the Serre section. The metapelites, which make up a large part of the lower crust section, have a restite character due to the extraction both of in situ crystallized leucosomes and of some of the late-Hercynian peraluminous granites which intrude the upper crust. The dominant calc-alkaline granitoids seem to require the contribution of hydrated basic lower crust and/or mantle-derived magmas for their genesis. As a result it appears that the melting processes which affected the lower crust about 300 Ma ago produced a crustal differentiation contributing to the generation of granitoids which migrated toward high crustal levels. The occurre...
Late Cenozoic volcanic rocks on the island of Sardinia are mildly alkalinetransitional lavas, dominantly hawaiites, mugearites, and transitional basalts with minor phonolites and trachytes, which form ~80% of the entire sample population. Tholeiitic basaltic andesites form the remaining 20% of the analyzed rocks. The oldest lavas, the ca. 6.6–4.4 Ma radiogenic Pb volcanic group, are in southern Sardinia; they have geochemical characteristics very similar to most Circum-Mediterranean Anorogenic Cenozoic Igneous Province rocks. After a gap of ~0.5 m.y., volcanism occurred in central and northern Sardinia, from ca. 3.9 to ca. 0.1 Ma. These products, the unradiogenic Pb volcanic group, are geochemically very different. Their geochemical characteristics (relatively high SiO2, low CaO, and CaO/Al2O3, relatively high Ni, relatively low high fi eld strength elements, low heavy rare earth elements, high Ba/Nb and La/Nb, slightly high 87Sr/86Sr, and unradiogenic 143Nd/144Nd and 206Pb/204Pb ratios) are considered to be derived from an orthopyroxene-rich lithospheric mantle source. The origin of this enrichment in orthopyroxene is a consequence of SiO2-rich melt derived from delaminated and detached ancient lower continental crust reacting with mantle peridotite. The presence of two distinct groups of rocks (unradiogenic Pb volcanics and radiogenic Pb volcanics) in a very close geographic position is related to the existence of a lithospheric discontinuity running roughly E-W in southern Sardinia.