Pubblicazioni

Articoli su rivista

Massonne H-J, Cruciani G., Franceschelli M. (2013) Geothermobarometry on anatectic melts – a high-pressure Variscan migmatite from northeast Sardinia. International Geology Review, vol. 55, p. 1490-1505
Cruciani G., Franceschelli M., Foley S.F., Jacob D.E. (2014) Anatectic amphibole and restitic garnet in some Variscan migmatite from NE Sardinia, Italy: understanding partial melting processes through the trace and rare earth element composition of mineral phases. European Journal of Mineralogy, in press. DOI: 10.1127/0935-1221/2014/0026-2376.
Cruciani G., Fancello D., Franceschelli M., Scodina M., Spano M.E. (2014) Geothermobarometry of Al-silicate-bearing migmatites from the Variscan chain of NE Sardinia, Italy. A P-T pseudosection approach. Periodico di mineralogia, vol. 82, in press.
Cruciani G., Franceschelli M, Groppo C., Oggiano G., Spano M.E. (2014). Re-equilibration history and P-T path of eclogites from Variscan Sardinia, Italy: a case study from the medium-grade metamorphic Complex. Submitted.
Carosi R., Cruciani G., Franceschelli M., Montomoli C. (2014). 29th Himalaya, Karakoram and Tibet Workshop, Lucca, Italy. 5-8 September 2014, Field trip in Sardinia (in review)

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Massonne H-J, Cruciani G., Franceschelli M. (2013). Geothermobarometry on anatectic melts – a high-pressure Variscan migmatite from northeast Sardinia. International Geology Review, vol. 55, p. 1490-1505.

We studied a high-pressure amphibole-bearing migmatite cropping out along the northeastern coast of Sardinia, a few kilometres northeast of Olbia, in order to improve our knowledge about its evolution using pressure–temperature (P–T) pseudosections. Thermodynamic calculations with PERPLE_X were undertaken in the system Na2O–K2O–CaO–FeO–MnO–MgO–Al2O3–TiO2–SiO2–H2O using a haplogranitic melt model. Calculations were conducted for average compositions of the protolith and the tonalitic leucosome with different contents of H2O to construct various pseudosections in the P–T range 2–20 kbar and 550–900◦C. We demonstrate that the molar ratios Na/K and Si/Al of the anatectic melt are of high geothermobarometric value. Particularly the melt field in the P–T pseudosection for the protolith, contoured by Si/Al isolines, shows a significant decrease in this ratio with rising pressure. This ratio is only weakly dependent on variable H2O contents of the amphibole-bearing migmatite as demonstrated by contoured T–H2O pseudosections. The application of the Na/K and Si/Al ratios in melt yielded P–T conditions close to 13 kbar and 700◦C, assigned as the conditions of partial melting. Evaluation of contoured P–T pseudosections for the leucosome composition resulted in conditions of about 10.5 kbar and 700◦C for the crystallization of amphibole in the leucosome melt, and 9 kbar and 680◦C for complete crystallization of this melt.

Cruciani G., Franceschelli M., Foley S.F., Jacob D.E. Anatectic amphibole and restitic garnet in some Variscan migmatite from NE Sardinia, Italy: understanding partial melting processes through the trace and rare earth element composition of mineral phases. European Journal of Mineralogy, in press. DOI: 10.1127/0935-1221/2014/0026-2376.

We report results of a Laser-ICP-MS investigation of trace element contents in the main constituent minerals of an amphibole-bearing migmatite from the Variscan orogen in northeastern Sardinia. The migmatite is associated with migmatised orthogneiss and Al-silicate-bearing pelitic migmatites. The protolith of the amphibole-bearing migmatite was a mid-Ordovician igneous rock of intermediate composition characterised by a biotite + plagioclase + quartz assemblage. The migmatite consists of mesosomes and tonalitic (or, less frequently, granodioritic) leucosomes, characterised by amphibole crystals (potassian ferropargasite) up to 2cm in size. The tonalitic leucosomes are made up of quartz, plagioclase, ± K-feldspar, biotite, ± amphibole, garnet. The mesosomes are foliated rocks made up of the same minerals with different modal proportions. In leucosomes, amphibole is the most abundant mafic mineral, occurring as euhedral crystals rich in plagioclase, quartz, and small garnet inclusions. Garnet occurs as corroded and fractured grains in the matrix or within the amphibole. Zircon forms euhedral bipyramidal grains up to a few hundreds of microns in size. Some amphibole rims have higher REE, and negative Eu anomalies whereas cores exhibit lower REE and positive Eu anomalies. Garnet has strongly fractionated REE patterns with chondrite-normalised abundances up to 2000 for HREE. Plagioclase has flat REE patterns with pronounced positive Eu anomalies. Zircon displays fractionated REE patterns with HREE enrichment, LREE depletion, positive anomalies for Ce and negative ones for Eu. Monazite shows high REE abundances, LREE enrichment, HREE depletion and negative Eu anomalies. Garnet is mostly a restitic phase, as indicated by significant variation in HREE concentrations between grains in the mesosome, the absence of a noticeable Eu anomaly, and Y depletion in the leucosomes as compared to the mesosomes. In the leucosomes and mesosomes, the cores of zoned amphibole are characterised by positive Eu anomalies: these crystallised from or in the presence of melt produced by anatexis of the original Bt+Pl+Qtz protolith. Adjacent rims with negative Eu-anomalies developed in coexistence with a Eu-depleted melt that had experienced plagioclase fractionation.

Cruciani G., Fancello D., Franceschelli M., Scodina M., Spano M.E. (2014). Geothermobarometry of Al-silicate-bearing migmatites from the Variscan chain of NE Sardinia, Italy. A P-T pseudosection approach. Periodico di mineralogia, vol. 82, in press.

This paper investigates Al-silicate-bearing migmatite from NE Sardinia by using the P-Tpseudosection approach with the aim to determine the P-T conditions of partial melting and those of melt crystallization. P-T pseudosections were calculated in the NCKFMASH system within the P-T range 500-800 °C, 0.1-1.5 GPa by using the average compositions of metapelitic greywacke, average mesosome and average trondhjemitic leucosome, respectively. The P-T pseudosections calculated for the average metapelitic greywacke and for the average mesosome, contoured for melt volume %, Si/Al and Na/K molar ratios in melt point to P-T conditions ~ 700-740 °C, 1.1-1.3 GPa which are indicative of partial melting. The P-T pseudosection calculated for the average composition of trondhjemitic leucosomes, contoured for kyanite and biotite modal content and for XMg ratio in biotite indicates P-T conditions of 660-730 °C, 0.75-0.90 GPa for the crystallization of the melt. The comparison between the Na/K and Si/Al ratios in leucosomes, and the same ratios modeled for the anatectic melt by an haplogranitic melt model is thus a powerful tool for the reconstruction of P-T conditions of partial melting also in pelitic rocks, provided that leucosomes represent pure melts and are not contaminated by restitic phases or feldspar cumulates.