Post-collisional late Variscan granites of southern Sardinia: evidences of contrasting suites

The 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.