Abstract

Massonne H-J, Cruciani G., Franceschelli M. (2012) Anatetic evolution of an amphibole-bearing migmatite from the Variscan basement of NE Sardinia – insights from modeling with PERPLE_X. European Mineralogical Conference, Vol.1, EMC 2012-261. September 2-6 Frankfurt-Main, Germany.
Cruciani G., Franceschelli M., Groppo C., Oggiano G., Spano M.E. (2013) Re-equilibration history of retrogressed eclogites hosted in the medium-grade methamorphic Complex from north-central Sardinia, Italy. X International Eclogite Conference, Abstract Volume pag.19. September 2-10 Courmayeur, Italy.
Cruciani G., Franceschelli M., Langone A., Spano M.E. (2013) Nature and age of protolith of retrogressed eclogite from the Variscan basement of north-central Sardinia, Italy. X International Eclogite Conference, Abstract Volume pag.20. September 2-10 Courmayeur, Italy.
Cruciani G., Franceschelli M., Groppo C., Scodina M., Spano M.E. (2014) Microstructures in granulitized kyanite-eclogite from N-E sardinia, Italy: insights into the reconstructions of metamorphic evolution. Rend. Online Soc. Geol. It., Suppl. n. 1 al Vol. 31 (2014).
Columbu S., Costamagna L.G., Cruciani G., Elter F.M., Fancello D., Franceschelli M., Scodina M. (2014) Petrographic features of foliated leucocratic body in the Migmatite complex from northeastern Sardinia. Rend. Online Soc. Geol. It., Suppl. n. 1 al Vol. 31 (2014).
Costamagna L.G., Cruciani G., Elter F.M., Fancello D., Franceschelli M., Massa F., Spano M.E. (2014) The high-grade metamorphics from Pittulongu to Golfo Aranci (NE Sardinia): an attempt of lithological reconstruction. Rend. Online Soc. Geol. It., Suppl. n. 1 al Vol. 31 (2014).
Franceschelli M., Cruciani G. (2014) ⁴⁰Ar-³⁹Ar results on amphibole and biotite of an anatectic amphibole-bearing migmatite from the Variscan basement of NE Sardinia, Italy. Rend. Online Soc. Geol. It., Suppl. n. 1 al Vol. 31

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Massonne H-J, Cruciani G., Franceschelli M. (2012). Anatetic evolution of an amphibole-bearing migmatite from the Variscan basement of NE Sardinia – insights from modeling with PERPLE_X. European Mineralogical Conference, Vol.1, EMC 2012-261. September 2-6 Frankfurt-Main, Germany.

Cruciani G., Franceschelli M., Groppo C., Oggiano G., Spano M.E. (2013). Re-equilibration history of retrogressed eclogites hosted in the medium-grade methamorphic Complex from north-central Sardinia, Italy. X International Eclogite Conference, Abstract Volume pag.19. September 2-10 Courmayeur, Italy.

Metabasite with eclogite facies relics are hosted within the Low- to Medium-Grade Metamorphic Complex near Giuncana, north-central Sardinia (Carmignani et al., 2001). The retrogressed eclogite is a medium- to fine-grained rock with garnet and amphibole as the most abundant mineral phases along with clinopyroxene, plagioclase, quartz, biotite, chlorite, epidote, ilmenite, rutile and titanite. Accessory minerals are apatite and zircon. Based on microstructural relationships and mineral assemblages four stages (I, II, III, IV) of mineralogical re-equilibration have been distinguished. The stage I is characterized by the occurrence of omphacite (XNa enclosed in garnet. Garnet is slightly zoned (Alm57-61,Pyr10-16,Grs24-30,Sps1-3). Garnet porphyroblasts contain rutile, epidote, quartz, amphibole (pargasite to Mg-hornblende with XMg and plagioclase inclusions; plagioclase (bytownite/andesine) usually associated to epidote, probably represents a prograde relict. The stage II is characterized by the breakdown of omphacite into two types of symplectitic microstructures: (i) amphibole (pargasite to Mg-hornblende) + quartz symplectite, and (ii) clinopyroxene (diopside, XMg=0.6-0.8) + plagioclase (oligoclase/albite) ± amphibole (mostly Mghornblende) symplectite. The first symplectite type, always replaces omphacite included in garnet. The clinopyroxene + plagioclase ± amphibole symplectite, by far the most common, is widespread in the rock matrix. Worth of note is the occurrence of biotite (XMg=0.5-0.6) + plagioclase (oligoclase/andesine) symplectites in the matrix, likely derived from the breakdown of former phengite. The stage III is characterized by the widespread formation of amphibole. This amphibole occurs: (i) as zoned porphyroblasts in the matrix, (ii) as corona-type microstructure (Mghornblende to pargasite) replacing garnet. The thickness of the amphibole corona around garnet is from a few tens of microns up to a maximum of about 100. Subordinate plagioclase (oligoclase) is also present in the amphibole corona. The stage IV is characterized by biotite replacing garnet at its rim, actinolite growing mostly at the rim of matrix amphibole, and by the growth of chlorite, albite and titanite in the matrix. P-T pseudosections calculated with Perple_X software in the NCKFMASH+Ti system give P-T conditions > 600°C, 1.9 < P < 2.2 GPa for the stage I, according to Na content in omphacite, and XMg and XCa content in garnet. Grt-Cpx geothermometer (Ellis and Green, 1974) applied on omphacite and garnet core compositions, yields temperature of 600-650°C. After the stage I, pressure decrease and temperature increase led to P-T conditions of the amphibolite-facies stage III (T > 600- 670; P= 0.7 -1.0 GPa). P-T conditions of the latest stage IV are in the range of greenschist-facies. The inferred clockwise P-T path suggests that burial was followed by decompression with slight temperature increase and subsequent cooling-unloading process. The P-T path of the retrogressed eclogite hosted in the medium-grade micaschist and paragneiss of Giuncana recalls the P-T trajectory of retrogressed eclogite hosted in the migmatite Complex of northern Sardinia. This aspect and their geodynamic implications will be discussed in the framework of the Variscan Orogeny.

Cruciani G., Franceschelli M., Langone A., Spano M.E. (2013). Nature and age of protolith of retrogressed eclogite from the Variscan basement of north-central Sardinia, Italy. X International Eclogite Conference, Abstract Volume pag.20. September 2-10 Courmayeur, Italy.

Metabasite with eclogite facies relict are hosted in micaschist and paragneiss of the Low- to Medium-Grade Metamorphic Complex of north-central Sardinia. The metabasite body is hectometric in size, it has sub-rounded shape and crops out one km SW of the phyllonitic belt of Giagazzu-Tungoni area. The metabasite consist of two main types of rocks: retrogressed eclogites and amphibolites. The retrogressed eclogites consist mainly of garnet with omphacite inclusions, clinopyroxene + plagioclase ± amphibole symplectite, along with amphibole, biotite, ilmenite, and titanite. Some metre sized blocks are completely re-equilibrated under amphibolite P-T conditions. Amphibolites mainly consist of amphibole, plagioclase, garnet, ± clinopyroxene, ilmenite and titanite. Bulk chemistry of retrogressed eclogites and amphibolites does not show significant differences. SiO2 ranges between 48.3-50.7 wt.%, K2O 0.11-0.26 wt%, Na2O 2.3-2.6 wt.%, TiO2 1.80-2.05 wt.%, and P2O5 0.1-0.3 wt.%. In the Nb/Y vs. Zr/TiO2 classificative diagram after Winchester and Floyd (1977) the samples fall in the field of andesite/basalt. The samples show 1 to 5 MORB normalized trace element abundances with the exception of a slight enrichment for Cs, Rb, Ba. Chondrite normalized REE patterns have flat trends about 10-40 times relative to chondrite values (Sun and Mc Donough, 1989). Several discriminant diagrams (Ti/100-Y-Zr*3; Ta/Yb vs. Th/Yb among the others) for the tectonic setting suggest a tholeiitic, MORB-type signature. LA-ICP-MS zircon analyses were performed on both mineral separate and directly on thin sections. According to CL images, zircon grains are mainly homogeneous. Dark cores surrounded by lighter rims are also common. More complex internal features are rare. A total of 38 U-Pb analyses have been performed on 28 different crystals. U-Pb data range mainly from 428 to 477 Ma with three older data at 522 ± 28, 600 ± 25, 604 ± 25 Ma and two younger data at 400 and 316 Ma. Zircon cores and rims do not show systematic age differences. The major cluster shows a weighted age of 454 ± 6 Ma (MSWD=1.6) that we interpret as the age of the protolith. The older U-Pb data (> 500 Ma) have been obtained from cores showing discordant CL features with respect to the rims and are interpreted as inherited ages. The youngest value of 316 Ma may represent partial or total U-Pb resetting during the Variscan metamorphic events. Similar ages for the protolith of the retrogressed eclogite from Punta de Li Tulchi (453±14 Ma) and for the amphibolitized eclogites from Golfo Aranci (460±5 Ma) cropping out in the Migmatite Complex of northern Sardinia, have been reported by Palmeri et al. (2004) and Giacomini et al.(2005), respectively. The geological implications of these new findings will be discussed in the framework of the Variscan Orogeny in Sardinia.

Cruciani G., Franceschelli M., Groppo C., Scodina M., Spano M.E. (2014) Microstructures in granulitized kyanite-eclogite from N-E sardinia, Italy: insights into the reconstructions of metamorphic evolution. Rend. Online Soc. Geol. It., Suppl. n. 1 al Vol. 31 (2014).

The studied kyanite-eclogites are massive to poorly foliated rocks cropping out a few kilometers north of Olbia, NE Sardinia. They consist of garnet, kyanite, clino/orthopyroxene + plagioclase sympectites and amphibole. Other minerals identified are sapphirine, spinel, corundum, epidote, biotite, ilmenite, titanite and chlorite. Garnet occurs as millimetersized zoned porphyroblasts (core: Grs27-26, Pyr23; rim: Grs18, Pyr37). The garnet crystals preserve armoured relics of kyanite and omphacite as well as several inclusions of quartz, epidote, amphibole, apatite, and rutile. Kyanite is also found in rounded to elongated crystals in the matrix. The kyanite-eclogites are characterized by the occurrence of amazing microstructures rarely observed in other eclogitic samples from Variscan Sardinia (Cruciani et al., 2012). These striking microstructures include: (i) spinel, sapphirine and corundum coronitic assemblages around kyanite, and (ii) double layered-coronas of amphibole and plagioclase around garnet. In the first ones a variable degree of kyanite replacement can be observed in different microdomains. When kyanite is still preserved, the corona around kyanite consists of a thin layer of spinel+plagioclase (Ab5-10) symplectite, in turn surrounded by sapphirine + plagioclase (Ab~20) symplectite. When kyanite is no more present, in the nucleus of the corona an intergrowth of acicular corundum crystals and Ca-rich plagioclase (Ab~13) is observed. All these microstructures are surrounded by a continuous thin layer of Na-rich plagioclase (Ab~60). The double layer-coronitic microstructures around garnet consist of an inner corona of Ca-rich plagioclase (bytownite) and Al-rich amphibole (Al-pargasite, tschermakite or Mg-hornblende, XMg: 0.7-0.8) adjacent to the garnet, in turn surrounded by an outer corona made up of Ca-Na plagioclase (andesine) and amphibole of the same composition. Preliminary P-T conditions estimated by thermodynamic modelling (method of Cruciani et al., 2008) indicate that garnet core and omphacite formed at P-T conditions of 1.8-2.0 GPa and ~ 650°C. The occurrence of orthopyroxene and sapphirine indicates a temperature of 700-800°C and P < 1.5 GPa for the symplectitic assemblages. The composition of coronitic amphibole and plagioclase points to P-T conditions of 0.9-1.1 GPa and 630-730°C for the development of coronas in contact with garnet.

Columbu S., Costamagna L.G., Cruciani G., Elter F.M., Fancello D., Franceschelli M., Scodina M. (2014) Petrographic features of foliated leucocratic body in the Migmatite complex from northeastern Sardinia. Rend. Online Soc. Geol. It., Suppl. n. 1 al Vol. 31 (2014).

At Punta Bados, a few kilometers north of Olbia city, an elongated foliated leucocratic body striking N160°-170°,characterized by coarse-grained garnets up to 4-5 cm in diameter is hosted in a sequence of igneous and sedimentary derived migmatite and fibrolite-rich metapelites with minor metabasite and metaultramafics (Cruciani et al., 2002,2008). The leucocratic body shows an alternation between poorly and moderately foliated domains, as well as a great variability in garnet size and abundance. Rock grain size is strongly variable reaching up, locally, a pegmatitic texture. The contact between the leucocratic body and the metapelites is marked by a 30 cm-thick cataclastic band with S-C structures and kinematic indicators suggesting a dextral shear component. It mainly consists of quartz, plagioclase, microcline, muscovite, biotite and garnet, with zircon, apatite and tourmaline as accessory minerals. Plagioclase is mainly unzoned albite (An~5). Garnet is unzoned to poorly zoned with composition Alm75, Pyr10, Sps15, Grs<1mol.%. It is free from inclusions and/or from other microstructural relics. Muscovite occurs as centimetre-sized crystals with Si content near to 6.15 a.p.f.u. and XNa = 0.10. The whole-rock composition of a few representative samples shows following major element content ranges: SiO2: 72.8-74.7; Al2O3: 15.0- 15.4; Fe2O3tot: 0.5-0.7; MnO: 0.05-0.08; MgO: 0.1-0.7; CaO: 0.4-2.0; Na2O: 4-5; K2O: 3.2-4.8 wt.%. The rocks are peraluminous with normative composition matching those of granite/trondhjemite. Preliminary data suggest that barium ranges from 71 to 634 ppm, whereas Rb and Sr vary between 36-252 and 54-327 ppm, respectively. Rare earth element (REE) content of the leucocratic body is ~50 ppm with moderately fractionated pattern for light and medium REE, and flat for heavy REE (CeN/YbN ~1.5, LaN/SmN ~1.5, GdN/YbN ~1.0) with no Eu anomaly. Field geological data and petrographic features indicate that the leucocratic felsic body of Punta Bados resembles the foliated leucogranites described in Golfo Aranci and Arzachena areas (northern Sardinia) interpreted as anatectic bodies emplaced during the late deformative event related to the exhumation of the metamorphic basament.

Costamagna L.G., Cruciani G., Elter F.M., Fancello D., Franceschelli M., Massa F., Spano M.E. (2014) The high-grade metamorphics from Pittulongu to Golfo Aranci (NE Sardinia): an attempt of lithological reconstruction. Rend. Online Soc. Geol. It., Suppl. n. 1 al Vol. 31 (2014).

In NE Sardinia, along the coastline from Olbia to the Capo Figari southern side, crops out a high-grade metamorphic complex pertaining to the Axial Zone of the Southern Variscan chain. The main outcrops are formed by migmatitic paragneisses, orthogneisses, eclogites, and mafic to ultramafic amphibolites (Franceschelli et al., 2002, Cruciani et al., 2008). Orthogneisses dated 469 ± 3.7 Ma prevail in the northern part (Capo Figari), while migmatitic paragneisses, minor amphibolites with relic eclogite parageneses (this latter with an age of 460 ± 5 Ma) and rare lenses of orthogneisses are present in the southern part. In the paragneisses, swarms of calc-silicatic nodules probably representing former carbonate beds (?) now completely shattered are locally concentrated. By a detailed geological mapping along the coast, an attempt has been made to unravel the pre-metamorphic sequence: we tried to refer every metamorphic rock to its former protolith. The mapping evidenced the presence of alternations of metamorphic rocks passing frequently from each other through gradual or alternated boundaries: this could be related to the original boundaries of the former sedimentary or igneous protoliths, thus probably suggesting the original composition of the succession. This may be composed by acidic and basic volcanics and immature siliciclastics. The geometrical upper part of the succession may be located in the Capo Figari area (Cala Moresca), where the thick augen-orthogneiss outcrop may be related to the Middle Ordovician volcanic complex widely outcropping along all the Variscan Sardinian transect. These rocks show also the oldest radiometric age ever found in the area and are characterized by primary/interdigitated contacts with paraderivates (paragneiss with calcsilicate nodules). In the lower part of the succession, in the M. Nieddu-Pittulongu beach area, alternations of migmatitic paragneiss with calcsilicate nodules and amphibolites could be related to former immature siliciclastics intercalated by carbonate beds (?) and basic volcanics.Thus, this work could allow to relate the low- to middle-grade metamorphics of the Variscan Nappe Zone with the ones of the Variscan Axial Zone and to confirm the local persistence in the Olbia area of the same sedimentary environments existing during lower Paleozoic times in the south and central part of the island.

Franceschelli M., Cruciani G. (2014) ⁴⁰Ar-³⁹Ar results on amphibole and biotite of an anatectic amphibole-bearing migmatite from the Variscan basement of NE Sardinia, Italy. Rend. Online Soc. Geol. It., Suppl. n. 1 al Vol. 31

The amphibole-bearing migmatite forms a decametric lens-shaped body at Punta Sirenella, a few km north of Olbia in NE Sardinia. The leucosomes are coarse-grained granoblastic rocks occurring as elongated, tightly folded leucosomes, ranging in thickness from 2 to 4 cm, or as pods or patches up to 30-50 cm long. Both leucosome and mesosome mainly consist of variable proportions of quartz, plagioclase, biotite, ± amphibole, ± garnet. Accessory minerals are apatite, zircon, titanite, Fe-oxides, Fe-sulphides, and epidote. The mesosomes show an oriented fabric defined by the alignment of biotite and amphibole parallel to the D2 foliation. Amphibole abundance is strongly variable from one leucosome to the other, or even along the same leucosome. In amphibole-rich leucosomes, amphibole occurs as large, green porphyroblasts (up to 4 cm in size) with several inclusions mainly of plagioclase, quartz, garnet, and biotite (Cruciani et al., 2008). According to Massonne et al. (2013), the migmatite protolith was an intermediate igneous rock metamorphosed at high pressure. At the final prograde metamorphic stage, the migmatite was heated to reach P-T conditions of about 1.3 GPa and 700°C. Close to these P-T conditions, melt separated from the rock to form leucosomes. Subsequently, pressure release and slight cooling resulted in the crystallization of the leucosome melt to form, among other phases, relatively large amphibole crystals that were partially resorbed later. Probably, the resorption process occurred at about 0.9 GPa and 680°C, just prior to the final crystallization of the leucosome melt. This implies that melt must have resided in the rock from about 45 km to 30 km depth. Pb–Pb zircon dating yielded a mean value of 452±3 Ma and an isochron age of 461±12 Ma which is interpreted as the emplacement age of the migmatite protolith (Cruciani et al., 2008). In order to improve our knowledge on the age of the amphibole-bearing migmatite, we analysed by the 40Ar-39Ar laser step-heating method amphibole and biotite separates. Amphibole yielded a slightly discordant age profile, characterized by a concordant segment representing ~75 % of the 39ArK released, yielding an error-weighted mean age of 317.4±2.0 Ma. We interpreted the measured age of amphibole as the age at which the migmatite passed, during exhumation, through the temperature 450-520 °C. In contrast, biotite gave a hump-shaped age spectrum, most probably due to the presence of minor interlayered chlorite. The total gas age is ~283 Ma and is taken as a minimum argon age for the biotite.