Gabriele Cruciani

Transformation of cumulate mafic rocks to granulite and re-equilibration in amphibolite and greenschist facies in NE Sardinia, Italy

Abstract The ultramafic amphibolites hosted in the Hercynian migmatite of NE Sardinia consist of three main compositional layers (A, B, C) from a few metres to a few decametres thick. Layer A is made up of coarse-grained olivine, chlorite, amphibole, spinel, minor pyroxene, garnet and, rarely, plagioclase. Layer B is made up of coarse-grained plagioclase, olivine, pyroxene, spinel, garnet and amphibole. Layer C consists mainly of porphyroblastic garnet, pyroxene, large amphibole grains (up to 5 cm) and minor plagioclase. On the basis of mineral assemblages and microstructures, three stages of mineralogical re-equilibration can be recognised: granulite, amphibolite and greenschist. Primary igneous olivine and anorthite reacted under granulite conditions to produce coronas consisting of orthopyroxene, clinopyroxene, green spinel and garnet. The amphibolite stage is characterised by the formation of brown and green clinoamphiboles (between pyroxene and corona garnet), anthophyllite, talc, Mg-rich chlorite, plagioclase and Cr-bearing spinel. Greenschist stage minerals, mostly replacing the mafic minerals, consist of tremolite, fayalite, epidote, albite, calcite, dolomite and serpentine. The history of the ultramafic amphibolites started with igneous crystallisation and continued through granulite ( T =700–740 °C, P =∼8–10 kbar), amphibolite ( T =580–640 °C, P =4–6 kbar) and greenschist facies ( T =∼330–400 °C, P Evaluation of P – T conditions indicated a P – T path from the protolith to granulite stage, characterised by an increase in pressure and temperature, and then from the granulite facies through amphibolite to the greenschist stage, characterised by a decrease in pressure and temperature. The petrological evolution of the ultramafic amphibolites and the P – T time path is discussed in the context of the Hercynian orogeny in Sardinia.

U–Pb zircon dating and nature of metavolcanics and metarkoses from the Monte Grighini Unit: new insights on Late Ordovician magmatism in the Variscan belt in Sardinia, Italy

In the external units of the Sardinian Variscides Nappe Zone, volcanic and volcanoclastic successions of Middle Ordovician age follow Lower Paleozoic calc-alkaline magmatism developed at the northern Gondwana margin. We present geochemical and zircon U–Pb isotopic data for the Truzzulla Formation, a low-to-medium-grade metamorphic volcanic–volcanoclastic succession belonging to the Monte Grighini Unit, the deepest unit in the Nappe Zone. Geochemical and radiometric data allow us to define a Late Ordovician (Katian) magmatic (volcanic) event of calc-alkaline affinity. These new data, in conjunction with previously published data, indicate that in the Sardinian Variscides, the age of Lower Paleozoic Andean-type calc-alkaline magmatism spans from Middle to Late Ordovician. Moreover, the age distribution of calc-alkaline volcanics and volcanoclastic rocks in the Nappe Zone is consistent with a diachronous development of Middle–Late Ordovician Andean-type magmatic arc through the portion of the northern Gondwanian margin now represented by the Sardinian Variscides. This reconstruction of the Sardinian Variscides reflects the complex magmatic and tectonic evolution of the northern margin of Gondwana in the Lower Paleozoic.

Geothermobarometry on anatectic melts – a high-pressure Variscan migmatite from northeast Sardinia

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.

Metabasite from the Variscan belt in NE Sardinia, Italy: within-plate OIB-like melts with very high Sr and low Nd isotope ratios

A metabasite lens with eclogite-facies relics discovered within high-grade gneisses, at Punta Orvili (Posada) in NE Sardinia, has proven to be the first example of within-plate basalt among the eclogite-derived, tholeiitic metabasites from the Variscan belt in Sardinia. The composition of the Punta Orvili metabasite is unusual: SiO 2 2 O 3 2 O 3tot >13 wt%, Cr > 550 ppm, Ni > 270 ppm, Mg # 68–70. Detailed investigation of the major and trace element content rules out any significant geochemical modification of the protolith by alteration and/or metamorphic processes. Normative contents of mafic minerals up to 55–79 wt%, very small amounts of Al 2 O 3 and Na 2 O and very high concentrations of MgO, Cr and Ni suggest a protolith of picritic nature. The steeply sloping REE patterns, with La N /Yb N = 13.0–17.3, suggest low-degree partial melting and the presence of garnet, as retention phase of HREE in the mantle source. Enrichment factors are 60–180 times higher for LREE, 30–60 times for MREE, 10–20 times for HREE. The primitive manntle-normalized trace element abundance patterns of the Punta Orvili metabasite show enrichment in Ba, Th, Nb, La, Ce, compared to eclogite-derived metabasites. Values of La/Nb, La/Ta, Ce/Nb, Th/Nb, Sm/Yb, Ce/Sm, Th/Y, Nb/Y, Th/Yb and Ta/Yb differ significantly from those of average crust, upper, lower and mid-crust, indicating an OIB-like source. Two values of 87 Sr/ 86 Sr (i) were obtained around 0.709 with e Sri = +69–71. The corresponding values of the 143 Nd/ 144 Nd (i) are about 0.51197 with e Ndi ~ −1.50 assuming an age of 460 Ma, i.e . the mean age of Sardinian eclogite. Isotope and geochemical data suggest that the Punta Orvili protolith was generated by interaction of OIB-like melts with an enriched sub-continental lithospheric mantle, similar in composition to Group II kimberlites and/or to their garnet peridotite xenoliths.

Continental collision from two perspectives: a review of Variscan metamorphism and deformation in northern Sardinia

Structural, petrological and geochronological data from the Variscan chain of northern Sardinia are discussed, with the aim to improve our knowledge on the tectono-metamorphic evolution of the northern Sardinian basement and its role in the tectonic puzzle of the southern Variscan massifs. In northern Sardinia, which corresponds to the Inner zone of the chain, the metamorphic grade rapidly increases towards the NE from biotite to sillimanite+K-feldspar zone. The highest grade rocks cropping out in the Migmatite zone reached amphibolite facies conditions in the sedimentary and igneous-derived migmatite and eclogite/granulite facies in the enclosed metabasite lenses. Evidence of HP metamorphism has also recently been found more to the south in micaschist from the garnet zone. The D 1 deformation can be observed in the southern part of the Inner zone, whereas, moving progressively northwards, the D 2 deformation progressively obliterates the previous foliation. The other, less pervasive, deformation phases (D 3 , D 4 , D 5 ) are sometimes recognizable in the outcrop localities, but they are not as ubiquitous as the previous D 1 and D 2 . HP metamorphism in metabasites and schists is related to the collision and/or northwards-directed subduction of the continental crust, which was part of the northern Gondwana or peri-Gondwanian terranes in Early Palaeozoic times. The HP metamorphism, which is pre- to syn-D 1 , was followed by Barrovian metamorphism. The stacking of metamorphic sequences with the building up of the nappe pile was a consequence of the High Grade Metamorphic Complex exhumation. This was activated by NW-SE striking and top-to-the S and SW shear zones and faults with a major dip-slip component of movement, coeval with the late phase of the D 1 deformation. Transpressional deformation then developed until the High-Grade Metamorphic Complex continued to be exhumed and the Low- to Medium-Grade Metamorphic Complex was underthrust to the north. Finally, the extensional tectonic regime was accompanied by local occurrences of HT-LP metamorphism. Deformation and metamorphism in the Sardinian Variscides were accompanied by the intrusion of igneous suites of the Sardinia-Corsica batholith, which was emplaced during the Middle Carboniferous/Permian post-collisional phase.

Anatectic amphibole and restitic garnet in Variscan migmatite from NE Sardinia, Italy: insights into partial melting from mineral trace elements

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 2 cm 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 micrometres 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.

Geological Map of the Monte Grighini Variscan Complex (Sardinia, Italy)

The study area, belongs to the Nappe zone of the Sardinian Variscan chain at the NW termination of the Flumendosa antiform. The Monte Grighini Complex is a NW-SE trending metamorphic complex made up of three tectonic units and synkinematic magmatic intrusions that show a section of the deepest portion of the Nappe zone. The tectonic units stacked and folded under lower greenschist and upper amphibolite facies conditions, were finally juxtaposed by a kilometer-wide NW-SE trending dextral transtensive shear zone during the Late Variscan tectonics. The 1:25,000 scale geological map, cross sections and shear zone deformation map illustrate the tectonic and metamorphic setting of the area, resulting from the polyphasic Variscan collisional evolution including early nappe stacking and later strike slip and extensional tectonics coeval with a late Carboniferous magmatism. Deformation structures and metamorphic assemblages recorded by the tectonic units as well as synkinematic intrusions, provide constraints of fundamental importance for the reconstruction of southern Variscan chain tectono-metamorphic history.

Mineral re-equilibration and P-T path of metagabbros, Sierra de San Luis, Argentina: insights into the exhumation of a mafic-ultramafic belt

At La Melada, Sierra de San Luis, Argentina, two lenses of mafic-ultramafic rocks, mainly consisting of metagabbros, are made up of igneous and metamorphic minerals in varying combinations. These rocks are part of a narrow NNE-SSW trending belt along the eastern flank of the Sierra and were affected by Famatinian metamorphism and deformation followed by shear deformation. La Melada metagabbros show a wide variety of microstructures including actinolite + chlorite + quartz pseudomorphs, epidote coronas and epidote + quartz symplectites, actinolite + quartz intergrowths and calcite + chlorite aggregates. These microstructural characteristics witness a complex metamorphic history consisting of a progressive cooling and decompression associated with regional and shear deformation from M0 down to the M1 to M3 greenschist/sub-greenschist stage. Cooling from the M0 stage ( T ≈780°C and 6.5 P T = 650–670 °C, P T P ≈ 3.8 kbar). The third re-equilibration stage led to the formation of epidote coronas and epidote + quartz symplectite around igneous plagioclase (M3 stage, 420 T P P-T path previously reported in the literature, La Melada rocks indicate that mineral re-equilibration reached P-T conditions as low as ~420–470 °C, with epidote development under (sub-)greenschist-facies P-T conditions. The resulting P-T path is in agreement with the P-T path reconstructed for the country rocks.

Nature and age of pre-Variscan eclogite protoliths from the Low- to Medium-Grade Metamorphic Complex of north–central Sardinia (Italy) and comparisons with coeval Sardinian eclogites in the northern Gondwana context

U–Pb zircon data on retrogressed eclogites sampled in the Giuncana locality from the Sardinian Medium-Grade Metamorphic Complex yielded a weighted age of 454 ± 6 Ma. This is in agreement with U–Pb zircon ages of 453–460 Ma obtained from eclogites from the High-Grade Metamorphic Complex. The Giuncana eclogites are very similar to the other well-known Sardinian eclogites. All of the Sardinian eclogites show positive K, Rb, Ba, U and Pb anomalies and negative Nb, La, Ce and Sr anomalies. Th is depleted in the Giuncana eclogites and enriched in those from Punta de Li Tulchi and Punta Tittinosu. All these data reveal clear crustal contamination of the Sardinian Ordovician mantle. REE patterns typical of normal mid-ocean ridge basalt (N-MORB) characterize all of the Sardinian eclogites. The supply of crustal and calc-alkaline materials to the Sardinian mantle during the Ordovician is further confirmed by the fact that most Sardinian eclogites plot on the left side and well above the mantle array in a Th/Yb v. Nb/Yb diagram. In the general Variscan framework of northern Gondwana, the Sardinian eclogites are witness to the most recent back-arc basins generated by the northward opening of the Rheic Ocean.

Low-temperature metamorphic evolution of a pre-Variscan gabbro: a case study from the Palaeozoic basement of northwest Sardinia, Italy

Abstract A lenticular metagabbro crops out in an early Palaeozoic metasedimentary sequence at Nurra, northwest Sardinia. The metagabbro consists of variable proportions of early-formed coarse-grained albite, chlorite, epidote and apatite, later biotite and amphibole, and late stilpnomelane. Clinopyroxene and ilmenite are rare relict igneous minerals; albite has completely replaced primary plagioclase. The metamorphic evolution of the Nurra metagabbro has been investigated by pseudosection modelling for a fixed bulk-rock composition in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2 -H2O (NCKFMASH) model system with added Ti and Mn in the P-T range 1-11 kbar and150-450ºC. The P-T path of the metagabbro is a loop with a prograde segment overprinted by later metamorphic re-equilibration. The pressure peak was at ≤7 kbar and ~400ºC. The subsequent temperature peak, at ~440ºC, was accompaniedby a decrease in pressure to ~3 kbar. The final P- T evolution of the metagabbro is characterizedby near-isobaric cooling to 250-300ºC, with the formation of stilpnomelane. The P-T path of the Nurra gabbro is typical of continental orogenic belts that have undergone crustal thickening.