Pre‐Variscan metagabbro from NW Sardinia, Italy: evidence of an enriched asthenospheric mantle source for continental alkali basalts

Franceschelli, Marcello; Cruciani, Gabriele; Puxeddu, Mariano; Utzeri, Daniela

doi:10.1002/gj.937

Cited by 6 publications

(3 citation statements)

References 24 publications

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“…From Ordovician to Early Carboniferous, three main magmatic cycles took place according to Memmi et al (1983), Di and Franceschelli et al (2003). The first one consists of basaltic to rhyolitic calc-alkaline volcanic rocks emplaced during the Early to Middle Ordovician in the sequences of inner and Nappe zones.…”

Section: Geological Settingmentioning

confidence: 99%

Metabasites with eclogite facies relics from Variscides in Sardinia, Italy: a review

Franceschelli

Puxeddu

Cruciani

et al. 2006

Int J Earth Sci (Geol Rundsch)

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Metabasites with eclogite facies relics occur in northern Sardinia as massive to strongly foliated lenses or boudins embedded within low-to medium-grade rocks (Anglona) and migmatites (NE Sardinia). U-Pb zircon dating yielded 453  14 Ma, 457  2 Ma and 460 ± 5 Ma as the protolith ages; 400 ± 10 and 403 ± 4 Ma have been interpreted as the ages of the HP event and 352 ± 3 Ma and 327 ± 7 Ma as the ages of the main Variscan retrograde events.\ud A pre-eclogite stage is documented by the occurrence of tschermakite, zoisite relics within garnet porphyroblasts (Punta de li Tulchi) and an edenite-andesine inclusion within a relict kyanite porphyroblast (Golfo Aranci). Four main metamorphic stages have been distinguished in the eclogite evolution: 1) Eclogite stage, revealed by the occurrence of armoured omphacite relics within garnet porphyroblasts. The Golfo Aranci eclogites also include kyanite, Mg-rich garnet and pargasite; 2) Granulite stage, producing orthopyroxene and clinopyroxene-plagioclase symplectites replacing omphacite. At Golfo Aranci, the symplectitic rims around relict kyanite consist of sapphirine, anorthite, corundum and spinel; 3) Amphibolite stage, leading to the formation of amphibole-plagioclase kelyphites between garnet porphyroblasts and pyroxene-plagioclase symplectites and to the growth of cummingtonite on orthopyroxene. Tschermakite to Mg-hornblende, plagioclase, cummingtonite, ilmenite, titanite and biotite are coexisting phases; 4) Greenschist to sub-Greenschist stage, defined by the appearance of actinolite, chlorite, epidote ss, titanite, sericite and prehnite.\ud The following P-T ranges have been estimated for the different stages. Eclogite stage: 550°-700°C; 1.3-1.7 GPa; Granulite stage: 650°-900°C; 0.8-1.2 GPa, clustering in the range 1.0-1.2 GPa; Amphibolite stage: 550°-740°C; 0.3-0.7 GPa; Greenschist stage: 300°-400°C; 0.2-0.3 GPa. Comparable ranges characterise the other Variscan massifs in Europe: Eclogite stage: T=530°-800°C; P from 0.7-1.1 to 1.7 ± 0.3 GPa; Granulite stage : T=760°-870°C and P from 7.2-9.9 to 1.1-1.4 GPa, clustering around 1.0-1.2 GPa.\ud Whole-rock chemistry: Sardinian eclogites are N- to T-MORB; European ones N- to E-MORB or calc-alkaline.\u

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Section: Geological Settingmentioning

confidence: 99%

Metabasites with eclogite facies relics from Variscides in Sardinia, Italy: a review

Franceschelli

Puxeddu

Cruciani

et al. 2006

Int J Earth Sci (Geol Rundsch)

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“…According to Franceschelli et al (2003), the within-plate alkaline affinity of the Nurra metagabbro is demonstrated by: (1) negative K and Rb anomalies referred to appropriate chondrite-normalized diagrams, coupled with high TiO 2 and P 2 O 5 contents; (2) positive Nb, Ta, La and Ce anomalies referred to the appropriate chondrite-normalized multivariate diagram of incompatible elements; (3) strong LREE enrichment and HREE depletion; and (4) plotting in the within-plate basalt field of diverse discriminant diagrams.…”

Section: Bulk-rock Compositionsmentioning

confidence: 99%

“…Samples MA1 and SA5 are fromFranceschelli et al (2003). { AVG is the average composition calculated for samples MA5, MA10 and eight calcite-free samples fromFranceschelli et al (2003).…”

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confidence: 99%

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

Cruciani¹,

Franceschelli²,

Massonne³

2011

Mineral. mag.

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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.\ud \ud The metamorphic evolution of the Nurra metagabbro has been investigated by pseudosection modelling for a fixed bulk-rock composition in the Na(2)O-CaO-K(2)O-FeO-MgO-Al(2)O(3)-SiO(2)-H(2)O (NCKFMASH) model system with added Ti and Mn in the P-T range 1-11 kbar and 150-450 degrees C. 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 kbar and similar to 400 degrees C. The subsequent temperature peak, at similar to 440 degrees C, was accompanied by a decrease in pressure to similar to 3 kbar. The final P-T evolution of the metagabbro is characterized by near-isobaric cooling to 250-300 degrees C, with the formation of stilpnomelane. The P T path of the Nurra gabbro is typical of continental orogenic belts that have undergone crustal thickenin

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Layered amphibolite sequence in NE Sardinia, Italy: remnant of a pre-Variscan mafic silicic layered intrusion?

Franceschelli

Puxeddu

Cruciani

et al. 2005

Contrib Mineral Petrol

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A banded amphibolite sequence of alternating ultramafic, mafic (amphibolite) and silicic layers, tectonically enclosed within Variscan migmatites, outcrops at Monte Plebi (NE Sardinia) and shows similarities with leptyno-amphibolite complexes. \ud The ultramafic layers consist of amphibole (75-98%), garnet (0-20%), opaque minerals (1-5%) and biotite (0-3%). The mafic rocks are made up of amphibole (65-80%), plagioclase (15-30%), quartz (0-15), opaque minerals (2-3%) and biotite (0-2%). The silicic layers consist of plagioclase (60-75%), amphibole (15-30%) and quartz (10-15%). Alteration, metasomatic, metamorphic and hydrothermal processes did not significantly modify the original protolith chemistry, as proved by a lack of K2O-, Rb-enrichment, CaO-, MgO- depletion and by no shift in the REE patterns.\ud Field, geochemical and isotopic data suggest that ultramafic, mafic and silicic layers represent repeated sequences of cumulates, basic and acidic rocks similar to macrorhythmic units of mafic silicic layered intrusions. The ultramafic layers recall the evolved cumulates of Skaergaard and Pleasant Bay mafic silicic layered intrusions. Mafic layers resemble Thingmuli tholeiites and chilled Pleasant Bay mafic rocks. Silicic layers with Na2O: 4-6 wt%, SiO2: 67-71wt% were likely oligoclase-rich adcumulates common in many mafic silicic layered intrusions. Some amphibolite showing a strong Ti-, P- and REE- depletion are interpreted as early cumulates nearly devoid of ilmenite and phosphates.\ud All Monte Plebi rocks have extremely low Nb, Ta, Zr, Hf content and high LILE/HFSE ratios, a feature inherited from the original mantle sources. The mafic and ultramafic layers show slight and strong LREE enrichment respectively. Most mafic layer samples plot in the field of continental tholeiites in the TiO2-K2O-P2O5 diagram and are completely different from N-, E- and T-MORB as regards REE patterns and Nd, Sr isotope ratios but show analogies with Siberian, Deccan and proto-Atlantic rift tholeiites. Comparisons with Thingmuli, Skaergaard and Kiglapait rocks and with experimental data suggest that the Monte Plebi intrusion was an open-to-oxygen system with fO2  FMQ. \ud Mafic and ultramafic samples yielded Nd(460) = + 0.79/ + 3.06 and 87Sr/86Sr = 0.702934 - 0.703426, and four silicic samples Nd(460) = -0.53/-1.13; 87Sr/86Sr = 0.703239 - 0.703653. Significant differences in Nd isotope ratios between mafic and silicic rocks prove that both groups evolved separately in deeper magma chambers, from different mantle sources, with negligible interaction with crustal material, and were later repeatedly injected within a shallower magma chamber. The spectrum of Sr and Nd isotope data is consistent with a slightly-enriched mantle metasomatized during an event earlier than 460 Ma. The metasomatising component was represented by alkali-Th-rich fluids of crustal origin rather than by sedimentary materials, able to modify alkali and Sr-Nd isotope systematics.\ud Monte Plebi layered amphibolites might represent the first example of a stro...

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Pre‐Variscan metagabbro from NW Sardinia, Italy: evidence of an enriched asthenospheric mantle source for continental alkali basalts

Cited by 6 publications

References 24 publications

Metabasites with eclogite facies relics from Variscides in Sardinia, Italy: a review

Metabasites with eclogite facies relics from Variscides in Sardinia, Italy: a review

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

Layered amphibolite sequence in NE Sardinia, Italy: remnant of a pre-Variscan mafic silicic layered intrusion?

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