Petrology and geochronology of metabasites with eclogite facies relics from NE Sardinia: constraints for the Palaeozoic evolution of Southern Europe

Lithos 82 (2005) 221 – 248 www.elsevier.com/locate/lithos

Petrology and geochronology of metabasites with eclogite facies relics from NE Sardinia: constraints for the Palaeozoic evolution of Southern Europe Folco Giacomini*, Rosa Maria Bomparola, Claudio Ghezzo Dipartimento di Scienze della Terra, Universita` di Siena, Siena, Italy Received 30 September 2003; accepted 25 October 2004 Available online 10 February 2005

Abstract In the Golfo Aranci high-grade metamorphic basement of NE-Sardinia (Italy), a fragment of the southern part of the Variscan chain, several metabasic rocks with relic eclogitic parageneses are interlayered with a dominant sequence of migmatitic paragneisses and felsic orthogneisses. Petrologic, geochemical and geochronological data on selected samples permitted the reconstruction of a long history starting from pre-Variscan magmatic activity and progressing with a polyphase metamorphism during the Variscan orogenic event. U–Pb LAM-ICPMS dating of zircon domains from two eclogites allowed us to constrain the emplacement of the mafic protolith in Ordovician times (460F5 Ma). The basic rocks were buried in a subduction-related environment with formation of Ky-bearing eclogite parageneses (undated event, ~650 8C and ~1.9 GPa). Subsequently, the eclogites underwent a strong reequilibration first under the granulite facies (sapphirine-bearing parageneses: 700–800 8C, ~1.0 GPa) and then under the amphibolite facies with pervasive growth of brown amphibole and resetting of the U–Pb system in zircon (352F3 Ma). This high-pressure metamorphic basement and its strong analogies with the basement slices of Corsica, Provence, of the Ligurian Alps and the intra-Alpine massifs indicate the presence of a HP migmatite–eclogite belt extending for several hundreds of kilometers at the southern margin of the Variscan realm. D 2005 Elsevier B.V. All rights reserved. Keywords: Variscan; Eclogite; Geochronology; Zircon; Sapphirine

1. Introduction The Sardinia–Corsica basement, little affected by Alpine tectonics and metamorphism, is one of the * Corresponding author. Tel.: +39 577233831; fax: +39 577233938. E-mail address: [email protected] (F. Giacomini). 0024-4937/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2004.12.013

best preserved Variscan remnants in the Mediterranean region and its geology is nowadays well constrained (see Carmignani, 2001 for a comprehensive summary, geological maps and references). Nevertheless, due to the scarceness of combined petrologic and high-precision geochronological studies, the protoliths, emplacement ages and P–T–t history of the metamorphic rocks from the axial

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zone of the chain are still poorly known. This hampers an accurate reconstruction of the palaeogeography in pre-Variscan and Variscan times and the geodynamic evolution of the collisional belt. The purpose of this study is to present and discuss new petrologic and geochronological data on the highgrade polymetamorphic basement of Golfo Aranci (north-eastern Sardinia) and to improve the knowledge on the geodynamic scenario of northern Sardinia in the framework of the southern branch of the Variscan belt. To decipher the complex geological history of the basement, from the protolith formation to the post-collisional exhumation, a

collection of field, petrologic and geochronological data was implemented. Sardinia and Corsica islands (Fig. 1) represent a section of the southern realm of the Variscan chain (Menot and Orsini, 1990), with a continuous polarity from the almost unmetamorphosed foreland basins in southern Sardinia to the high-grade metamorphic core of northern Sardinia and Corsica (Carmignani et al., 1992; Ricci, 1992). Sardinia is commonly divided into three main tectonometamorphic complexes: (1) the south-western part and external zone of the chain, mainly constituted by folded and thrust anchimetamorphic sedimentary sequences of Cambro-Carboniferous

Corsica Sardinia

Asinara Island

Study Area 0

50km

Post-Variscan cover Alpine nappes Variscan Batholith External Zone (anchimet.) Internal Nappe Zone (low grade) Internal Nappe Zone (medium grade) High grade Metamorphic Complex

Main thrusts Regional faults Posada-Asinara Line Fig. 1. Simplified geological map of Sardinia and Corsica islands (modified after Carosi and Palmeri, 2002) and their actual position (upper right) with respect to the European Variscides (dotted, modified after Stampfli et al. 2002).

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age; (2) the central part constituted by a nappe pile of low- to medium-grade metamorphosed sedimentary (Palaeozoic) and volcano-sedimentary (early Ordovician to Caradocian) sequences; (3) the northern axial zone, known as bHigh-Grade Metamorphic ComplexQ (HGMC), mainly constituted by upper amphibolite to granulite facies metamorphic rocks (Ferrara et al., 1978; Carmignani et al., 1992; Ricci, 1992). The central and axial zones are separated by the Posada– Asinara Line (PAL): this is a several-kilometer-wide mylonitic belt, made up of metasedimentary rocks and minor metabasites lenses, which is interpreted by several authors as a Hercynian suture zone, separating the pre-Cambrian Armorican continental margin from the Gondwana (Cappelli et al., 1992; Carmignani et al.,

223

1992). In late Carboniferous and early Permian times, shallow level, mainly felsic plutons intruded the whole chain from the southwestern foreland to the High-grade Metamorphic Complex in N Sardinia and Corsica. After the Variscan orogenic event, the CorsoSardinian microplate was separated from the European margin as a result of the opening of the LiguroProvencal basin in late Oligocene–early Miocene times (Arthaud and Matte, 1977; Speranza et al., 2002).

2. Geological setting The Golfo Aranci metamorphic basement (Fig. 2) is a part of the HGMC and is made up mainly by

Fig. 2. Geological map of the study area.

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morphism (Carmignani et al., 1992; Ricci, 1992; Franceschelli et al., 1998). At the outcrop scale, the migmatitic paragneisses are usually coarse-grained metapsammites–pelites with stromatic fabrics and well-developed foliation defined by alternating lensoidal leucosomes and mesosomes. Nebulitic–agmatitic textures are less abundant. Sparse small calc-silicate nodules are in places observed within the migmatites. Felsic orthogneiss bodies (mainly two micas, K-feldspar gneisses with augen texture) occur within the paragneisses. Foliation strikes commonly N180–1408 and is often subvertical or steeply E dipping. Banded amphibolites with minor ultramafic layers and amphibolitised eclogites crop out in the region as large boudins (up to 2 km long) within the migmatites and their setting is concordant with the main regional foliation. Amphibolites are foliated and layered rocks: the layers are defined by varying relative modal amounts of amphibole (Fgarnet) and plagioclase. The amphibolitised eclogites (hereafter called also overprinted eclogites or simply eclogites) are Cpx–Grt–Amp rocks (mineral abbreviations according to Kretz, 1983) and are easily distinguishable from the amphibolites by the large modal amount of garnet porphyroblasts. Also, the eclogites often display a compositional layering outlined by garnet-rich and garnetpoor levels.

Grt rich layer Grt poor layer

a)

Prg

Ap p Om

Grt

0.6mm

b)

Fig. 3. Sample H343 from the Iles outcrop (a) well-developed compositional banding consisting of garnet-rich and garnet-poor layers; (b) thin-section photomicrograph of a garnet-rich microdomain with relic eclogite paragenesis.

migmatitic paragneisses, felsic orthogneisses (Gneiss Complex) and minor metabasite lenses with relic eclogitic parageneses. The presence of mafic rocks equilibrated at medium to high pressure has been attested south of the study area in several papers, but there is no agreement either on the emplacement age of the magmatic protolith (960 Ma: Cappelli et al., 1992; 453 Ma: Palmeri et al., 2004), or on the peak PT metamorphic conditions and the origin of HP meta-

a)

b)

Ky Ab61

Cpx-Opx-Pl sympl.

Amp (Ed)

Ky

20µm Grt

c)

An92

An35

Spr Crn

Spl+An sympl.

Spl Ky Spr

40µm

Spr+Spl+ +Crn+An sympl.

An92 An35

150µm

Fig. 4. Back-scattered electron images of a relic Ky grain in sample H341; (a) possible pre-eclogitic inclusion within Ky; (b and c) composite symplectitic coronas around Ky relics.

F. Giacomini et al. / Lithos 82 (2005) 221–248

225

3. Petrography

a)

3.1. Amphibolitised eclogites Grt

The best preserved eclogitic relics are found in the Iles area (samples H342, H343 and H213). The eclogitic parageneses are mostly preserved within the garnet-rich layers (Fig. 3a) and consist of centimeter-scale relics within strongly reequilibrated domains. Under the microscope, the garnet-rich layers also contain omphacite, zoisite, rutileFkyanite, amphibole and apatite (Fig. 3b). Garnets (up to 2 mm large) are subhedral and contain small inclusions of Rt, Ap and Zrn at the core. Omphacite is usually consumed at the rim by Cpx–Pl symplectites; towards garnet grains, it may develop a bluish-green amphibole rim. Bluish-green amphibole is also present as small groundmass crystals. Zoisite relics are hosted within polycrystalline anorthite aggregates spotted by opaque mineral inclusions. Kyanite is commonly devoid of inclusions, but in one kyanite crystal from sample H341 we found a well-preserved plagioclase+amphibole inclusion with negative crystal shape (Fig. 4a and b). Kyanite shows composite coronitic textures consisting of an outer coarse-grained Pl– Cpx–Opx symplectite, an intermediate polycrystalline plagioclase domain and an inner symplectitic rim. SEM-EDS microanalyses demonstrate that the inner symplectite is made up of a sapphirine–anorthite (FcorundumFspinel) changing to spinel–anorthite (Fcorundum) in the innermost part in contact with relic kyanite (Fig. 4b and c). Within the garnet-poor layers (Fig. 3a), the eclogitic relics are rarely preserved and the texture is dominated by Cpx–Pl symplectites, quartz and sparse garnet porphyroblasts (Fig. 5a). The quartz grains are rimmed by granoblastic clinopyroxene aggregates. Garnet porphyroblasts (up to 4 mm large) display often a pale rose core, rich in small inclusions (Zrn, Ap, Rt, Omp) and a colourless poikiloblastic mantle rich in inclusions of brown amphibole, Cpx-Pl symplectites and quartz. Brown amphibole is present also in the groundmass and usually overgrows the Cpx–Pl aggregates or forms symplectitic rims with plagioclase around garnet. In several samples (es. H90, H313, H340), the modal amount of amphibole strongly exceeds that of the other phases, so that the eclogites are almost completely transformed into

Qtz

Cpx-Pl sympl. Ilm Mg-Hbl + Pl

0.5 mm

b) Grt relic Cpx+Pl sympl.

Pl 1mm

Mg-Hbl

Fig. 5. (a) Photomicrograph of a garnet-poor layer dominated by Cpx–Pl symplectites and large garnet porphyroblasts. (b) Photomicrograph of strongly amphibolitised sample H313.

garnet-bearing amphibolites (Fig. 5b): in such a case, the rock mainly consists of brown hornblende, plagioclase and quartz (Filmenite) with rare occurrence of relic Cpx–Pl symplectites. Within some strongly retrogressed samples, late overgrowths of pale green amphibole on brown amphiboles, as well as rims of titanite over ilmenite and small veins filled with prehnite and epidote, are common. 3.2. Amphibolites We classified as amphibolites all the rocks that do not contain textural (i.e., symplectites) or mineralogical (i.e., paragenetic relics) evidence of a former eclogitic stage. Amphibolites are commonly foliated

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rocks and often display banded textures with alternating amphibole-rich and plagioclase-rich layers. The main constituents of the best preserved samples are pleochroic green hornblende and plagioclase with apatite and ilmenite as accessory phases. Ilmenite is commonly skeletal and often rimmed by titanite. Garnet occurs in some samples and is mostly subhedral, devoid of inclusions and rimmed by a thin plagioclase corona towards amphibole grains. Polygonal textures are common in several samples, but interlobate grain boundaries also occur: in these samples amphibole is a greenish hornblende and titanite is the main Ti-bearing mineral. Late epidote crystallisation (in veins or intergrown with amphibole and plagioclase), sericite replacing plagioclase and chlorite consuming garnet are common features. Ultramafic rocks are not frequent. Those cropping out at M. Nieddu (Fig. 2) are clearly cumulate layers, indicating an original intrusive nature of the whole mafic sequence. For a detailed description of the M. Nieddu ultramafic rocks, refer to Ghezzo et al. (1979) and Franceschelli et al. (2002). 3.3. Gneiss complex The host paragneisses display variable textures, ranging from fine- to coarse-grained stromatic migmatites, to nebulites–agmatites. They are composed mainly of Qtz, Bt, Pl, Ms plus minor Kfs, Ky, Grt and Sil. The leucosomes are trondhjemitic to granitic in composition and usually display lobate grain boundaries, but pseudogranoblastic fabrics are observed in the Ky- or Sil-bearing samples. Kyanite is found in almost all outcrops commonly concentrated within the mesosomes or the melanosomes and armoured by plagioclase or rimmed by muscovite (Fig. 6). In some samples sillimanite occurs in small prismatic crystals often intergrown with biotiteFquartz or in late fibrolite mats. Muscovite commonly rims kyanite or overgrows the migmatitic foliation. The rare calcsilicate boudins within the paragneisses are mainly composed of Qtz, Cpx, Grt, Pl and Amp, with the modal amount of amphibole strongly increasing at the contact with the migmatites (Ghezzo et al., 1979). The westernmost migmatite outcrops at the contact of the late-orogenic calc-alkaline granite intrusions of the Sardinian Batholith are characterised by the postkinematic blastesis of andalusite+cordierite.

1mm fibrolite Bt Ky

Ms rim Qtz Grt

Fig. 6. Photomicrograph of migmatitic paragneiss H143 with BtFKy on the main foliation and Ky rimmed by a thin Ms corona.

The felsic orthogneisses are strongly deformed rocks, with predominant monzogranitic composition and are mainly composed of Qtz, Kfs, Pl, Bt and MsFGrt. They show typical augen texture due to the presence of large Kfs porphyroclasts embedded by Bt (FMs) septa.

4. Analytical techniques 4.1. Mineral chemistry Electron microprobe analyses were carried out at the Centro di Studio per il Quaternario e l’Evoluzione Ambientale (CNR-Rome, Italy) with a Cameca SX50 equipped with five WDS spectrometers and one EDS spectrometer (Link Analytical eXL). A CAMEBAX SX50 electron microprobe (GEMOC Key Centre, Macquarie University, Sydney) was used to obtain BSE images as well as to analyse selected major and minor elements on zircons (Zr, Si, Hf, Y, U and Th). Operating conditions were 15 kV accelerating voltage and a beam current of 20 nA. 4.2. Whole-rock chemistry The best preserved samples of amphibolite and eclogite were analysed for major, trace-element and REE composition by ICP-AES spectrometry at SARM CRPG-CNRS of Nancy, France.

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4.3. Zircon dating and chemistry Zircon grains were concentrated using standard techniques. Zircons were analysed for U, Th and Pb isotopic compositions using a 213-nm laser ablation microprobe (LAM) coupled to an Agilent 4500, series 300, ICP-MS at the GEMOC Key Centre, Macquarie University, Sydney. Samples and standards were ablated in a custom-built sample chamber using He as carrier gas. GEMOC/GJ-1 zircon (609 Ma) was used as external standard and 91500 and Mud Tank were analysed with the unknowns in every run as an independent control on reproducibility and instrument stability. The signals of masses 206 Pb, 207Pb, 208Pb, 232Th and 238U were acquired using the instrument’s time-resolved analysis data acquisition software. 235U was calculated from the ratio 235U=238U/137.88. Raw data were processed using GLITTER, an in-house data reduction program developed by Van Achterberg et al. (1999). Concordia ages were determined using Isoplot 2.32 (Ludwig, 2000). Details of the analytical technique, instrumental conditions and measurement procedures are given in Belousova et al. (2001). The precision (1r) of the method (using a spot size of 40 Am and a pulse energy of 0.25 mJ) evaluated on the GJ-1 zircon is close to 1% for the 206/238, but somewhat lower for 207/235 ratios, because of the lower signals for 207Pb. Two age measurements were performed using a 213-nm laser ablation microprobe, coupling a magnetic sector ICP-MS at CNR-Istituto di Geoscienze e Georisorse of Pavia, Italy. Ablation of samples and standards was performed in an inhouse built ablation cell, using He as the carrier gas and 91500 zircon (1065 Ma) as external standard. GJ-1 zircon was used as independent control. The signals of masses 206Pb, 207Pb, 235U and 238U were acquired in the magnetic scan mode (B-scan) at a spatial resolution of 40 Am. Details of the analytical technique, experimental conditions, data accuracy and precision are given in Tiepolo (2003). Traceelement data for the same set of zircons were performed on the same LA-ICPMS system at CNR-Pavia, Italy. Thirty-seven elements were determined, adopting NIST-610 as external standard to correct for mass bias and laser-induced elemental fractionation, and normalisation of each analysis to the electron probe data for Zr as an internal standard.

227

A detailed description of the method is in Tiepolo et al. (2002).

5. Mineral chemistry 5.1. Pyroxene Following the classification of Morimoto (1988), clinopyroxene in the eclogite relics is commonly an omphacite (CpxI) with medium jadeite content (Jd31– 36), characterised by an average Fe/(Fe+Mg) of 0.21. A single high-Jd omphacite (Jd44) was found as inclusion within a garnet porphyroblast in sample H342. Clinopyroxene in the coronas around quartz grains and in the symplectitic pseudomorphs after CpxI is diopside (CpxII) with low jadeite component (Jd3–9). Orthopyroxene within symplectitic aggregates around Ky is En66 in composition (Table 1). 5.2. Garnet Several garnet crystals have been analysed and zonation profiles have been measured on garnets from different samples and from different micro-structural domains within the same sample. Complex zonation patterns have been often observed. In the overprinted eclogites garnet composition strongly depends on the bulk-rock composition. In addition, within compositionally banded rocks (Fig. 3a) garnet composition is strongly dependent on the microstructural domain in which garnet crystallised. Within Grt-rich layers, garnet is commonly low in Prp component (Prp21–27, Alm48–51, Grs21–28, Sps0.7–1.55) and, where garnet is in mutual contact with omphacite, zonation profiles are relatively flat (Fig. 7a). Conversely, a sharp chemical zoning at mantle–rim boundary is observed where garnet is in contact with secondary CpxII–Pl symplectites. The crystals from the garnet-poor layers are richer in the Prp component (Prp28–33, Alm42–49, Grs19–21, Sps0.1–1.2) and again display high Prp content in the presence of Omp inclusions and a strong zonation at mantle–rim boundary (Fig. 7b and c). The mantle–rim transition is in both cases marked by a decrease of the Prp component followed by a new Prp increase at the outer rim. In the amphibolites, the small garnet relics are unzoned and very different from garnets in the

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Table 1 Representative clinopyroxene analyses Sample Mineral Analysis location

H342 Omp Crystal core

H343 Omp Crystal core

H342 Omp Incl. Grt

H92 Omp Incl. Grt

H342 Cpx Sympl.

H341 Cpx Sympl.

H341 Opx Sympl.

wt.% SiO2 TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O Sum

54.91 0.12 9.45 0.02 2.27 2.26 – 9.69 15.84 5.59 0.02 100.16

54.83 0.09 8.83 0.04 1.50 2.46 – 10.35 16.45 5.10 – 99.51

56.49 0.32 12.92 0.16 – 4.05 – 7.88 11.78 6.52 – 100.12

54.08 0.23 8.00 0.05 1.75 5.77 0.03 9.01 15.94 4.87 0.03 99.75

53.10 0.11 5.45 – 0.09 6.25 0.05 12.13 21.56 1.74 – 100.50

53.87 0.09 2.75 – – 4.52 0.04 14.70 23.72 0.72 – 100.42

53.74 0.04 0.83 – – 21.20 0.29 23.66 0.31 – – 100.07

1.96 0.04

1.97 0.03

1.99 0.01

1.97 0.03 – 0.31 – 0.05 0.18 – 0.49 0.62 0.34 – 4.00

1.94 0.06

1.96 0.04

1.98 0.02

Cations Si AlIV Ti AlVI Cr Fe3+ Fe2+ Mn Mg Ca Na K Sum

–

– 0.36

–

– 0.34

– 0.06 0.07

–

– 0.04 0.07

– 0.52 0.61 0.39

–

– 0.12 –

0.55 0.63 0.36 –

4.00

0.52

0.41 0.44 0.44 –

4.00

3.96

–

– 0.17

– –

– 0.08

– – 0.19

–

0.14 –

0.66 0.84 0.12 –

0.65 –

0.80 0.93 0.05 –

4.00

0.02 – –

4.00

1.30 0.01 – – 4.00

Formula normalisation to six oxygens and four cations. Sympl.: CpxII–Pl symplectite; incl.: inclusion.

amphibolitised eclogites: they are markedly enriched in the Sps component (Sps5.7–7.5) and have low Grs contents (Grs10–15). Garnets from one migmatitic gneiss (H143) were also analysed: they are almandine-rich (rim-core values: Alm68–73) and strongly zoned, with low-Mn core and high-Mn rim and opposing Mg behaviour (rim–core values: Sps15–5, Prp17–12). Representative garnet analyses are shown in Table 2. 5.3. Plagioclase Several texturally different plagioclases occur within the overprinted eclogites: plagioclase as inclusion within kyanite relics is andesine in composition (An39). Plagioclase within symplectitic aggregates after omphacite is mainly oligoclase and becomes slightly more calcic (low-Ca labradorite)

within amphibole–plagioclase symplectites around garnet. Plagioclase aggregates around zoisite and kyanite are nearly pure anorthite. In the amphibolites, plagioclase is mainly andesine in composition. Representative analyses are reported in Table 3. 5.4. Amphibole Only calcic amphiboles (Leake, 1997) were found in the analysed samples (Table 4). One amphibole crystal of edenitic composition was found in association with plagioclase as inclusion within a kyanite porphyroblast in the overprinted eclogite H341.Within relic eclogitic domains amphibole is pargasite in composition with relatively high Na contents (0.5–0.9 a.p.f.u.) and M4 occupancy (up to 0.41 a.p.f.u.). Amphibole from the post-eclogitic

F. Giacomini et al. / Lithos 82 (2005) 221–248

a)

rim Omphacite

229

rim Cpx-Pl sympl

core

1.60

a.p.f.u.

m-r

1.20

0.80

0.40 0

250

500

750

1000

Distance µm

b)

rim

core

rim

1.60

Omp incl.

a.p.f.u.

m-r

1.20

0.80

0.40 0

600

1200

1800

2400

Distance µm 2+

Fe

Mg

+ Ca

Fig. 7. Rim to rim zonation profiles from two different garnets of sample H343. (a) Garnet from a partially preserved eclogitic domain: flat profile (left side) where Grt is in contact Omp and sharp mantle–rim (m-r) transition where Grt is in contact with secondary Cpx–Pl symplectites (right side). (b) Garnet from a strongly overprinted eclogitic domain, preserving an omphacite inclusion in its mantle (left).

overprinted domains is Mg-hornblende in composition, with lower Na contents (0.2–0.5 a.p.f.u.) and M4 occupancy (~0.2 a.p.f.u.). Within the high-grade domains of the amphibolites, mainly hastingsite and Mg-hastingsite are found. 5.5. Mica Micas within migmatitic gneisses are biotites with low Ti content (0.34 a.p.f.u.) and Fe/(Fe+Mg)=0.55.

White mica is muscovite with about 10% of paragonite component and low Fe and Mg contents (0.1 a.p.f.u.). 5.6. Sapphirine and spinel Sapphirine has never been observed before in the Sardinian eclogites: it is strongly peraluminous and close to the average composition of the very peraluminous sapphirine from the kyanite-bearing eclogite of the Pays de Leon (France) reported by Godard and Mabit (1998). Its composition is slightly beyond the

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Table 2 Representative garnet analyses Sample Lithotype Analysis location

H341 Ov. ecl. Crystal rim

H341 Ov. ecl. Mantle–rim transition

H341 Ov. ecl. Crystal core

H342 Ov. ecl. Crystal rim

H342 Ov. ecl. Crystal core

H343 Ov. ecl. Crystal rim

H343 Ov. ecl. Mantle–rim transition

H343 Ov. ecl. Crystal core

H217 Amph.

H143 Migm. Crystal rim

H143 Migm. Crystal core

wt.% SiO2 TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O Sum

39.29 – 21.58 0.11 1.34 19.13 0.26 10.22 7.58 – 0.02 99.52

39.09 0.09 21.99 0.03 0.92 19.89 1.15 8.27 9.05 – – 100.50

39.59 0.08 21.90 0.22 0.65 20.02 1.17 8.80 8.23 0.02 – 100.66

38.80 0.01 21.44 0.04 1.22 20.99 0.35 7.23 9.78 – 0.02 99.87

39.21 – 21.73 0.02 0.88 21.53 0.28 7.97 8.61 – – 100.23

38.69 0.05 22.46 0.07 1.79 22.15 0.73 7.14 8.42 – – 101.50

38.62 – 22.03 – – 24.07 0.54 4.94 10.67 0.02 – 101.02

38.82 – 21.62 – 0.87 22.41 0.51 6.73 9.09 – – 100.06

37.81 0.04 21.28 – 1.05 26.21 3.27 4.21 6.90 0.06 – 100.83

37.18 – 21.22 – 1.10 31.06 6.74 3.07 1.71 0.04 – 102.11

37.58 – 21.54 – 0.31 33.31 2.41 4.15 1.86 – – 101.17

Cations Si AlIV Ti AlVI Cr Fe3+ Fe2+ Mn Mg Ca Na K Sum Prp Alm Sps Uv Grs Adr

2.98 0.02 – 1.92 – 0.08 1.22 0.02 1.16 0.62 – – 8.00 38.49 40.43 0.56 0.33 16.37 3.82

2.97 0.03 – 1.94 – 0.05 1.26 0.07 0.94 0.74 – – 8.00 31.11 41.97 2.46 0.10 21.72 2.64

2.99 – – 1.95 0.01 0.04 1.27 0.07 0.99 0.67 – – 7.99 33.08 42.20 2.49 0.65 19.74 1.84

2.98 0.02 – 1.93 – 0.07 1.35 0.02 0.83 0.81 – – 8.00 27.55 44.89 0.77 0.12 23.15 3.52

2.99 – – 1.94 – 0.05 1.37 0.02 0.91 0.70 – – 7.99 45.77 30.18 0.61 20.84 0.07 2.53

2.94 0.06 – 1.95 – 0.10 1.41 0.05 0.81 0.69 – – 8.00 27.44 47.73 1.59 0.20 18.07 4.97

2.97 0.03 – 1.97 – – 1.55 0.04 0.57 0.88 – – 8.00 18.67 51.08 1.16 – 29.00 –

2.99 0.01 – 1.95 – 0.05 1.44 0.03 0.77 0.75 – – 8.00 25.77 48.13 1.11 – 22.49 2.51

2.97 0.03 – 1.94 – 0.06 1.72 0.22 0.49 0.58 – – 8.02 16.36 57.15 7.22 – 16.17 3.11

2.95 0.05 – 1.93 – 0.07 2.06 0.45 0.36 0.15 – – 8.03 12.02 68.20 14.99 – 1.51 3.30

2.97 0.03 – 1.98 – 0.02 2.20 0.16 0.49 0.16 – – 8.01 16.25 73.16 5.36 – 4.32 0.91

Formula normalisation to 12 oxygens, Fe3+ recalculation method based on site occupancy. Ov. ecl.: overprinted eclogite; Amph.: amphibolite s.s.; Migm.: Migmatitic gneiss.

13(Mg,Fe2+)O : 19(Al, Fe3+, Cr3+)2O3 : 5SiO2 endmember. Spinel composition is very homogeneous with the hercynite component varying between 0.5 and 0.52. See Table 5 for representative analyses of sapphirine and spinel.

6. Whole-rock chemistry All analysed samples have normative-calculated modal compositions ranging from Qtz-poor tonalite to

gabbro (Table 6). Several samples have distinctive chemical features related to differentiation-cumulus processes. The selected most primitive mafic rocks have low alkali contents and a subalkaline, tholeiitic affinity as shown by the FeOT –MgO–alkali diagram in Fig. 8a. No substantial chemical difference can be envisaged between the overprinted eclogites and the amphibolites. The chondrite-normalised REE compositions are reported in Fig. 8b. The metabasites show nearly flat (LaN/YbN=0.62–1.31) or slightly enriched (LaN/YbN=2.40–7.32) patterns, a common positive Eu

F. Giacomini et al. / Lithos 82 (2005) 221–248

231

Table 3 Representative plagioclase analyses Sample Lithotype Analysis location

H341 Ov. ecl. Incl. in Ky

H341 Ov. ecl. Sympl. around Ky

H341 Ov. ecl. Cpx–Pl sympl.

H340 Ov. ecl. Rim around Grt

H342 Ov. ecl. Rim around Zo

H216 Amph.

H217 Amph.

wt.% SiO2 TiO2 Al2O3 FeO CaO Na2O K2O Sum

58.54 0.03 26.52 0.07 8.11 7.15 0.04 100.49

45.04 – 34.48 0.33 18.64 1.16 0.01 99.69

59.56 – 25.24 0.05 7.34 7.42 0.12 99.76

58.33 – 26.02 0.29 8.25 7.19 0.02 100.10

44.42 – 35.51 – 19.08 0.60 – 99.61

47.28 0.04 33.57 0.36 16.38 2.28 0.03 99.95

56.92 – 27.76 0.22 9.11 6.60 0.08 100.69

2.61 – 1.39 – 0.39 0.62 – 5.01

2.09 – 1.88 0.01 0.93 0.10 – 5.02

2.66 – 1.33 – 0.35 0.64 – 5.00

2.61 – 1.37 0.01 0.40 0.62 – 4.99

2.06 – 1.94 – 0.95 0.05 – 5.00

2.17 – 1.82 0.01 0.81 0.20 – 5.02

61.32 38.43 0.25

10.08 89.84 0.08

64.19 35.11 0.70

52.26 47.50 0.25

5.38 94.62 –

20.10 79.70 0.20

Cations Si Ti Al Fe2+ Ca Na K Sum End-m. % Ab An Or

2.54 – 1.46 – 0.44 0.57 – 5.02

56.50 43.00 0.50

Formula normalisation to 8 oxygens and 5 cations. Ov. ecl: overprinted eclogite; Amph.: amphibolite; sympl.: symplectite; incl.: inclusion.

anomaly and REE total content ranging between 5 and 70 chondrite: this feature is consistent with protolith of N- to T-MORB affinity. Moreover, the Th/Yb vs. Ta/Yb plot (Fig. 8c) suggests that the basic rocks did not originate from subduction-related mantle sources. Cappelli et al. (1992) and Franceschelli et al. (1998) propose a similar chemical fingerprint for the metabasic lenses of the Posada–Asinara mylonitic belt and for the eclogites of Punta de li Tulchi. The major and trace element composition of investigated samples points to a probable continental rifting environment as involved by Ricci and Sabatini (1978).

7. Mineral parageneses and PT estimation The general disequilibria observed in the analysed samples are of great importance for unravelling the P–T–t path followed by the rocks, but they make difficult the application of geothermometers and

geobarometers to mineral pairs. Additional problems are given by the presence of phases like garnet and amphibole that are stable over a long part of the metamorphic history. Geothermobarometry was therefore carried out on local microequilibria at the grain scale or on mineral pairs inferred to be in equilibrium during the metamorphic overprint. The peak pressure and temperature conditions for the different metamorphic assemblages were estimated by means of conventional thermobarometry combined with the use of THERMOCALC 3.1 software (Holland and Powell, 1998), and by comparison of well calibrated reaction curves on petrologic phase diagrams. Table 7a is a summary of the P–T conditions for the inferred equilibria observed within the analysed lithotypes. 7.1. Amphibolitised eclogites The overprinted eclogites are rather complex rocks preserving relics of mineral parageneses that can be

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Table 4 Representative amphibole analyses Sample Lithotype Classif. Analysis location

H341 Ov. ec. Ed Inclusion in Ky

H342 Ov. ec. Prg Eclogitic domain

H343 Ov. ec. Prg Eclogitic domain

H343 Ov. ec. Mg–Hbl Overgrowth on sympl.

H92 Ov. ec. Mg–Hbl Rim around Grt

H340 Grt. amph. Mg–Hbl Overgrowth on sympl.

H90 Grt. amph. Mg–Hbl

H216 Amph. Hs Crystal rim

H217 Amph. Mg–Hs

wt.% SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Sum

50.20 0.35 10.15 0.25 2.12 – 18.70 12.10 2.51 0.10 96.48

43.74 0.91 12.98 0.1 12.75 – 13.25 10.33 2.76 0.21 97.03

40.42 0.14 16.99 0.12 13.52 0.14 11.02 11.16 3.17 0.04 96.72

48.18 0.09 8.51 – 11.82 0.19 14.72 11.13 1.87 – 96.51

45.20 1.41 10.88 0.12 13.87 0.03 12.11 11.49 2.01 0.05 97.17

46.59 0.60 12.14 0.12 10.09 0.12 14.28 12.48 2.01 0.08 98.51

46.38 0.93 9.27 – 15.43 0.12 12.09 11.60 0.95 0.09 96.86

40.52 0.84 13.80 0.18 20.24 0.19 7.66 11.93 1.26 0.61 97.23

40.26 0.82 15.07 – 18.08 0.36 8.79 10.79 1.46 0.60 96.23

Cations Si AlIV Ti AlVI Cr Fe3+ Fe2+ Mn Mg Ca Na K Sum

7.07 0.93 0.04 0.76 0.03 – 0.25 – 3.93 1.83 0.69 0.02 15.55

6.30 1.70 0.10 0.51 0.01 0.98 0.55 – 2.85 1.60 0.77 – 15.37

5.94 2.06 0.02 0.88 0.01 0.71 0.95 0.02 2.41 1.76 0.90 – 15.66

6.94 1.06 0.01 0.38 – 0.70 0.72 0.02 3.16 1.72 0.52 – 15.24

6.61 1.39 0.16 0.49 0.01 0.39 1.30 – 2.64 1.80 0.57 – 15.37

6.64 1.37 0.06 0.67 0.01 0.17 1.03 0.01 3.03 1.90 0.56 0.02 15.47

6.77 1.23 0.10 0.37 – 0.74 1.14 0.02 2.63 1.82 0.27 0.02 15.10

6.12 1.88 0.10 0.58 0.02 0.74 1.81 0.02 1.73 1.93 0.37 0.12 15.42

6.00 2.00 0.09 0.65 – 1.18 1.08 0.05 1.95 1.72 0.42 0.11 15.26

Formula normalisation to 23 oxygens and 13 cations excluding Na, K, Ca. Ov. ecl.: overprinted eclogite; Grt. Amph.: garnet amphibolite; Amph.: amphibolite s.s.; sympl.: symplectite.

related to the progressive burial and exhumation path followed by the rock during its history. In the studied samples, the oldest event is most likely attested by the presence of the edenite–andesine pair found as small inclusion within a kyanite porphyroblast in sample H341. The negative crystal shape of the inclusion (Fig. 4a and b) suggests that the assemblage Pl–Amp–Ky was in equilibrium during kyanite growth and therefore could represent a prograde, amphibolite facies metamorphic relic. It was not possible to calculate a pressure value for this paragenetic relic using conventional barometry. Hbl–Pl thermometry (calibration of Holland and Blundy, 1994) suggests for this stage temperatures of about 580–605 8C (assumed pressure for calculation: 0.5–1.0 GPa).

7.1.1. Omphacite +garnet +rutile +zoisiteFkyaniteF pargasite paragenesis A plagioclase-free, HP metamorphic event is documented by the relic eclogitic assemblage found in several samples. The general flat profile of garnets close to the best preserved omphacite crystals or around omphacite inclusions and the relic microtextures suggesting the former presence of triple junctions Grt–Omp–Zo or Grt–Omp–Rt are good indications that equilibrium conditions were attained during metamorphism. From textural and chemical evidence (see also Messiga et al., 1992), we assumed that the bluish-green pargasite (Prg) found in the groundmass developed during the eclogitic event. Therefore, we considered the assem-

F. Giacomini et al. / Lithos 82 (2005) 221–248 Table 5 Representative accessory minerals analyses Sample Lithotype Mineral

H341 Ov. ecl. Spr

H341 Ov. ecl. Spr (avg. 5 analyses)

H342 Ov. ecl. Zo

wt.% SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Sum

11.32 – 66.77 0.23 5.40 – 15.81 0.08 – – 99.62

10.95 0.02 67.64 0.15 5.96 0.04 15.54 0.11 – 0.01 100.43

39.12 0.06 30.84 – 2.45 – 0.13 24.13 0.04 – 96.76

0.67 – 4.65 0.01 0.27 – 1.39 – – – 7.00

0.64 – 4.69 – 0.29 – 1.36 – – – 7.00

3.04 – 2.82 – 0.16 – 0.02 0.01 2.01 – 8.05

Cations Si Ti Altot Cr Fe2+ Mn Mg Ca Na K Sum

Formula normalisation to 10 oxygens and 7 cations (Spr), 13 oxygens and 8 cations (Zo). Ov. ecl.: overprinted eclogite; Grt. Amph.: garnet amphibolite.

blage Qtz, Omp (CpxI), Rt, Ky, Zo, high-Mg garnet coresFPrg to be in equilibrium during the eclogitic event. Since petrographic analysis suggests that the HP stage was plagioclase-free, the minimum pressure of equilibration for this event was estimated as a function of the Jd content in Omphacite: indeed, the amount of Jd-component in clinopyroxene coexisting with quartz and albite is controlled by pressure (Holland, 1980). The reaction curve Ab=Jd+Qtz was determined with THERMOCALC, considering the compositions of analysed omphacites (Jd44 and Jd36) and the composition of plagioclase (Ab61) found within kyanite and inferred to be a preeclogitic relic. Pressure estimates (Table 7a) are in the range 1.4–1.72 GPa for temperatures of 550–700 8C (1.32–1.63 GPa for the most common Jd36 omphacite). By selecting a plagioclase with theoret-

233

ical composition inferred from CIPW norm calculation (Ab41), the curve shifts towards slightly higher pressures (about 0.1 GPa in the same temperature interval). Temperatures of equilibration were calculated by means of Cpx–Grt thermometry (calibration of Berman et al., 1995) on omphacite and Mg-rich garnet cores (samples H342 and H343) and are in the range 580–690 8C (1.3bPb1.8 GPa). The program THERMOCALC used in baverage PTQ mode on the inferred eclogitic equilibrium assemblage (Grt–Jd36–Zo–Ky–Prg–Qtz, a H2O=0.5) gave consistent values of 677 8C and 1.89 GPa. The choice of the a H2O=0.5 was made in order to minimise the THERMOCALC fit value for 95% of confidence. Table 7b reports activities and relative standard deviations of mineral end-members used for PT estimates (AX activity composition calculation program by Holland and Powell, 1998). The pressure and temperature during the eclogitic stage are moreover constrained by the absence of paragonite (Pg) in the paragenesis: paragonite at such PT conditions is stable only with very high water activities (a H2O=0.7–0.9) in the system. These results are in good agreement with those reported by Franceschelli et al. (1998) for the eclogites of Punta de li Tulchi cropping about 30 km SSE of Golfo Aranci. 7.1.2. Diopside +plagioclaseForthopyroxene and sapphirine +spinel +corundum +anorthite symplectites The HP minerals are often nearly completely overprinted by new assemblages, which developed under granulite and then amphibolite facies conditions. The CpxII–Pl symplectites are known to be the result of destabilisation of former omphacite in the granulite facies field (Mysen and Heier, 1972; Boland and van Roermund, 1983) following the reaction: Omp þ SiO2 ¼ Pl þ Di In absence of free quartz, SiO2 required for the reaction derives from the decomposition of kyanite, following the reaction: Omp þ Ky ¼ Crn þ Pl þ Di A similar interpretation is given for the symplectitic intergrowths around kyanite, which are considered to grow during pressure decrease at relatively

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Table 6 Representative chemical analyses of metabasite samples Sample Lithotype

H212 Ov. ecl.

H213 Ov. ecl.

H342 Ov. ecl.

H92 Ov. ecl.

wt.% SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI Total

49.06 2.10 13.73 5.72 8.31 0.20 6.15 10.10 2.87 0.19 0.31 1.26 100.00

47.61 0.92 15.02 1.99 9.55 0.15 8.53 12.62 2.42 0.02 0.02 1.15 100.00

46.35 1.82 14.26 6.64 6.94 0.21 7.69 12.67 2.27 0.04 0.30 0.81 100.00

46.33 2.85 14.40 2.15 12.20 0.24 7.29 11.94 2.15 – 0.05 0.65 100.25

ppm Ba Rb Sr Ta Nb Hf Zr Y Th U Cr Ni V La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

51.0 9.0 173.0 0.4 4.6 4.7 170.0 50.0 0.4 0.7 125.0 53.0 378.0 7.3 20.4 3.4 16.6 5.3 1.8 6.0 1.2 7.9 2.0 5.3 0.8 5.3 0.9

18.2 1.6 68.7 0.1 1.2 0.5 11.9 18.0 0.4 0.6 110.7 46.2 328.0 2.3 6.0 1.1 5.7 1.9 0.9 2.0 0.4 2.7 0.7 2.1 0.3 2.2 0.3

20.6 4.5 177.0 0.3 2.8 0.8 20.6 29.5 0.3 0.6 221.0 60.3 364.0 2.6 7.6 1.3 6.9 2.4 1.1 2.8 0.6 4.7 1.2 3.1 0.5 3.0 0.5

6.2 0.8 33.4 0.2 1.5 0.5 14.4 13.4 0.1 0.1 208.0 47.4 523.0 0.5 1.4 0.3 1.6 0.7 0.6 1.4 0.3 2.2 0.5 1.4 0.2 1.5 0.2

H313 Grt Amph. 49.09 2.04 14.94 4.25 8.13 0.21 6.04 10.82 3.00 0.27 0.22 0.45 99.46

124.0 6.4 215.0 0.5 6.7 5.0 192.0 49.0 0.8 0.7 216.0 48.0 389.0 9.2 25.3 3.8 19.3 6.1 2.0 7.4 1.3 8.6 1.7 5.1 0.8 5.1 0.8

H162 Grt Amph.

H45 Amph

S306 Amph

51.54 1.31 16.56 2.44 8.30 0.22 4.79 9.16 2.86 0.86 0.24 1.72 100.00

51.53 1.08 14.73 2.46 7.95 0.18 7.13 10.18 2.21 0.75 0.12 1.68 100.00

49.35 2.58 13.89 2.85 11.11 0.22 5.50 9.85 1.78 0.39 0.38 2.10 100.00

185.0 20.0 237.0 0.5 7.2 3.2 127.0 26.0 2.1 0.8 81.0 39.0 258.0 8.2 24.4 2.9 13.7 4.0 1.4 4.1 0.7 4.4 1.0 2.5 0.4 2.4 0.4

388.0 26.0 115.0 0.2 1.7 2.0 68.0 29.0 0.6 0.6 195.0 58.0 304.0 3.7 8.5 1.6 8.0 2.9 1.2 3.6 0.7 4.7 1.2 3.0 0.5 3.1 0.5

86.0 8.5 181.0 1.1 14.7 4.8 190.0 40.0 1.6 0.5 134.0 42.0 366.0 14.7 34.9 4.9 23.0 6.4 2.2 6.4 1.2 7.1 1.6 4.2 0.6 3.9 0.6

Ov. ecl.: overprinted eclogite; Grt. Amph.: garnet amphibolite; Amph.: amphibolite.

high temperatures (Bard and Caruba 1982; Godard and Mabit, 1998; Mo¨ller, 1999; Elvevold and Gilotti, 2000; Carrigan et al., 2002). Kyanite reacts with clinopyroxene (Fzoisite) and breaks down to anorthite, corundum, spinel, sapphirine and orthopyroxene (refer to Mo¨ller, 1999 for a detailed

discussion of the kyanite breakdown reactions). There is no agreement in the literature about the PT conditions of the kyanite breakdown: Carrigan et al. (2002) state that the breakdown takes place at relatively high pressure (1.4–1.2 GPa and temperatures of 7008–800 8C), whereas Mo¨ller (1999)

F. Giacomini et al. / Lithos 82 (2005) 221–248

a)

235

b)

FeOT

Sample / C1 Chondrite

1000

Tholeiitic

Calcalkaline H313 H213

100

10

1 Na2O+K2O

MgO

La Ce Pr Nd

Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

c) 10

tra

-p

lat

eb

EM

Oceanic island arcs

as

alt

s

Active continental margins

Th/Yb

In

1.0

DM

0.1

RB

O

M

0.1

1.0

10

Ta/Yb Fig. 8. Whole-rock compositions of analysed metabasites. White dots represent the overprinted eclogites, stars represent the dated samples and the grey field represents the amphibolite samples.

points to lower pressure conditions at similar temperatures (1.2–0.9 GPa, 700–800 8C). Equilibration conditions at this granulitic stage were therefore calculated by means of conventional thermobarometry on the assemblage CpxII–Opx–Pl and low Prp garnet compositions from the garnet mantle–rim transition. Indeed, we interpreted the Prp decrease at garnet mantle–rim boundary as the result of garnet growth in competition with other Mg-rich phases like sapphirine, orthopyroxene and spinel during the granulitic overprint. Calculated pressure (Table 7a for references) is always in the range 0.85–1.25 GPa: this relatively large spread of values is due to differences in CpxII compositions containing variable amounts of jadeitic component (Jd3– Jd9), related to the gradual pressure decrease during

symplectite development. Grt–Cpx pairs yield temperatures of 650–790 8C for the majority of the investigated samples. The calculated PT equilibration conditions of the post-eclogitic granulitic metamorphic event are consistent with those calculated by Franceschelli et al. (2002) and Ghezzo et al. (1979) for the ultramafic rocks of M. Nieddu (700–750 8C, ~1.0 GPa). 7.1.3. Hornblende +plagioclase assemblage The post-HP reequilibration continues with an almost complete transformation of the Cpx–Pl–Grt granulites into garnet amphibolites: brown amphibole overgrows the CpxII–Pl symplectites and develops, together with plagioclase, around garnet porphyroblasts. Reactions controlling the growth of amphibole

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Table 7a Calculated PT conditions for the inferred equilibria in the studied lithotypes Method Overprinted eclogites Eclogite THERMOCALC facies stage calculation of all reactions THERMOCALC calculation of all reactions Cpx–Grt

Granulite facies stage

Amphibolite facies stage

Assemblage

Calibration

T (8C)

Jd44 Ab(41– 61) Qtz

Holland and Powell (1998)

550 –700

7

1.4–1.72

0.17

Jd36 Ab(41– 61) Qtz

Holland and Powell (1998)

550 –700

8

1.32–163

0.17

Omp – Grt cores

Berman et al. (1995) Holland and Powell (1998)

580 – 690

THERMOCALC Grt Omp (Jd36) average PT mode Zo Ky Prg Qtz a H2O=0.5 GADS CpxII – Pl sympl. – Grt mantle – rim transition Grt– Cpx– Opx–Pl Opx – CpxII – Pl sympl.– Grt mantle – rim transition Cpx–Grt CpxII – Grt mantle – rim transition Grt–Amp–Pl Grt rims–brown Hbl–Pl Hbl–Pl Hbl–Pl pairs

Newton and Perkins(1982); Eckert et al. (1991) Paria et al. (1988)

677

S.D. P (T) (GPa)

S.D. Fit ( P)

Correlation

at 1.3–1.8 40

1.89

at 650 –750

0.85–1.25

at 700 –750

1.15–1.2

Berman et al. (1995)

650 –790

at 1.0–1.2

Kohn and Spear (1990) Holland and Blundy (1994)

at 650 –750

0.9–1.1

660 –740

at 0.5–1.0

720–830

at 0.5–1.0

at 680–750

0.8

680–750

at 0.8

0.2

1.35 0.039

Amphibolites Hbl–Pl

Gneiss complex GASP Granulite facies stage (migmatite formation)

Amphibolite facies stage (retrograde)

Hbl–Pl pairs

Holland and Blundy (1994)

Grt Holland and core–Bt–Ky–Pl–Qtz Powell (1998)/ Ganguly and Saxena (1984) Grt–Bt Grt cores–Bt Thompson (1976); Ferry and Spear (1978); Perchuck and Lavrent’eva (1983); Bhattacharya et al. (1992) THERMOCALC Bt Grt cores Ky Holland and average PT mode Kfs Pl Qtz Powell (1998) THERMOCALC Bt Grt rims Sil Ms Holland and average PT mode Qtz a H2O=0.5–1.0 Powell (1998)

788

149

1.02

0.25 0.4

0.885

582 663

26 32

0.6 0.7

0.11 0.53 0.875 0.13 0.45 0.881

F. Giacomini et al. / Lithos 82 (2005) 221–248

237

Table 7b Activities (a) and standard deviations (S.D.(a)) of mineral end-members used in the THERMOCALC average PT calculations Overprinted eclogite

Gneiss complex

Eclogite facies stage at 650 8C

Granulite facies stage at 650 8C

a Garnet Prp Grs Alm Clinopyroxene Di Jd Acm Hed Amphibole Tr Fe-Act Ts Prg Gln Quartz Kyanite Zoisite H2O

S.D.(a)

0.06100 0.02700 0.07800

0.01350 0.00791 0.01530

0.51000 0.36000 0.06200 0.06600

0.05110 0.03580 0.01400 0.01450

0.07720 0.00006 0.00180 0.16700 0.04080 1 1 1 0.5

0.01930 0.00004 0.00200 0.04170 0.01020

a Garnet Prp Grs Alm Sps Biotite Phl Ann East Plagioclase Ab An Quartz Kyanite

Amphibolite facies stage at 550 8C S.D.(a)

0.00760 0.00028 0.33000 0.00016

0.00293 0.00015 0.05000 0.00008

0.03200 0.04100 0.04100

0.00903 0.01050 0.01060

0.82000 0.26000 1 1

0.41200 0.02840

involve both Cpx–Pl symplectites and garnet porphyroblasts as follows: CpxII þ Na  rich Pl þ Grt þ H2 O ¼ Amp þ Ca  rich Pl Water required for amphibole growth might derive in part from zoisite breakdown; nevertheless, due to the large modal amount of brown amphibole in the most retrogressed samples with respect to the hydrous phases in the eclogitic assemblages, water for this reaction is most likely provided from dehydrating metapelites (see below and Poli, 1993). Temperature conditions for this fluid-assisted metamorphic event were calculated on amphibole– plagioclase pairs (calibration of Holland and Blundy, 1994) mainly on symplectites around garnet or on topotactic overgrowths after the CpxII–Pl symplectites: inferred temperatures vary between 660 and 740 8C. The equilibration pressure for this stage was calculated on garnet–amphibole–plagioclase triplets (Kohn and Spear, 1990): P values range between 0.9 and 1.1 GPa at 650–750 8C (Table 7a).

Garnet Prp Grs Alm Sps Biotite Phl Ann East Muscovite Ms Pg Cel Plagioclase Ab An Quartz Sillimanite K-feldspar H2 O

a

S.D.(a)

0.00280 0.00017 0.27000 0.00310

0.00124 0.00009 0.04100 0.00137

0.03500 0.04500 0.04500

0.00942 0.01130 0.01130

0.73000 0.61400 0.00890

0.04120 0.06140 0.00290

0.82000 0.26000 1 1 1 0.5–1.0

0.02840 0.07300

7.1.4. Sericite+prehnite+epidote+albite paragenesis The final metamorphic evolution is marked by diffuse growth of green amphibole, epidote, sericite and prehnite that are the result of sample reequilibration within the epidote amphibolite and then the greenschist facies field. 7.2. Amphibolites When compared to the eclogites, amphibolites are rather monotonous rocks. The inequigranular, polygonal texture and the lack of mineral zoning within several samples point to relatively high temperature of equilibration and/or high fluid activities during metamorphism within the high-T amphibolite facies field. The simple modal composition of these rocks hampers precise thermobarometric calculations for the peak PT conditions: equilibration temperatures were calculated using the amphibole–plagioclase thermometer of Holland and Blundy (1994). Calculated temperatures are 770–830 8C for a pure Amp–Pl amphibolite (H216), whereas they are lower (720–770 8C) for sample H217, which contains sparse garnet relics. Amphibolites with lobate grain boundaries yielded

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F. Giacomini et al. / Lithos 82 (2005) 221–248

lower equilibration temperatures (550–650 8C) in agreement with that observed by Franceschelli et al. (2002) for the medium-grade amphibolite stage of M. Nieddu ultramafites. 7.3. Gneiss complex 7.3.1. Kyanite +biotite +K-feldspar +garnet +quartz paragenesis Metamorphic fabrics and parageneses from the migmatitic paragneisses point to a peak equilibration

in the high-T amphibolite to granulite facies field. Petrographic evidence suggests that the assemblage Bt+Ky+Pl+Kfs+Grt+Qtz was stable during the peak metamorphic conditions. The presence of kyanite and K-feldspar is most likely the result of a dehydration melting reaction involving muscovite, plagioclase and quartz (Patin˜o Douce and Harris, 1998): Ms þ Pl þ Qtz ¼ Bt þ Kfs þ Al2 SiO5 þ Melt This is also demonstrated by the almost total absence of muscovite within the Ky-bearing, high50 µm

100 µm

a) H213-13

b) H213-17

50 µm

100 µm

c) H313-39

d) H313-6 50 µm

50 µm

e) H313-44

f) H313-14

Fig. 9. BSE images of zircons from the analysed samples. Black and white circles are location of U–Pb and trace-element LAM-ICPMS analyses, respectively. (a and b) Zoned magmatic zircons; (c) metamorphic structureless zircon; (d–f) metamorphic zircons with bright zones and cores.

F. Giacomini et al. / Lithos 82 (2005) 221–248

grade metamorphic assemblages. Garnet cores–biotite pairs seem to have equilibrated at relatively high temperature ranging between 680 and 750 8C for pressures of 0.8 GPa (calibrations of Thompson, 1976; Ferry and Spear, 1978; Perchuck and Lavrent’eva, 1983; Bhattacharya et al., 1992). Pressure was estimated by means of GASP geobarometry using the calibration of Holland and Powell (1998) and the garnet mixing model of Ganguly and Saxena (1984). To corroborate these data, we ran THERMOCALC in average PT mode considering the compositions (Table 7b) of the inferred equilibrium phases mentioned above: peak conditions estimates

239

are 788 8C and 1.02 GPa. The large temperature standard deviation (Table 7a) is most likely due to uncertainties in the calculation of mineral activities and correct Fe3+ estimation in both garnet and biotite (Table 7b). Nevertheless, the obtained values are in good agreement with the published data (Ghezzo et al., 1979) and with the PT grids for the pelitic system (Spear et al., 1999; Patin˜o Douce and Harris, 1998): at temperature between 750 and 800 8C, muscovite breaks down producing Kfs, Al2SiO5, Bt and melt. Moreover, orthopyroxene was never observed in the migmatites even in the Bt-rich melanosome layers. The lack of Opx in the peak

Table 8 Laser ablation ICPMS U–Th–Pb isotope data and calculated ages for zircons from the Golfo Aranci metabasites Sample

Isotopic ratios 207

Pb/ 206 Pb

RSD

Ages 206

Pb/ 238 U

RSD

207

Pb/ 235 U

RSD

H213 13.1 z 13.2 z 17.1 z 17.2 z 18.1 z 21.1 s 23.1 z

0.0562 0.0565 0.0566 0.0546 0.0570 0.0564 0.0562

1.14 2.76 1.41 2.25 1.28 1.56 1.28

0.0738 0.0743 0.0766 0.0743 0.0693 0.0746 0.0727

1.03 0.74 1.06 0.58 1.07 1.10 1.03

0.5716 0.5786 0.5979 0.5588 0.5451 0.5802 0.5633

1.14 2.36 1.39 2.04 1.28 1.53 1.26

H313 4.1 s 5.1 s 6.1c s 8.1 s 12.1 s 13.1 s 14 .1 bp 15.1 s 16.1 s 17.1 s 18.1 s 19.1 s 20.1 s 21.1 s 22.1 s 22.1 s 22.1 s 22.1 s 22.1 s 22.1 s 22.1 s 22.1 s 22.1 s

0.0555 0.0535 0.0537 0.0547 0.0540 0.0525 0.0539 0.0583 0.0544 0.0539 0.0569 0.0536 0.0535 0.0546 0.0521 0.0574 0.0542 0.0537 0.0536 0.0534 0.0537 0.0534 0.0535

3.33 2.95 3.55 3.18 5.32 11.59 1.04 4.96 4.38 6.61 2.69 2.29 4.34 5.90 4.36 4.97 5.83 3.59 2.76 3.76 2.55 4.81 4.39

0.0552 0.0546 0.0556 0.0554 0.0543 0.0527 0.0568 0.0549 0.0574 0.0568 0.0565 0.0561 0.0564 0.0516 0.0555 0.0568 0.0564 0.0564 0.0557 0.0543 0.0573 0.0543 0.0571

1.47 1.34 1.46 1.39 1.58 2.81 1.06 1.79 1.64 1.76 1.22 1.18 1.60 1.80 1.41 1.69 1.76 1.35 1.28 1.35 1.24 1.75 1.56

0.4221 0.4025 0.4118 0.4170 0.4038 0.3818 0.4224 0.4406 0.4307 0.4220 0.4437 0.4150 0.4155 0.3880 0.3982 0.4494 0.4216 0.4175 0.4110 0.3993 0.4244 0.4001 0.4212

3.28 2.92 3.46 3.09 5.23 11.34 1.07 4.80 4.23 6.47 2.61 2.22 4.19 5.72 4.28 4.81 5.68 3.51 2.71 3.70 2.50 4.66 4.25

z: zoned domains; s: structureless domains; bp: bright patches.

208

Pb/ 232 Th

RSD

207

Pb/ Pb

1r

206

0.0223

1.26

0.0226

1.95

0.0242 0.0236 0.0229

1.53 2.12 1.40

0.0150 0.0107 0.0166 0.0158 0.0090 0.0281 0.0209 0.0167 0.0144 0.0180 0.0163 0.0088 0.0108 0.0134 0.0281 0.0177 0.0184 0.0135 0.0167 0.0174 0.0208 0.0171 0.0207

9.06 11.27 7.95 7.20 43.25 55.87 3.64 13.47 20.06 15.27 11.11 34.05 22.66 44.37 7.37 7.93 18.12 10.01 3.84 9.84 4.76 18.03 11.81

459 470 475.4 392 492.3 467.2 460.9

432 349 360 398 370 309 367 540 387 366 489 356 350 395 289 506 380 358 352 345 358 346 352

206

Pb/ U

1r

238

25 61 31 50 28 34 28

76 68 82 73 123 266 24 111 101 153 61 53 101 136 102 112 135 83 64 87 59 112 96

207

Pb/ U

1r

4 9 5 7 5 6 5

235

459 462 475.8 462 432.1 464.1 452.1

5 3 5 3 4 5 5

459.1 464 475.9 451 441.8 464.6 453.7

346 343 349 347 341 331 356 344 360 356 354 352 353 324 348 356 354 354 349 341 359 341 358

5 4 5 5 5 9 4 6 6 6 4 4 5 6 5 6 6 5 4 4 4 6 5

358 343 350 354 344 328 358 371 364 357 373 352 353 333 340 377 357 354 350 341 359 342 357

208

Pb/ Th

1r

232

10 9 10 9 15 32 3 15 13 19 8 7 12 16 12 15 17 10 8 11 8 14 13

445

6

451.7

9

483.9 471 457.8

7 10 6

301 214 333 317 182 561 418 335 289 361 327 177 217 269 560 354 368 271 334 348 416 344 415

27 24 26 23 78 309 15 45 58 55 36 60 49 119 41 28 66 27 13 34 20 61 49

240

F. Giacomini et al. / Lithos 82 (2005) 221–248

assemblage constrains the maximum temperature to less than about 820 8C for pressures of about 1.0 GPa.

probably the result of back-reaction of partial melt pods with the restitic layers of the migmatite (Kriegsmann, 2001; Spear et al., 1999) during retrograde metamorphism. Muscovite rims around kyanite and the strong compositional zoning of garnet relics (with increasing Mn contents towards the rim) represent the best evidence of the retrograde equilibration under the medium-T amphibolite

7.3.2. Muscovite +sillimanite +biotite +quartz assemblage The late development of sillimanite in close association with muscovite, biotite and quartz is

Mean = 460 ± 5 [1.1%] 95% conf. MSWD = 1.6, probability = 0.18 (error bars are 2σ)

0.085

540

a)

500

206

Pb/

238

U

460 0.075

420

470

0.065 460

380 450 440

0.055 0.4

0.5

0.6

0.7

Mean = 351.7 ± 2.9 [0.81%] 95% conf. MSWD = 2.1, probability = 0.003 (error bars are 2σ)

0.07

440

b)

400 0.06

206

Pb/

238

U

360

320 0.05 380

280

370 360

0.04

350

240

340 330

200

320

0.03 0.2

0.3

0.4 207

Pb/

0.5 235

0.6

U

Fig. 10. (a) U–Pb zircon age of the partially overprinted eclogite H213, plotted on a concordia diagram. (b) Zircon ages of the strongly overprinted eclogite H313.

F. Giacomini et al. / Lithos 82 (2005) 221–248

facies conditions. THERMOCALC P–T estimates are in the range 582–663 8C and 0.6–0.7 GPa (0.5ba H2Ob1.0).

241

8.1. Ages 8.1.1. Partially overprinted eclogite H213 Zircons in the overprinted eclogite H213 are rare and often concentrated in the garnet cores. They show prismatic, euhedral to sub-rounded shapes, a maximum length of 250 Am and weak oscillatory zoning in BSE images (Fig. 9). Few unzoned homogeneous BSE domains are also present. Inherited cores are absent. Seven spots were analysed on six zircon grains. Five of these are from zircon domains containing slightly fainted planar zoning; the other two spots lie in structureless domains. The U–Pb data for the analysed zircons are shown in Table 8 and plotted on a Concordia diagram in Fig. 10a. The weighted mean ages and the errors (given at the 95% confidence level) were calculated using 207Pb/206Pb concordant ages. On the Concordia diagram, five of the seven analyses combine in a cluster with a mean age of 460F5 Ma. The two remaining spots show, respectively, older and younger ages (475.8F3.8 and 436.8F3.4 Ma). The young age is slightly discordant and probably reflects a minor Pb loss event. No real difference in age exists between weak oscillatory zoned and structureless domains.

7.3.3. Chlorite +sericite development The exhumation stage proceeds finally with a moderate equilibration in the greenschist facies with the development of chlorite and sericite on biotite and plagioclase, respectively.

8. Geochronology: U–Pb LAM-ICPMS results U–Pb LAM-ICPMS dating of zircon crystals was carried out on two samples selected to be representative of a partially overprinted eclogite (H213) and a strongly amphibolitised eclogite (H313). Sample H213 preserves eclogitic relics within dominant granulite facies CpxII–Pl symplectites. Brown amphibole overgrowths are limited to garnet rims. Conversely, sample H313 displays a strong overprinting in the amphibolite–facies field and consists mainly of brown amphibole, garnet and plagioclase. Only rare relics of CpxII–Pl symplectites are present whereas relic eclogitic parageneses are absent.

10000

Zircon/Chondrite

1000

100

10

1

H313 structureless, homogeneous domains H313 bright patches H213 planar zoning domains

0.1

0.01 La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Fig. 11. Zircon–chondrite normalised REE distribution from the partially overprinted eclogite H213 and the strongly overprinted eclogite H313.

242

Table 9 Representative trace-element contents (ppm) of analysed zircons Sample

P

Ti

H213 13.1 z 18.1 z 22.1 s 23.1 z 13.2 z

1091 1474 1117 1428 1362

35.2 1.123 302.9 4.070 20.4 3.660 21.2 2.110 67.4 12.39

104.0 66.3 40.9 64.0 44.8 74.3 58.2 49.2 51.9 32.2 108.0 33.6 32.6 52.9 31.3 36.7 49.8 26.3 52.4 40.3 44.7 45.1 188.6 36.7 69.6 44.5

10.7 80.2 2.5 16.1 4.6 14.6 2.6 3.7 2.8 8.5 27.6 4.3 8.4 2.7 2.6 4.9 4.4 11.8 6.2 0.9 14.0 5.3 60.2 2.6 4.2 5.9

1.118 1.232 0.506 0.572 0.569 0.650 0.430 0.592 0.635 0.679 1.490 0.463 0.629 0.542 0.410 0.616 0.519 1.132 0.630 0.466 0.458 0.635 1.740 0.644 0.717 0.613

Y

Nb

Ba

La

2036 3759 2671 2778 2936

1.833 3.630 2.540 2.890 3.940

1.620 25.11 19.80 16.02 8.070

2.310 1.846 1.513 2.039 1.634 2.208 1.785 1.860 1.748 1.718 2.370 1.778 1.749 1.895 1.597 1.568 1.563 1.412 1.882 1.472 1.828 1.654 1.793 1.435 1.992 1.433

1.738 1.880 0.091 0.105 bd.l. 0.171 0.148 bd.l. 0.207 0.106 1.810 bd.l. bd.l. 0.182 0.502 0.143 0.276 7.260 bd.l. bd.l. 0.675 0.136 2.070 0.286 0.640 0.121

761.9 137.4 228.3 188.9 214.9 951.8 105.5 151.7 95.7 78.4 708.1 218.2 195.2 99.5 262.5 156.6 165.8 90.6 92.1 131.3 110.8 178.0 474.6 173.7 178.5 210.2

Ce

Pr

Nd

0.060 1.307 1.443 6.650 3.040 15.47 1.018 5.070 4.140 16.64

0.080 1.223 2.970 0.897 2.166

0.105 0.611 0.202 0.996 bd.l. 0.902 0.076 1.141 0.007 0.915 0.026 10.21 0.016 0.761 bd.l. 0.668 0.011 0.581 0.019 0.104 0.198 1.796 0.020 0.750 0.029 0.721 0.014 0.688 bd.l. 0.655 0.032 0.753 0.036 0.657 0.029 2.230 0.015 0.676 0.013 0.658 0.076 0.646 0.024 0.608 0.779 9.120 0.012 0.624 0.016 0.819 0.012 0.830

0.216 0.083 0.005 0.034 0.007 0.044 0.009 0.008 0.019 0.002 0.165 0.004 bd.l. bd.l. bd.l. 0.011 0.025 0.007 bd.l. bd.l. 0.021 bd.l. 0.631 0.011 0.032 0.014

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Hf

Ta

1.345 3.87 12.22 18.49 19.54 17.23 5.930 9.04 15.87 15.04

0.098 0.661 3.720 0.498 1.219

25.76 85.25 52.88 44.98 66.36

13.58 32.99 23.87 20.34 25.78

190.8 414.2 271.1 271.3 297.8

80.31 149.8 104.5 110.4 112.6

378.2 669.7 471.0 508.9 490.5

92.3 133.8 102.8 109.4 102.6

938 1263 1022 1133 956

142.9 231.7 184.9 178.0 157.9

9073 10176 11872 11109 9073

0.384 0.595 0.151 0.269 0.036 0.961 0.043 0.025 0.081 0.053 0.990 0.082 0.077 0.080 0.046 0.087 0.096 0.069 0.046 bd.l. 0.108 0.082 4.240 0.055 0.074 0.067

0.235 3.23 0.133 1.04 0.136 1.42 0.147 1.51 0.167 1.72 1.248 14.62 0.06 0.64 0.059 0.80 0.075 0.80 0.082 0.31 0.261 3.75 0.220 1.32 0.228 1.59 0.078 0.60 0.282 2.41 0.098 1.05 0.074 1.07 0.05 0.32 0.036 0.63 0.089 1.00 0.104 0.59 0.114 1.40 1.135 9.54 0.112 1.37 0.096 1.32 0.148 1.77

13398 13398 10769 10769 11957 11957 11363 12889 11193 9073 28153 10939 11702 11702 11109 10091 11278 11363 12380 9921 11193 10515 10176 9582 13907 10176

0.399 0.468 0.160 0.174 0.300 2.050 0.052 0.119 0.074 0.060 1.020 0.249 0.063 0.190 0.356 0.080 0.164 0.123 0.151 0.092 0.179 0.147 2.860 0.155 0.190 0.092

2.075 0.524 0.754 0.713 0.754 5.620 0.395 0.494 0.333 0.231 2.280 0.882 0.717 0.375 1.067 0.555 0.575 0.355 0.406 0.415 0.452 0.646 2.910 0.778 0.620 0.733

c: core; r: rim; z: zoned domains; s: structureless domains; bp: bright patches; bz: bright zones.

37.82 7.23 12.45 10.67 12.31 72.34 5.66 8.42 5.11 3.39 32.53 12.93 11.15 4.96 14.37 8.36 8.92 5.33 5.51 7.44 6.33 10.29 33.75 10.56 10.16 11.09

24.59 168.2 4.25 25.93 6.57 39.75 5.43 30.84 6.18 37.78 31.97 155.0 3.19 18.23 4.36 26.94 2.98 16.76 2.49 14.79 17.91 121.8 6.40 37.90 5.44 32.79 2.71 18.23 7.67 44.58 4.42 29.58 4.91 29.05 2.42 15.49 2.80 16.20 3.88 23.47 3.26 19.79 5.41 31.04 13.94 69.67 5.40 28.96 5.44 32.50 5.84 34.82

57.54 7.78 12.82 8.41 11.54 40.58 5.80 7.98 5.06 4.99 37.55 10.74 10.97 5.59 13.51 8.45 8.86 4.32 4.98 7.31 5.91 9.77 17.99 8.89 9.52 10.13

835.8 190.7 101.3 30.84 186.3 43.06 108.4 29.24 154.9 39.77 482.9 106.1 76.43 18.76 111.0 28.86 69.79 17.04 68.78 17.81 530.1 157.4 151.0 39.17 148.7 37.81 73.87 18.44 180.2 45.20 117.9 29.94 115.3 29.38 58.37 15.59 67.14 17.31 98.92 26.20 78.76 20.49 139.0 35.65 217.9 50.08 118.5 30.00 124.3 32.35 138.5 35.28

Pb

Th

U

Th/ REETOT Eu/ PrN/ U Eu* GdN

GdN/ YbN

0.580 3.44 56.00 299.8 1.561 27.42 175.24 877.3 0.737 19.07 98.62 577.8 0.956 8.18 114.12 461.4 1.137 11.98 267.46 484.9

0.19 0.20 0.17 0.25 0.55

1868 3021 2295 2399 2265

0.02 0.04 0.35 0.06 0.10

0.007 0.032 0.125 0.044 0.073

0.022 0.055 0.042 0.032 0.056

0.400 0.70 1.83 0.586 3.64 1.55 0.183 0.47 2.46 0.184 1.34 11.08 0.206 0.28 3.65 0.326 10.63 327.25 0.194 1.67 2.54 0.188 0.20 2.19 0.193 0.67 2.27 0.138 2.13 0.18 1.161 4.81 5.86 0.191 0.34 2.97 0.228 0.75 3.14 0.301 0.57 2.43 0.151 0.39 2.95 0.250 1.19 2.34 0.275 1.01 1.58 0.691 3.63 1.25 0.221 0.59 2.64 0.151 0.23 2.29 0.197 0.32 3.60 0.174 0.70 2.41 0.325 17.59 474.58 0.212 0.22 2.87 0.259 0.95 2.64 0.141 0.18 2.58

0.01 1322 0.08 181 0.07 304 0.10 197 0.11 266 0.54 924 0.07 130 0.07 190 0.06 119 0.03 113 0.01 908 0.10 262 0.07 250 0.08 126 0.09 310 0.12 201 0.03 199 0.06 105 0.08 116 0.08 169 0.09 137 0.09 234 0.82 435 0.09 205 0.06 217 0.04 239

0.44 0.56 0.59 0.59 0.55 0.51 0.60 0.44 0.59 1.49 0.36 0.94 0.96 0.64 0.69 0.60 0.40 0.74 0.30 0.55 0.89 0.50 0.60 0.50 0.43 0.55

0.150 0.178 0.008 0.050 0.010 0.007 0.032 0.022 0.053 0.012 0.099 0.007 0.000 0.000 0.000 0.023 0.051 0.052 0.000 0.000 0.080 0.000 0.148 0.017 0.053 0.017

0.003 0.008 0.006 0.011 0.009 0.025 0.007 0.006 0.009 0.004 0.006 0.007 0.009 0.007 0.011 0.007 0.008 0.004 0.008 0.008 0.006 0.008 0.035 0.009 0.009 0.010

308.1 19.88 35.89 106.6 33.65 603.8 35.37 31.64 37.02 6.95 922.7 29.69 43.66 31.91 32.39 18.77 48.62 22.24 32.81 28.01 40.99 25.67 576.2 32.14 44.20 66.97

F. Giacomini et al. / Lithos 82 (2005) 221–248

H313 3.1c,bp 3.2 r,s 4.1 s 5.1 s 6.1 c,s 6.2 r,bz 9.1 s 10.1 s 12.1 s 13.1 s 14.1 bp 15.1 s 20.1 s 21.1 s 23.1 s 26.1 s 36.1 s 38.1 s 39.1 s 42.1 s 43.1 s 44.1 c, s 44.1 r,bz 46.1 s 47.1 s 58.1 s

Sr

F. Giacomini et al. / Lithos 82 (2005) 221–248

8.1.2. Strongly amphibolitised eclogite H313 Sample H313 contains a homogeneous zircon population. Zircons are typically 100–300 Am long and exhibit subhedral short prismatic to almost equant sub-rounded shapes (Fig. 9). Some fluid inclusion trails are found in few crystals. Zircons are commonly hosted in plagioclase and quartz. The BSE images show structureless homogeneous grains. Only three crystals were found containing bright patches in the core or thick bright zones (15–25 Am) towards the rim of the grains. No traces of inherited cores were observed. Twenty-three analyses were carried out on this sample. U–Pb results are given in Table 8 and plotted on Concordia plots in Fig. 10b. All LAM-ICPMS analyses are concordant. Twenty-one of the twentythree analysed grains define a relatively narrow cluster of concordant ages giving a weighted average value of 352F3 (2r) Ma. Two analyses of grains showing similar shape and BSE response yielded younger ages with a mean value of 326.7F9.1 (2r) Ma. The position of points on the reverse Concordia plot suggests a minor Pb loss. Bright domains show U–Pb ages similar to the structureless darker domains.

243

Zircons from sample H313 have a distinctive chemical composition, characterised by an overall low abundance of all the trace elements and Th/U ratios. Leaving out the trace-element composition of bright zones occurring in 3 of the 23 analysed grains, a strong depletion in U, Th, Y, Hf, P and REE contents (Fig. 11) is observed, and Th/U ratios often lower than 0.1 (0.02–0.12). The chondrite-normalised patterns of REE show more pronounced positive Ce anomalies and usually absent or only slightly positive Eu anomalies (Eu/Eu*=0.30–1.49). Furthermore, a slightly higher fractionation of HREE relative to MREE (lower GdN/YbN ratios) occurs in sample H313 with respect to H213. Bright domains in zircons of H313 show intermediate contents of most of the trace elements (P, Y, REE, Th, U) between zircons from sample H213 and the structureless domains of zircons from sample H313 (Th/U ratios commonly lower than 0.1).

9. Discussion 9.1. Age interpretation

8.2. Rare-earth and trace-element composition of zircons LAM-ICPMS trace-element analyses were performed on zircons from the metabasites in order to relate the calculated ages to the metamorphic history of the studied rocks. Recent works outlined the importance of trace-element and REE compositions to relate geochronological data to different metamorphic paragenesis and conditions (Bea and Montero 1999; Schaltegger et al., 1999; Rubatto 2002; Whitehouse and Platt, 2003; Rubatto and Hermann, 2003). Ablation spots, about 25 Am in size, were sited near the spots used for U–Pb analyses. Trace-element analyses of the analysed zircons (Fig. 11, Table 9) revealed significant differences in composition between the two samples. Sample H213 is characterised by relatively high contents of all the trace elements (U, Th, Hf, Y, P and REE) and Th/U ratios with a wide range of values but always higher than 0.1 (0.17–0.5). Chondrite-normalised REEs (Fig. 11) exhibit profiles with depleted light REE relative to heavy REE, together with pronounced Eu anomalies (Eu/Eu*=0.02–0.34).

Weak planar zoning as revealed by BSE images, together with trace-element compositions, suggest a magmatic origin for zircons of the overprinted eclogite H213. Indeed, prismatic elongated shapes, oscillatory zoning, high Y, U and Th contents and Th/ U ratios, highly fractionated REE patterns and negative Eu anomalies are often described as magmatic features. The weakness of oscillatory planar zoning in the BSE images of the described zircons is probably related to the mafic composition of the magma (see also Rubatto and Gebauer, 2000). The age of 460F5 Ma is therefore here interpreted as dating the emplacement of the basaltic (gabbroic) protolith, in agreement with the age obtained from Punta de Li Tulchi eclogites (Palmeri et al. 2004). However, the weak scatter in ages with a minimum value of 436.8F3.4 (1r) Ma suggests a minor Pb loss phenomenon, possibly related to a younger metamorphic event. No further evidence of resetting under eclogite or granulite facies conditions was found in the zircons of this sample. Palmeri et al. (2004) suggest a possible age of 400F10 Ma for the Sardinian eclogites of Punta de li Tulchi.

244

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In sample H313, the unzoned domains and frequent stubby shapes can be interpreted as metamorphic. The overall low Th/U ratios (often b0.1) and Y, U, Th and P contents strongly support a metamorphic origin for the studied zircons. The age of 352F3 is regarded as the timing of a complete resetting of the zircon’s U–Pb and trace-element systematics. The high MSWD of 2.1 for the main group of 21 analyses (352F3 Ma) related to the relatively large spread of ages along Concordia, and the two ages of 327F9 Ma could be thus the result of a long-lasting thermal reequilibration of zircon. What is more difficult is to assess whether the 352F3 Ma old age is related to the eclogitic or to the post-eclogitic metamorphic equilibration. The rare occurrence of REE and U, Th and Y enriched zones (bright domains in BSE images) in three of the analysed grains suggests that a process of metamorphic solid-state recrystallisation is responsible for the formation of some metamorphic zircons, as proposed by Hoskin and Black (2000). However, the completely reset zircon domains (dark zones in BSE images), the absence of preserved primary textures, of mixed isotopic ages and the overall depletion in all the trace elements seem consistent with a process of local dissolution–reprecipitation promoted by fluids. The marked positive Ce anomaly shown by zircons of sample H313 suggests the presence of Ce in the oxidised state during zircon growth: literature data indicate that Ce4+ is very unlikely to occur in magmatic systems (Schreiber et al., 1980) as it is reduced by Fe3+, but is expected in more oxidising conditions such as fluid-assisted metamorphic or hydrothermal and near-surface processes. The magnitude of Eu anomaly in zircon depends not only on fO2 conditions (Eu3+ more compatible in zircon than Eu2+), but also on the stable mineral phases in the system prior or during zircon crystallisation. A zircon REE pattern without Eu negative anomaly could therefore indicate high fO2 conditions and/or a system particularly enriched in Eu (plagioclase absence or breakdown). In this second case, typical of eclogite facies rocks, metamorphic zircons grown in equilibrium with garnet display relatively depleted HREE patterns (Rubatto, 2002; Schaltegger et al., 1999). The high Ce contents observed in zircons from H313 are always associated to absent or slightly

positive Eu anomalies and no HREE depletion, thus suggesting high fO2 conditions during the zircon recrystallisation. As a conclusion, zircon characteristics (shape, internal structure and trace-element composition) and the comparative petrographic study of samples H213 and H313 imply that the age of 352F3 Ma relates to the fluid-assisted upper amphibolite facies overprint that strongly affects sample H313. 9.2. P–T–t path The result of petrologic analyses indicates that the metabasite lenses (Fig. 12a), including the ultramafic rocks of M. Nieddu (Ghezzo et al., 1979; Franceschelli et al., 2002) and the Gneiss Complex (Fig. 12b), share the same PT evolution starting from the metamorphic overprint in the granulite down to the amphibolite (and greenschist) facies. Zircon dating gives further constraints and proves that the metabasite protoliths emplaced in the Ordovician (460F5 Ma). The relic eclogite parageneses attest that at least some of the metabasites were forced into subduction to minimum depths of 50–60 km (undated event). The eclogites were then exhumed towards shallower crustal levels and incorporated within the migmatite complex. This is demonstrated by the early development of mostly anhydrous granulite facies parageneses and by the following equilibration within the upper amphibolite facies field under high fluid activity conditions (zircon resetting at 352F3 Ma). The mineral assemblages of the host gneisses do not carry any evidence that these rocks underwent high-pressure conditions: Libourel and Vielzeuf (1988) and our unpublished data indicate PT conditions of 1.3–1.7 GPa and 800 8C for the felsic granulites of the Southern Corsican basement, which represents the northern prosecution of the Golfo Aranci area. The Golfo Aranci gneisses record a high temperature–medium pressure event characterised by the muscovite dehydration melting at about 1.0 GPa and 750 8C. This event probably relates to the Rb/Sr whole rock age at 344 Ma found by Ferrara et al. (1978) on one migmatite from N Sardinia and is most likely responsible for the pervasive fluid infiltration documented by the enclosed metabasite lenses at about 350 Ma. Subsequently, the migmatites show the

F. Giacomini et al. / Lithos 82 (2005) 221–248

a)

245

b)

?

2.0

2.0

tz

19

E

b 61 A

66

Jd 3

G

Corsican HP Libourel&Vielzeuf (1988)

R&

2) t( ou Pg

+Q 6

1.5

1.5

(2)

o Pg

Ms o ut

(1)

1.0

PR-A

HT-Amp

1.0

? Ky

Ky

MT-Amp

Sil

Retrograde white mica 320-300 Ma

Sil

Granite emplacement 310-300Ma (Paquette et al. 2003)

And

0

Rb/Sr W-R +7 344 Ferrara et al. (1978)

MT-A MT-Amp T-Amp T-Am

0.5

Protoliths 460+ - 5 Ma

Ms ou t (1)

GR

Zrn resetting + 3 Ma 352-

Opx in

P(GPa)

ut

GR

(Di Vincenzo et al., 2004) And

600

800

1000

0

600

800

1000

T(˚C) Fig. 12. P–T–t path for the analysed samples. Grey fields represent the calculated PT conditions for the inferred equilibrium assemblages. PRAmp=prograde amphibolite; E=eclogite; GR=granulite; HT-Amp=high-temperature amphibolite; MT-Amp=medium-temperature amphibolite. (a) Metabasites. Pg out 1 and Pg out 2 are the paragonite-out curves (a H2O 0.1 and 0.9, respectively) calculated with THERMOCALC considering the reactions PgYKy+Jd36+H2O and PgYKy+Ab+H2O. R and G1966 is the eclogite–granulite boundary after Ringwood and Green (1966). (b) Gneiss complex. bMs outQ curves are the dehydration melting curves following (1) Spear et al. (1999) and (2) Patin˜o Douce and Harris (1998). Opx-in reaction curve is from Spear et al. (1999). Dotted circle after Libourel and Vielzeuf (1988) for the felsic granulites of Solenzara (Corsica).

development of lower temperature, muscovite–sillimanite-bearing assemblages over the high-T foliation. Di Vincenzo et al. (2004) dated several post-peak muscovite from the northern Sardinia basement at 320–300 Ma (in situ 40Ar–39Ar on Ms). The emplacement at shallow crustal levels of the calc-alkaline granite of the Sardinian batholith (310–300 Ma, Del Moro et al., 1975; Paquette et al., 2003) gives indication on the age of the final exhumation stage of the basement after the Variscan collision.

10. Conclusions 10.1. Metamorphic history and zircon dating The Golfo Aranci basement is a polyphase composite migmatite–orthogneiss–metabasite com-

plex showing relics of an eclogitic metamorphic overprint followed by a continuous decompressional evolution through the granulite and amphibolite facies field. The high-precision zircon geochronology on two eclogites showing different degrees of metamorphic overprinting during exhumation yielded two different ages corresponding, respectively, to the magmatic crystallisation of the basaltic protoliths in Ordovician times (~460 Ma) and to their complete resetting during the lower Carboniferous (~350 Ma) under high-temperature amphibolite facies conditions. The preservation of magmatic zircons in eclogites is a clear indication of good resistance of zircon in high-P, medium-T anhydrous conditions. The complete resetting of zircon during the fluidassisted medium-P, high-T Hercynian overprinting is, on the contrary, a confirmation of the key role played by fluids in promoting a strong mobility of

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trace elements and isotopes in otherwise refractory zircons. 10.2. Geodynamic implications The presence of eclogite bodies several kilometers to the north of the Posada Asinara Line suggests that this tectonic lineament most likely does not represent a Variscan suture (see also Helbing, 2003), separating the southern bfold and thrust zoneQ (Gondwana) from the northern baxialQ zone (Armorica). Our new results indicate that the whole NE Sardinia high-grade basement exhibits strong lithostratigraphic, geochronological and tectonometamorphic analogies with the basements units of Corsica (Menot and Orsini, 1990), Maures–Esterel (Bard and Caruba, 1981; Crevola and Pupin, 1994) and intra-Alpine massifs (Paquette et al., 1989; von Raumer et al., 1999; Rubatto et al., 2001). These analogies are consistent with the tentative reconstructions of the North Gondwanan margin proposed by von Raumer et al. (2002), Stampfli et al. (2002) and von Raumer et al. (2003) and strongly support the existence of a high-pressure belt incorporating several relics of eclogitic rocks (Matte, 1988) extending from Sardinia to the Alps, through Corsica and the Maures–Esterel. We propose that this HP belt is the result of the Variscan subduction, collision, crustal thickening and exhumation related to the convergence between the passive northern Gondwana margin and the Hunic terranes.

Acknowledgements Zircon dating and trace-element analyses could not have been possible without the collaboration of Bill Griffin (GEMOC Key Centre, Macquarie University, Sydney) and Massimo Tiepolo (CNR-Istituto di Geoscienze e Georisorse of Pavia, Italy). Rock samples and geological mapping largely derive from the field work of Hans Guldbransen. We gratefully acknowledge M. Serracino (CNR-IGAG, Rome, Italy) for assistance during microprobe analyses. Constructive reviews by Marian Janak, Jan Kosler and Jana Kotkova significantly improved the manuscript. Financial support from MIUR-COFIN 1997Sassi and MIUR-COFIN 2001-Cortesogno is acknowledged.

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