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

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1 Lithos 63 (2002) Transformation of cumulate mafic rocks to granulite and re-equilibration in amphibolite and greenschist facies in NE Sardinia, Italy M. Franceschelli a, *, G. Carcangiu b, A.M. Caredda a, G. Cruciani a, I. Memmi c, M. Zucca a a Dipartimento di Scienze della Terra, via Trentino 51, Università degli Studi di Cagliari, I Cagliari, Italy b Centro Studi Geominerari e Mineralurgici C.N.R., Piazza d Armi, Cagliari, Italy c Dipartimento di Scienze della Terra, via Laterina 8, Università degli Studi di Siena, I Siena, Italy Received 15 March 2001; accepted 12 April 2002 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 = jc, P = f 8 10 kbar), amphibolite (T = jc, P = 4 6 kbar) and greenschist facies (T = f jc, P < 2 3 kbar). 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. D 2002 Published by Elsevier Science B.V. Keywords: Cumulate mafic rocks; Granulite; Multistage re-equilibration; P T path; Hercynian chain 1. Introduction * Corresponding author. Tel.: ; fax: address: francmar@unica.it (M. Franceschelli). The metabasite enclosed in the Hercynian metamorphic basement of Montiggiu Nieddu, NE Sardinia, consists of a sequence of plagioclase-banded amphib /02/$ - see front matter D 2002 Published by Elsevier Science B.V. PII: S (02)

2 2 M. Franceschelli et al. / Lithos 63 (2002) 1 18 olites with minor Mg-rich mafic/ultramafic cumulates known as ultramafic amphibolites (Ghezzo et al., 1979). The metabasite contain relics of igneous and granulite facies assemblages and they are particularly interesting because they can provide useful information about the peak metamorphic conditions and the mineralogical and microstructural transformations that occurred during exhumation. The aim of this study is to give an example of mineralogical transformation from cumulate ultramafics to granulite and subsequent retrogression to amphibolite and greenschist facies. The P T conditions of the various stages of metamorphic re-equili- Fig. 1. Geological sketch map of the Montiggiu Nieddu area.

3 M. Franceschelli et al. / Lithos 63 (2002) bration and the P T path will be discussed in the context of the Hercynian evolution of the Sardinia chain. 2. Geological setting The metamorphic basement of NE Sardinia (Fig. 1) consists of a Hercynian Migmatite Complex made up of gneisses, migmatites, orthogneisses (emplacement age 458 F 31 Ma; Ferrara et al., 1978) and metabasites which preserve relics of eclogite and granulite facies assemblages and intercalations of calc-silicate rocks, calc-silicate pods and rare marbles (Elter et al., 1986; Franceschelli et al., 1989; Carmignani et al., 1994). The eclogite and granulite rocks have been considered allochthonous bodies within the migmatite (Franceschelli et al., 1998). The 344 F 7 Ma Rb/Sr Fig. 2. Field photographs of the metabasite outcrop at Montiggiu Nieddu: (a) contact between ultramafic and plagioclase amphibolites (white in the photo). The pencil indicates the axis of a mesofold. (b) Layer C of ultramafic amphibolites. A garnet-rich vein lies perpendicular to the hammer.

4 4 M. Franceschelli et al. / Lithos 63 (2002) 1 18 Fig. 3. Photomicrographs (a,b) and BSE images (c,d) showing various stages of evolution of ultramafic amphibolites of Montiggiu Nieddu: (a) opaque minerals, mainly Fe-oxide, enclosed in amphibole and pyroxene crystals, forming oriented trails that resemble exsolution microstructures from earlier Fe-richer orthopyroxene. One polar; (b) igneous clinopyroxene (Cpx 0 ) replaced by brown (Cam 1 ) and green amphiboles (Cam 2 ); (c) corona microstructure developed at the interface between igneous olivine (Ol 0 ) and plagioclase crystals (Pl 0 ); (d) microstructure of garnet (Grt 2 ), amphibole (Cam 2 ), and spinel (Spl 2 ).

5 Fig. 4. BSE images (a,b) and photomicrographs (c,d) showing various stages of evolution of the ultramafic amphibolites: (a) talc (Tlc) at the contact with orthopyroxene, Oam 1 amphibole and igneous olivine (Ol 0 ); (b) interleaved grains of Chl 1 chlorite and anthophyllite (Oam 2 ); (c) Cam 3 amphibole in a Chl 1 chlorite matrix; (d) Cam 4 replacing and/or rimming Cam 2 amphibole. Mineral abbreviations as in Kretz (1983) and Spear (1993). M. Franceschelli et al. / Lithos 63 (2002)

6 6 M. Franceschelli et al. / Lithos 63 (2002) 1 18 Table 1 Selected microprobe analyses of olivine Sample type Z6 Z16 Z39 OI 0 Ol 1 Ol 0 Ol 0 SiO Al 2 O TiO FeO tot MnO MgO CaO Total Si Al Ti Fe Mn Mg Ca Total X Mg Ol 0 = igneous olivine; Ol 1 = metamorphic olivine. Structural formulae were calculated on the basis of four oxygens. The iron was assumed to be divalent. age (Ferrara et al., 1978) of a layered migmatite in NE Sardinia could provide the minimum age of the Hercynian thickening stage. In the Montiggiu Nieddu area the metabasite occurs as a lenticular body approximately 2 km in length and about m in thickness (Fig. 1). Two main types of metabasites are distinguished: ultramafic amphibolites and plagioclase-banded amphibolites. The contact between the mafic and ultramafic rocks appears to be igneous. The ultramafic amphibolites (Fig. 2a,b) form a dark-green to black, massive to weakly schistose body, made up of three main compositional layers (A, B, and C). Layer A (20 m thick) consists of grey to brownish, moderate schistose chlorite-rich amphibolites; layer B ( f 5 m thick) consists of plagioclase-rich greenish amphibolites; layer C consists of dark-green garnet-rich amphibolites. Small-scale compositional layering (up to cm) is frequently observed in layers A, B and C. Some of these thin layers contain up to 90 95% of coarse-grained amphibole. Garnet-rich nodules (1 15 cm in diameter) and garnet-, amphibole-, and epidote-rich veins are also observed (Fig. 2b). The plagioclase-banded amphibolites form an approximately N S body, up to 100 m thick. They are characterised by dark green (amphibole-rich) and white (plagioclase-rich) alternating layers ranging in thickness from a few decimetres to a few decametres. The dark-green bands consist mainly of green medium-grained amphibole, plagioclase, garnet and relics of orthopyroxene and clinopyroxene, chlorite, biotite, epidote and sphene. The white plagioclase bands consist mainly of plagioclase, large green amphibole, garnet and rare clinopyroxene. No relics of igneous minerals have been observed. The plagioclase-banded amphibolites are frequently crossed by Table 2 Selected microprobe analyses of plagioclase Sample type Z5 Z16 Z20 Z25 Z35 Z39 Pl 1 Pl 0 Pl 0 Pl 1 Pl 1 Pl 1 Pl 1 Pl 0 SiO Al 2 O FeO tot CaO Na 2 O K 2 O Total Ab An Or Structural formulae were calculated on the basis of eight oxygens. Pl 0 = igneous plagioclase; Pl 1 = crystal enclosed in garnet or plagioclase in matrix.

7 M. Franceschelli et al. / Lithos 63 (2002) mafic dykes, acidic dykes and coarse-grained quartz veins ranging from a few decimetres to a few metres in thickness. 3. Petrography The ultramafic amphibolites retain relics of igneous phases. Igneous minerals are plagioclase (Pl 0 ), olivine (Ol 0 ), clinopyroxene (Cpx 0 ) and orthopyroxene (Opx 0 ) (Fig. 3a c). Medium-grained igneous orthopyroxene commonly contains many oriented opaque mineral trails, mainly Fe-oxide, usually concentrated in the pyroxene core (Fig. 3a). The rocks of layer B show spectacular corona textures around igneous olivine and plagioclase. Olivine grains (1 5 mm in size) are surrounded by a shell of orthopyroxene (Fig. 3b,c) (Opx 1 ) and an irregular discontinuous patch of clinopyroxene (Cpx 1 ) (Fig. 3c). Igneous plagioclase (Pl 0 ) (Fig. 3c) is very rich in spinel inclusions. Plagioclase is rimmed by small grains of green spinel (Spl 1 ), which form an irregular shell, followed by a larger layer of garnet (Grt 1 ). Spl 1 also occurs enclosed in garnet. Small quartz crystals and plagioclase are occasionally enclosed in the corona garnet. Coronitic garnet contains very small, strongly altered aggregates of spinel, corundum and Al-rich trioctahedral chlorite and amphibole. Igneous plagioclase is replaced by amphibole, chlorite and epidote. Corona textures are rare in the rocks of layer A. Coarse-grained igneous olivine (Ol 0 ) is surrounded by a sporadic and discontinuous thin shell of orthopyroxene (Opx 1 ). The rocks of layer A consist of igneous orthopyroxene (Opx 0 ), clinoamphibole, orthoamphibole, idioblastic chlorite (Chl 1 ) and a green spinel forming worm-like structures with the green amphibole and minor garnet and plagioclase (Pl 1 ). No corona texture is observed in the rocks of layer C. They consist of coarse-grained amphibole, poikilo- Table 3 Selected microprobe data for clinopyroxene and orthopyroxene Sample type Z3 Z5 Z16 Z39 Opx 1 Opx 1 Opx 0 Cpx 0 Opx 1 Cpx 1 Opx 1 Cpx 1 SiO Al 2 O TiO FeO MnO MgO CaO Na 2 O Total Si Al IV Al VI Ti Fe Mn Mg Ca Na Total X Mg Structural formulae were calculated on the basis of six oxygens. All iron was assumed to be divalent. Opx 0 and Cpx 0 = relics of igneous orthopyroxene and clinopyroxene, respectively; Cpx 1 = clinopyroxene in coronas around orthopyroxene or in matrix; Opx 1 = orthopyroxene in coronas around olivine or in matrix.

8 8 M. Franceschelli et al. / Lithos 63 (2002) 1 18 blastic garnet (Grt 2 ), plagioclase (Pl 1 ), orthopyroxene and minor clinopyroxene and olivine. Grt 2 garnet is very rich in spinel (Spl 2 ) and plagioclase (Pl 1 ) inclusions and it is replaced by small crystals of tremolite, epidote, and chlorite. A common feature of the rocks of layers A, B, and C is the great variety of morphological and textural features of the amphiboles (Figs. 3b,d and 4). Four types of clinoamphibole can be distinguished: brown amphibole (Cam 1 ), green amphibole (Cam 2 ), green zoned amphibole (Cam 3 ) and late stage Cam 4 amphibole. Cam 1 grows on igneous orthopyroxene and clinopyroxene (Fig. 3b) and rarely between the garnet and pyroxene layers of the coronas. Cam 2 grows on pyroxene and/or as a thin layer between Grt 1 and Cpx 1 /Opx 1 of the coronas. In samples from layer C, Cam 2 occurs as medium-sized grains associated with large poikiloblasts of garnet (Grt 2 ). Cam 2 and Grt 2 form worm-like microstructures with Cr-bearing green spinel (Spl 2 ) (Fig. 3d). Cam 3 occurs in veins or in the chlorite-rich amphibolites as medium-grained compositionally zoned crystals (Fig. 4c). Cam 4 replaces Cam 2, igneous plagioclase, pyroxene, coronitic and poikiloblastic garnet. In some samples, Cam 4 associated with epidote and Fe-rich chlorite (Chl 2 ) grows on and around the rim of Cam 2, usually forming a sharp boundary (Fig. 4d). Two textural types of orthoamphiboles have been detected (Fig. 4a,b): Oam 1 is present in the rocks of layer B as small grains at the contact between Ol 0, talc and Opx 1. In the rocks of layer A, Oam 2 occurs as small grains or interleaved with Mg-rich chlorite (Chl 1 ) and is usually associated with Cam 2 and orthopyroxene. Oam 2 also grows around the rim of Cam 2. Chl 1 chlorite is widespread, occurring as medium-sized grains in the rocks of layer A. Among the opaque minerals are magnetite, hematite, ilmenite, pyrite and blende. Other minerals are fayalite (Ol 1 ), rutile, calcite, dolomite, albite, K-feldspar and kaolinite. 4. Mineral chemistry Selected electron microprobe analyses of olivine, plagioclase, pyroxene, garnet, amphibole, spinel and chlorite are reported in Tables 1 6. The chemical Table 4 Selected microprobe analyses of garnet Sample Z3 Z5 Z16 Z20 Z25 Z35 Z39 Type Grt 2 Grt 2core Grt 2rim Grt 1 Grt 2core Grt 2rim Grt 2 Grt 2 Grt 1 SiO TiO Al 2 O FeOtot MnO MgO CaO Total Si Al IV Al VI Ti Fe Mn Mg Ca Total X Fe Structural formulae were calculated on the basis of 12 oxygens, assuming all iron to be divalent. Core and rim compositions are reported for samples Z5 and Z20. Textural types are: Grt 1 = garnet in coronas around plagioclase; Grt 2 = poikiloblastic garnet.

9 M. Franceschelli et al. / Lithos 63 (2002) composition of the minerals was determined with a fully automated ARL-SEMQ electron microprobe at Centro Studi Geominerari e Mineralurgici C.N.R., Cagliari, using the method described by Franceschelli et al. (1998). All mineral abbreviations are from Kretz (1983) and Spear (1993). Olivine Coarse-grained igneous Ol 0 olivine ranges in composition from Fo = 69 to Fo = 71 mol%. Mn and Ca content is negligible. Ol 1 is a metamorphic fayalite with a fosterite content of 0.02%. Plagioclase The anorthite content of igneous plagioclase (Pl 0 ) is greater than 89%. Metamorphic plagioclase (Pl 1 ) ranges from An = 47 to An = 81 mol%. The plagioclase crystals are unzoned or poorly zoned. Albite is occasionally observed. Clinopyroxene Cpx 0 and Cpx 1 are diopside. The Na content of igneous clinopyroxene (Cpx 0 )is f 0.01 a.p.f.u. and the X Mg ratio is near Cpx 1 composition is fairly constant. The maximum Na observed in Cpx 1 is 0.01 a.p.f.u. with X Mg near Orthopyroxene Coarse-grained igneous Opx 0 is a bronzite with X Mg between 0.74 and There are no significant compositional differences between cores and rims. The composition of Opx 1 shows little Fig. 5. Representative profiles for Mn, Ca, Fe, Mg (a.p.f.u.) across a selected garnet crystal from sample Z20.

10 10 M. Franceschelli et al. / Lithos 63 (2002) 1 18 variation from one corona to another of the same sample. Al tot content of Opx 1 ranges from to a.p.f.u. and X Mg from 0.73 to Cr content of Opx 0 and Opx 1 is negligible. Garnet Two textural types of garnet were analysed: corona garnet around plagioclase (Grt 1 ) from layer B and poikiloblastic garnet (Grt 2 ) from layer C. Grt 1 is almandine-rich (37 40 mol%) with high contents of grossularite (28 32 mol%) and pyrope (29 30 mol%) and a subordinate content of spessartine (1 2 mol%). This type of garnet shows no compositional zoning. Grt 2 is almandine-rich (45 62 mol%) with higher pyrope (16 38 mol%) but lower grossularite (10 17 mol%) content than Grt 1. Although spinel and amphibole replace most Grt 2 crystals, some of them maintain compositional growth zoning (Fig. 5). The larger inner zone (core) is characterised by an increase in Mg and Fe content, a gradual decrease in X Fe and a progressive decrease in Ca content towards the rim. Mn content is constant. The outer zone (rim) shows an increase in Mn and Fe contents and a decrease in Mg and Ca content. We interpreted the inner zone as relic growth zoning and the outer zone as diffusion zoning, superimposed on the primary chemical zoning of the garnet. Amphibole Selected analyses of Cam 1, Cam 2, Cam 3, Cam 4, Oam 1 and Oam 2 are listed in Table 5. Structural formulae were calculated on the basis of 23 oxygens assuming all iron to be divalent. The different textures correspond to different mineral chemistries. Cam 1, Cam 2 and Cam 4 are calcic according to the classification of Leake et al. (1997). The plot of Al IV vs. (Na + K) (Fig. 6a) of calcic amphiboles shows a linear trend of isomorphic substitution between tremolite and an intermediate mem- Table 5 Selected microprobe analyses of amphiboles Sample Z6 Z16 Z20 Z25 Z35 Z39 Z79 Z56 type Cam 2 Cam 2 Cam 4 Cam 1 Cam 2 Cam 2 Cam 2 Cam 2 Cam 2 Cam 1 Cam 2 Cam 3core Cam 3rim Oam 2 SiO TiO Al 2 O FeO Cr 2 O MnO MgO CaO Na 2 O K 2 O Total Si Al IV Al VI Cr Ti Mg Fe Mn Ca Na K Total Structural formulae were calculated on the basis of 23 oxygens. All iron was assumed to be divalent. Cam 1 = brown amphibole; Cam 2 = green amphibole; Cam 3 = medium-grained compositionally zoned amphibole. Cam 4 = green amphibole around Cam 2 ; Oam 2 = anthophyllite.

11 M. Franceschelli et al. / Lithos 63 (2002) Fig. 6. Composition of amphiboles: (a) plot of Al IV content vs. (Na + K) atoms; (b) Na/Ca + Na vs. Al/Al + Si ratios of calcic amphiboles. Envelopes delimit the composition of medium-pressure and low-pressure amphiboles from Dalradian (Scotland), Haast River (New Zealand) and Abukuma (Japan), respectively (Laird and Albee, 1981). Composition of amphiboles, not reported in Table 5, is also shown; (c) compositional zoning and Mg/(Mg + Fe) ratio across a selected Cam 3 crystal. ber between pargasite and tschermakite. The tschermakite and pargasite substitutions are approximately in the ratio 1:1. The composition of brown amphiboles (Cam 1 ) ranges from magnesiohornblende to edenite (according to Leake et al., 1997). They are characterised by a large variation in Al IV ( a.p.f.u.), high X Mg ( ) and Na content up to 0.61 a.p.f.u. The composition of Cam 2 ranges widely from tschermakite and magnesiohornblende to pargasite. They are characterised by high Al IV ( a.p.f.u.) and Al VI ( a.p.f.u.) content, low Ti (

12 12 M. Franceschelli et al. / Lithos 63 (2002) 1 18 a.p.f.u.) and Na content up to 0.70 a.p.f.u. X Mg ranges from 0.50 to Cam 3 is a compositionally zoned magnesiohornblende. A representative chemical zoning profile of a Cam 3 crystal is shown in Fig. 6b. From the core to the rim, Ca content is almost constant, Al IV, Al VI and Na increase while X Mg decreases. Cam 4 is a tremolite with a very low Na content ( < a.p.f.u.). In the Na/(Na + Ca) vs. Al/(Al + Si) diagram of Fig. 6c, all the calcic amphiboles plot within the medium-pressure field of Dalradian (Scotland) amphiboles. Oam 1 and Oam 2 are anthophyllites with X Mg = 0.70 and 0.77, respectively. Spinel Two textural types of spinel were analysed: Spl 1 in plagioclase-olivine coronas and green Spl 2 forming vermicular structures with garnet and amphibole. Both belong to the spinel s.s-hercynite series. Spl 1 shows little Mg Fe chemical zoning. X Mg is near 0.5 and Cr content is negligible. X Mg of Spl 2 is between 0.40 and Spl 2 is sometimes zoned with Table 6 Selected microprobe data for spinel and chlorite Sample Z16 Z25 Z56 Z6 Z20 Z56 type Spl 1 Spl 2 Spl 2 Chl 1 Chl 2 Chl 1 SiO Al 2 O TiO Cr 2 O FeO tot MnO MgO Total Si Al IV Al VI Ti Cr Fe Mn Mg Total X Mg Sp 1 = spinel in plagioclase and garnet; Sp 2 = spinel associated with garnet and amphibole; Chl 1 = Mg-rich chlorite; Chl 2 = Fe-rich chlorite replacing mafic minerals and plagioclase. Structural formulae were calculated on the basis of 8 and 28 oxygens, respectively, assuming all iron to be divalent. higher Mg and Al at the rim and higher Fe and Cr at the core. Chlorite and talc Chl 1 is a clinochlore with X Mg f 0.86 and Al tot f 4.90 a.p.f.u. (Table 6). Chl 2 is an Fe-rich chlorite with X Mg f 0.40 and Al tot f 5.8 a.p.f.u.. Chlorite growing on spinel is an Al-rich trioctahedral chlorite with chemical composition similar to corundophyllite. Talc shows Fe content up to 0.40 a.p.f.u. and low Mn. Magnetite, ilmenite, rutile Magnetite shows an FeO content of 91% with TiO 2 near 2% and Al 2 O 3 near 1.5%. Ilmenite shows an MnO content of 0.92%. Rutile is almost pure TiO 2. Epidote shows an Fe tot content of about 0.80 a.p.f.u. and negligible Mn content. Corundum contains a small amount of iron and titanium. 5. Reaction history and mineralogical re-equilibration The formation of coronas and mineral re-equilibration in the ultramafic amphibolite of Montiggiu Nieddu can be illustrated by a schematic evolution involving three stages of mineralogical crystallization. First stage (corona stage) The typical arrangement of phase in space Ol 0! Opx 1! Cpx 1! Grt 1! Spl 1! Pl 0 at the olivine plagioclase interface in the rocks of layer B can be described by the generalized reaction: 4Pl 0 þ 6Ol 0 Z 2Opx 1 þ 3Cpx 1 þ Grt 1 þ 2Spl 1 ð1þ proposed by McLelland and Whitney (1980) for the formation of corona in the Mg-rich metagabbros. In our rocks the mineral layers could easily correspond to a steady state of reaction progress dominated by a decreasing rate of the diffusing component. The spinel forms in an Al-rich reaction boundary corresponding to the plagioclase side, and the sequence of the corona layers from plagioclase to olivine is mainly controlled by the diffusion of aluminium. Second stage This metamorphic stage is essentially dominated by the formation of clinoamphibole, chlorite, spinel and orthoamphibole. Cam 1,2 clinoamphiboles mainly overgrow Cpx 0 or Opx 0 and/or develop between the corona garnet and the pyroxene layers.

13 M. Franceschelli et al. / Lithos 63 (2002) This texture suggests that the formation of clinoamphibole occurred through the destabilisation of the garnet + pyroxene pair via the following simplified reaction: Grt 1,2 þ Cpx 0,1 þ H 2 O Z Cam 1,2 þ Spl 2 FPl 1 ð2þ Spl 2 is Cr-bearing, belongs to the spinel s.s-hercynite series and forms vermicular structures with the Cam 2 amphibole. Several compositional features indicate control exerted by the microtextural site on Cam 1 composition. In fact the lower Al tot content and similar X Mg and Na M4 amount in Cam 1 with respect to Cam 2 are presumably due to Cam 1 growth in microdomains at the expense of igneous pyroxene. In the rocks of layer A the close association of Cam 2 amphibole and chlorite (Chl 1 ) also suggests the following schematic Cam 2 amphibole-forming reaction: Ol 0 þ Spl 1 þ Cpx 0,1 þ Opx 0,1 þ H 2 O ¼ Cam 2 þ Chl 1 ð3þ This stage is also characterized by the presence of talc and/or anthophyllite in the Ol 0 +Opx 1 + Cam 2 + Spl 1 FAth+Tlc and Opx+Cam 2 +Ath+Chl 1 assemblages. Third stage The minerals formed during this stage replace garnet, olivine, pyroxene, amphibole and plagioclase. Four main types of replacement reactions can be envisaged: (i) a reaction involving Cam 2 and forming chlorite (Chl 2 ), epidote and Cam 4 via the simplified reaction : 25Tschermakite in Cam 2 ¼ 13Tremolite in Cam 4 þ 22H 2 O þ 7Chlorite Chl 2 þ12epidote þ 14quartz ð4þ (ii) a reaction involving olivine and forming serpentine: Ol 0 þ SiO 2 þ H 2 O ¼ Srp ð5þ (iii) a reaction involving clinopyroxene and a CO 2 -bearing fluid forming tremolite and calcite by the reaction: 5Cpx þ 3CO 2 þ H 2 O ¼ Cam 4 ðtrþþ3cal þ 2Qtz ð6þ (iv) a reaction involving spinel and forming chlorite and corundum. In brief, the reaction history and the occurrence of orthopyroxene, clinopyroxene and garnet support the hypothesis that the corona texture around igneous olivine and plagioclase was formed in a granulite metamorphic stage. The destabilization of the granulite assemblage and the widespread formation of amphibole indicate a subsequent mineralogical crystallization in an amphibolite stage. Finally, the formation of tremolite, epidote and chlorite and the retrogression of all the mafic minerals show that the last mineralogical crystallization occurred during a greenschist metamorphic stage. 6. Geothermobarometry Montiggiu Nieddu ultramafic amphibolites contain a variety of mineral assemblages that could constrain the P T conditions of the granulite and amphibolite stages of metamorphic recrystallization. Taking into account the uncertainties involved in the estimation of the Fe 3+ content, and because the Fe 3+ content of ultramafic minerals is generally considered to be negligible (Krogh and Carswell, 1995), temperature and pressure conditions were calculated assuming all Fe to be divalent. Granulite stage Although corona textures usually indicate a state of global disequilibrium, several studies on the mechanism of corona formation have established that equilibrium conditions may be met at certain reaction interfaces and that equilibrium may be achieved at least at a domain scale (Bethume and Davidson, 1997 and references therein). In the case of our samples (the clinopyroxene between garnet and orthopyroxene forms an irregular discontinuous patch), the temperature and pressure were obtained at the rim of the garnet in contact with clinopyroxene

14 14 M. Franceschelli et al. / Lithos 63 (2002) 1 18 and orthopyroxene. Plagioclase is rarely in mutual contact with orthopyroxene and/or clinopyroxene. The composition of plagioclase in contact with garnet (inner rim) was used in the thermobarometric work. The olivine composition when in contact with either garnet (very rarely) or orthopyroxene is very homogeneous. P T conditions were estimated using mineral data from a single corona domain. However, for each sample, the P T conditions were calculated for at least three corona domains. Fig. 7a,b summarises the results for two corona microdomains from samples Z16 and Z39. Temperatures estimated with the garnet clinopyroxene thermometer, using the calibrations of Ellis and Green (1979) and Powell (1985), range from f 700 to 740 jc. Temperatures estimated with the garnet orthopyroxene thermometer, after calibrations according to Sen and Bhattacharya (1984), are from f 650 to f 700 jc. A higher temperature ( f 50 jc) is obtained with the Lee and Ganguly (1988) calibration. The garnet orthopyroxene barometer, based on the Al content of orthopyroxene using the Harley (1984) calibration, gives a pressure from 8 to 9 kbar. A higher pressure is obtained with the Harley and Green (1982) calibration. Pressure estimates with the garnet orthopyroxene thermometer are subject to uncertainties arising from the presence of Fe 3+ in the orthopyroxene. However, in the orthopyroxenes used for ther- Fig. 7. P T conditions calculated for samples from Montiggiu Nieddu. The following calibrations were used. Thermometers: (1) Grt Cpx (Ellis and Green, 1979); (2) Grt Opx (Sen and Bhattacharya, 1984); (2a) (Lee and Ganguly, 1988); (3) Grt Hbl (Perchuk et al., 1985); (3b) (Graham and Powell, 1984). Barometers: (4) Grt Opx (Al Opx) (Harley, 1984); (5) Grt Opx Pl Qtz (Newton and Perkins, 1982); (5a) Grt Opx Pl Qtz (Eckert et al., 1991); (6) Grt Hbl Pl Qtz (Kohn and Spear, 1990, tschermakite Fe model); (6a) (Kohn and Spear, 1990, tschermakite Mg model); (7) 3Fa + 3An = 2Alm + Grs (reaction equilibria calculated with TWQ 2.02; Berman, 1991). See text for the explanation.

15 M. Franceschelli et al. / Lithos 63 (2002) mobarometry the amount of Fe 3+ is negligible as emerges from the calculation of the structural formula based on charge balance. Additional information on P T conditions in some coronas can be obtained from the actual composition of garnet and clinopyroxene. The maximum jadeite content of clinopyroxene (Holland, 1980, 1983) for a temperature of 700 jc suggests a metamorphic pressure lower than 10 kbar. The almandine and grossularite contents of garnet, calculated with the reaction Fa + An = Alm + Grs calibrated experimentally by Bohlen et al. (1983), provide a minimum metamorphism pressure of 6 8 kbar for a temperature of 700 jc. The position of the reaction is reported in Fig. 7a,b. P T estimates in the corona assemblages show considerable uncertainty and should be considered with caution. Therefore, the P T conditions of the granulite stage were also determined in granoblastic mafic/ultramafic amphibolites. Two groups of samples are useful for thermobarometry. One consists of garnet, orthopyroxene, amphibole, spinel and F olivine; the other group of samples, which is rather rare, consists of garnet, plagioclase, orthopyroxene, amphibole, and F quartz. In the first group of samples temperature estimates with the garnet orthopyroxene thermometer yield temperatures ranging from 550 to 750 jc with a cluster of values between 650 and 700 using the calibrations of Sen and Bhattacharya (1984) and Lee and Ganguly (1988). Pressure between 7 and 10 kbar has been obtained with the garnet Al orthopyroxene barometer using the Harley (1984) calibrations. P T estimations in the sample Z3 from the first group are P f 8 kbar and T f 690 jc. P T estimations on the second group of samples were based on the composition of garnet core, orthopyroxene and plagioclase enclosed in porphyroblastic garnet (Grt 2 ). Garnet especially at its rim shows an incipient growth of green amphibole but in the core it contains numerous inclusions of plagioclase and more rarely orthopyroxene. The composition of plagioclase enclosed in the garnet core is very similar to that of the matrix plagioclase. Garnet is slightly zoned. From core to rim it shows an increase in the Fe and Mg content counterbalanced by a decrease in Ca content (Table 4). The temperatures estimated in three samples with the garnet orthopyroxene thermometer using the calibration of Sen and Bhattacharya (1984) and Lee and Ganguly (1988) range from 680 to f 750 jc. The Newton and Perkins (1982) calibration used for the garnet orthopyroxene plagioclase quartz barometer yields pressures around 8 9 kbar; the calibration of Eckert et al. (1991) and Perkins and Chipera (1985) gives slightly lower pressures. P T estimation in the sample Z5 representative of the second group is reported in Fig. 7c. The P T estimates made on the coronas of the ultramafic amphibolite are consistent with the temperature ( jc) and pressure (8 10 kbar) conditions estimated in the granoblastic mafic/ultramafic amphibolites, apparently escaping re-equilibration in the amphibolite stage. These data are also consistent with the T f 750 jc and P f 10 kbar estimates, based on mineral assemblages, made by Ghezzo et al. (1979). This suggest that both the coronite and the mafic/ultramafic samples record the P T conditions of the granulite stage that are probably close but not at the peak of metamorphism. Amphibolite stage The P T conditions of the amphibolite stage were based on the Grt 2 + Cam 2 + Pl 1 +Qtz assemblages in mutual contact of deeply re-equilibrated samples. In the Grt 2+ Cam 2+ Qtz + Pl 1 assemblage of samples Z35, Z20 and Z25, the calibrations of the garnet hornblende thermometers by Graham and Powell (1984) and Perchuk et al. (1985) give temperatures of jc (Fig. 7d f). The pressures obtained with the garnet amphibole plagioclase quartz barometer are approximately 4 6 kbar. The pressures obtained using calibrations involving the Mg or Fe tschermakite model are usually lower than those determined with the calibration involving the Mg Fe pargasite model by Kohn and Spear (1990). Worthy of note is the fact that in some samples, especially in the plagioclase-free samples, the garnet green amphibole couple at times gives a wide range of temperature values greater than 700 jc. Most probably the higher temperature values may be explained with the fact that garnet was formed in the granulite stage and its rim composition does not perfectly re-equilibrate with green amphibole during the amphibolite stage. On the basis of mineral assemblages, Ghezzo et al. (1979) gives Tf650 jc and Pf5 kbar for the amphibolite stage. This datum is in agreement with the P T estimation determined in several studied samples.

16 16 M. Franceschelli et al. / Lithos 63 (2002) Discussion 7.1. P T path The textural features and P T estimations reported above indicate that the ultramafic rocks of Montiggiu Nieddu experienced a complex P T history. In order to trace the P T path in Fig. 8, it is necessary to examine the correspondence between P T estimates and microstructural evolution. The P T path of the amphibolitic rocks of NE Sardinia has two stages: (1) from igneous protolith to granulite and (2) from granulite through amphibolite to greenschist. The P T conditions of emplacement of the igneous protolith are still doubtful. Igneous orthopyroxene and clinopyroxene appear to re-equilibrate during the granulite stage. Assuming that the rocks re-crystallized from a basic magma at temperatures in the range jc, the pressure conditions during igneous crystallization must have been lower than 6 kbar (Green and Ringwood, 1972) because of the coexistence of forsteritic olivine, anorthite, orthopyroxene and clinopyroxene. Fig. 8. P T path for ultramafic amphibolites of Montiggiu Nieddu. Aluminium silicate triple points from Holdaway (1971). Pmp + Chl + Qtz = Ep + Tr + V and Ep + Chl + Tr + Qtz = Hbl + V after Liou et al. (1987). The reactions: Atg = Tlc + Fo + V, Fo + Tlc = Ath + V, Chl = Spl + Fo + En + V, Ath + Fo = En + V, Fo + An = Spl + Opx + Cpx were calculated with TWQ 2.02 (Berman, 1991). Reaction curve (1) represents incoming garnet in a typical quartz tholeiite composition and separates high-pressure (HP) from medium-pressure (MP) granulites (Green and Ringwood, 1967); reaction (2) represents incoming garnet in an undersaturated basalt composition (Ito and Kennedy, 1971) separating medium- and low-pressure (LP) granulites.

17 M. Franceschelli et al. / Lithos 63 (2002) There is some evidence indicating the thermal history between the crystallization of cumulate rocks and the granulite facies metamorphism. The widespread occurrence of expurgated cloudy to platy Feoxide phases in igneous clinopyroxene and orthopyroxene (Fig. 3a) suggests that the rocks underwent a cooling period after igneous crystallization. The coronas between igneous olivine and plagioclase presumably formed during a period of heating at P T conditions of 8 10 kbar and jc. These data are similar to the findings of Ghezzo et al. (1979) for the peak of the granulite stage of northern Sardinia. Considering the re-equilibration history the thermobarometric results would have provided only minimum conditions for the peak metamorphism. During the amphibolite stage, pervasive hydration led to destabilization of corona pyroxene and garnet, with widespread formation of Cam 2. The amphibolite stage generally involved decompression of 4 6 kbar accompanied by a temperature drop of Cj from the peak of metamorphism. The zoning of Cam 3 in Al IV,Na A and Al VI can be tentatively related to some variation in metamorphic conditions occurring during the final exhumation of the rocks, probably due to the emplacement of the late tectonic Hercynian granitoids. New formation of amphibole is suggested by the growth of Cam 4, coexisting as patches or thin rims on Cam 2. The very low Na M4, (Na + K) A and Al IV contents of Cam 4 indicate that it developed under the P T conditions of the greenschist facies. Coexistence of Chl 2 chlorite, tremolite and epidote suggests a re-equilibration at temperatures ranging from f 330 to 400 jc. The empirical barometer based on the Na M4 content (Brown, 1977) of tremolite growing on hornblende yields pressure conditions less than 2 3 kbar. In summary, the P T path inferred for the ultramafic amphibolites requires prograde evolution up to the granulite stage, followed by a P T decrease during the amphibolite and greenschist stages. The shape of the post-peak P T path provides an indication of the tectonic processes involved in the origin and evolution of granulites. It shows P T evolution similar to that caused by tectonic thickening of the crust followed by exhumation dominated by erosion. (England and Thompson, 1984). The age of granulite facies metamorphism in NE Sardinia is unknown. The 344 F 7 Ma Rb/Sr age of a banded migmatite (Ferrara et al., 1978) could represent the time when the granulitic lenses were tectonically incorporated in the country rocks during Hercynian crustal thickening. The re-equilibration age of granulite in the amphibolite facies can be considered contemporaneous with that of migmatite formation (i.e. f 344 Ma), while the greenschist facies re-equilibration developed during late Hercynian extensional tectonics (Franceschelli et al., 1998) in response to the gravitational equilibration of the collisional structure. Acknowledgements We greatly appreciated the detailed comments and critical review by Bruno Messiga and an anonymous referee. Financial support was provided by the Italian MURST project: Terrestrial materials and synthetic analogues at high pressure and high temperature: physical, chemical, and rheologic properties Cofin 98 (National coordinator C.A. Ricci). References Berman, R.G., Thermobarometry using multi-equilibrium calculations: a new technique with petrological applications. Can. Mineral. 29, Bethume, K.M., Davidson, A., Greenville metamorphism of the Sudbury diabase dyke-swarm: from protolith to two-pyroxene garnet coronite. Can. Mineral. 35, Bohlen, S.R., Wall, V.J., Boettcher, A.L., Experimental investigation and application of garnet granulite equilibria. Contrib. Mineral. Petrol. 83, Brown, E.H., The crossite content of Ca amphibole as a guide to pressure of metamorphism. J. Petrol. 18, Carmignani, L., Carosi, R., Di Pisa, A., Gattiglio, M., Musumeci, G., Oggiano, G., Pertusati, P.C., The Hercynian chain in Sardinia (Italy). Geodin. Acta 5 (4), Eckert, J.O.J., Newton, R.C., Kleppa, O.J., The DH of reaction and recalibration of garnet clinopyroxene plagioclase quartz geobarometers in the CMAS system by solution calorimetry. Am. Mineral. 76, Ellis, D.J., Green, D.H., An experimental study of the effect of Ca upon garnet clinopyroxene Fe Mg exchange equilibria. Contrib. Mineral. Petrol. 71, Elter, F.M., Franceschelli, M., Ghezzo, C., Memmi, I., Ricci, C.A., The geology of Northern Sardinia. IGCP No. 5, Newsl., Special Issue.

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