Miocene to Recent ... http://go.to/permin Per. Mineral. (2003), 72, SPECIAL ISSUE: , 73-104

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CHAPTER 2

Elba Island: intrusive magmatism

SERGIO ROCCHI1*, ANDREA DINI2, FABRIZIO lNNOCENTI1, SONIA TONARINI2 and DAVID S. WESTERMAN3

1 Dipartimento di Scienze della Terra, Universita di Pisa, Via S. Maria, 53, Pisa, 56126, Italy 2 CNR, lstituto di Geoscienze e Georisorse, Via Moruzzi, 1, 56124, Pisa, Italy 3 Norwich University, Department of Geology, Northfield, Vermont 05663, USA

2.1 HISTORICAL PERSPECTIVE little island in the Etruscan region where metals, such as iron and copper, were mined, and he The first inhabitants of Elba Island were calls this island «Aethalia» (i.e. sooty) owing to Musterian/Neanderthal men, about 50,000 years the smoke from smelting furnaces. ago. At that time, during the Wiirm glaciation, At the beginning of the fourth century B.C., the sea level was some 100 m lower, and it was the Greeks coming from Siracusa conquered possible to walk from mainland Tuscany to Elba. About a century later, the Romans took Elba, Gorgona, Capraia, Montecristo and control of the island and exploited its iron ores. Pianosa islands. About 18,000 years ago, Their techniques were rough, leaving up to 70% individuals of Homo Sapiens were living in of the iron in the scoria, while the Etruscans had Elba. Some 12,000 years ago the isthmus been able to extract twice this quantity. joining the Elba to Tuscany was submerged, but Moreover, the Romans almost completely no people remained in Elba. During the destroyed the woods of Elba to supply their Neolithic, some 5,000 years B.C., people came furnaces, whereas Etruscans cut the trees again to Elba, and 2,000 years ago they started according to their age. Then, during the II to exploit the copper ores from ophiolitic century B.C., the Romans started to take metal sequences of western, central and eastern Elba. from Sardinia, Spain, Germany and France. Thus, Elba Island, with its potential of iron ores, Finally, during the I century B.C., the Romans joined the history of the main Mediterranean finally swept the threat of the pirates out of the civilisations and is reported in the legend of Mediterranean Sea and proudly called it Mare Jason and the Argonauts. During the VIII Nostrum (i.e. Our Sea). At this time, Romans century B.C. the Etruscans with their king, built beautiful villas on the islands of the Tyrrhenus, arrived in Tuscany. At around 650 Tuscan Archipelago, and the fi rst Emperor, B.C., when the wars in the Middle East Augustus, established that no more trees could discontinued the iron supply from the Anatolian region to the Mediterranean area, the Etruscan be cut on Elba to supply furnaces. The Romans mines of Elba Island and southern Tuscany started to quarry the granite of western Elba, became the major producers of iron and other and columns from there can be admired today at metal s such as copper, lead, zinc, silver. the Pantheon and Saint Paul's cathedral in Aristotle refers to Elba when he speaks about a Rome. At this time Plinius and Virgil referred to the island as «llva», a name deriving from a Corresponding author, E-mail: [email protected] people coming from Ligury and living in Elba. 74 S. RoccHI, A. DINI, F. INNOCENT!, S. ToNARINI and D.S. WESTERMAN

The documented history of Elba begins with extensional processes behind the eastward Christianity and the Middle Ages, and the name migrating compressive regime in the hanging «Elba» was first used by Pope Gregori us wall of the eastward retreating Apennine slab Magnus. In the eleventh century, Elba became a (Malinverno and Ryan, 1986; Fig. 1 ). The possession of the Republic of Pisa, who started backbone structure of the Apennine mobile belt to exploit the iron ores again and defended the was constructed during an early Miocene island against the Saracen pirates with compressive phase in the collision zone mountain- and sea-towers, fortresses (Volterraio between the Sardinia-Corsica block and the and Marciana) and churches. From the end of Adria plate (Keller and Pialli, 1990). This the XIII century to the French Revolution, Elba orogenic system evolved diachronously as the Island was dominated by ever changing powers, regime of extension migrated from west to east, i.e. Genoa, Spain, the Medici of Florence, the trailing the retreat of the compressive regime Lorraines of Tuscany, the Bourbons of Naples. (Brunet et al., 2000) and giving way to the During the years of Napoleon's wars, Elba opening of the Tyrrhenian basin. In this Island suffered complex events, and finally, framework, magmas were generated in the after the defeat of Lipsia, Napoleon arrived in mantle and interacted with crust-derived felsic exile at Elba on May 1814. He built streets, set magmas to generate the variety of Tuscan down administrative rules, supported the public Magmatic Province intrusive and extrusive health, and imposed a new development of the products exposed over about 30,000 km2 of mining industry. On February 26, 1815, southern Tuscany and the northern Tyrrhenian Napoleon left the island with 1100 followers Sea (Juteau at al., 1986; Gil·aud et al., 1986; taking advantage of the absence of the English Ferrara et al., 1989; Poli et al., 1989; Pinarelli, Commissioner. He would never come back, but 1991; Poli, 1992; Innocenti et al., 1992; Elba will never forget him. He left here his two Innocenti et al., 1997; Westerman et al., 1993). residences, a town house, Villa dei Mulini, and This igneous activity migrated from west (14 a country house, Villa San Martino. Ma) to east (0.2 Ma) in an extensional ensialic After Napoleon's departure and his defeat, back-arc setting as the west-dipping Adriatic Elba was annexed to the Grand Duchy of plate delaminated and rolled back to the east Tuscany in 1815 and it became part of the (Serri et al., 1993). In late Miocene time, the newly born Italy by plebiscite in 1860. The extensional processes affected the area of Elba Italian Elba became an important iron centre Island (Bouillin et al., 1993; J olivet et al., and it went through a fl ourishing trade 1994), and several laccoliths, a major pluton, development that also brought a stream of and an extensive dyke swarm were emplaced immigrants from the mainland. This industrial within stacked tectonic complexes. development was also favoured by the loss of excessive bureaucracy due to the unification of 2.2.2 Geological outlines of Elba Island Italy. Yet, a new threat was already hanging Elba Island was constructed from five over Elba, and the outbreak of the World War tectonic complexes that were thrust onto each II caused the destruction of Portoferraio. In the other by about 20 Ma (see Part Ill, Chapt. 1; post-war period the island started up its tourist Deino et al., 1992). The lower three complexes trade, leaving forever the industrial activities. have continental features, while the upper two are oceanic in character (Trevisan, 1950; Trevisan, 1953; Pen·in, 1975; Keller and Pialli, 2.2 GEOLOGIC SETTING 1990; Pertusati et al., 1993). Complex IV consists of Jurassic oceanic lithosphere of the 2.2.1 Regional geology western Tethys Ocean (peridotite, gabbro, Elba Island is located at the northern end of pillow basalt and ophiolite sedimentary the Tyrrhenian Sea, a region affected by breccia) and its upper Jurassic-middle Elba Island: intrusive magmatism 75

��:;:).. th��front thinned 100 110 I2u cant crust

J\N\ 0 "-\ 50km -- -..o;,;;j

Corsica

42f --- TyrrhenianI �Sea ,----=::::�-:----;;-----L ------I rocks of the Tuscan Magmatic Province j m-ain exten sional fau lts intrusive rocks of the Tuscan Magmatic Province Capraia- Elba- Montecristo Ridge a volcanic rocks of the Roman Magmatic Province (- 'Tuscan Magmatic Province -../

Fig. 1 - Location map for the Tuscan Magmatic Province. Also reported are the younger potassic-ultrapotassic volcanic rocks of the Roman Magmatic Province. Ages after the compilation of Serri et al. (200 1 ).

Cretaceous sedimentary cover ( chert, Western Elba is separated from central Elba limestone, and argillite interbedded with by the Eastern Border fault that roughly siliceous limestone). Complex V consists of parallels the east side of the Monte Capanne argillite, calcarenite and sandy marl of pluton, truncating its contact aureole and Palaeocene to middle Eocene age, overthrust dipping moderately to steeply eastward (Fig. 2 by an upper Cretaceous flysch sequence and 3). For the most part, the fault is marked by (Bellincioni, 1958; Raggi et al., 1965; Keller a distinct plane separating a western footwall and Pialli, 1990). breccia of hornfelsed Complex IV rocks Large-scale faults subdivided Elba Island ( ophiolitic material and deep marine cover into three geographic areas: western, central rocks) plus fragments of the Monte Capanne and eastern Elba (Fig. 2 and 3). Western Elba pluton, from an eastern hanging wall breccia consists of the Monte Capanne monzogranitic made of Complex V fl ysch and megacrystic pluton and its thermometamorphic carapace of San Martino porphyry. Complex IV rocks that contain hypabyssal Central Elba consists of Complex V flysch porphyry intrusions. and enclosed porphyry intrusions, and is S. RoccHJ, A. DIN!, F. INNOCENT!, S. ToNARINI and D.S. WESTERMAN 76

Monte Capanne pl uton Complex V and enc losed intrusive po rphyries Complex IV and enclosed intrusive po rphyries Top-to-theeast thrust fault s (Oiigocene-Miocene) Complex Il l Complex 11 Complex I Top-to-the east detachment fault s (CEF: Central Elba fault, late Miocene), (ZF: Zuccale fault, early Pliocene)

East� side down high ang le normalfault (EBF: eastem Borderfault, late Miocene)

Fig. 2 - Tectonic sketch map of Elba Island.

Porto Azzurro pluton � Orano porphyry * Leucogranites Monte Capanne 'i ate felsic dykes Cotoncello dyke fi >Sant'Andrea facies Monte acies 'i ne ��=� ��=���:�i�� J ���f0� San Martino porphyry

Portoferraio porphyry

:--.lonk Capannc pluton

Fig. 3 - Geological map of Elba Island. Elba Island: intrusive magmatism 77 separated from eastern Elba by the low-angle compositions (Fig. 4). It occurs in western Elba Central Elba fault (Fig. 2 and 3) expressed as a within Complex IV as four adjacent, but tectonic mElange of rocks from Complexes IV isolated, caps on a ridge (Fig. 3 ), interpreted as and V (Trevisan, 1950; Bellincioni, 1958; an original sill dismembered by younger Pen·in et al., 1 975), including rocks whose intrusions. The outcrops in central Elba make equivalents crop out in western Elba. Thus, the up a structurally higher tourmaline-rich rocks of central Elba were most likely laccolith layer (Westerman et al., submitted). displaced about 10 km eastward from their Whole rock-muscovite Rb-Sr isochrons (Ms original position by way of movement on the phenocrysts >350 �m selected to avoid Central Elba fault (Westerman et al., secondary sericite) yielded cooling ages of 7.95 ± 0. Ma and 7.91 ± 0.1 Ma for samples from submitted). I Eastern Elba consists of a stack of tectonic western and central Elba, respectively (Dini et complexes. The upper portion of the stack, al., 2002). A 40Ar-39Ar age older than 8.5 Ma including part of the contact aureole of the for late magmatic muscovite from central Elba upper Pliocene Porto Azzurro pluton, was is reported by (Maineri et al., 2003). In minor displaced eastward by 5-6 km along the outcrops of Capo Bianco aplite which Zuccale fault (Keller and Pialli, 1990; Pertusati experienced eastward tectonic translation to et al., 1993) whose activity is geometrically eastern Elba (Pertu s ati et al., 1993), the and kinematically similar to that inferred for original rock is affected by hydrothermal the Central Elba fault. recrystallisation of albite to sericite, a process dated at 6.7 ± 0.1 Ma (Maineri et al., 2003). The isotopic ratios of the Capo Bianco aplite 2.3 THE INTRUSIVE UNITS AND SEQUENCE are, then, corrected to 8 Ma.

Within Complex IV (western Elba) and 2.3.2 Nasuto Microgranite Complex V (central Elba), distinction of the The Nasuto microgranite, with a various intrusive units and their correlation syenogranitic composition (Fig. 4 ), crops out between exposures is based on the consistent over an area of 0.5 km2 al ong the northern textural and mineralogical aspects of each unit shore of western Elba (Fig. 3). It is entirely (Table 1 ), and on consistent crosscutting surrounded and intruded by the younger relationships. Additional support comes from Portoferraio porphyry. mineral chemistry and from major and trace element whole-rock data. All the different rock 2.3.3 Portoferraio porphyry types are classified with the intrusive nomenclature after the Q'-ANOR diagram (Fig. The Portoferraio porphyry has dominantly 4; Streckeisen and Le Maitre, 1979). Indeed, monzogranitic, with minor syenogranitic, despite the occurrence of high-temperature compositions (Fig. 4), with biotite having Fe# phases in some porphyritic dyke s, all the ranging from 0.45 to 0.50 (Fig. 5). These studied units belong to a unique intermediate- to porphyries occur as major laccolith layers up to 700 m thick (Fig. 3), commonly interconnected, shallow-level intrusive complex. and as numerous dykes (Westerman et al., Igneous products in eastern Elba are submitted). discussed here for thoroughness, but have not A Rb-Sr wr-Bt isochron points to an age of been the focus of detailed study by the authors. 8.4±0. 1 Ma, but internal isotopic disequilibrium is suggested by the fact that 2.3.1 Capo Bianco aplite initial 87Srf86Sr ratios for plagioclase and K­ The Capo Bianco aplite is a white feldspar deviate significantly from the isochron porphyritic rock with alkali feldspar granite intercept (Dini, 1997). Thus, the interpretation TABLE -.) 1 00 Summary of petrographic and chronological features of the late Miocene intrusive units frmn Elba Island (nwdified after Dini et al., 2002).

Intrusive unit Rock type Texture Paragenesis Accessories MME Xenoliths Age (Ma)

Monte Castello shoshonite sub-aphyric pheno: Cpx, 01 Mag, Chr no no 5.83±0.14 dyke ghosts xeno: PI, Kfs, Qtz ;:.n gm: Cpx, PI, 0:;o San n n ? � Porto Azzuno monzogranite Qtz±Pl PI, Qtz, Kfs, Bt Zrc no no 5.9±0.2 0 pluton phenocrysts -� medium­ grained :-n equigranular z 0 n matrix rn z Orano granodioritic granodiori te porphyritic pheno: PI, Bt, Ap, Zrc, Aln, common rare 6.83±0.06 � porphyry type to qtz (<20%) very rare Amph, Thr, Mag, Ilm, (l-3cm) 6.87±0.30 ;:.n 0-l monzodiorite fine-grained Cps relics Per z >­ gm xeno: Kfs, Qtz :;o monzogranitic monzogranite porphyritic gm: PI, Mg-Bt, � i:ll type (25-35%) fine- Qtz, Kfs ::::1 0.. grained 0 0 Late felsic Leucogranite syenogranite medium grain Qtz, Kfs, PI, Ap, Zrc, Mnz, no no coeval with :E dykes dykes size locally Bt, Ms ± Crd Tur Monte Monte -l[;5 associated with anisotropic or Grt Capanne rn :;o the Monte pluton � Capanne Contoncello syenogranite very low % PI, Qtz, Kfs, Bt Ap, Zrc, Mnz, common no z pluton dyke megaKfs fine- Ilm (1-3 cm)

------�! �������:______San Piero monzogranite very low % PI, Qtz, Kfs, Bt Ap, Zrc, Mon, low vol% scattered facies to granodiorite megaKfs fine­ Aln, Tur (5 cm- 1 m) (1-5 cm) medium grained equigranular matrix TABLE 1 CONTINUED

Intrusive unit Rock type Texture Paragenesis Accessories MME Xenoliths Age (Ma)

Monte Capanne Sant'Andrea monzogranite -20-30 vol% PI, Qtz (up to Ap, Zrc, Mon, variable, up to scattered 6.9 (see text pluton facies megaKfs 15 mm), Kfs Aln, Ilm Tur 70 vol% (1-5 cm) for details) mediurn -coarse (up to 15 cm), (0.1-lOm) grained Bt (up to 5 mm)

San Francesco monzogranite variable% PI, Qtz, Kfs, Bt Ap, Zrc, Mon, variable vol% scattered facies megaKfs (5-9 Aln, Tur n 11m2 (1-5 cm) cm) medium- (5 cm-5m) gr. equigranular

matrix c::;-� �

San Martino monzogranite porphyritic mega: San (3- Ap, Zrc, Mon common common 7.2±0. 1 � ( 40-50%) fine­ 15%; cm); ±Aln±Tur 1/10 m2 (1-10 cm) § porphyry -s 7.44±0.08 grained gm pheno: Qtz, PI, (2 cm - 2 m) � ::::1' miarolitic Bt; gm: Qtz, ....: cavities Kfs ±PI S. � Portofenaio monzogranite porphyritic pheno: San, Ap, Zrc,Aln, no rare (1-3cm) 8.4±0. 1 OQi5 porphyry (minor (25-50%) fine­ Qtz, PI, Bt; Mon, Thr :::: syenogr.) grained gm gm: Qtz, Kfs � ±PI �·

Nasuto monzogranite porphyritic pheno: Qtz, PI, no no microgranite (25-30%) Kfs, Bt; gm: microgran ular Qtz, Kfs, PI gm

Capo Bianco alkali fe ldspar porphyritic pheno: Qtz, PI, Tur abundant no no 7.91±0. 1 aplite granite trachytoid gm Kfs, Ms; gm: in central Elba; 7.95±0. 1 Ab Xen, Mon, Nb­ > 8.5 Ta oxides

Abbreviations: Ab: albite; Aln; allanite; Ap: apatite; Bt: biotite; Crd: cordierite; Grt: garnet; Ilm: ilmenite; Mag: magnetite; Mon: monazite; Ms: muscovite; Per: perrierite; PI: plagioclase; Qtz: quartz; San: sanidine; Thr: uraniferous thorite; Tur: tourmaline; Xen: xenotime; Zrc: zircon; mega: megacrysts (>2 cm); pheno: --.] phenocrysts (< 2 cm); xeno: xenocrysts; gm: groundmass. \0 80 S. RoccHI, A. DIN!, F. INNOCENT!, S. ToNARINI and D.S. WESTERMAN

0 Capo Bianco aplite 0 Cotoncello dyke V Nasuto microgranite / �matic microgranular enclaves 5 !'!. Portofen·aio porphyry 1/ within ivlonteCapanne pluton, 0 San l'vlartino and Orano porphyry 19 !'!. San l'vlartinoporphyry Orvlonte C:apannepluton + Orano porphyry (granodioritic) 18

17

16

15 20

QMD 0.30 0.40 0.50 0.60 10 � no. 5 0 10 20 30 40 0 60 Fe ANOR Fig. 5 - Al701 vs. Fe# (Fe/(Fe+Mg) atomic ratios) for

- biotites fron1 the Elba intrusive units. Modified after Dini Fig. 4 Q' -ANOR classification diagram (Streckeisen and Le Maitre, 1979). Abbreviations of rock names: afG, alkali et al. (2002). feldspar granite; SG, syenogranite; MG, monzogranite; GD, granodiorite; QM, quartz monzonite; QMD, quartz et al. (2002). monzodiorite. Modified after Dini the whole rock. Nevertheless, this Rb-Sr date is quite close to a sanidine 40 Arf39Ar isochron age of 7.44 ± 0.08 Ma (Dini and Laurenzi, 1999). of the Rb-Sr age is constrained by the field Initial isotop ic ratios are then corrected to 7.4 evidence showing the Portoferraio porphyry as Ma. Isotopic dates are in agreement with field younger than the Capo Bianco ap lite and older observations and indicate that the San Martino than the San Martino porphyry. For the purpose porp hyry was emplaced after a significant of this work, we chose to correct isotopic ratios period of quiescence. for the Portoferraio porp hyry to 8 Ma. Correction to 8.5 Ma would lead to deviations 2.3.5 Monte Capanne pluton in 87SrJS6Sr of 2-3 times the analytical error, The Monte Capanne monzogranitic pluton is and to deviations in I43Ndfl44Nd within the the largest of those exposed in the Tuscan error. Magmatic Province (Marinelli, 1955; Marinelli, 1959; Poli et al., 1989; Bussy, 1990; 2.3.4 San Martino p01phyry Poli, 1992). It is roughly circular in plan (about The San Martino porphyry has 10 km in diameter), and is bordered along two monzogranitic compositions (Fig. 4), and is thirds of its perimeter by contact characterised by prominent K-feldspar metamorphosed rocks mainly after Complex IV megacrysts and biotite with Fe# = 0.53 - 0.58 protoliths (Fig. 3). (Fig. 5), This unit occurs in western Elba as The pluton shows minor variability of its dykes cutting the Capo Bianco ap lite and petrographic features, but internal facies with Portoferraio porp hyry (Fig. 3 ), whereas in diffu se transitions have been identified in the central Elba, it occurs as three main laccolith field (Fig. 3). They include as end members the layers up to 400 m thick and as minor «Sant' Andrea facies» and the «San Piero crosscutting dykes (Westerman et al., facies», separated by a wide zone of «San submitted). Francesco facies» having transitional A Rb-Sr wr-Pl-Bt isochron indicates an age characteristics. The two extreme facies are of 7.2 ± 0.1 Ma (Dini, 1997). However, easily distinguished by their textures: the significant Sr isotopic disequilibrium exists Sant' Andrea facies exhibits high percentages of within K-feldspar, and between megacrysts and coarse phenocrysts (Kfs, Qz, Bt) while the San Elba Island: intrusive magmatism 81

Piero facies appears mostly as a homogeneous, 2.3.6 Late felsic products associated �with the fi ne- to medium-grained rock almost devoid of Monte Capanne pluton early, coarse phenocrysts, while amphibole clots The main pluton is cut by several felsic units, replacing former pyroxene are found. This identified by their field characteristics as the petrographic subdivision of the pluton is Cotoncello dyke, the leucogranite dykes and supported by structural studies (Boccaletti and the aplite-pegmatite veins and dykes. At Punta Papini, 1989) that identified a strong NNE-SSW del Cotoncello (Fig. 3 ), the syenogranitic preferred orientation of minerals in the south­ Cotoncello dyke exhibits a modest K-feldspar eastern region (roughly cmresponding to the San megacryst content and a distinctively finer­ Piero facies ), in contrast with the more inegular grained matrix than the Sant' Andrea facies of fabrics found in the northwestern part of the the main pluton which it cuts. This dyke strikes pluton. This pattern is also mimicked by those roughly parallel to the external contact of the derived from AMS data (Bouillin et al., 1993). pluton over a distance of more than 500 m, The whole intrusive mass is characterised by with a maximum thickness of about 100 m. the widespread occurrence of mafic The leucogranite dykes also have microgranular enclaves of varying amounts and syenogranitic compositions (Fig. 4 ), and they sizes. These enclaves vary in composition from occur mainly close to the pluton' s contact, tonalite to monzogranite and consist principally within both the pluton and its of fi ne-grained plagioclase laths and biotite, thermometamorphic aureole. They commonly with varying quantities of generally rounded have thicknesses up to tens of meters. These and resorbed xenocrysts of quartz and K­ dykes were emplaced late in the crystallisation feldspar. Plagioclase typically exhibits sequence of the Monte Capanne pluton, and are oscillatory zoning (An content up to 45% ), and locally cut by dykes of the Orano porphyry. We amphibole clots replacing former pyroxene are consider their emplacement age as common. Two distinct types of megacrystic K­ indistinguishable from that of the Monte feldspar xenocrysts occur in these enclaves, one Capanne pluton. Aplite-pegmatite veins and with poikilitic margins including matrix dykes occur commonly as thin (0.1 to 2 m) and minerals, and the other with andesine-oligoclase short (up to a few m) masses, in some places borders. Mafic microgranular enclaves are crosscutting the leucogranite dykes. characteristically less abundant in the south­ eastern facies than in the main (normal). 2.3. Orano porphyry Dates for emplacement of the Monte 7 Capanne pluton are quite scattered (Juteau et The Orano porphyry unit is an E-W -trending al., 1984; Ferrara and Tonarini, 1985), with swarm of nearly 100 darkly coloured dykes that Rb-Sr and U-Pb dates between 5.8 and 7.0 Ma. crosscut all the other intrusive units of the None the less, dates obtained for the late, post­ sequence (Dini et al., 2002). They are restricted plutonic Orano porphyry dykes (6.83 6.87 in western Elba to the northwestern portion of Ma, e.g. Section 3.g.) suggest that the most the Monte Capanne pluton and its contact likely emplacement age for the Monte Capanne aureole (approximately 6 dykes/km2); in pluton is close to the highest literature values, central Elba, the Orano dykes crop out only in i.e. 6.8 - 7.0 Ma (Dini et al., 2002). the northern part of the area (Fig. 3). Dyke Additionally, two samples from the San Piero thicknesses range from less than a meter up to a facies display full Sr isotopic equilibrium and maximum of 50 m, and contacts with host rock Rb-Sr wr-Pl-Kfs-Bt cooling ages of 6.88 ± 0.1 are sharp and planar, although commonly with and 6.75 ± 0.07 (lnnocenti et al., 1992). abrupt changes in orientation. Both zoned and Therefore, initial isotopic ratios of samples unzoned dykes are recognised, the former from the Monte Capanne pluton have been being generally thicker and internally corrected to 6.9 Ma. differentiated with abrupt transitions between 82 S. RoccHI, A. DIN!, F. INNOCENT!, S. TONARINI and D.S. WESTERMAN

inner and outer zones. Outer border zones are composition of samples (Conticelli et al., typically a few 1 0' s of centimetres thick and 200lb) is similar to the most acidic portions of are distinguished from the inner zones by (i) the Monte Capanne pluton, and the finer grained groundmass, (ii) lower content of emplacement age is constrained at 5.9 Ma K-feldspar and quartz xenocrysts, mafic (Maineri et al., 2003). microgranular enclaves and xenoliths, and (iii) higher ferromagnesian mineral concentrations. 2.3.9 Monte Castello dyke The inner portions of zoned dykes have A grey-brownish porphyritic dyke is found in monzogranitic compositions (Fig. 4 ), and their eastern Elba (Conticelli et al., 2001b). The rock xenocrysts are petrographically very similar to is quite altered, and the original phenocryst minerals of the Monte Capanne pluton. The borders of zoned dykes are petrographically assemblage was constituted of plagioclase, comparable to the unzoned dykes and have clinopyroxene, olivine and scattered K-feldspar granodiorite to quartz-monzodiorite megacrysts of likely exotic origin. The dyke compositions (Fig. 4). These unzoned Orano had an original shoshonite composition, and an dykes show evidence of early magma mingling emplacement age of 5.8 Ma (Conticelli et al., including (i) three populations of plagioclase 2001 b). Its petrographic and geochemical phenocrysts having strong differences in feature resemble those of the Orano dykes, and composition and texture, (ii) groundmass also the close age relations to the Porto biotite attaining lower Fe# with respect to Azzurro pluton resemble those between the phenocrysts (Fig. 5), (iii) rounded and embayed Orano dyke swarm and the Monte Capanne quartz and Kfs xenocrysts, and (iv) coexisting pluton. Thus, similar pluton-mafi c dykes quartz and olivine plus clinopyroxene history occurred in western Elba at 6.9-6.8 Ma, phenocrysts (generally replaced by tremolite­ an in eastern Elba 1 myr later. actinolite, Cr-clinochlore and Mg­ phyllosilicates ). 2.3.10 Summary The Orano porphyry is the youngest intrusive The igneous sequence of western-central unit in both western and central Elba (Dini et Elba started with construction of a laccolith al., 2002). A western Elba sample, bearing complex, first by emplacement of the Capo compositionally homogeneous biotite, yielded Bianco aplite and Nasuto microgranite, a wr-Bt Rb-Sr internal isochron of 7.06 ± 0.07 followed in succession by the Portoferraio Ma, but minor Sr isotopic disequilibrium exists porphyry and, after a time lag of ea. 1 myr, the between phenocrysts and the whole rock. San Martino porphyry. The deepest layers of However, such disequilibrium is rather this complex were then intruded and/or restricted, and a wr-Pl-Kfs-Bt Rb-Sr internal deformed by the Monte Capanne pluton and its isochron gave an age of 6.87 ± 0.28 Ma associated late leucocratic dykes and veins. (MSWD = 16). This date matches the sanidine Finally, the Orano dyke swarm was emplaced, 40Ar/39Ar isochron age of 6.83 ± 0.06 Ma cutting through the entire succession. Shortly obtained for a central Elba Orano dyke (Dini thereafter, the upper part of the sequence, along and Laurenzi, 1999). Initial isotopic ratios were with its host of Complex V flysch, was corrected to 6.85 Ma, accounting for a cooling tectonically translated eastward so that the age for Orano dykes that is essentially the same lower part is presently found in western Elba as that of the Monte Capanne pluton. while the upper part is in central Elba (Rocchi et al., 2002). 2.3.8 La Serra-Porto Azzurro pluton The emplacement of a pluton-mafic dyke Small outcrops of a monzogranite body are association and tectonic translation of the upper interpreted as evidence for the occunence of a part of the complex took place also in eastern pluton in the Porto Azzurro area. The Elba, 1 myr later. Elba Island: intrusive magmatism 83

2.4 GEOCHEMISTRY AND PETROGENESIS San Martino porphyry have overlapping major element characteristics, with combined Si02 2.4.1 Geochemistry and Al203 values higher than the Monte Capanne pluton and with a steeper Al20/Si02 Major elements. The intrusive units of the slope (Fig. 7). The Nasuto microgranite is the Elba igneous complex display limited intra-unit most peraluminous of the three units (ASI = compositional variations, but significant inter­ 1.29) and has slightly higher alkali content unit geochemical variability (Dini et al., 2002; (Fig. 6). Major element chemistry of the Table 2, Figs. 6, 7). All the intrusive units of monzogranitic Portoferraio porphyry (ASI = the laccolith complex, which were emplaced 1.18 ± 0.07) and San Martino porphyry (ASI = before the Monte Capanne pluton, together 1.22 ± 0. 16, Table 2) overlap in every respect, with the pluton itself and its associated late although Portoferraio rocks are, on average, felsic products, are made up of acidic rocks more alkaline and richer in Si02 and Al20 having Si02 in the range from 66.2 to 75.7 3 wt%. In contrast, the younger Orano porphyry (Figs 5, 6) and have biotites with lower Fe# 5). is more mafic and shows lower Si02 contents (Fig. between 62.8 and 66.9 wt %. The late-plutonic felsic products are The Monte Capanne pluton shows restricted chemically distinct from the Monte Capanne chemical variations (Figs. 6, 7). The whole pluton and from each other (Figs 5, 6). The pluton has monzogranite composition (Si02 peraluminous Cotoncello dyke sample (ASI = between 66 and 70 wt% ), and slightly 1.16) plots close to the silica-rich end of the Monte Capanne field and overlaps the peraluminous character (average ASI = 1.11 ± 0.05 1 sd). Although three facies are recognised compositional fields of the Portoferraio and petrographically, they differ only slightly from San Martino porphyries. In contrast, the each other chemically and thus are reported as leucogranite dykes have the highest Si07 and a single field in Figures 4, 6 and 7. Due to the lowest Al203 contents of the analysed ;ocks relative chemical homogeneity of the pluton from the Elba magmatic complex, and their and to its volumetric importance with respect to Al20/Si02 trend plots as an extension of the all the other intrusive units, descriptions of Monte Capanne pluton field. chemical data will be made in relation to the Trace elements. The trace element Monte Capanne pluton except where otherwise distribution of the Monte Capanne pluton has noted. low variability (Fig. 7) and the distribution of The intrusive units of the pre-pluton laccolith REE is homogeneous throughout the pluton complex are more acidic than the pluton. Even (Dini et al., 2002; Fig. 8). Again, the Capo though the entire intrusive assemblage shows Bianco aplite shows peculiar features. The Be MgO, FeOtot' CaO and total alkalis vs. Si02 and Cs contents of the Capo Bianco aplite are with general negative correlations (Figs. 6, 7), notably high (up to 67 and 131 ppm, the Si02 vs. Al203 diagram discriminates respectively); Rb, Nb and Ta have the highest between the intrusive units on the basis of both concentrations among the Elba igneous rocks, elemental abundances and elemental ratios while Y ranges from the lowest to the highest (Fig. 7). values; Sr is low, and Zr, Ba and Th have the The Capo Bianco aplite, with the highest lowest values for Elba intrusives (Table 2; Fig. combined Si02 and Al203, defines a 6). The flat chondritic REE patterns (LaN/YbN completely separate narrow field with the = 1.3 - 2.2) with a very deep Eu trough

steepest Al20)Si02 slope (Fig. 7). These rocks (Eu/Eu* = 0.04 - 0.14) readily distinguish the also have extremely low Ti02, FeOtot' MgO Capo Bianco aplite from all the other units. and CaO, and a strong peraluminosity (average Worth noting is the anomalously low Nd

ASI = 1.42 ± 0.10, Table 2). The Nasuto content, with NdN/SmN <1. The Nasuto microgranite, the Portoferraio porphyry and microgranite has high Rb relative to rocks of 00 +>--

TABLE 2 Average nwjor elenzent chemical compositions for late Miocene intrusive units from Elba Island

unit Capo Nasuto Porto­ San Monte Monte Orano Orano Porto Monte Bianco fenaio Martino Capanne Capanne normal hybrid Azzurro Castello aplite microgr. porphyry porphyry pluton leucogranites dyke swarm dyke swarm pluton dyke � ;;Cl average stdev PP-706 average stdev average stdn· average stdev average stdev average stdev average stdev average stdev average stdev n0 n , n. samples 6 10 10 23 9 28 8 4 2 :5 ?> Si02 73.07 0.66 69.10 69.92 1.38 68.78 1.41 67.46 0.83 74.38 1.17 65.08 1.49 67.46 1.17 70.35 0.22 50.30 0.71 0 ,3 Ti02 0.02 0.01 0.36 0.29 0.04 0.36 0.03 0.56 0.05 0. 17 0.07 0.60 0.05 0.48 0.05 0.44 0.04 0.79 0.01 :-n Al203 z 16.59 0.57 16.45 15.82 0.62 16.28 0.47 15.89 0.27 14.09 0.43 15.76 0.43 15.8 1 0.35 15.40 0.41 15. 15 0.35 z n0 Fe20 m 3 0.23 0.11 0.40 0.50 0.18 0.52 0.20 0.79 0.20 0.53 0.21 1.08 0.42 0.90 0.34 0.95 0.51 2.28 0.08 z ,:l FeO 0.23 0.08 1.54 1.24 0.23 1.56 0.30 2.35 0.25 0.49 0.29 2.74 0.32 2. 15 0.51 1.67 0.55 4.57 0.41 � >--3 MnO 0 0.05 0.03 0.04 0.03 0.01 0.04 0.01 0.06 0.01 0.03 0.01 0.06 0.01 0.05 0.01 0.04 0.01 0. 13 0.01 z > MgO ;N 0.09 0.07 0.99 0.85 0.17 0.94 0.16 1.4 1 0.29 0.36 0.19 2.62 0.76 1.6 1 0.29 0.92 0.15 6.23 0.14 3; CaO 5 0.22 0.14 0.83 1.57 0.29 2. 11 1.09 2.62 0.24 0.94 0.26 3.15 0.62 2.22 0.36 1.76 0.28 6.63 0.45 0.. 0 Na20 4.24 0.35 3.82 3.60 0.28 3.38 0.30 3.48 0.34 3.20 0.36 3.22 0.22 3.55 0.49 3.24 0.14 2.24 0.53 0 =E K20 4.06 0.10 4.7 1 4.50 0.26 4.23 0.42 4.08 0.23 5.07 0.73 3.65 0.30 4.04 0.66 4.2 1 0.10 3.4 1 0.56 ...,Bl m P20s ;N 0.02 0.01 0.06 0. 10 0.02 0.14 0.02 0.20 0.03 0.06 0.03 0. 19 0.04 0.19 0.03 0. 17 0.04 0. 15 0.01 s:: > LOI z 1.26 0.08 1.69 1.60 0.80 1.99 0.95 1.08 0.26 0.68 0.22 1.75 1.12 1.54 0.51 0.85 0.15 8. 14 0.54 ASI 1.42 0.10 1.29 1.18 0.07 1.22 0.16 1. 10 0.04 1. 15 0.05 1.09 0.09 1. 15 0.08 1.22 0.10 8. 14 0.54

Abbreviation: ASI: Alumina Saturation Index, corrected for the apatite content. Orano normal: thin dikes and border facies of thick dikes. Orano hybrid: inner facies of thick dikes. Porto Azzurro: data from Conticelli et al. (200 I) and unpublished data from the authors. monte Castello dyke: data from Conticelli et al. (200 I). Elba Island: intrusive magmatism 85

0 Capo Bianco aplik D Cotonccllo dvkc The post-plutonic Orano dyke swarm V Kasuto microgranile / 'mttfic m icrogranular cncla\'cs includes two compositional groups, /::,. Portofcrrain porphyry 1-1 \Yilhin l\1out� Capannc pluton, San ll!artino and Orano porphyry monzogranitic samples from the core of thick A San !v!artino porphyry Oil!on le Capannc pluton + Orano porphyry (granodioritic) zoned dykes, and granodioritic samples from 0 'donk Capannc lcucogr. <> Orano porphyry (monzngranitic) borders of those dykes and from unzoned dykes (Table 1; Fig. 6). Compositions of the monzogranitic samples are intermediate between the Monte Capanne pluton and the granodioritic Orano group, and these +' I xenocryst-rich rocks are interpreted as mixtures of Orano melt and solids from the Monte s.o�b�) Capanne crystal mush, formed during --� emplacement of the dykes. The Orano (a) porphyry has the lowest Si02 content of the 60 65 70 ,�75 80· Elba intrusive units, with values similar to Si02 wt% those of mafic microgranular enclaves hosted

Fig. 6-(a) Total alkali vs. silica (TAS) classification in the Monte Capanne pluton, the San Martino diagram (Le Bas et al., 1986). (b) TAS enlargement for porphyry, and the Orano dykes themselves. Orano dykes; tie-lines connect cores (larger symbol) and MgO (Fig. 6), as well as FeOtot and Ti02, rims (smaller symbol) of individual dykes. Modified after Dini et al. (2002). shows strong variation over a rather restricted range of Si02 (62 - 68 wt%). The Orano porphyry has very distinctive overall trace the Portoferraio and San Martino porphyries element distribution. Some Orano porphyry (Fig. 6), but shows an overall REE distribution samples display extreme enrichment in Sr, Ba, similar to these units, albeit with a deeper Eu and LREE, which is oddly coupled with the highest Ni and Cr contents (Table 2; Fig. 6). anomaly (Eu/Eu* = 0.27). The Portoferraio porphyry has uniform LREE distribution Also, REE fractionation reaches extreme values (LaN/YbN up to 59), coupled with (LaN/SmN = 3.5 - 4.4), whereas the HREE fractionation is quite variable as shown by moderate Eu anomalies (Eu/Eu* = 0.53 - 0.72). Despite the high variability of major element GdN/YbN = 2.1 - 3 .9) and high and low Y contents (Fig. 6) which also correlate chemistry of the mafic microgranular enclaves negatively with the Eu/Eu* ratios. The San (Figs. 6, 7), the analysed Orano enclave has Martino porphyry has constant REE REE distribution identical to samples from the distribution (LaN/YbN = 17.4 - 17.8 and Eu/Eu* Monte Capanne pluton, and is very similar to

= 0.51 - 0.52) with a pattern that is that of a San Martino enclave (Fig. 8). indistinguishable from that of the Monte Sr-Nd isotope composition. The Elba Capanne pluton. Among the late felsic facies igneous products display wide variation in Sr associated with the Monte Capanne pluton, the isotopic ratios and bimodal distribution for Nd Cotoncello dyke has a HREE pattern (GdN/YbN isotopic ratios (Dini et al., 2002). The Capo

= 3.7) more fractionated than the Monte Bianco aplites, Nasuto microgranite, Capanne rocks and similar to the low Y-HREE Portoferraio porphyry and Cotoncello dyke samples of the Portoferraio porphyry (Fig. 7). make up a group with low ENct(t) values On the other hand, the leucogranite dykes show between -9.5 and -10.0 and strongly variable Sr considerable overlap with the Capo Bianco isotopic ratios (0.7115 - 0.7228). The Monte aplite for many geochemical parameters, with Capanne pluton and its leucogranite dykes, REE fractionation intermediate between that of along with the San Martino porphyry and all Monte Capanne pluton and the Capo Bianco the mafic microgranular enclaves in the aplite. complex, make up a second group having S. RoccHI, A. DIN!, F. INNOCENT!, S. ToNARINI WESTERMAN 86 and D.S.

19 I'

18 AI203 0 • " " 17 " " " 0 o o • " i'" --- " fA" oo 16 -,,. + 0

' -- '• • • �t• ._ " " 15 " " O o �·· 0

14 0 0 0 0 0

13

78 G2 G6 70 74 78

1600 700 �------, Sr 0 1400 Rb 0 600 0 1200 o 0

500 0 1000 0 0

0 800 400 0 0

GOO 300 lr 0

400

200 200 '-

100

62 GG 70 74 78 f32 GG 70 74 78 ao �------,40 0 0 Nb 25 0 y 0 0 0 0 30 0 0 0 20 0 0 o8 0 0 15 20

" 0 " 10 liYA "" " o" 0 0 10 " 0 + " " + " "" "

G2 6G 70 74 78 G2 G6 70 74 78

100 • �------� 200 Ce Ni + 80

150

GO

62 66 70 74 78

o .-""\mat1c microg:nmular enclaves o 0 Capo Bianco aplite QMonte Capanne pluton ( Si02wt ; /within M nt Capannc plnton, Si02wt% "f a u o microgranite 0 Monte Capannc leucogr. o e N s t San Martino and Orano porphyry Le:. Portofcrraio porphyry 0 Cotonccllo dyke + Orano porphyty (granodioritic) A San Mm1ino porphyry * Orano porphyty (monzogranitic)

Fig. 7-Variation diagrams for western-central Elba intrusive units, after Dini et al. (2002). 100

$ :ac 10 10 _g �(.) e UJ UJ 1 a:

Nasuto microgranite Portoferraio porphyry 0.1 _l_-r--T"---r-·1-·:-...... ,···-·i-----,--� -·r--r-·····T--r-1---·-·T··-·- 0.1 0.1 Sm Eu Gd Tb Dy Ho Er

PP-113 e PP-118 100 100 100 0 PP·1G6 \:1"� B PP-705 !::, $ !::,s: c :::: �0 10 10 10 .r::. -�� � � e �- UJ 1 UJ ;::::. a: �

San Martino porphyry Monte Capanne - Cotoncel/o dyke 0.1 0.1 -L---y----,-,--,--,--,----,---y----,-,..--,---,---,----,----y-- 0.1 -'------c---,-,..--,.---,--.,-----r----y----,--.,---·�-,---,------r----r- La Ge Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Ho Tm Lu �!2. c;;· ::1

100 100 100 (::::�:.. . -�Cl 8-_ c .=:!_-�����-.c.<"'<:::: .:::�,�"'. ' . . · ��- - - ��--:*,:::::_ _g - . 10 13-.l:J.-_____ ,\ �' . . . .. - .. --==� 10 10 � _ - e w UJ - - - - UJ a:

Monte Capanne -leucogranite dykes maficmicrogranular enclaves Orano porphyry . 0.1 ...... L ...... , ...... ,...... r······· ·r········.,.----·r·----r· ·-·..,--·····1--······r--···.,-········r·····r·······-r········T··········· 0.1 0.1 La Ge Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb La Ge Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Fig. 8 - Chondrite-normalised REE patterns (McDonough and Sun, 1995) for Elba intrusives. The shaded field represents the REE compositional field for the Monte Capanne pluton. After Dini et al. (2002). 00 -..1 S. ROCCHI, A. DIN!, F. INNOCENT!, S. TONARINI and D.S. WESTERMAN 88

relatively high c:NctCt) values between -8.1 and These units were emplaced at ea. 7 Ma, and -8.9, and 87SrJ86Sr varying in the range 0.7131 - represent the bulk of the complex with a 3 0.7162. Samples of Orano porphyry constitute a volume on the order of about 170 km . Group 3 third group that displays significant variation of rocks, namely the Orano dykes, constitute the ENct(t) between -7.0 and -10.1, while initial Sr youngest units of the complex and are characterised by strongly variable isotopic isotopic ratios have a relatively restricted compositions, including the lowest Sr and variation (0.7114 0.7138). The Orano highest Nd isotope ratios of the complex. porphyry has the lowest Sr and highest Nd isotopic ratios within the whole Elba intrusive Group 1 - Crustal messengers. All the older complex. Isotopic ratios of mafic microgranular (ea. 8 Ma) pre-plutonic intrusive units, and the enclaves in the San Martino and Orano rocks younger, late-plutonic Cotoncello dyke, have are uniform and independent of the host. compositional features matching those of both Enclaves from the San Martino porphyry have natural and experimental melts derived from lower Sr and higher Nd isotopic ratios with metasedimentary crustal sources. These features respect to their host while enclaves from the are mainly represented by strong peraluminosity Orano porphyry have higher Sr and lower Nd and high Si02 content, coupled with low isotopic ratios than their host. In contrast, most concentrations of ferromagnesian elements of the analysed enclaves overlap those of the (Patina Douce, 1999). In the Capo Bianco Monte Capanne host pluton. Interestingly, some aplite, significant internal geochemical mafic enclaves display unusual low c:NctCt) [- variations exist that can be explained by mineral 9.84

fractionation. The major element compositions would be able to both flatten a typical granite­ of the samples with extremely high Rb/Sr ratios like REE pattern (see Monte Capanne REE can be modelled by removing less than 1 0 wt% pattern of Figure 7) and produce the unusually of a plagioclase-dominated assemblage from the low Nd content relative to Pr and Sm. This is highest-Sr Capo Bianco sample. In the same not, however, a physically viable process, way, the Rb/Sr vs. Sr and Ba trends are unless accessories are hosted in biotite (Bea, accounted for by removing about 20 wt% of the 1996), a phase typically absent in the Capo above assemblage (Fig. 1 0). On the other hand, Bianco aplite. Additionally, examination of the rocks with the lowest Rb/Sr ratio could have Si02 vs. Al203 for the Elba intrusives (Fig. 6) been produced by two alternative processes, shows that the Capo Bianco aplites define an either crystal fractionation from a less evolved isolated high silica high aluminium field. parent magma, or direct melting of a crusta! Given the high aluminium content of feldspars, source. For the first alternative, extreme it is clear that the Capo Bianco aplites did not fractionation of K-feldspar, plagioclase, quartz form by feldspar-dominated fractionation since and biotite would explain the major element any required parent magma would be compositions and the strongly negative Eu unrealistically aluminous. anomaly of these samples. Furthermore, The second alternative, with direct melting of removal of accessory minerals like monazite a crusta! source, is a viable process. Indeed,

35 ,__ Cl) 0 Capo Bianco aplite --30 Monte Capanne pluton .0 a: Monte Capanne leucogr. 25 crustal melts (Harris and lnger, 1992) 20

15

10

5 15

0 10 0 50 100 150 200 250 300 350 400 Bappm '< '-

0 50 100 150 200 250 300 Sr ppm

Fig. 10 - Rb/Sr vs. Ba and Sr diagrams to compare major and trace element compositions of possible crust-derived melts from Elba to calculated compositions (Harris and Inger, 1992). Fractional crystallisation lines for internal variations of Capo Bianco aplite and Monte Capanne microleucogranites are solid and dashed, respectively. Dotted lines shows fractional crystallisation of the average Monte Capanne pluton composition. Removed fractions are as discussed in the text; ticks are at 5% intervals. Modified after Dini et al. (2002). S. RoccHI, A. DINI, F. INNOCENT!, S. TONARINI and D.S. WESTERMAN 90 both major and trace element compositions of Group 2 Hybrids. The Monte Capanne those rocks having the lowest Rb/Sr ratio, pluton is an acidic, voluminous body with Al203 content, and Na20/K20 value (ea. 1), are homogeneous geochemical features. Its low comparable with compositions obtained for silica content, along with a ubiquitous presence experimental melts from muscovite dehydration of abundant mafic microgranular enclaves with melting of pelitic sources at 0.8 GPa and 800°C isotopic ratios equal those of the pluton, (Patifio Douce and Harris, 1998). Moreover, amphibole clots after pyroxene, and plagioclase muscovite dehydration melting can account for with calcic cores, all point to the involvement the deep negative Eu anomaly in a flat REE of more than one magma in the genesis of this pattern with low LREE content. In fact, pluton (Poli, 1992). The youngest pre-plutonic monazite, apatite and zircon are commonly unit, the San Martino porphyry, displays armoured by biotite and, therefore, do not geochemical and petrographic features similar supply trace elements to the liquid, thus to the Monte Capanne pluton, although the producing a Zr-P-LREE-poor melt (Nabelek former is slightly more peraluminous and and Glascock, 1995). Such a melt would have a richer in Si02. Tonalitic mafic microgranular negative Nd anomaly linked to the high enclaves are common in this porphyry and the partition coefficient of Nd in the restitic Sr-Nd isotopic composition of one of them is monazite, relative to its neighbours Ce-Pr and indistinguishable from that of the Monte Sm (Yurimoto et al., 1990). In summary, Capanne pluton. This indicates that hybrid samples of the Capo Bianco aplite are melts were available and mingled with interpreted as a direct product of muscovite peraluminous melts during emplacement of the dehydration melting of a metapelitic source. San Martino magmas. The late leucogranite The N asuto micro granite and Portoferraio dykes of the Monte Capanne pluton show porphyry also belong to the 8 Ma emplacement significant intra-unit geochemical variations stage, and are, like the Capo Bianco aplite, (Figs. 6, 7, 8) that can be explained by strongly peraluminous. Such a strong «crustal» invoking a process of mineral fractionation imprint suggests a direct derivation by crustal (e.g. Poli, 1992). Major element compositions melting, and the significant content of of the samples with extremely high Rb/Sr ratios ferromagnesian elements agrees with a process can be satisfactorily modelled by removing that involved biotite dehydration melting. from the least acidic, highest- Sr samples, 13 Furthermore, the CaO enrichment in these rock, wt% of a solid assemblage made up of relative to ferromagnesian components, is plagioclase (80%; An16), biotite (13%) and K­ consistent with derivation from a biotite­ feldspar (7%). The Rb/Sr vs. Sr and Ba trends plagioclase-rich source such as metagreywacke, are also accounted for by removing up to 25 rather than a simple biotite-rich metapelite wt% of such an assemblage (Fig. 1 0). On the (Patifio Douce, 1999). The involvement of other hand, major element compositions of the biotite together with its accessory minerals (e.g. least evolved leucogranites can be satisfactorily monazite) can explain the more typical, modelled by removal of a biotite-andesine «crusta!» REE pattern of these units. Finally, assemblage from the average composition of the Cotoncello dyke is also definitely Monte Capanne pluton equal to 30 - 35 wt% of peraluminous and, as in the case of the Nasuto that parent. The lack of K-feldspar in this micro granite and Portoferraio porphyry, its fractionating assemblage makes the San Piero distribution of major elements and REE pattern facies the most likely parent for the are consistent with derivation from a biotite­ leucogranite melts since it was only in that plagioclase-rich source. Furthermore, its high Sr facies that K-feldspar entered the liquidus isotopic composition points to a crustal origin assemblage late. Thus, we envisage the and rules out derivation as a fractionation possibility that only low amounts of product of the Monte Capanne pluton. leucogranite melt were effectively separated Elba Island: intrusive magmatism 91

from the residue, and the bulk composition of the San Piero facies was not strongly modified. In conclusion, the leucogranites are interpreted as a series of fractionation products from a magma having characteristics similar to those Capraia K- andesites of the San Piero facies of the Monte Capanne (4 samples) pluton, a hypothesis further supported by the overlapping Sr and Nd isotopic compositions of the Monte Capanne pluton and the leucogranite dykes. Group 3 - Mantle messengers. The Orano

dyke swarm consists of two petrographic types. Cs Ba Th Ta L Pr Nd HI Ti T Y Er Yb The monzogranitic type is rich in xenocrysts Rb U K Nba Ce Sr Z Sm Gdb Dy Ho Tm Lu captured from an acid crystal mush that matches well with the Monte Capanne pluton, so this group is not, therefore, a suitable source of information about original magmas. The granodioritic to quartz monzodioritic type, with only minor xenocrysts, still displays evidence TMP lamproites (7 samples) for melt hybridism. However, these melts did / contain phlogopite, as well as olivine and/or clinopyroxene, as a part of their liquidus assemblage. Additionally, these rocks have high Ni and Cr content coupled, in some samples, with extreme enrichment in Sr, Ba and LREE, and strong REE fractionation. Two samples from the Orano porphyry have been Fig. 11 - N-MORB normalised spidergrams (Sun and McDonough, 1989) of the most incompatible element­ compared with mantle-derived magmas from enriched Orano dyke (2 samples), compared with K-rich the Tuscan Magmatic Province, i.e. the K-rich andesites from Capraia Island (D'Orazio, pers. comm.) and andesites from Capraia Island (e.g. Poli, 1992) lamproites from the Tuscan Magmatic Province (Conticelli et al., 1992). et al. (2002). and lamproites from mainland Tuscany (Fig. Modified after Dini 11). These products all have similar overall distributions of incompatible trace elements, with highly fractionated NMORB normalised coupled with very high Sr and Nd contents patterns, very high contents of the most (1461 and 94 ppm respectively). Because incompatible elements, and high LILE/HFSE crustal materials generally have much lower Sr ratios. However, the Orano dykes most closely and Nd contents, we conclude that these Orano resemble the Capraia K-andesites with which isotopic ratios are very close to the original they share overlapping Th/Ta values (Fig. 11). values of mantle-derived magma. Therefore, In addition, they lack the negative Sr anomaly the anomalous isotopic ratios and trace element which characterises the lamproites. distribution are seen as evidence for an origin Regarding the isotopic data, 87Srf86Sr(t) and of Orano melts from a strongly modified I43Ndfl44Nd(t) values of the Orano porphyry mantle source, as is generally envisaged for the constitute the limits found in Elba intrusive Tuscan Magmatic Province (Serri et al., 1993; rocks, i.e. 0.71145 and 0.51227, respectively, Peccerillo, 1999). with the exception of the San Piero gabbroic Sr and Nd isotope constraints on enclave. These values are quite extreme for petrogenesis. The isotopic compositions of the mantle-derived magmas; however, they are crust-derived Group 1 units are significant in 92 S. RoccHJ, A. DIN!, F. INNOCENT!, S. ToNARINI and D.S. WESTERMAN understanding the nature of the crust involved 1993; Harris and Ayres, 1998), depending on as sources for the Elba magmas. These units the modal proportions of the phases entering have equivalent low ENctCt)values, coupled with the melt (mainly K-feldspar, plagioclase and strongly variable Sr isotope ratios (Fig. 12). mica) and on their Sr concentrations. Two main processes can be invoked to explain The modelling of Bat·bero et al. ( 1995) these isotopic features, namely melting of shows that when plagioclase makes up >30 independent crusta} sources with different 87Sr/ modal% of the phases entering the melt, the 86Sr ratios and uniform ENct(t) values, or melt has a 87Srf86Sr ratio lower than that of the melting of a Sr-Nd isotopically homogeneous protolith, with a difference up to 0.004. This crustal source under conditions able to type of process could be invoked to explain the differentiate the Sr isotopic ratios of the melt. differences in Sr isotopic ratios between the The latter process can occur in the case of Capo Bianco, Nasuto and Portoferraio units incongruent fluid-absent melting. Muscovite ( <0.003), but it is not able to explain the large and/or biotite dehydration melting also difference in 87Srf86Sr ratio (0.011) occurring involves K-feldspar and plagioclase, which between the Capo Bianco and Cotoncello units. have Rb/Sr ratios substantially lower than those On the basis of the above discussion, it is of micas. The lack of a fluid phase can yield possible to conclude that the crusta} sources melts with a 87Srf86Sr ratio either higher or activated at around 8 Ma and 7 Ma were lower than that of the source (Inger and Han·is, different, at least isotopically (Dini et al.,

Group 1 Group 2 0 Capo Bianco aplite A San tdmlino pmphy1y P01tofenaio ]hllphyrv Monte Capannc plnton 0.5126 A 0

UR

0.5125

"0 z 0.5124 '<:!' '<:!' 0.730 ' "0 z 0.5123 (") '<:!' Roccastrada rhyo/ltes

0.5122

Montecristo late porphyritic dykes 0 0.5121

0.5120 .J-,-���-r--r���-.---...��"--r--���---,--1 0.706 0.708 0.710 0.712 0.714 0.716 0.718 0.720 0.722 0.724 S7Srf86Sr (t)

12 Fig. - Initial I43Ndfl44Ncl vs. 87Srf86Sr plot for studied samples and Tuscan Magmatic Province samples from the literature. Source of data: (Juteau et al. , 1984; 1986; Ferrara et al. , 1989; Pinarelli et al., 1989; Westerman et al. , 1993; Innocenti and Tonarini (unpublished data). After Dini et al. (2002). Elba Island: intrusive magmatism 93

2002). A unique crustal source (Tuscan Crust­ enclaves is also attested by the isotopic C1) could have been activated in the initial composition of the micro-gabbroic enclave phases of melting to produce the Capo Bianco, from San Piero. Nasuto and Portoferraio magmas. In contrast, Additional insight into the evolution of the the source for the Cotoncello dyke represents a hybridised rocks of the Monte Capanne pluton separate component (Tuscan Crust-C2) that is and San Martino porphyries comes from isotopically comparable with the crust-derived examination of a 87Srf86Sr vs. 1000/Sr diagram San Vincenzo rhyolites of the Tuscan (Fig. 13a), in which most samples cluster at Sr Magmatic Province (Feldstein et al., 1994; ea. 200 ppm and 87Srf86Sr ea. 0.7145. These Ferrara et al., 1989; Pinarelli et al., 1989). values can, therefore, be regarded as the most Most of the Elba igneous rocks belong to the likely to have developed during the isotopically homogeneous Group 2, thought to hybridisation process as suggested by Poli be primarily generated by a hybridisation (1992). This cluster of hybrid compositions can process. This hybrid system had to be large and be obtained by a mixing process involving the homogeneous in order to sustain production of Cotoncello dyke magma and Capraia K­ such significant amounts of compositionally andesites (close in space and time to the Elba constant magma. The general petrogenetic igneous activity) whose source is here called relationships of the materials involved in this Tuscan Mantle-Ml. Moreover, the Capraia­ process are illustrated in the 1 43Ndf144Nd vs. Cotoncello mixing trend and its end-members 87Srf86Sr diagram of Figure 12, where the Elba are very similar to those inferred to explain San data are plotted together with isotope variation Vincenzo magmatism, where hybridism is well fields of other Tuscan Magmatic Province constrained (Ferrara et al., 1989). rocks. Overall, the available data define two Examination of the plot of 1 43Ndf1 44Nd vs. curvilinear trends pointing on the right toward 1 000/Nd (Fig. 13 b) confirms the above two distinct crustal end-members with high Sr­ interpretation and suggests further insights and low Nd-isotope ratios, and, on the left, concerning the origin of isotopic variations toward a mantle component with relatively low internal to the Orano porphyry whose samples Sr and high Nd isotope ratios. The upper trend plot well off the Capraia-Cotoncello mixing corresponds well with the mixing trend trend. The analysed samples of Orano dykes described for the San Vincenzo volcanics that are distributed along a main linear trend with a relates mafic microgranular enclaves and their few samples shifted horizontally into the field cordierite-bearing rhyolite host, considered as a of hybrid magmas (Capraia-Cotoncello mixing nearly pure crustal melt (Ferrara et al., 1989). trend). The steep trend starts from the LILE­ The lower trend points to a crustal component richest sample previously identified as with a lower Nd isotopic ratio, comparable isotopically similar to the mantle end member with that observed in some Tuscan basement for Orano magmas (from Tuscan Mantle-M2), rocks (outcrops, cores and xenoliths; Fig. 12). and points to a felsic end member with a very Therefore, the occurrence of the two trends low Nd isotopic ratio, even lower than the suggests that more than one crustal component Group 1 end members, thus identifiable as has been involved in Tuscan Magmatic coming from a different crusta} source (Tuscan Province magmatism. Regarding the mantle­ Crust-C3). A possible representative of this derived components involved in the mixing, K­ melt can be found in the late crustal rich andesites and basalts from Capraia leucogranite of Giglio Island (Le Scole (Tonarini, unpubl. data), and mafic intrusion; Westerman et al., 1993). The microgranular enclaves from the San Vincenzo horizontally shifted (Nd depleted) samples are rhyolites seems to be the most likely interpreted as originating from a solid-liquid candidates. The involvement of a mantle­ mixture with Monte Capanne magmas. Thus, derived magma similar to San Vincenzo mafic Orano magma does not represent the mantle 94 S. Roccm, A. DJNI, F. INNOCENT!, S. ToNARINJ and D.S. WESTERMAN

Group 2 0.7210 A San l\lmlinoJlOiphyiy 0 Monte Capmme phtton -...... 0 hlonte Capanne leucl)gt lo.- e ma:fi�: micro�u. enclave� Group 3 c.o(f) • Orano pmphy1y

� 0.7160 Sisco 1'-­(f) /amproite c:o 0 0 �'':" . • 0 0.71 10

(a) +'���-�-�� -�-��-�-+'---�-��-����--' o. 7060 o 5 � o 50 1 oo

scale� change

0.5126 .------,I

0.5125 s 0.5125 f 0.5124

'0-o .5123 0.51 24 f 0.5122 0.5121 -o Coloucello- Capraia z 0.5123 lrnzd -.::!" 0.70 0.712 0.716 0.720 0.724 -.::!" 8 T"" 87 Sr/86Sr (t) ...... _ -o z 0.5122 Cl) -.::!"

0 0

0.5121 TMP1:;0 /amproites (b)

0·5120 +--�-�-��-.,so.-�-��-�-1or-o---�---.-�-�o-o �. ��. �1·-'ooo 0 5 � 1 000/Nd scale change

Fig. 13 - (a) 87Srf86Sr vs. 1000/Sr diagram showing possible crusta] and mantle-derived components, along with possible mixing trajectories able to produce the cluster of hybrid samples with Sr ea. 200 ppm and 87Srf86Sr ea. 0.7 145. Note that a mixing plus fractional crystallisation process with a petrologically likely bulk Dsr close to 1, would lead to the same path as pure mixing. (b) I43Nd/144Nd vs. 1 000/Nd diagrams showing possible crusta! and mantle-derived components, along with possible mixing trajectories able to produce the cluster of hybrid samples. Nd content for Giglio Le Scale extrapolated from La and Ce contents (Westerman et al. , 1993). In the inset, ellipses labelled C and M represent crusta! and mantle component discussed in the text. C I: crusta! source for Capo Bianco aplite. C2: main crusta! end-member for hybridism processes in the Tuscan Magmatic Province (exemplified by Cotoncello dyke and some San Vincenzo rhyolites). C3: low 143Ndfl44Nd crusta! source, responsible for deep hybridism of Orano primary melt (exemplified by the Giglio-Le Scale intrusion). M 1: main mantle end-member for hybridism processes in the Tuscan Magmatic Province (exemplified by the Capraia K-andesites and the mafic microgranular enclaves from San Vincenzo rhyolites). M2: mantle source for primary Orano magma. M3: mantle source for the lamproites from Tuscan Magmatic Province, not involved in the genesis of Elba magmas. After Dini et al. (2002). Elba Island: intrusive magmatism 95

component of the Group 2 hybridisation microgranite and Portoferraio porphyry likely process, and its internal variations are instead originated via biotite dehydration melting of a linked to shallow level, more or less thorough metagreywacke source. The earliest magmas mingling with a granitic crystal mush, probably were produced without any chemical during emplacement of the dykes as they contribution of mantle melts, with melting passed through the Monte Capanne pluton. In probably linked to late Miocene lithospheric summary, the S7Srf86Sr vs. 1 000/Sr and thinning and decompression, following earlier 1 43Ndfl44Nd vs. 1 000/Nd diagrams illustrate Oligocene-Miocene orogenic overthickening. that the most voluminous Group 2 rocks After a period of quiescence, the represent a hybridisation trend between Capraia volumetrically important phase of hybrid (from M1) and Cotoncello (from C2) end­ magmatism began, involving mantle-derived member magmas. Group 3 Orano rocks mafic magmas and peraluminous crusta! melts. represent a mix between a different mantle­ Mantle magma involved in this phase is never derived magma (from M2) having directly represented, being present most geochemical-isotopic characteristics prominently as hybrid products preserved in intermediate between Capraia K-andesites and mafic microgranular enclaves. This mantle Tuscan Magmatic Province lamproites, and a magma is thought to be similar to the nearby different crus tal magma (from C3; low coeval Capraia K-andesites and has been 1 43Ndf144Nd isotopic ratio), with additional identified as M1. The crustal component of the varieties derived from mingling with Group 2 hybridisation (C2) was derived from a different magmas. crusta! source than the peraluminous melts of The possible involvement of a magma the first phase, and is represented by the having lamproitic affinities (i.e. high Rb/Sr) is Cotoncello dyke. The first voluminous also suggested by the trace element zoning of intrusion of the hybrid group, the San Martino K-feldspar megacrysts, which indicate that porphyry, was produced by incipient melt such a magma could have contributed to the hybridism. Next came the Monte Capanne genesis of the Monte Capanne pluton pluton from a fully mature hybrid system, with (Gagnevin et al., 2002). Mafic microgranular internal facies representing emplacement enclaves displaying unusual £Nd(t) could be the pulses, and leucogranite dykes derived by result of such interaction with lamproitic fractional crystallisation. Finally, Orano magmas. In the same framework, the Group 1 magmas were generated from strongly Capo Bianco aplite, Nasuto microgranite and modified mantle (M2), as products distinctly Portoferraio porphyry are crusta! melts (from different than those involved in the earlier main C 1) that did not contribute to the main hybridisation process. During their ascent, they hybridisation processes (Dini et al., 2002). were first variably hybridised by mixing with a unique crusta! material (C3), and then further 2.4.3 Sununary modified by capturing material from the Monte The general igneous framework of western­ Capanne system. central Elba consists of an acidic laccolith The magma formation processes recorded complex intruded by a monzogranitic pluton between ea. 8 and 6.8 Ma by the Elba and a slightly younger swarm of more mafic magmatism, changed from crust-, to hybrid-, to dykes. Figure 14 schematically summarises the mantle-dominated, as the Apennine fold belt sequence of events that produced the Elba was progressively thinned, heated and intruded igneous complex (Dini et al., 2002). Capo by mafic magmas during late Miocene time. Bianco aplite melts were produced first, Very unusual melts emplaced at the beginning apparently by muscovite dehydration melting and at the end of the igneous activity were not of a metapelitic source, identified as C1. As the volumetrically significant and did not anatectic process continued, the Nasuto contribute to the generation of main hybrid 96 S. ROCCHI, A. DIN!, F. INNOCENT!, S. TONARINI and D.S. WESTERMAN

0.4 low 28 volume (km3) 24 -200 low verylow low no no no mafic enclaves common common. variable % no small common 1.42 1.29 1.18 average ASI 1.22 1.11 1.15 1.16 0.94 (min) 0.71 1 5·0.7133 0.7146 0.7144 0.71 62 0.7145 . 0.7147 0.71 45 0.7228 0.71 15 (min) 87Sr/86Sr(t) -9.6 -9.9 -10.0 -8.8 ·8.1 . ·8.8 ·8.6 -9.7 -7.0 max) I>Nd(t) .. (

inferred high-K andesite Sr i.r.=0.708

8 Ma magmatic ph ase 7 Ma magmatic ph ase

Fig. 14 - Schematic model for the genesis of Elba magmatic products throughout the time. Abbreviations: MME: mafic microgranular enclaves; i. r. : initial isotopic ratio. Modified after Dini et al. (2002).

magmas. They do, however, emphasise the geometry is typical of a multilayered laccolith highly variable nature of crusta! and mantle complex, constituting an outstanding example sources that can be involved, during a short of shallow-level nested Christmas-tree time span, in post-collisional, extension laccoliths (Corry, 1988; Figs. 3, 16, 17). magmatism. These relationships allow determination of the geometric parameters for all the layers (Rocchi et al., 2002; Table 3). Maximum 2.5 EMPLACEMENT OF MAGMA thickness (T) of the nine most significant intrusive layers, along with thicknesses of the 2.5.1 The nested Christmas-tree laccolith intervening and overlying host rock (Thr), were complex estimated by measurement in cross sections Shape and dimension. The main bodies of (e.g., Figs. 16 and 17). T values of individual Capo Bianco aplite, Portoferraio porphyry, and layers vary over an order of magnitude, from San Martino porphyry have laccolithic shapes 50 to 700 m (Rocchi et al., 2002; Table 3). (Rocchi et al., 2002): (1) contacts of intrusions Intrusion floors were assumed to be roughly share the strike and dip of host flysch bedding circular on the basis of the observation that (Fig. 15) or tectonic surfaces in the nappe diameter differences in a single laccolith are sequence, (2) the sheets clearly taper out at generally less than two times (Corry, 1988). their visible eastern ends (Fig. 15), (3) detailed The diameter (L) for each layer was taken as mapping and cross sections show that the nine the maximum length of the body as measured main layers have convex-upward roofs and flat on the geological map (Fig. 3). In central Elba, or convex-upward floors. the determined axes strike N-S with lengths between 2.4 and 0 km, whereas in western The layers of each unit are connected by I small dykes, and major dykes below the Elba, they have NE strikes with lengths overlying sheets are interpreted as feeders (Fig. between 1.6 and 9.3 km (Table 3). The latter 3, north of Marciana: SMF). The overall was estimated by reconstruction of the Elba Island: intrusive magmatism 97

Fig. 15 - View from the south of the WNW-ESE oriented cliff bounding the north of the Gulf of Marina di Campo. SM3 and P4 refer to the laccolith layers as reported in Table 3 and Figure 16.

geometry of layer P3 prior to deformation and and to lift the overburden and (2) magma truncation by the Monte Capanne pluton, batches with the same composition reached whose intrusion produced mylonitic foliation depths as different as 1 km. Furthermore, the along its northeast and southwest contacts (Fig. occurrence of intrusive layers along surfaces of 3). The nine main Elba laccolith layers show strength anisotropy points out that crustal diameters varying by nearly an order of magma traps (Hogan et al., 1998) played a magnitude, and they all have large aspect ratios prominent role in controlling the emplacement (LIT varies from 12 to 33). Volumes were level of magma at Elba (Rocchi et al., 2002). calculated by approximating the shape of each Laccolith grmvth. On the basis of geological laccolith layer to a spherical cap with height and geophysical reconstruction of the shapes of equal to T and diameter equal to L. The 125 laccoliths, an empirical power-law resulting values vary over more than two orders relationship linking width (L) and thickness (T) of magnitude, from 0.1 to 24 km3 (Rocchi et of these intrusive bodies has been proposed al., 2002; Table 3). (McCaffrey and Petford, 1997). The

Emplacement depth and structural level. By relationship has the form L = kTa (where k is a measuring the overburden from cross sections constant and a is the slope of a regression line (plus 800 m, based on an estimate of the in a Log-Log plot), suggesting a scale­ present mean erosion rate for Italy of 0.1 independent mechanism of growth. A similar mm/yr), the emplacement depths of Elba scale-invariant distribution of tabular shapes laccolith layers have been calculated to range has been documented for 66 plutons (Cruden from 1.9 to 3.7 km (Rocchi et al., 2002; Table and McCaffrey, 2001). The mechanism 3). The laccolith layers were emplaced along generally acknowledged as most likely for strong crusta} heterogeneities such as thrust laccolith growth is a two-stage process. First, surfaces between tectonic complexes, the magma spreads laterally at the secondary thrusts inside Complex IV, and emplacement level with a < 1 (the stage of strong contrasts of strength between arenaceous horizontal elongation) until an initial sill is and argillaceous sequences within the flysch of formed having a very high aspect ratio (L >> Complex V. The switch from vertical to T), and a width nearly equal that of the future horizontal magma movement was not intrusion. Then the thin intrusion thickens by exclusively controlled by reaching the neutral dominantly upward inflation and roof lifting buoyancy level because ( 1) the magma at its along a growth line with a > 1. Experiments on emplacement level had sufficient residual vertical growth due to floor subsidence suggest driving pressure to exceed the vertical stress that tabular plutons make room by floor 98 S. RoccHI, A. DIN!, F. INNOCENT!, S. ToNARINI and D.S. WESTER!v!AN

TABLE 3 Dimensional parameters of intrusive units and host rock of the laccolith complex of western-central Elba (after Rocchi et al., 2002) .

Unit Layer Label Host rock Intrusive rock layers Depth Thr T L V LIT (m) (m) (km) (km3) (m)

erosion loss 800 Central Elba flysch (Complex V) 1100

San Martino porphyry layer 3 SM3 700 8.3 19.1 12 1.9 flysch (Complex V) 50

San Martino porphyry layer 2 SM2 100 2.4 0.23 24 2.0 flysch (Complex V) 250

San Martino porphyry layer 1 SMl 200 5.0 1.97 25 2.2 flysch (Complex V) 400

Portoferraio porphyry layer P4 400 10.0 15.7 25 2.6 4 Capo Bianco aplite layer 2 CB2 �120 3.5 0.58 29 2.6 Western Elba hornfels (Complex IV) 400

Portoferraio porphyry layer 3 P3 700 9.3 23.9 13 3.1 Capo Bianco aplite layer 1 CB I 50 1.6 0.05 32 3.0 ophiolite (Complex IV) 280

Portoferraio porphyry layer 2 P2 75 �1.9 0.11 25 3.3 ophiolite (Complex IV) 210

Portoferraio porphyry layer 1 PI 75 �2.5 0.18 33 3.7 Totals 2690 2420 62

Label: label reported on each layer on the geological map (Fig. 1 C). Thr = thickness of intervening host rock layers, as determined from cross sections. T = thickness of individual laccolith layers, as determined from cross sections. L = length of individual layers, as measured on the geological map. V = volume of the individual layer, calculated by assuming a laccolith shape equal to the shape of a spherical cap, with maximum thickness = T and diameter = L: V=pT(3 (L/2)2+T2)16. LIT = aspect ratio. Depth: emplacement depth.

Fig. 16 - Geological section across central Elba Island. Labels of intrusive layers as in Table 3. Modified after Rocchi et al. (2002). Elba Island: intrusive magmatism 99

Orano dike swarm (ODS) 11 San Martino laccolith (SML) Portoferraio laccolith (PFL) D Capo Bianco laccolith (CBL)

complex IV

Fig. 17 Schematic reconstruction of the western Elba Christmas-tree Iaccolith complex, before tectonic-gravitational decapitation.

depression and grow from an initial thin sill of a vertical inflation stage in laccolith growth along a line with slope of ea. 6.0 (Cruden and (Rocchi et al., 2002). McCaffrey, 2001). Finally, the dimensional parameters for each The dimensional parameters collected for Christmas-tree laccolith as a single unit can be the Elba intrusive layers allow testing of these calculated. The basal diameter has been hypotheses. A significant power-law assumed equal to (1) the average diameter or correlation exists between thickness (T) and (2) the maximum diameter of the layers diameter (L): on a Log-Log plot of T versus L, composing it. In case ( 1 ) , the resulting 2 this correlation results in a linear fit (r = 0.93) parameters for the Portoferraio and San with the equation T (0.026 ± Martino laccoliths plot on the regression line 0.006)UI .36±0. 14) (Rocchi et al., 2002; Fig. 18). for pluton dimensions of Cruden and Owing to the strong coherence of the Elba data McCaffrey (2001), as do the parameters of the set, this scaling curve could replace that of Monte Capanne pluton (Fig. 18). In case (2), McCaffrey and Petford ( 1997) to approximate the dimensions of Portoferraio and San Martino the general power-law for laccolith laccoliths plot on the regression line for dimensions. However, each intrusive layer is laccolith dimensions (Rocchi et al., 2002; Fig. thought to represent only part of a complete 18). These results indicate that in the case of laccolith, and the dimensional parameters of coalescence of the layers to form three single­ all the layers plot in the horizontal elongation layer intrusions, the latter would have field even though the slope (a > 1) of the dimensions typical of laccoliths or plutons, regression line is typical of vertical self-affine suggesting that the growth of laccoliths and inflation (i.e. intrusions have the same type of plutons goes on through the amalgamation of shape at different scales, but L and T do not smaller sheet-like bodies. Additionally, it is retain the same proportions as the intrusions likely that the multilayer laccoliths at Elba grow; McCaffrey and Petford, 1997). These failed coalescence due to an abundance of observations led us to interpret the collected lithologic and/or tectonic discontinuities that Elba data as the first record of the occurrence acted as magma traps (Rocchi et al., 2002). 100 S. RoccHI, A. DIN!, F. INNOCENT!, S. ToNARINI and D.S. WESTERMAN

Capanne pluton is on the order of 80 km, about separate amalgamated amalgamated layers layers, layers, 4 times that of the San Martino laccolith. average L maximum L Monte Capanne pluton ·{( Sufficient lithological variation of protoliths in an Martin a laccolith • Ill • 1 O.O js the aureole preserves the reactions (i) f i Portoferraio laccolith 0 0 <) andalusite = sillimanite, (ii) talc forsterite = Jcapo Bianco laccolith 0 o <) + anthophyllite H 0, and (iii) the breakdown of + 2 T= 0.60 L060 muscovite quartz (Thompson, 1974; Spear fit lines for piu tons + 1.00 / and Cheney, 1989; Tracy and Frost, 1991 ). T= Loso Although these reactions depend on the compositions of fl uids and solid solution phases, taken together they suggest peak contact metamorphic conditions with temperatures in excess of 600°C at a pressure of 0.1 - 0.2 GPa (Dini et al., 2002). This environment is consistent with palinspastic 0.01 +----�---�--� �----����-�·-�·-.-,--��·�..-.-.-l reconstructions that provide an emplacement 0. 1 100 depth of 4.5 km (Rocchi et al., 2002).

Fig. 18 - Log-Log plot of T vs. L, where T = thickness of Faults are mapped along parts of the western individual laccolith layers and L = diameter of individual and southern contacts of the pluton (Barberi et laccolith layers (Table 3). The regression lines for al, 1967b ). Close to the western and northern laccoliths' dimensional parameters (McCaffrey and Petford, 1997), for plutons (McCaffrey and Petford, 1997; contacts, the Portoferraio porphyry exhibits a Cruden and McCaffrey, 2001), and for Elba laccoliths strong mylonitic foliation in the groundmass, (Rocchi et al. , 2002) are shown. Grey and white areas with quartz phenocrysts having subgrain represent vertical-inflation and horizontal-elongation fields of McCaffrey and Petford (1997), respectively. The boundaries, plagioclase cracks cemented by inferred dimensions of Monte Capanne pluton are shown. micrographic quartz plus K-feldspar, and Dimensions for each Christmas-tree laccolith as a whole biotite as oriented polycrystalline aggregates. are calculated as explained in the text. Light grey arrows depict the two-stage development of tabular intrusions Thermal metamorphism along all these faulted reconstructed for Elba laccolith layers, with growth during contacts is strong, so movement was the second stage along a slope a = 1.36. Modified after significantly less than the thickness of the Rocchi et al. (2002). aureole. Leucogranite dykes and aplite-pegmatite veins and dykes occur throughout the Monte The filling time of a pluton like that formed Capanne pluton, although somewhat more by the amalgamation of San Martino laccolith abundantly in the south-eastern half where they layers ( �22 km3) can be estimated from the have a strong preferred orientation trending dimensions of its main feeding dyke ( � 1500 m NNE (Boccaletti and Papini, 1989). Elsewhere long x 10-20 m thick, Fig. 1C, north of they are either randomly oriented or trend Marciana: SMF). A magma-ascent rate as low generally perpendicular to the contact. These

as 3 x 10-3 ms-I would result in filling on a patterns correspond with those reported for time scale of <102 yr (Rocchi et al., 2002). magnetic anisotropy studies which has led to an interpretation that emplacement occurred in an 2.5.2 Monte Capanne pluton and younger ex tensional setting (Bouillin, et al., 1993). This dykes pattern of dykes suggests extension in the ESE direction which, along with other evidence, has With an exposed diameter of 10 km and an led to speculation (Jolivet et al., 1994) that pre­ estimated thickness of 2 km on the basis of to syn-plutonic detachment faulting «opened preliminary magnetic modelling (0. Faggioni, the door» for the pluton to rise. However, the pers. comm.), the total volume of the Monte presence of Monte Capanne hornfels as clasts Elba Island: intrusive magmatism 101

in the fault breccia of the Central Elba fault, many cases, halted the supply of magma which located some 10 km off to the east of the pluton then filled another layer. Laccoliths from Elba (see below), as well as clear evidence that can be envisaged as sheet-like intrusions that unroofing of the pluton occurred following did not coalesce to form single laccoliths or emplacement of the younger Orano dyke plutons with dimensions typically observed swarm, argue for post-emplacement elsewhere. This in turn suggests that laccoliths detachment. and plutons, at least in some cases, grow by About 100 Orano dykes have been mapped, amalgamation of smaller sheet-like bodies all restricted to the northern portions of western (Rocchi et al., 2002). and central Elba and exhibiting a strong E-W orientation (Westerman et al, submitted). They occur up to 50 m thick and 2.5 km long, as well 2.6 TECTONIC EVOLUTION as throughout the section from the lowest (SYN- TO POST-INTRUSIVE) exposed levels in western Elba up to the top of the exposed section in central Elba, a total Large-scale faults subdivide Elba Island into section of several km. Orano magmas three main zones: western, central and eastern interacted with the Monte Capanne magmatic Elba (Fig. 2), and these faults are the key to the system from which it captured xenocrysts and reconstruction of the original emplacement enclaves, however the dykes sometimes clearly geometry of the intrusive complexes. All the exploited brittle joint structures, testifying to intrusive units now located in western and the «solid» character of their host. central Elba were emplaced within the tectonic Complexes IV and V, which were piled up 2.5.3 Eastern Elba magmatisnz above the present western Elba (Fig. 19a; Westerman et al., submitted). Then, shortly In comparison to the Monte Capanne pluton, after the intrusion sequence was completed, the much less is known about the emplacement of upper part of the igneous-sedimentary complex the Porto Azzurro pluton due to its very limited was tectonically translated eastward along the exposure. Recent mapping shows outcrops in a Central Elba Fault (Fig. 19b-c ), so that the general E-W trending belt about 2 km long, but lower part is presently found in western Elba data from boreholes (Bortolotti et al, 2001 ). while the upper part is in central Elba (Fig. Widespread distribution of contact 19d). The fault gently dips to the west, so that metamorphic effects attributed to this pluton the stratigraphically highest part of the central indicate a broader extent at depth , with Elba section occurs at the western edge, where emplacement conditions producing peak it is cut by the high angle brittle Eastern Border metamorphism at 600-650°C and about 2 kbar. fault. This reconstruction of tectonic dismemberment is based on the following 2.5.4 Summary evidence: (i) the igneous layers and their host Favourable tectonic conditions at Elba Island rocks of central Elba are a complex bounded at have allowed a detailed study of a 5-km thick its base by a low angle fault, the Central Elba crusta! section including nine intrusive layers fault, (ii) the footwall melange of the Central of late Miocene age that built up three Elba fault (Trevisan, 1950; Perrin, 197 5) multilayer Christmas-tree laccoliths. The contains fragments of rocks typical of western dimensional parameters of the intrusive layers Elba outcrops, such as thermally fit a power-law distribution indicating that, metamorphosed serpentinite and basalt after a likely first stage of horizontal (Marinelli, 1955), garnet- and wollastonite­ expansion, the layers underwent a second stage bearing marble from the Monte Capanne of dominantly vertical inflation. The contact aureole (Vom Rath, 1870), tourmaline­ abundantly available crustal magma traps, in free aplite porphyry equivalent to Capo Bianco 102 S. RoccHI, A. DIN!, F. INNOCENT!, S. TONARINI and D.S. WESTERMAN

a) 1km Ma �l km A6.8 - 7.0

/ / CEF

1 km b) )1 km A 6 Ma

Porto Azzurro Piu ton

c) 1 km }1 km A 6.0 -3.5 Ma

1 km d) �! km Present

Monte Capanne Piu ton Piuton

Orano porphyry ...------, Monte Capanne pluton complex V !�! San Martino porphyry ,______. � Portoferraio porphyry � c=J complex IV Capo Bianco aplite

Fig. 19- Tectonic-gravitational dismemberment of the western Elba intrusive complex (after Westerman et al., submitted). Elba Island: intrusive magmatism 103 aplite from western outcrops, and K-feldspar Capo Bianco aplite and Portoferraio porphyry phenocryst-bearing porphyry; (iii) petrographic layers were concentrated just above the Central and geochemical features of the intrusive units Elba Fault, and the most logical mechanism to cropping out in central Elba fully match those expose them without exposing the overlying of intrusive units cropping out in western Elba; San Martino porphyry units was by raising the (iv) in both western and central Elba the Orano Central Elba fault surface as Complex V was dykes crop out only in the northern area and translated eastward. The rate of displacement is have strong preferred E-W orientation. constrained by the time between beginning of After the eastward translation on the Central the main movement along Central Elba fault Elba fault, a «west side up» movement (6.8 Ma) and the time cobbles were produced occurred along the Eastern Border fault with a and deposited (late Messinian), that is about 2 throw of 2 to 3 km. The Eastern Border fault myr. Allowing for erosion and transport, a roughly parallels the east side of the Monte maximum estimate for the time available for Capanne pluton, truncating its contact aureole. movement on Central Elba fault could be about The fault has wide variation in attitude, with 1.5 myr. Thus the eastward translation of about overall NNE-SSW strike and moderate to steep 7-8 km occurred at an average rate of about 5-6 eastward dip (Figs. 2, 3). For the most part, the mm/yr, a movement rate consistent with the fault is marked by a distinct plane separating a typical rates of tectonic movements associated western footwall breccia of hornfelsed with magma emplacement. Complex IV rocks ( ophiolitic material and A similar sequence of events occurred also deep marine cover rocks) plus fragments of the for the tectonic evolution and pluton Monte Capanne pluton, from an eastern exhumation in eastern Elba (Keller and Pialli, hanging wall breccia made of Complex V 1990; Pertusati et al., 1993 ). The common flysch and megacrystic San Martino porphyry. history for both regions began initially with The amount of displacement along the dominantly, sub-horizontal movement on a Central Elba fault is constrained by the distance low-angle detachment fault with top-to-the east from leading edge of the fault to the pluton' s sense of shear (Central Elba and Zuccale aureole of about 7 to 8 km. The timing of faults), translated the overlying rocks eastward displacement is constrained by the occurrence while trimming out part of the contact aureole in the footwall melange of the Central Elba of the pluton with which it is associated. Then, fault of fragments of the thermometamorphic high-angle structures were activated at the carapace of the Monte Capanne pluton: the eastern edge of the pluton, i.e. the Eastern main displacement occurred after the Border fault east of the Monte Capanne pluton, emplacement of Monte Capanne pluton and and off-shore faults east of the Porto Azzurro development of its thermometamorphic pluton. In this picture, pluton emplacement aureole, i.e. after 6.8 Ma. However, minor low­ occurred before the main faulting, and angle normal faulting on the Central Elba fault promoted its activation. This inference occurred during the emplacement of the oldest challenges the hypotheses of pull-apart opening (8 Ma) units, because close to the fault, Capo (Bouillin et al., 1993) or top-to-the-east shear Bianco aplite and Portoferraio porphyry above the pluton (Joli vet et al., 1998; Daniel fragments from western Elba show evidence of and J olivet, 199 5; Rossetti et al., 2000) syn-intrusive deformation. The timing in this yielding room to the magma and controlling scenario is further supported by the occurrence the level of emplacement of the Monte of abundant cobbles and boulders of tourmaline Capanne and Porto Azzurro plutons. clots-bearing Capo Bianco aplite and The eastward displacement of the upper part Portoferraio porphyry in late Messinian of the complex is at least partly linked to conglomerates on mainland Italy some 50 km gravitational instability. Indeed, in a time span to the east (Marinelli et al., 1993). Indeed, of about 1 myr, a 2700 m thick 104 S. RoccHI, A. DIN!, F. INNOCENT!, S. TONARINI and D.S. WESTERMAN tectonostratigraphic section was inflated by brittle regime, truncating the Central Elba fault addition of a total of 2400 m of laccolithic which has since been eroded in western Elba intrusions, leading to a total thickness for the and lies almost completely buried below new section of about 5000 m. A dome with a central Elba. 10 km diameter and a height of 2.5 km, was produced with a surface slope of about 25° This paper is based on the following works: (assuming originally flat surface). The A. DINI, F. INNOCENT!, S. ROCCHI, S. TONARINI and emplacement of Monte Capanne pluton led to D.S. WESTERMAN - The magmatic evolution of the the oversteepening of the dome and triggered laccolith-pluton-dyke complex of the Elba Island, the main eastward displacement of the upper Italy, in Geological Magazine 139, 257-279, 2002. S. RoccHI, D.S. WESTERMAN, A. DIN!, F. INNOCENT! section. Once significant movement began, and S. TONARINI - Tw o-stage laccolith growth at transfer of the load from above Monte Capanne Elba Island (Italy), in Geology 30, 983-986, 2002. towards central Elba promoted movement on D.S. WESTERMAN, A. DIN!, F. INNOCENT! and S. the east-dipping Eastern Border fault as the RoccHI - Rise and fa ll of a nested Christmas-tree unloaded pluton rose and the thickened central laccolith complex, Elba Island, Italy, submitted for Elba section subsided. Final movement on the publication in Geological Society Special Eastern Border fault took place entirely in the Publication, N. Petford editor.