Alkali metasomatism as a process for trondhjemite genesis: evidence from Aspromonte Unit, north-eastern Peloritani, Sicily

Mineralogy and Petrology (2005) 84: 19–45DOI 10.1007/s00710-005-0073-9

Alkali metasomatism as a processfor trondhjemite genesis: evidencefrom Aspromonte Unit, north-easternPeloritani, Sicily

P. Fiannacca1, P. Brotzu2, R. Cirrincione3,P. Mazzoleni1, and A. Pezzino1

1 Dipartimento di Scienze Geologiche, Universit�aa di Catania, Catania, Italy2 Dipartimento di Scienze della Terra, Universit�aa Federico II, Napoli, Italy3 Dipartimento di Scienze della Terra, Universit�aa della Calabria, Cosenza, Italy

Received April 26, 2004; revised version accepted December 20, 2004Published online March 1, 2005; # Springer-Verlag 2005Editorial handling: G. Hoinkes

Summary

Rocks of trondhjemitic composition are widespread in the North-Eastern PeloritaniBelt within the Aspromonte Unit, a Hercynian medium- to high-grade metamorphiccomplex intruded by late-Hercynian peraluminous granites and later affected byMP=LT Alpine metamorphism. Among these trondhjemitic bodies, the Pizzo Bottinotrondhjemites form one of the largest, outcropping over about 6 km2 and up to 400 mthick. These rocks display concordant to discordant relationships with associated meta-morphic rocks and are often cut by late-Hercynian leucogranitic dykes. The field, petro-graphic and geochemical features of these trondhjemites are consistent with an igneousorigin. Petrographic and geochemical evidences suggest that the trondhjemitic charac-ter of the Pizzo Bottino rocks is due to an alkali metasomatism process involvingcationic exchange of Na and Ca for K and consequent replacement of K-feldspar byoligoclase in the original granitoids. The major and trace element contents of thePizzo Bottino trondhjemites are in fact comparable to those of the peraluminous late-Hercynian granitoids from the southern Calabrian-Peloritani Arc (CPA), when the ele-ments directly involved in the alkali metasomatism process (Na, Ca, K, Sr, Ba, Rb) arenot considered. The behaviour of REE elements, plus Th and U, also seems to bepartially controlled by metasomatic processes, because their abundances vary withthe K=Na ratio. Metasomatism seems to be the only viable mechanism involved inthe genesis of the Pizzo Bottino trondhjemites. Other trondhjemite generation pro-cesses such as fractionation from basaltic parents and partial melting of metabasaltic or

metasedimentary sources are ruled out on geological, petrographic and isotopic (Sr, Nd)grounds. Lastly, regional considerations place the metasomatic event during the lateHercynian, after the emplacement of the original granitoids and preceding the intrusionof the leucogranitic dykes, which are not affected by metasomatism.

Introduction

Trondhjemites are rocks with a low K=Na ratio, mainly occurring in Archean greygneiss complexes (Barker, 1979; Drummond and Defant, 1990; Martin, 1999).However, significant occurrences of trondhjemitic rocks are also reported fromProterozoic to Cenozoic continental margins (Drummond and Defant, 1990; Johnsonet al., 1997) and, with slightly different features, from island arcs and ophioliticcomplexes (Phelps, 1979; Coleman and Donato, 1979). Minor volumes of trondh-jemitic rocks are also found in high-grade metamorphic terrains (Barker, 1979;Drummond et al., 1986). Although trondhjemites are mainly regarded as productsof partial melting of metabasaltic (Drummond and Defant, 1990; Rapp and Watson,1995; Petford and Atherton, 1996) or metasedimentary sources (Conrad et al.,1988; Pati~nno Douce and Harris, 1998), they may also be generated by pervasivemetasomatic reworking of granitic precursors, as documented in metamorphic set-tings from North America and South Australia (Drummond et al., 1986; Elburget al., 2001).

Trondhjemitic rocks are widespread in the north-eastern Peloritani Belt both aslarge bodies and as sills and leucosomes. They have been interpreted as the resultof isochemical metamorphism of arkoses (Atzori et al., 1974), partial melting ofbiotitic paragneisses (Atzori et al., 1984a) and fluid-assisted metamorphic differ-entiation or metasomatic alteration of muscovitic schists (D’Amico et al., 1972;Lo Giudice et al., 1985).

In this paper, we describe the petrographic and geochemical features of thePizzo Bottino trondhjemitic body in order to better define its petrogenetic pro-cesses. The overall data set indicates that alkali metasomatism is the most suitablepetrogenetic process which best explains the field, petrographic and geochemicalfeatures of the Pizzo Bottino rocks. The present work contributes to the conceptthat metasomatism is not only a secondary alteration process but may also be animportant petrogenetic one.

Geological setting

The Peloritani Mountain Belt is located in north-eastern Sicily (Fig. 1). It con-stitutes the southern part of the Calabrian-Peloritani Arc (CPA), an Alpine belt rep-resenting the connection between the Southern Apennine and Maghrebid Chains.A segment of the Hercynian belt, made up of very low to high-grade Paleozoicmetamorphic rocks intruded by late-Hercynian granitoids, outcrops in the CPA.The evolution and geodynamic significance of the Calabrian-Peloritani Arc are stillsubject of numerous and contrasting interpretations, yielding three main hypoth-eses: a) this domain is a fragment of the original European continental paleo-margin (Dewey et al., 1989; Knott, 1987, 1994); b) it is a fragment of the Alpineorogenic belt, belonging to the African domain, emplaced on the Apennine

20 P. Fiannacca et al.

domains during the Neogene (Amodio Morelli et al., 1976; Bonardi et al., 1993);c) it represents a microcontinent, originally located between the European andAfrican continents and later involved in the Europe-Adria collision (Critelli,1999; Piluso et al., 1998).

Fig. 1. Geological sketch map of north-eastern Peloritani (after Atzori et al., 1974, modified)

Alkali metasomatism as a process for trondhjemite genesis 21

The Peloritani Belt is composed of a nappe pile, largely consisting ofHercynian basement rocks with metamorphic grade increasing northwards andMeso-Cenozoic cover units (Lentini and Vezzani, 1975). It may be subdivided intotwo complexes with different tectonic-metamorphic histories (Atzori et al., 1994;Cirrincione and Pezzino, 1994). The lower complex is exposed in the southern partof the Peloritani Belt and consists of three tectonic units made up of very low-tolow-grade metamorphic sequences and unmetamorphosed Mesozoic-Cenozoiccover rocks. The upper complex, in the north-eastern part of the belt, consists oftwo tectonic units (Mandanici Unit, Aspromonte Unit) showing low- to high-gradeHercynian metamorphism and an Alpine metamorphic overprint. The AspromonteUnit, which also outcrops in southern Calabria, is composed of amphibolite-faciesrocks dominated by biotitic, sometimes migmatitic paragneisses and augengneisses as well as by minor micaschists, amphibolites and marbles. These Hercynianmetamorphites are intruded by several peraluminous granitoids framed in thelate-Hercynian peraluminous granitic suite of CPA (D’Amico et al., 1982; Rotturaet al., 1990, 1993). Many other leucocratic bodies of granitic–granodioritic andtrondhjemitic composition outcrop within the Aspromonte Unit. Preliminary stud-ies on some of these bodies indicate that their origin is associated with late-Hercynian peraluminous magmatism (Atzori et al., 1984a, b; Lo Giudice et al.,1985) but the majority of the leucocratic rocks are not yet fully characterisedand are, at present, the subject of considerable work.

P–T conditions for the Hercynian metamorphism in the Peloritani sector of theAspromonte Unit are not well constrained, and available thermobarometric dataare still limited. Ioppolo and Puglisi (1989) identified two different Hercynianevents of blastesis, respectively syn-deformational and post-deformational, inparagneisses from north-eastern Peloritani. The P–T determinations, based onmicroprobe analyses (Ioppolo and Puglisi, 1989 and references therein), indicatedthat in the Capo Rasocolmo area both syn-deformational and post-deformationalevents took place at T¼ 600–650 �C and P¼ 3.2–4.0 kbar and in the Pizzo Chiarinoarea, located southwards, at somewhat lower conditions of T� 550 �C andP¼ 3.0–3.6 kbar. Atzori et al. (1990) indicated a common metamorphic historyfor the augen gneisses and associated biotitic paragneisses from north-easternPeloritani, with Rb=Sr cooling ages on micas of 280–292 Ma. U–Pb monaziteages for two upper crustal paragneisses of the Aspromonte Unit in SouthernCalabria indicated a metamorphic peak at 295� 2 Ma, coeval with the lowercrustal metamorphism reaching a peak temperature of 700–800 �C (Graessneret al., 2000).

Late-Hercynian magmatic activity was widespread in the CPA, and several gran-itoid complexes were emplaced in low- to high-grade metamorphic rocks, usually athigh crustal levels. These granitoids belong to two different suites: a main calc-alkaline suite, metaluminous to weakly peraluminous, and a highly peraluminoussuite, with less extensive intrusions. The calc-alkaline association, representing�70%of exposed granitoids, displays a broad compositional range (SiO2¼ 48–70%) andis biotite-dominated; tonalites and granodiorites are the dominant rock types.The peraluminous granitoids have a more restricted range of composition(SiO2¼ 67–76%) and contain two micas�Al-silicates. The granitoids are late-to post-tectonic and were probably emplaced along ductile shear zones connected

22 P. Fiannacca et al.

with an extensional regime (Rottura et al., 1990). In particular the unfoliated toweakly foliated peraluminous and calc-alkaline granitoids intruded in a brittledomain whereas strongly foliated calc-alkaline types intruded earlier at somewhatdeeper structural level (Rottura et al., 1990). The calc-alkaline suite is probablyof mixed origin, generated by the interaction with mantle derived magmas withcrustal components (Rottura et al., 1991). The granitoids of the peraluminoussuite have been interpreted as either of mixed mantle-crust origin (Rottura et al.,1991, 1993), or as typical S-type granites (D’Amico et al., 1982; Rottura et al.,1990). The peraluminous suite is the only one outcropping in the Aspromonte Unit.Magmatism occurred in the time-span of 295� 2 Ma to 270� 5 Ma, based onRb–Sr whole-rock and mineral ages and U–Pb zircon ages (Borsi and Dubois,1968; Borsi et al., 1976; Schenk, 1980; Del Moro et al., 1982). Graessner et al.(2000) determined U–Pb intrusion ages of 303–302� 0.6 Ma for two peralumi-nous granites outcropping in Serre and Aspromonte (Southern Calabria).

Tectonometamorphic Alpine overprints locally affected both metamorphitesand magmatites, generating pseudotachylites and variably sized bands of cataclas-tic to mylonitic rocks.

Field description and petrography

The Pizzo Bottino trondhjemites form a body of about 6 km2 which is clearlydistinguished in the field from the surrounding biotitic paragneisses by its contrast-ing colour. In the southern sector, a migmatitic complex extends a few hundredmetres, and leucosomes of trondhjemitic to leucogranitic composition occur,whereas in the northern sector the contacts with biotitic paragneisses are sharp.Several metasedimentary blocks up to 5–6 metres long and enclaves of centimetreto metre size occur within the trondhjemitic body, as frequently reported for late-Hercynian peraluminous granitoids outcropping in several sectors of the southernCalabrian-Peloritani Arc (D’Amico et al., 1982; Rottura et al., 1990, 1993; Fornelliet al., 1994). It is important to note that decimetre to decametre-sized trondhjemiticdykes have also been observed within the surrounding metamorphic rocks; locally,the trondhjemitic dykes are cut by late-Hercynian leucogranitic aplite dykes (Atzoriet al., 1984a, b; Lo Giudice et al., 1985).

The Pizzo Bottino trondhjemites are whitish, coarse to very coarse-grained het-erogranular rocks; locally, medium-fine grained facies also occur. Studied rocksexhibit subhypidiomorphic to autoallotriomorphic texture. Partial recrystallizationoverprints the original magmatic features that remain preserved as structural relicswithin domains of variable size (Fig. 2a, b, c). Metasedimentary enclaves of restitic=xenolithic significance often occur. Locally, cataclastic to protomylonitic structuresare developed.

The main rock forming minerals (up to 90% by volume) are plagioclase andquartz, followed by very small amounts of biotite, muscovite, sillimanite andK-feldspar. Accessory phases are tourmaline, apatite, zircon, monazite and Fe–Tioxides. According to the IUGS classification (Le Maitre, 1989), all the sampledPizzo Bottino rocks are trondhjemites. Their colour index is �10% with biotite astypical mafic phase, plagioclase of oligoclase composition, quartz �20% of leuco-cratic minerals, and K-feldspar <10% of total feldspars.

Alkali metasomatism as a process for trondhjemite genesis 23

On petrographic basis, the following two main trondhjemitic types are identi-fied in the Pizzo Bottino body:

– A-type trondhjemite, consisting of plagioclase=interstitial quartz, often withlobate textural relationships, and variable amounts of tiny metamorphic xenolithscomposed of muscoviteþ sillimanite and biotiteþmuscovite� sillimanite�quartz� plagioclase� apatite� oxides. Clots are sometimes essentially com-posed of biotite and show isotropic or decussate texture. Biotite and muscovite,may also occur enclosed in plagioclase, as discrete euhedral plates of variablesize with frequently corroded or fringed rims. Microcline, in interstitial patchesand=or homoaxial scattered inclusions in large plagioclase, is present;

– B-type trondhjemite, consisting of plagioclase and quartz and very little biotite,muscovite and sillimanite. K-feldspar is very scarce and only occurs as inclu-sions in plagioclase.

Trondhjemitic rocks with intermediate characteristics between A- and B-typesoften occur. In all rock types, plagioclase occurs as anhedral to subhedral megacrysts,up to 5–6 cm long. Only in the A-type rocks smaller plagioclase crystals with mag-matic features occur. Quartz mainly occurs as medium to large anhedral discrete

Fig. 2. Representative textures in the trondhjemitic rocks. a Magmatic plagioclase (pl1) withidiomorphic zoning in a weakly sheared sample. b Interstitial microcline (Kfs) enclosingcorroded magmatic plagioclase (pl1). Secondary plagioclase (pl2) partly replaces both micro-cline and pl1 plagioclase. c Interstitial microcline (Kfs) partly replaced by secondary plagio-clase (pl2). Myrmekite, optically continuous with newly formed plagioclase, occurs at theplagioclase rim. In the upper left corner a muscovite–sillimanite (ms–sill) metamorphic enclaveis visible. d Typical feature of trondhjemites: plagioclase megacrystals (pl2, up to 5–6 cmlong) with more or less abundant inclusions of variably sized microcline (Kfs) and quartz (qtz)

24 P. Fiannacca et al.

grains or as glomerocrystic aggregates; anhedral or rounded quartz also occurs withinthe plagioclase. Quartz and tartan-twinned microcline inclusions are sometimes veryabundant within the plagioclase megacrysts, so that the latter resemble a chessboardtexture. This is a typical texture of rocks modified by alkali metasomatism withreplacement of K-feldspar by sodic plagioclase (Callegari and De Pieri, 1967; Raith,1970; Pirinu et al., 1996). In some samples plagioclase replaces both older micro-cline and magmatic plagioclase (Fig. 2b); these latter representing, together withquartz and some biotites and muscovites, relics of the original magmatic assemblage.Myrmekites are diffused and plagioclase megacrysts often show myrmekitic rimsinvading adjacent K-feldspar (Fig. 2c). In the extreme stage of replacement relicmicrocline only remains as scattered inclusions in plagioclase megacrysts (Fig. 2d).

Evidences for muscovite breakdown under anhydrous conditions, according tothe reaction: muscoviteþ quartz¼K-feldsparþ sillimaniteþH2O, are shown bytextural relationships and mineral assemblages occurring in the metasedimentaryenclaves. These latter are considered as xenoliths=anatectic relics and are wide-spread within the peraluminous granitoids outcropping in the CPA (D’Amico et al.,1982; Rottura et al., 1993).

Among the accessory phases, apatite mainly occurs as rounded grains asso-ciated with biotite; sillimanite occurs as fibrolite, almost invariably associated withmuscovite and=or biotite; zircon and monazite appear as very small rounded inclu-sions, mainly in biotite, but the former also as euhedral rectangular and largerbipyramidal crystals; tourmaline is brown in thin section and often occurs in largecrystals in aggregation with other accessory phases and biotite; epidote is fre-quently encountered, deriving from plagioclase and biotite alteration or frommicro-fracture filling. Other abundant secondary phases are chlorites, from partialto total replacement of biotite and sericite from plagioclase alteration.

Geochemistry

Analytical methods

Major and trace element analyses for selected samples were performed by ICP-MSat Actlabs in Ancaster, Canada. Reported relative errors are 5% or less for majorelements (>1 wt.%) and about 5–15% for most minor and trace elements.

Sr and Nd isotopic compositions were determined using a Finnigan MAT 262 Vmulticollector mass spectrometer (at IGG-CNR, Pisa) after conventional ion-exchange procedures for Sr and Nd separation from the matrix. Measured 87Sr=86Srhave been normalized to 86Sr=88Sr¼ 0.1194; 143Nd=144Nd ratios to 146Nd=144Nd¼0.7219. During the collection of isotopic data, 10 replicate analyses of SRM 987(SrCO3) standard gave an average value of 0.710243� 8 (2� mean) and 11 mea-surements of La Jolla standard yielded an average 143Nd=144Nd¼ 0.511857� 4(2� mean). All 87Sr=86Sr data were normalized to a value of 0.71025 for theSRM 987 standard.

Major and trace elements

Major and trace element compositions of representative trondhjemitic rocks aregiven in Table 1. According to Barker (1979) and Drummond and Defant (1990),

Alkali metasomatism as a process for trondhjemite genesis 25

Table 1. Chemical compositions of selected Pizzo Bottino trondhjemites

GC 1 GC 2 GC 4 GC 5 GC 7 GC 8 GC 10

SiO2 wt.% 75.28 76.01 73.36 74.93 75.74 71.31 74.53TiO2 0.12 0.10 0.16 0.10 0.09 0.19 0.06Al2O3 14.31 15.04 15.97 15.06 14.66 16.55 15.37Fe2O3TOT 0.95 0.73 0.87 0.70 0.71 1.35 0.55MnO 0.02 0.01 0.01 0.02 0.01 0.02 0.01MgO 0.45 0.34 0.43 0.31 0.31 0.65 0.22CaO 2.83 1.99 2.24 2.31 2.03 2.93 2.78Na2O 4.73 4.26 4.79 4.96 4.85 5.26 5.51K2O 0.60 1.10 1.22 1.16 0.99 0.89 0.58P2O5 0.04 0.04 0.04 0.12 0.03 0.02 0.02LOI 1.03 0.91 0.99 0.76 1.04 1.09 0.44Total 100.36 100.53 100.08 100.43 100.46 100.25 100.07ASI 1.05 1.27 1.20 1.11 1.15 1.11 1.04

CIPW norms (wt.%)

Q 39.01 42.38 35.75 36.56 39.15 30.23 33.98or 3.56 6.51 7.26 6.86 5.87 5.28 3.43ab 40.11 36 40.73 41.9 41.1 44.57 46.55an 13.86 9.64 10.95 10.76 9.93 14.46 13.68C 0.81 3.3 2.79 1.69 1.98 1.64 0.66Hy en 1.13 0.85 1.08 0.77 0.78 1.63 0.55Hy fs 0 0 0 0 0 0 0.2mt 0.79 1.04 0.59 0.94 0.76 1.41 0.8he 0.41 0.01 0.47 0.05 0.19 0.38 0il 0.24 0.18 0.3 0.2 0.18 0.35 0.11ap 0.09 0.09 0.09 0.26 0.07 0.04 0.04

Ba ppm 256 552 671 609 506 340 248Rb 25 33 37 35 28 31 16Sr 544 483 514 510 507 661 538Y 4 3 3 9 3 1 2Zr 102 34 66 142 28 65 116Nb 3 2 2 2 2 3 1Cr 22 <20 <20 <20 <20 <20 <20Ni 48 <15 <15 <15 <15 <15 <15Th 3 5 13 8 3 1 0U 1 1 1 2 0 0 1La 11.4 14.7 40.2 26 10.9 3.76 2.42Ce 20.7 28.5 79.2 53.9 20.5 6.6 4.2Pr 2.29 3.25 8.89 6.12 2.3 0.69 0.42Nd 8.22 12.4 33.7 23.6 8.91 2.44 1.57Sm 1.43 2.08 5.21 4.08 1.52 0.41 0.29Eu 1.26 0.82 0.96 0.85 0.72 0.82 0.82Gd 1.17 1.57 3.73 3.3 1.19 0.35 0.25Tb 0.14 0.19 0.3 0.39 0.14 0.04 0.05Dy 0.66 0.79 1.02 1.83 0.66 0.21 0.27Ho 0.11 0.12 0.12 0.3 0.11 0.04 0.05Er 0.33 0.29 0.22 0.83 0.28 0.11 0.18Tm 0.05 0.04 0.02 0.11 0.03 0.02 0.03Yb 0.37 0.2 0.09 0.77 0.21 0.11 0.24Lu 0.06 0.03 0.02 0.10 0.03 0.02 0.05Rb=Sr 0.05 0.07 0.07 0.07 0.06 0.05 0.03Eu=Eu� 2.98 1.39 0.67 0.70 1.63 6.59 9.35

(continued)

Table 1 (continued)

GC 25 GC 27 GC 30 GC 31 GC 32 GC 34 GC 36

SiO2 wt.% 74.12 71.73 71.53 75.30 73.21 71.91 72.81TiO2 0.06 0.08 0.16 0.07 0.10 0.10 0.04Al2O3 15.58 16.98 16.02 15.03 15.66 15.90 15.97Fe2O3TOT 0.40 0.61 1.31 0.51 0.83 0.87 0.73MnO 0.01 0.01 0.02 0.01 0.01 0.01 0.01MgO 0.21 0.28 0.56 0.27 0.40 0.35 0.15CaO 3.03 3.20 3.11 3.04 2.82 3.03 2.89Na2O 5.72 5.85 5.05 5.09 5.57 5.72 5.51K2O 0.33 0.72 1.08 0.56 0.66 0.59 0.92P2O5 0.03 0.02 0.05 0.04 0.03 0.04 0.03LOI 0.51 0.76 0.80 0.54 1.01 0.62 0.47Total 100.00 100.24 99.68 100.46 100.30 99.14 99.53ASI 1.02 1.05 1.06 1.04 1.04 1.02 1.04

CIPW norms (wt.%)

Q 32.94 27.85 30.72 36.64 31.91 29.83 31.18or 1.96 4.27 6.41 3.3 3.92 3.53 5.48ab 48.46 49.56 42.82 42.91 47.27 48.85 46.91an 14.9 15.81 15.19 14.82 13.88 14.96 14.27C 0.36 0.79 0.99 0.6 0.71 0.42 0.71Hy en 0.53 0.7 1.4 0.67 1 0.88 0.38Hy fs 0.1 0 0.16 0.1 0 0 0mt 0.58 0.71 1.91 0.74 0.7 1.19 0.58he 0 0.12 0 0 0.35 0.06 0.34il 0.11 0.15 0.3 0.14 0.19 0.19 0.08ap 0.07 0.04 0.11 0.09 0.07 0.09 0.07

Ba ppm 182 272 454 235 334 298 461Rb 7 19 35 15 14 22 17Sr 705 659 575 624 559 620 584Y 2.7 2.6 24.9 0.8 1.8 1.6 2.7Zr 48 167 34 104 45 25 25Nb 1.6 1.9 4.3 0.9 1.7 2.3 1.4Cr <20 <20 <20 <20 <20 <20 <20Ni <15 <15 <15 <15 <15 <15 <15Th 3.53 3.39 9.26 0.12 0.21 0.4 3.46U 0.41 0.63 1.05 0.21 0.3 0.3 0.42La 12.1 11 32.1 1.9 2.1 3.39 11.3Ce 23 20.3 64.2 2.9 3.7 5.8 21.8Pr 2.55 2.18 7.42 0.3 0.41 0.64 2.46Nd 9.44 8.06 28.9 1.02 1.54 2.34 9.37Sm 1.75 1.36 5.77 0.16 0.37 0.47 1.72Eu 1.06 0.998 1.08 0.68 0.765 0.89 0.895Gd 1.34 1.05 5.42 0.14 0.32 0.42 1.4Tb 0.16 0.12 0.82 0.02 0.06 0.06 0.17Dy 0.68 0.55 4.47 0.12 0.31 0.32 0.71Ho 0.09 0.08 0.8 0.02 0.06 0.05 0.09Er 0.22 0.23 2.28 0.08 0.18 0.13 0.19Tm 0.026 0.034 0.306 0.014 0.026 0.02 0.023Yb 0.18 0.24 1.8 0.11 0.17 0.11 0.14Lu 0.031 0.045 0.232 0.019 0.024 0.016 0.021Rb=Sr 0.01 0.03 0.06 0.02 0.03 0.04 0.03Eu=Eu� 2.12 2.55 0.59 13.88 6.79 6.12 1.76

the Pizzo Bottino rocks are high-Al trondhjemites (Al2O3>15 wt.% at 70 wt.% ofSiO2), with high Sr (>400 ppm) and low Rb=Sr (typically �0.07), K=Rb (<500),Nb (<4.3 ppm), Y (typically <10 ppm) and Ni and Cr (almost entirely below de-tection limits). On the basis of major and trace element contents and normativecompositions, these rocks are continental-type trondhjemites, with subalkalinecharacter and very low K2O=Na2O (Arth, 1979). They are mildly peraluminous,with an Al saturation index (molar ratio Al2O3=CaOþNa2OþK2O) showing posi-tive correlation with the K=Na ratio.

In Harker diagrams, Al2O3 and, to a lesser extent CaO, Na2O and Sr, arenegatively correlated with SiO2, whereas the other elements are more or less

Fig. 3. SiO2 variation diagrams for trondhjemitic rocks compared with fields definedby calc-alkaline (dashed lines) and peraluminous (full lines) granitoids from southernCalabrian-Peloritani Arc (fields after D’Amico et al., 1982; Rottura et al., 1990, 1991)(oxides in wt.%; elements in ppm)

28 P. Fiannacca et al.

scattered (Fig. 3). With respect to the late-Hercynian granitoids of the CPA, thePizzo Bottino trondhjemites show trends similar to those of peraluminous gran-itoids for some elements (D’Amico et al., 1982; Rottura et al., 1990, 1991). Inparticular, the SiO2, Al2O3, Fe2O3, MgO, TiO2 contents of the trondhjemitic rocksplot inside the peraluminous suite fields, but trondhjemites display higher Na2O,CaO and Sr and lower K2O, P2O5, Ba and Rb.

In ORG-normalized diagrams (Pearce et al., 1984; not shown), the PizzoBottino trondhjemites show spiked patterns (e.g., K, Rb, Ba, Th, Ce and Nb, andHf–Zr) typical of orogenic magmas.

Rare earth elements

The Pizzo Bottino trondhjemites show extremely variable REE contents, withLREE ranging from 10 to 100 times chondrite and a LaN=YbN ratio in the range14–20; typically, Eu anomalies (calculated according to Taylor and McLennan,1985) vary from negative to highly positive values (Fig. 4). These differences inREE patterns are closely correlated with petrographic features. A-type rocks, char-acterised by higher modal contents of micas, show high REE values (LaN>80�chondrite), with a mildly negative Eu anomaly and moderate fractionation in twosamples (LaN=YbN¼ 17). Instead, the pattern of sample GC4 is highly fraction-ated (LaN=YbN¼ 301); the trough for Yb probably being an analytical artefact.B-type rocks show a high positive Eu anomaly (Eu�=Eu>6), less fractionation(LaN=YbN¼ 14.4) and the lowest REE contents, with LaN¼ 18.6 times chondrite andHREE<5 times chondrite. The rocks with intermediate petrographic characteristicsbetween A and B-types show a moderately positive Eu anomaly (Eu�=Eu¼ 1–6)and patterns roughly parallel to the former group (LaN=YbN¼ 20.1), but with lowerREE abundances (LaN¼ 28 times chondrite). It is to be noted that some samplesshow HREE abundances lower than chondritic values. The REE patterns of A-typesamples are very similar to those of some peraluminous granitoids of the CPA, suchas the Serre, Cittanova and Capo Vaticano granodiorites (Rottura et al., 1990, 1991;Fornelli et al., 1994).

There is no variation of Eu=Eu� with silica; on the contrary, Eu=Eu� increasesas the other REE elements decrease. In the trondhjemitic rocks, Eu= Eu� ratios arepositively correlated with CaO, Na2O and Sr, and negatively with K2O, Rb, Ba, Ce,La, Th, U and Y. These trends clearly indicate strong control of REE behaviour byaccessory phases such as apatite, monazite, uraninite and xenotime. This fits dataindicating that the REE contents of 90–95% in peraluminous systems are hosted inaccessory phases, whereas major phases play an indirect role, mainly as hosts forinclusions of accessory minerals (Bea, 1996).

Sr, Nd isotopes

Sr and Nd isotopic data for selected trondhjemitic rocks and associated biotiticparagneisses and leucogranitic dikes are given in Table 2. The Sr and Nd isotopiccompositions of the trondhjemites have been calculated for 290 Ma, in order tomake direct comparisons with igneous and metamorphic rocks from the southernCPA. An age of 290 Ma is in fact considered by many workers as the age of the end

Alkali metasomatism as a process for trondhjemite genesis 29

of Hercynian metamorphism and of the emplacement of the granitoid bodies(Schenk, 1980; Rottura et al., 1990, 1993; Atzori et al., 1990; Caggianelli et al.,1991; Fornelli et al., 1994; Del Moro et al., 2000). The Pizzo Bottino trondhjemitesshow low (87Sr=86Sr)290 and ("Nd)290, respectively in the ranges 0.7073–0.7076 and�6.68 to �6.90. It is to be noted that trondhjemitic rocks belonging to outcropssome tens of kilometres away from Pizzo Bottino gave rather different isotopicdata. In fact, two trondhjemite samples from Dinnamare have (87Sr=86Sr)290 valuesof 0.7104 and 0.7124 and ("Nd)290 values of �8.15 and �8.49, and one from Colle

Fig. 4. Chondrite-normalizedREE patterns (Taylor andMcLennan, 1985) fortrondhjemitic rocks of A, Band transitional type. Topdiagram also shows varia-tion field of REE patternsfor Serre (hatched area) andCapo Vaticano and Cittanovagranitoids (shaded area)Reference REE patterns areafter Rottura et al. (1990)and Fornelli et al. (1994)

30 P. Fiannacca et al.

S. Rizzo has intermediate values of 0.7081 and �7.72 (Fiannacca, 2000). In the"Nd vs. (87Sr=86Sr)i diagram (Fig. 5) the Sr–Nd isotopic compositions of tron-dhjemites plot within the field of CPA peraluminous granitoids. Sr–Nd data fortwo samples of aplitoid leucogranites from Pizzo Bottino and Colle S. Rizzo(Fiannacca, 2000), also fall in the same field. In particular, the values of "Nd

and (87Sr=86Sr)i of trondhjemites are similar to those of granites from Cittanova,Villa S. Giovanni and Capo Rasocolmo and, to a lesser extent, those of the CapoVaticano and Serre granitoids (Del Moro et al., 1982; Rottura et al., 1990, 1993;Fornelli et al., 1994), except for trondhjemites that are less enriched in radiogenicSr. Besides general similarities between trondhjemites and CPA peraluminousgranitoids, it was noted that the CPA metamorphites contain granulitic metapelites

Table 2. Rb–Sr and Sm–Nd analytical results for selected trondhjemites and associatedleucogranites and paragneisses

Rb ppm Sr ppm 87Rb=86Sr 87Sr=86Sr � 2� (87Sr=86Sr)290 ("Sr)290

Trondhjemites

GC 2 33.0 483.0 0.1977 0.7085 � 12 0.7076 49GC 5 35.0 510.0 0.1986 0.7083 � 11 0.7074 47GC 34 22.0 620.0 0.1027 0.7077 � 14 0.7073 44GC 12� 33.0 489.0 0.1954 0.7132 � 11 0.7124 118GC 19� 23.0 470.0 0.1416 0.7110 � 12 0.7104 89GC 49�� 30.0 474.0 0.1831 0.7089 � 12 0.7081 56

Leucogranites

GC 9 107.0 100.0 3.1006 0.7235 � 12 0.7107 93GC 47�� 238.0 66.0 10.4813 0.7545 � 11 0.7112 101

Biotite paragneisses

GC 39 76.0 205.0 1.0738 0.7191 � 10 0.7147 150GC 13� 183.0 248.0 2.1373 0.7185 � 14 0.7097 79

Sm ppm Nd ppm 147Sm=144Nd 143Nd=144Nd � 2� (143Nd=144Nd)290 ("Nd)290

Trondhjemites

GC 2 2.1 12.4 0.1028 0.5121 � 14 0.5119 �6.94GC 5 4.1 23.6 0.1050 0.5121 � 8 0.5119 �6.68GC 34 0.5 2.3 0.1314 0.5122 � 10 0.5119 �6.70GC 12� 2.1 11.0 0.1154 0.5121� 10 0.5118 �8.15GC 19� 0.8 4.8 0.1007 0.5120 � 23 0.5118 �8.49GC 49�� 0.3 2.0 0.0907 0.5120 � 17 0.5119 �7.72

Leucogranites

GC 9 2.5 9.6 0.1574 0.5122� 11 0.5119 �7.68GC 47�� 3.5 12.7 0.1666 0.5121 � 10 0.5118 �9.54

Biotite paragneisses

GC 39 7.5 39.1 0.1159 0.5119 � 9 0.5117 �11.22GC 13� 7.6 38.2 0.1203 0.5120� 9 0.5118 �8.74

� Samples from Dinnamare; �� samples from Colle S. Rizzo

Alkali metasomatism as a process for trondhjemite genesis 31

and metagraywackes of northern Serre (Del Moro et al., 2000), showing isotopicfeatures compatible with those of studied rocks in terms of "Nd, although moreenriched in radiogenic Sr (87Sr=86Sr)290>0.7099). These granulitic rocks hostweakly peraluminous leucotonalitic leucosomes, with isotopic features comparableto those of trondhjemites.

Discussion

Geological and petrographic constraints

Field relations indicate that the trondhjemitic rocks are related to the late-Hercy-nian magmatism in the CPA, and mainly show intrusive-like contacts. Preliminarypetrographic and geochemical data on the trondhjemitic leucosomes bordering thesouthern portion of the Pizzo Bottino body indicate differences with respect to thetrondhjemites of the main body and of the dykes. For example, the leucosomes aredefinitely richer in K-feldspar than the studied trondhjemites and also differ in theirK2O content (1.12 to 1.96 wt.% vs. 0.33 to 1.22 wt.% in trondhjemites).

For the Pizzo Bottino trondhjemites, an original igneous origin is suggested by:a) the occurrence of fine- to medium-sized euhedral and zoned plagioclase; b)intrusive-like contacts; c) xenolithic=restitic clots reflecting rock-magma interac-tion; d) close similarity of mineralogical relict assemblages of both trondhjemites

Fig. 5. "Nd290 Ma vs. (87Sr=88Sr) diagram for trondhjemites (empty circles are samples fromDinnamare and Colle S. Rizzo areas), leucogranites (triangles) and biotitic paragneisses(diamonds). Fields of CPA peraluminous granitoids and range (rectangle) and mean com-position of Calabrian metasedimentary crust (MSC) after Rottura et al. (1991, 1993)

32 P. Fiannacca et al.

and peraluminous granitoids. The original magmatic texture is partially to entirelyobliterated by superimposed late crystallization and=or recrystallization processes.The most important textural aspects linked to the latter processes are: 1) growth ofplagioclase at the expense of large microcline crystals and older magmatic plagio-clase, 2) myrmekitic intergrowths invading and replacing older microcline. In 1),homoaxial islands of microcline often occur in plagioclase megacrysts, that cannotbe related to post-magmatic exolution. In the latter case K-feldspar crystals shouldshow more regular distribution, mainly occurring, for instance, in the outer portionsof normally zoned plagioclases. Similar textures have been well documented inmagmatic trondhjemites, in which scarce potassic feldspar occurs as an interstitialphase, as inclusions in quartz or as antiperthites; the latter only occurring in themore albitic plagioclase rims (Payne and Strong, 1979; Phelps, 1979). Instead, inthe studied trondhjemites, microcline islands are scattered, without any relation-ship with the composition of the host plagioclase.

Geochemical constraints

Variation diagrams and spiderdiagrams indicate abundances and trends which, forSiO2, Al2O3, TiO2, MgO and Fe2O3 are substantially comparable with those dis-played by the late-Hercynian peraluminous granitoids of the CPA. Significant dif-ferences were found for Na2O, CaO, K2O, P2O5, Sr, Ba and Rb abundances. Thelatter also define more or less scattered trends. On these grounds it is not possible torecognize some genetic relationship between trondhjemites and the above granit-oids in terms of magmatic differentiation or partial melting models.

REE patterns and Eu anomaly correlations with major and trace elements areconsistent with the petrographic features; in particular, the trondhjemites of type Acontaining relatively abundant biotite, muscovite, sillimanite and accessory phases,are richer in �REE than rocks of type B, which are poor in these mineral phases.Notably, the latter rocks, dominated by newly-formed plagioclase, show strongpositive Eu anomalies. It is to be noted that the change of sign and amplitude ofthe Eu anomaly is only a consequence of the progressive decrease of the other REEs;the EuN value is indeed nearly stable at around 10 times chondrites for all types ofrocks.

REE patterns may be explained in terms of disequilibrium melting models. It hasbeen demonstrated that high field strength elements such as Zr, Th and REE do notalways reach equilibrium concentrations in partial melts because their concentra-tions are controlled by accessory phases which are refractory or hosted in residualmajor phases during melting (Watt and Harley, 1993; Bea, 1996). Disequilibriummelting may then be responsible for both the decrease in REE contents and theincrease in positive Eu anomaly. However, a similar result may also be achieved byfluid-induced destabilisation of the same accessory phases. For instance, in musco-vitic granites, respectively fresh and hydrothermally altered, Gomes and Neiva (2000)reported subparallel REE patterns with progressively lower REE contents, particu-larly LREE, in the altered granites; REE decrease was mainly attributed to mon-azite and zircon loss. Moreover moderate to strong mobility of REE has been oftenobserved in alteration processes involving alkali metasomatism (e.g. Cathelineau,1986; Charoy and Pollard, 1989; Petersson and Eliasson, 1997).

Alkali metasomatism as a process for trondhjemite genesis 33

In order to verify if the geochemical patterns shown by the Pizzo Bottinotrondhjemites were influenced by alkali metasomatism processes, major, traceand REE elements were related to the ratio K=Na (Fig. 6). The negative correla-tions of CaO and Sr, together with positive correlations of Rb and Ba, may indicatemetasomatic replacement of potassic feldspar by oligoclase. This is also indicatedby the different distribution of data in the MgO vs. K=Na ratios, reflecting thebehaviour of biotite. In spite of data scattering, La, Ce, Th and U contents arepositively correlated with K=Na ratios (correlation coefficient r>0.707 for LREEand Th, and 0.521 for U). All this suggests control, at least partial, of elementabundances by alkali metasomatism processes, because decrease of elementswhich are associated with REE-rich phases in peraluminous systems (monazite,uraninite, apatite, zircon, Th-orthosilicates) is linked with loss of K and gain of Na.

Fig. 6. Variations of selected major and trace elements and of Eu anomaly with K=Na ratio

34 P. Fiannacca et al.

As regards isotopic data, the distribution of trondhjemite samples in the "Nd vs.(87Sr=86Sr)290 diagram (Fig. 5) is consistent with a variety of processes, includingmixing between crustal melts and basaltic components (Rottura et al., 1991, 1993),disequilibrium melting of metasediments with more radiogenic Sr and less radiogenicNd (Knesel and Davidson, 1996; Del Moro et al., 2000) and simple melting of lowRb=Sr lower crustal metasediments (Taylor and McLennan, 1985; Del Moro et al.,1999). Although the probable resetting of Sr and Nd isotopic systems limits thepetrological use of the isotopic ratios, it is possible to draw some petrogenetic con-siderations. In particular, 87Sr=86Sr and 143Nd=144Nd initial ratios seem to differ fromthose normally found in trondhjemites directly generated from metabasaltic sources.The latter are characterised by initial Sr<0.7057 and positive "Nd (Barnes et al.,1992; Petford and Atherton, 1996; Johnson et al., 1997). The origin of Pizzo Bottinotrondhjemites by simple partial melting of a mafic source thus appears improbable,due to their isotopic composition and particularly to the strongly negative "Nd values,indicating a dominant metasedimentary contribution. The latter may be linked toterrains similar to the Serre granulitic rocks, hosting weakly peraluminous leucoto-nalitic leucosomes with isotopic features comparable to those of trondhjemites.These leucosomes, interpreted by Del Moro et al. (2000) as partial melts generatedby H2O-fluxed melting of metapelitic sources, have major and trace element compo-sitions and REE patterns unlike those of the studied trondhjemites, but it cannot beexcluded that some of the trondhjemites outcropping in the Peloritani Belt are thedirect result of similar processes of H2O-fluxed melting of metasedimentary rocks.

Petrogenetic models

In order to define the origin of the Pizzo Bottino trondhjemites, the main petroge-netic models of these rock-types proposed up to now are reviewed here.

The main petrogenetic models are: a) differentiation from a parental melt orpartial melting of a mafic source, b) partial melting of metasedimentary rocks, c)metamorphic differentiation, d) alkali metasomatism. Fractionation suites are quiterare and do not match the field relationships and geochemical features displayed bythe Pizzo Bottino trondhjemites. Metamorphic differentiation is a typical mechanismable to produce small volumes of trondhjemitic leucosomes; this process is unlikelyfor our trondhjemites, owing to the large volume of the Pizzo Bottino body.

Melting of metabasaltic rocks is not consistent with isotopic composition of thestudied rocks. Moreover, although these latter show some geochemical analogieswith trondhjemites produced by slab melting (Drummond and Defant, 1990) andby partial melting of continental lower crust (Barnes et al., 1992; Petford andAtherton, 1996), they differ in other important geochemical features. Trondhje-mites generated by partial melting of metabasaltic (eclogitic or amphibolitic) crustare normally richer in FeOtotal and K2O than the Pizzo Bottino trondhjemites andalso show high LaN (>40) and highly fractioned REE patterns, due to retention ofgarnet in the residue. Lastly, trondhjemites generated in both types of environmentsare space- and time-related with other magmatic products ranging in compositionfrom quartz diorites to leucogranodiorites and, frequently, have mafic enclaves(Payne and Strong, 1979; Phelps, 1979; Barnes et al., 1992). The Pizzo Bottinotrondhjemites contain exclusively metasedimentary enclaves.

Alkali metasomatism as a process for trondhjemite genesis 35

Melts of trondhjemitic composition have also been obtained by experimentalmelting of metapelitic rocks. In particular, Pati~nno Douce and Harris (1998) showedthat dehydration melting of muscovitic schists produces leucogranitic melts, butthe addition of small amounts of H2O causes the generation of trondhjemitic melts;indeed, with increasing aH2O, the plagioclaseþ quartz solidus falls progressivelyuntil melting starts, inside the stability field of muscovite. In this case, melting iseutectic, and muscovite dissolves congruently in the melt, without forming the K-feldsparþ sillimanite intergrowths typical of dehydration melting. A shift to trondh-jemitic compositions is favoured by melting at conditions of temperature low enoughand pressure high enough to maintain muscovite in the residue, more sodic composi-tions being obtained at 10 kbar and 700 �C with the addition of 4% H2O. The com-positions of the Pizzo Bottino trondhjemites are still richer in Na and Ca and haveNa2O=K2O much greater with respect to experimental melts (Fig. 7). Moreover, theoccurrence of residual muscovite associated with sillimanite and K-feldspar withinthe metasedimentary enclaves, is suggestive of melting under fluid absent conditions(Harris and Inger, 1992; Pati~nno Douce and Harris, 1998). The above model is,moreover, mainly applicable to migmatitic associations, because water-saturatedmelting does not seem to produce large quantities of melts. It then readily leads to

Fig. 7. CIPW normative classification of the studied rocks, after Barker (1979). Trond-hjemites do not correlate well with the fields defined by compositions of experimentalglasses produced by dehydration melting and H2O-fluxed melting of metapelitic schists,at different P conditions (fields of experimental melts after Pati~nno Douce and Harris, 1998)

36 P. Fiannacca et al.

generation of abundant leucosomes, but probably not of large-scale bodies (Whitneyand Irving, 1994; Thompson, 1999; Del Moro et al., 2000; Fornelli et al., 2002).

Trondhjemitic rocks may also be generated by alkali metasomatism processeson different, but mainly granitic, protoliths (Drummond et al., 1986; Elburg et al.,2001). Other than the above mentioned petrographic features typical of rocksaffected by sodic metasomatism, major and trace element data also strongly sup-port subsolidus metasomatic alteration processes (Fig. 8). In the Q1F1 diagram[Q1¼ Si=3� (KþNa); F1¼K�Na], trondhjemites follow theoretical trends of‘‘quartz dissolution accompanied by albitisation’’, as observed in several alteredperaluminous granites of the Variscan chain (Cathelineau, 1986). The trend dis-played by the trondhjemitic rocks clearly indicates that quartz dissolution did notprecede but only partly accompanied replacement of potassic feldspar by sodicplagioclase. Involvement in a Na metasomatic process is also demonstrated in theNa=K vs. (NaþK)=(Al� 2Ca) diagram (McCaig et al., 1990), in which the roleplayed by late muscovite growth is also shown through the (NaþK)=(Al� 2Ca)ratio. In this diagram the Pizzo Bottino trondhjemites show a dominant verticaltrend of simple albitisation towards extremely high Na=K ratio values. In bothdiagrams, the CPA peraluminous granitoids may be assumed as starting products.

The occurrence of newly formed oligoclase imposes some constraints regardingthe temperature of the metasomatic process. At low temperature, plagioclase isstable only in solutions with unrealistically high Ca2þ concentrations and, as aconsequence, water-rock interactions at these temperatures should cause extensivedestruction of calcic plagioclase, with the formation of albite (Morteani et al.,

Fig. 8. a) Distribution of trondhjemites (dots) in the millication Q1F1 diagram(Cathelineau, 1986), compared with theoretical trends and fields representative for quartzdissolution accompanied by: dominant albitisation (IIA), albitisation-chloritisation (IIB)and microclinisation (III). IIa represents a real trend of quartz dissolution combinedwith albitisation shown by Portugese granites (after Cathelineau, 1986). b) Na=K vs.(NaþK)=Al(� ) diagram (McCaig et al., 1990) for trondhjemitic rocks (dots). In bothdiagrams, squares represent mean compositions of CPA peraluminous granitoids (CapoRasocolmo, Villa S. Giovanni, Capo d’Orlando, Delianuova, Cittanova, Capo Vaticano;data from D’Amico et al., 1982; Rottura et al., 1991)

Alkali metasomatism as a process for trondhjemite genesis 37

1986). At higher temperatures (>300–400 �C) more calcic plagioclase is thermo-dynamically favoured.

On the basis of the overall data set of the trondhjemites, their protoliths may beframed within the CPA late-Hercynian peraluminous magmatism. Notwithstandingthe lack of geochronological data, it is presumed that the emplacement of theoriginal granitoids occurred directly after the high-grade metamorphic peak, whenlarge volumes of fluids derived from devolatilisation of metasediments, were stillcirculating (Kent et al., 2000). It is also possible that the intrusion of the granitoidrocks of both calc-alkaline and peraluminous suites may have played an importantrole in triggering or facilitating the devolatilisation reactions of the surroundingmetasediments. The generation of Na-rich fluids from metasediments may haveinvolved consumption of paragonitic mica. Muscovite of metasedimentary rockssurrounding the trondhjemites or outcropping at the same structural level in thePeloritani mountain belt contain, at least, a paragonite component of about10 mol% (Ioppolo and Puglisi, 1989).

Fluids of metamorphic derivation are often considered responsible for metaso-matic processes, localised or pervasive (Bickle et al., 1988; Selverstone et al., 1991;Kent et al., 2000). As regard the mechanism of metasomatism, the nearly completereplacement of K-feldspar by sodic plagioclase suggests, as also indicated byDrummond (1986), an infiltration metasomatism. On the contrary, in case of diffu-sion metasomatism only approximately 10 to 20 mol% of Or component can bereplaced by Ab in feldspar (Hoffmann, 1972). In general, during regional meta-morphism, fluids flow upward, exchanging alkalies in the pathway (Wood andWalter, 1986; Guidotti and Sassi, 1998). In such a scenario replacement of micro-cline by oligoclase may follow the process of alkali ion exchange between vapourand feldspar phases (Orville, 1963), with K and Na moving to the cooler and hotterregions, respectively. So, due to the infiltration of a metamorphic fluid, a hot gran-itoid body will tend to become rich in Na, transferring K to the fluid. Etheridge et al.(1984) estimated that metamorphic fluids are able to cover up to 380 km in a time-span of 1–10 Ma, fitting the occurrence of metasomatised rocks over large areas.Indeed, evidence of metasomatic processes comes from various zones of the Aspro-monte Unit of CPA; for example, Messina et al. (1974) reported samples from theVilla S. Giovanni (southern Calabria) leucogranodiorites with K-feldspar fullyreplaced by chessboard albite, and Ferla (1994) reported albitised augen gneissesoutcropping a few kilometres south of the Pizzo Bottino trondhjemitic body.

Nature of the protoliths

Geological, petrographical and geochemical data indicate that the protoliths oftrondhjemites may be leucogranites and=or leucogranodiorites outcropping in thesouthern Calabrian-Peloritan Arc (e.g., Capo Vaticano, Cittanova, Delianuova, VillaS. Giovanni, Capo Rasocolmo granitoids). Mass balance calculations indicate thattrondhjemitic rocks may be obtained from one of these granitoids by a loss of 3.30–3.75 wt.% of K2O and an addition of 2.25–3.25 wt.% of Na2O; CaO also increasesby about 1:2 with respect to Na2O (1:4 at Capo Vaticano), whereas MgO and FeOtotal

slightly decrease. Mass balance results are graphically reported (Fig. 9) using isocondiagrams (Grant, 1986). On this type of diagram, data points falling on a straight line

38 P. Fiannacca et al.

passing through the origin, represent elements which remained immobile duringalteration. Such a line is called isocon and is expressed by the equation:

CA ¼ ðMO=MAÞCO

Fig. 9. Isocon diagrams (Grant, 1986) for Pizzo Bottino trondhjemites (a–d). In absenceof actual unaltered protoliths, mean compositions of Capo Vaticano, Cittanova, CapoRasocolmo and Delianuova late-Hercynian peraluminous granitoids (data from D’Amicoet al., 1982; Rottura et al., 1991) have been used to represent compositions of potentialprotoliths. In e and f, isocon diagrams only for CPA granitoids unaffected by metasomaticprocesses are also shown. Major elements and trace elements are in weight percentageoxides and in ppm, respectively, and are scaled randomly as labelled (the insets representenlargements of portions of diagrams for oxide concentrations lower than 1.25%)

Alkali metasomatism as a process for trondhjemite genesis 39

where C is concentration, M is mass and the O and A superscripts refer to theoriginal and to the altered rocks, respectively. Element concentrations plottingabove and beneath the reference isocon represent gains and losses during altera-tion, respectively. We considered as CO, representative for the unaltered protoliths,the averaged elemental concentrations of above mentioned peraluminous gran-itoids (data from D’Amico et al., 1982; Rottura et al., 1991) and as CA, represen-tative for the altered rocks, the averaged elemental concentrations of Pizzo Bottinotrondhjemites. All concentrations are scaled by random factors (Grant, 1986) asshown in Fig. 9, and isocons are based on the assumption of constant alumina. Inthe diagrams for the trondhjemitic rocks (Figs. 9a–9d) deviations from the isoconare interpretable by the addition of Na2O, CaO and Sr and removal of K2O, Ba andRb and, to a lesser extent, FeO and MgO, to CPA granitoid protoliths with geo-chemical features comparable with that of Capo Vaticano, Cittanova, CapoRasocolmo and, particularly, Delianuova. It is interesting to note that, in the isocondiagrams (Figs. 9e, 9f) concerning granitoid rocks not modified by metasomaticprocesses (e.g., Capo Vaticano vs. Capo Rasocolmo; Villa S. Giovanni vs. CapoRasocolmo), none of the elements shows important deviations from the isocon.

In summary, the origin of the trondhjemites through alkali metasomatic pro-cesses on granitoid rocks similar to that outcropping in the southern CPA (e.g.,Delianuova granites, Aspromonte) is very likely.

Conclusions

The main genetic models nowadays invoked for the origin of trondhjemitic rocksare partial melting of mafic sources (Drummond and Defant, 1990; Barnes et al.,1992; Rapp and Watson, 1995; Beard, 1995; Petford and Atherton, 1996) and H2O-fluxed melting of metasedimentary rocks (Conrad et al., 1988; Pati~nno Douce andHarris, 1998).

On the basis of petrographic and geochemical features, the Pizzo Bottinotrondhjemites may be referred to a different genetic process. Homoaxial, scatteredinclusions of microcline in plagioclase, plagioclases with chessboard texture,together with myrmekitic intergrowths and late plagioclase growing on largemicrocline and older euhedral plagioclase are all typical of rocks affected by sodicmetasomatism. In addition, the studied trondhjemites show relic assemblages nor-mally observed in late-Hercynian peraluminous granitoids from the southern CPA.Relic petrographic features suggest an origin of the protolith via dehydration melt-ing processes of metasedimentary sources. Major and trace elements show abun-dances and trends comparable with those of CPA peraluminous granitoids, exceptfor elements directly involved in a process of alkali metasomatism (Na, Ca, K, Sr,Ba, Rb), the abundances of which are very well correlated with variations in theK=Na ratio. The REE patterns for these trondhjemites seem to be mainly controlledby the behaviour of accessory phases by means of processes, possibly combined, ofdisequilibrium melting and metasomatic alteration. The Sr–Nd isotopic composi-tions of trondhjemites are similar to those of the CPA peraluminous granitoids,except that the trondhjemites tend to be less enriched in radiogenic Sr. Probableresetting of Sr and Nd isotopic systems limits the petrological application of87Sr=86Sr and 143Nd=144Nd ratios. Nevertheless, beyond possible variations due

40 P. Fiannacca et al.

to subsolidus modifications, the initial 87Sr=86Sr and 143Nd=144Nd ratios seem to beunlike to those normally found in trondhjemites directly generated from metaba-saltic sources. The consistent geological, petrographic and geochemical datastrongly suggest derivation of the Pizzo Bottino trondhjemites by sodic metasoma-tism at the expense of original peraluminous leucogranites and=or leucogranodio-rites similar to those outcropping in the southern CPA. Emplacement of originalgranitoid plutons may have occurred in late-Hercynian times, before the intrusionof the leucogranitic dykes which sharply cut the trondhjemites and which are inturn part of the picture of CPA peraluminous late-Hercynian magmatism (Atzoriet al., 1984a, b; Lo Giudice et al., 1985). Lastly, a late-Hercynian age is alsosuggested for the alkali metasomatism which probably occurred in the time-spanbetween the emplacement of the trondhjemite protoliths and the intrusions of leu-cogranitic rocks, which were not significantly affected by metasomatic alteration.

Acknowledgements

The authors wish to thank A. Dini (IGG-CNR, Pisa) for Rb–Sr and Sm–Nd isotopic analyses.Constructive criticism and comments by G. Hoinkes, F. Melcher and an anonymous reviewerare gratefully acknowledged.

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Authors’ addresses: P. Fiannacca (corresponding author; e-mail: [email protected]), P.Mazzoleni and A. Pezzino, Dipartimento di Scienze Geologiche, Universit�aa di Catania, CorsoItalia 55, 95129 Catania, Italy; P. Brotzu, Dipartimento di Scienze della Terra, Universit�aa diNapoli, Via Mezzocannone 8, 80138 Napoli, Italy; R. Cirrincione, Dipartimento di Scienzedella Terra, Universit�aa della Calabria, 87036, Arcavacata di Rende, Cosenza, Italy

Alkali metasomatism as a process for trondhjemite genesis 45