TheMontecristomonzogranite (Northern Tyrrhenian Sea, Italy):a collisional pluton in an extensional setting

FABRIZIO INNOCENTI 1�, DAVID S. WESTERMAN 2, SERGIO ROCCHI 1

and SONIA TONARINI 3

1Dipartimento di Scienze della Terra, UniversitaÁ di Pisa, Italy2Department of Geology, Norwich University, North®eld, Vermont, USA

3Istituto di Geocronologia e Geochimica Isotopica, CNR, Pisa, Italy

The Montecristo monzogranite (MM) is a near-circular peraluminous monzogranite pluton occupying the entire10 km2 of Montecristo Island. Outcrops of country rock are scarce, and are mainly roof pendants of metagabbros andcalcsilicate hornfels of the Apenninic ophiolite sequence. Emplacement of the pluton (Rb±Sr age � 7�1+0�2 Ma),following the early Miocene onset of continental collision, occurred during an extensional phase which migratedeastward via a combined process of subduction±delamination. The MM rocks are strongly porphyritic, the assemblagebeing composed of alkali-feldspar, quartz, plagioclase (all occurring as mega- or phenocrysts), biotite and minorcordierite. Accessory minerals include tourmaline, apatite, zircon, ilmenite, allanite, monazite, rutile and hellandite.Reconstructed crystallization histories for the mineral phases reveal a polybaric crystallization starting at about 5 kb.Textural variations of MM occur in sharp contact with each other; darker types often form globular masses containingfewer megacrysts and more abundant ma®c microgranular enclaves. Geochemical, isotopic and petrographic dataindicate that the MM magma was produced by anatectic melting of an intermediate to deep pelitic crustal source. Onthe basis of the geochemical and mineralogical characteristics of the enclaves, modi®cation of their parent magmaoccurred by crystal fractionation coupled with mixing and mingling of components from the MM magma. The limitedgeochemical variation in MM is interpreted as due to crystal fractionation processes during the magma's ascent.Younger porphyritic dykes with more potassic and alkaline a�nities cut the pluton; these dykes are concentrated in amajor fracture zone and are associated with contemporaneous pseudotachylites.

Received 24 June 1996; Revised 18 October 1996; Accepted 26 November 1996

Geol. J., 32, 131±151 (1997)No. of ®gures: 12 No. of tables: 2 No. of refs: 41

KEY WORDS Montecristo Island; Tuscany; Apenninic Orogeny; collisional granitoid; ma®c microgranular enclave

1. INTRODUCTION

The Northern Apenninic belt in central Italy is the result of a convergence between the Eurasian (Corsican)and Adriatic plates. Following the main phase of continental collision (early Miocene), convergencecontinued to the present via a combined process of delamination±subduction, causing an eastward migra-tion of the belt and contemporaneous extension in the inner (western) regions (Serri et al. 1993). Magmatismaccompanied formation of this ensialic back-arc basin, generating a complex volcano±plutonic associationknown in the literature as the Tuscan Magmatic Province (Marinelli 1967; Innocenti et al. 1992).

CCC 0072±1050/97/020131±00$17�50 # 1997 by John Wiley & Sons, Ltd.

GEOLOGICAL JOURNAL, VOL. 32, 131±151 (1997)

�Correspondence to : F. Innocenti, Dipartimento di Scienze della Terra, UniversitaÁ di Pisa, Pisa 56126, Italy.

Contract grant sponsor: Centro di Studio per la Geologia, Strutturale e Dinamica dell'Appennino, CNR.Contract grant number: NSF EPSCOR OSR 9350540.

After minor lamproitic subvolcanic episodes, igneous activity in the Tuscan Magmatic Province wassubsequently (7.3±6.0 Ma) dominated by acidic plutonism forming rocks now recognized on the islands ofMontecristo and Elba (Figure 1) and on the seamount of Mt. Vercelli in the central Tyrrhenian Sea. Only avery general description is available for the Montecristo pluton (Mittempergher 1954); sporadic descriptionsand chemical data on scattered samples (Poli et al. 1989), along with a Rb±Sr whole rock±biotite age(7.3 Ma: Ferrara and Tonarini 1985), have subsequently been published. This paper is intended to ®ll thegap of geologic knowledge for the Montecristo pluton, presenting new geological, petrological and chemicaldata based on a limited but systematic ®eld study. The petrogenesis of the Montecristo pluton will bediscussed in the context of the geodynamic evolution of the region.

2. GEOLOGICAL SETTING

Montecristo Island is an isolated, uninhabited and protected nature reserve situated along the westernmargin of the Tuscan Archipelago. It rises from the sea to a height of 645 m with an near-constant slope of258 in most pro®le sections (Figure 2). The island is located near the southern terminus of the shallow Tuscanplatform (water depths less than 200 m), and at the north end of a highly elongated and pronounced set ofN±S trending ridges and basins (Figure 1). These features, trending parallel to the Corsican coast, ®rst began

Figure 1. Regional location map for the Tuscan Archipelago region

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to develop by normal listric faulting in late Tortonian time (Sartori 1989). A secondary set of normal faults isaligned E±W (CNR 1983), producing an orthogonal system, with both Montecristo and Giglio located onrectangular upthrown blocks.

Access to Montecristo Island is discouraged by regulation and by the shoreline cli�s that rise steeply forseveral tens of metres. Montecristo's nearest neighbours are to the east Giglio (50 km), to the north Pianosa(30 km), and to the west Scoglio d'A�rica (9 km) and then Corsica (60 km) (Figure 1). The steep slopes ofthe island ¯atten abruptly in all directions to 1.58 within 300 m of the shoreline. Although the contact withthe surrounding country rock is under water, this geometry suggests that the pluton has a near-circular mappattern about 4 km in diameter.

The regional geologic and tectonic setting for the Montecristo intrusion is very similar to that of the otherTuscan Magmatic Province plutons, namely Monte Capanne and Porto Azzurro on Elba Island 60 km tothe north, and the Giglio granitic intrusions to the east. Those plutons were emplaced in a highly deformedallochthonous complex of Mesozoic ophiolites and predominantly argillaceous sedimentary rocks thatinclude some distinctive carbonates (Biancone and Tucci 1984; Keller and Pialli 1990; Westerman et al.1993).

Seismic studies (Fabbri et al. 1981) have revealed the pronounced character of the structurally controlled,N±S trending Elba Ridge located between Montecristo and Scoglio d'A�rica, where Quaternary limestoneoverlies a thin cover of Eocene ¯ysch (Bertini et al. 1969; Wezel et al. 1981). Other prominent N±Stopographic features south of Montecristo and east of Elba Ridge include the Montecristo Basin and theEtruscan Ridge (Figure 1). The axis of the pronounced N±S magnetic lineament in Figure 1 has beentraced for more than 300 km along the eastern edge of the Elba Ridge, through Montecristo to Elba Island,where it bends slightly to the west and continues northward (AGIP 1983). This lineament has beenattributed to a near-surface assemblage of ophiolitic rocks within the allochthonous Ligurian cover(Bartole et al. 1992).

Several buried granitoid intrusions, comparable in size to the Montecristo and Giglio bodies (Figure 1),have been interpreted on the basis of seismic re¯ection studies to occur within the ridges south ofMontecristo (Zitellini et al. 1986). Gravimetric and deep seismic studies constrain depths to the Moho andtop of the asthenosphere in the Montecristo region (Bartole et al. 1992; Scarascia et al. 1994). Althoughinterpretations of these data vary, there is general agreement that along a section through Montecristo

Figure 2. Montecristo Island, looking towards the south

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between Corsica and mainland Italy, crust and lithosphere thicknesses under the Tuscan Platform reachminimum values of approximately 25 and 50 km, respectively.

3. FIELD RELATIONS AND PETROGRAPHY

Montecristo Island consists mainly of acid intrusive rocks dominated by a medium-grained monzogranitebody containing abundant alkali-feldspar and quartz megacrysts with subordinate phenocrysts ofplagioclase (Montecristo monzogranite, MM). The properties of the MM range continuously and varietiesof this unit occur in sharp contrast with each other in the ®eld, displaying distinct di�erences in colour index,megacryst content, or the abundance of ma®c microgranular enclaves (MME). All these plutonic rocks arecut by aplite and greyish porphyritic dykes (Montecristo porphyritic dykes). Figure 3 presents a mapshowing the distribution of geological and structural features, along with sample locations. Some generalchemical characteristics of these samples are depicted in Figure 4A, the total alkali silica (TAS) diagram afterLe Bas et al. (1986).

3a. Montecristo monzogranite (MM)

The MM has a uniform mineralogy throughout, with gradational variations linked to the distribution ofalkali-feldspar and quartz megacrysts, biotite, and to di�erences in matrix grain size. The abundance ofbiotite, the only ma®c mineral, ranges from 8 to 12% (determined by automatic image analysis of photo-micrographs); rocks with higher percentages appear to have fewer megacrysts and more ma®c microgranularenclaves. Varieties of MM are often in sharp contact. In these cases, the darker variety frequently occurs ashighly rounded masses (5 to 20 m in diameter) immersed in the paler one (Figure 5).

The texture of MM is strongly porphyritic with ubiquitous alkali-feldspar (up to 13 cm in length),subordinate quartz megacrysts and plagioclase phenocrysts (Figure 6). According to the terminology ofBryon et al. (1994), these large crystals are suspended in a medium-grained, plagioclase/quartz/alkali-feldspar/biotite framework, with a matrix consisting of quartz and alkali-feldspar (Figures 7A and 7B).

Megacryst and phenocryst phasesEuhedral alkali-feldspar megacrysts are elongated with maximum elongation ranging from 2 to 13 cm andwith aspect ratios between 2 : 1 and 10 : 1. Local c-axis alignment is common but shows no consistent patternfrom place to place. Alkali-feldspar megacryst content in the MM is typically between 15 and 25% byvolume (®eld estimates according to Westerman et al. 1993). Locally, zones of exceptionally high concen-trations (70% vol.) of densely packed and generally aligned alkali-feldspar megacrysts occur in irregularelongated streaks up to 2 m in width. Zones with similar dimensions containing almost no alkali-feldsparmegacrysts but with quartz megacrysts tend to occur nearby, sometimes with biotite concentrations at theirmargins.

Microscopically, alkali-feldspar megacrysts exhibit irregular, indented and fringed outer edges. Carlsbadtwins are prominent and the crystals contain stringy microperthitic exsolution lamellea; their compositionsare in the range Or77±86 . These compositions and the others reported below were determined by the JEOLJXA-8600 electron microprobe (15 kV accelerating potential, 10 nA cup current, 2 mm spot size) ofGNV-CNR at Firenze University. Cores and intermediate portions carry rare euhedral inclusions of biotiteand normally zoned plagioclase. Towards the margins, quartz joins plagioclase (An37± > 20) and biotite�XFe � 0�63� to form zones of aligned inclusions. Also present as inclusions are anhedral to dendritictourmaline, as well as minor crystals totally transformed to a sericite±chlorite aggregate (probably formercordierite), apatite and sphene.

Ubiquitous, generally rounded quartz megacrysts reach 3 cm in diameter (Figures 7A and 7B) withinclusions of small anhedral biotite and plagioclase as well as very rare blue tourmaline and zircon.

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Figure 3. Geological and structural map of Montecristo Island with location of analysed samples (see inset)

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Plagioclase phenocrysts (An25±19) range in size gradationally up to 1 cm long, often as glomerocrysts. Corestend to have patchy zoning, while overgrowths exhibit oscillatory patterns. Inclusions are rare, restricted tobiotite, smaller plagioclase and very rare zircon.

Framework phases

The main crystal framework consists dominantly of euhedral to subhedral plagioclase (An17±26) exhibitingoscillatory zoning and containing inclusions of biotite parallel to the prismatic and domal borders. Alkali-feldspar occurs as perthitic subhedral crystals of varying dimensions. Reddish-brown biotite crystals in theframework have compositions similar to those included in alkali-feldspar megacrysts (XFe � 0�64±0�65).Sparsely scattered in most sections are rounded areas, 1±2 mm in diameter, of a very ®ne-grained aggregateof muscovite, biotite and chlorite, representing former cordierite crystals which are typically present in otherTuscan Magmatic Province rocks such as at Giglio Island (Westerman et al. 1993).

Matrix phases

Fine-grained perthitic alkali feldspar and quartz dominate the matrix, often showing vermicular graphicrelations. Biotite and zoned plagioclase occur in the matrix and as inclusions in alkali feldspar and quartz.Compositions of biotite in the matrix (XFe � 0�62±0�63) are only slightly more iron-rich than elsewhere(Figure 4B).

Accessory minerals include tourmaline, apatite, zircon (with radiogenic halos) and ilmenite, with minorsphene, monazite, allanite and rutile. Apatite and blue-green/yellow-brown tourmaline are ubiquitous andthe most abundant accessories. Additionally, a highly birefringent red-brown phase occurs, which on thebasis of the scanning electron microscopy±energy dispersive spectroscopy analytical data appears to be therare earth element-yttrium silicate hellandite (Mellini and Merlino 1977).

Summary

Based on textural relations, a sequence of crystallization for felsic minerals has been established as follows:(1) plagioclase, alkali-feldspar and quartz as mega- or phenocrysts; (2) plagioclase and alkali-feldspar asframework crystals (accompanied by partial resorption of quartz); and (3) matrix crystallization (oftenvermicularly) of interstitial perthitic alkali feldspar and quartz. Biotite crystallized throughout this sequenceas inclusions in phenocrysts, framework crystals, and ¯akes in the matrix.

Figure 4. (A) Enlarged portion of the TAS diagram (Le Bas 1986), also showing the Irvine and Baragar (1971) division of alkaline andsubalkaline ®elds. (B) Biotite compositions reported on FeO�±Al2O3±MgO, with the ®eld of biotites typical of calc-alkaline (C),

peraluminous (P) and alkaline (A) rocks (after Abdel-Rahman 1994)

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3b. Ma®c microgranular enclaves (MME)

MME at Montecristo occur sporadically throughout the pluton, but most abundantly within the darkervarieties of MM. Most enclaves have maximum dimensions measured in tens of centimetres with ellipsoidalshapes, but attenuated forms also occur. Colours range from very dark to medium grey, primarily as afunction of biotite concentration. The texture is characteristically microgranular with randomly orientedplagioclase laths in an equigranular, non-foliated matrix (Figure 7C). Scattered coarse xenocrysts arecommon (as much as 10% by volume) with alkali-feldspar crystals up to 10 cm in length and quartz up to1 cm in diameter. Xenocrysts exhibit macroscopic and microscopic characteristics of their counterparts inthe surrounding MM; similar relationships are well displayed nearby in the Monte Capanne pluton on ElbaIsland (Poli 1992).

In some MME, particularly near their margins, randomly oriented macroscopic biotite (1±1.5 mm) andplagioclase (3±5 mm) are observed with ovoid shapes, presumably resulting from partial dissolutionfollowing `capture' from the surrounding magma. Elsewhere, viscous ®ngering has produced swirly patchesof enclave material within a few millimetres of the elliptical enclave borders.

The matrix mineralogy of MME is, for the most part, the same as in the MM, but the proportionsof minerals vary signi®cantly within di�erent enclaves. Plagioclase and biotite dominate, with quartzor alkali-feldspar sometimes present only as minor constituents. The largest plagioclases are oftenstrongly zoned with calcic cores (An58±51) and more sodic margins (An48±40), and they locally containabundant oriented biotite and minor quartz in the core portion. The smallest plagioclase form weaklyzoned laths.

Randomly oriented biotite is generally abundant as small ¯akes, but sometimes occurs replacing largercrystals, probably hornblende based on the pseudomorphic outlines. The least evolved enclave sample(MW16; SiO2 � 58�5%) contains abundant ®brous pale amphibole in knots a few millimetres in diameter,

Figure 5. Globular masses of darker Montecristo monzogranite (maximum dimension 10 m) in the more typical paler variety(Cala Giunchitelli)

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apparently replacing pyroxene. In that sample, biotite is MgO-rich (XFe � 0�57±0�58) with respect to MMbiotite and plots in the calcalkaline ®eld of Figure 4B; a more evolved enclave (MW18; SiO2 � 67�6%) hasbiotite with composition practically indistinguishable from MM biotite. Quartz occurs interstitially withconsertal texture or, more rarely, as irregular mosaic patches up to 1.5 mm across; alkali-feldspar in thematrix is weakly perthitic.

Accessory minerals in ma®c microgranular enclaves include those found in the MM, with acicular apatitepresent in greatest abundance. Ilmenite is commonly included in biotite along with minor zircons havingwell-developed radiogenic halos. Sphene, as in the MM, occurs only rarely, locally as web-like intergrowths

Figure 6. Varieties of Montecristo monzogranite in sharp contrast. Note in (A) the higher concentration of megacrysts in the palervariety (lens cap � 55�2 mm), and in (B) the MME in the darker variety and the cross-cutting left-lateral pseudotachylite ®lled fault

(pen � 15 cm)

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Figure 7. (A) Photomicrograph of Montecristo monzogranite illustrating a rounded quartz megacryst in a framework of euhedral plagioclase, alkali-feldspar and biotitesuspended in a mosaic matrix dominated by quartz and alkali-feldspar; (B) illustration of the textural relationships between mega- and phenocryst, framework and matrixphases in MM, constructed from a scanned photomicrograph (P � plagioclase; Q � quartz; K � alkali-feldspar; B � biotite); (C) photomicrograph of ma®cmicrogranular enclave at the contact with the monzogranite; (D) photomicrograph of a Montecristo porphyritic dyke (MW5) illustrating two populations of plagioclase

phenocrysts (note the ¯ow fabric)

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enclosing the main phases. Allanite occurs with moderately strong pleochroism in pinkish purple andpale pink.

Zoned black and white nodules up to 10 cm in diameter occur in two enclaves (MW2a/b) near thenorthern border of the E±W trendingMaestra±CorfuÁ Fracture Zone (Figure 3). These nodules have cores ofpoikiloblastic schorl enclosing plagioclase and quartz crystals; the cores are surrounded by white borders upto 1 cm thick in which quartz and plagioclase occur with little or no tourmaline (as in the nodule) or biotite(as in the enclaves). These nodules probably formed by in®ltration of boron-rich hydrothermal ¯uids withinthe Maestra±CorfuÁ Fracture Zone (discussed in Section 4).

3c. Dykes

Aplites occur primarily as white, thin (10±30 cm), ®ne-grained dykes occasionally exhibiting splaystructures, en echelon patterns and lattice-like structures (occupying orthogonal joints sets). These dykes aregenerally tourmaline-rich and sometimes muscovite-bearing with strongly perthitic, untwinned, alkali-feldspar. Some dykes exhibit a strong schistosity due to aligned muscovite `shreds', entrained tourmaline thatis commonly ruptured (and healed with quartz) and aligned plagioclase (also often as fractured crystals). Thematrix in these strained aplites has a very ®ne-grained mosaic texture.

A second group of dykes, the Montecristo porphyritic dykes (MPD), weather to a greyish-brown colourand occur less frequently than the aplites, but their outcrop thicknesses are signi®cantly greater (up to 12 m).The largest of these dykes, ®rst described by Mittempergher (1954) as greenish-grey, granitic porphyry withresorbed quartz phenocrysts, trends E±W for more than 1 km along the southern shore (Figure 3). MPDemplacement was generally along steeply dipping planar fractures, but locally subhorizontal intrusions ofMPD material form irregular masses capping small knolls. Most MPD were observed in the southern thirdof the island within or near the Maestra±CorfuÁ Fracture Zone.

Although MPD vary considerably in their texture, mineralogy and degree of low-temperature alteration,they appear to represent two distinct types, both of which contain biotite and plagioclase as phenocrysts andhave a felty to microgranular groundmass. The most frequently observed type (MPD1) are recognized bytheir macroscopic resorbed quartz phenocrysts as well as variable amounts of alkali-feldspar and apatite asphenocrysts. These dykes sometimes contain inclusions of stubby alkali-feldspar xenocrysts, rounded ®ne-grained xenoliths, and anisotropic garnet xenocrysts from hornfelsed garnetiferous tactite (like that in thelocal country rock).

Plagioclase phenocrysts in MPD1 vary from euhedral to rounded. Glomerophyric aggregates are commonand most crystals show the e�ects of sericitization. Biotites tend to have `ragged' margins, suggestinginstability after initial growth; biotite from the most acidic MPD1 (MW22a) has a composition(XFe � 0�58±0�59) similar to that of the most basic MME (Figure 4B).

The second type of porphyritic dykes (MPD2), represented by MW5, are distinctly di�erent texturally andmineralogically in that they contain magnetite as phenocrysts in addition to the dominant plagioclase andbiotite which are, in this case, sharply euhedral (Figure 7D). The larger plagioclase crystals (up to 3 mm)exhibit oscillatory zoning, sometimes with resorbed cores surrounded by more basic rims. The dominantsmaller plagioclase are highly elongated with some parallel alignment, and they are moderately to stronglyzoned (An56±41). Biotite in MW5 is particularly enriched in Mg relative to all the other Montecristo biotites(XFe � 0�41, Figure 4B) and falls well within the calcalkaline ®eld of Abdel-Rahman (1994).

3d. Country Rocks

Exposures of country rock material are rare, with limited outcrops occurring near the southern shoreline(Figure 3). Some of these are disjointed foundered blocks and lack any systematic orientation; locally,however, exposures with bedding oriented parallel to the ground surface are thought to be unrotated roofpendants. These xenolithic blocks comprise an association of metagabbros, brown or black quartzites, and

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metapelites. Metagabbros are characterized by the occurrence of Mg-rich amphibole often with augite cores,partially albitized relict plagioclase, and minor talc, chlorite and epidote. Metasedimentary rocks are ®ne-grained granoblastic quartzites and albite-muscovite-epidote hornfels. These rocks are cut by numerousveinlets of the lilac-coloured borosilicate ferroaxinite (Mg=�Mg � Fe� � 0�19±0�21) and/or epidote withminor adularia; scattered scheelite is also generally present as inclusions in axinite. Fine-grained wollastoniteoccurs locally along vein margins.

Gently dipping, well-bedded metasedimentary rocks are exposed over more than 1000 m2 on two cupolasabove Punta Rossa (elevation 150 m); they have consistent bedding orientations (N66W, 27S to N81E, 18S)and are interpreted as roof pendants. This sequence is dominated by calcsilicate hornfels with variableproportions of calcite, garnet, diopside and quartz, commonly in granoblastic texture; rare biotite andplagioclase are also observed. Peculiar in this sequence is the occurrence of garnetiferous layers as muchas 40 cm thick, which are almost completely composed of a birefringent garnet characterized by rhombicdodecahedral habit, sector twinning and concentric zoning, with grossular-rich composition (Gr59±67And38±29Alm3±4). Texture and paragenesis are representative of medium-grade contact metamorphism, locallyfollowed by boron metasomatism.

Overall, the metamorphic association is thought to have been derived from an Apenninic ophiolitesequence which is typically composed by ma®c and ultrama®c rocks overlain by a sedimentary cover oflimestones, cherts and pelagic shale (with siliceous limestone interbeds). Ophiolitic sequences form one ofthe highest structural levels of the Apenninic chain. This relationship, coupled with the absence of anyevidence of regional metamorphism in the country rocks, indicates a shallow emplacement level of the MMpluton as previously interpreted for the other Tuscan Archipelago granitoids (Marinelli et al. 1993; Pertusatiet al. 1993).

4. STRUCTURAL OUTLINES

The most abundant structures in the Montecristo pluton are high-angle joints, often developed parallel tofaults (with or without mineralization or pseudotachylite development) and dykes (especially MPD). Inaddition, very well-developed sheeting commonly occurs in the MM, dipping towards the sea at 258 or less.

All observed faults and most MPD are located in the southern third of the island in the Maestra±CorfuÁFracture Zone, a wide belt rich in mylonitic structures. Northwest of Punta Rossa an extremely hard,silici®ed mylonitic breccia of MM shows maximum strain with porphyroclasts of quartz and alkali-feldsparin a ®ne- to medium-grained ribbon-like matrix of biotite, `shredded' quartz and feldspar (MW13a). Withlower degrees of strain at this location, discrete shear surfaces separate angular zones (tens of centimetres indiameter) having varying degrees of foliation, mylonitization and cataclasis. Very abundant disseminated®ne-grained tourmaline occurs locally where quartz and unzoned plagioclase phenocrysts are fragmented ina recrystallized matrix. These brecciated and recrystallized rocks include both ductile and brittle textures,indicating that deformation occurred within a broad range of conditions with respect to temperature and/orstrain rates.

The orientations of selected structural features, namely those involving either faulting or MPD emplace-ment, are presented in Figure 8. Included as faults are: (i) fractures with demonstrable o�set; (ii) zones ofcataclasis and/or mylonitization; and (iii) mineralized slip surfaces with lineations. Mineralization on faultsurfaces is commonly black tourmaline, less commonly quartz, and rarely a black sooty assemblage ofsulphides with associated malachite. Fractures containing pseudotachylite are plotted separately in Figure 8although they, too, represent fault surfaces. These pseudotachylites occur as dykes up to 10 cm thick with ablack glassy matrix containing visible quartz, feldspar and lithic fragments of the MM host. They are oftenassociated with tourmaline-coated joints and have been observed o�setting aplite dykes at two locations.The most prominent characteristic of this data is the correspondence between the strong (N50W) and weak

(N80E) trends of MPD and the various faults. In fact, all the MPD belong to these trends, as do half of the

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pseudotachylites plus all the other faults. The remaining pseudotachylites may represent a separatepopulation trending N20W with steep to moderate eastward dips. This pronounced geometric corres-pondence between MPD and faults implies that the emplacement of MPD only brie¯y preceded thedevelopment of the more pervasive brittle fault fabric in the Maestra±CorfuÁ Fracture Zone.

In Figure 8, measured fault and MPD orientations are plotted in the western quadrants and theorientations of 67 topographic lineaments (> 300 m) are plotted in the eastern quadrants as half-rosediagrams. It is interesting to note the correlation between the two data sets, but it is perhaps more importantto call attention to the absence of the regional N±S normal fault pattern that reportedly dominates the region.This may be due to di�erences in spatial and temporal characteristics of the two settings. The main fracturesets in theMMdeveloped in a narrowwindow of time near the close of crystallization in a small, rheologicallyhomogeneous pluton. Regional fracture systems, on the other hand, developed over a more extended periodwithin an extensive sheet of heterogeneous crust. In addition, the regional pattern has broad spacing betweenfaults (CNR 1983) so that the pluton may have escaped the in¯uence of this faulting (Figure 1).

5. RADIOMETRIC DATA

A cooling age for the Montecristo monzogranite of 7�07+0�07 Ma has been calculated by Rb±Sr methodsusing whole-rock±biotite (wr±Bt) data from the freshest sample collected (MW27, Table 1). Alkali-feldsparand plagioclase isotopic data from the same sample have been plotted in Figure 9. The data show little initialSr isotopic disequilibrium (Table 1), and the mineral±wr York®t gives a mean square weight deviation of 30,indicating that the errors on measured ratios are not the only cause of the scatter (York 1969). These resultssuggest some uncertainty regarding the biotite 87Sr/86Sr initial ratio. Taking into account that mineral phasesshowing slightly di�erent initial 87Sr/86Sr are coeval, we can use the Model-3 York®t for isochron calculation(McIntyre et al. 1966; Ludwig 1988). This model takes into account the `geological scatter or error' andyields a more realistic result with respect to the two-point isochron and the mineral±wr York®t. Thiscalculation gives an age of 7�07+0�15 Ma with a variation in initial isotopic composition among wr andmineral phases of 0.00027 (2s). The result di�ers from the wr±Bt date only in the amount of the associated

Figure 8. Structural elements of Montecristo Island

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error, indicating that mineral disequilibrium does not signi®cantly a�ect the calculated age. The most correctcooling age to adopt is thus 7�1+0�2 Ma which falls between the K±Ar (7�0+0�2 Ma) and Rb±Sr(7�3+0�3 Ma) ages reported by Ferrara and Tonarini (1985).

For the MPD, we obtained a wr±Bt Rb±Sr isochron for sample MW5, which indicates a cooling date of8�8+0:1 Ma. This result is unrealistic since cross-cutting ®eld relations show the dyke to be younger than theMM. The initial 87Sr/86Sr ratio for the biotite, corrected for a geologically realistic maximum age of 7.1 Ma,is 0.71233, distinctly higher than that of the whole rock containing it (0.70962). These values indicate astrong Sr isotopic disequilibrium between biotite phenocrysts and the host dyke. In order to test the likeli-hood of a reduction of the whole-rock Sr±isotope ratio by secondary crystallization of cryptic carbonates,we determined a whole-rock 87Sr/86Sr ratio after leaching with 6.2 N HCl. This ratio is indistinguishable,

Table 1. Rb±Sr isotope data on whole rock and mineral phases

Sample no. Material Rb (ppm) Sr (ppm) 87Sr=86Sr+2s 87Rb/86Sr (87Sr/86Sr)i

MM (Montecristo monzogranite)MW19 wr 349 111 0�71586+2 9.10 0.71495MW24a wr 379 114 0�71499+3 9.63 0.71402MW24b wr 356 91.6 0�71530+2 11.3 0.71417MW27 wr 382 111 0�71566+4 9.96 0.71466MW27 Kfs 602 111 0�71604+3 15.7 0.71446MW27 Plg 66.9 216 0�71482+4 0.897 0.71473MW27 Bt 1652 1.71 1�00343+6 2876 0.71345

MPD (Montecristo porphyritic dykes)MW5 wr 285 484 0�70979+2 1.70 0.70962MW5 wr (L) 0�70975+2MW5 Bt 826 20.5 0�72406+9 116 0.71233

Rb and Sr were determined by isotope dilution. Analytical error on 87Rb/86Sr ratio is 1% (95% con®dence limit). Sr isotopiccompositions were measured by VG-Micromass 54E single collector mass spectrometer. (87Sr/86Sr�i � initial ratio calculated at7.07 Ma; (87Sr/86Sr)i ratio was normalized to 86Sr=88Sr � 0�1194. Abbreviations: wr � whole rock; wr �L� � wr powder leached withhot 6.2 N HCl for 1 h; Bt � biotite; Plg � plagioclase; Kfs-K-feldspar

Figure 9. 87Sr=86Sr vs: 87Rb=86Sr diagram for whole-rock and mineral phases separated from Montecristo monzogranite (MW27).Straight line is the whole rock±biotite isochron

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within analytical error, from the non-leached measurement (Table 1), strongly suggesting that biotitecrystallized from a melt having a higher Sr isotopic composition that the dyke in which it occurs.

6. CHEMISTRY

Chemical analyses of major and trace elements are presented in Table 2 for 19 selected samples ofMontecristo monzogranite, ma®c microgranular enclaves and Montecristo porphyritic dykes (see Figure 3for locations). Petrographic examination of these samples revealed little evidence of chemical alteration withthe exception of MW13a, which was hydrothermally altered with silici®cation. In the alkali vs. silica diagram(Figure 4A), the most noticeable characteristics of the three groups are the pronounced positive linear trendof the MME samples, the distinct negative trend of the MM rocks, and the somewhat scattered character ofthe alkali-rich MPD.

Figure 10. Harker variation diagrams for (A) major elements and (B) trace elements; symbols as in Figure 4

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Table 2. Chemical analyses of rocks from the island of Montecristo

MM (Montecristo monzogranite) MME (ma®c enclaves) MPD (porph. dykes)

No. MW3 MW8 MW17 MW19 MW21 MW22b MW23 MW24a MW24b MW26 MW27 MW13a MW2a MW2b MW16 MW18 MW5 MW15 MW22a

SiO2 (wt %) 69.24 70.69 69.77 70.18 69.40 70.31 69.79 70.11 73.10 71.25 70.88 74.77 64.48 70.61 58.51 67.64 66.84 67.21 69.86TiO2 0.49 0.37 0.49 0.52 0.49 0.50 0.45 0.47 0.41 0.46 0.44 0.18 0.86 0.43 1.53 0.64 0.78 0.49 0.37Fe2O2 15.75 16.46 15.59 15.40 15.52 15.79 15.69 15.51 14.34 15.31 15.39 14.32 17.79 15.27 17.01 16.14 16.33 15.76 15.41Al2O3 0.87 0.98 0.74 0.97 0.82 0.91 0.94 0.78 0.82 0.65 0.75 0.79 1.12 0.52 1.88 0.79 1.11 1.90 0.99FeO 1.73 1.22 1.93 1.87 1.87 1.85 1.46 1.62 1.37 1.71 1.67 0.51 2.99 1.43 6.10 2.42 1.45 1.20 0.61MnO 0.05 0.05 0.05 0.05 0.04 0.05 0.04 0.04 0.04 0.04 0.03 0.04 0.07 0.04 0.08 0.05 0.02 0.02 0.01MgO 0.71 0.45 0.72 0.74 0.64 0.65 0.65 0.66 0.59 0.64 0.68 0.23 1.20 0.60 3.14 0.88 1.11 0.74 0.49CaO 1.64 1.12 1.63 1.75 1.82 1.53 1.59 1.68 1.30 1.47 1.68 0.60 2.13 1.27 4.19 2.03 2.00 0.24 0.40Na2O 3.93 3.97 3.85 3.74 3.96 3.48 3.70 3.36 3.37 3.49 3.60 3.92 4.06 3.56 2.59 3.88 3.51 2.49 3.52K2O 4.69 3.78 4.16 3.82 4.65 4.02 4.80 4.69 3.73 3.82 4.06 3.86 3.30 5.31 2.83 4.20 5.46 8.12 7.40P2O5 0.21 0.17 0.23 0.22 0.25 0.23 0.20 0.20 0.18 0.20 0.19 0.15 0.24 0.15 0.30 0.29 0.32 0.21 0.22LOI 0.70 0.75 0.84 0.74 0.54 0.69 0.69 0.88 0.75 0.95 0.63 0.63 1.76 0.82 1.83 1.04 1.08 1.61 0.72

V (ppm) 24 18 26 26 21 25 23 23 21 23 24 8 32 17 124 31 52 28 20Cr 12 12 12 13 12 12 10 13 9 12 13 8 8 8 44 15 26 11 11Co 5 4 6 5 6 6 4 5 4 4 5 1 8 4 24 8 7 7 4Ni 7 5 6 6 6 7 5 6 6 5 7 4 4 3 14 12 13 6 6Rb 411 504 387 364 360 388 359 415 376 364 396 540 393 453 464 380 297 399 368Sr 133 76 129 120 145 118 137 125 97 119 119 62 164 96 195 163 515 168 178Y 31 33 30 30 29 28 29 30 27 26 32 29 40 33 40 32 31 28 27Zr 145 111 153 157 158 135 139 131 132 143 136 70 310 175 164 225 171 169 149Nb 19 24 18 17 16 16 15 16 17 17 16 17 22 15 15 18 15 17 14Ba 284 85 281 190 314 257 363 350 177 239 234 131 550 487 379 501 653 761 600La 31 45 30 35 32 34 34 32 29 31 32 14 58 38 39 38 51 33 15Ce 59 86 54 65 58 53 61 56 52 56 57 18 99 64 66 65 85 64 32

Analytical techniques: MgO, Na2O determined by AAS; FeO by titration; LOI in microwave mu�e furance at 10008C, assuming complete oxidation of Fe2�: analyst M.Bertoli, DST Univ. of Pisa. Other major elements and trace elements determined by full matrix correction procedure on powder pellets: analyst M. Menichini, CSGSDA,CNR, Pisa, after Franzini et al. (1975) and Leoni and Saitta (1976). For trace elements the precision is estimated better than 5% for abundances greater than 10 ppm.

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Variations in abundance of major and trace elements with respect to SiO2 are presented in Harkerdiagrams (Figure 10). Although total chemical variation within the MM rocks is restricted (SiO2 ranges from69 to 73%, exclusive of silici®ed MW13a with 75%), plots show a negative correlation of all major elementsand silica. The pluton is peraluminous, with aluminium saturation index values ranging from 1.04 to 1.30;the maximum value is for MW8 which contains only minor alkali-feldspar megacrysts. Trace elements alsoshow small variability, with ratios between maximum values generally from 14% (Rb) to 21% (Nb) greaterthan minimum values. Only Sr and Ba have higher ranges, with maximum values being 33% and 51%greater than minimum values, respectively. These two trace elements are also the only ones showing a clearnegative correlation with SiO2 (others show dispersion, especially Nb and Y).

The analysed MME show greater compositional variability (SiO2 � 58�5 to 70�6%). They plot in thesubalkaline ®eld, where they de®ne a positive linear trend (Figure 4A). Other elements display complexbehaviour: CaO, FeOtot , TiO2 , MgO, and the trace elements Sr and V correlate negatively with SiO2

exhibiting linear or near-linear trends, while K2O correlates positively (Figure 10).Analysed samples of both MPD1 andMPD2 plot on the TAS diagram well above the trends de®ned by the

MM and MME (Figure 4A), and their K2O/Na2O ratios, as well as Ba and Sr contents, are distinctly higherthan for the MM and MME (Figure 10). MPD1 samples, which range in SiO2 between 67.2 and 69.9%, arenotably high in K2O (average 7.8%), approximately twice the values for typical MM. Na2O content of thesesamples is variable and CaO is distinctly low; these latter characteristics may be due, at least in part, to low-temperature alteration processes.

The chemistry of the MPD2 is represented by the freshest of all dyke samples (MW5). This rock plots onthe alkaline/subalkaline line in Figure 4A, intermediate in value between the MPD1 and MM samples. It isalso distinguished from all the MPD1 and typical MM samples by its greater abundance of Ca and P and, toa lesser extent, Al, Mg and Ti; furthermore, it is enriched in La, Sr and V, while being depleted in Rb.

7. DISCUSSION

7.1 The pluton

The Montecristo monzogranite intrusion, with its minimum emplacement age of 7.1 Ma and stronggeologic and petrologic similarity with the megacrystic Monte Capanne pluton of Elba, was producedduring the second stage of magmatic activity that a�ected the Northern Apennines between 7.3 (Elba) andabout 6 Ma (Capraia). This igneous activity generated mainly acid intrusives with only minor volcaniccentres made up of rhyolites (S. Vincenzo and Roccastrada). On the basis of geochemical and petrologicaldata, all the products of this phase are considered to be dominantly of crustal origin. The data collected onthe MM further support this petrogenetic model since these rocks are peraluminous, contain Al-richminerals such as cordierite, have biotite compositions typical of peraluminous magmas, and have relativelyhigh 87Sr/86Sr (Table 1) and low 143Nd/144Nd ratios (S. Tonarini, unpublished data). However, thecompositional variability exhibited by Tuscan Magmatic Province intrusives and their Sr and Nd isotopicratios have been interpreted as evidence of mixing between crustal melts, represented by some rhyolites fromS. Vincenzo and Roccastrada, and a basic mantle-derived magma, probably with 87Sr/86Sr ratio less than0.711, whose occurrence is revealed by the magmatic enclaves in both plutonic and volcanic rocks (Ferraraet al. 1989; Poli 1992; Serri et al. 1993).

Within the MM, evidence for interaction with ma®c magma includes the presence of MME, whichthemselves exhibit a range of compositions and provide physical evidence of mingling with the granitic melt.Although the 87Sr/86Sr ratios of the MM samples are well within the range of other Tuscan MagmaticProvince rocks, they are relatively low compared to the values for deep crustal sedimentary rocks of theregion (generally > 0�720 at 7 Ma: Ferrara and Tonarini 1985). This relationship is further indicative ofmixing with a basic component having a lower 87Sr/86Sr value but whose other geochemical characteristicsare not easily established. However, taking into account the subalkaline character, the relatively high K2O

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content and the K2O/Na2O ratio (>1) of the MME samples, we suggest that the basic magma had high-Kcalcalkaline a�nity. Although it is possible that the alkalis in MME re¯ect exchange between a basic magmaand a granitic one (Van der Laan andWyllie 1993), the presence on the island of Capraia of high-K andesitesand dacites with similar chemical a�nities and age (Serri et al. 1993) further supports our suggestion.

The plutonic rocks at Montecristo show only limited chemical variability but they display well-de®nedtrends of major and trace elements which can be evaluated by mixing±mingling and/or fractionationprocesses. Because these trends are strongly linear and in opposition to possible mixing lines with the mostma®c MME sample (Figures 4A and 10), we conclude that the observed trends for MM rocks were notproduced by simple mixing±mingling of ma®c material with the MM host.

Mixing of the two `end member' MM rocks (MW3 and MW24b) has been evaluated as a possiblemechanism to account for the variability of MM samples. For each of the intermediate samples in the series,a model composition was calculated minimizing the di�erences between the observed and calculateddistributions of determined elements. The measured/calculated concentration ratios, averaged for all theelements in each sample, are in the range 1.01±1.08 (standard deviation 0.10±0.15). These numbers indicatethat the observed chemical variability is compatible with mixing±mingling mechanisms but ®eld observa-tions of sharp contacts between varieties of MM rock, along with their occurrence as globular masses,indicate that these magma types did not mix readily.

Observed variations between the least-evolved (MW3) and most-evolved (MW24b) rocks of the MM arecompatible with a crystal fractionation model. This mechanism has been tested by a least-squares massbalance program (Stormer and Nicholls 1978). The calculations predict the separation of about 27% solids,consisting of mainly alkali-feldspar (12%) associated with plagioclase (10%) and biotite (5%). The ®t of theresulting data is very good, with the sum of the square residual at 0.25; the inclusion of a small amount ofaccessory minerals, such as apatite, results in an even better ®t (sum of the square residual � 0�14). Afractionation model of the Rayleigh type using the same amounts and kinds of solids noted above, and withpartition coe�cients currently used for granitic rocks (Henderson 1982; Nash and Crecraft 1985), is in goodagreement with the distribution of nearly all trace elements determined. However, a good ®t for Zr, Nb, Laand Ce requires the involvement of very small amounts (< 1%) of accessory minerals such as apatite andzircon. Abundances of the fractionated phases generated by this model are in agreement with the abundancesobserved in the framework phases of the MM. Taken all together, the evaluations above show thatfractionation of the framework phases is a viable explanation for the observed chemical variations within theMM, perhaps accompanied by mingling to transfer mega- and phenocryst material between magma varieties.

In order to evaluate the conditions under which crystallization occurred at Montecristo, the texturalrelations and compositions of our samples are examined with respect to the Q±Ab±Or±H2O system. Thissystem represents the MM well since normative An is consistently below 7%. All of the MM rocks showevidence that at the onset of crystallization, quartz and two feldspars were liquidus phases representing acotectic assemblage. Textures of these rocks indicate that the magma composition was very close to theminimum melt at the onset of crystallization, so that the precipitation of the three felsic phases was, for allintents and purposes, contemporaneous. Low nucleation density at this stage resulted in the stronglymegacrystic texture of the pluton. The successive stage was characterized by the conspicuous segregation ofplagioclase and alkali-feldspar and the resorption of the previously crystallized quartz, requiring a distinctchange in the physical±chemical conditions of the magma. Crystallization of the alkali-feldspar andplagioclase in this new regime occurred with a higher nucleation density, building the crystal framework anddriving the residual liquid towards a new minimum. Finally, interstitial crystallization of quartz and alkali-feldspar occurred.

Using the data of Ebadi and Johannes (1991), the least-evolved MM rocks plot near a minimum of 5 kb,with water activity of approximately 0.4 (Figure 11A). The trend of the samples shows migration towards theQ corner of the diagram while maintaining a near-constant Ab/Or ratio of 0.8. This trend is suggestive of avariation of the physical conditions during crystallization, with a signi®cant decrease in total pressure andincrease in water activity, and an associated reduction in the stability ®eld of quartz. These changes in

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conditions are consistent with the resorption of megacrystic quartz prior to the reappearance of quartz as amatrix phase.

The separation of framework phases from the mega- and phenocrysts which crystallized previously duringthe ®rst deep stage could have been achieved during the ascent of the magma along channelways towards itsrelatively shallow ®nal emplacement level. While ¯owage di�erentiation concentrated mega- and phenocrystsin the inner parts of the rising mass, sidewall crystallization, induced by heat transfer from the magma to thecountry rocks, produced the separation of the mineral assemblage that was crystallizing under the changingpressure conditions (mainly framework phases). A limited range of chemical variability in theMM rocks wasgenerated in this manner, with discrete masses of the variants entering the uppermost magma chamber,mingling on a grand scale and completing their solidi®cation.

7b. The enclaves

Chemical variations of MME are rather di�cult to explain. Their morphological and textural features,particularly their ellipsoidal form and inclusions of xenocrysts from MM, indicate an igneous origininvolving interaction of largely molten basic magma with partially crystallized MM magma. It is enticing tolook for a simple evolutionary model of mixing±mingling to explain MME variations, particularly given thelinearity of TAS plots for these samples (Figure 4A), but variations of individual elements on binarydiagrams are not always linear (Figure 10). Some elements (Al2O3 , Na2O, Nb, Zr and Ba) show a positivepeak plotted with respect to SiO2 , while others (Rb and La) show a negative peak.

MME samples are assumed here to have evolved from a single parent magma. The overall chemicalvariation between the most basic enclave MW16 (SiO2 � 58�5%) and the intermediate one MW2a(SiO2 � 64�5%) is compatible with a process of crystal±liquid fractionation. However, the increases inalkalis and Al2O3 indicate that the removed solids included an Al-poor phase which is not present in theexisting assemblage. Mass balance calculations show that the removal of a solid fraction composed of basicplagioclase and Ca-poor pyroxene, approximately in equal proportion and associated with minor opaques,®ts the observed chemical variations well. The inclusion in the fractionation model of pyroxene is, moreover,suggested by petrographic observations indicating the occurrence in the least-evolved enclave (MW16) ofequidimensional aggregates of ®ne-grained amphibole replacing a previous phase which is thought to havebeen pyroxene. The trace element contents of MME samples are compatible with this model, taking intoaccount the uncertainty of crystal/liquid partition coe�cients as reported in the literature (Henderson 1982;Nash and Crecraft 1985).

Figure 11. (A) Normative Q±Ab±Or compositions of Montecristo monzogranite (MM) samples; data illustrating the evolutionof granitic melts at di�erent P and with decreasing water activity are from Ebadi and Johannes (1991). (B) discrimination diagram for

Rb vs.Y � Nb (after Pearce et al. 1984)

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The transition from intermediate towards the more evolved enclaves (SiO2 � 65 to 72%), can be modelledby separation of phases now occurring in the rocks, i.e. intermediate plagioclase, subordinate biotite andminor ilmenite. However, trace element distribution is inconsistent with this model. In fact, the markeddecrease of both high ®eld strength elements (Zr, Nb and Y) and large ion lithophile elements (La, Ce and, toa lesser extent, Sr) point to a more complex petrogrenetic process. Poli and Tommasini (1991) and Poli(1992), studying the whole compositional spectrum of enclaves in Tuscan Magmatic Province granitoids,concluded that the main mechanism causing their chemical variations is mixing of an acidic melt with a basicmagma as the ma®c melt evolves by fractional crystallization. Such a process of mixing±mingling coupledwith fractional crystallization during dynamic interaction between the two magmas is thought to havegenerated the chemical characteristics of Montecristo enclaves.

7c. The porphyritic dykes

The MPD provide evidence of magmatic activity at Montecristo that followed the main phase of plutonemplacement, but this activity does not appear to be the result of late-stage injections of di�erentiationproducts derived from the MM. In the case of the MPD1 , their strong alkaline character with K, Ba and Srenrichment, relatively low SiO2 , relative Fe-poor biotite (XFe � 0�58), and the presence of microinclusionsof grossular hydro-garnet (like that in contact rocks of the MM), strongly suggest that these rocks werederived from outside the pluton. The case for an external origin of MPD2 is even stronger since those rocksare even poorer in SiO2 and contain relatively Ca-rich plagioclase and Fe-poor biotite. These characteristicsexclude the possibility that MPD2 were derived from the MM magma by fractionation processes; also,cumulate enrichment with biotite and/or feldspar cannot account for the observed data.

The sum of these observations, even if not conclusive, indicates that the MM magma did not serve as asource for theMPD dykes, but that they were derived instead from a source or sources having distinctly morepotassic and alkaline petrogenetic a�nities. We recall that on the island of Capraia, the high-K calc-alkalinevolcanics are cut by shoshonitic products of a signi®cantly younger age (Serri et al. 1993). This indicates thatwithin the Tuscan Magmatic Province, acidic subalkaline products and K-alkaline products are known tohave been emplaced at the same location in distinctly di�erent times.

8. CONCLUSIONS

The regional sequence of tectonic events leading to the genesis of the magmatic rocks at Montecristo beganin the lower Miocene with the onset of continental collision between the Corsican and Adriatic plates. As thezone of maximum compression migrated eastwards by delamination and rollback of the subductedlithosphere, an extensional regime developed in the Tuscan Archipelago region with thinning of the over-lying lithosphere. As has been previously proposed (Serri et al. 1993), this thinning was accompanied byupwelling of hot asthenosphere to replace the sinking lithosphere, with resulting generation of mantle-derived basaltic melts which interacted with the overlying crustal rocks and provided the heat source foranatexis to generate the Tuscan Magmatic Province magmas.

The evidence accumulated from the rocks of Montecristo Island supports this general scenario and allowsus to make the following conclusions.

(1) Mantle-derived ma®c magma is represented at Montecristo Island as a component of ma®c micro-granular enclaves whose broad chemical range resulted from fractionation±contamination as well asmixing and mingling with components of an acidic magma.

(2) The Montecristo monzogranite magma was produced by anatectic melting of a pelitic crustal source,and the resulting igneous rocks retain a geochemical signature characteristic of an overall collisionalenvironment (Figure 11B; Pearce et al. 1984), as do other Tuscan Magmatic Province rocks.

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(3) Crystallization of this partial melt product initially formed megacrysts of alkali-feldspar and quartz,along with phenocrysts of plagioclase (Figure 12, Stage 1), perhaps contemporaneously with anatexisabove (Huppert and Sparks 1988) and mixing±mingling below.

(4) Stage 2 of crystallization (Figure 12) occurred during the ascent of the MM magma to its ®nal shallowlevel of emplacement; ¯owage di�erentiation concentrated mega- and phenocryst phases, allowingcrystal fractionation of framework phases to occur along marginal contacts, while progressive reductionin the stability ®eld of quartz led to partial resorption of quartz megacrysts.

(5) Matrix crystallization (Figure 12, Stage 3) occurred following ®nal emplacement of the magma.(6) Shortly after complete solidi®cation of the MM, magmas derived from a source(s) having more potassic

and alkaline a�nities were emplaced as porphyritic dykes (MPD1 and MPD2) in the developingMaestra±CorfuÁ Fracture Zone.

ACKNOWLEDGEMENTS

Financial support for this work came from Centro di Studio per la Geologia Strutturale e Dinamicadell'Appennino, CNR, MURST, NSF EPSCoR OSR 9350540, and Norwich University. We wish to thankR. A. Wiebe and two anonymous reviewers for constructive criticism of the manuscript.

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