Reaction microtextures of REE–Y–Th–U accessory minerals

in the Monte Capanne pluton (Elba Island, Italy):

a possible indicator of hybridization processes

A. Dinia,*, S. Rocchib,1, D.S. Westermanc,2

aIstituto di Geoscienze e Georisorse, CNR, Pisa, Italy

bDipartimento di Scienze della Terra, Universita` di Pisa, Pisa, Italy

cDepartment of Geology, Norwich University, Northfield, Vermont, USA

Received 4 March 2003; accepted 23 April 2004

Available online 9 September 2004

doi:10.1016/j.lithos.2004.04.045

Lithos 78 (2004) 101– 118

Elsevier B.V. All rights reserved.

www.elsevier.com/locate/lithos

Abstract

The study of accessory minerals in granitoid rocks can provide clues to the history of magmatic processes. In particular, the textural–chemical characteristics of accessories could represent effective markers of hybridization processes. The concomitant occurrence of contrasting reaction microtextures of REE–Y–Th–U accessory minerals in the Monte Capanne anatectic–hybrid pluton suggests the occurrence of transient chemical conditions (alumina saturation up and down in the same system) in the early stages of crystallization. Incongruent dissolution of apatite produced microcrystal clusters of huttonitic monazite, while monazite-(Ce) crystals were replaced by allanite-(Ce)±apatite assemblage at the same location. A magma mingling process, involving acidic peraluminous and mafic metaluminous end-members, can provide the expected initial strong differences in alumina saturation that are able to induce such contrasting reactions. The proposed double exchange of accessory minerals between the two magmas strongly suggests a dynamic setting (stirring and straining of crystal-rich melts) in which anatectic and hybridization processes evolved.

Keywords: Accessory minerals; Granite; Hybridization; Reaction textures; Elba Island

1. Introduction

A variety of geological and geochemical evidence points out the hybrid nature of many granitoid plutons and batholiths that were fed by multiple batches of anatectic and subcrustal magma. In turn, the preservation in the rock record of geochemical heterogeneity, or its obliteration by homogenization of successive batches of melt, will depend on many factors including the interplay between rates of melting, segregation, diffusion, fractional crystallization, mixing, mingling, ascent and solidification. When hybridization processes go to the extreme, the character of individual magma batches can be obscured, producing large volumes of metaluminous and/or slightly peraluminous magmas solidified as apparently homogeneous monzogranitic, granodioritic and tonalitic plutons (e.g., Poli et al., 1996; Patiño Douce, 1999). The lack of distinctive geochemical and mineralogical variability or internal structures in these cases is not sufficient to rule out a history of complex open-system processes, requiring that less obvious, but still effective, markers of such processes be searched for.

Among the petrographic features that can contribute to unveil complex igneous histories, accessory minerals commonly played a secondary role. However, it is worth noting that the assemblages of accessories are more variable in granitic rocks than are fundamental phases, and most felsic magmas are saturated in at least some accessories early in their history (Watt and Harley, 1993). Nevertheless, the magmatic stability of key accessories like zircon, apatite and monazite-(Ce) is now reasonably well known, but experimental data are still lacking on the stability, in magmatic conditions, of important phases such as allanite-(Ce) (see Poitrasson et al., 2002 for a recent overview on accessory minerals). The stability of accessory minerals is susceptible to a range of important parameters, most notably temperature, melt composition and fO₂. As a consequence, the textural record of growth, chemical zoning and dissolution of such phases provides indications about the magmatic history; in particular, stability relations between monazite, allanite-(Ce) and apatite seem to be strongly controlled by the activity of Ca and the degree of peraluminosity of the magma (e.g., Wolf and London, 1995; Robinson and Miller, 1999; Broska et al., 2000).

An intrusion for which a crustal–subcrustal hybrid origin is documented (Dini et al., 2002) is the Monte Capanne pluton at Elba Island (Tuscany). Although its petrographic and chemical features are quite homogeneous, textural–chemical features of accessory minerals do record early mingling–mixing processes and provide clues to the history of the hybridization processes in this slightly peraluminous, anatectic–hybrid pluton. Th–U–REE–Y accessory minerals are involved in some unusual microtextures that could be indicative of reactions between minerals and melts that were far from equilibrium; mafic–acidic magma interactions (mingling, mixing) are invoked to explain such disequilibrium conditions during the very first formation stage of magmas that fed the Monte Capanne plutonic system.

2. Geological setting

2.1. Elba Island

Elba Island is located at the northern end of the Tyrrhenian Sea (Fig. 1), a region affected by extensional processes behind the eastward progressing front of the Apennine mobile belt, built by the collision of the Sardinia–Corsica block with the Adria plate (Malinverno and Ryan, 1986). The extensional regime migrated from west to east, trailing the movement of the compressive front (Brunet et al., 2000) and giving way to the opening of the extensional ensialic backarc Tyrrhenian basin.

Igneous activity associated with extensional processes also migrated from west (14 Ma) to east (0.2 Ma) as the west-dipping Adriatic plate delaminated and rolled back to the east (Serri et al., 1993). Intrusive and extrusive hybrid products from mantle and crustal sources built the Tuscan Magmatic Province, spreading over about 30,000 km2 in southern Tuscany and the northern Tyrrhenian Sea (Poli, 1992; Westerman et al., 1993; Innocenti et al., 1997; Dini et al., 2002).

The structure of Elba Island consists of five tectonic units stacked onto each other by about 20 Ma during the Apenninic convergence (Deino et al., 1992). The lower three units have continental features, whereas the upper two are oceanic in character. Extensional processes affected the area of Elba Island during late Miocene (Jolivet et al., 1994) and largescale faults separated Elba Island into the western, central and eastern zones.

During the late Miocene, between about 8 and 6.8 Ma, an intrusive complex was progressively emplaced within the tectonic stack of the western Elba Island with multiple injections of magma (Dini et al., 2002). The igneous sequence started with construction of a laccolith complex (Rocchi et al., 2002) by emplacement (8.0–7.4 Ma) of almost pure anatectic magmas. The deepest layers of this complex were then intruded by the hybrid–anatectic Monte Capanne pluton and its associated late leucogranite–pegmatite dykes (6.9 Ma). Finally, mantle-derived hybrid magmas were emplaced to produce a dyke swarm (Orano porphyries, 6.85 Ma) that cut the entire succession. The magma formation processes recorded by the Elba magmatism changed from crust-, to hybrid-, to mantle-dominated as the Apennine fold belt was progressively thinned, heated, and intruded by mafic magmas during late Miocene time (Dini et al., 2002). Approximately 1 million years later, the next locus of igneous activity developed further east in eastern Elba where the mainly anatectic Porto Azzurro pluton, its associated aplite–pegmatite dykes, and some mafic dykes were emplaced at about 5.9 Ma (Westerman et al., 2004; Maineri et al., 2003, Conticelli et al., 2001; and reference therein). During the same time span, a K-andesite/shoshonite volcanic complex was built up at Capraia Island, 30 km northwest of Monte Capanne plunton (Poli and Perugini, 2003).

2.2. The Monte Capanne pluton

The Monte Capanne monzogranite is the largest of the exposed, Miocene–Pliocene plutons in the Tuscan Magmatic Province (Innocenti et al., 1992), and is, along with its

Fig. 1. Geologic sketch map of Monte Capanne pluton. The preliminary contacts between the different intrusive facies are indicated as dashed lines (field mapping project in progress). Topography of Monte Capanne intrusion is shown using a DEM base (courtesy of M.T. Pareschi, IGGCNR Pisa).

leucocratic products, the most extensively studied petrologically. Several petrographic and geochemical characteristics of the intrusion cannot be fully explained invoking a purely anatectic origin and the contribution for subcrustal magma has been documented (Poli, 1992, Dini et al., 2002).

According to new field observations, coupled with petrography, mineral chemistry and geochemistry, the Monte Capanne intrusion can be described as a composite pluton fed by multiple hybrid melt batches that experienced further mingling at the emplacement level. Three main facies are recognised (Fig. 1 and Table 1, Dini et al., 2002): the silicic “Sant’Andrea facies” (NW part) and the relatively less silicic “San Piero facies” (SE part), separated by the wide transition zone of “San Francesco facies”.

The two extreme facies can be easily distinguished by textural characteristics: The Sant’Andrea facies exhibits 20–40 vol.% of coarse phenocrysts (euhedral/subhedral K-feldspar megacrysts, quartz, plagioclase and biotite) developed as a crystal suspension, during an early stage of crystallization; the San Piero facies is almost devoid of early coarse phenocrysts and mostly appears as a homogeneous, fine- to medium-grained rock where K-feldspar crystallization started at a relatively late stage to produce large anhedral– subhedral, extremely poikilitic K-feldspar crystals. The character of the San Francesco facies is highlighted by its transitional texture, intermediate between Sant’Andrea and San Piero. In fact, this facies has an intermediate porphyricity with scattered K-feldspar megacrysts frequently showing late (San Piero-like) anhedral K-feldpar overgrowths (mm to cm thick) in optical continuity with early crystallized (Sant’Andrea-like) subhedral/euhedral cores. The Monte Capanne pluton shows restricted chemical variations (Fig. 2). The whole pluton has monzogranite composition (SiO₂ between 66 and 70 wt.%) and slightly peraluminous character (average Alumina Saturation Index [ASI]=1.11±0.05 1S.D.). However, samples from San Piero and Sant’Andrea facies respectively occupy the less silicic and more silicic parts of Monte Capanne field when plotted in Harker diagrams (Fig. 2). The San Francesco facies defines an intermediate field partially overlapping both the extreme facies.

As reported by Westerman et al. (2003), the hybrid character and the peculiar petrographic and geochemical features of the two extreme facies were acquired somewhere at depth before the final emplacement of magmas in the pluton body. On the other hand, the present gradational transition between Sant’Andrea and San Piero facies, with the formation of the so-called San Francesco facies, indicate that further stirring occurred after/during emplacement of the two already hybrid batches of magma. Geochemical and isotopic studies (Poli, 1992; Dini et al., 2002; Westerman et al., 2003) indicate that the hybrid magmas that fed the Sant’Andrea and San Piero facies were originated by hybridism between a dominant peraluminous, crustal magma (similar to San Vincenzo rhyolites, mainland Tuscany and Cotoncello leucogranite, Elba Island) and a mantle-derived magma similar to the nearby, coeval K-andesites and shoshonites of Capraia Island. Strongly modified witnesses of such mantle component can be identified in the widespread mafic microgranular enclaves from all the Monte Capanne granite facies. Finally, an even more metasomatised mantle was later involved in Elba magmatism providing mantle-derived magmas (Orano porphyries) having geochemical–isotopic characteristics intermediate between Capraia Island K-andesites and Tuscan lamproites (Dini et al., 2002; Serri et al., 1993).

Monte Capanne leucogranite dykes and aplite– pegmatite veins occur mainly close to the pluton’s eastern contact, within both the pluton and its metamorphic aureole (Fig. 1).

Fig. 2. (a) Total alkali vs. silica (TAS) classification diagram of western Elba magmatic products. (b) MgO vs. SiO2 and (c) Sr vs. SiO2 Harker diagrams showing the slight differences between facies of Monte Capanne monzogranite.

Leucogranites are syenogranitic in composition and show the highest SiO₂ contents (74–77 wt.%) among the Elba intrusive units. Dini et al. (2002), on the basis of major and trace element modeling and Sr–Nd isotopic data, interpreted the leucogranites as fractionation products from a magma having characteristics similar to those of the San Piero facies. Aplite–pegmatite veins are abundant along the whole pluton perimeter, but only the veins from the eastern contact are strongly enriched in B, Li, Cs and Ta, as well as being characterized by a peculiar Ti–Nb–Y–U-rich mineralogical assemblage (Aurisicchio et al., 2002). The tight spatial and structural relationships existing between leucogranites and rare element pegmatite veins along the eastern edge of Monte Capanne pluton could be indicative of a possible genetic link, as proposed by Pezzotta (2000).

3. Samples and methods

Fifteen samples, five for each facies of Monte Capanne pluton, were selected from the large set used by Dini et al. (2002) for the petrologic study of the western Elba intrusive complex. The petrographic and chemical investigation of accessory phases was carried out by SEM with backscattered imaging and EDS, coupled with EPMA-WDS analyses (representative EPM analyses are reported in Tables 2 and 3). Backscattered imaging and EDS qualitative analyses were carried out by means of the Philips XL30 equipped with EDAX DX4 housed at Dipartimento di Scienze della Terra, Pisa University. Quantitative analyses of accessory minerals were made with a JEOL JXA 8600 electron microprobe at the IGGCNR—Florence, operating in the wavelength-dispersive mode. The operating conditions during analyses were an acceleration voltage of 15 kV, a beam current of 50 nA

(measured on the Faraday cup) and a spot size of 4 µm. The counting times on the peak is 40 s, and the same amount of time for background counts on both sides of the peak. Ka lines were used for Si, Ti, Al, Fe, Mg, Ca and P, La lines for Y, La and Ce, and Lb lines for Pr, Nd, Sm and Gd. Problems due to interferences of ThMh with UMa were eliminated by using the ThMa and UMh. Primary standards included kaersutite for Si, Al, Fe, Mg and Ca, rutile for Ti, pure metals for Th and U, synthetic cubic zirconia for Y, monazite for P and La, Ce, Pr and Nd, and REE glasses prepared by Drake and Weill (1972) for Sm and Gd. Detection limits were approximately 500 ppm for Si, Al, Ti, Fe, Mg, Ca and P, 150–250 ppm for REE, and 100 ppm for Y, Th and U.

4. Accessory minerals: textures and chemistry

All the facies are characterized by very similar accessory mineral assemblages dominated by

Table 3

Representative EPM analyses (oxides and halogens as wt.%) of apatites from Monte Capanne pluton, Orano porphyry and Capraia

ubiquitous apatite and zircon, followed in abundance by monazite-(Ce) and rare U-rich thorite inclusions in zircon (Fig. 3). These minerals form small euhedral crystals (20–500 µm) preferentially hosted in biotite. Rare crystals of late xenotime-(Y), uraninite and unidentified Ti–Nb–Y–U oxides, hosted in interstitial quartz, have been only observed in San Piero and San Francesco facies (Fig. 3f). Another peculiarity of these two facies is the presence of: (1) euhedral monazite- (Ce) crystals replaced, to different degrees, by allanite-(Ce) and apatite, and (2) Th-rich monazite microcrystal clusters. Finally, ilmenite occurs as a

relatively common accessory mineral in the Sant’Andrea facies, while it is absent or very rare in the San Piero and San Francesco facies.

A detailed petrographic study was carried out to characterize the different types of monazite group minerals occurring in the San Piero and San Francesco facies. Indeed, in spite of the abundant and ubiquitous occurrence of euhedral monazite-(Ce) in all the intrusive facies (Fig. 3e), the peculiar character of San Piero and San Francesco facies is the frequent occurrence of two contrasting types of reaction microtextures involving monazite group minerals. Type-I texture (Fig. 3a,b) is represented by aggregates of allanite-(Ce) and minor apatite replacing, to different degrees, original euhedral monazite-(Ce) crystals which remain as skeletal relics. Type-II texture is represented by clusters (50–100 µm) of Th-rich monazite microcrystals (1–10 µm) pointing out the former presence of a euhedral prismatic crystal of an unknown mineral (Fig. 3c,d). The space between the Th-rich monazite microcrystals is filled by interstitial quartz and K feldspar

Fig. 3. Selected SEM-BSE images of accessory minerals of Monte Capanne pluton. Photos (a) and (b) show Type-I reaction microtexture. Photos (c) and (d) show Type-II reaction microtexture. Photo (e) is a typical euhedral crystal of monazite-(Ce), and photo (f) shows an intergrowth of xenotime-(Y) and an unidentified Ti–Nb–Y–U oxide that grew late over an early zircon crystal. Abbreviations: Aln: allanite-(Ce), Ap: apatite, Bt: biotite, hut-Mnz: huttonitic monazite, Kfs: K-feldspar, Mnz: monazite-(Ce), Pl:plagioclase, Qtz: quartz, Xen: xenotime-(Y), Zrc: zircon.

plus some anhedral grains of apatite. The Th-rich monazite microcrystals, sometimes are physically interconnected defining a weakly packed cluster, sometimes are spatially separated. Type-I texture can be frequently observed in most samples, whereas the Type-II texture is relatively rare and it seems to be less widespread.

Composition of monazite group minerals observed in Monte Capanne pluton are shown in Figs. 4 and 5. Stable euhedral crystals occurring in all the facies and skeletal relics from the Type-I microtexture in San Piero and San Francesco facies show similar compositions (monazite component >85 mol%) ranging in the field of monazite-(Ce). At the scale of the single euhedral crystal, the composition is quite homogeneous, although, under high-contrast BSE imaging, few crystals showed oscillatory zoning (Th content variations). The extent of huttonite substitution in Th-rich monazite microcrystals from the Type-II microtexture ranges from 25 to 60 mol%; most of the analysed spots plot in the range 25–35 mol% REEPO₄. According to Förster (1998), these crystals can be classified as huttonitic monazite. Some representative analyses of monazite-(Ce) and huttonitic monazite are reported in Table 2 and in Figs. 4 and 5.

Composition of allanite-(Ce) replacing monazite-(Ce) in Type-I microtexture is shown in the plot (Fig. 6) of total Al vs. REE (+Y+Th+U) as suggested by Petrík et al. (1995). Alumina content displays significant variations ranging from high values, typical of allanite-(Ce) from peraluminous granite, to low values, typical of metaluminous and

Fig. 4. Compositional variation of monazite group minerals in Monte Capanne pluton. Base diagram modified after Förster (1998).

Fig. 5. Chondrite normalized REE variation diagram for selected analyses of the accessory minerals discussed in the text. The

variation field for the Monte Capanne pluton granites is also reported for comparison. Heavy REE were not determined for

minerals due to the very low concentrations below or near to the detection limit. However, it can be assumed that their behavior

approximates that of Y (included in the place of Ho). Normalization values used are those of McDonough and Sun (1995).

peralkaline granitoids. Data are scattered but they approximate an elongated trend approaching, at low Al content, the compositional field of allanite-(Ce) found in the more mafic products (mafic microgranular enclaves and mantle-derived late dykes; Dini, 1997; Viviani, 1997) associated with Monte Capanne pluton. Some representative analyses of allanite-(Ce) are reported in Table 2, and REE patterns in Fig. 5.

Apatite played a role as both a probable precursor of Type-II microtexture, and a minor reaction product in Type-I microtexture. Therefore, in order to discuss Type-I and Type-II textural and chemical data, the mineral chemistry of apatite of selected magmatic rocks from Elba and Capraia islands was investigated (Table 3 and Fig. 5): (1) euhedral microcrystals from Orano porphyries; (2) euhedral microcrystals from mafic microgranular enclaves in San Piero facies; (3) euhedral microcrystals from Capraia K-andesites; (4) euhedral microcrystals from Monte Capanne monzogranite (San Piero and Sant’Andrea facies); (5) anhedral grains (replacement product) in Type-I microtexture; (6) anhedral grains (relics?) in Type-II microtexture. All the analysed apatites show a high F content and can be classified as fluorapatite (F up to 3.5 wt.%; see Table 3). They can be divided in two groups: (i) a first group, including euhedral apatites from Monte Capanne monzogranite and replacive apatite (Type-I microtexture), is characterized by an almost flat REE pattern (from La to Sm) with La_n/ Sm_n>0.6–0.8 (few analysis spots slightly over 1); (ii) a second group, including euhedral apatites from Orano porphyry, San Piero mafic microgranular enclaves and Capraia K-andesites as well as the apatite relics in Type-II microtexture, show a more fractionated REE pattern with La_n/Sm_n>2–4. Apatites from the second group have a higher content in REE and a detectable amount of Th and U. Yttrium is always above the detection limit in all the analysed apatites, but it does not show any systematic distribution (Y₂O₃ in the range 0.05–0.50 wt.%).

5. Discussion

5.1. Relationships between microtextures and chemical reactions

In the Tuscan Magmatic Province, young anatectic–hybrid granites (8–4.4 Ma) exposed at the surface provide the opportunity to investigate both magmatic and hydrothermal processes in rocks that did not suffered multiple magmatic–metamorphic–hydrothermal events typical of older orogenic belts. The plutons of Gavorrano and Campiglia Marittima, as well as the buried bodies of Larderello and Castel di Pietra, offer a record of strong hydrothermal processes that make them suitable for investigation of the late- to postmagmatic history (e.g., Dini et al., 2003; Boyce et al., 2003; Franceschini et al., 2000; Villa et al., 1997). On the other hand, the Monte Capanne and Montecristo plutons (Dini et al., 2002; Innocenti et al., 1997) lack pervasive hydrothermal modifications and are representative of either "dry" magmatic/well-drained systems, or systems poorly connected with meteoric circulation. Therefore, in the latter intrusions, delicate textures involving accessory minerals have been preserved together with the overall original petrographic and chemical features of the magmatic rocks.

A magmatic origin for the reaction microtextures observed in the San Piero and San Francesco facies of Monte Capanne pluton is supported by the lack of any significant hydrothermal effects (e.g., chloritization) in the samples studied. Furthermore, studies of

Fig. 6. Compositional variation of allanite-(Ce) replacing monazite-(Ce) in Type-I microtexture. Base diagram and reference fields of metaluminous and peraluminous granites modified after Broska et al. (2000). Aln: allanite-(Ce), Clz: clinozoisite, Epi: epidote, ferri-Aln: ferriallanite.

coeval monzogranitic products exposed in central Elba and locally affected by pervasive hydrothermal effects (San Martino Porphyry, Dini, 1997) pointed out that the alteration products of accessory minerals differ from the products observed in reaction microtextures of Monte Capanne pluton. Indeed, in those cases, hydrothermal fluids produced strong chloritization of biotite and sericitization of feldspars, as well as the partial breakdown of monazite-(Ce) in secondary REE fluoro-carbonates.

An interpretation of primary origin and early appearance of reaction microtextures, as well as of the euhedral crystals of monazite-(Ce) and other accessory minerals, is supported by their shape and textural position in the rock. Further support comes from the hosting of such minerals not only in the quartz–feldspar mass, but especially at the cores of early euhedral–subhedral biotite and plagioclase crystals. Based on these textural observations, we consider these reaction microtextures as the witnesses to processes that occurred in the early evolutionary stages of magmas that fed Monte Capanne pluton. Owing to these observations, variation in melt chemistry induced by subsequent crystallization of main phases can be ruled out as an effective process influencing the formation of such reaction microtextures. Finally, a restitic origin of these microtextures from the metamorphic– anatectic protolith is argued against by the euhedral shape of the phases (Fig. 3), in contrast to the general anhedral shape observed in reacting accessory minerals from high grade migmatitic rocks (Bea and Montero, 1999; Watt and Harley, 1993).

On the basis of our present knowledge of accessory minerals, derived from both experimental studies and direct observations in natural systems, an explanation of the observed reaction microtextures has been proposed. Most fertile crustal lithologies contain a

sufficient amount of REE, Y, Zr, P to saturate an anatectic melt in these components and allow early precipitation of accessory phases (e.g., Watt and Harley, 1993). In turn, stability relationships between accessory phases such as monazite-(Ce), allanite-(Ce) and apatite are mainly controlled by the activity of Ca and degree of peraluminosity of the melt. Allanite-(Ce) (and/or titanite) in particular should be stabilized in metaluminous melts with CaO>1 wt.% (Cuney and Friedrich, 1987); in contrast, monazite-(Ce) is a stable phase in peraluminous melt owing to its low solubility in such melts (Rapp andWatson, 1986). Peraluminosity also control solubility of apatite: increasing the ASI values in melts increases the stability of the AlPO₄ complex, which, in turn, enhances the solubility of apatite (Wolf and London, 1995). The key to understand why allanite-(Ce) is the stable LREE phase in metaluminous granites whereas monazite plays the same role in peraluminous granites is not the magma phosphorus contents, which may be higher in allanite-bearing granites, but the excess of Ca over Al, which stabilizes minerals such as epidote and allanite. Thus, on a qualitative way, early crystallisation of apatite and allanite-(Ce) (and/or titanite) is favoured in metaluminous melts, while early crystallization of monazite-(Ce) and/or xenotime should be observed in peraluminous melts. However, an intermediate field of melt composition can be identified in which all the phases could coexist. This field is represented by many slightly peraluminous hybrid granitoids, having intermediate Ca content (around 2.5 wt.%), that resemble the actual compositions of products in Monte Capanne pluton. Therefore, the actual composition of Monte Capanne products is compatible with the observed coexistence of monazite-(Ce), allanite-(Ce) and apatite, but cannot explain the reaction microtextures involving these minerals. The formation of such microtextures needs "extreme" melt compositions in order to achieve disequilibrium conditions for the accessory minerals.

We interpret reaction microtextures of accessory minerals in Monte Capanne pluton as follows. For Type-I microtexture, products and reactants were maintained and the general chemical reaction (1) can be proposed:

monazite-(Ce) from the melt allanite-(Ce) apatite

LREEPO₄ + (CaO+FeO+Al₂O₃+SiO₂+H₂O) Û (Ca,LREE)₂(Al,Fe)₃(SiO₄)₃OH ± Ca₅(PO₄)₃(F,OH)

¾¾®

(Decreasing ASI – increasing Ca activity in the melt)

(1)

Replacement of monazite-(Ce) by allanite-(Ce)±apatite, has been reported to occur in both metamorphic and igneous rocks (Finger et al., 1998; Broska and Siman, 1998; Ward et al., 1992) as a result of either late hydrothermal reactions or fluid-driven metamorphic reactions. In the Monte Capanne pluton, owing to the hybrid character of its products and the absence of significant hydrothermal effects, a magmatic process controlled by the interaction of melts with contrasting chemical compositions seems more likely. In this scenario, we hypothesize that euhedral monazites from a strongly peraluminous melt (pure anatectic melt?) got enclosed in a high-Ca, metaluminous melt during the hybridization process, thus suffering varying degrees of replacement (occurring mostly as a volume-for-volume process) by allanite-(Ce) and apatite. Moreover, REE and Th contents of allanite-(Ce) in Type-I microtexture (Table 2) are typical of magmatic allanites (e.g., Bea, 1996; Broska et al., 2000) and significantly higher than observed in allanites from hydrothermal veins crosscutting Tuscan granitoids (P. Orlandi, personal communication) as well as other granites elsewhere (e.g., Exley, 1980).

In contrast, the product in Type-II microtexture (huttonitic monazite) has been preserved, while the reactant was almost totally dissolved (rare anhedral apatite grains might be interpreted as relics of former euhedral crystals). The petrographic character of Type-II microtexture indicates a replacement process with volume loss, consistent with an incongruent dissolution mechanism. Wolf and London (1995) obtained similar textures in experiments as precipitation of monazite microcrystal clusters (defining the former shape of apatite crystal) occurred during the incongruent dissolution of apatite crystals that were placed in a strongly peraluminous melt. Based on the results of these authors, formation of Type-II microtexture could be explained assuming the general chemical reaction (2):

apatite from the melt huttonitic monazite to the melt

(Ca, LREE)₅(PO₄)₃(F,OH) + (Al₂O₃ + SiO₂) Û (LREE, Th) (PO₄) (SiO₄) + (CaO + AlPO₄ +F+H₂O)

¾¾®

(Increasing ASI – decreasing Ca activity in the melt)

(2)

Also in this case, as noted previously for Type-I textures, a magmatic process involving mingling of magmas with contrasting compositions seems to be the most reasonable for introducing apatite crystals (from a mafic melt) into a strongly peraluminous melt. In apatites, REE and Th contents are significant (from hundreds to thousands of parts per million; e.g., Bea, 1996), but do not represent an essential structural constituent. This characteristic, coupled with the high stability of AlPO4 in peraluminous melts and the low diffusivity of REE and Th for huttonitic monazite growth, can explain the replacement with volume loss producing a non-continuous microcrystal cluster of huttonitic monazite. This feature strongly contrasts with Type-I microtexture derived by reaction (1), in which both reactant and product contain REE, Th and Y as essential structural constituents.

However, Harlov et al. (2002) recently reported experimental results where (Y+REE) phosphate minerals nucleated in apatite in the presence of aqueous fluids. Such an interpretation for Monte Capanne apatites, involving late fluid interactions (with high T, strongly evolved, water-rich melts or hydrothermal fluids), cannot be completely ruled out, but is, thus far, not supported by field and petrographic evidence.

In summary, the observed textures (Type-I and -II) are indicative of reactions between minerals and melts that were far from equilibrium; we suggest that such disequilibrium conditions were created by mafic–felsic magma interactions (mingling, mixing) during the very first formation stage of magmas that fed the Monte Capanne system.

5.2. Behaviour of REE, Y, Th and U in reactions involving accessory minerals

Because accessory minerals control the REE, Y, Th and U budget in granitic rocks, further discussion is needed to describe the behavior of such elements during reactions (1) and (2). Discussion of Type-II reaction needs some consideration about mineral chemistry of apatite and monazite. The proposed derivation of huttonitic monazite by incongruent dissolution of apatite must be tested verifying if the characteristic element ratios are maintained from reagents to products (e.g., REE/Th). Magmatic apatite, even from a variety of geologic settings (e.g., Bea, 1996; Sha and Chappel, 1999), usually shows a significant REE and Y content (hundreds to thousands of parts per million) but low Th and U content (tens to hundreds of parts per million), thus, their REE/Th ratios are significantly higher (»20–30) than those observed in typical magmatic monazites (»5–15). As previously outlined by Förster and Harlov (1999), the high REE/Th ratios of apatites could be used specifically in igneous rocks distinguishing between monazite formed by incongruent dissolution of apatite during magma formation (high REE/Th ratio) and Th-rich monazite grown later from the melt (relatively lower REE/Th ratio). In the former case, the authors assumed a steady system, where new monazite grains inherited the REE/Th ratio of former apatite, i.e., no fractionation of Th and REE was permitted in the crystallizing medium (melt and/or aqueous fluid). Type-II huttonitic monazite has textural characters consistent with incongruent dissolution of apatite but also has an extremely enriched Th composition. This last feature is apparently in contrast with the Fo¨rster and Harlov’s steady system hypothesis. However, if the different geochemical behavior of REE and Th is considered, formation of huttonitic monazite in Monte Capanne pluton can be explained as the result of Th–REE fractionation during an open system reaction. As suggested by Watt (1995), formation of Th-rich monazite during disequilibrium melting of metapelites is driven by the fluorine-rich character of the melt derived by biotite breakdown. Formation of F–REE complexes lowers REE activities and results in reduced REE crystal–melt partition coefficients (Ponander and Brown, 1989).

In Table 3, the chemical composition of apatites from mafic products of Elba and Capraia islands (Orano porphyries, mafic microgranular enclaves and K-andesites) are reported; they have a significant F content, up to 3.2 wt.%, in agreement with the dominance of fluorapatite in magmatic systems (e.g., Sha and Chappel, 1999). Such fluorapatites can be tentatively assumed as representative of apatites of the mafic magma involved in hybridization of San Piero facies, thus, they represent a potential "reagent" phase for Type-II textures. Incongruent dissolution of fluorapatites released a significant amount of F, and a Th–REE fractionation process could has been active during crystallization of huttonitic monazite. REE retention occurred in the melt due to F complexation (Förster, 2000), while increased substitution of Si and Th occurred via the charge-balanced substitution reaction Th⁴⁺ + Si⁴⁺ = REE³⁺ + P⁵⁺ in the growing "monazite", due to the high silica composition of the host melt. Such a process could account for the sudden change in REE/Th from the high values of "mafic" fluorapatites (»20–30) to the low values recorded by huttonitic monazite (»1–3). Finally, if we compare the Th/U ratios in "granitic" apatites, usually ranging between 0.01 and 3.00 (see Bea, 1996; Sha and Chappel, 1999), with the high Th/U ratios of huttonitic monazite (Table 2), an additional process involving differential mobilization of Th and U must be considered. Also in this case, fractionation of Th and U can occur owing to the preferential complexation of U by F, CO₂ and Cl dissolved in granitic melts (Keppler and Wyllie, 1990), coupled with the limited U substitution allowed into the huttonite structure (Kucha, 1980). Element fractionation could have been enhanced by a higher F content in the anatectic melt than in the mafic magma. A rough estimate of F content in the magma can be attempted using a mineral monitor such as biotite (London, 1997). Iron and fluorine content in biotites (data from Dini, 1997) are higher in Sant’Andrea facies (Mg#»0.45; F»0.8–1.1 wt.%) than in the relatively more mafic San Piero facies (Mg#»0.55; F»0.6–0.7 wt.%). Fluorine concentration in magmas has been calculated from these data using the partition coefficient proposed by Icenhower and London (1997). Calculated F content in Sant’Andrea and San Piero facies are 0.2–0.3 and 0.1–0.15 wt.%, respectively. Such data are intermediate between values calculated for Orano and Capraia products (F in melt »0.05–0.1; biotite–phlogopites with Mg#»0.5–0.7 and F»0.2–0.7 wt.%) and for Elba, pure, anatectic rocks (F in melt»0.15–0.4; Fe-rich biotites with Mg#»0.4–0.45 and F»0.7–1.5 wt.%). These estimates indicate that the anatectic melts were richer in fluorine than the mafic products, and were able to drive REE–Th fractionation.

Incongruent dissolution of apatite coupled with F-driven fractionation processes could account for the formation of huttonitic monazite microcrystals. However, an estimate of the amount of huttonitic monazite that can be produced by dissolution of a unit volume of the starting apatite is required to test this hypothesis. A precise mass balance calculation for mineral phases involved in the Type-II texture is beyond the aim of this paper. Moreover, the rare occurrence, the extreme variability of microcrystal packing, the impossibility to define the original volume of the reagent phase, and the large chemical variation observed in huttonitic monazite preclude any quantitative mass balance calculation. Nevertheless, a rough mass balance test was performed for Th, U, Y and REE. Firstly, the amounts of such elements made available from a unit volume of reacting apatite were calculated (apatite composition: Orano and Capraia in Table 3; apatite density=3.3 g/cm3). Then, point counting on BSE images gave a cumulative volume of huttonitic monazite microcrystals of 5–16 vol.% of the starting apatite volume as defined by the envelope of the microcrystal clusters. Finally, the cumulative amount of Th, U, Y and REE in the microcrystal clusters was calculated (huttonitic monazite composition: Table 2; density=5.8 g/cm3). The amounts of Th, U, Yand REE supplied by a unit volume of apatite are significantly lower than those calculated for the huttonitic monazite microcrystal clusters, even if the apatites with the highest content of Th, U, Yand REE data are considered. In particular, Th shows calculated values in apatites 10–40 times lower than those hosted into the microcrystal clusters. Uranium, Y and REE show minor deficit, up to four times.

The mass balance results raise some questions on the starting hypothesis. However, such discrepancies can be explained in several ways: (1) the original apatite crystals had a Th content significantly higher (thousands of ppm) than those measured in apatites from Orano and Capraia mafic rocks. However, magmatic apatites rarely show Th content higher than 600–700 ppm (ThO₂ in the range 0.07–0.08 wt.%; e.g., Sha and Chappel, 1999); (2) the starting apatites contained microinclusions of Th-rich minerals, as suggested by the occurrence of tiny disseminations (<2–3 µm) of euhedral, tetragonal thorite within apatite crystals from Orano porphyry; (3) significant amount of Th was provided by the melt hosting the reaction; (4) the volume of starting apatite calculated from the envelope of the microcrystal clusters is underestimated, owing to the microcrystals collapse possibly occurred during the reaction. (a radius collapse of 30–60% implies that an optimum Th budget for huttonitic monazite formation is provided, whereas the extra amount of U, Y and REE provided by apatite have to be mobilized to the melt). In summary, the process leading to the formation of huttonitic monazite microcrystals could consist in incongruent dissolution of apatite as suggested by petrographic observations, although some inconsistencies in mineral chemistry requires further investigation.

On the other hand, reaction (1), responsible for Type-I texture, proceeds as a volume-for-volume replacement involving minerals which have LREE, Y, Th and U as essential structural constituents. However, monazite-(Ce) contains a higher concentration (more than twice) of REE, Y, Th and U than does allanite-(Ce). Thus, less than half the original REE, Y, Th and U in monazite-(Ce) was maintained in allanite-(Ce), and a significant quantity of these elements was made available in the melt. Apatite is part of the product assemblage and its low Th, U, Y and REE content (Table 3), it enhances such effect. The net result of this reaction is the large availability of such elements for the growth of allanite-(Ce)±apatite.

These considerations suggest that although the REE, Y, Th and U were locally fixed by formation of new phases in reactions (1) and (2), they were partially fractionated and mobilized to the melt. Owing to the main role played by accessory minerals in the geochemistry of REE, Y, Th and U in granitoid rocks, such reactions are able to modify the geochemistry of hybridized magmas, giving back to the melt elements that commonly are extracted by early liquidus accessory phases. The restricted occurrence of xenotime, uraninite and Ti–Nb–Y–U oxides in the two most hybrid facies of Monte Capanne pluton (San Piero and San Francesco) could be tentatively ascribed to the temporary availability of REE, Y, U and Th in those magmas. Moreover, such processes could reverse the behavior of this group of trace elements in peraluminous granitic melts; instead of their normal early extraction, they could be maintained at significant concentrations up to the final stage of the hybrid facies evolution. Late extraction of residual melts, still enriched in Y, U, Th and REE, could be responsible for the presence of rare-element enriched pegmatites (Aurisicchio et al., 2002; Pezzotta, 2000) only at the southeast edge of the intrusion (close to the most hybridized San Piero facies) and not at the northwest edge where the Sant’Andrea facies crops out.

5.3. Insights to the petrogenesis of the Monte Capanne pluton

Although subdivision of Monte Capanne pluton into three facies is supported geochemically and petrographically (e.g., SiO₂, MgO, Sr, Ba, Ni; distribution of K-feldspar and quartz megacrysts, etc.), the main geochemical parameters controlling the stability of monazite-(Ce)–allanite-(Ce)–apatite assemblages in acidic magmas, the ASI value and Ca content, vary only minimally (ASI=1.05–1.15; CaO=2–3 wt.%). The significant and consistent slight peraluminosity of these rocks agrees well with the widespread occurrence of euhedral monazite-(Ce) in the three main facies of Monte Capanne Pluton, and represents the dominant geochemical character of magmas that fed the different portions of the pluton. However, the occurrence of reaction microtextures like those discussed above suggests that the peraluminosity and Ca activity of the pre-emplacement magmas forming the San Piero (and San Francesco) facies differed significantly from the values recorded for rocks now exposed at the emplacement level.

On the basis of the petrochemical hybrid character of rocks from these two facies, the apparent contrasting conditions suggested by the microtextures described above (ASI up and down in the same magma) might be explained by a magma mingling process in which the interplay of acid and mafic end-members provided the expected initial strong difference in ASI values (Fig. 7). Such magmas can be identified with a peraluminous, crustal, magma similar to the Cotoncello leucogranite (Elba Island) or San Vincenzo rhyolites (mainland Tuscany), and a mantle-derived magma similar to the nearby, coeval K-andesites

Fig. 7. Sketch diagram of Alumina Saturation Index (ASI) vs. time showing the proposed genetic history of accessory minerals in San Piero and San Francesco facies of Monte Capanne pluton (see the text for detailed discussion). Ap: apatite, Mnz: Monazite-(Ce). Magma A=San Vincenzo/Cotoncello-like magma; Magma B=Capraia/Orano-like magma.

and shoshonites of Capraia Island (Dini et al., 2002). In this scenario, the microcrystal clusters of huttonitic monazite might represent relics of dissolved apatite crystals coming from a mafic magma and trapped by mingling in an acid melt with high peraluminosity. In contrast, the concomitant presence of monazite-(Ce) replaced by allanite-(Ce)±apatite might be ascribed to the resorption of monazite-(Ce) crystals coming from the peraluminous acid magma and entrapped in a metaluminous mafic one. Both of the textures must have developed during the very first stages of hybridization because, in later stages, crystallization of well-preserved, stable, euhedral monazite-(Ce) was permitted.

The double exchange (mingling) of accessory minerals between the two magmas before they reached the equilibrium conditions for mixing could be a proof of the dynamic setting (stirring and straining of crystal-rich melts) in which anatectic and hybridization processes evolved. Finally the absence of such dissolution textures in the Sant’Andrea facies (which shows evidence of hybridism between the same two end members) does not rule out the role of mingling and mixing processes in its generation but rather emphasizes the importance of

rates and timing at which melting, segregation, diffusion, fractional crystallization, mixing, mingling, ascent and solidification worked. During the emplacement of the Monte Capanne pluton, the San Francesco facies progressively grew as a transitional facies derived by the interaction of the two main magma batches (San Piero- and Sant’Andrea-like). Such a process is responsible for the scattered occurrence of the microtexture also in this plutonic facies. Then, serendipitous conditions probably allowed the persistence of such delicate microtextures into the San Piero (and San Francesco) facies.

6. Conclusion

Accessory minerals often play an "accessory" role in the description and interpretation of igneous rocks. However, they have proved useful both in discriminating facies in otherwise homogeneous intrusive bodies, and in unraveling complex, open-system histories of plutons. The evaluation of physical–chemical parameters of igneous processes, should be mostly based on experimental work. Unfortunately, few experimental data exist on the stability of monazite-(Ce), and data for allanite-(Ce) and huttonitic monazite are still lacking. For these reasons, any attempts to interpret textures involving accessory minerals must be performed cautiously.

Nevertheless, studies of natural samples should be the starting point to give a full picture of natural conditions, and the young, unaltered hybrid rocks of Monte Capanne pluton provide a natural laboratory to investigate the magmatic processes that acted in the early stages of an anatectic–hybrid system. The concomitant occurrence of contrasting reaction microtextures of REE–Y–Th–U accessory minerals in the Monte Capanne pluton suggests the existence of transient chemical conditions (ASI up and down in the same system) in the early stages of crystallization. Incongruent dissolution of apatite produced microcrystal clusters of huttonitic monazite while monazite-(Ce) crystals were replaced by allanite-(Ce)±apatite assemblage at the same location. A magma mingling process, involving acidic peraluminous and mafic metaluminous end-members, can provide the expected initial, strong differences in ASI value that are able to induce such contrasting reactions. The dynamic environment in which anatectic and hybridization processes evolved, with active stirring and straining of crystal-rich melts, could promote the double exchange of accessory minerals between mingling magmas.

The proposed interpretation must be considered in light of the limited knowledge about stability of REE-bearing accessory minerals in melts having contrasting compositions. Alternative interpretations involving late interactions with high T, strongly evolved, water-rich melts or hydrothermal fluids cannot be completely rule out, but are not supported by field and petrographic evidences. This contribution is intended to stress the role of accessory minerals as a possible indicator of hybridization processes occurring in the early stages of the evolution of anatectic–hybrid granitic systems. Of course, a more complete understanding of igneous processes based on data from accessory phases call for additional detailed observations in similar intrusions and specifically planned experimental

data.

Aknowledgements

F. Olmi (IGG-CNR, Florence) performed specific procedure for EPM analysis of accessory phases. F. Colarieti (DST, University of Pisa) is thanked for support during SEM analytical work. The paper benefited from the thoughtful reviews of F. Bea and D. London. The whole work was carried out with funding from the Ministry for the University of Italy, the National Council for Research (Italy) and Norwich University (VT, USA).

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