717

§ E-mail address: cabella@dipteris.unige.it

The Canadian MineralogistVol. 39, pp. 717-727 (2001)

AUTHIGENIC MONAZITE AND XENOTIME FROM PELITIC METACHERTSIN PUMPELLYITE–ACTINOLITE-FACIES CONDITIONS, SESTRI–VOLTAGGIO ZONE,

CENTRAL LIGURIA, ITALY

ROBERTO CABELLA§, GABRIELLA LUCCHETTI AND PIETRO MARESCOTTI

Dipartimento per lo Studio del Territorio e delle Sue Risorse, Università di Genova, C.so Europa, 26, I-16132 Genova, Italy

ABSTRACT

Monazite-(Ce) and xenotime-(Y) were found in the metacherts (pumpellyite–actinolite-facies conditions: 3 ≤ P ≤ 4 kbars and300 ≤ T ≤ 350°C) of the Monte Figogna Unit, Sestri–Voltaggio Zone, Liguria, in Italy, as scattered submillimetric grains in peliticbeds and as euhedral crystals in both millimeter-thick quartz veins and open fissures. Their composition is comparable to thoseof known occurrences in metapelites from greenschist-facies conditions. The textural evidence points to a synmetamorphicauthigenic origin of these phases related to local mobilization and reconcentration of the REE during the metamorphicrecrystallization of the clay fraction.

Keywords: monazite, xenotime, metapelite, chert, rare-earth elements, Liguria, Italy.

SOMMAIRE

Nous avons découvert la monazite-(Ce) et le xénotime-(Y) dans des métacherts (conditions du faciès à pumpellyite–actino-lite: 3 ≤ P ≤ 4 kbars et 300 ≤ T ≤ 350°C) de l’ensemble de Monte Figogna, zone de Sestri–Voltaggio, en Ligurie, Italie. Cesminéraux se présentent sous forme de grains submillimétriques épars dans des couches pélitiques, et de grains idiomorphes dansdes veines de quartz millimétriques et dans des fissures encore ouvertes. Leur composition est comparable à celle d’exemplesconnus dans des métapélites équilibrées aux conditions du faciès des schistes verts. D’après les indications texturales, ces minérauxont une origine synmétamorphique et authigène, résultant d’une mobilisation locale et d’une reconcentration des terres rares lorsde la recrystallisation métamorphique de la fraction argileuse.

(Traduit par la Rédaction)

Mots-clés: monazite, xénotime, métapélite, chert, terres rares, Ligurie, Italie.

INTRODUCTION

Members of the monazite and xenotime groups(Mandarino 1999, Buck et al. 1999) occur as accessoryminerals in a variety of geological environments, suchas in felsic and alkaline magmatic rocks and in hydro-thermal veins and open fissures. In metamorphic rocks,they have been reported as authigenic phases mainlyfrom metapelites, mostly from high- to intermediate-grade conditions (i.e., from granulite to greenschist fa-cies; Franz et al. 1996, Bea & Montero 1999). Atsubgreenschist-facies P–T conditions, xenotime hasnever been reported, whereas monazite is rare, and itsauthigenic origin is still in doubt (Overstreet 1967,Kingsbury et al. 1993, Ohr et al. 1994).

In this paper, we discuss the first documented oc-currence of authigenic monazite and xenotime in peliticbeds within metacherts equilibrated under conditions of

the pumpellyite–actinolite facies (300 ≤ T ≤ 350°C, 3 ≤P ≤ 4 kbar) from the Monte Figogna Unit, Sestri–Voltaggio Zone, central Liguria, Italy.

GEOLOGICAL SETTING

The Sestri–Voltaggio Zone is located at the easternmargin of the Voltri Group and consists of three dis-tinct tectonometamorphic units: Trias–Lias unit,Cravasco–Voltaggio unit, and Monte Figogna unit(Fig. 1). All these units were involved in the Alpine sub-duction-related tectonic events and underwent metamor-phic re-equilibration under different conditions of peakmetamorphism: a) blueschist facies, T ≈ 350°C; P ≥ 6kbar (Trias–Lias and Cravasco–Voltaggio units), and b)pumpellyite–actinolite facies, 300 ≤ T ≤ 350°C; 3 ≤ P≤ 4 kbar (Monte Figogna unit) (Cortesogno & Haccard1984).

717 39#3-juin-01-2247-02 24/07/01, 11:05717

718 THE CANADIAN MINERALOGIST

The Monte Figogna unit is composed of meta-ophiolites (mainly serpentinite, ophicalcite, andmetabasalt) and their sedimentary covers (metachert,metacalcarenite, pelitic schist, and shale). It representsthe metamorphic equivalent of the Eastern Liguriaophiolitic sequences (Cortesogno & Haccard 1984,Crispini & Capponi, in press).

The metacherts are variable in thickness, from tensof centimeters to some meters, and consist of rhythmicalternations of reddish and greenish centimetric beds,both characterized by synmetamorphic schistosity andmultistage veins and fractures.

The reddish beds consist of hematite-bearing sili-ceous mudstones containing deformed radiolarian testscommonly filled by microcrystalline quartz. The green-

ish beds are chlorite- and phengite-bearing siliceouspelites containing scattered ophiolitic clasts. Moreover,dark brown millimetric hematite-rich levels are presentboth in reddish and greenish beds.

ANALYTICAL METHODS

Identification of monazite and xenotime wasachieved by means of optical microscopy and X-raydiffraction (XRD) analyses of hand-picked single crys-tals using a DIFFLEX II diffractometer equipped with aGandolfi camera (Ni-filtered CuK� radiation, 35 kV,25 mA, duration of exposure 20 hr). Unit-cell data wereobtained by a least-squares fit of about 15 reflections.

VOLTAGGIO

Genova

Isoverde

Cravasco

2 km

MtFigogna

Monte Figogna Unit

Cravasco-Voltaggio Unit

Trias-Lias Unit

N

SESTRIPONENTE

Sestri-Voltaggio Zone

) (

250 m

Mt Poggio841

Bocchetta Pass772

Eluvium and colluvium

Monte FigognaUnit

Palombini shales

Serpentinites

Ophicalcites

Metabasalts

Metacherts

Cravasco-VoltaggioUnit

Calcschists

Pelitic schists

MetachertsUndifferentiatedmetaophiolitesMetagabbros

FIG. 1. Location and geological maps of the Sestri–Voltaggio Zone. Redrawn from Cortesogno & Haccard (1984).

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MONAZITE AND XENOTIME, PUMPELLYITE–ACTINOLITE FACIES 719

Qualitative analyses were performed on six polishedthin sections with a scanning electron microscope(SEM, Philips 515) equipped with an X-ray energy-dis-persion analyzer (EDAX PV9100) using an accelerat-ing voltage of 20 kV and a beam current of about 2 nA.A complete investigation using back-scattered electrons(BSE) was carried out in order to identify the rare-earth-element (REE) minerals, to illustrate their textural rela-tionships and to evaluate their modal abundance.

Quantitative analyses were performed with an ARL–SEMQ electron microprobe in wavelength-dispersionmode (WDS) using an accelerating voltage of 20 kV(sample current of 20 nA) with a beam size of about 4�m, and peak and background counting times of 40 and10 seconds, respectively. The reference standards REE1,REE2, REE3, and REE4 (Drake & Weill 1972), syn-thetic glasses (ThO2 100%, Y2O3 100%), synthetic com-pounds (apatite, zirconia, NdSi, PrSi, Ce2YSi2,

FIG. 2. Textural location of monazite and xenotime in assemblages P1 (white box) and P2(black box) within the pelitic beds. Abbreviations: qtz: quartz, chl: chlorite, hem:hematite.

717 39#3-juin-01-2247-02 24/07/01, 11:05719

720 THE CANADIAN MINERALOGIST

La2YSi2), and natural phases [monazite-(Ce), albite],were used as standards. The lines used for analysis, theinterfering elements, and detection limits for each ele-ment are reported in Table 1. Interference effects due topeak overlaps were minimized using the PROBE 5.2software (Donovan & Rivers 1990). The accuracy andprecision of the analyses and the interference-correctioneffects at different levels of concentration were checkedusing the same analytical standards as the unknownsduring the analytical runs. The observed analytical er-ror (E) varies markedly in each case, depending on theconcentration (X) of the element: X ≥ 10%, E ≤ 1%; 1 ≤X ≤ 10%, E ≤ 15%, and X ≤ 1%, E > 15%.

Bulk-rock analyses were carried out by X-ray fluo-rescence (XRF) on fused glass disks (major elements);instrumentation consisted of a Philips PW1480 with Rhtube. Replicate analyses of rock standards show that themajor-element data are precise to within 0.5% to 2.5%,and between 3% and 5% for Al, Na, and P. Concentra-tions of the REE were determined using InductivelyCoupled Plasma – Mass Spectrometry (ICP–MS); thesamples were prepared with a total dissolution method(perchloric, nitric, hydrofluoric acids). The accuracy ofthe REE determinations on replicate analyses is between2% and 3%.

OCCURRENCE

The rare-earth-element phosphates occur in meta-cherts within the greenish beds and in veins or open fis-sures cross-cutting them (Fig. 2). Other mineralstrapping the REE (allanite, apatite, and zircon) arepresent in trace amounts, along the main schistosity asscattered irregular grains. On the basis of textural rela-

tionships, two populations of REE phosphates have beendistinguished, hereafter named P1 and P2.

P1 grains are present exclusively in greenish bedsassociated with hematite and chlorite. They occur asscattered irregular aggregates (up to 5 �m) (Fig. 3A).Owing to their small dimensions, quantitative analysesof P1 crystals were not possible; nevertheless, qualita-tive energy-dispersion spectra suggest that they are REEphosphates, most likely monazite and xenotime.

P2 grains consist of microscopic euhedral crystalsof monazite and xenotime, the latter invariably subordi-nate (xenotime:monazite ratio ≈ 1:10). They occurwithin open fractures (up to 1 mm wide) and veins (upto 0.5 mm wide) cross-cutting the main schistosity(Fig. 3B), associated with dark green sheaves of lamel-lar chlorite, euhedral quartz, and dark yellow, steeplypyramidal crystals of anatase. Monazite (Fig. 4A) ispresent as light yellow tabular and prismatic crystals,up to 200 �m in length, whereas xenotime (Fig. 4B)occurs as very pale honey-yellow elongate prismaticcrystals, up to 100 �m in length.

ANALYTICAL RESULTS

Results of bulk chemical analyses of two samples ofmonazite- and xenotime-bearing metachert are reportedin Table 2. Because the separation of the greenish andreddish beds was impossible, the analytical results per-tain to the whole metachert samples. As a consequence,in comparison with a pure chert, these compositionscontain significant amounts of elements such as Fe, Al,and Mg that are concentrated in the pelitic fraction.

Patterns of REE distribution normalized with respectto the NASC (North Atlantic Shale Composite: Grometet al. 1984) are very flat, without a significant positiveor negative anomaly (Fig. 5A). The generally lower REEcontent in our samples compared to the NASC is possi-bly due to the dilution effects of the cherty (nearly puresilica) reddish beds.

717 39#3-juin-01-2247-02 24/07/01, 11:05720

MONAZITE AND XENOTIME, PUMPELLYITE–ACTINOLITE FACIES 721

The chondrite-normalized REE distribution patterns(Fig. 5B) show relatively steep LREE (La–Eu) distribu-tion, with a slight Eu negative anomaly, and relativelyflat HREE (Gd–Lu) and Y patterns.

XRD data and quantitative chemical analyses wereobtained only from euhedral crystals of the P2 suite,owing to the submicroscopic dimension of monazite andxenotime from population P1.

X-ray-diffraction patterns and the calculated unit-cell parameters confirmed that the analyzed REE phos-phates correspond to monazite [a 6.795(4), b 7.036(4),c 6.483(3) Å, � 103.91(3)°] and xenotime [a 6.856(4)Å, c 5.986(3) Å].

In the P2 suite, monazite and xenotime crystals donot exhibit any systematic core–rim chemical zoning,but significant chemical variations are present amongcrystals.

FIG. 3. SEM micrographs of monazite (mnz). The white boxes indicate the enlarged areas(right side): A) Irregular grain of P1 monazite along the schistosity in pelitic beds; B)euhedral monazite crystal (P2) in open fissure.

717 39#3-juin-01-2247-02 24/07/01, 11:05721

722 THE CANADIAN MINERALOGIST

On the basis of chemical compositions (Table 3), allmonazite crystals correspond to monazite-(Ce) with Ceinvariably the dominant REE (0.42–0.48 apfu). Theother light REE (LREE) occurring in significant amountsare La (0.16–0.23 apfu) and Nd (0.18–0.24 apfu); mi-nor amounts of Pr (0.03–0.05 apfu), Sm (0.02–0.04apfu) and trace amounts of Eu (<0.01 apfu) also havebeen detected. The heavy REE (HREE) are invariablybelow the detection limits, excluding Gd (about 0.02apfu); Y is present, up to 0.05 apfu.

In our samples, La and Sm (Fig. 6A) show a weaknegative correlation and fall within the field reportedfor the occurrence of monazite in greenschist-faciesrocks (Franz et al. 1996). As had been concluded byFranz et al. (1996), no correlations are found among theother LREE. Y and HREE contents are quite homoge-neous, both ranging from ~1 to ~2 wt% of the relevantoxide. Yttrium contents are generally higher than mona-zite from greenschist-facies rocks, whereas the HREEcontents are comparable (Fig. 6B).

The chemical analyses of xenotime (Table 4) revealvery homogeneous Y contents (0.79–0.81 apfu). Dys-

prosium (from 0.06 to 0.08 apfu), Gd (from 0.04 to 0.05apfu), and Er (from 0.02 to 0.03 apfu) also are presentin significant amounts, whereas only minor amounts ofSm (~0.01 apfu) and Lu (<0.01 apfu) have been de-tected. The other REE are generally below their limitsof detection.

The chondrite-normalized patterns of REE distribu-tion confirm that monazite and xenotime are respec-tively LREE- and HREE-selective (Fig. 7), with the REEbetween La to Nd strongly partitioned into monazite,and the REE between Gd to Lu preferentially accom-modated in the xenotime structure.

Only low amounts of Th (0.01–0.02 apfu), Ca (≤0.01apfu), and Si (0.01–0.03 apfu) are incorporated in mona-zite and xenotime, whereas U has not been detected. Thepresence and the relative abundance of these elementsseem related to two different types of isomorphous sub-stitutions that, as previously described by Franz et al.(1996), can be represented by the exchange vectors

FIG. 4. SEM micrographs of REE phosphates from open fis-sures (P2). A) Tabular prismatic crystals of monazite (mnz)together with quartz (qtz); B) prismatic crystals of xenotime(xen) together with quartz grains and lamellar chlorite (chl).

1

10

102

103

LaCe

PrNd

SmEu

GdTb

Dy YHo

ErTm

YbLu

* Chondritic values fromAnders & Ebihara (1982).

rock/chondrite

*

0.1

1

10

102

LaCe

°PrNd

SmEu

GdTb

Dy Y°Ho

Er°Tm

YbLu

rock/N

ASC*

*NASC values from Gromet et Al. (1984)and ° Haskin et al. (1968). A

B

FIG. 5. Bulk-rock patterns of REE distribution in monazite-and xenotime-bearing metacherts. A) Patterns normalizedwith respect to the NASC (North Atlantic Shale Compos-ite); B) chondrite-normalized patterns.

717 39#3-juin-01-2247-02 24/07/01, 11:05722

MONAZITE AND XENOTIME, PUMPELLYITE–ACTINOLITE FACIES 723

CaThREE–2 in monazite and ThSiREE–1P–1 in xenotime,respectively. A comparison with a large set of monaziteand xenotime data (Fig. 8) reveals that in our samples,only limited substitutions are present along these vec-tors. In contrast with what is expected, Th contents aregenerally higher in xenotime than in monazite.

DISCUSSION AND CONCLUSIONS

Monazite and xenotime found in metacherts from theSestri–Voltaggio Zone are restricted to two modes ofoccurrence, both within pelitic greenish beds, i.e., alongthe main schistosity associated with chlorite + hematite(P1), and within veins and fissures together with chlo-rite, quartz, and anatase (P2).

The occurrence of authigenic monazite and xenotimefrom medium- to high-grade metamorphic assemblageshas been reported by many authors (Overstreet 1967,

Kingsbury et al. 1993, Franz et al. 1996, Bea & Montero1999). Franz et al. (1996) reported authigenic monaziteand xenotime occurring in metapelites from a regionalmetamorphic area in northeastern Bavaria, Germany,over a range of P–T conditions spanning from green-schist to almost granulite facies. Xenotime has neverbeen reported from subgreenschist-facies assemblages.On the other hand, monazite is rare, and its authigenicorigin is still doubtful; in most of the cases, it is consid-ered a detrital mineral (Overstreet 1967, Kingsbury etal. 1993, Ohr et al. 1994).

The selective appearance of monazite and xenotime(assemblage P1) in the phyllosilicate-rich greenish bedssuggests that the REE were concentrated almost exclu-sively in the pelitic fraction of the metacherts. More-over, as suggested by textural evidence, the rhythmicalternation of reddish and greenish beds in the meta-cherts presumably reflects the primary sedimentary

717 39#3-juin-01-2247-02 24/07/01, 11:05723

724 THE CANADIAN MINERALOGIST

layering; thus it is likely that these minerals formed as aconsequence of local mobilization and reconcentrationprocesses during the metamorphic recrystallization ofthe clay fraction, without the necessity of involving anyexternal source.

The genesis of monazite and xenotime in veins andfissures (assemblage P2) cannot unequivocally be con-strained as being synmetamorphic. Nevertheless, theconcentration of monazite and xenotime exclusively inveins cross-cutting the greenish pelitic beds suggeststhat they probably have formed as a consequence oflocal mobilization of REE from the adjoining rocks.

However, the relatively constant Y/Dy ratio (mean value≈ 11.8, Table 4), which approaches the value in theEarth’s crust (11.5: Taylor & McLennan 1985), is infavor of the absence of complexing agents in the depos-iting solutions (Gramaccioli et al. 1999).

The composition of P2 monazite and xenotime is ingood agreement with the constraints inferred by fieldand experimental data (Franz et al. 1996, Gratz &Heinrich 1997, Heinrich et al. 1997) relative to low-grade metamorphic conditions. In particular, our dataindicate a very low content of HREE in monazite and ofLREE in xenotime, as observed in samples from

717 39#3-juin-01-2247-02 24/07/01, 11:05724

MONAZITE AND XENOTIME, PUMPELLYITE–ACTINOLITE FACIES 725

greenschist-facies conditions elsewhere (Franz et al.1996). Moreover, the Th content of monazite is gener-ally very low, as observed at low temperature inmetapelites and hydrothermal veins (Overstreet 1967,Franz et al. 1996).

These data suggest that monazite and xenotimefound in metacherts from the Sestri–Voltaggio Zonepossibly represent the first occurrence of these mineralsunder subgreenschist metamorphic conditions (pumpel-lyite–actinolite facies; 300° ≤ T ≤ 350°C, 3 ≤ P ≤ 4 kbar),thus extending the stability field of these phosphateminerals to a wider range of metamorphic conditions.

ACKNOWLEDGEMENTS

This study was supported by the Ministero dell’Uni-versità e della Ricerca Scientifica e Tecnologica: project“Crystal chemistry of mineral species: use of advancedtechniques for a modern systematics”. The electron-microprobe work at the Dipartimento di Scienze dellaTerra, University of Modena, was made possible by fi-

nancial support of the CNR of Italy. The authors thankS.L. Hanson, C.M. Gramaccioli, and R.F. Martin fortheir careful reviews.

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This study

La2O3 (wt%)

Sm2O

3 (w

t%)

A

0.0

1.0

2.0

3.0

5.0

4.0

5.0 7.0 9.0 11.0 13.0 15.0 21.017.0 19.0

∑HREE2O3 (wt%)

Y2O

3 (w

t%)

B

0.0

1.0

2.0

3.0

5.0

4.0

0.0 1.0 2.0 3.0 4.0 5.0

Tm

10

105

107

LaCe

PrNd

SmEu

Gd Dy Y

* Chondritic values fromAnders & Ebihara (1982)

min

era

l/chondrite

*

106

104

103

102

monazite

Tb

xenotime

HoEr Yb

Lu

FIG. 6. REE chemical variations in monazite. A) La2O3 –Sm2O3; B) HREE2O3 (except Y) – Y2O3. Grey fields (fromFranz et al. 1996) represent monazite from metapelites inlow-grade metamorphic conditions.

FIG. 7. Chondrite-normalized patterns of REE distribution formonazite and xenotime.

717 39#3-juin-01-2247-02 24/07/01, 11:05725

726 THE CANADIAN MINERALOGIST

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FIG. 8. Distribution of non-REE elements in monazite and xenotime in terms of the binary plot (REE + P) – (Th + U + Si);ThSiREE–1P–1 and CaThREE–2 exchange vectors are represented. Literature data (open circles and squares) are taken fromÅmli (1975), Bea & Montero (1999), Bogoch et al. (1992), Demartin et al. (1991a, b), Franz et al. (1996), Hanson et al.(1992), Mannucci et al. (1986), Montel (1993), Pan et al. (1994), Sevigny (1993), Ward & Miller (1993), and Watt (1995).

REE+P (a.p.f.u.)

Th+

U+

Si

(a.p

.f.u

.)0.2

0.1

0.0

1.8 1.9 2.0

REE+P (a.p.f.u.)

Th+

U+

Si

(a.p

.f.u

.)

CaThREE-2

ThSiREE-1 P

-1

0.6

0.5

0.4

0.3

0.2

0.1

0.0

1.61.51.4 1.7 1.8 1.9 2.0

xen

mnz

717 39#3-juin-01-2247-02 24/07/01, 11:05726

MONAZITE AND XENOTIME, PUMPELLYITE–ACTINOLITE FACIES 727

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Received October 22, 2000, revised manuscript acceptedMarch 22, 2001.

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