metamorphism and anatexis in the maﬁc complex contact …€¦ · facies metamorphism preceded...

JOURNAL OF PETROLOGY VOLUME 41 NUMBER 8 PAGES 1307–1327 2000

Metamorphism and Anatexis in the MaficComplex Contact Aureole, Ivrea Zone,Northern Italy

SCOTT A. BARBOZA∗ AND GEORGE W. BERGANTZUNIVERSITY OF WASHINGTON, DEPARTMENT OF GEOLOGICAL SCIENCES, SEATTLE, WA 98195, USA

RECEIVED APRIL 16, 1999; REVISED TYPESCRIPT ACCEPTED JANUARY 14, 2000

Emplacement of mantle-derived magma (magmatic accretion) is the total thermal budget of the continental lower crust,often presumed or inferred to be an important cause of regional produce regional granulite facies metamorphism, andgranulite facies metamorphism and crustal anatexis. The jux- generate Y- and heavy rare earth element (HREE)-taposition of mafic cumulates and regionally distributed granulite depleted granitoids (Ellis, 1987). Accretion of maficfacies rocks has led some to consider the Ivrea zone (northern Italy, magma and migration of partial melt will internallySouthern Alps) as an important exposure that demonstrates this stratify, chemically differentiate, and deplete the con-causal relationship. However, regional PTt paths indicated by tinental lower crust in large ion lithophile elementsmetamorphic reaction textures and PT conditions inferred from (LILE). Magmatic accretion has been invoked to providegeothermobarometry indicate that the emplacement of mafic plutonic the heat and mass necessary for sustained magmatism inrocks (Mafic Complex) at the Ivrea zone occurred during de- tectonic settings such as Phanerozoic extensional terranescompression from ambient pressures at the regional thermal maximum. (Lister et al., 1986; Gans, 1987; Fountain, 1989; MareschalField and petrographic observations, supported by PT estimates, & Bergantz, 1990; Jarchow et al., 1993) and magmaticindicate that regional retrograde decompression and emplacement of arcs (Hamilton, 1981; Kay & Kay, 1981; Bohlen &the upper parts of the Mafic Complex probably accompanied Lindsley, 1987; Hildreth & Moorbath, 1988).extension during the Late Carboniferous–Early Permian. A spatially However, as few exposed sections of lower continentalrestricted decompression-melting event accompanied final em- crust show contiguous mafic intrusions and regionallyplacement, depleting supracrustal rocks enclosed by an >2–3 km distributed granulite facies rocks, estimates of the extentaureole overlying the upper Mafic Complex by 20–30% granite of anatexis and metamorphism accompanying magmaticcomponent. The upper Mafic Complex provided the thermal energy accretion have relied on numerical and analog sim-to reset mineral assemblages and locally overprint the regional ulations. These models yield disparate results dependingprograde metamorphic zonation. The limited extent of the contact on whether heat transfer within the mafic intrusionaureole suggests that magmatic accretion may not inexorably cause and surrounding country rocks is primarily convectiveregional metamorphism and crustal anatexis. (Campbell & Turner, 1987; Huppert & Sparks, 1988) or

conductive (Marsh, 1989; Bergantz & Dawes, 1994;Barboza & Bergantz, 1996). A comparison with fieldevidence is required to discriminate between the modelKEY WORDS: Granulite facies; Ivrea zone; underplating; thermobarometry;results. In the Ivrea zone, an exposed section of maficmigmatite; mafic complexplutonic rocks (the Mafic Complex) has been interpretedto represent deformed cumulates of the accreted magmasthat caused regional granulite facies metamorphism (Ri-

INTRODUCTION valenti et al., 1975, 1980; Schmid & Wood, 1976; Sills,1984; Pin, 1990). The continuous exposure betweenThe accretion of mantle-derived magma at or near the

base of the crust (Wells, 1980; Bohlen, 1987) can enhance granulite facies crustal rocks and the mafic intrusion that

∗Corresponding author. Present address: ExxonMobil Upstream Re-search Company, Hydrocarbon Systems Analysis Division, PO Box2189, Houston, TX 77252-2189, USA. Telephone:+1-(713)431-7357.Fax: +1-(713)431-7265. e-mail: [email protected] © Oxford University Press 2000

JOURNAL OF PETROLOGY VOLUME 41 NUMBER 8 AUGUST 2000

is thought to have provided the heat during regional Austroalpine and Penninic crustal rocks to the north andwest (Berckhemer, 1968; Giese et al., 1982; Hirn et al.,metamorphism has led some to consider the Ivrea zone

a particularly important example of magmatic accretion 1989). This geophysical model is consistent with structuraldata indicating that steep tilting accompanied em-(e.g. Voshage et al., 1990).

However, Barboza et al. (1999) provided field and placement during Alpine collision and closure of theTethys (Schmid et al., 1987; Nicolas et al., 1990). Thegeochemical data supporting an alternative model (Zingg

et al., 1990; Schmid, 1993), in which regional granulite mafic plutonic rocks exposed along the Insubric Line(the Mafic Complex) represent the originally deepestfacies metamorphism preceded the emplacement of the

upper parts of the Mafic Complex. In this paper, we levels of exposed crust, and the shallowest levels arerepresented by supracrustal rocks (the Kinzigite For-present whole-rock major- and trace-element com-

positional data, an inferred sequence of metamorphic mation) juxtaposed with the Strona–Ceneri zone (Fig. 1).Although alternative interpretations have been proposedreactions, and PT estimates derived from thermo-

barometry for supracrustal rocks proximal to the Mafic (see Boriani et al., 1990b), steep tilting of the Ivrea zoneis supported by the regional increase in metamorphicComplex. These new data suggest that the emplacement

of a major part of the Mafic Complex occurred during grade to the NW (Schmid, 1967; Mehnert, 1975; Zingg,1980) and the regional baric gradient according todecompression from ambient pressures at the thermal

maximum during the regional granulite facies episode. thermobarometry (Sills, 1984; Henk et al., 1997; De-marchi et al., 1998).Final emplacement caused anatexis and metamorphism

only within a narrow (>2–3 km) aureole in the proximalsupracrustal rocks. These events overprint the regionalamphibolite to granulite facies prograde metamorphic

ROCK TYPESzonation.Mafic ComplexThe Mafic Complex, commonly interpreted to have beenthe heat source for regional metamorphism, consists of

GEOLOGICAL SETTING mafic plutonic rocks and mantle peridotite that form aRegional framework belt spanning the length of the Ivrea zone along its

western and northwestern margin. For the purpose ofThe Ivrea zone is one of three fault-bounded sections ofPaleozoic basement exposed in the Southern Alps. From this study, we make an important distinction between

the upper and lower parts of the Mafic Complex. Upperthe originally deepest to shallowest crustal levels, thePaleozoic Southern Alpine crust comprises the Ivrea, Val Grande and Val d’Ossola expose several hundred

meters to 1 km thickness of banded pyroxene–hornblendeStrona–Ceneri (or Serie dei Laghi in the Italian literature),and Val Colla zones. To the north and west, the Ivrea and hornblende granofels (‘pyribolite’; Schmid, 1967).

Similar rocks are exposed in upper Val Sesia and Valzone (Fig. 1) is separated from rocks of the Penninic beltby the Insubric Line, a major, NW-dipping, Neogene Strona di Omegna (Mehnert, 1975; Rivalenti et al., 1975).

On the basis of field relationships (Quick et al., 1994),shear zone that juxtaposes pre-Alpine and Alpine struc-tures and rocks (Gansser, 1968; Schmid et al., 1987). To compositional trends (Sinigoi et al., 1994), and micro-

structures, J. E. Quick (personal communication, 1999)the south and east, the Cremosina, Cossato–Mergozzo–Brissago (CMB), and the Pogallo Lines sep- suggested that these rocks may be related and represent

a phase of magmatism earlier than that of the upperarate the Ivrea zone from plutonic rocks and amphibolitefacies orthogneiss and paragneiss of the Strona–Ceneri Mafic Complex. These rocks may have been derived

from a compositionally distinct parent magma and havezone (Boriani & Sacchi, 1973; Boriani et al., 1977, 1990a;Hodges & Fountain, 1984; Handy, 1987; Schmid et al., undergone a different deformational history from the

upper Mafic Complex (Rivalenti et al., 1975; Zingg et al.,1987).Most regional studies have interpreted the Ivrea zone 1990). Therefore, we distinguish these rocks (referred to

collectively hereafter as the lower Mafic Complex) fromas a cross-section through attenuated lower continentalcrust (Fountain, 1976; Burke & Fountain, 1990). This the overlying deformed cumulates of the upper Mafic

Complex (Fig. 1). As the field relationships between theinterpretation is based on gravity and seismic studies thatindicate the Ivrea zone is the surface expression of a lower Mafic Complex and the overlying supracrustal

sections are obscured by the (possibly) younger phase oflarge, high-density body (the Ivrea geophysical body)situated where the fossilized (pre-Alpine) Southern Alpine magmatism, we confine our interpretations to the timing

of regional granulite facies metamorphism and em-Moho comes closest to the surface (Mueller et al., 1980;Giese et al., 1982). Geophysical modeling suggests that placement of the upper Mafic Complex.

We focus our study on the southern Ivrea zone (Fig. 1)the Ivrea body may be a NNE-striking, SE-dipping sliverof Southern Alpine crust and mantle juxtaposed against where the Mafic Complex reaches its maximum exposed

1308

BARBOZA AND BERGANTZ MAFIC COMPLEX, IVREA ZONE

Fig. 1. Geological map of the southern Ivrea and Strona–Ceneri zones west of Lago d’Orta. Inset shows location of study area in northernItaly. Contact between Mafic Complex and Kinzigite Formation based on our own mapping and that reported by Quick et al. (1994). Geometryof the boundary between the Ivrea zone and the Strona–Ceneri zone between Val Strona di Omegna and Val d’Ossola taken from Boriani etal. (1990a). The muscovite–K-feldspar isograd in sillimanite-bearing paragneiss, and the first appearance of orthopyroxene in mafic rocks areindicated by the dashed lines (Schmid, 1967; Zingg, 1980). Enclosed areas within the Kinzigite Formation indicate mapping transects alongwhich samples were collected. Detail sample location maps for R. Forcioula and F. Duggia are provided in Fig. 2.

thickness of 10 km. In this area, the upper Mafic Complex field and compositional relationships, these researchersinterpreted the upper Mafic Complex to have beencomprises leucodioritic to gabbroic rocks intercalated

with ultramafic rocks and layers of banded granulite emplaced during Late Carboniferous–Early Permian ex-tension within the Ivrea zone. Space for the Mafic(Quick et al., 1994; Sinigoi et al., 1994). On the basis of

1309


Complex may have been partially accommodated by Geochronologynorthward transport and ductile attenuation of the over- Geochronology indicates a Late Carboniferous–Earlylying Kinzigite Formation (Quick et al., 1994; Snoke et Permian crystallization age for the upper Mafic Complex.al., 1999). South of Val Sesia, lensoidal bodies of tonalite Pin (1986) reported six fractions of nearly concordantto granodiorite diatexite Ζ200 m thick, which contain zircon U–Pb data with an upper intercept of 285 + 7/blocks and schlieren of metabasite and metapelite, sep- –5 Ma from a diorite sample from the upper Maficarate the Mafic Complex from the overlying Kinzigite Complex. A Carboniferous–Early Permian age for em-Formation (Bürgi & Klötzli, 1990; Quick et al., 1994). placement is supported by Sm–Nd mineral isochrons andPhase relations and PT estimates indicate that em- a whole-rock Rb–Sr age (Bürgi & Klötzli, 1990; Voshageplacement of the Mafic Complex occurred at crustal et al., 1990). Microanalytic U/Pb ages of newly growndepths from 4–5 kbar to 8–10 kbar (Demarchi et al., zircon within two minor members of the lower Mafic1998). Complex are 299 ± 5 Ma (Vavra et al., 1999).

The imposition of the regional metamorphic zonationprobably occurred within 20 My of emplacement of theupper Mafic Complex (see Hunziker & Zingg, 1980).Vavra et al. (1996, 1999) suggested regional granulite

Kinzigite Formation facies metamorphism occurred in a single event before299 ± 5 Ma, on the basis of an ion microprobe studyThe Kinzigite Formation comprises amphibolite-to-gran-of zircons from granulite within the lower Mafic Complexulite facies metapelite and metapsammite, with sub-and from supracrustal rocks in upper Val Strona diordinate metacarbonate and metabasite. MetabasiteOmegna. A Late Carboniferous age for the regionalsouth of Val Sesia (Fig. 1) occurs as 25 cm–50 m thick,granulite facies metamorphism is consistent with U–Pbfoliation-parallel lenses or layers intercalated with me-ages on monazite (Köppel, 1974; Henk et al., 1997) andtapelite and minor metacarbonate. In Val d’Ossola andthe inferred age of lead loss of 99% (granulite faciesVal Strona di Omegna, metabasite is more commonrocks) to 85% (amphibolite facies rocks) from discordantand thick layers of metabasite and metapelite alternatezircon populations (Köppel & Grünfelder, 1971; Köppel,(Schmid, 1967; Zingg, 1980; Sills & Tarney, 1984). In1974). These studies indicate regional metamorphismamphibolite facies rocks, metabasite contains nemato-before 275± 2 Ma (Köppel, 1974), 292± 2 Ma (Henkblastic amphibole + plagioclase ± clinopyroxene. Or-et al., 1997), and 285 ± 10 Ma (Köppel & Grünfelder,thopyroxene appears in granoblastic amphibole- and1971; Köppel, 1974), respectively, and may reflect coolingplagioclase-bearing metabasite with the transition to thebelow 600°C (monazite) and conditions close to thermalgranulite facies. Metacarbonate occurs in lenses andpeak (zircon). Although Henk et al. (1997) and Vavrabands Ζ40m thick, which contain a variety of mineral& Schaltegger (1999) differed in their interpretation ofassemblages (Schmid, 1967; Zingg, 1980).decreasing monazite ages from 292 ± 2 Ma near theThe most common lithology of supracrustal rock isPogallo Line to 276 ± 2 Ma near the Insubric Line,high-Al migmatitic metapelite with sillimanite as theboth studies support a Late Carboniferous age for thestable aluminosilicate polymorph. In lower Val Stronaregional thermal peak of metamorphism.di Omegna, metapelite commonly contains biotite +

quartz + plagioclase ± garnet ± sillimanite ± mus-covite ± K-feldspar. Primary muscovite is restricted tothe lowest grade rocks in lower Val Strona di Omegna

RESULTSand Val d’Ossola. Toward the NW, in upper Val Stronadi Omegna, the modal proportions of garnet and K- Sample suitefeldspar increase at the expense of biotite and muscovite To constrain the relationship between emplacement of(Schmid & Wood, 1976; Schmid, 1978–1979). With the the upper Mafic Complex and the regional metamorphicexception of rare occurrences in lowermost Val Strona PTt history of the Ivrea zone, we collected samples fordi Omegna (Zingg, 1980) and Val d’Ossola (Peyronel petrographic study and geochemical analyses on fourPagliani & Boriani, 1967), cordierite and hercynitic spinel traverses across the regional metamorphic zonationare restricted to metapelite proximal to the contact with through the Kinzigite Formation (Fig. 1). Sample loc-the upper Mafic Complex (Fig. 1). In agreement with ations were selected from our mapping of the supracrustalSills (1984), we observed orthopyroxene with garnet, section overlying the upper Mafic Complex south of Valbiotite, and plagioclase in one Mg-rich and Al-poor Sesia and the maps of Bertolani (1959–1965) and Schmidmetapelite. Common accessory minerals include rutile, (1967) for Val Strona di Omegna and Val d’Ossola,ilmenite, zircon, graphite, apatite, monazite, pyrite, chal- respectively. We compared whole-rock major- and trace-

element compositions, metamorphic reaction textures,copyrite, and corundum.

1310


and mineral compositions in samples collected south of Inferred reaction sequenceVal Sesia (proximal to the upper Mafic Complex) with Between upper and lower Val Strona di Omegna and Valmore distal samples collected in lower Val Strona di d’OssolaOmegna and Val d’Ossola. Sample locations are in-

The increase in metamorphic grade to the NW in Valdicated in Figs 1 and 2.Strona di Omegna and Val d’Ossola with increasingoriginal crustal depth characterizes the regional am-phibolite to granulite facies metamorphism in the Ivreazone. We present new data and review the relevantmineralogy and regional reaction textures for comparisonWhole-rock geochemistrywith rocks proximal to the upper Mafic Complex only.Analytical techniques Interested readers are directed to other studies for a

The migmatites were separated for analysis by first sawing complete analysis and review (Schmid; 1967, 1993;the sample into slabs of leucosome and melanosome. A Mehnert, 1975; Schmid & Wood, 1976; Zingg, 1980;representative split of 100–500 g of powdered me- Sills, 1984; Zingg et al., 1990; Henk et al., 1997).lanosome sample was then taken for X-ray fluorescence On the basis of a compilation of previous petrographic(XRF) analysis. Major and trace elements were analyzed work, Zingg (1980) located mineral isograds that representusing a Philips PW 1404 XRF spectrometer. Corrections prograde reactions produced during the high-grade,were based on a calibration using 40 international stand- regional metamorphic episode (Fig. 1). These isogradsards according to procedures described by Franzini et al. include the muscovite–K-feldspar isograd in sillimanite-(1975) and Leoni & Saitta (1975). The detection limits bearing metapelite, the first appearance of coexistingare 0·01% for major elements and 10 ppm for trace pyroxenes in metabasite, and the upper stability limit ofelements. Analytical uncertainties are±1% of the value calcite–quartz–actinolite in calcsilicate. A textural changefor major elements, except for Mg and Na, for which in metapelite attends the replacement of lepidoblasticthey are±5%. Analytical uncertainties of trace-element muscovite and biotite by granoblastic K-feldspar andanalyses are±5–10 ppm for the concentrations at>100 garnet with increasing grade (Schmid, 1967; Schmid &ppm and ±10–20 ppm for those at 30–50 ppm. Wood, 1976; Hunziker & Zingg, 1980). In the granulite

facies rocks of upper Val Strona di Omegna and Vald’Ossola, quartz+ hypersthene± garnet granulite and

Results calcsilicate are interlayered with granoblastic graphite+sillimanite + garnet gneiss, interpreted to be residuaNew whole-rock, and major- and trace-element com-from the melting of metapelite (Schmid & Wood, 1976;positions of selected metapelite melanosomes are listedSighinolfi & Gorgoni, 1978; Schmid, 1978–1979; Sch-in Table 1. Locations for these selected samples arenetger, 1994). Schmid & Wood (1976) attributed theindicated in Figs 1 and 2. Whole-rock compositionsprogressive increase in modal garnet at the expense ofprojected from muscovite (Val d’Ossola samples) and K-biotite to the continuous reactionfeldspar (F. Duggia and R. Forcioula samples) are de-

picted in Fig. 3. AFM projections of melanosome com-biotite + sillimanite + quartz ⇒ garnet +positions indicate that some samples are enriched in Al

K-feldspar + rutile + H2O(1)

over Fe and Mg relative to typical high-Al metapelites.Mass balance calculations indicate a systematic depletion

Two phases of muscovite growth are apparent SE ofof SiO2, the alkalis, and incompatible trace elements inthe muscovite-out isograd (Fig. 1) in lower Val Stronamelanosomes overlying the upper Mafic Complex relativedi Omegna and Val d’Ossola. Muscovite2 is distinguishedto lower-grade metapelites in lower Val d’Ossola andfrom muscovite1 in that its long axis is oblique to theVal Strona di Omegna (Barboza, 1998). Leucosomefoliation defined by muscovite1, biotite, and sillimaniteadjacent to the melanosome samples collected within(Fig. 4a). This relationship suggests that muscovite2 mayKinzigite Formation overlying the upper Mafic Complexhave appeared during retrograde cooling and hydrationtypically are tonalitic in composition, lack mafic phases,through the reactionand are depleted in K2O relative to the inferred melt

extracted from the melanosomes (Barboza, 1998; Barbozasillimanite + K-feldspar + H2O ⇒et al., 1999). Banded granulite intercalated with the lower

muscovite + quartz (2)Mafic Complex and exposed in upper Val Strona diOmegna and Val d’Ossola are depleted in incompatibleelements and enriched in compatible elements relative to an inference supported by common textures indicatingtheir amphibolite facies equivalents (Sighinolfi & Gorgoni, the direct replacement of sillimanite + K-feldspar by

symplectitic intergrowths of muscovite2 + quartz.1978; Schnetger, 1994; Barboza et al., 1999).

1311


Fig. 2. Detail sample location maps along R. Forcioula and F. Duggia (see Fig. 1). Underlined sample numbers indicate samples used forthermobarometry. Across-strike baric gradient along the R. Forcioula transect with distance from the contact between the Kinzigite Formationand the upper Mafic Complex is also shown.

Val Duggia (Figs 1 and 2). Mineral assemblages forBetween lower Val Sesia and Val Strona di Postuaselected samples collected along these transects are listedThe longest sections of continuous exposure south of Valin Table 2. Metamorphic grade increases to the SWSesia that extend across the contact between the Kinzigitefrom the amphibolite facies in lower Val Strona diFormation and the upper Mafic Complex are along Rio

Forcioula in Val Strona di Postua and Fiume Duggia in Omegna, parallel to the regional fabric toward Val

1312


muscovite + quartz ⇒ sillimanite +K-feldspar + H2O (5)

or an H2O-conserved melting reaction such as

muscovite + plagioclase + K-feldspar +quartz ⇒ sillimanite + melt. (6)

At pressures below 6 kbar, either reaction (5) or (6) inintermediate XMg metapelite indicates that temperaturesproximal to the contact between the upper Mafic Com-plex and the Kinzigite Formation exceeded >600°C(Vielzeuf & Holloway, 1988).

In lower Val Strona di Omegna, biotite is the majormineral in metapelite. However, biotite is irregularlydistributed in metapelite overlying the Mafic Complexsouth of Val Sesia: some samples contain 2–3 km wide aureole overlying the upper Mafic+ Sill = Grt + Crd is indicated by the appearance of cordierite

Complex (Fig. 1). We interpret these relationships towithin metapelite proximal to the upper Mafic Complex.indicate that rocks proximal to the eastern magmaticcontact with the upper Mafic Complex south of ValStrona di Omegna reached final equilibration within theSesia. In the Kinzigite Formation south of Val Sesia,stability field of cordierite. AFM projections of the bulkorthopyroxene commonly coexists with plagioclase incompositions of metapelite melanosome (Fig. 3) illustrateamphibole-bearing metabasites, indicating final equi-the change from the three-phase field defined by garnet–libration of these rocks in the granulite facies.biotite–sillimanite in lower Val Strona di Omegna toZingg (1980) reported the local presence of relict stau-garnet–cordierite–sillimanite south of Val Sesia.rolite grains enclosed by cordierite. Relict kyanite has

Cordierite is typically heavily pinitized, but, whenalso been reported in two localities south of Val Sesiapreserved, at least two generations are present (Capedri(Bertolani, 1959; Boriani & Sacchi, 1973), one locality& Rivalenti, 1973). Cordierite1, found in minor amounts,in Val Sesia (Demarchi et al., 1998), and one locality inencloses fibrolite and is elongate parallel to the foliationVal Mastallone (Capedri, 1971). Fibrolite is the dominantdefined by biotite, sillimanite, and the long axes of garnettextural variety of sillimanite but, with increasing grade,augen. This observation implies that cordierite1 growthcoarse-grained prismatic sillimanite is common. Fibrolitefollowed reaction (4). We infer that cordierite1 may havecommonly occurs on basal sections of biotite, a textureformed from the continuous reactionwe associate with the nucleation and growth of sillimanite

on biotite indicating the breakdown of staurolite withingarnet + sillimanite + quartz ⇒ cordierite1. (7)the stability field of sillimanite (Fig. 4a). The reactions

The more abundant cordierite2 occurs as up to 5 mmkyanite ⇒ sillimanite (3)prisms that cut the foliation defined by biotite and sil-limanite (Capedri & Rivalenti, 1973). Idiomorphic cor-followed bydierite2 is present in leucotonalite leucosomes (Barbozaet al., 1999) within migmatitic metapelite and occursstaurolite + quartz + muscovite ⇒ garnet +abundantly with K-feldspar porphryoblasts in the tonalitebiotite + sillimanite + H2O (4)to granodiorite diatexite (Bürgi & Klötzli, 1990; Quicket al., 1994) separating the upper Mafic Complex fromprobably document the prograde transition from thethe overlying Kinzigite Formation south of Val Sesia.kyanite + staurolite to the sillimanite stability field.These K-poor leucosomes are restricted to the 2–3 kmThe muscovite-out isograd in metapelite lies to the NEwide aureole that surrounds the upper Mafic Complex,of the Val Sesia region (Fig. 1), so the limits of muscovitecoincident with the appearance of cordierite and her-stability were exceeded within the stability field of sil-cynitic spinel in the melanosome assemblage. Theselimanite. Accordingly, metapelite that overlies the upperrelationships indicate that cordierite2 was a peritecticMafic Complex contains sillimanite + K-feldspar, butphase produced during the melting reaction from whichlacks primary muscovite (Zingg, 1980). These ob-

servations imply the reaction the leucotonalite leucosomes were derived.

1313


Tabl

e1:

Who

lero

ckm

ajor

-an

dtra

ce-e

lemen

tcom

posit

ions

ofrep

resen

tativ

em

etape

lite

mela

noso

mes

Sam

ple

8049

680

496

8049

680

496

8059

680

596

6209

762

097

7019

770

197

7019

762

497

6249

761

797

6179

761

797

6179

782

395

no.:

-11

-13

-16

-21

-03

-05

-05

-02

-03

-05

-10

-01

-03

-01

-03

-07A

-09

-01

Loca

tion

RF

RF

RF

RF

RF

RF

RF

RF

VD

VD

VD

VD

VD

VD

OV

DO

VD

OV

DO

VS

O

Maj

orox

ide

abun

danc

esby

X-r

ayflu

ores

cenc

ean

alys

is(w

t%

)

SiO

254

·72

47·2

246

·49

42·3

742

·33

38·6

841

·78

52·7

253

·75

59·5

61·6

155

·41

41·7

858

·58

57·9

654

·86

46·8

753

·80

TiO

21·

751·

821·

982·

592·

132·

582·

311·

651·

651·

341·

311·

432·

311·

401·

041·

211·

620·

96

Al 2O

320

·52

23·6

330

·829

·29

20·7

724

·93

32·3

825

·32

22·6

220

·24

17·3

023

·62

32·3

818

·13

21·9

321

·83

26·1

519

·00

FeO

T11

·21

14·5

613

·61

14·7

518

·17

18·5

514

·28

11·6

812

·55

10·7

98·

7210

·87

14·2

810

·39

8·53

9·32

12·8

712

·30

MnO

0·16

0·21

0·16

0·18

0·28

0·21

0·19

0·11

0·09

0·09

0·10

0·13

0·19

0·09

0·22

0·08

0·23

0·77

MgO

4·39

6·19

4·35

5·24

6·91

8·00

4·60

3·6

3·81

3·15

4·26

3·24

4·6

3·12

2·35

2·75

3·50

2·50

CaO

2·40

0·91

0·43

0·54

2·80

0·59

1·06

0·37

0·37

0·37

1·08

0·4

1·06

0·31

0·63

0·39

0·45

1·44

Na 2

O1·

310·

510·

20·

141·

770·

160·

660·

380·

230·

220·

680·

220·

660·

571·

481·

430·

571·

79

K2O

1·81

2·71

1·58

1·93

3·14

3·53

1·41

2·48

3·31

2·18

2·97

2·85

1·41

4·40

3·51

4·79

4·14

4·36

P 2O

50·

100·

030·

06n.

d.0·

040·

030·

070·

020·

050·

040·

210·

050·

070·

100·

140·

100·

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1314


Fig. 4. Electron microprobe backscatter images of (a) retrograde muscovite2 nearly perpendicular to the foliation defined by muscovite1, biotite,and sillimanite; sillimanite–garnet–biotite schist 82395-1, Val Strona di Omegna; (b) hercynitic spinel and ilmenite with sillimanite corona incordierite2 core; sillimanite–cordierite–garnet gneiss 80596-5, Val Strona di Posuta; (c) retrograde replacement of cordierite + K-feldspar bysymplectitic intergrowths of biotite+ sillimanite+ quartz; sillimanite–cordierite–garnet gneiss 62097-3; (d) close-up of cordierite2 with reactionrim of radiating biotite + quartz + sillimanite symplectite; sillimanite–cordierite–garnet gneiss 62097-3.

Garnet grains in metapelite from lower Val Strona augen in metapelite proximal to the upper MaficComplex commonly exhibit inclusion-rich cores sur-di Omegna and Val d’Ossola are typically idiomorphic,

exhibit interior zoning, and are restricted to rounded by inclusion-free rims. We interpret these tex-tural relationships to indicate at least two episodes


Table 2: Mineral assemblages

Sample Location Qtz Kf Plg Ga Bi Sil Cd Spi Rt Il Pyr Mn Ap Zr

80496-6 RF X X X X X X X X X

80496-11 RF X X X X X X X X X X X X

80496-13 RF X X X X X X X X X

80596-3 RF X X X X X X X X

80596-5 RF X X X X X X X X X X

62097-5 RF X X X X X X X X X

62097-3 RF X X X X X X X X X X X

IV96-1 VD X X X X X X X X X

70197-10 VD X X X X X X X X X

62497-3 VD X X X X X X X X X

Location abbreviations as in Table 1 X indicates mineral present in sample.

biotite + sillimanite + quartz ⇒ garnet + Biella in Valle Mosso (Bertolani, 1959; Sacchi, 1962).Although Zingg (1980) interpreted andalusite to be acordierite2 + K-feldspar + melt (8)relict phase, Capedri & Rivalenti (1973) inferred that

occurred within metapelite restricted to a 2–3 km aureole andalusite grew after sillimanite, on the basis of micro-surrounding the upper Mafic Complex. structural relationships. This implies a retrograde trans-

Grains of hercynitic spinel with sillimanite corona ition from the sillimanite to the andalusite stability field:are occasionally present and are always enclosed within

sillimanite ⇒ andalusite. (12)cordierite2 (Fig. 4b). This observation is consistent withthe prograde reaction

biotite + sillimanite ⇒ cordierite2 + hercynitic spinel Thermobarometry+ K-feldspar + ilmenite} + melt (9) The widespread occurrence of metapelite in the Kinzigite

Formation provides favorable assemblages for thermo-in locally quartz-deficient domains, or the retrograde barometric constraints upon the regional PTt paths. Wereaction obtained pressure and temperature estimates with the

GASP (3An⇒ Grs+ 2Als+ Qtz) reaction and Fe–Mgsillimanite + melt ⇒ cordierite2 + exchange between garnet and biotite. Calculations werehercynitic spinel. (10)conducted with both MacPTAX ( J. Lieberman, un-published program, 1991) using a 1991 update of Ber-In agreement with the observations of Capedri &man’s thermodynamic database (Berman, 1988) andRivalenti (1973), we observed occasional neoblasts ofTWQ 2·02 (Berman, 1991). MacPTAX adopts the ac-fine-grained, idiomorphic, inclusion-free garnet, perhapstivity models of Berman (1990) for garnet, McMullin etindicating an episode of post-tectonic garnet growth.al. (1991) for biotite, and Fuhrman & Lindsley (1988) forRetrograde replacement of the assemblage cordierite1,2+plagioclase. TWQ 2·02 adopts the activity models ofK-feldspar is indicated by the development of symplectiticBerman & Aranovich (1995, 1996) for garnet and biotite,intergrowths of biotite + sillimanite + quartz (Fig. 4cand Fuhrman & Lindsley (1988) for plagioclase.and d). This observation implies retrograde hydrationMacPTAX typically returns temperatures and pressuresvia reaction with a vapor phase or reaction with residual>50–100°C and >1–2 kbar lower than TWQ 2·02.melt through the equilibrium

We estimated PT conditions for nine samples collectedon traverses along the Rio Forcioula (6) and Fiumegarnet + cordierite2 + K-feldspar + H2O/melt ⇒Duggia (3) (Fig. 2). To estimate ‘peak’ conditions, webiotite + sillimanite + quartz. (11)compared garnet core compositions free of local zoningnear inclusions (determined by X-ray mapping andAndalusite has been reported between lower Val Sesia

and Val Sessera (Capedri & Rivalenti, 1973; Zingg, microprobe analyses) with plagioclase cores and neigh-boring matrix biotite not in contact with garnet. Mineral1980; Demarchi et al., 1998). The reported abundance

of andalusite increases to the SW, and it is common near compositions and garnet and plagioclase zoning patterns

1316


Table 3: Representative∗ matrix biotite analyses

Sample 80496 80496 80496 80596 80596 62097 62097 IV96 70197 62497

no.: -06 -11 -13 -03 -05 -05 -03 -01 -10 -03

Location RF RF RF RF RF RF RF VD VD VD

SiO2 36·78 36·66 36·91 36·76 35·62 36·52 35·34 36·28 36·45 36·28

TiO2 5·56 5·04 3·94 5·23 4·63 4·28 3·78 4·84 4·45 3·47

Al2O3 16·57 16·31 16·59 17·21 16·84 17·66 18·55 17·59 18·11 16·51

FeOT 13·60 13·08 12·91 14·08 14·15 16·06 19·11 15·39 14·00 15·08

MgO 12·14 13·43 14·33 12·51 13·57 11·96 9·82 11·75 12·37 13·55

Na2O 0·09 0·05 0·14 0·09 0·40 0·11 0·13 0·10 0·16 0·11

K2O 9·48 9·83 9·48 9·68 9·80 9·61 9·23 9·80 9·83 9·51

Total 94·22 94·4 94·3 95·56 95·01 96·2 95·96 95·75 95·37 94·51

Si 5·51 5·49 5·51 5·45 5·36 5·42 5·33 5·41 5·41 5·47

Ti 0·63 0·57 0·44 0·58 0·52 0·48 0·43 0·54 0·50 0·39

Al 2·93 2·88 2·92 3·01 2·99 3·09 3·30 3·09 3·17 2·93

AlIV 2·49 2·51 2·49 2·56 2·64 2·58 2·67 2·60 2·59 2·53

AlVI 0·44 0·36 0·43 0·45 0·34 0·51 0·63 0·49 0·58 0·40

Fe 1·57 1·51 1·48 1·60 1·64 1·83 2·22 1·76 1·60 1·75

Mg 2·71 3·00 3·19 2·76 3·04 2·65 2·21 2·61 2·74 3·04

Na 0·03 0·01 0·04 0·02 n.d. 0·03 0·04 0·03 0·05 0·03

K 1·81 1·88 1·80 1·83 1·88 1·82 1·78 1·86 1·86 1·83

XFe† 0·29 0·28 0·27 0·30 0·30 0·34 0·40 0·33 0·30 0·32

XMg 0·50 0·56 0·57 0·51 0·55 0·48 0·40 0·48 0·51 0·54

XTi 0·12 0·10 0·08 0·11 0·09 0·09 0·08 0·10 0·09 0·07

XAlVI 0·09 0·07 0·08 0·08 0·06 0·09 0·11 0·09 0·11 0·07

∗All mineral analyses obtained using the JEOL 733 electron microprobe at the University of Washington. The term‘representative’ indicates that data are actual analyses (not averages) and are characteristic of the sample analyzed.†Mole fraction Z = XZ/(XFe + XMg + XTi + XAlVI).

were obtained by X-ray mapping and electron micro- equilibria tend to have different closure temperatures(Frost & Chacko, 1989), our calculation of ‘peak’ con-probe analyses. Guided by the X-ray maps and micro-

probe transects, we analyzed 3–5 points per mineral and ditions should be regarded as a minimum estimate ofthe PT conditions during a local thermal maximum.4–6 inferred near-peak equilibrium mineral combinations

(garnet–biotite–plagioclase) per sample. The com- High temperatures are also indicated by a change ofgarnet zoning patterns with proximity to the upper Maficpositional variability of minerals within a sample results

in a range of calculated pressures and temperatures. Complex. South of Val Sesia, the predominant garnetsare medium- to coarse-grained augen, elongate parallelSample locations are provided in Fig. 2, and rep-

resentative mineral analyses are listed in Tables 3–5. In to the foliation defined by sillimanite and biotite. At leastthree phases of garnet growth are evident. Garnet augenaccordance with Guidotti & Dyars’ (1991) empirical

model, we assumed that ferric iron accounted for 8% of consist of inclusion-rich cores, surrounded by inclusion-free rims. Late, fine-grained, inclusion-free, idiomorphicFeOT in biotite. We assumed FeOT = FeO in garnet.

The results of the thermobarometric calculations are garnet cuts the foliation defined by biotite, sillimanite,and the long axes of garnet augen. Garnet grains fromdepicted in Fig. 5 and tabulated in Table 6. Although

estimated pressures (>3–6 kbar) and temperatures lower Val Strona di Omegna and Val d’Ossola samplesare weakly zoned in grossular and, to a lesser extent,(650–750°C) are consistent with the mineral assemblages,

these conditions probably do not correspond to a local spessartine contents with the most extreme zonationbeing within>5–10 !m of their rims. In contrast, garnetthermal maximum. As a result of retrograde Fe–Mg

exchange and the fact that exchange and net transfer grains in samples collected south of Val Sesia are virtually

1317


Table 4: Representative garnet core compositions.

Sample 80496 80496 80496 80596 80596 62097 62097 IV96 70197 62497

no.: -06 -11 -13 -03 -05 -05 -03 -01 -10 -03


SiO2 38·13 38·44 38·46 38·72 38·16 38·78 37·84 37·96 38·19 38·78

TiO2


Table 5: Representative matrix plagioclase analyses.

Sample 80496 80496 80496 80596 80596 62097 62097 IV96 70197 62497

no.: -06 -11 -13 -03 -05 -05 -03 -01 -10 -03


SiO2 57·64 55·51 58·97 59·72 58·37 58·92 60·41 57·76 56·80 58·57

Al2O3 26·04 27·79 25·93 25·36 25·68 26·16 25·15 25·49 27·44 25·54

CaO 7·60 8·58 7·81 7·25 8·42 7·54 6·08 6·68 9·10 7·41

Na2O 7·35 6·43 6·68 7·12 7·09 6·94 8·11 7·70 6·15 7·23

K2O 0·21 0·14 0·14 0·26 0·09 0·19 0·12 0·24 0·15 0·14

FeO n.d.


Fig. 5. (a) Shaded polygons depict the results of our thermobarometric calculations (TWQ 2·02) for metapelite overlying the upper MaficComplex. Dimensions of the polygons determined by the compositional range of each sample. The results of the TWQ 2·02 calculations correlatewith the results of Demarchi et al. (1998). The results of their study are summarized by the unfilled polygons that enclose the intersections ofGASP and garnet–cordierite equilibria with PH2O = Ptotal (a) and PH2O = 0 (b) calculated for metapelite overlying the upper Mafic Complex(Demarchi et al., 1998). Insets (b) and (c) are example calculations for two samples illustrating the difference between the results of the twoalgorithms. Calculated PT positions of thermobarometric equilibria from individual garnet–biotite and garnet–plagioclase pairs bound the shadedregions. Continuous lines indicate MacPTAX calculations; dashed lines indicate TWQ 2·02 calculations. Complete results of MacPTAX andTWQ 2·02 calculations are given in Table 6.

& Holloway, 1988), indicating that the region followed (e.g. Schmid & Wood, 1976; Hunziker & Zingg, 1980;a near-isobaric cooling path after regional retrograde Sills, 1984; Bürgi & Klötzli, 1990; Schnetger, 1994;decompression and magmatism (Fig. 6b). The presence Sinigoi et al., 1996). This interpretation is based onof retrograde andalusite [reaction (11)] probably doc- geochronology, an inferred temperature gradient acrossuments a retrograde transition from the sillimanite to the the southeastern margin of the upper Mafic Complexandalusite stability field. (Schmid & Wood, 1976), and a textural gradation from

hypidiomorphic–granular to granoblastic with increasingoriginal depth within the upper Mafic Complex (Rivalentiet al., 1980; Quick et al., 1994). Rivalenti et al. (1980)DISCUSSION AND CONCLUSIONSand Pin (1990) suggested that this textural gradation is

Timing of regional and contact consistent with cooling subsequent to synmetamorphicmetamorphism intrusion.Previous interpretation However, some PT estimates (Sills, 1984) do not in-

dicate a regional temperature gradient with proximity toAlthough Zingg et al. (1990), Schmid (1993), and Barbozathe upper Mafic Complex. Alternative interpretations foret al. (1999) took an alternative view, most regional studiesthe origin of the textural gradation within the upperhave presumed or inferred that final emplacement of the

upper Mafic Complex caused regional metamorphism Mafic Complex (Zingg et al., 1990; Quick et al., 1994)

1320


Table 6: Calculated metamorphic conditions

Sample Location MacPTAX TWQ 2·02

T (°C) P (kbar) T (°C) P (kbar)

80496-06 RF 600–650 3·0–4·5 700–750 5·0–6·5

80496-11 RF 600–650 3·5–4·5 675–725 5·5–6·5

80496-13 RF 550–625 3·0–4·0 650–700 5·0–6·0

80596-03 RF 600–650 3·0–5·0 650–725 5·0–7·0

80596-05 RF 600–650 3·0–4·0 650–700 5·0–5·5

62097-05 RF 650–750 4·0–6·0 650–800 4·5–6·5

62097-03 RF 575–625 3·0–4·0 625–800 4·5–5·5

IV96-01 VD 600–700 3·0–5·0 675–700 5·0–5·5

70197-10 VD 600–775 3·0–5·5 700–825 5·0–7·0

62497-03 VD 565–625 3·0–5·0 650–750 3·5–6·0

Location abbreviations as in Table 1.

obviate the need for a causal relationship between mag- prograde path (650–700°C (Henk etComplex occurred late relative to the regional granulite al., 1997), implying a quasi-steady-state thermal gradientfacies episode (Zingg et al., 1990; Barboza et al., 1999). of >40–60°C/km.Our interpretation that the low-pressure assemblage These temperatures are elevated with respect to anproximal to the upper Mafic Complex overprints the average crustal geotherm and may require elevated heatprograde regional metamorphic zonation supports the flow through a combination of magmatic accretion, aque-conclusion that the regional thermal maximum preceded ous fluid flow, and/or lithospheric extension (De Yoreoemplacement of the upper Mafic Complex. et al., 1991). Stable isotopes, however, indicate limited

fluid infiltration (Baker, 1988) and the metamorphismwe associate with the emplacement of the upper MaficComplex overprints the regional metamorphic zonation.We suggest that crustal attenuation through tectonicTectonic implicationsextension (Fig. 7b and c) could explain the moderateFigure 7 depicts a schematic illustration of our new modelpressures along the prograde PT path and the elevatedfor metamorphism and magmatism at the Ivrea zone.geotherm at the regional thermal maximum. PassiveThe presence of relict kyanite and staurolite may suggestupwelling of the asthenosphere accompanied the ex-that an early phase of crustal thickening accompaniedtension leading to a steepening of the geotherm, therebyprograde metamorphism (Fig. 7a). Although not theenhancing the lower-crustal heat budget (Furlong &only interpretation of the early prograde metamorphicLonde, 1986; De Yoreo et al., 1991) and imposing gran-evolution of the Ivrea zone (see Zingg, 1983; Pin, 1990;ulite facies conditions at relatively shallow crustal levels.Handy & Zingg, 1991), such a prograde contractionalLachenbruch & Sass (1978) inferred a geotherm of >event is consistent with evidence of an early phase of>40°C/km for the Battle Mountain High in the Basinhigh-pressure metamorphism in the Strona Ceneri zoneand Range Province, a thermal gradient sufficiently steep(Boriani & Villa, 1997).to explain the PT conditions at the regional thermalThe ubiquitous presence of metapelite with sillimanitemaximum at the Ivrea zone (Fig. 5).in the matrix and as inclusions within garnet indicates

Decompression subsequent to the regional thermalthat, subsequent to the early contractional phase ofmaximum is documented by the appearance of a lower-metamorphism, much of the prograde PTt path occurredpressure assemblage (cordierite, hercynitic spinel) withinwithin the stability field of sillimanite. The limits of bothdepleted metapelite proximal to the upper Mafic Com-muscovite and staurolite stability were probably exceededplex. The absence of these phases and the presence ofwithin the stability field of sillimanite, further indicating

that moderate pressures prevailed along much of the retrograde muscovite2 require that cooling accompanied

1321


Fig. 6. (a) Partial petrogenetic grid for intermediate XMg metapelite. Single lines represent divariant fields for clarity. Reaction boundariesadapted after Jones & Brown (1990) and references therein. Reaction Chl + Ms(ss) ⇒ St + Bt + V from Xu et al. (1994). (b) Proposed PTtpath. The two paths represent the PTt paths of rocks proximal (path A) and distal (path B) to the upper Mafic Complex. (I), PT conditions andage constraints at the regional thermal maximum; (II), thermal maximum for rocks proximal to the upper Mafic Complex discussed in text;(III), cooling age below >300°C based on K–Ar biotite ages (Bürgi & Klötzli, 1990). Alternative interpretations of the prograde metamorphicevolution reflect the range of uncertainty in the dashed portion of the prograde PTt path.

decompression in rocks distal to the upper Mafic Com- to reset proximal mineral assemblages. Mineral as-semblages in distal rocks were incompletely reset andplex. Emplacement of the upper Mafic Complex occurred

during retrograde decompression from the regional ther- preserve evidence of higher pressures at the regionalthermal maximum (Fig. 5).mal maximum (Fig. 7c) and provided the thermal energy

1322


Fig. 7. Inferred tectonic evolution of the Ivrea zone (see text for discussion). As in Fig. 6, points A and B represent rocks at approximately thesame crustal level, but situated proximal (A) and distal (B) to the upper Mafic Complex. Placement of the isotherms and isobars is schematicand based on our inferred regional PTt path.

We attribute the decompression from the regional baric et al., 1989). Finally, Snoke et al. (1999) reported evidencesupporting progressive, non-coaxial, ductile deformationgradient at the regional thermal maximum to crustal

thinning by continued tectonic extension. This in- within the carapace of supracrustal rocks overlying theupper Mafic Complex, an observation consistent withterpretation is consistent with other observations in the

Ivrea zone. Henk et al. (1997) inferred a baric gradient ductile attenuation through extension during em-placement. Although compositional variability and theof >0·41 kbar/km in Val Strona di Omegna. Although

the uncertainty in baric gradients calculated from mineral thin (>3 km) carapace of Kinzigite Formation overlyingthe upper Mafic Complex south of Val Sesia preclude aequilibria is large, this estimate exceeds the lithostatic

gradient of >0·3 kbar/km (Burke & Fountain, 1990), direct calculation of the baric gradient in Val Strona diPostua, significant crustal attenuation can be ac-suggesting crustal attenuation by a factor of >1·4 sub-

sequent to the closure temperature of the net transfer commodated by our results (Fig. 2).We conclude that extension characterized the Ivreareactions from which pressure was estimated. Regional

extension is consistent with Brodie & Rutter’s (1987) zone from the Late Carboniferous to the Early Permian.The peak thermal episode of regional granulite faciesinference that 2 km of E–W-directed attenuation within

the lowermost 5 km of the Ivrea zone occurred along metamorphism resulted from elevated heat flow duringextension and was perhaps enhanced by magmatic ac-mylonitic shear zones. Oriented symplectic intergrowths

of orthopyroxene, plagioclase, and spinel in metapelite cretion below the current level of exposure (Henk etal., 1997). Remnants of the prograde and near-peakfrom upper Val Strona di Omegna support an episode of

regional, near-isothermal decompression (Brodie, 1995). magmatism that accompanied extension and regionalgranulite facies metamorphism may be preserved inHandy (1987) estimated 8–10 km of E–W-directed ex-

tension along normal faults near the boundary between parts of the lower Mafic Complex (Vavra et al., 1999).Emplacement of the upper Mafic Complex accompaniedthe Ivrea and Strona–Ceneri zones. Quick et al. (1994)

interpreted the arcuate structure of relict magmatic fo- decompression from ambient pressures at the regionalpeak of metamorphism during continued extension. Theliation in the upper Mafic Complex as implying em-

placement into a zone of active extension, an inference heat released during crystallization of the upper MaficComplex provided the thermal energy for contact meta-consistent with age estimates for the onset of tectonic

extension along the regional retrograde PTt path (Brodie morphism up to the granulite facies in proximal crustal

1323


rocks. The occurrence of cordierite, hercynitic spinel, Complex represents the thermal equilibration of crustalrocks with heat released during the final stages of em-and coarse-grained garnet augen preserves evidence ofplacement of the upper Mafic Complex. Large-scalelow-P, high-T contact metamorphism.regional metamorphism resulting from emplacement,Crustal attenuation after the regional thermal max-however, is not apparent, demonstrating that the in-imum led to decompression melting of metapelite over-trusion of large volumes of mafic magma within the lowerlying the upper Mafic Complex and its depletion bycontinental crust may not inexorably imply regional-scaleup to >30% granite component (Barboza, 1998). Themetamorphism and anatexis of crustal rocks. Referencedepleted accumulations of cumulate and peritectic min-to studies that predict regional granulite facies meta-eral phases that document this melting event are probablymorphism and extensive crustal anatexis are a necessaryrepresented by the abundant leucotonalitic leucosomesconsequence of magmatic accretion (Wells, 1980;intercalated with metapelite within the upper Mafic Com-Campbell & Turner, 1987; Wickham, 1987; Huppert &plex contact aureole (Barboza, 1998; Barboza et al., 1999;Sparks, 1988) should be tempered by an awareness thatSnoke et al., 1999). Foliation-parallel melt movement andsuch effects are not apparent in the Ivrea zone, oftenmelt segregation to higher crustal levels may have beenregarded as an important example of such a process.directed by penetrative deformation within the carapace

of supracrustal rocks overlying the upper Mafic Complex(Snoke et al., 1999). Weakly deformed granitic intrusivebodies in the eastern Ivrea zone may represent themobilized melt extracted from the contact aureole (Snoke ACKNOWLEDGEMENTSet al., 1999). The cessation of extension and high-grade Dr J. Quick, Professor S. Sinigoi, Professor M. Brown,conditions in rocks proximal to the upper Mafic Complex Dr L. Burlini, and Professor A. Boriani are thankedmay have coincided with its final emplacement (Demarchi for fruitful discussions and logistical support. Informalet al., 1998) and was followed by near-isobaric cooling reviews by Professor B. W. Evans and Dr Quick, and(Fig. 7d) until Tertiary orogenic activity led to em- formal reviews by Professors A. W. Snoke, B. R. Frost,placement of the Ivrea zone within the upper crust. R. Schmid, and B. Hacker substantially improved the

manuscript. The detailed editorial assistance of Dr SorenaSorensen is also greatly appreciated. Partial funding forthis work was provided by National Science FoundationMagmatic accretion modelsgrant EAR-9508291, and by a Royalty Research FundOur field, petrographic, and geochemical evidence im-grant from the University of Washington.plies that anatexis and metamorphism caused by em-

placement of the upper Mafic Complex was confined toa >3 km aureole in supracrustal rocks that overlie theintrusion. Models that rely upon rapid cooling and con-

REFERENCESvective heat transfer during magmatic accretion (Wells,Baker, A. J. (1988). Stable isotope evidence for limited fluid infiltration1980; Campbell & Turner, 1987) overpredict the extent

of deep crustal rocks from the Ivrea zone, Italy. Geology 16, 492–495.of crustal heating and anatexis we observe in the IvreaBarboza, S. A. (1998). Anatexis and metamorphism during under-zone. Huppert & Sparks (1988) calculated that em-

plating: field and numerical results. Ph.D. Thesis, University ofplacement of basaltic magma at 1200°C into crust at Washington, Seattle.500–850°C would generate a volume of silicic magma Barboza, S. A. & Bergantz, G. W. (1996). Dynamic model of de-roughly 2·0–0·6 times that of the basalt. This amount hydration melting motivated by a natural analogue: applications toexceeds the anatexis we have documented within the the Ivrea–Verbano zone, northern Italy. Transactions of the Royal Society

of Edinburgh 87, 23–31.upper Mafic Complex contact aureole. Even simulationsBarboza, S. A., Bergantz, G. W. & Brown, M. (1999). Regional granulitebased on a conductive model for heat transfer (Barboza

facies metamorphism in the Ivrea zone: is the Mafic Complex the& Bergantz, 1996) overestimate the extent of the contactsmoking gun or a red herring? Geology 27, 447–450.

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Carrington, D. P. & Harley, S. L. (1995). Partial melting and phaseBerman, R. G. & Aranovich, L. Y. (1995). Reanalysis of the garnet–relations in high-grade metapelites: an experimental petrogeneticclinopyroxene Fe–Mg exchange thermometer. II. Thermodynamicgrid in the KFMASH system. Contributions to Mineralogy and Petrologyanalysis. Contributions to Mineralogy and Petrology 119, 20–42.120, 270–291.Berman, R. G. & Aranovich, L. Y. (1996). Optimized standard state

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