Metasomatism of continental crust during subduction: The UHP whiteschists from the Dora-Maira Massif (Italian Western Alps

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The ultrahigh-pressure pyrope whiteschists from the Brossasco-Isasca Unit of the Southern Dora-Maira Massif represent metasomatic rocks originated at the expense of post-Variscan granitoids by the influx of fluids along shear zones. In this study,
  Metasomatism of continental crust during subduction: theUHP whiteschists from the Southern Dora-Maira Massif (ItalianWestern Alps) S. FERRANDO, 1 M. L. FREZZOTTI, 2 M. PETRELLI 3 AND R. COMPAGNONI 1 1 Department of Mineralogical and Petrological Sciences, University of Torino, Via Valperga Caluso 35, I-10125 Torino, Italy( 2 Department of Earth Sciences, University of Siena, Via Laterina 8, I-53100 Siena, Italy 3 Department of Earth Sciences, University of Perugia, P.zza Universita ` 1, I-06100 Perugia, Italy ABSTRACT  The ultrahigh-pressure pyrope whiteschists from the Brossasco-Isasca Unit of the Southern Dora-MairaMassif represent metasomatic rocks srcinated at the expense of post-Variscan granitoids by the influxof fluids along shear zones. In this study, geochemical, petrological and fluid-inclusion data, correlatedwith different generations of pyrope-rich garnet (from medium, to very-coarse-grained in size) allowconstraints to be placed on the relative timing of metasomatism and sources of the metasomatic fluid.Geochemical investigations reveal that whiteschists are strongly enriched in Mg and depleted in Na, K,Ca and LILE (Cs, Pb, Rb, Sr, Ba) with respect to the metagranite. Three generations of pyrope, withdifferent composition and mineral inclusions, have been distinguished: (i) the prograde Prp I, whichconstitutes the large core of megablasts and the small core of porphyroblasts; (ii) the peak Prp II, whichconstitutes the inner rim of megablasts and porphyroblasts and the core of small neoblasts; and (iii) theearly retrograde Prp III, which locally constitutes an outer rim. Two generations of fluid inclusions havebeen recognized: (i) primary fluid inclusions in prograde kyanite that represent a NaCl-MgCl 2 -rich brine(6–28 wt% NaCl eq  with Si and Al as other dissolved cations) trapped during growth of Prp I (type-Ifluid); (ii) primary multiphase-solid inclusions in Prp II that are remnants of an alumino-silicate aqueoussolution, containing Mg, Fe, alkalies, Ca and subordinate P, Cl, S, CO 32- , LILE (Pb, Cs, Sr, Rb, K,LREE, Ba), U and Th (type-II fluid), at the peak pressure stage. We propose a model that illustrates theprograde metasomatic and metamorphic evolution of the whiteschists and that could also explain thegenesis of other Mg-rich, alkali-poor schists of the Alps. During Alpine metamorphism, the post-Variscan metagranite of the Brossasco-Isasca Unit experienced a prograde metamorphism at HPconditions (stage A:  1.6 GPa and  £  600   C), as indicated by the growth of an almandine-rich garnet insome xenoliths. At stage B (1.7–2.1 GPa and 560–590   C), the influx of external fluids, srcinated fromantigorite breakdown in subducting oceanic serpentinites, promoted the increase in Mg and the decreaseof alkalies and Ca in the orthogneiss toward a whiteschist composition. During stage C(2.1 <  P  < 2.8 GPa and 590 <  T   < 650   C), the metasomatic fluid influx coupled with internaldehydration reactions involving Mg-chlorite promoted the growth of Prp I in the presence of the type-IMgCl 2 -brine. At the metamorphic peak (stage D: 4.0–4.3 GPa and 730   C), Prp II growth occurred inthe presence of a type–II alumino-silicate aqueous solution, mostly generated by internal dehydrationreactions involving phlogopite and talc. The contribution of metasomatic external brines at themetamorphic climax appears negligible. This fluid, showing enrichment in LILE and depletion in HFSE,could represent a metasomatic agent for the supra-subduction mantle wedge. Key words:  antigorite breakdown; HP metasomatic fluid; multiphase-solid inclusions; serpentinite;trace-element geochemistry. INTRODUCTION Pyrope whiteschists from the Brossasco-Isasca Unit(BIU) of the Southern Dora-Maira Massif (WesternAlps) – i.e. talc-phengite-kyanite-pyrope-quartz(   ⁄   coe-site) rocks characterized by an extremely high Mg   ⁄   Feratio, high Si 2 O and the virtual absence of Ca and Na — became well known to the scientific communitybecause of the first record of coesite in crustal litho-logies (i.e. the first evidence that continental crust cansubduct to depths of at least 100 km; Chopin, 1984).Since this discovery, a number of studies have beencarried out on Dora-Maira whiteschists to characterizethe metamorphic evolution of continental crust duringsubduction to ultrahigh-pressure (UHP) conditions.The composition, crystal structure, and sometimesalsothestabilityfieldofmajor(e.g.pyrope,phengite)andminor (e.g. bearthite, ellenbergerite, Mg-dumortierite,  J. metamorphic Geol. , 2009 doi:10.1111/j.1525-1314.2009.00837.x   2009 Blackwell Publishing Ltd  1  wagnerite) minerals have been characterized (e.g.Chopin, 1986; Schertl  et al. , 1991; Chopin  et al. , 1986,1995; Chopin & Schertl, 1999; Chopin & Ferraris, 2003 and references therein). Schertl  et al.  (1991) distin-guished prograde pyrope megablasts and peak smallerpyrope. Prograde pyrope megablasts, formed at theexpense of chlorite + talc + kyanite, have inclusionsof kyanite, talc, chlorite, ellenbergerite, glaucophane,phlogopite (usually retrogressed to vermiculite), rutile,zircon, tourmaline, apatite and Mg-dumortierite. Thesmaller peak pyrope crystals, formed at the expense of talc + kyanite, include coesite, kyanite, talc, phengite,rutile, zircon and tourmaline.Recent petrological and experimental studies of BIUwhiteschists, in particular from the Po Valley in Italy,have constrained a  P–T   evolution characterized by sixmain stages: (i) a poorly constrained prograde stage at £  600   C and   1.6 GPa (Schertl  et al. , 1991; Com-pagnoni & Hirajima, 2001), (ii) a second progradestage at   700–720   C and 2.9–3 GPa (Schertl  et al. ,1991; Simon  et al. , 1997), (iii) a UHP peak stage at730   C and 4.3 GPa, (iv) an early decompression stageat 4–3.5 GPa, (v) a first decompression and coolingstage at 700–670   C and 3–2.5 GPa, and (vi) a seconddecompression and cooling stage at 600   C and1.1 GPa (Hermann, 2003). The UHP peak has beendated at  c . 35 Ma by U-Pb and Lu-Hf analyses onellenbergerite, zircon, monazite and garnet (Tilton et al. , 1989; Ducheˆne  et al. , 1997; Gebauer  et al. , 1997;Vaggelli  et al. , 2006).There is a general agreement concerning thepetrology and geochronology of the Dora-Mairawhiteschists, but the origin of the unusual chemicalcomposition of these rocks is still a matter of debate.Chopin (1984) and Schertl  et al.  (1991) proposed asedimentary protolith (a clay-evaporite composed of quartz, Mg-chlorite and illite), whereas Compagnoni &Hirajima (1992) first proposed a metasomatic srcin,suggesting a fluid influx in the metagranite along ductileshear zones during prograde UHP evolution. Stable-isotope and fluid-inclusion studies have providedfurther data useful for understanding the origin of whiteschistsandthefluid-rockinteractionprocesses.Onthe basis of stable isotope data, Sharp  et al.  (1993)interpreted the whiteschists as metasomatic rocksthat srcinated by the influx of metasomatic fluids fromthe subducting oceanic lithosphere (in addition, seeDeme ´ny  et al. , 1997). They also concluded that duringpyrope growth the water activity was low ( a H 2 O  = 0.4– 0.75) andthatduringtheprograde, peakandretrogrademetamorphism  X  CO 2 was low (<0.02). In pyropemegablasts from whiteschists, the fluid-inclusion studyof  Philippot  et al.  (1995) allowed the recognition of aprograde   ⁄   peak brine represented by fluid inclusionsconsistingofmagnesite,Mg-phosphate,chloride,talcorMg-chlorite and an opaque mineral. This brine wasinterpreted as a residual fluid that had remained afterprograde  in-situ  generation of a melt, which should berepresented by the    jadeite quartzite   closely associatedwith the whiteschists in the field. The presence of a melt,already suggested by Sharp  et al.  (1993) to explain thelow  a H 2 O  in the system, was subsequently excluded byHermann (2003).After the isotopic study of  Tilton  et al.  (1989), whichrevealed that whiteschists still retain a strong crustalPb-Nd-Sr isotope signature similar to that of granites,the metasomatic origin of the whiteschists at theexpense of the hosting orthogneiss was conclusivelyconfirmed by cathodoluminescence and SHRIMPstudies on zircon (Gebauer  et al. , 1997). However,these authors disagreed with Compagnoni & Hirajima(1992) and Sharp  et al.  (1993) about the timing of themetasomatism (they considered it to be earlier thansubduction) and the origin of the metasomatic fluid(they considered it to be derived from evaporiticmetasediments).More recently, Compagnoni & Hirajima (2001)reported superzoned garnet characterized by a reddish-brown zoned almandine core (Alm 70-58 Prp 25-37 ) and apinkish, zoned pyrope rim (Alm 58-14 Prp 38-84 ) from aUHP whiteschist interpreted as a former xenolithwithin the orthogneiss. The strong change in compo-sition and mineral inclusions from the core to the rimallowed Compagnoni & Hirajima (2001) to concludethat the metasomatic event occurred during progradehistory at pressures from 1.6 GPa up to conditionsclose the UHP climax. Based on this study and on Clstable isotope data on serpentinites, Sharp & Barnes(2004) proposed that the unusual bulk composition of the whiteschists, especially the high-Mg content, wasdue to the infiltration of high-salinity fluids generatedduring subduction by dehydration of antigorite fromthe underlying oceanic crust. In their recent paper,Schertl & Schreyer (2008), comparing whole-rockmajor- and trace-element data from a number of theBIU whiteschists and of the host orthogneisses   ⁄   metagranites, accepted the metasomatic origin andconcluded that the whiteschists derived from agranitoid   ⁄   orthogneiss protolith through a significantincrease in Mg and a decrease in Na, Ca, Fe, Cu, P,Rb, Ba and Sr. However, they did not give a conclusiveexplanation of the nature of the metasomatic fluid.In this paper, we use new petrological, geochemicaland fluid inclusion data to constrain a model for themetasomatic formation of the whiteschists and theirsubsequent UHP history. The study concentratesmainly to the different generations of pyrope-richgarnet and on the associated minerals and fluid inclu-sions present in samples collected from the classiclocality of Case Ramello (i.e. Case Parigi of  Chopin,1984) and from a new sampling site located SSW of Case Ramello (Po valley). GEOLOGICAL BACKGROUND The Dora-Maira Massif (DMM), together with theGran Paradiso and the Monte Rosa, belongs to theInternal Crystalline Massifs of the Pennine Domain of  2  S. FERRANDO  ET AL.   2009 Blackwell Publishing Ltd  the Western Alps (Fig. 1a). Detailed field and labora-tory investigations of the southern DMM (seeCompagnoni & Rolfo, 2003 and Compagnoni  et al. ,2004 for reviews) showed that this portion of themassif consists of a nappe pile composed of continent-derived tectonic units – including both the Variscancrystalline basement and its Triassic cover – juxtaposedduring the Alpine orogeny to thin, ocean-derived units(calcschists and metaophiolites) of the Piemonte Zone(Fig. 1a). Among these units, only the Brossasco-Isasca Unit (BIU) experienced an early Alpine UHPmetamorphism, peaking at 730   C and 4.0–4.5 GPa(Hermann, 2003; Castelli  et al. , 2007), followed by aretrograde greenschist facies recrystallization. The BIUhas been subdivided into a   PolymetamorphicComplex   derived from Alpine tectono-metamorphicreworking of the Variscan amphibolite faciesmetamorphic basement and a   MonometamorphicComplex   consisting of orthogneiss (mainly aug-engneiss with very minor metagranites) derived fromAlpine tectono-metamorphic reworking of Permiangranitoids (Compagnoni  et al. , 1995; Fig. 1b).The Monometamorphic Complex locally alsoincludes peculiar Mg-rich and Na-Ca-poor lithologies,named   pyrope-whiteschists   (Fig. 1b; Compagnoni et al. , 2004), which occur within the orthogneisses aslayers from a few centimetres to   20 m in thicknessand from a few metres to hundreds of metres in length(e.g. Compagnoni  et al. , 1995). The contact betweenthe whiteschist and the hosting orthogneiss is marked (a) (b) Fig. 1.  (a) Simplified tectonic sketch-map of the Southern Dora-Maira Massif (modified from Castelli  et al. , 2007). Tectonic Units of the Massif are structurally listed from the lowest Pinerolo Unit to the highest Dronero-Sampeyre Unit. The inset shows the location of the Southern Dora-Maira Massif within a simplified tectonic sketch-map of the Western Alps. Helvetic-Dauphinois domain: MB,Mont Blanc-Aiguilles-Rouges. Penninic domain: SB, Grand St. Bernard Zone; MR, Monte Rosa; GP, Gran Paradiso; DM, Dora-Maira; V, Valosio; PZ, Piemonte zone of calcschists with meta-ophiolites. Austroalpine Domain: DB, Dent Blanche nappe; ME,Monte Emilius klippe; SZ, Sesia-Lanzo zone; SA, Southern Alps; EU, Embrunais-Ubaye flysch nappe; PF, Penninic thrust front; CL,Canavese line. (b) Simplified geological map of the coesite-bearing Brossasco-Isasca Unit (BIU) (modified from Castelli  et al. , 2007).Undifferentiated units: graphite-rich schists and metaclastics of the epidote-blueschist facies   Pinerolo Unit  ;   San Chiaffredo Unit   and  Rocca Solei Unit   with pre-Alpine basement rocks overprinted by Alpine quartz-eclogite facies metamorphism. The white stars showthe two locations of the studied whiteschists. CONTINENTAL CRUST METASOMATISM IN SUBDUCTION  3   2009 Blackwell Publishing Ltd  by a few decimetres to several metres of a phengite-rich, banded gneiss locally preserving relics of theigneous K-feldspar phenocrysts (Compagnoni  et al. ,1995). ANALYTICAL TECHNIQUES Chemical analyses of minerals were obtained using aCAMECA SX50 electron microprobe at the IGAG,CNR of Roma. Operating conditions were 15 kVaccelerating voltage, 15 nA beam current and 10 scounting time for each element. The used natural andsynthetic standards were: orthoclase (K), wollastonite(Ca, Si), native manganese (Mn), corundum (Al), jadeite (Na), magnetite (Fe), olivine (Mg), nativechromium (Cr), native zirconium (Zr), rutile (Ti),apatite (P), potassium chloride (Cl) and phlogopite (F).At the above operating conditions, values below 0.05wt% for minor elements must be considered onlyindicative. Areal analyses of 50–10  l m 2 have beencollected on mica.Whole-rock major- and trace-element compositionswere analysed at Chemex Laboratories (Canada) usingICP-AES (major elements) and ICP-MS (trace ele-ments). The precision for the analyses is better than1% for major elements and better than 5% for traceelements. Trace-element patterns reported in the fig-ures are plotted with the P ETROGRAPHETROGRAPH  software byPetrelli  et al.  (2005).In situ trace-element analysis of minerals (garnet,white mica, ellenbergerite, Mg-dumortierite) and mul-tiphase-solid (MS) inclusions were performed ondoubly polished 100- l m-thick sections by using theLaser Ablation-Inductively Coupled Plasma-MassSpectrometer (LA-ICP-MS) installed at the Depart-ment of Earth Sciences, University of Perugia (SMA-Art facilities). The instrumentation consists of a NewWave UP213 frequency-quintupled Nd:YAG laserablation system coupled with a Thermo Electron X7quadrupole-based ICP-MS. All LA-ICP-MS mea-surements were carried out using time-resolved analy-sis operating in a peak jumping mode. Each analysisconsisted of    40 s of measurement of instrumentalbackground – i.e. analysis of the carrier gas with nolaser ablation – followed by   60–80 s of data acqui-sition with the laser on. The laser-beam diameter, therepetition rate and the laser energy density were fixedto 30–40  l m, 10 Hz and   10 J cm ) 2 respectively.Helium was preferred over argon as a carrier gas toenhance transport efficiency of ablated aerosol (Eggins et al. , 1998). The helium carrier exiting the ablationcell was mixed with argon make-up gas before enteringthe ICP torch to maintain a stable and optimumexcitation condition. Two main LA-ICP-MS analyticalprotocols have been adopted here: (i) mineral analysison the sample surface, and (ii) MS inclusions analysisin the depth of the sample. For mineral analysis, thetime-resolved spectra are characterized by  40 s of gasbackground followed by   60 s of mineral signal.External calibration was performed using NIST SRM610 and 612 glass standards in conjunction withinternal standardization using  29 Si, previously deter-mined by WDS electron microprobe following themethod proposed by Longerich  et al.  (1996). Datareduction was performed using the Glitter software(van Achterbergh  et al. , 2001). The US GeologicalSurvey (USGS) reference standard BCR2G (a fusedglass of the Columbia River Basalt) was analysed ineach analytical run as quality control in order to assessthe accuracy and the reproducibility of the analyses.Further details on the analytical method can be foundin Petrelli  et al.  (2007, 2008). For MS inclusions (10–30  l m in diameter), analysis of the time-resolvedspectra is characterized by   40 s of gas background,followed by the host-mineral (Prp II) signal for a timedepending upon the depth of MS inclusions and finallyby the host-Prp II-plus-MS-inclusion signal. At the endof the MS inclusion, ablation signals return to thehost-mineral values. The host-Prp II-plus-MS-inclu-sion signal was characterized for major and traceelements using the USGS standard BCR2G (USGS,2009) as calibrator and normalizing the results to afixed-oxide total following the procedure described inGuillong  et al.  (2005) and Halter  et al.  (2002). Theconcentration of one major element ( 29 Si) was subse-quently used as the internal standard to reduce trace-element concentrations from count-per-second signalsfollowing the method proposed by Longerich  et al. (1996). External calibration for trace elements wasperformed using the USGS standard reference materialBCR2G (USGS, 2009) following the method proposedby Longerich  et al.  (1996). No correction for the hostPrp II contribution was performed. This proceduredoes not allow the real chemical composition of MSinclusions to be obtained, but it does make it possibleto clearly recognize elements that are enriched in MSinclusions with respect to the Prp II withoutperforming assumptions or modelling to correct theanalyses for the host-mineral contribution.The composition of minerals within MS inclusionswas investigated with a Cambridge Instruments SEMStereoscan 360 equipped with an EDS Energy 200 anda Pentafet detector (Oxford Instruments) at theDepartment of Mineralogical and PetrologicalSciences, University of Torino. Operating conditionswere 15 kV accelerating voltage and 50 s countingtime. SEM-EDS quantitative data (spot size = 2  l m)were acquired and processed using the MicroanalysisSuite Issue 12 (INCA Suite version 4.01) then the rawdata were calibrated on natural mineral standards andthe  F q Z correction (Pouchou & Pichoir, 1988) wasapplied.Microthermometric measurements of fluid inclu-sions within doubly polished, 100- l m-thick sectionswere performed with a Linkam THM 600 heating-freezing stage coupled with an Olympus polarizingmicroscope (100-x objective) at the Department of Mineralogical and Petrological Sciences, University of  4  S. FERRANDO  ET AL.   2009 Blackwell Publishing Ltd  Torino. The instruments were calibrated by a set of synthetic fluid inclusions (SYNFLINC) with an esti-mated accuracy of about ±0.1   C at the triple point of CO 2  (-56.6   C), at the triple point of H 2 O (0.01   C)and at the critical temperature of H 2 O (374   C). Fluid-inclusion compositions were determined by using thepackage FLUID (Bakker, 2003; Bakker & Brown,2003).Raman microspectroscopy analyses were collected atthe Department of Earth Sciences of Siena (with aconfocal Labram multichannel spectrometer by JobinYvonInstruments,characterizedbyanexcitationlineat514.5 nm produced by an Ar + -ion laser, a Notchholographical filter with a spectral resolution of 1.5 cm ) 1 , and a grating of 1800 grooves mm ) 1 ) and atthe Department of Mineralogical and PetrologicalSciences of Torino (with an integrated micro   ⁄   macroRaman LABRAM HRVIS by Horiba Jobin YvonInstruments, characterized by an excitation line at532.11 produced by solid-state Nd laser, a Super NotchPlus filter with spectral resolution of 1 cm ) 1 and agratingof600 groovesmm ) 1 ).Thebeamwasfocusedtoaspotsizeof   1–2  l musinga100-xobjective.Thedailycalibration was performed using the 1332-cm ) 1 diamond band or the 520.6-cm ) 1 Si band. PETROGRAPHY AND MINERAL CHEMISTRY The studied UHP whiteschists consist of pyrope – frommedium to very-coarse grain size – embedded in amatrix of variable amounts of quartz (from formercoesite), kyanite, phengite, talc and accessory rutile,apatite, zircon and monazite. Mg-chlorite, muscovite,phlogopite, paragonite, unusual Mg-rich mineralsand accessories occur as inclusions in pyrope and   ⁄   orkyanite (see below).Three kinds of zoned garnet have been distinguished(Fig. 2): (i) megablasts – from 10 to 20 cm indiameter – characterized by a wide, reddish core (Prp I;abbreviations after Fettes & Desmons, 2007), a thininner rim (Prp II) and a thinner outer rim (Prp III); (ii)porphyroblasts – from 2 to 10 cm in diameter – char-acterized by a small Prp I core, a wide Prp II inner rimand a thin Prp III outer rim; and (iii) neoblasts, <2 cmin diameter, characterized by a wide Prp II core and athin Prp III rim. Prp I (Prp 69-81 Alm 16-26 ; Fig. 3a;Table 1) is interpreted as a prograde garnet (see alsoSchertl  et al. , 1991) because it includes kyanite, talc,phlogopite (totally retrogressed to vermiculite), azoned ellenbergerite with little P-ellenbergerite com-ponent (Si   ⁄   P = 43 in the core; Si   ⁄   P = 263 in the rim),and Mg-chlorite [mg# = Mg   ⁄   (Mg + Fe) = 0.95],Mg-dumortierite, rutile, zircon, apatite and monazite,but never coesite   ⁄   quartz or phengite. Prp II (Prp 82-98 Alm 1-16 ; Fig. 3a; Table 1) is interpreted as a peakgarnet (see also Schertl  et al. , 1991) because it containsinclusions of coesite, kyanite, phengite, rutile, zircon,monazite and rare talc, but never Mg-chlorite andphlogopite. Prp III (Prp 86-67 Alm 4-11 ; Fig. 3a; Table 1),usually pseudomorphically replaced by phengite, bio-tite, talc, kyanite and chlorite (e.g. Schertl  et al. , 1991;Hermann, 2003), is considered retrograde because itincludes phengite, kyanite, rutile and zircon. Allpyrope generations always contain P (up to 0.012a.p.f.u.), Ti (locally, up to 0.008 a.p.f.u.), Zr (up to0.004 a.p.f.u.) and Cr (up to 0.002 a.p.f.u.) and, morerarely, F (up to 0.011 a.p.f.u.) and Cl (up to 0.001a.p.f.u.; see Table 1).Three kyanite generations have been distinguished inthe whiteschists: (i) prograde porphyroblastic Ky I,usually occurring within Prp I and including quartz,rutile, zircon and minor talc, muscovite, paragonite,and Mg-chlorite; (ii) peak porphyroblastic Ky II,which occurs both in the matrix and locally also in PrpII and includes phengite and polycrystalline quartzfrom coesite; and (iii) retrograde Ky III, which occursin Prp III and, together with quartz, in the rock matrix.Three types of white mica have been distinguished:(1) prograde muscovite (Mus: Si = 3.185 a.p.f.u.,Mg = 0.216, Na = 0.352, Ti = 0.014; F = 0.005,Cl = 0.001; Fig. 3b; Table 1) included in Ky I in PrpI; (2) peak phengite (Phg I: Si = 3.548–3.580 a.p.f.u.,Mg = 0.603–0.642, Na = 0.013–0.027, F = 0.005– 0.036, Cl up to 0.001; Fig. 3b; Table 1), which formsthe core of coarse-grained, zoned mica-fishes in thematrix and, locally, is also included in Prp II; and (3)retrograde phengite (Phg II: Si = 3.495–3.540 a.p.f.u.,Mg = 0.568–0.624, Na = 0.008–0.025, F = 0.009– 0.037, Cl up to 0.001; Fig. 3b; Table 1), which occursin the matrix as the rim of Phg I mica fishes and asfiner-grained, retrograde neoblasts. Little P (up to0.003 a.p.f.u.), Zr (up to 0.004 a.p.f.u.) and Cr (up to0.002 a.p.f.u.) were locally measured in white mica(Table 1). (a) (b)(c) Fig. 2.  Simplified sketch showing the three types of pyroperecognized in the studied UHP whiteschists and their chemicalzoning: (a) megablast, (b) porphyroblast, (c) neoblast. CONTINENTAL CRUST METASOMATISM IN SUBDUCTION  5   2009 Blackwell Publishing Ltd
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