Petrological study of Taba migmatites, east central Sinai, Egypt

M.E.R.C. Ain ShamsUniv., Earth Sci.Ser.,Vol. 17,2003,p. 58-72.

Petrological studyof Taba migmatites,east centralSinai, Egypt

DorisSADEKGHABRIALGeological& GeophysicalDepartment,NationalResearchCentre,12622Dokki,Cairo,Egypt

ABSTRACT. A petrologicalstudywascarriedoutonmigmatitesofTaba.Sinai.toconstraintheP-T conditionsof metamorphismandpartialmelting.Mostof migmatitesexposedthereareofstromatictype,althoughdiatexitesare locallypresent.Theformer is typifiedby a layeredstructure,wherelayersof leucosomesare concordantto thehostrockfoliation.Othersareclearlydiscordantandimplymeltinjection.TheparentrocksofthesemigmatitesareregardedtobethesurroundingTabaschists.Leucosomesare trondhjemiticin composition(plagioclase+ quartzof:biotite).TheK-poorcompositionof thisproductcouldimplymeltingunderhighwateractivitycondition.Suchcondi-tioncausesa decreasein thetemperatureofplagioclase+ quartzsolidusandbeginstheirmeltingwithinthestabilityfieldofmuscovite.whichinthiscasedissolvescongruentlyin themelt(PatinoDouce& Harris 1998).Themigmatitemesosomescontainassemblageplagioclase+quartz+biotiteof:garnetof:raresillimanite.Melanosomeswithhigherbiotitecontentmayoccuradjacenttoleucosomelayers.Assemblageofgarnet+plagioclase+quartz+sillimaniteor biotiteaswellasgarnet+biotiteareusedtoprovideinformationfor pressureandtemperaturecalculations,respectively.Theseresultssuggestthatmigmatitesbeformedatmediumpressurecondition(about6 kbars)andattemperaturesnear700C.Assuminguncertainitiesofof:lkbar,thepressuremaybe in accordwithlow/mediumpressureconditions.

INTRODUCTION

Continentalcrustmetamorphosedunderupperamphiboliteandgranulitefaciesconditionstypicallyis migmatitic,characterizedby segregationof leucosomein adeformedmetamorphichost(Brown1997).Experimentalstudiesshowedthatmetape-liteandmetagraywackeyieldeda felsicmeltwithinthetemperaturerange650-900C(Vielzeuf& Holloway1988,Vielzeuf& Montel1994,PatinoDouce& Harris1998).Theproducedmelthaseithera granitic(alkalifeldspar-quartz)or a trondhjemitic(plagioclase-quartz)composition,implyingmeltingunderconditionsof low andhighwateractivity,respectively(PatinoDouce& Harris1998).

Migmatitesaremorphologicallycomplex.This complexityarisesbecauseastheamountof meltincreases,thereis a progressivechangein thephysicalnatureof thepartiallymoltenrockfromsolid-dominated(metatexite)tomelt-dominated(diatexite)rheology.Thedistributionandaccumulationof themelt(i.e.leucosome)arenotonlycontrolledbytheamountof meltsegregated,butalsobytherockfabricsandstructures(Brown1997,2001).

Migmatitesarewidespreadin manyPrecambrianShields,includingthemigmatitesofTabametamorphicbeltintheArabian-NubianShield.Thesemigmatiteswereshownto betheproductof metamorphicdifferentiationof originalTabametasedimentaryrocks(Eyal1980),whosesourceterrainwasdatedbyKroneretal.(1990)atc.800-820Ma.Theageof thehighestgrademetamorphismin Elatarea,whichoccursin theNEextensionoftheTababelt,fallswithintherange620::1:10Ma(Coscaetal.1999),i.e.theagerangeofthePan-Africanorogeny(Jackson& Ramsay1980).

PETROLOGY OF TABAMIGMATITES 59

Thepresentworkdealswiththemetamorphismandpartialmeltingof migmatites,whichoccurovera relativelylargearea(about4 km2)in theTababelt,eastcentralSinai.Petrographicalstudyandelectronmicroprobeanalyseswerecarriedoutonthesemigmatitesandtheiressentialmineralstoestimatethephysicalconditionsatthemeta-morphicpeak.

GEOLOGIC SETTING

TheTabametamorphicbeltis about25kmlongand2-10kmwide,andconsistspredominantlyof metamorphosedgreywackesandplutonicrocks(tonalitetogranitegneisses).The northernpartof theTababeltwasmetamorphosedin the lowerormiddleamphibolitefacies,butthecentrereachedtheupperamphibolitefacies,wheremigmatitesweredeveloped(Eyal 1980,Abu EI-Enenetal. 1999,Samuel& SadeKGhabrialinprep.).

Theoverallstructureof thebeltis synformalroughlytrendingE-W, wherethemigmatitesoccuralongitssouthernlimb.Eyal(1980)concludedthatthebeltsufferedfourdeformationalstages(DI-D4).Themainphaseof migmatizationoccurredpriortoD2, andcontinuedin D2. DuringD2, themigmatitesunderwentextensivefoldingformingisoclinalandptygmaticfolds,whichwererefoldedandaccompaniedbythedevelopmentsof newfoldsduringD3.RetrogrademetamorphismtookmainlyplacebetweenD3andD4.D4phaseincludesbrittledeformation,wherekinkfoldsappearedinmicaandchloriteanddeformationtexturesinquartz.

MigmatitesaretheoldestrockunitinWadiMinarish,envelopedfromall sidesbygranite.Theyaretectonicallyborderedto thenorthby garnetbiotiteandhornblendebiotitegneisses,whichbelongtotheoldergraniteof Egypt(AbuEI-Enenetal. 1999).To thesouthandwest,theyareintrudedbyyoungergranite(Fig. 1).Dykes,mainlyofacidictype,arealsocommoncrossingmostmigmatites.

MORPHOLOGY OF PRINCIPAL ROCK TYPES

Themetasedimentaryrocksin thestudiedareaaretransformedintomigmatites,althoughrelictlayersof metasedimentaryrocksmaypreservewithinthemigmatites.TheLattershowaconsiderablerangeinmorphologyandattheoutcropscaletheprin-cipalvariationfromoutcroptooutcropis intheproportionof leucosomes.Thefollow-ingprincipalrocktypescanbedistinguishedwithinthestudiedarea,althoughtheycommonlygradeintooneanother.

Metasedimentaryrocks.-Theyarenon-migmatizedrocksof sedimentaryorigin,occurringasrare,smallrelictswithinthemigmatitesin thewesternpartof thestudiedarea.Theyconsistmainlyofdarkgrey,micaschistof graywackeparentageandhaveastrongfoliationdefinedbythepreferredorientationofbiotite.

Metatexitemigmatites(stromatic).-Mostmigmatitesexposedin Taba belt areconsideredasmetatexitesin thebroadsenseof Mehnert(1968)andBrown(1973),wherethepre-migmatizationfabrics,suchaslayeringandfoliation,arestillvisibleandsurvivedpartialmelting.Thesemigmatitesaremainlyof stromatictype,typifiedbyalayeredstructure.Theleucosomelayersareorientedconsistentlyatoutcrop,concordanttothehostrockfoliation.Thisobservationsuggeststhattheyhavebeenformedinsitu.The leucosomesarefew millimetersto few centimetersthick,andmayhavethin

60 D. SADEK GHABRIAL

biotite-richselvages.Less commonly,theleucosomeforms-smallveinsorientedat highanglesto the stromaticstructureand linked to layer-parallelleucosomes,suggestingmelt-filled fractures.These fractures,which act as low-pressuresites,were activityfilled assoon as themeltwas generated(Brown & Rushmer1997).Locallyandwiththeincreasein theamountof melt,themigmatitesmayoccuras isolatedblockswithinirregularleucosome,resultingin raftmigmatitesthatrepresentthemetatexite/diatexitetransition.

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Diatexitemigmatites.-Diatexitesrepresentrocksinwhichthemeltfractionwaslarge.Theylocallyoccurin thenorthernpartof thestudiedareaasa smallconcordantbodywithinthestromaticmigmatites.In comparisonwiththelatter,theyaremorehomoge-neousanduniformattheoutcrop,wherethelayeredstructureof theparentalmetasedi-mentaryrocksandmetatexiteshasbeenlargelydestroyed.Moststudieddiatexitesarecharacterizedby discontinuous,weaklydefmedmicafoliation,and bothmesosomesandleucosomescannotbeseparated,typifiedbydiffuseboundaries.

Discordantleucosomes.-They are coarse-grainedand possessalmostpegmatitetexture,reachingup to 60 emin thickness,andoccasionallyobservedwithinthemetatexites.In handspecimen,somemembersof this leucosomesuiteappearto belayerparallel,butonthescaleof anoutcrop,theycanbeseentobediscordanttohostrockfoliation.In otherleucosomes,discordanceisclearlyapparent.Bothtypespossess.sharpcontactswiththehostrockandhaveneithermesosomesnormelanosomes.Thenatureof theseleucosmesimpliesmeltinjectionandtheylargelypost-datethestro-maticleucosomes.

PETROGRAPHY

Metasedimentaryrocks.-Therockscomprisetheassemblageplagioclase+quartz+biotite:i:garnetwith tracesof zirconandiron oxides.Muscoviteoccursas rare,randomlyorientedanhedralflakes(Fig.2),whilebiotitedefinesthefoliation.Plagio-claseoccursassub-idoblasticgrains,muchof themarealteredtowhitemica.Quartzvariesin sizefromfineroundedgrainstocoarsepatchesshowingunduloseextinction.Gametisalwayscoarse,xenoblasticandcontainsunorientedinclusionsofquartz.

Metatexitemigmatites.-Stromaticmigmatitesconsistof lenticlesor sheetsofleucosome,withorwithoutmelanosomewallsin mesosome.Themesosomeis schis-toselayered,hasroughlythesamemineralassemblageasthemetasedimentaryrocks,butis characterizedby minordepeletionin plagioclaseandquartz.Additionally,silli-maniteisrarelyobservedasscatteredneedlesinthematrix(Fig.3),whilemuscoviteiscompletelyabsent.Coarse,saussuritizedplagioclasegrainsarecommonlycorrodedandcracked,withquartz-filledcracks(Fig.4).Suchtextureis usedto inferthepresenceofameltin themigmatites(Blumenfeld& Bouchez1,988).Biotiteoccurscommonlyinthematrixandas inclusionsin garnetandplagioclase.Thematrixbiotiteexhibitsapreferredorientationandisrarelyalteredtochlorite.Garnetwasonlyobservedinsomesamplesas porphyroblastsand mediumgrainsdispersedin the matrix.Somephenocrystsarexenomorphicwithsievestructureandsimilarin appearanceto thatobservedin themetasedimentaryrocks,butenclosefewerquartzinclusions.Theydisplaycompositionalzoningandarereplacedalongtheircracksbychlorite.Themela-nosomeisawell-foliatedportionoftherockrichinbiotite,butpoorinthefelsicminer-als.Theleucosomeis composedofquartz,plagioclaseandminorbiotite,andistextur-allymorehomogeneousandcoarsergrainedthanthemesosomeormelanosome.Grainsizeincreaseswith increasingthethicknessof theleucosomes.Locallyit possessesawell-definedigneoustexturewherehypidiomorphicplagioclasedevelopscrystalfacesagainstquartz(Fig.5).Othercriteriaindicativeofamagmatictextureincludescuspateshapeofplagioclasegrains(Fig.6)(Marchildon& Brown2002).

,62 D. SADEK GHABRIAL

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Figures2-7.2, Unorientedmuscovitein metagraywake.C.N., approximatewidth of thephotois0.8mm.3, Sillimaniteneedlesin stromaticmigmatite.P.P.L., approximatewidth of thephotois0.3mm.4, Plagioclasegrainwith quartz-filledcracksin stromaticmigmatite.C.N., approximatewidth of the photo is 0.3 mm. 5, Plagioclase developscrystal faces againstquartz. C.N.,approximatewidth of thephotois 0.8 mm.6, Cuspateshapeof plagioclase.C.N., approximatewidthof thephotois 0.3mm.7, Leucosomedomainin theschlierenof diatexitecontainingsmallcrystalsof plagioclaseandquartz.C.N., approximatewidthof thephotois 0.8mm.

Diatexitemigmatites.-Diatexitesaremediumtocoarsegrainedrockswitha relativelyuniformtexture.Mainphasesin thesemigmatitesareplagioclase,quartzandbiotitewithchlorite,muscoviteandepidoteformedatexpenseof biotite.Thenotableminera-logicaldifferencebetweenthemetatexitesanddiatexitesistheabsenceofgamet,whichmaybe dueto thevariationin protolith composition.Discontinuousbiotite-rich


schlierenarepresentandweaklydefinethefoliation,whereasbiotiteof thematrixoutsidetheschlierenis generallysmallerin sizeandoccursas unorienteddiscreteflakesor in smallaggregates.The leucocraticdomainsin schlierennearlyresemblethoseformingtheframeworkof thehostdiatexites;therearealsosmallercrystalsofquartzandplagioclase(Fig.7),whichmayrepresentthecrystallizationproductsof melttrappedbetweenthebiotiteflakes(Milord& Sawyer2003).Locallydiatexitesshowevidenceof post-anatecticdeformation,suchas unduloseextinctionin quartzandbendingof biotiteflakes,butgenerallytheypreservetheirhypidiomorphicgranulartexture.

Discordantleucosomes.-Theyhavea trondhjemiticcompositionlargelyresemblingthatof thestromaticleucosomes.Theyconsistmainlyof plagioclaseandquartz,some-timeswithminorbiotite.Plagioclaseoccursassubhedralto euhedral,saussuritizedcrystals,reachingupto 1cmin diameter.Quartzis foundascrystalaggregateswithserratedor straightoutlines,andmaydisplayunduloseextinction.Onlyonesamplecontainsfewinterstitialorthoclasecommonlycorrodingplagioclase.

MINERAL COMPOSITION

ThedifferentmineralphasespresentinthemigmatiteswereanalysedwithAMRAY18101scanningelectronmicroprobeattheDepatmentof PetrologyandGeochemistry,EotvosUniversity,Budapest.RepresentativeanalysesarereportedinTables1-4.

Plagioclase.-Thecompositionof theplagioclasein theanalysedsamplesshowsnoremarkablevariation,whereit rangesfromAn28to An30and~ 2-4 mole% Or (Table 1).Notableplagioclasezoningis notobserved,andCaOconcentrationvarieswithin


Biotiteenclosedin thegarnetporphyroblastshasXMg,Alvl andTi contentssimilartomatrixbiotite.Garnet.-Itis almandinerichin thecompositionalrangeAlms8.7oSpSI3-29PYP7-13Grs4.9(Table3).Garnetis theonlymineralphasewithawell-developedzoningshowingapatterncharacterizedby largehomogeneouscoressurroundedbythinrims(Fig.8 andTable3).Thereis anoverallincreasein Fe/Fe+Mgtowardstherims,coupledwithanincreaseinMncontent,whileCavariesirregularly.

T

Chlorite.-ItisrepresentedbyripidolitewithAIIVcontentaveraging2.500(Table4).ItshowsawiderangeinMg/Mg+Feratios,varyingITom0.29to0.46.

METAMORPHIC CONDITIONS IN TABA MIGMATITES

PartialmeltingMost migmatiteswereformedduringthepartialmeltingof continentalcrust,

althoughmetamorphicdifferentiationis anotherpossibleformingmechanism(Robin1979).Theinterpretationof anatectic,aswellasnon-anatecticsubsolidusoriginsformigmatitesis commonlyrevealedby theirmacrostructureandmicrostructureandpermittedby estimatesof prevailingP-T conditions.The followingfeaturesin theleucosomesintheTabamigmatitesfavourformationbypartialmelting:1) Thediscordantnatureof someleucosomeveinsthatarenot relatedto intrudedgranitesrequiresproductionandsegregationofmelt(Kaltetal.1999).2)Variablethicknessof leucosomes,andtheiroccasionallargethickness(Robin1979).3) Therestiticcharacterof melanosomespointsatmeltextraction(Kalt etal. 1999,Sawyer1999).4) Leucosomespreservetexturesindicatingthepresenceof a melt(seesectiononpetrography).5)Semi-quantitativeestimatesofpeakconditions(seesectionongeobarothermometry)indicatethattemperaturewasenoughtoinducemelting.

iable2. Chemicalcompositions(wt%)andionic formulaof biotite * =biotiteinclusion).SampleNo. 4e 6dPoints I 2 3 4* 5 6 7 8Si02 35.73 36.23 35.56 35.89 35.98 35.71 34.45 35.63Ti02 3.81 3.54 3.74 3.58 4.13 3.38 3.42 3.58AhOJ 18.14 18.54 17.75 18.04 18.19 18.16 17.24 18.03FeO 21.43 21.50 22.09 21.78 21.79 21.93 22.12 21.71MnO 0.21 0.29 0.22 0.35 0.32 0.30 0.31 0.34MI!O 7.49 7.76 7.48 7.86 7.89 7.93 7.15 7.80K20 9.07 8.96 8.85 8.96 8.98 8.86 8.74 8.89Total . 95.88 96.82 95.69 96.46 97.28 96.27 93.43 95.98

.. - .. --- .-.Numberofcationsonthebasisof22(0

Si 5.447 5.457 5.449 5.445 5.410 5.430 5.432 5.434AI" 2.553 2.543 2.551 2.555 2.590 2.570 2.568 2.566AI" 0.706 0.748 0.654 0.671 0.634 0.685 0.636 0.674Ti 0.437 0.401 0.431 0.408 0.467 0.387 0.405 0.410Fe'. 2.732 2.708 2.831 2.764 2.740 2.789 2.917 2.769Mn 0.027 0.037 0.029 0.045 0.041 0.039 0.041 0.044MI! I.702 I.742 I.708 1.778 I.768 I.797 1.681 I.773K 1.764 1.721 1.730 I.734 1.722 1.719 I.758 1.729Total 15.368 15.357 15.382 15.400 15.372 15.415 15.439 15.400

XM. 0.38 0.39 0.38 0.39 0.39 0.39 0.37 0.39


T

II-c, 12,13& 14-r=zonedgarnetgrainfromcoretorim.

PossiblemeltingreactionsandpoTconditionsThemigmatitezonerecordsthehighestmetamorphicconditionsin theTababelt.

This zoneis characterizedby theoccurrenceof leucosomesegregations,whicharethoughttohavebeendrivedmostlybypartialmeltingof Tabametasedimentaryrocks.Somemeltingreactionsarepossibleforthedevelopmentof thesemigmatites.Musco-viteis virtuallyabsent,butbiotiteis stillpresentandshowsnosignof destablization.Hencemeltingof theTabarocksoccurredlargelywithinthestabilityfieldof biotite,

----

- -. - n n- -- - ---- n---_n- __n- -_m- n----_'n- -- _--_n- - un, - - un .

Sample 4e 6dNo.points I 2 3 4 5 6-c 7-r 8 9Si02 36.71 37.29 36.98 36.49 36.50 37.16 37.44 36.66 36.54Ah03 20.55 20.80 2050 20.30 20.63 21.08 20.67 20.49 20.22FeO 29.07 28.99 29.35 30.87 30.97 28.86 27.41 32.31 31.62MnO 7.53 7.56 7.62 6.68 6.70 7.78 11.00 5.93 5.91MI!O 2.63 2.80 2.92 2.99 3.14 2.83 1.79 3.06 2.99CaO 3.00 2.92 254 1.83 1.51 3.02 2.23 1.75 1.70Total 99.49 100.36 99.91 99.16 99.45 100.73 10054 100.20 98.98

Numberof cationsontbebasisof 12(0)Si 2.973 2.991 2.982 2.969 2.959 2.967 3.021 2.954 2.980Al 1.962 1.966 1.948 1.947 1.971 1.984 1.966 1.946 1.943Fe" 1.969 1.944 1.979 2.101 2.100 1.927 1.850 2.177 2.157Mn 0517 0513 0520 0.460 0.460 0.526 0.752 0.405 0.408M2 0.318 0.335 0.351 0.363 0.379 0.337 0.215 0.367 0.363Ca 0.260 0.251 0.219 0.160 0.131 0.258 0.193 0.151 0.149Total 7.999 8.000 7.999 8.000 8.000 7.999 7.997 8.000 8.000

Aim 64.2 63.9 64.6 68.1 68.4 63.1 61.5 70.2 70.1Spes 16.9 16.9 16.9 14.9 15.0 17.3 25.0 13.1 13.3Pvr 10.4 11.0 11.4 11.8 12.3 ILl 7.1 11.8 11.8Gros 85 8.2 7.1 5.2 4.3 8.5 6.4 4.9 4.8

Sample 6dNo.points 10 ll-c 12 13 14-r 15-c 16-r 17-c 18-rSi02 36.41 36.60 36.69 36.22 37.24 3754 37.42 36.46 37.07Ah03 20.33 20.07 20.10 20.33 20.60 20.10 20.73 20.26 20.15FeO 30.55 29.08 29.13 28.70 27.02 29.90 25.29 32.24 28.17MnO 7.82 8.74 8.70 9.25 11.83 8.46 12.73 5.81 10.32MI!O 2.82 252 2.57 2.42 1.77 2.56 1.79 3.31 2.20CaO 2.27 1.99 2.04 2.12 1.79 1.65 1.91 1.80 1.90Total 100.20 99.00 99.23 99.04 100.25 99.21 99.87 99.88 99.81

Numberofcationsontbebasisof 12(0)Si 2.937 2.992 2.991 2.960 3.018 2.983 3.031 2.944 3.013Al 1.933 1.934 1.931 1.958 1.968 1.934 1.979 1.928 1.930Fez' 2.061 1.988 1.986 1.961 1.831 2.042 1.713 2.177 1.915Mn 0.534 0.605 0.601 0.640 0.812 0585 0.873 0.397 0.710M2 0.339 0.307 0.312 0.295 0.214 0.312 0.216 0.398 0.266Ca 0.196 0.174 0.178 0.186 0.155 0.144 0.166 0.156 0.165Total 8.000 8.000 7.999 8.000 7.998 8.000 7.978 8.00 7.999

Aim 65.8 64.6 64.6 63.6 60.8 66.2 57.7 69.6 62.7Spes 17.1 19.7 19.5 20.8 27.0 19.0 29.4 12.7 23.2Pvr 10.8 10.0 10.1 9.6 7.1 10.1 7.3 12.7 8.7Gros 6.3 5.7 5.8 6.0 5.1 4.7 5.6 5.0 5.4


i.e.attemperatures


Reaction(1) is widelyconsideredappropriatefortheproductionof a graniticmeltthatis differentincompositionfromTabamigmatiteleucosomes.Suchcompositionaswellastheabsenceof K-feldsparandretrogrademuscovite,whichmayappearin theconsumptionofK-feldspar(Spearetal.1999)suggestthatthisreactiondidn'toccur.

It is clearthattheH2O-fluxedmeltingreaction(2) contrastssharplywith thedehydrationmeltingreaction(1).In thecaseof H2Ofluxing,themeltsproducedwithsmallamountsof addedH2Omaybeof trondhjemiticcomposition,whichis virtuallysimilartoTabaleucosomes.Thisresultedfromthedecreasein thetemperatureof theplagioclase+quartzsoliduswithincreasingH2Oactivity,whichstartsmeltingwithinthestabilityfieldof muscovite.Therefore,theamountof meltgeneratedis generallylarge,limitedbyprotolithplagioclasecontent,however,plagioclaseis consumedin agreaterproportionthanmuscovite,whichinthiscasedissolvescongruentlyinthemeltwhenpresent.In contrast,dehydrationmeltingconsumesmuscovitein greaterpropor-tionthanplagioclasebecauseH2Oissuppliedbyincongruentbreakdownof muscovite.To producetrondhjemitesthroughmelting,aplagioclase-richandmuscovite-poorrockis needed.Hencemetapsammitesandmetagraywackesrepresenta morefertilesourcethanmetapelites.It canbeindeedobservedthatmostTabametasedimentaryrocksaremetagraywackesanddonotcontainmuscovitein excess,andthereforereaction(2) isproposedastheprincipalmelt-formingreactionintheTabamigmatites.

Anotherpossiblemeltingreactionformigmatitesprovidedthatawater-richfluidisavailableis

Ms+Ab+Qtz+H2O=Sil +melt(3)Thisreactionproducessillimanite,whichisrarelyrecognizedin fewsamples,most

likelyreflectingthatreaction(3) wasof minorimportancein thepartialmeltingoftheserocks.Suchreactionrequiresatemperatureabove650Cat4-6kbars(Spearetal.1999)in theNaKFMASH modelsystem.Thepresenceof oligoclasein thestudiedsamplesis likelyto raiseslightlytheminimumtemperatureforthisreactionin relationtotheNaKFMASHgridofSpearetal.(1999).

Thecommonoccurrenceof chlorite,replacingbiotiteandgarnetin Tabamigma-tites,isatypicalfeatureof retrogradealterationacrossthetransitionfromamphibolite-togreenschistconditions,attemperatures


temperaturevaluesin therangeof 633-654C,whereasgarnetcore-biotitepairsyieldvaluesin therangeof 696-719C.Theseestimatedtemperaturesareobviouslyclosetothoseobtainedwiththecalibrationof Indares& Martignole(1985)withmaximumdifferencesof 45C,whereasthecalibrationsof Ferry& Spear(1978)andLoomis&Nimick(1982),correctedfornonidealmixingof garnetaccordingtoHodges& Spear(1982),alwaysgivehighertemperatureswithdeviationsof upto 95C.Ti-in-biotitethermometerof Henry& Guidotti(2002)generallygivestemperaturesof about700C,whichis in agreementwiththegarnet-biotitetemperaturesaccordingto Holdaway's(2000)geothermometer.

Theresultsof geothermometryindicatethatthetemperaturesobtainedfromgarnetrimsarelowerthanthoseobtainedfromgarnetcores.Theseinconsistenciesaremostlikelytheresultof latestagere-equilibrationof thephases.Bucher& Frey(1994)mentionedthatanomalouslylowgarnet-biotitetemperatureswereobtainedfromgarnetrim/matrixbiotitein theupperamphiboliteandgranulitefaciesrocks.This effectisattributedto retrogradeFe-Mg exchangeduringcoolingthatcausedconsiderablechangesin garnetrim composition,whilethematrixbiotitecompositionremainedalmostunchanged.Thereforereasonabletemperatureestimatescanbeobtainedbyusinggarnetcoreandmatrixbiotite,nearlyreflectingpeakmetamorphicconditions(Bucher& Frey1994).

Pressureestimatesfor thestudiedmigmatitescanbemadefromthegarnet-silli-manite-plagioclase-quartzgeobarometerof Holdaway(2001).Theseestimatesgivepressure.of 5.6-6.1kbars(at650C)forgarnetrimsandof6.1-9.2kbars(at700C)forcores,assumingsillimaniteasthecoexistingAI-silicatemineral.Additionalpressureestimateswereobtainedby usingthegarnet-plagioclase-biotite-quartzgeobarometercalibratedbyHoisch(1990),whichyieldspressuresin therangeof 6.1-6.9kbars(at650C)andof 6.4-8.9kbars(at700C)forgarnetrimsandcores,respectively,consis-tentwiththatof Holdaway's(2001)calibration.It isobservedthatthepressureobtainedbygarnetcoresinsample4eishigherthanexpected,becausethelackof kyaniteindi-catesmaximumpressuresof about8kbars(Pattison1992).Theunreasonablepressureis likelycausedby disequilibriumbetweengarnetcoresandplagioclase.Generally,averagepressuresbetween6.1and6.4kbarswererecordedforthestudiedmigmatites.Assuminguncertainitiesof :1:1kbar,thepressuremaybein accordwith low/mediumpressureconditions.

- ---. .. - .,n .- --_u - -- ,-..-- - n- -______n_n. --- no-.------ nn ----,..-..SampleNo. 4e 6d

Position Gamet-core Gamet-rim Gamet-core Gamet-rim

Garnet-biotitethermometer

Ferry& Spear(1978) 796 616 760 653

Loomis& Nimick(1982) 814 656 784 690

Indares& Martignole(1985) 765 628 728 684

Holdaway(2000) 719 633 696 654

Garnet-AI-silicate-quartz- plagioclasebarometer

Holdaway(2001) 9.15 6.08 6.1 5.56

Garnet- plagioclase- biotite- quartzbarometer

Hoisch(1990) 8.92 6.87 6.4 6.05

. _!I-


Themigmatiteswerein partaffectedby minorretrogradealteration.Formationofchloriteattheexpenseofbiotiteandsaussuritizationofplagioclaseoccurredlaterundergreenschistfacies conditions.However,the chlorite-Allv geothermometerofCathelineau(1988)yieldstemperaturesbetween337and347C.

DISCUSSION AND CONCLUSION

Stromatictypeprevailsamongmigmatitesexposedin Tababelt,but diatexiteslocallyoccur.Therearetwogenerationsof leucosomes.Onesetis paralleltotherockfoliationandcrystallizedin situ.Theotheris discordantandimpliesmeltinjection,largelypost-datingthestromaticleucosomes.Leucosomesareof trondhjemiticcompo-sitionsuggestingmeltingunderhighwateractivitycondition.

Unlike the othermigmatiticterrains,theTaba migmatitesdo not havelow-metamorphicgrademargins.Theyuncommonlycontainrelictsof unmigmatitizedmetasedimentaryrocks,andareseparatedfromtheTabaschistsin thenorthbyvarioustypesof graniticgneisses.Themainmineralogicalcomponentsof themigmatitesandtheassociatedmetasedimentaryrelictsareplagioclase,quartz,biotiteandgarnet,obviouslysimilarto thoseof the Taba schistsat Wadi Morakh.Moreover,themel,},edimentaryrelicts,fromwhichthemigmatiteswereformed,arechemically

. indl~lmguishablefromtheTabaschists(Table6),althoughtheformerhaveslightlyhigherTi02andFe203t,consistentwiththeabundanceof biotite(Le.theycontainmorepeliticmatrix).Thus,theprotolithof themigmatitesmightbegraywackesaffiliatedtothenorthernTabaschists,in agreementwith theformeropinionof Eyal (1980),however,theseschistswereprovedto bethelowermemberof Hammamatclasticsediments(Samuel& SadekGhabrialinprep).

Table6.Averagemajorelementdataofthemetasedimentaryrelicts(MSR)withinstudiedmigmatites.MST=averagecompositionofWadiMorakhschistsfromSamuel& SadekGhabrial

Generally,metamorphicgradein theTababeltincreasestowardsthesouth,andmigmatitesarecharacteristicof thecentralpart.At WadiTwiebato thenorth,stauro-lite-cordieriteschistsareobservedinterlayeredwiththemorecommonbiotite-garnetschists(Samuel& SadekGhabrialin prep.).In thisparticularareaAbu El-Enenetal.(2003)reportedandalusiteandsillimanitenearthewadientrance.Geothermobarometryappliedto theseschistsyieldeda temperaturerangeof 550-580Cat about3 kbars.SouthwardsatWadiMorakh,biotitegarnetschistsis onlyfoundandgavesomewhathighertemperaturesof about600Catabout4 kbars(AbuEl-Enenetal.2003).Farthersouth,therockswereremobilizedandtransformedintomigmatitesathighertempera-tures(-700C)andtransitionaltothemedium-PITmetamorphictype,representingadeepercrustallevel.

Theoccurrenceof hightemperaturemineralsandmigmatitescommonlyassociatedwithvariousgraniticgneissesin theTababeltis somewhatsimilarto thosedescribedfromtheHammamatsedimentsatWadiKid area,southeastSinai(Reymeretal. 1984),butthe lattergavelowerpressureestimatesof 3.2 kbarsat Wadi Kid andtheir

.

in prep., BTG= averagecompositionof garnetbiotitegneissfromAbu EI-Enen etal. (1999).Rocks SiOz TiOz AhO. FezO.t MoO MgO CaO NazO KzO LO.I.

MSR 64.2 1.02 16.2 6.9 0.1 2 2.9 3.8 2.7 1.5

MST 64.8 0.75 16.5 5 0.09 2.2 2.8 4.1 2.4 1.2

BTG 65 0.91 15.8 6.5 0.1 1.9 2.8 3.7 2.8 0.7

- -:-

~ "-


metamorphismwasdescribedas low-pressure"regionalmetamorphism".The low-pressuremetamorphismaffectedtheHammamatsedimentsatUrnHadarea,EasternDesert(El-Gabyetal. 1988,El Kalioubi1988)is shownto becomparableto thatofWadiKid area.In bothoccurrences,themetamorphosedrocksgradefromstaurolitezonethroughananatecticzoneintogranitegneiss.Thegranitegneissis consideredaspartof a concealedgraniticdiapir,whichresultedin thedevelopmentof thehightemperaturemetamorphismandis intrusiveintothesurroundinglower-gradeschists(Reymeret al. 1984).ThestudiedTabarocksappearto haveformedby thesamemechanism,namelyinthermaldomesassociatedwithgranitoiddiapir,butatsomewhathigherpressure.

Thegarnetbiotitegneiss(thetonaliticFjordgneissof Eyal 1980)borderedthemigmatitesto thenorthis similarin compositionandchemistryto migmatites(Table6),butthereis nogradationbetweenthemtojustifYtheircommonorigin.However,theborderbetweengneissandmigmatitesis structurallycontrolled.ApplyingNi-Zr/TiOzandMgO/CaO-PzOs/TiOzdiscriminantdiagrams(not shownhere)introducedbyWinchesteretal.(1980)andWerner(1986),respectively,todistinguishbetweenortho-andpara-quartzofeldspathicrocks,mostFjordgneisssamplesanalyzedby Abu El-Enenetal. (1999)plotonorcloseto theboundarybetweenthefieldsof igneousandsedimentaryrocks.Basedonfieldappearance,in someplacessedimentaryoriginfortheFjord gneissis suggestedby theabsenceof obviousintrusivefeaturesandthepresenceof concordantintercalationsof schist.In otherplacestowardsthenorth,theFjordgneissintrudestheschistwithsharpcontactandschistxenolithsarefoundwithinit, indicatingigneousorigin.The ambiguousresultsof geochemicalandoutcropcharacteristicssuggestthatpartof Fjordgneissresultedfromrecrystallizationof theschistthatinteractedwiththerisingmagmaticdiapir,whichintroducedtheprincipalheatforanatexisandintrudedintotheschistinthenorth.

ACKNOWLEDGEMETS

Theauthorwishto thankProf.Dr. SamirEl-Gaby,GeologyDepartment,AssiutUniversity,Egypt,for hishelpin revisionof themanuscript,andProf.Dr. Mike J.Holdaway,SouthernMethodistUniversity,U.S.A., for his helpin estimationof P-TconditionsusingGASPgeobarometerandGB geothermometer.

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Petrological study of Taba migmatites, east central Sinai, Egypt

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