Comment on Bybee et al. (2014): Pyroxene megacrysts in Proterozoic anorthosites: Implications for tectonic setting, magma source and magmatic processes at the Moho.

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Comment

on

Bybee

et

al.

(2014):

Pyroxene

megacrysts

in

Proterozoic

anorthosites:

Implications

for

tectonic

setting,

magma

source

and

magmatic

processes

at

the

Moho

Jacqueline

Vander

Auwera

a

,

∗

_,

_Bernard

_Charlier

a

_,

_Jean

_Clair

_Duchesne

a

_,

_Bernard

_Bingen

b

_,

John

Longhi

c

_,

_Olivier

_Bolle

a

a_{DepartmentofGeology,UniversityofLiège,B-4000Liège,Belgium} b_{NorwegianGeologicalSurvey,N-7040Trondheim,Norway} c_{Lamont-DohertyEarthObservatory,Palisades,NY10964,USA}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received1April2014 Accepted4June2014 Availableonlinexxxx Editor: T.M.Harrison

In their recent paper, Bybee et al. (2014) present new iso-topic and geochronological data on High-Alumina Orthopyrox-ene Megacryts (HAOM) sampled in three different Proterozoic massif-type anorthosites, namely, the Mealy Mountains Intrusive Suite (MMIS), the Nain Plutonic Suite (NPS) and the Rogaland AnorthositeProvince(RAP).TheSm–Ndisochronagesobtainedon HAOMfromthesethreeprovincesindicatethatthemost alumina-richsamples,i.e.thosethatcrystallizedatthehighestpressure,are 80–120 Maolder than their host anorthosites. Moreover, Sm–Nd isochrons between plagioclase lamellae, host orthopyroxene and whole-rockfromtheNPSandRAPhavedecompressedatagesthat correspondto the crystallization agesof the hostanorthosites at mid crustal level, as recorded by zircon U–Pb data (i.e. Emslie, 1990; Myers et al., 2008; Schärer et al., 1996). There is thus a 70–100 Ma gapbetween the crystallization of the HAOM at the baseofthecrust, onthe onehand,andtheir decompression and anorthosite crystallizationin the rising diapirs (or crystalmush), on the other hand. The authors interpret thesegeochronological data by proposing that HAOM crystallized from mantle derived magmaspondedatthebaseofthecrustsome100 Mabeforethe crystallization and emplacement of the anorthosite andthat the 100 Matimeintervalwasspentpartlyinaconvectingand recharg-inglowercrustal magmachamberandpartlyintheascentofthe anorthosite.AsthediapiricriseoftheRAPanorthositeswas mod-elledtolastforabout2.5 Ma(Barnichonetal.,1999),mostofthe

DOIoforiginalarticles:http://dx.doi.org/10.1016/j.epsl.2013.12.015, http://dx.doi.org/10.1016/j.epsl.2014.06.032.

*

Correspondingauthor.

100 Ma timespanisleft inthiscaseforthelifetimeofthelower crustalmagmachamber.

These new geochronological data bring very important con-straints on the petrogenesis of Proterozoic massif anorthosites (2.12–0.92 Ga: Hamilton et al., 1998; Ryan et al., 1999; Vander Auwera etal., 2011).Weagreewiththeauthorsthatseverallines of evidence support that their isochronsrecord crystallization of the HAOM. These evidences include the consistent ca. 100 Ma timespanbetweenHAOMcrystallizationandanorthosite emplace-ment obtainedinthreedifferentanorthositic provinces.However, we think that there is an alternative explanationto this 100Ma timespanthat hasnotbeenexplored bytheauthors.Wewill dis-cuss this alternative below with a focus on the RAP for which extensive datahavebeen presented onthe regionalgeology, iso-topicmeasurementsfortheanorthosite massifandthe surround-ing rocks, completed by extensive experimental study for phase equilibria.

There isan ongoing debateaboutthe mantleorlower crustal source of the anorthosite parent magma. Experimental data ac-quired on a high-Al basalt (HLCA), parent magma of Proterozoic anorthosites provide a strongconstraint asthey indicate that, at the lower crustal pressures (10–13 kbar) necessary to produce the characteristic HAOM, this high-Al basalt lies on a thermal divide of the plagioclase

+

pyroxene liquidus surface requiring that it was produced by partial melting ofa gabbronoritic lower crustalsource(Duchesneetal.,1999; Longhietal.,1999).Vander Auwera et al. (2011) identified several possible precursor mafic magmatic events in the Sveconorwegian orogen that could po-tentially have provided the lower crustal source of the younger RAP (0.93 Ga): the 1.05 Ga Feda suite mafic facies, the volcanic http://dx.doi.org/10.1016/j.epsl.2014.06.031

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2 J. Vander Auwera et al. / Earth and Planetary Science Letters_•••(_••••)_•••–_•••

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Fig. 1. A.(206_Pb/204_{Pb)ofRAPHAOMversustime(Ma).B.(}207_Pb/204_{Pb)ofRAPHAOMversustime(Ma)samesymbolsasinA.DatafromBingenetal. (1993),Bybeeetal.} (2014)andWeis (1986).

sequencesofGjuve–Morgedal (1.16 Ga),Valldal(1.26 Ga) and Ve-mork (1.50 Ga). Vander Auwera et al. (2011) favoured the Feda suite because the Nd and Pb isotopic composition of the mafic facies of this suite is very close to the isotopic composition of the high-Al gabbros of the RAP. The 1.04 Ga age obtained by Bybee etal. (2014) forthecrystallizationofthe RAPHAOM with thehighestAl2O3 content(>8 wt.%)corresponds within errorto the age of the Feda suite of southern Norway (1.05 Ga: Bingen and van Breemen, 1998). Moreover, the Pb isotopic composition of the RAP HAOM recalculated back at 1.05 Ga (supplementary data of Bybee et al., 2014) (206_Pb/204_Pb1

.05 Ga

=

14.91–17.88; 207_Pb/204_Pb1

.05 Ga

=

15.32–15.53) perfectly overlaps the initial Pb isotopic composition of the Feda suite (206_Pb/204_Pb1

.05 Ga

=

16.92–17.32; 207_Pb/204_Pb1

.05 Ga

=

15.43–15.50) (Bingen et al., 1993; Vander Auwera et al., 2011) (Fig. 1). This is also clear when average compositions are compared: Feda (30 samples) (206_Pb/204_Pb1

.05 Ga

=

17.06; 207Pb/204Pb1.05 Ga

=

15.46), HAOM (13 samples) (206_Pb/204_Pb1

.05 Ga

=

17.03; 207Pb/204Pb1.05 Ga

=

15.47).Therangesof

ε

Nd1.05 GadisplayedbytheRAPHAOM(

−

0.9 to

+

3.4) (Bybee etal., 2014) and the Fedasuite (

−

0.7 to

+

3.8) (Bingenetal.,1993; VanderAuwera etal.,2011) perfectlyoverlap too.TheRAPHAOMwiththehighestAl2O3content(>8 wt.%)thus crystallizedatthesametimeastheFedasuiteandfromamagma thathadthesameisotopiccompositionthantheFedasuite. More-over,availableisotopicdataonthehigh-Algabbros(3 samples)of theRAP(VanderAuwera etal.,2011; Weis,1986) recalculatedback at 0.93 Ga display an isotopic composition (206_Pb/204_Pb0

.93 Ga

=

17.28–17.37; 207_Pb/204_Pb0

.93 Ga

=

15.48–15.49) that is in the range of the Pb isotopic composition of the RAP HAOM also calculated back at 0.93 Ga (206_Pb/204_Pb0

.93 Ga

=

15.39–18.21; 207_Pb/204_Pb0

.93 Ga

=

15.37–15.57)(Bybee etal.,2014) (Fig. 1).At 0.93Ga, theRAP HAOMwere thus inchemical, asshown bythe experimental data (Fram and Longhi, 1992; Longhi et al., 1999), andisotopicequilibriumwiththehigh-Albasalts(Fig. 1).

The new isotopic data of Bybee et al. (2014) combined with the experimental constraints (Fram and Longhi, 1992; Longhi et al.,1999) thussupportatwo-stepmodelfortheoriginoftheRAP Proterozoicanorthosites.Duringthefirststep,theHAOM(>8 wt.% Al2O3)crystallizedat1.05 Gaatthebaseofathickenedcrustmost probablyfromthe parentmagmaoftheFedasuite.120 Malater, during a second step, the lower crustal gabbronoritic cumulates including the HAOM were partially meltedto produce the high-Al basalts. According to this model, the HAOM could be restitic mineralsofthe lowercrustal source.Thiswouldsuggest thatthe HAOM displaying the highest Al2O3 content (and also possibly themega-plagioclasethatlocallydisplayophitictextureswiththe

HAOM)observedandsampledintheRAParerepresentative frag-ments of thesource, unmelted eitherbecause they were crystals too large (up to 1 m) to melt or they were originally incorpo-ratedinmonomineraliclayers.TheFedasuitehasbeeninterpreted as resulting from the crystallization of a mantle-derived magma ofcalc-alkaline affinitymixedwithcrustalmeltsproducedduring thelowercrustalunderplating ofthebasalts(Bingenetal.,1993). Experimental data (Müntener etal., 2001) and field observations (Hermannetal.,2001) indicatethatthistypeofmagmaproduces significant amountsofgabbronoritic cumulateswhencrystallizing atthebaseofthecrust. Asnotedby Bybeeetal. (2014),the ma-jor element composition ofthe HAOM isgenerallysimilar to the composition ofthe experimentally obtainedorthopyroxeneswith, however, significant differences in their Mg#s and FeO concen-trations, Mg#s (0.49–0.66: Bybee et al., 2014) and FeO concen-trations (11.65–19.6 wt.%: Bybee et al., 2014) of the HAOM are respectivelylowerandhigherthantheMg#s(0.88–0.77)andFeO (7.7–13.7 wt.%)concentrationsoftheexperimentalorthopyroxenes (Münteneretal.,2001).Thisdifferencecannotbeexplainedbythe early crystallizationofolivine asthismineralisnotablyabsent in theexperimental highpressurecumulates(Münteneretal.,2001) due to theshrinking of the olivine stability field withincreasing pressure.InagreementwithwhatwasinitiallyproposedbyLonghi etal. (1999),wesuggestthatHAOMwerepartofthemoreevolved cumulates,containingmoreferroanpyroxenestogetherwithFe–Ti oxidesandphosphate,thatwerepreferentiallyfounderedor down-thrustedbecauseoftheirhigherdensity.

AspreviouslymentionedbyLonghietal. (1999)andrecalledby Bybeeetal. (2014),theabovemodelrequiresextensivemeltingof the lower crustat hightemperature. The recentwork ofDrüppel et al. (2013) on thepetrology of sapphirine-bearing gneisses oc-curring just north of the RAP supports this interpretation. In-deed, zircon U–Pb geochronology, phase modelling and geother-mobarometryindicate thatthesapphirinegranulitesrecord ultra-hightemperaturemetamorphicconditionsofabout1000◦_Candca. 0.75 GPaat1.01 Ga,i.e.afterintrusionoftheFedasuiteandbefore emplacementoftheRAP,followedbynearisothermal decompres-sionat0.55 GPa. Theseresultsthusindicatethatatthattime,the temperatureinthelowercrustwaswellabovec.1000◦_C. Consid-ering a lower crustal pressure of 1.25 GPa (Charlier et al., 2010; Longhi et al., 1999) and a conservative geothermal gradient of 20◦_{C/km, the temperature at the base of the crust can be} es-timated at 1390◦_{C. Clearly, improved geochronology of regional} metamorphism is requiredto link pressure–temperaturepaths in thecrusttomagmaticeventsandpossibleheatsources.

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Inconclusion, we think that the high-Al basalts,parent mag-masof massif-type anorthosites are product of meltingof a dry lower crustal protolith, in accordance with available experimen-tal data. The data of Bybee et al. (2014) can be interpreted to supportcrystallizationofa gabbronoriticlower crust followedca. 100 Malaterbyremeltingandproductionoftheanorthosite par-entmagma.Intermsofsecularevolutionoftheearth,itmaymean thattheProterozoicwasa timeweretemperatureandconvection ofthemantlewas favourableforthe remeltingoffreshly crystal-lizedcrustalunderplates.

EditorialAcknowledgement

TheEditor thanksthe reviewer,Carol Frost,forher thoughtful andbalancedreviewsofbothCommentaryandReply.

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