Earth and Planetary Science Letters 389 (2014) 74–85

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Earth and Planetary Science Letters

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Pyroxene megacrysts in Proterozoic anorthosites: Implications fortectonic setting, magma source and magmatic processes at the Moho

G.M. Bybee a,b,∗, L.D. Ashwal a, S.B. Shirey b, M. Horan b, T. Mock b, T.B. Andersen c

a School of Geosciences, University of the Witwatersrand, Private Bag 3, Wits, 2050, South Africab Department of Terrestrial Magnetism, Carnegie Institute for Science, 5142 Broad Branch Road NW, Washington, DC 20015, USAc Center of Earth Evolution and Dynamics (CEED), University of Oslo, P.O. Box 1047, Blindern, 0316, Oslo, Norway

a r t i c l e i n f o a b s t r a c t

Article history:Received 1 September 2013Received in revised form 6 December 2013Accepted 11 December 2013Available online xxxxEditor: T.M. Harrison

Keywords:Proterozoic anorthositesmegacrystcrustal differentiationmagma pondingmagmatic processes

Proterozoic anorthosites from the 1630–1650 Ma Mealy Mountains Intrusive Suite (Grenville Province,Canada), the 1289–1363 Ma Nain Plutonic Suite (Nain–Churchill Provinces, Canada) and the 920–949 MaRogaland Anorthosite Province (Sveconorwegian Province, Norway), all entrain comagmatic, cumulate,high-alumina orthopyroxene megacrysts (HAOMs). The orthopyroxene megacrysts range in size from 0.2to 1 m and all contain exsolution lamellae of plagioclase that indicate the incorporation of an excess Ca–Al component inherited from the host magma at pressures in excess of 10 kbar at or near Moho depths(>30–40 km). Suites of HAOMs from each intrusion display a large range in 147Sm/144Nd (0.10 to 0.34)making them amenable for precise age dating with the Sm–Nd system. Sm–Nd isochrons for HAOMsgive ages of 1765 ± 12 Ma (Mealy Mountains), 1041 ± 17 Ma (Rogaland) and 1444 ± 100 Ma (Nain), allof them older by about 80 to 120 m.y. than the respective 1630–1650, 920–949 and 1289–1363 Macrystallization ages of their host anorthosites. Internal mineral Sm–Nd isochrons between plagioclaseexsolution lamellae and the orthopyroxene host for HAOMs from the Rogaland and Nain complexes yieldages of 968 ± 43 and 1347 ± 6 Ma, respectively – identical within error to the ages of the anorthositesthemselves. This age concordance establishes that decompression exsolution in the HAOM was coincidentwith magmatic emplacement of the anorthosites, ∼100 m.y. after HAOMs crystallization at the Moho.Correspondence of Pb isotope ages (206Pb/204Pb vs. 207Pb/204Pb) with Sm–Nd ages and other stronglines of evidence indicate that the older megacryst ages represent true crystallization ages and not theeffects of time-integrated mixing processes in the magmas. Nd isotopic evolution curves, AFC/mixingcalculations and the age relations between the HOAMs and their anorthosite hosts show that the HAOMsare much less contaminated with crustal components and are an older part of the same magmatic systemfrom which the anorthosites are derived. Modeling of these anorthositic magmas with MELTS indicatesthat their ultramafic cumulates would have sunk in the magma and been sequestered at the Moho,where they may have sunk deeper into the mantle resulting in large-scale compositional differentiation.The HAOMs thus represent a rare example of part of a cumulate assemblage that was carried to theupper crust during anorthosite emplacement and, together with the anorthosites, illustrate the dramaticinfluence that magma ponding and differentiation at the Moho has on residual magmas traveling towardsthe surface. The new geochronologic and isotopic data indicate that the magmas were derived by meltingof the mantle, forming magmatic systems that could have been long-lived (e.g. 80–100 m.y.). A geologicsetting that would fit these temporal constraints is a long-lived Andean-type margin.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Near monomineralic plagioclase-rich intrusions, up to 18 000km2 in areal extent, known as Proterozoic massif-type anorthosites,have been studied for decades, yet many questions remain regard-

* Corresponding author at: School of Geosciences, University of the Witwater-srand, Private Bag 3, Wits, 2050, South Africa. Tel.: +27 11 717 6633.

E-mail address: grant.bybee@wits.ac.za (G.M. Bybee).

ing their tectonic setting, magma source and temporal restriction(1800–1100 Ma; Ashwal, 1993; Morse, 1982). These batholithswere emplaced at upper crustal levels as crystal-laden mag-mas carrying with them giant (up to 1 m in length), cogenetic,high pressure (10–15 kbar), high-Al orthopyroxene megacrysts(HAOMs; 3–9 wt% Al2O3; Fig. 1; Emslie, 1975; Emslie et al.,1994). Proterozoic anorthosites are known to have crystallizedfrom high-Al basaltic magmas and are so plagioclase-rich thatthey are widely believed to be missing 30–40% mafic minerals

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G.M. Bybee et al. / Earth and Planetary Science Letters 389 (2014) 74–85 75

Fig. 1. (Color on web.) The field setting and microscopy of HAOMs. a. One large(∼30 cm), euhedral HAOM crystal forming part of aggregate/pod from the MMIS.b. Equant and subhedral megacryst aggregation from the NPS (photo courtesy B.Ryan). c. Rounded HAOM aggregate from RAP. d. Elongate (∼40 cm) orthopy-roxene megacryst from RAP possibly fractured during magma ascent/transport.e. Two HAOMs from RAP showing interstitial relationships with plagioclase grains.f. Crossed-polarized photomicrographs of HAOM showing plagioclase exsolutionlamellae (RAP). g. Plane polarized microphotographs of HOAMs (RAP).

(Charlier et al., 2010; Emslie, 1990; Emslie et al., 1994; Framand Longhi, 1992; Heinonen et al., 2010; Mitchell et al., 1995).The comagmatic HAOMs, which may represent a small portion ofthis missing component, are trapped fortuitously by high-viscosityanorthositic magma mushes and transported to upper crustal lev-els. HAOMs thus provide a rare look at magmatic processes atthe crust–mantle interface, where evidence is usually sequestered30–40 km below the surface, and can help explain the processesthat operate at the Moho.

We explore possible constraints on crustal differentiation pro-cesses as revealed by the HAOMs and associated anorthosites fromthree classic Proterozoic anorthosite massifs: the Mealy MountainsIntrusive Suite (MMIS), Nain Plutonic Suite (NPS, both in Labrador,Canada, Fig. 2a) and Rogaland Anorthosite Province (RAP, Nor-way, Fig. 2b). Direct petrologic and geochemical evidence for theponding of upwelling magmas at the Moho is documented us-ing the Nd and Pb isotopic systems. At Moho depths cumulatescan form, be sequestered, and sink across the Moho due to theirhigh density. The geochemical and geochronological informationfor these HAOMs and their host anorthosites indicates a man-tle source and Andean-type arc setting for Proterozoic massif-typeanorthosites.

1.1. The enigmatic petrogenesis of Proterozoic anorthosites

Proterozoic massif-type anorthosites remain an enigmatic modeof anorthosite occurrence. Ongoing debate surrounds the tec-tonic setting, parental melt compositions, source and emplacementmechanisms of these predominantly felsic (80–90% plagioclase;An contents = 50 ± 10), temporally restricted, batholithic intru-sions. Competing hypotheses describing the source of anorthosite-forming magmas in the Proterozoic have existed for decades(e.g. Berg, 1969; Morse, 1969; Philpotts, 1969) and in the sim-plest sense the two schools of thought argue that the meltswere derived either from the mantle or the lower crust. Geo-chemical and petrologic studies investigating not only Protero-zoic anorthosites, but also associated jotunites/monzonites andmarginal basaltic/high-Al gabbroic rocks, pointed to several petro-logic and geochemical lines of evidence indicating that the mag-mas were derived from the mantle (Ashwal and Wooden, 1983;Emslie, 1978; Icenhower et al., 1998; Mitchell et al., 1995, 1996;Morse, 1982; Olson, 1992; Wiebe, 1990). Key to many of themantle-derivation proposals is the recognition that crustal as-similation played a role in developing the petrologic and geo-chemical composition of the magmas. Stable and radiogenic iso-tope data support the proposals that the magmas are derivedfrom melting of the depleted mantle combined with all-importantcrustal contamination (Emslie et al., 1994; Peck and Valley, 2000;Peck et al., 2010). In recent years lower crustal melting, throughunderthrusting of lower crustal material into the mantle, has be-come popular based on several experimental investigations, geo-chemical arguments and field/geophysical observations (Duchesne,2001; Duchesne et al., 1999; Longhi, 2005; Longhi et al., 1999;Sauer et al., 2013; Schiellerup et al., 2000; Taylor et al., 1984).

A number of potential tectonic settings have, over the years,been proposed for Proterozoic anorthosites such as meteorite im-pacts, extensional regimes, convergent margins and even anoro-genic settings (Anderson, 1975; Ashwal, 1993; Berg, 1977; Corriganand Hanmer, 1997; Dewey and Burke, 1973; Emslie, 1978; Hoff-man, 1989; Morse, 1982; Scoates and Chamberlain, 1997; Vigner-esse, 2005). Proposals that anorthosites may represent part of thedeep roots of Andean-arc systems were favored for a time becausethese settings could account for long, linear arrays of anorthosites(e.g. Grenville Province, Eastern Ghats Belt; Ashwal, 1993). Withincreasing geochronological evidence linking anorthosite emplace-ment to the diminishing stages of collisional orogeny, consensuson the tectonic setting of Proterozoic anorthosites has shifted tothat of a late- to post-collisional orogenic setting (Martignole andSchrijver, 1970; McLelland et al., 2010).

1.2. High-Al orthopyroxene megacryst geobarometry

Igneous relationships between HAOMs and surrounding mega-crystic plagioclase (Emslie, 1975) and geochemical modeling(Charlier et al., 2010) indicate a cogenetic relationship with theirhost anorthosites (Fig. 1). Four independent studies using generalpetrography/geochemistry (Wiebe, 1986), Al-in-orthopyroxene geo-barometry (Emslie, 1975), experimental work (Longhi et al., 1993;Fram and Longhi, 1992) and major element modeling (Charlieret al., 2010), show that HAOMs crystallized from fractionat-ing magmas, beginning at pressures of 10–15 kbar (30–40 km).These crystallization depths are at, or near, the Moho, an inter-val seldom sampled by other geological processes, and are fardeeper than the emplacement depths of the host anorthosites(<10–20 km; Berg, 1977; Valley and O’Neil, 1982). Although Alcontent in many mafic systems is not the sole indicator of depthof crystallization, detailed calibration and studies on pyroxenesfrom the anorthositic system show that Al content is a well-calibrated geobarometer (Emslie, 1975; Fram and Longhi, 1992;

76 G.M. Bybee et al. / Earth and Planetary Science Letters 389 (2014) 74–85

Fig. 2. a. The geological setting of the Nain Plutonic Suite (NPS) and Mealy Mountains Intrusive Suite (MMIS) in Labrador, the easternmost province of Canada. b. The RogalndAnorthosite Province is located on the southwestern coast of Norway and intrudes into rocks of the Sveconorwegian Orogen.

Longhi et al., 1993 and Charlier et al., 2010). Dymek and Gromet(1984) and Morse (1975) have alternatively suggested that theseHAOMs were products of rapid in-situ crystallization and an in-creased incorporation of Al2O3 into orthopyroxene at the liquid–crystal interface as a result of sluggish diffusion of Al2O3 into theliquid. Longhi et al. (1993) showed through experiments and cal-culations that compatible elements like Cr2O3 would have beendepleted in the crystals if rapid, low pressure crystallization hadtaken place. However, a natural, positive correlation of Cr2O3 andAl2O3 is observed (Longhi et al., 1993). Consensus has thereforesettled on the fact that these HAOMs are products of crystalliza-tion of parental magma to anorthosites at upper mantle to lowercrustal depths, providing a window into primitive magmatic pro-cesses operating near the source of these magmas.

1.3. Regional geology of a trio of Proterozoic anorthosite intrusions

The Mealy Mountains Intrusive Suite (MMIS), Nain PlutonicSuite (NPS), and Rogaland Anorthosite Province (RAP) intruded intopre- and early-Labradorian orthogneisses of the Grenville Province,Archaean ortho- and paragneisses from the Torngat Orogen andgneisses/amphibolites of the Sveconorwegian Orogen, respectively,are all classic examples of Proterozoic massif-type anorthosites(Fig. 2).

The Mealy Mountains Intrusive Suite (MMIS; Fig. 2, S1a), whichforms part of a deep-level, thrust-stacked block in the exte-rior thrust belt of the Grenville Province known as the Mealy

Mountains Terrane, is the largest of several anorthosite mas-sifs in the easternmost section of the Eastern Grenville Province(Emslie, 1976; Emslie et al., 1983; Gower et al., 2008a, 2008b).The MMIS lies approximately 200 km southeast of the GrenvilleFront and has been emplaced into relatively juvenile and im-mature, pre-Labradorian (1800–1770 Ma) and early-Labradorian(1710–1655 Ma) orthogneisses and paragneisses that make upa significant portion of the NE sector of the Grenville Province(Emslie and Hegner, 1993; Gower et al., 2008b; Hegner et al.,2010). Two distinct massifs make up the anorthositic mem-ber of the suite: leuconorites and pyroxene-bearing anorthositesof the Etagaulet massif and leucotroctolites and olivine-bearinganorthosites of the Kenemich massif (Emslie and Bonardi, 1979).Geochemically, the pre-Labradorian and early-Labradorian gneissesof the Mealy Mountains Terrane, into which the anorthositicsuite has intruded, are juvenile with 143Nd/144NdI at 1640 Ma =0.51040–0.51051.

The Nain Plutonic Suite is a mid- to upper-crustal, coalescedbatholithic assemblage of more than 12 anorthositic, granitic, troc-tolitic and gabbronoritic intrusions that were emplaced in twodistinct sequences spanning 1363–1289 Ma (Fig. 2, S1b; Myers etal., 2008; Ryan, 2000; Xue and Morse, 1993). A roughly north–south trending tectonic junction, the Torngat Orogen, plays hostto the NPS and also marks a zone of oblique collision, transcurrentshear and subsequent uplift between the Archaean Nain Provinceand the Palaeoproterozoic Churchill/South Eastern Rae Province be-tween 1860 and 1740 Ma (Ryan, 2000; Van Kranendonk, 1996).

G.M. Bybee et al. / Earth and Planetary Science Letters 389 (2014) 74–85 77

The anorthositic lithologies, of which leuconorites and troctolitesare dominant, separate granitoid lithologies to the west and Ar-chaean gneisses of the Nain Province to the east (Ryan, 2000).Small plutons, dykes and sheets of ferrodiorite are also commonin the NPS as are layered troctolitic intrusions (Ryan, 2000).

The Rogaland Anorthosite Province (Fig. 2) formed during theMeso- to Neoproterozoic within the Sveconorwegian Orogen onthe western margin of the Fennoscandian plate (within the largerBaltica or East European Craton; Bingen et al., 2008; Bogdanovaand et al., 2008; Rivers and Corrigan, 2000). Three separate massifs,the Egersund-Ogna, Håland-Helleren and Åna-Sira bodies, makeup the anorthositic end member of the RAP. The Egersund-Ognamassif, from which the majority of the HAOMs in this study arederived, is a mantled dome with margins consisting predomi-nantly of deformed leuconoritic lithologies (Fig. S1c). The centralregions of the body are composed of a leuconoritic–noritic faciesknown as the anorthositic–noritic complex that displays granu-lated, 1–3 cm plagioclase grains and hosts substantial concentra-tions of sub-ophitic pyroxene and plagioclase megacryst aggregates(Charlier et al., 2010). These lithologies are emplaced into granulitefacies, migmatitic gneisses that represent metamorphosed grani-toids and volcanic lithologies that formed at 1.5 Ga during oneof the first recorded magmatic, crust-forming events in the Sve-conorwegian Orogen (Slagstad et al., 2013; Vander Auwera et al.,2011). These gneisses show similar juvenile characteristics to thosein the Mealy Mountains Terrane with 143Nd/144NdI at 930 Ma =0.51127–0.51142 compositions (Menuge, 1988).

2. Analytical methods

Major element data were obtained from the School of Geo-sciences Earth Lab at the University of Witwatersrand using XRF.A full suite of trace elements was analyzed at the Departmentof Geological Sciences at the University of Cape Town. Radio-genic isotopic abundances and concentration data for Sm, Nd andPb were determined at the Department of Terrestrial Magnetism(DTM) of the Carnegie Institute of Washington using traditionalisotope dilution, ion-exchange chromatography and a combinationof Thermal Ionisation Mass Spectrometry (TIMS; for Nd) and Multi-Collector Inductively Coupled Mass Spectrometry (MC-ICPMS; forSm and Pb). A detailed analytical procedure can be found in Sec-tion 1.3 of the Supplementary Information.

3. Results

3.1. Megacryst and anorthosite petrology, geochemistry, and isotopiccompositions

HAOMs are ubiquitous in Proterozoic massif-type anorthositesand in both the MMIS and RAP, these megacrysts either occuras single, euhedral to subhedral crystals or form part of aggre-gates of several subhedral megacrysts (Fig. 1a–e). HAOM habit inthe NPS is more varied and in addition to occurring as single,curved crystals or giant aggregate pods, these phases also occuras single, angular pyroxene crystals with an intercumulus rela-tionship to surrounding plagioclase. In the MMIS, field observa-tions, in addition to collations of previous field workers’ obser-vations, shows that the orthopyroxene megacrysts are restrictedto leuconorite-bearing lithologies and appear to be absent inthe olivine-bearing Kenemich massif. Similar correlations betweenorthopyroxene-bearing lithologies and megacrysts are also notedin the leuconoritic Egersund-Ogna massif, which hosts the mostHAOMs of the three intrusive suites making up the RAP. HAOMsstudied in this contribution show the typical microscopic featuressuch as plagioclase exsolution lamellae and pervasive inclusionsof Fe-oxide material. HAOMs from the MMIS are undeformed and

Fig. 3. Chondrite-normalized (Anders and Grevesse, 1989) REE element diagramcomparing HAOMs and anorthosites from the three localities sampled in this study.Numbers adjacent to HAOMs from the MMIS represent Al2O3 contents (wt.%).

pristine and, while the outer margins of the Egersund-Ogna massifdo preserve deformed and recrystallized megacrysts, the only sam-ples collected from the RAP were pristine and showed no signs ofpost-intrusion alteration. On the other hand, many of the samplesfrom the NPS show signs of recrystallization and some show kink-banding caused by deformation.

Microscopically, these HAOMs are unzoned crystals of orthopy-roxene with abundant, thin, planar, calcic plagioclase exsolutionlamellae (average An80) occasionally incorporating olivine, ilmeniteand biotite (Fig. 1f–g; Emslie, 1975). Light rare earth element en-richment (LREE) and Eu anomalies vary substantially within andbetween sample sets with the highest Al, most LREE depleted sam-ples showing no or very small Eu anomalies, whilst more LREEenriched samples display prominent negative Eu anomalies (Fig. 3and S5). Based on experimental data (Fram and Longhi, 1992;Longhi et al., 1993), an Al-in-orthopyroxene geobarometer forProterozoic anorthosites calibrated by Emslie et al. (1994), sug-gests that our sampled HAOMs formed at pressures between12.14–5.85 ± 0.4 kbars (RAP), 9.67–4.52 ± 0.4 kbars (MMIS), and9.36–7.18 ± 0.4 kbars (NPS; Fig. S2).

The isotopic compositions of the different suites are an im-portant first order observation (Supplementary Data Tables). TheMMIS, NPS and RAP display remarkably different isotopic composi-tions with the MMIS and RAP showing positive εNd,T compositions(2.3 to 3.6 and 2.2 to 4.4, respectively) whilst the NPS presents arange of negative values (−1.6 to −8.5; Fig. 4a). The systematicdifferences and the precision of the isotopic analyses in the MMISallow us to differentiate different rock types isotopically (Fig. 4ainset). The anorthosites (sensu stricto) have the highest εNd,T com-positions, followed by leucotroctolites and then leuconorites (al-though the compositional ranges of the leuconorites and leucotroc-tolites overlap substantially when plotted with 2 sigma errors). Ona εNd-208Pb plot, suites of HAOMs have approximately constant εNdcompositions, but show variations in 208Pb (Fig. 4b). In the RAP,lower-Al megacrysts are displaced, in varying degrees, to lower εNdand 208Pb compositions on a vector trending towards upper crustalcompositions. Anorthositic lithologies from the MMIS are displacedto slightly lower εNd than the HAOMs although many lithologiesoverlap. HAOMs from the NPS have been displaced to much lowerεNd and slightly lower 208Pb compositions than the other HAOMsuites, while the host anorthosites show similar 208Pb but lowerεNd compositions.

78 G.M. Bybee et al. / Earth and Planetary Science Letters 389 (2014) 74–85

Fig. 4. a. A depleted mantle origin for HAOMs and Proterozoic anorthosites. εNdvs. time illustrating how emplacement of anorthositic mushes into terranes of dif-ferent ages dramatically influences the isotopic composition of the anorthosites. Inyoung terranes, where crust is relatively juvenille, the initial megacrystic isotopiccompositions (large triangles, derived from isochron regression) are consistentlyonly slightly displaced from depleted mantle evolution, indicating magma derivationfrom depleted mantle. Linear fields indicate evolution of potential crustal contami-nants (potential contaminants for the NPS include both Churchill and Nain Provincegneisses, due to intrusion into the province-suturing Torngat Orogen). (Nägler andKramers, 1998.) b. εNd vs. (208Pb/204Pb)i plot illustrating isotopic variation betweenmegacrysts and host anorthosite. The 232Th–208Pb system is chosen as a bettertracer of contamination because of increased concentrations of Th in the continen-tal crust. Depleted mantle composition determined from Rehkämper and Hofmann(1997) and the representative lower crustal composition is taken from Ben-Othmanet al. (1984). HAOM and anorthosite compositions are plotted at the ages shown inTable 1.

Fig. 5. a. 143Nd/144Nd vs. 147Sm/144Nd illustrating the isochronous relationshipsbetween high-Al megacrysts from three different Proterozoic anorthosite massifs.Each isochron defines an age approximately 110–130 m.y. older than the rec-ognized age range of the anorthosite intrusions. 2σ errors are all smaller thanthe symbol size. Isochron regressions produce εNd values for MMIS: εNd;1765 Ma =+3.17 ± 0.31, RAP: εNd;1041 Ma = +4.83 ± 0.61 and NPS: εNd;1444 Ma = −3.16 ± 2.55.The Churchill and Nain Province Gneisses cluster around the following isotopic com-positions not shown on the graph: 143Nd/144Nd: 0.511421–0.510451; 147Sm/144Nd:0.107025–0.085978 (Supplementary Data Tables). MSWD: mean square of theweighted deviates. b. Four point isochrons (including repeat analyses of whole-rock megacryst) between plagioclase lamellae, host orthopyroxene and whole-rockmegacrysts indicating that the ascent of HAOMs in plagioclase-rich mushes to finalemplacement levels in the upper crust requires significant lengths of time – similarto the age differences between HAOMs and the host anorthosites.

3.2. High-Al orthopyroxene megacryst geochronology

Isochronous relationships amongst HAOMs using the Sm–Ndsystem demonstrate that the highest pressure HAOMs, in eachsuite, are 110–130 m.y. older than their host, comagmatic anortho-sites (Fig. 5a; Table 1). The Sm–Nd isochron age for the HAOMsfrom the MMIS is very precise at 1765 ± 12 Ma and statisticallysignificantly older by 115–130 m.y. than the anorthosite crys-tallization age as given by the precise U–Pb zircon and badde-leyite ages on the MMIS anorthosites themselves (Emslie, 1990;Gower et al., 2008b; this study). The Sm–Nd isochron age for theHAOMs from the NPS is 1444 ± 100 Ma, older by 81–155 m.y.

than the anorthosite crystallization age as given by the U–Pb zir-con, baddeleyite, and apatite ages of 1289 to 1363 Ma on the NPSanorthosites directly and the very precise megacryst exsolution ageof 1347 ± 6 Ma (see summary in Myers, 2008; this study). Simi-larly, the Sm–Nd isochron age for the HAOMs from the RAP with>8 wt% Al2O3 of 1041±17 Ma is 92 to 121 m.y. older than the in-ferred anorthosite crystallization age of 920 to 949 Ma as given byU–Pb ages of zircon and baddeleyite separated from the cogenetic,high-Al orthopyroxene megacrysts (Andersen and Griffin, 2004;Sauer et al., 2013; Schärer et al., 1996; this study). That threedifferent anorthosite massifs, formed at different times, show con-sistent relative age relations tied to identical mineralogy can only

G.M. Bybee et al. / Earth and Planetary Science Letters 389 (2014) 74–85 79

Table 1Comparative table of megacryst isochron age (this study) vs. emplacement age of the host anorthosite (U–Pb zircon/baddeleyite; see geochronological sources below) andSm–Nd megacryst exsolution ages (this study). Point values in the parentheses indicate the number of points creating the isochron.

Intrusion MMIS NPS RAP (>8 wt% Al2O3)

Megacryst isochron age(Ma)

1765 ± 12 1444 ± 100 1041 ± 17(n = 6 this study) (n = 9, this study) (n = 5; >8 wt% Al2O3, this study)

Megacryst exsolution age(Ma; see Fig. 4)

–* 1346.9 ± 5.9 968 ± 43(n = 4, this study) (n = 4, this study)

Anorthosite emplacementage (Ma)

1650–16301 1363–12892 949–9203

1 U–Pb zircon/baddeleyite ages from anorthosites and granitoids (Gower et al., 2008b; Emslie, 1990).2 U–Pb zircon/baddeleyite/apatite, Ar–Ar ages from multiple sources summarized in (Myers et al., 2008).3 U–Pb zircon/baddeleyite in high-Al megacrysts (Sauer et al., 2013; Schärer et al., 1996) and Storgangen orebody of the RAP (Andersen and Griffin, 2004).* Insufficient plagioclase could be separated from megacrysts in the MMIS to perform exsolution geochronology.

be explained by a process general to all three massifs and perhapsto all anorthosites. Fig. S2 in the Supplementary Information indi-cates the calculated pressure of crystallization for the HAOMs thatform these isochrons and Section 1.4 highlights more detailed dis-cussions surrounding the geochronology.

Previous U–Pb geochronology and detailed field studies revealthat these comagmatic, coalesced anorthosite massifs were em-placed over at least a 12–80 m.y. period (Gower et al., 2008a;Myers et al., 2008; Schärer et al., 1996). HAOMs from RAP withlower Al2O3 contents, as well as intercumulus orthopyroxene dis-play various degrees of isotopic disequilibrium with the HAOMs(Fig. 5a). Recrystallization has affected, and is unavoidable, in manyof the samples from the Nain Plutonic Suite, creating a largerisochron age error. Age data from Pb–Pb isochrons are of poorerquality, perhaps due to the ease with which Pb remobilization oc-curs. In one case (RAP), however, data trends are preserved in theU–Pb system, and although not a true isochron, this array producesa comparable age to the isochron in the Sm–Nd system (Fig. S4).

Four-point Sm–Nd isochrons between plagioclase lamellae, hostorthopyroxene and whole-rock HAOM compositions from twoHAOMs in the NPS and RAP (Fig. 5b, Table 1) show that de-compression exsolution occurred at 1346.9 ± 5.9 Ma and 968 ±43 Ma, respectively – ages that correspond (within error) withanorthosite crystallization age ranges in both massifs. Plagio-clase exsolved from these HAOMs 70–100 m.y. after crystalliza-tion, suggesting that significant timescales are required for the20–30 km ascent from crystallization depth to plagioclase exso-lution depth. The low Al2O3 contents (less than 3 wt%) and lackof plagioclase exsolution lamellae in intercumulus orthopyrox-ene of the host anorthosites indicate that these lithologies musthave crystallized at shallow, upper crustal depths (Berg, 1977;Valley and O’Neil, 1982) over the time spans indicated above and110–130 m.y. after HAOM formation. Interestingly, Sm–Nd pla-gioclase exsolution ages in HAOMs from the RAP overlap withU–Pb ages of zircon and baddeleyite separated from the cogenetic,HAOMs (Table 1). These U–Pb ages are interpreted as the emplace-ment age of the anorthosites in the RAP by Andersen and Griffin(2004), Sauer et al. (2013) and Schärer et al. (1996) and the im-plications of this seemingly contradictory overlap are discussed inSection 4.3.

Viewed in isolation, the ∼100 m.y. differences between HAOMsand anorthosite ages could be interpreted as a result of differencesin closure temperature between the Sm–Nd system in the prim-itive megacrysts and U–Pb in anorthositic zircon that crystallizedat a late-stage (or even at sub-solidus conditions) in zirconium de-pleted magmas. However, associated with Proterozoic anorthositesare suites of coeval granitoids (Ashwal, 1993), which are morelikely to be saturated in zircon, confirming that 100 m.y. differ-ences between megacryst and younger anorthosites and granitoidscannot be the effect of different closing temperatures and slowcooling.

3.3. True isochrons vs. pseudo-isochronous mixing lines?

One interpretation of the isotopic arrays created by the HAOMs(Fig. 5a) may be as time-integrated mixing lines created by ini-tial INd variations due to either mixing or assimilation-fractional-crystallization (AFC) with crustal material during magma crystal-lization (Davidson et al., 2005; DePaolo, 1981). In theory, whilethis hypothesis could explain the data, several lines of evidence(described below) disprove the “mixing” hypothesis and indicatethat these arrays are true isochrons, ∼100 m.y. older than the hostanorthosites.

3.3.1. Isotopic disequilibrium in lower-Al2O3 megacrysts and inintercumulus orthopyroxene

In the RAP, HAOMs with Al2O3 content >8 wt% form a ca.1041 Ma isochron, but lower Al2O3 megacrysts (3–7 wt%) are indisequilibrium with this isochron and are displaced to lower Ndisotopic compositions (Fig. 5a). Regression analysis reveals that thislower-Al suite forms an errorchron with a shallower slope in Sm–Nd space (Section 1.4, Supplementary Information). Isotopic dis-equilibrium between the higher- and lower-Al compositions sug-gests that magmatic differentiation processes, like AFC, affectedonly the lower-Al megacrysts. Had the mixing between two sep-arate components occurred during an AFC process that affectedall the megacrysts, it would be expected that they all plot onan isochron. This is not the case. The fact that a distinct suiteof higher Al2O3 content megacrysts retains an older isochron agesuggests that this age dates the crystallization of these minerals,otherwise they should not yield an isochron either.

3.3.2. Coincidence in Pb–Pb isochron ageAlthough much of the Pb–Pb isotopic data appears reset, due

perhaps to later orogenic activity or secondary effects, HAOMsfrom the RAP form a linear array on a 207Pb–206Pb plot with an ageof 1062 ± 330 Ma (MSWD = 258; Fig. S4). Although not strictly anisochron, perhaps due to Pb remobilization, the age produced pro-vides a useful comparison to the Sm–Nd isochrons (Fig. 5a). Giventhe differences in fractionation behaviors and half-lives betweenthe Sm–Nd and U–Pb isotope systems, it is highly unlikely thatsimilar age differences between the HAOMs and host anorthositeswould result from initial crustal mixing or assimilation (Davidsonet al., 2005). The close correspondence between Sm–Nd and Pb–Pbisochron ages, in at least one intrusion, disproves the hypothesisthat the isochrons result from variation in the initial isotopic ratioof the magma.

3.3.3. Anorthosite isochrons and U–Pb agesSm–Nd isotopic arrays for anorthosite whole-rock compositions

from the MMIS, although not isochronous, produce an age equiv-alent to the U–Pb zircon/baddeleyite ages determined for these

80 G.M. Bybee et al. / Earth and Planetary Science Letters 389 (2014) 74–85

intrusions (1653 ± 150 Ma vs. 1650–1630 Ma, respectively). If ini-tial contamination had affected the magmas and produced oldisochrons in the megacrysts, one would expect the anorthosites,which crystallize from the same, if not more contaminated magma,to display similarly old ages relative to the U–Pb ages. This is notthe case, providing further evidence against the HAOM arrays rep-resenting pseudoisochrons. Anorthosites from the NPS do not formarrays with any meaningful age and so no comparison can be madein this case.

3.3.4. Consistent isochron ages between intrusionsThe RAP, MMIS and NPS intrusions are each of significantly

different age (ca. 930 Ma, 1650–1630 Ma, 1363–1289 Ma, respec-tively; see Table 1) and intrude into crust with significantly differ-ent isotopic compositions and age. If the arrays do represent mix-ing or AFC trends between melt and a crustal assimilant, the diver-sity of age and crustal compositions would be highly unlikely toproduce the observed, self-consistent age differences between themegacrysts and the comagmatic host anorthosites (100–144 m.y.,see Table 1).

3.3.5. Time-integrated AFC/Mixing forward modelsUsing megacrysts from the RAP, where a range of Al2O3 con-

tents are preserved in the megacrysts and reasonable constraintson the isotopic composition of the lower continental crust exist(Othman et al., 1984), we forward model the effect of variationsin INd ratio due to mixing or assimilation on aged isotope data(Section 1.5 of Supplementary Information). All of the “isochron”ages produced by AFC and pure mixing using constraints appro-priate to the RAP are between 285 m.y. and over a billion yearsolder than the anorthosite, illustrating that it is not possible tocreate isochrons only ∼100 m.y. older than the anorthosites. Onlyin a simulation where no fractionation occurs (r = 1), representingpure mixing, do array ages approach those of the actual megacrystisochrons (1215 Ma cf. 1041 Ma; Fig. S3c). A scenario involvingpure mixing is highly unlikely (DePaolo, 1981) and not applica-ble in the case of the RAP as variations in Eu anomalies amongstthe isochronous HAOMs (<8 wt% Al2O3) indicate that the magmawas fractionating orthopyroxene and began fractionating plagio-clase during HAOM crystallization (Fig. S5).

These various and diverse lines of evidence all disprove thehypothesis that the isochronous Sm–Nd data arrays could be pro-duced by time-integrated initial INd variations due to mixing orAFC processes in a magma chamber. The more likely scenario forthe creation of isochronous data arrays in all three anorthosite-hosted megacryst suites is directly related to magmatism near theMoho.

4. Discussion

4.1. Direct geochemical evidence for magma ponding at the Moho

Given our evidence that HAOM isochron ages represent truecrystallization ages, we can now place HAOM crystallization ina plausible sequence of events to better understand Proterozoicanorthosite petrogenesis and general magmatic processes operat-ing in the upper mantle and lower crust.

Broadly basaltic high-Al, tholeiitic melts, known to be theparental magmas of Proterozoic anorthosites (Charlier et al.,2010; Emslie, 1990; Emslie et al., 1994; Fram and Longhi, 1992;Heinonen et al., 2010; Mitchell et al., 1995), initially crystallizeolivine and clinopyroxene while rising towards the base of thecrust (Müntener and Kelemen, 2001; Müntener and Ulmer, 2006).Interactions between this magma and partial melts of the maficlower crust would induce a limited pulse of more silicic lowercrustal contamination, not only inducing orthopyroxene formation

due to peritectic olivine reaction (Müntener and Kelemen, 2001),but lowering and shifting the Nd and Pb isotopic composition ofthe high-Al orthopyroxene cumulates, as we observe (Fig. 4a, b).Although most Proterozoic anorthosite massifs commonly preserveorthopyroxene megacrysts, rare examples of the occurrence ofclinopyroxene megacrysts together with HAOMs have been notedin the Labrieville and Grenville Township massifs (Quebec; Ashwal,1993; Owens and Dymek, 1995; Philpotts, 1969). These observa-tions illustrate that most clinopyroxene (and olivine) is likely tohave crystallized before, and in rare cases, with orthopyroxeneand plagioclase at high pressures. The paucity of clinopyroxene(and olivine) megacrysts in anorthosite massifs suggests that thesephases formed (and perhaps sank in the magma chamber) beforesufficient, buoyant plagioclase mush had formed – a vital ingredi-ent for delivering some HAOMs to upper crustal depths with theanorthosite. The isochronous isotopic composition of the highest-Al megacrysts (Fig. 5a) indicates that they were in equilibriumwith a fractionating magma that received compositionally indis-tinct lower crustal contamination and/or magmatic recharge atupper mantle or lower crustal pressures. Ponding and subsequentcrystallization of a rising basaltic magma at a suitable interface inthe lithosphere allows creation of such a locus for such crystal-lization. The Moho provides the required rheological contrast tostall an upwelling mafic magma at the calculated depths of HAOMcrystallization on modern-day Earth (Artemieva, 2011). Depths tothe Moho in arc/orogenic environments range between 27–60 km(with the maximum value typical of continent–continent colli-sional zones; Artemieva, 2011). Ponding of magma at the Moho isa commonly proposed, but poorly documented phenomenon whendescribing magma ascent from source to surface through the con-tinental lithosphere. Our new data for HAOMs, however, providedirect, geochemical and petrological evidence for these processesat the Moho. Our findings are supported by recent seismic re-fraction studies by Richards et al. (2013) imaging large ultramaficbodies (V p ∼ 7.4–8.0 km/s) at or above the Moho below olderoceanic hotspots formed beneath thick oceanic lithosphere. Thesefindings, together with our results, illustrate the importance of cu-mulate formation in magmas that pond at the Moho. Implicationsof these processes are discussed in Section 4.2.

The scarcity of negative Eu anomalies in some of the highest-Almegacrysts, in addition to their high Al content, suggest that pla-gioclase was not yet part of the crystallizing assemblage at thesepressures (Fig. 3, Fig. S5). These magmas are so plagioclase richthat if the HAOMs were crystallizing in equilibrium with them,they would be expected to have prominent negative Eu anoma-lies as are seen in lower-Al megacrysts. Positive-buoyancy insta-bilities develop once sufficient plagioclase has formed, allowingplagioclase-rich mushes (defined as magmas with volume frac-tion crystallinity of 0.25–0.55) to begin rising through the crust(Kushiro and Fuji, 1977). Both lower-Al megacrysts and intercumu-lus orthopyroxene in the host leuconorites are in isotopic disequi-librium with the highest-pressure megacrysts, indicating that poly-baric assimilation and fractional crystallization characterize theremainder of the ascent of these buoyant plagioclase mushes totheir final level of emplacement (Fig. 5a). Later pulses of magmathat crystallize olivine and plagioclase may also pond at the Mohoand create younger troctolitic mushes (as observed; Gleißner etal., 2011; Morse, 2006) that rise along similar pathways to thefinal levels of emplacement. These magmas are not significantlycontaminated by crustal material (yielding the most radiogenic,mantle-like compositions; Fig. 4a inset) because earlier anatexisdepleted and sealed the country rock. This explains the restric-tion of HAOMs to orthopyroxene-bearing massifs, as we observe,particularly in the MMIS.

In the model presented here, HOAMs form in a ponded magmaat the Moho ∼100 m.y. before anorthosite emplacement in the up-

G.M. Bybee et al. / Earth and Planetary Science Letters 389 (2014) 74–85 81

per crust, but we do not envisage these crystals remaining at theMoho for 100 m.y., and suggest that part of this time period wasspent in a convecting, recharging magma at the Moho with the re-mainder of the period involving the ascent of the magma mushto mid-crustal levels. The ascent of magmas, sufficiently saturatedwith plagioclase to be positively buoyant, may take significantlengths of time – longer than predicted for less-viscous basalticmagmas (Kushiro, 1980).

4.2. Implications for magma chamber processes in the deep crust andmantle

Recent work has shown that equilibrium crystallization ofbasaltic magmas produces, at most, 50–60% of plagioclase (Mün-tener and Ulmer, 2006) in the crystallizing assemblage. Basedon our maximum estimates, any given anorthositic massif con-tains 10–15% of mafic phases (olivine, orthopyroxene, clinopyrox-ene and magnetite/ilmenite), suggesting that 35–40% of the maficmineralogy is unaccounted for. Considering our evidence for cu-mulate formation in ponded magmas at the Moho, we proposethat ultramafic crystallization and sequestration in ponded mag-mas at these depths could account for the missing componentin Proterozoic anorthosites. Calculations show that, in arc envi-ronments, ∼21% ultramafic cumulate formation from primitivemantle-derived magmas occurs below the Moho (Conrad and Kay,1984) and the observation of high-pressure ultramafic xenolithsin lavas from the Aleutian arc (Conrad and Kay, 1984) and theNógrád-Gömör Volcanic Field in northern Hungary (Zajacz et al.,2007) provide corroboratory evidence that upper mantle cumulatesdo form in other environs, but are rarely brought to the surface.HAOM compositions also compare favorably with fields of exper-imentally determined, high-pressure (12 kbar) basaltic arc cumu-lates (Fig. S6; Müntener and Kelemen, 2001). HAOMs are slightlymore evolved than these experimental cumulates, suggesting thatolivine fractionation did occur prior to HAOM crystallization. Theselines of evidence point to similar ultramafic cumulates formingin major continental crust generation zones where basaltic mag-mas pond at the Moho. HAOMs, therefore, are not peculiaritiesrestricted to Proterozoic anorthosites, but instead are vestiges ofubiquitous and significant magmatic processes operating at theMoho, fortuitously brought to the surface as a result of high viscos-ity plagioclase-rich mushes which form Proterozoic anorthosites.

Arndt and Goldstein (1989), working on the assumption thatmafic magmas pond, crystallize and assimilate lower crust atthe Moho (as we have demonstrated), show that olivine andpyroxene cumulates would sink back into the mantle due tonegative density contrasts with the surrounding mantle. MELTS(Ghiorso and Sack, 1995) modeling was performed to quantifythe density contrasts between cumulates, magmas and the man-tle at the Moho using starting conditions expected for Protero-zoic anorthosite formation. An experimentally produced high-Al basalt was used as the parental magma (Fram and Longhi,1992), with close similarities to natural samples found in vari-ous anorthositic massifs and reported in Charlier et al. (2010).Models were run under equilibrium and fractional crystallizationscenarios at 10 and 15 kbar. Pyroxenes formed between 10 and15 kbar will be denser than both the crystallizing magma and thesurrounding mantle (e.g. #ρmagma-pyroxene [10 kbar] = −0.62 g cm−3;#ρPREMmantle-pyroxene [10 kbar] = −0.07 g cm−3) and will sink, notonly in the ponded magma, but also into the mantle (Fig. 6). Thedensity contrasts between the cumulate pyroxene and mantle areless pronounced at 15 kbar, although still exhibit relatively largedensity contrasts with the magma. The small negative density con-trast between pyroxene cumulates and the surrounding mantle atboth pressures will be exacerbated as the cumulates transition to

eclogite facies. These results demonstrate that ultramafic mate-rial formed at the Moho, under conditions and using chemistrieslikely to crystallize anorthosites, would be denser than the mag-mas and the surrounding mantle and would consequently sink intothe mantle, or at least be sequestered beneath the Moho. TheseMELTS models (particularly at 10 kb) also produce positively buoy-ant plagioclase (with geologically reasonable An contents; An±50)that will accumulate at the top of a ponded magma and even-tually rise diapirically through the crust, as predicted in modelsof anorthosite petrogenesis (Ashwal, 1993; Charlier et al., 2010;Emslie et al., 1994). Jull and Keleman (2001) modeled and quanti-fied density contrasts between cumulates/lower crust at the Moho,using different methods, and demonstrated that ultramafic ma-terial (olivine clinopyroxenites) present at Moho conditions (onarc-type geotherms) would have density contrasts with the man-tle between 0.05–0.1 g/cm3 and would sink into the mantle ontimescales of 10 million years, supporting the MELTS models cre-ated in this study.

The fate of these ultramafic cumulates is crucial. Although itis not known whether bulk crustal differentiation occurs by sink-ing of mantle-derived cumulates, or foundering of lower crustalmaterial into the mantle (Kay and Kay, 1991, 1993; Lee et al.,2006), our results indicate that cumulate formation and founder-ing at the Moho is a very real and significant mechanism forgenerating discrepancies between single-stage melts and observedmagmatic products (Fig. 7). Similar processes of cumulate forma-tion and delamination are inferred to have occurred in other arcenvironments from studies in exposed arc sections (e.g. Kohis-tan Arc, N. Pakistan) and represent analogs to the processes wehave observed in Proterozoic anorthosites (Jagoutz et al., 2006;Jagoutz and Schmidt, 2012). As shown in Fig. 7, the effects of ul-tramafic cumulate sequestration in ponded magmas at the Mohoplays a major role in developing the bulk, felsic composition ofProterozoic anorthosites and we suggest that the same process iscrucial for development of the intermediate composition of thebulk continental crust. The ultimate fate of these cumulates maybe remelting and creation of chemical heterogeneities elsewherein the convecting mantle, followed by subsequent recycling intothe sources of MORB and OIB (Tatsumi, 2000).

4.3. The tectonic setting and source of Proterozoic anorthosites

A variety of tectonic settings have been proposed for Pro-terozoic anorthosites, with opinion divided between intraplate,divergent and convergent plate tectonic settings (McLelland etal., 2010). Anorthositic magma production, in terranes such asthe Grenville–Labrador region (over 1600 km long), spans about500 m.y. and it is difficult to envisage even the largest con-tinental rifts supplying magma to a region for this length oftime without initiating continental break-up (Ashwal, 1993). Insome tectonic models of anorthosite formation (Martignole etal., 1993; McLelland et al., 2010), the magmas are produced bypost-collisional lithospheric delamination followed by ascent andpartial melting of upwelling asthenospheric mantle. Such modelsare possible for some anorthosite-bearing terranes (e.g. GrenvilleProvince), but fail for others (e.g. Nain), where no collisional eventsof the appropriate ages have been recognized. Furthermore, post-collisional delamination models imply short-lived magmatism, incontrast to our results, which suggest longer-lived magmatic sys-tems on the order of ∼100 m.y.

We suggest that one geodynamic setting capable of deliveringsufficient, geographically-focused quantities of magma (and heat)to the base of the crust for ∼100 m.y., or more, is a convergent arcenvironment. Well-known convergent systems such as the Califor-nian Arc were in operation for >150 m.y., and produced volcanicand plutonic magmatism in only two short episodes (10–20 m.y.

82 G.M. Bybee et al. / Earth and Planetary Science Letters 389 (2014) 74–85

Fig. 6. Extracted magma and cumulate densities from MELTS modeling of high-Al basalt crystallization at Moho pressures. The starting composition is the same in each modelpermutation. a. Model fractionating solids at 1 GPa (10 kbars). b. Model not fractionating solids at 1 GPa (10 kbars). c. Model not fractionating solids at 1.5 GPa (15 kbars).d. Model fractionating solids at 1.5 GPa (15 kbars).

in duration; Ducea, 2001). Subduction-related plutonism is alsorecorded for a period of over 150 m.y. in Late Jurassic to Neo-gene granitoids constituting the South Patagonian Batholith (Hervéet al., 2007). In the RAP, the formation of magmas in an arc envi-ronment is supported by the overlap in HAOM crystallization ageand the onset of calc-alkaline magmatism, forming the Sirdal Mag-matic Belt in the Sveconorwegian Orogen, both ∼100 m.y. beforeanorthosite emplacement (Bybee et al., in preparation; Slagstad etal., 2013). As shown in this paper, Sm–Nd plagioclase exsolutionages in HAOMs record the times of anorthosite massif emplace-ment. This final anorthosite emplacement in the mid- to uppercrust, ∼100 m.y. after magma generation, may be facilitated byextensional regimes common to post-collisional tectonics settingsby, for instance, weakening the crust through orogenic collapse, orwarming the collapsed crust to near-solidus temperatures, therebyfacilitating ascent of viscous magmas. Correspondence of Sm–Ndplagioclase exsolution ages and U–Pb zircon-in-megacryst agesmay be due to zircon, like plagioclase, being an exsolution prod-uct in the megacrysts and consequently dating an exsolution andmid-crustal emplacement event.

Batholithic bodies of massif-type anorthosite have not yet beenfound in Phanerozoic or modern-day arc-related terranes. It hasbeen suggested that this might be due to insufficient levels of ero-sion in young arc terranes (Hamilton, 1981); perhaps a search inpoorly explored plutonic parts of young arcs, such as in southern

Patagonia, may reveal new anorthosite occurrences. However, thereare minor anorthosites that occur as cognate xenoliths in volcanicsfrom several magmatic arcs (Ashwal, 1993), although the plagio-clase in these is far more calcic (An70–100) than the typical An45–60found in Proterozoic massifs. If these occurrences are represen-tative of a general anorthosite-forming process, then this mightimply a difference in the compositional details between Protero-zoic and modern magmatic systems, perhaps involving volatiles.For instance, dry magmatism in Proterozoic arcs would enhanceplagioclase production, promote tholeiitic differentiation trendsand Fe enrichment, whereas higher water content in Phanerozoicarcs would suppress plagioclase crystallization, possibly leading tosmaller anorthosite bodies, with more calcic plagioclase. We dorecognize the petrological and compositional differences betweenour proposed anorthosite-producing continental arcs and modernarcs typified by calc-alkaline magmas, but we call attention tothe clear diversity in the magmatic products in modern arc en-vironments, which include rhyolitic volcanics and equivalent gran-itoid intrusions as well as basaltic and gabbroic bodies, spanningfour main magma series including low-K, tholeiitic compositions(Wilson, 2007). It is this part of the arc, characterized by low-K,tholeiitic compositions that could also conceivably produce Pro-terozoic anorthosites and their parental magmas.

Magmas forming Proterozoic anorthosites and their cogeneticHAOMs are considered to have been derived from melting of ei-

G.M. Bybee et al. / Earth and Planetary Science Letters 389 (2014) 74–85 83

Fig. 7. a. MgO vs. SiO2 illustrating the compositional discrepancy between single-stage melts (grey-shaded box) of peridotitic mantle [Stage 1] in arc environmentsand anorthosites and the bulk continental crust (CC; Rudnick and Gao, 2003 andreferences therein). Fractionation [Stage 2] of these peridotitic melts at the Mohoand associated sequestration of dense ultramafic cumulates (real megacryst com-positions, experimental cumulates and modeled MELTS phases) below the Moho,drives the composition of the residual magma to more intermediate compositions– a process operating at the Moho that is fundamental to generating, not only thecharacter of Proterozoic anorthosites, but all magmatic products that rise from thisdepth. Primitive mantle (PM) compositions are taken from (Palme and O’Neill, 2003)and references therein. Yellow pentagon represents the proposed parental magma ofProterozoic anorthosites (Charlier et al., 2010). Density contrast calculations (#ρ)between magma, mantle and cumulates derived from MELTS modeling. b. We showthat ultramafic cumulates formed in fractionating magmas ponded at the Moho aredenser than the melt and the surrounding mantle and may founder into the deepermantle. This process is crucial to the development of anorthositic mushes in arcenvironments as we propose. Furthermore, the effect of cumulate sequestration atthis depth cannot be discounted in models of continental crust differentiation andis likely to play an important role in developing the intermediate composition ofthe bulk crust.

ther the depleted mantle (Emslie et al., 1994) or of the lowercrust (Schiellerup et al., 2000). Data in this study support previ-ous workers’ conclusions (Ashwal et al., 1986) that the age andnature of the continental crust into which anorthositic magmas in-trude dramatically affects their isotopic composition, changing theconvecting or depleted mantle isotopic signature of these rockstoward more evolved compositions (Fig. 4a, b). The NPS, which in-trudes into old, isotopically evolved crust of the Nain and ChurchillProvinces (Archaean–Palaeoproterozoic) has Nd isotopic composi-tions up to 10 ε-units lower than the MMIS and RAP, whichintrude into lithosphere of the Palaeoproterozoic Grenvillian andMesoproterozoic Sveconorwegian provinces (Ashwal, 1993; Ashwalet al., 1986). In these younger terranes, the initial isotopic compo-sitions of the magmas from which the highest-pressure megacrystscrystallized has only been slightly displaced from depleted man-tle, indicating that the parental magmas forming both the HAOMsand anorthosites were derived from melting of the depleted man-tle. Melting of the lower crust alone to produce the necessarymafic melts requires unreasonably high degrees of melting and an

external source of heat from upwelling magmas (for which theconcomitant mantle-derived magmatism is not observed; Morse,1991). We suggest, therefore, that Proterozoic anorthosites formedfrom melting of the depleted mantle in long-lived continental-arcenvironments.

5. Conclusion

Nd and Pb isotopic geochronology and geochemistry of Protero-zoic anorthosites and ubiquitous, cogenetic high-pressure orthopy-roxene megacrysts suggests that these intrusions are derived frommelting of the depleted mantle in long-lived Andean-type arc sys-tems. The ca. 100 m.y. time differences between HAOM crystalliza-tion and anorthosite emplacement, as well as isochronous HAOMcompositions are evidence that these phases were in equilibriumwith a fractionating, ponded magma that received compositionallyindistinct lower crustal contamination and/or magmatic rechargeat upper mantle or lower crustal pressures. Although products ofcumulate formation in ponded magmas at Moho are rarely broughtto surface, the viscous plagioclase-rich magmas forming Protero-zoic anorthosites entrain evidence from the crust–mantle boundaryin the form of high-Al megacrysts, thereby providing an accessi-ble proxy for perhaps more widespread, but less visible crustaldifferentiation that occurs in silicic systems. Our results indicatethat crustal differentiation by cumulate formation and sequestra-tion at the Moho is an important mechanism that plays a signifi-cant role in creation of the distinctive composition of Proterozoicanorthosite massifs, but that could also explain the intermediate(SiO2 > 60%) compositions of the bulk continental crust. Thesemagmatic processes operating at the Moho should be taken intoaccount in future studies on the evolution and differentiation ofthe crust, along with more popular processes like crustal delami-nation.

Acknowledgements

G.M.B. acknowledges the School of Geosciences and Faculty ofScience at the Univ. of Witwatersrand for financial support as wellas the staff at the Department of Terrestrial Magnetism (CarnegieInstitute) for technical training and support during a pre-doctoralfellowship. C.F. Gower and B. Ryan are thanked for field expeditionassistance. L.D.A. acknowledges funding from the South African Na-tional Research Foundation. A Center of Excellence grant from theNorwegian Research Council to PGP funded T.B.A.

Appendix A. Supplementary information

Supplementary material related to this article can be found on-line at http://dx.doi.org/10.1016/j.epsl.2013.12.015.

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