Earth and Planetary Science Letters 401 (2014) 381–383
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Earth and Planetary Science Letters
www.elsevier.com/locate/epsl
Debating the petrogenesis of Proterozoic anorthosites –
Reply to comments by Vander Auwera et al. on “Pyroxene megacrysts
in Proterozoic anorthosites: Implications for tectonic 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
b
c
School of Geosciences, University of the Witwatersrand, Private Bag 3, Wits, 2050, South Africa
Department of Terrestrial Magnetism, Carnegie Institute for Science, 5142 Broad Branch Road NW, Washington, D.C., 20015, USA
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
Article history:
Received 2 May 2014
Accepted 4 June 2014
Editor: T.M. Harrison
We welcome the opportunity for discussion on the petrogenesis of Proterozoic anorthosites stimulated by a recent comment
by Vander Auwera et al. (2014). In their comment Vander Auwera et al. accept our new geochronologic and isotopic data on
comagmatic, high-aluminum orthopyroxene megacrysts (HAOMs)
and anorthosites (Bybee et al., 2014), but provide an alternate interpretation of the data, in line with their model of magma genesis
through melting of a mafic lower crust. In short, they suggest that
the similarity of the ages of the HAOMs and a potential lower
crustal source for the Rogaland Anorthosite Province (RAP) is evidence that the HAOMs crystallized at 1.05 Ga at the base of a thickened crust from the parent magma of the gabbronoritic Feda suite.
They suggest that this hypothesis is supported by overlapping Nd
and Pb isotopic compositions of the HAOMs and Feda suite. Vander Auwera et al.’s argument centers on their suggestion that
the HAOMs, observed in most Proterozoic anorthosites, are restitic
source material entrained in the melts from their proposed gabbronoritic source. This model relies on a two-stage petrogenesis,
wherein an underplating event forms lower crustal gabbronoritic
cumulates, followed approximately 100 million years later by a
lower crustal melting event that entrains HAOMs and then crystallizes vast amounts of plagioclase to form Proterozoic anorthosites.
We strongly disagree with this interpretation of our geochronologic
DOI of original article: http://dx.doi.org/10.1016/j.epsl.2013.12.015.
DOI of comment: http://dx.doi.org/10.1016/j.epsl.2014.06.031.
Corresponding author at: School of Geosciences, University of the Witwatersrand, Private Bag 3, Wits, 2050, South Africa. Tel.: +27 11 717 6633.
E-mail address: grant.bybee@wits.ac.za (G.M. Bybee).
*
http://dx.doi.org/10.1016/j.epsl.2014.06.032
0012-821X/© 2014 Elsevier B.V. All rights reserved.
and geochemical data for the reasons that are outlined in this reply.
The alternative that we proposed in Bybee et al. (2014) is that
comagmatic HAOMs crystallize from a depleted, mantle-derived
magma, which ponded at the Moho, allowing sufficient time for
accumulation of plagioclase. Crucial HAOM geochronology (combined with published anorthosite geochronology) indicates that
megacryst crystallization, lower crustal assimilation, polybaric ascent of plagioclase mushes through the crust and final emplacement at 5–10 km depths took between 100–130 million years,
thereby restricting the tectonic setting to long-lived environments
such as continental arcs (Bybee et al., 2014). In the subsequent
discussion we (1) establish points of agreement, and (2) evaluate
criteria that would support one or the other hypothesis, including
textural evidence preserved in the HOAMs, geochemical relationship of the high-Al HOAMs to other HOAMs, and the temperatures,
time frames and heat sources required by each model.
In some general principles, the two interpretations of these
anorthosite suites are not very different: both call on processes
at the Moho, both call on a convergent margin setting, both agree
that the HAOMs crystallized from magmas significantly older than
the anorthosites, both propose that the anorthosites rose diapirically at a much later stage than the crystallization of the HOAMs,
and both models operate in concert with extensive lower crust
melting. The chief disagreement centers on the petrogenesis of the
HAOMs and its interpretation for the anorthosites. In the Vander
Auwera et al. model, the HAOMs are related directly to the Feda
suite parental magmas based on isotopic compositions, but then
become restites of partial melting to produce high-alumina basaltic
parental magmas of the anorthosites some 120 Ma later. The issue
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G.M. Bybee et al. / Earth and Planetary Science Letters 401 (2014) 381–383
Fig. 1. a. Al2 O3 vs. calculated pressure (using the Al-in-orthopyroxene geobarometer) in suites of samples from the Rogaland Anorthosite Province, the Mealy Mountains
Intrusive Suite and the Nain Plutonic Suite. b. Mg# vs. calculated pressure (kbar) in the Rogaland Anorthosite Province (Norway). c. Al2 O3 content and Eu anomaly illustrating
that megacrysts formed at the same pressure (similar Al content) have variable Eu/Eu∗ .
reduces to whether these HAOMs represent preserved fragments of
a lower crustal source or are comagmatic cumulates. A problematic aspect of the Vander Auwera et al. model is how the HAOMs,
with their relatively evolved compositions could be restites and
especially how the HAOMs could retain such pristine, older ages
if they were. Furthermore, the Vander Auwera et al. model does
not take into account a source for the heat for these processes and
the importance of mass transfer across the Moho – two important
contributions of the Bybee et al. model.
A significant aspect of Vander Auwera et al.’s argument against
mantle-derived models is the observation that the HAOMs have
similar isotopic compositions to calc-alkaline magmas, manifest
at surface as the mafic Feda Suite (Vander Auwera et al., 2011)
and hypothetical lower crustal gabbronoritic equivalents. Although
they use this similarity in isotopic composition to propose that
the megacrysts are fragments of a lower crustal cumulate (complementary to the Feda Suite), these same observations could support
our model in which the megacrysts crystallized from a mantle derived, calc-alkaline magma formed in a subduction zone setting
at approximately the age we determined for HAOM crystallization
(1.041 Ga).
Vander Auwera et al.’s argument hinges on a suggestion that
HAOMs might represent restitic minerals of a lower crustal source.
One would predict, that these phases would display textures indicative of a melt-depleting event. To the best of our knowledge, no descriptions or discussions dealing with HAOMs in Proterozoic anorthosites (Dymek and Gromet, 1984; Emslie, 1975;
Morse, 1975; Wiebe, 1986; Xue and Morse, 1994) have documented any textural evidence for these minerals (and/or their
surrounding pegmatitic plagioclase) being restitic phases and displaying typical textural features of restites (as documented, for
example, by Vernon, 2004). All the aforementioned researchers
reach the same conclusion – all evidence is consistent with the
megacrysts crystallizing from magmas that were parental to the
anorthosites.
In contrast to Vander Auwera et al.’s restitic model, we provide geochemical evidence, supporting several previous studies
(Charlier et al., 2010; Emslie, 1975; Fram and Longhi, 1992),
showing that the all HAOMs are comagmatic with Proterozoic
anorthosites and therefore part of polybaric fractionation series.
Well-constrained Al-in-orthopyroxene geobarometry for HAOMs
from Proterozoic anorthosites indicates that Al2 O3 content is a reliable predictor of crystallization pressure of these HAOMs (Fig. 1a).
HAOMs crystallized at the highest pressure are the most primitive members of each suite (as indicated by their higher Mg#)
and as crystallization pressure decreases, the HAOMs become more
G.M. Bybee et al. / Earth and Planetary Science Letters 401 (2014) 381–383
evolved (Fig. 1b). An argument could be made that this covariation in Al2 O3 (vis. crystallization pressure) and MgO is indicative
of differentiation of source cumulates in a lower crustal chamber.
However, HAOMs with Al content greater than >8 wt% (i.e. those
that would have crystallized at approximately the same pressure),
show significant variation in Eu anomaly, with Eu/Eu∗ < 1 to > 1
(Fig. 1c), indicating that plagioclase joined the crystallizing assemblage during crystallization of these megacrysts. These variations
in Eu anomaly, at constant Al concentration, would not be plausible if the HAOM suite was simply a differentiated lower crustal
cumulate, not comagmatic with the anorthosites. This evidence
strengthens our assertions that HAOMs are comagmatic with the
Proterozoic anorthosites and crystallized from a magma ponded
at the Moho, undergoing fractional crystallization, and in which
plagioclase joins the crystallizing assemblage after some HAOM
crystallization has occurred.
In particular, very clear geochemical trends and correlations in
major and trace element geochemistry between higher and lower
pressure high-Al megacrysts in the RAP (Charlier et al., 2010),
in combination with geochemical modeling, show that the highalumina megacrysts are comagmatic and form part of a polybaric
fractionation sequence (Charlier et al., 2010). This result directly
supports our arguments.
One of the most problematic aspects of the lower crustal source
hypothesis are the thermal and geodynamic problems involved in
extensive melting of a gabbronoritic lower crust to yield highvolume, broadly basaltic magmas that could be parental to the
anorthosites. To contend with this, the commentators speculate
that lowermost crustal temperatures of 1390 ◦ C might be achieved
if a “conservative” geothermal gradient of 20 ◦ C/km can be linearly
extrapolated downward from the conditions of 1000 ◦ C at 0.75 GPa
(∼20–25 km) recorded in 1.01 Ga sapphirine granulites that outcrop near the Rogaland anorthosites (Drüppel et al., 2013). However, crustal geotherms are characteristically parabolic in shape,
and steepen dramatically as they approach the adiabatic gradients in the sub-lithospheric mantle. Therefore, lowermost crustal
temperatures should rarely exceed ∼1050 ◦ C, even if conditions
sufficient to produce sapphirine granulites are achieved at shallower depths.
Timing is also an issue in this model, as the required high
temperatures are constrained at 1.01 Ga, after which the terrane
must decompress isothermally. However, in the Vander Auwera et
al. model, melting to form parent magmas of anorthosite occurs
at approximately 930 Ma, after the terrane has isothermally decompressed to 0.5 GPa. It may not be appropriate to extrapolate
geothermal conditions at 1.01 Ga to lower crustal depths, as melting, in their model, occurs at a later time (as well as after terrane
decompression).
An ancillary, but important question, for the lower crustal
model is the lack of an adequate energy source to melt large volumes of the lower crust in order to generate the required voluminous magmas which ultimately crystallize Proterozoic anorthosite
massifs. The on- and off-shore surface area of the Rogaland
Anorthosite Province is ∼3500 km2 (Sigmond, 1992; Norges Geologiske Undersøkelse website, 2014), and assuming a conservative
thickness of 4 km, this massif would have a volume of 6712 km3 .
Given the notion that parental magmas of Proterozoic anorthosites
were broadly basaltic (Ashwal, 1993; Morse, 1982), approximately
40–50% of the mafic phases are missing. Consequently, one can infer that a complete melt would consist of ∼28 000 km3 of liquid.
Assuming 40% partial melting, the total volume of lower crust involved would be 70 000 km3 . To melt this significant volume of
crust, heat must be imported. The only reasonable mechanism to
induce the required volume of melting would be upwelling mantle.
If this were the case, we would expect to see equally, if not more
voluminous, coeval mafic melts (formed by decompression melting
383
of the upwelling mantle) together with the anorthosites – a feature which is simply not observed around Proterozoic anorthosite
massifs. The volume of melting required is a conservative estimate
given that the largest massifs have a surface area of ∼18 000 km3
and should be formed by the same processes (Ashwal, 2010). Furthermore, if the source material is not a cotectic composition, the
supporters of the lower crustal model would require near total
melting of a significant volume of the mafic lower crust to produce a broadly basaltic melt capable of forming anorthosites. In
any case, we conclude that extensive, dry, lower crustal melting
of mafic protoliths is unlikely to account for the generation of the
magmas parental to massif-type anorthosite, either in Norway or
elsewhere on Earth.
Acknowledgement
We thank Vander Auwera et al. for their interest in our new
data on Proterozoic anorthosites and the debate that their commentary has stimulated.
Editorial Acknowledgement
The Editor thanks the reviewer, Carol Frost, for her thoughtful
and balanced reviews of both Commentary and Reply.
References
Ashwal, L., 1993. Anorthosites. Springer-Verlag.
Ashwal, L.D., 2010. The temporality of anorthosites. Can. Mineral. 48, 711–728.
Bybee, G.M., Ashwal, L.D., Shirey, S.B., Horan, M., 2014. Pyroxene megacrysts in
Proterozoic anorthosites: implications for tectonic setting, magma source and
magmatic processes at the Moho. Earth Planet. Sci. Lett. 389, 74–85.
Charlier, B., Duchesne, J.C., Vander Auwera, J., Storme, J.Y., Maquil, R., Longhi,
J., 2010. Polybaric fractional crystallization of high-alumina basalt parental
magmas in the Egersund-Ogna massif-type Anorthosite (Rogaland, SW Norway) constrained by plagioclase and high-alumina orthopyroxene megacrysts.
J. Petrol. 51, 2515–2546.
Drüppel, K., Elsäber, L., Brandt, S., Gerdes, A., 2013. Sveconorwegian mid-crustal
ultrahigh-temperature metamorphism in Rogaland, Norway: U–Pb LA-ICP-MS
geochronology and pseudosections of Sapphirine granulites and associated
paragneisses. J. Petrol. 54, 305–350.
Dymek, R., Gromet, L., 1984. Nature and origin of orthopyroxene megacrysts from
the St-Urbain anorthosite massif, Quebec. Can. Mineral. 22, 297–326.
Emslie, R.F., 1975. Pyroxene megacrysts from anorthositic rocks: new clues to the
sources and evolution of the parent magmas. Can. Mineral. 13, 138–145.
Fram, M., Longhi, J., 1992. Phase-equilibria of dikes associated with Proterozoic
Anorthosite complexes. Am. Mineral. 77, 605–616.
Morse, S., 1975. Plagioclase Lamellae in Hypersthene, Tikkoatokhakh Bay, Labrador.
Earth Planet. Sci. Lett. 26, 331–336.
Morse, S.A., 1982. A partisan review of Proterozoic anorthosites. Am. Mineral. 67,
1087–1100.
Geologiske
Undersøkelse,
2014.
www.ngu.no/mineralforekomster/
Norges
bjerkreim-sokndal/English/the_rogaland_anorthosite_province.php. Date accessed – 1 July 2014.
Sigmond, E.M.O., 1992. Berggrunnskart, Norge med havområder – Målestokk 1:3
millioner (Bedrock map, Norway with Marine Areas – Scale 1:3 Million). Norges
Geologiske Undersøkelse.
Vander Auwera, J., Bolle, O., Bingen, B., Liégeois, J.P., Bogaerts, M., Duchesne, J.C.,
De Waele, B., Longhi, J., 2011. Sveconorwegian massif-type anorthosites and related granitoids result from post-collisional melting of a continental arc root.
Earth-Sci. Rev. 107, 375–397.
Vander Auwera, J., Charlier, B., Duchesne, J.C., Bingen, B., Longhi, J., Bolle,
O., 2014. Comment on Bybee et al. (2014): Pyroxene megacrysts in Proterozoic anorthosites: implications for tectonic setting, magma source and
magmatic processes at the Moho. Earth Planet. Sci. Lett. 401, 378–380.
http://dx.doi.org/10.1016/j.epsl.2014.06.031.
Vernon, R.H., 2004. A Practical Guide to Rock Microstructure. Cambridge University
Press.
Wiebe, R.A., 1986. Lower crustal cumulate nodules in Proterozoic dikes of the
Nain Complex: evidence for the origin of Proterozoic anorthosites. J. Petrol. 27,
1253–1275.
Xue, S., Morse, S., 1994. Chemical characteristics of Plagioclase and Pyroxene
Megacrysts and their significance to the Petrogenesis of the Nain Anorthosites.
Geochim. Cosmochim. Acta 58, 4317–4331.