High-pressure megacrysts and lower crustal contamination: probing a mantle source for Proterozoic massif-type anorthosites
Date
2014-03-05
Authors
Bybee, Grant Michael
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Abstract
Many aspects of Proterozoic massif-type anorthosite petrogenesis have been, and remain, controversial. Mafic lower crust
and depleted mantle have both been proposed as mutually exclusive sources of these near-monomineralic, temporally
restricted batholiths. The debate surrounding the magma source has also led to uncertainty regarding the tectonic setting of
these massifs, with a range of possibilities including convergent, divergent and anorogenic settings. The dramatic
geochemical effects of crustal contamination in these massifs are well known and strong crustal signatures are evident in
most, if not all, Proterozoic anorthosite massifs. The source debate, in the simplest sense, reduces to whether the ubiquitous
crustal signature is derived principally from melting of a lower crust or is an effect of crustal assimilation. The origin of this
crustal signature, and whether it obscures the original isotopic composition of the magmas or not, has fuelled the debate
surrounding the source of the anorthosites. Using major element, trace element and isotopic compositions, as well as energyconstrained
assimilation-fractional-crystallisation (EC-AFC) modelling from samples representing various stages of the
polybaric crystallisation history of the magmas, including high-pressure megacrysts, anorthosites and their internal mineral
phases, I remove the obfuscating effects of possible crustal contamination and probe the source of the magmas. In order to
assess the effects of crustal contamination, if any, anorthosites from three massifs – the Mealy Mountains Intrusive Suite,
Nain Plutonic Suite (both in eastern Canada) and Rogaland Anorthosite Province (Norway), have been analysed – all of
which intrude into crust of significantly different age and chemical character.
Sm-Nd geochronology of high-Al, high-pressure orthopyroxene megacrysts, as well as the comagmatic, host anorthosites,
indicate that the magmatic system is long-lived, with an age difference between the megacrysts and hosts of ~110-130
million years. Isotopic compositions of primitive megacrysts qualitatively show that the magmas were derived from melting
of the depleted mantle. Strong links between the isotopic offset from depleted mantle evolution and the age and composition
of the surrounding crust confirm that the geochemical nature of the crustal contaminant plays a significant role in the
petrogenesis of the anorthositic rocks. The geochronological indications of a long-lived magmatic system point to
Proterozoic anorthosite formation in a continental magmatic arc – one of the only environments capable of supplying
geographically-localised magma and heat to the base of the crust for over 100 million years. Proposed divergent or
‘anorogenic’ settings could not plausibly supply magma to the base of the crust for over 100 m.y. without initiating ocean
formation or continental break-up. Anorthosite emplacement at mid-crustal levels may coincide with late- to post-orogenic
events in several terranes, but evidence presented for a long-lived magmatic system is incongruent with this proposed
setting. In this thesis, I propose that the petrogenesis of these intrusives must span both orogenic and post-orogenic periods.
An overlap in megacryst crystallisation age with the onset of calc-alkaline orogenic magmatism in the Sveconorwegian
Orogen, both occuring ~100 m.y. before anorthosite emplacement, confirms that initial magma and megacryst formation
coincides with the main phase of magmatic and orogenic activity in a convergent magmatic arc. These geochronological
constraints have implications for regional geodynamics in the Sveconorwegian Orogen (and the Labrador region) with the
evidence providing corroboratory support for a long-lived accretionary orogen, as opposed to the widely-held view that the
Sveconorwegian orogeny was predominantly collisional.
Compositions of high-pressure megacrysts, anorthosites and analysis of internal isotopic disequilibrium indicates that lower
crustal contamination has a significant influence on the isotopic composition of the rocks, with relatively minor
contributions from the mid- to upper crust. Energy-constrained AFC modelling confirms that significant lower crustal
contamination occurs during ponding of magmas at the Moho and is able to reproduce the observed isochronous isotopic
compositions of the megacrysts as well as the compositions of the host anorthosites. Evidence of varying degrees of internal
isotopic disequilibrium reinforces the significant role that assimilation of crust of different age and chemical nature have on
the compositions of Proterozoic anorthosites. Unexpected patterns of isotopic disequilibrium show that anorthosite
petrogenesis is not a “simple” case of progressive crustal contamination during polybaric ascent of viscous, partially-molten
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magma mushes, but is more likely to involve significant differentiation and solidification at lower crust depths, followed by
ascent of high-crystallinity bodies (> 50 % crystallinity) to upper crustal levels.
Although the composition of the bulk continental crust is different to plagioclase-rich Proterozoic anorthosites, both are
missing a mafic component. It is unclear how this missing mafic component was generated in the continental crust, because
most of the evidence for these crustal differentiation processes is sequestered below or near the Moho. However, Proterozoic
anorthosites, formed by viscous, plagioclase-rich mushes, entrain rare cumulate megacrysts from these depths and
consequently preserve evidence of magmatic differentiation processes at the Moho. The evidence for the formation and
sequestration of dense ultramafic cumulates in ponding magmas at the Moho can not only explain the missing mafic
component in Proterozoic anorthosites, but also suggests that cumulate formation in crust-forming, arc environments is a
significant process and should be taken into account in models dealing with evolution and differentiation of the continental
crust.
Sampling and petrographic and geochemical analysis of five pegmatitic segregations, or “pods”, from anorthosites of the
Mealy Mountains Intrusive Suite reveal a diverse range of compositions from mafic, Fe-rich and Si-poor, to Fe-poor and Sirich
felsic compositions and from monzogranite through quartz-monzodiorite and monzodiorite to Fe-P-rich gabbronorite.
Each pod shows a range of noteworthy graphic, myrmekitic and symplectic textures on a variety of scales, along with
distinctive mineralogical assemblages and highly-enriched trace element compositions. Derivitive minerals (e.g. apatite and
zircon), high concentrations of Fe, Ti, P (and in some cases SiO2) and 10-1000 times chondrite enrichment suggest that
many of the pods are highly fractionated. U-Pb zircon geochronology reveals that all the pods are the same age as the
anorthositic hosts and confirms that the Mealy Mountains Intrusive Suite was emplaced between 1654 and 1628 Ma. Using
the aforementioned evidence, I show that the pods represent the fluid-bearing, late-stage crystallisation products of a residual
liquid in the massif anorthosite system and provide a window into the final stages of crystallisation in the anorthosite system.
A range of rock types (monzonites, monzonorites, ferrodiorites and jotunites) observed in similar pod-like structures, as well
as dykes and plutons, have also been documented in other Proterozoic anorthosite massifs. These have, at one time or
another, controversially been interpreted as the residual liquids of anorthosite crystallisation. The observation of in-situ pods
with similar compositions to all of the aforementioned rock types and displaying textures indicative of late-stage
crystallisation support the notion that these associated lithologic units are comagmatic with, but residual to, the anorthosites
and are not residual liquids of other crustally-derived rocks, immiscible liquids, parental magmas or cumulates. Isotopic
compositions of these highly-fractionated, late-stage pods also overlap with those of anorthosites, lending further evidence to
the case that upper crustal contamination plays only a minor role in developing the chemical signature of the anorthosites.
With these results I propose that the nature/composition of the residual liquids of Proterozoic anorthosite magmas can vary
dramatically, depending on geochemical differences in the original magma pulses and by mixing of mobilised,
independently-evolved segregations of residual liquids. This process could explain why so many varied rock types
associated with Proterozoic anorthosites have been suggested as residual liquids: these rocks all represent residual liquids
resulting from varying degrees of differentiation, subsequent mobilisation, mixing and final solidification as plutons or
dykes.
Proterozoic anorthosite petrogenesis is an inherently polybaric process and so by its very nature produces a range of
complicated and contradictory features which have clouded interpretation of numerous aspects of the rocks formation. In
analysing crystallisation products from numerous stages of the anorthosites polybaric history, I have been able to probe the
magmatic processes operating at different stages of Proterozoic anorthosite petrogenesis. In doing so I show that the magmas
are derived from melting of the depleted mantle in continental-arc-like settings – two controversial aspects of Proterozoic
anorthosite petrogenesis. These constraints on the source and tectonic setting will allow renewed investigation into the
ultimate question surrounding Proterozoic anorthosites: why are these rock types restricted to the Proterozoic and what clues
does this temporal restriction offer about Earth’s geodynamic evolution during this period? The assertion in this thesis that
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Proterozoic anorthosites formed in arc environments dictates that subduction processes or geodynamic conditions during the
Proterozoic favoured the production of voluminous masses of plagioclase, because modern-day magmatic arc terranes show
no evidence of anorthosites with similar compositions. However, calcic anorthositic inclusions and xenoliths are observed in
modern-day volcanic and continental arcs suggesting that anorthosites may be forming in these environments, but that
conditions such as water content or style of subduction are different to the Proterozoic, producing less and compositionally
different plagioclase and anorthosite. The results of this thesis shed new light on and refine the petrogenesis of Proterozoic
anorthosites, but the focus of research must now shift to explaining the temporal restriction of these intrusions and the
implications of this restriction for the geodynamic evolution on Earth during the Proterozoic.