High-pressure megacrysts and lower crustal contamination: probing a mantle source for Proterozoic massif-type anorthosites

Bybee, Grant Michael
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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 4 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 5 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.