An electron microscopic study of some twinning and exsolution textures in microcline amazonites

An electron microscopic study of the twinning and exsolution textures in one hydrothermal and two pegmatitic amazonites from southern Norway is reported. From observations of three types of interfaces: between twins, between domains twinned on different laws and between Na- and K-feldspar domains, perthitic textures involving low albite lenses up to 10 μm long in a twinned microcline matrix are shown to result from exsolution from the triclinic phase. Complex twinning on albite and pericline laws is suggested to be the result of strains induced by exsolution. Other textures, involving the late introduction of Na at low temperature in a pegmatite and the occurrence of Pb-rich and plagioclase crystallites in a hydrothermal amazonite, are described.     

Strontium isotope disequilibrium of plagioclase in the Upper Critical Zone of the Bushveld Complex: evidence for mixing of crystal slurries

We report in situ Sr isotope data for plagioclase of the Bushveld Complex. We found disequilibrium Sr isotopic compositions on several scales, (1) between cores and rims of plagioclase grains in the Merensky pyroxenite, the Bastard anorthosite, and the UG1 unit and its noritic footwall, (2) between cores of different plagioclase grains within thin sections of anorthosite and pyroxenite of the Merensky unit, the footwall anorthosite of the Merensky reef and the footwall norite of the UG1 chromitite. The data are consistent with a model of co-accumulation of cumulus plagioclase grains that had crystallized from different magmas, followed by late-stage overgrowth of the cumulus grains in a residual liquid derived from a different level of the compacting cumulate pile. We propose that the rocks formed through slumping of semi-consolidated crystal slurries at the top of the Critical Zone during subsidence of the center of the intrusion. Slumping led to sorting of crystals based on density differences, resulting in a layered interval of pyroxenites, norites and anorthosites.     

Origin of the Pegmatitic Pyroxenite in the Merensky Unit, Bushveld Complex, South Africa

The genesis of the pegmatitic pyroxenite that often forms thebase of the Merensky Unit in the Bushveld Complex is re-examined.Large (>1 cm) orthopyroxene grains contain tricuspidate inclusionsof plagioclase, and chains and rings of chromite grains, whichare interpreted to have grown by reaction between small, primaryorthopyroxene grains and superheated liquid. This superheatedliquid may have been an added magma or be due to a pressurereduction as a result of lateral expansion of the chamber. Therewould then have been a period of non-accumulation of grains,permitting prolonged interaction with the crystal mush at thecrystal–liquid interface. Crystal ageing and grain enlargementof original orthopyroxene grains would ensue. Only after thepegmatitic pyroxenite had developed did another layer of chromiteand pyroxenite, with normal grain size, accumulate above it.Immiscible sulphide liquids formed with the second pyroxenite,but percolated down as a result of their density contrast, evenas far as the footwall anorthosite in some cases. Whole-rockabundances of incompatible trace elements in the pegmatiticpyroxenite are comparable with or lower than those of the overlyingpyroxenite, and so there is no evidence for addition and/ortrapping of large proportions of interstitial liquid, or ofan incompatible-element enriched liquid or fluid in the productionof the pegmatitic rock. Because of the coarse-grained natureof the rock, modal analysis, especially for minor minerals,is unreliable. Annealing has destroyed primary textures, suchthat petrographic studies should not be used in isolation todistinguish cumulus and intercumulus components. Geochemicaldata suggest that the Merensky pyroxenite (both pegmatitic andnon-pegmatitic) typically consists of about 70–80% cumulusorthopyroxene and 10–20% cumulus plagioclase, with a further10% of intercumulus minerals, and could be considered to bea heteradcumulate. KEY WORDS: Bushveld Complex; Merensky Reef; pegmatitic textures; cumulate processes; heteradcumulates; recrystallization; incompatible trace elements     

http://dx.doi.org/10.1016/j.gsf.2016.06.005

The formation of anorthosites in layered intrusions has remained one of petrology's most enduring enigmas. We have studied a sequence of layered chromitite, pyroxenite, norite and anorthosite overlying the UG2 chromitite in the Upper Critical Zone of the eastern Bushveld Complex at the Smokey Hills platinum mine. Layers show very strong medium to large scale lateral continuity, but abundant small scale irregularities and transgressive relationships. Particularly notable are irregular masses and seams of anorthosite that have intrusive relationships to their host rocks. An anorthosite layer locally transgresses several 10 s of metres into its footwall, forming what is referred to as a "pothole" in the Bushveld Complex. It is proposed that the anorthosites formed from plagioclase-rich crystal mushes that originally accumulated at or near the top of the cumulate pile. The slurries were mobilised during tectonism induced by chamber subsidence, a model that bears some similarity to that generally proposed for oceanic mass flows. The anorthosite slurries locally collapsed into pull-apart structures and injected their host rocks. The final step was down-dip drainage of Fe-rich intercumulus liquid, leaving behind anorthosite adcumulates.     

Geochemistry and petrogenesis of Archaean metavolcanic amphibolites from Fiskenaesset,S. W. Greenland

The meta-volcanic amphibolites closely associated with the Fiskenaesset anorthosite complex can be subdivided on the basis of trace element patterns or mineral chemistry; by far the most abundant type has light rare-earth element (LREE) depleted REE patterns and displays a wide range in trace element abundances. Chemically comparable amphibolites can be recognised throughout the ca. 2800 M.yr. West Greenland terrain.The geochemistry of the basaltic amphibolites is dominantly controlled by fractional crystallisation processes, although variable degrees of partial melting may also be important. The required crystal extract (plagioclase dominated) in the proposed fractionation scheme is very similar to the primary mineralogy of cumulates of the Fiskenaesset complex and trace element models support a genetic relationship between the anorthosite complex and enclosing host amphibolites.The application of trace element discrimination to assign tectonic environment in the Archaean is arguable. However, details of the trace element chemistry (especially chondritic La/Ta ratios) are taken to suggest, out of a range of likely tectonic environments, an ocean floor, rather than island arc, affinity for the Fiskenaesset amphibolites. The large ion lithophile (LIL) elements display erratic distribution patterns, but are generally enriched relative to the REE. This appears not to be related to high-grade metamorphism but may be a relict feature of seafloor alteration. The association of the cumulate sequence with meta-volcanic amphibolites and metasediments probably represents an ocean floor assemblage emplaced into the lower crust during crustal accretion.     

The significance of magmatic erosion for bifurcation of UG1 chromitite layers in the Bushveld Complex

One of the most puzzling features of the UG1 chromitite layers in the famous exposures at Dwars River, Eastern Bushveld Complex, is the bifurcation, i.e. convergence and divergence of layers along strike that isolate lenses of anorthosite. The bifurcations have been variously interpreted as resulting from: (1) the intermittent accumulation of plagioclase on the chamber floor as lenses, terminated by crystallization of continuous chromitite layers (the depositional model); (2) late-stage injections of chromite mush or chromite-saturated melt along anastomosing fractures that dismembered semi-consolidated plagioclase cumulates (the intrusive model); (3) post-depositional deformation of alternating plagioclase and chromite cumulates, resulting in local amalgamation of chromitite layers and anorthosite lenses that wedge out laterally (the deformational model). None of these hypotheses account satisfactorily for the following field observations: (a) wavy and scalloped contacts between anorthosite and chromitite layers; (b) abrupt lateral terminations of thin anorthosite layers within chromitite; (c) in situ anorthosite inclusions with highly irregular contacts and delicate wispy tails within chromitite; many of these inclusions are contiguous with footwall and hanging wall cumulates; (d) transported anorthosite fragments enclosed by chromitite; (e) disrupted anorthosite and chromitite layers overlain by planar chromitite; (f) protrusions of chromitite into underlying anorthosite; (g) merging of chromitite layers around anorthosite domes. We propose a novel hypothesis that envisages basal flows of new dense and superheated magma that resulted in intense thermo-chemical erosion of the temporary floor of the chamber. The melting and dissolution of anorthosite was patchy and commonly inhibited by chromitite layers, resulting in lens-like remnants of anorthosite resting on continuous layers of chromitite. On cooling, the magma crystallized chromite on the irregular chamber floor, draping the remnants of anorthosite and merging with pre-existing chromitite layers excavated by erosion. With further cooling, the magma crystallized chromite-bearing anorthosite. Emplacement of multiple pulses of magma led to repetition of this sequence of events, resulting in a complex package of anorthosite lenses and bifurcating chromitite layers. This hypothesis is the most satisfactory explanation for most of the features of this enigmatic igneous layering in the Bushveld Complex.     

Geochemistry and petrogenesis of the Fiskenaesset anorthosite complex,southern West Greenland: Nature of the parent magma

The petrogenesis of the Fiskenaesset anorthosite body has been investigated using major and trace element data for a large range of rock types from each zone of the complex. The chemistry of these ultramafic to anorthositic cumulates is interpreted in terms of crystal fractionation of a parental, trace element impoverished, tholeiitic magma, involving crystallisation of the cumulus phases olivine, orthopyroxene, clinopyroxene and (dominant) plagioclase feldspar. Amphibole appears not to have been a significant cumulus phase at any stage of crystallisation of the body, the abundant amphibole found in the rocks of the complex being produced by primary intercumulus crystallisation, supplemented by secondary metamorphic recrystallisation. Similarly, magnetite is unlikely to have been a significant early cumulus phase, although, together with chromite, it crystallised as a cumulus phase at high stratigraphic levels in the complex. The metamorphism appears to be largely isochemical, although sub-solidus metamorphic re-equilibration of the REE can be demonstrated on a grain-size scale.The spatial and temporal association between the anorthosite complex and the bordering metavolcanic amphibolites is matched by a strong similarity between the observed trace element chemistry of the amphibolites and the trace element chemistry of calculated successive liquids for the complex. This is taken to suggest a genetic relationship between the evolution of the anorthosite complex and enclosing amphibolites. The presence of trace element impoverished amphibolites (which are not cumulates) with trace element abundances comparable to those of the suggested parental liquid to the anorthosite complex, is used to derive a major element composition for the primary Fiskenasset magma. This composition approximates a moderately aluminous tholeiitic basalt, which may have been generated by hydrous fusion of previously depleted mantle. This primary magma underwent crystal fractionation under low pressure conditions, allowing the development of extensive plagioclase cumulates.The Fiskenaesset anorthosite, and similar bodies, cannot represent a cumulate residue complementary to the enclosing voluminous tonalitic gneisses, which have a calc-alkaline chemistry controlled by high pressure crystal liquid fractionation. Rather, the association between the cumulate layered complex and bordering supracrustal sequence may imply an ancient ocean crust analogue for the development of this component of Archaean high-grade terrains. It is suggested that slices of such Archaean ocean floor may be emplaced laterally into the base of the continental crust during subduction of oceanic lithosphere at Cordilleran type continental margins.     

Composition of plagioclase feldspar in modern deep-sea sands: relationship to tectonic setting

Microprobe analysis of plagioclase in deep-sea sands from various tectonic settings gives averages ranging from 19% An for trailing-edge continental margins to 56% An for basins on the ocean side of an island arc. For each setting except island arcs, anomalously high amounts of pure albite are found. These albitic grains may be recycled from pre-existing sedimentary rocks. Potassium-rich plagioclase, thought to be indicative of a volcanic provenance, is less abundant than would be expected from the proportion of volcanic rock fragments. Perhaps such high-K plagioclases are less resistant to weathering than low-K plagioclase.     

Multiple,isotopically heterogeneous plagioclase populations in the Bushveld Complex suggest mush intrusion

The Bushveld Complex and other layered intrusions show significant initial isotopic heterogeneity, both between and within co-existing cumulate minerals. Various processes have been proposed to account for this, including (i) intrusion of variably contaminated crystal mushes from deeper staging chambers, (ii) blending of semi-consolidated crystal mushes as a result of subsidence during cooling, (iii) variable infiltration of contaminants into a partially solidified crystal mush, (iv) density-driven mixing of minerals from isotopically distinct magma pulses, (v) contamination of crystals at the roof of the intrusion and mechanical incorporation of such contaminated crystals into the lower crystallisation front as a result of gravitational instability at the upper crystallisation front, and (vi) late-stage metasomatic processes. In order to assess the likely process(es) responsible for initial isotopic heterogeneities within the Bushveld Complex, we analysed core and rim domains of 12 plagioclase crystals from the Main and Upper zones of the Bushveld Complex for their Sr-isotopic compositions. The data show the presence of multiple, isotopically heterogeneous populations of plagioclase occurring within the same rocks. The data presented here are best explained through the intrusion of variably contaminated crystal mushes derived from a sub-compartmentalized, sub-Bushveld staging chamber that underwent different degrees of contamination with crustal rocks of the Kaapvaal craton.     

Sense of shear in high-temperature movement zones from the fabric asymmetry of plagioclase feldspars

A new method for determining the sense of shear in plagioclase-bearing tectonites from the (010) orientation of plagioclase feldspar is presented. The method is based on the asymmetry of the (010) plane with respect to the structural frame (foliation and lineation) and the dominant activity of the (010) slip plane in the high-temperature plasticity of plagioclase feldspar. Using examples from the Zabargad gneisses (Red Sea) the method is applied to plagioclases of An25–An45 and compared with other methods of shear-sense determination (quartz c-axis fabrics and microstructural criteria).     

Petrochemistry and mineralogy of early Precambrian anorthositic rocks of the Limpopo belt, southern Africa

Two sections of the anorthosite ‘complex’ were examined at Messina, South Africa and at Pikwe, Botswana. Thirty XRF whole-rock analyses of samples in stratigraphic order show that alkalies at Messina increase upwards from leuco-gabbros to anorthosites, but no such correlation was found at Pikwe. Electron probe analyses of plagioclases in 33 samples indicate extensive normal zoning and variation (An₃₁-An₈₀) especially at Pikwe. The highest An-contents tend to vary monotonically with stratigraphic position, whereas the lowest values are erratic. Twenty-two amphibole analyses indicate that SiO₂, TiO₂ and MgO/FeO increase, decrease and increase, respectively, with height at Messina, but 22 analyses from Pikwe show no such trends. Their compositions are similar to those from the Fiskenaesset complex. Aluminous chromites are Fe-rich, also like the Fiskenaesset ones.

The Limpopo anorthositic rocks belong to a layered igneous body that was recrystallized by regional metamorphism and subjected to erratic metasomatism. The original Limpopo and Fiskenaesset bodies strongly resemble gabbroic and peridotitic relics in the Peninsular Ranges Batholith in S. California.     

Geochemical modelling of the Chilas Complex in the Kohistan Terrane,northern Pakistan

The Chilas Complex in the Kohistan Terrane, Pakistan, is a huge basic intrusion, about 300 km long and up to 40 km wide, which is regarded as tilted island-arc type crust. It has been interpreted as the magma chamber root zone of the Kohistan Island Arc. The Chilas Complex is composed mainly of gabbronorite (main facies) and several masses of ultramafic–mafic–anorthosite (UMA) association. The UMA association consists mainly of olivine-dominant cumulate (dunite, wehrlite, lherzolite) and plagioclase-dominant cumulate (troctolite, olivine gabbro, gabbronorite, anorthosite), with minor amount of pyroxene-dominant cumulate (clinopyroxenite, websterite).The major element geochemistry of the gabbronorite (main facies) and rocks of the UMA association, plotted on Harker diagrams, are explained by a cumulate and a non-cumulate model, respectively. Namely, the UMA association is explained as variable crystal cumulates from a primary magma and the gabbronorite of the main facies is explained as due to the fractionation of the residual melt. Chemical variations of major, trace and rare earth elements for the gabbronorite of the main facies in the Chilas Complex are explained by fractional crystallization and accumulation of plagioclase, orthopyroxene and clinopyroxene from the residual melt of the primary magma.     

Direct electron-microscopic proof of domain texture in orthoclase (KAlSi3O8)

A submicroscopic domain texture consisting of small lamellae twinned after a combination of albite and pericline laws (crosshatch), closely resembling the domain texture of optically monoclinic adularia, occurs in two orthoclases from Japan. It is concluded from the electron transmission and -diffraction data that these materials, though optically nearly monoclinic, are composed of triclinic domains and that they originally grew as sanidine crystals.     

Comment on “Geochemistry and petrogenesis of the Fiskenaesset anorthosite complex,southern West Greenland: Nature of the parent magma” by B. L. Weaver,J. Tarney and B. Windley

Evidence that REE have been metamorphically redistributed between plagioclase feldspar and mafic minerals, as suggested by Weaveret al. (1981) is not forthcoming from the six Fiskenaesset rocks used in the study. REE patterns of the separated feldspars and of two rocks containing small modal amounts of mafic minerals are consistent with a light REE-enriched magma during the formation of the Fiskenaesset Complex.     

Pipes of mafic pegmatite in the stratiform Fiskenaesset anorthosite complex, southwest Greenland

Pipes of hornblende gabbro and ultramafite cut across cumulate layering in the lower part of the Fiskenaesset complex. The pipes form irregular networks in which all vertical portions are massive and pegmatitic, whereas some horizontal portions are layered. Two kinds of layering occur: mineral-graded layers formed by precipitation of plagioclase and hornblende, and comb layers formed by the upward growth of tabular hornblende crystals. Alternations of these layers show that magma with plagioclase phenocrysts was flushed through the pipes in pulses, perhaps associated with erratic propagation of cracks and magmatic drilling.     

Large-scale magmatic layering in the Main Zone of the Bushveld Complex and episodic downward magma infiltration

The Bellevue drillcore intersects ~3 km of Main and Upper Zone cumulates in the Northern Limb of the Bushveld Complex. Main Zone cumulates are predominately gabbronorites, with localized layers of pyroxenite and anorthosite. Some previous workers, using bulk rock major, trace and isotopic compositions, have suggested that the Main Zone crystallized predominantly from a single pulse of magma. However, density measurements throughout the Bellevue drillcore reveal intervals that show up-section increases in bulk rock density, which are difficult to explain by crystallization from a single batch of magma. Wavelet analysis of the density data suggests that these intervals occur on length-scales of ~40 to ~170 m, thus defining a scale of layering not previously described in the Bushveld Complex. Upward increases in density in the Main Zone correspond to upward increases in modal pyroxene, producing intervals that grade from a basal anorthosite (with 5% pyroxene) to gabbronorite (with 30–40% pyroxene). We examined the textures and mineral compositions of a ~40 m thick interval showing upwardly increasing density to establish how this type of layering formed. Plagioclase generally forms euhedral laths, while orthopyroxene is interstitial in texture and commonly envelops finer-grained and embayed plagioclase grains. Minor interstitial clinopyroxene was the final phase to crystallize from the magma. Plagioclase compositions show negligible change up-section (average An₆₂), with local reverse zoning at the rims of cumulus laths (average increase of 2 mol%). In contrast, interstitial orthopyroxene compositions become more primitive up-section, from Mg# 57 to Mg# 63. Clinopyroxene similarly shows an up-section increase in Mg#. Pyroxene compositions record the primary magmatic signature of the melt at the time of crystallization and are not an artefact of the trapped liquid shift effect. Combined, the textures and decoupled mineral compositions indicate that the upward density increase is produced by the downward infiltration of noritic magma into a previously emplaced plagioclase-rich crystal mush. Fresh noritic magma soaked down into the crystallizing anorthositic mush, partially dissolving plagioclase laths and assimilating Fe-enriched pore melt. The presence of multiple cycles showing upward increases in density in the Bellevue drillcore suggests that downward magma infiltration occurred episodically during crystallization of the Main Zone.     

Microtextural characterisation of the Lower Zone in the western limb of the Bushveld Complex,South Africa: evidence for extensive melt migration within a sill complex

The Lower Zone of the Bushveld Complex comprises an up to 2-km-thick package of different ultramafic rock types with an approx. 90-cm-thick, sulphide-bearing noritic interval that occurs in the western and eastern limbs. The distribution and geometry of the zone are highly variable across the Complex, showing pronounced, yet laterally discontinuous layering on different scales. Together with the ubiquitous lack of large-scale fractionation in the Mg# of orthopyroxene, variable Sr isotope compositions and erratic Pt/Pd ratios, these observations strongly suggest an emplacement of the Lower Zone as a sill complex, as these contrasting geochemical characteristics are difficult to account for in a large Bushveld magma chamber, as previously suggested. It is more likely that these sills were episodically fed from a sub-Bushveld staging chamber, and variably contaminated, while passing through the crust before their final emplacement in the Lower Zone. Detailed mineralogical and microtextural work based on high-resolution elemental mapping of a set of samples, covering the entire Lower Zone stratigraphy of the western Bushveld shows that the variations in the late crystallising interstitial mineral mode are different from what would be expect, if all phases crystallised from a fixed initial mass of interstitial liquid. The interstitial mineral mode, represented by plagioclase, clinopyroxene and other late stage phases, shows variable ratios of these minerals ranging from ca. 21:15:64 to 75:17:8. In comparison to modelled expected ratios, most of the analysed rocks have higher amounts of early crystallising interstitial phases (e.g. plagioclase, clinopyroxene), relative to late crystallising phases (e.g. quartz, alkali feldspar). Therefore, interstitial melt must have migrated at different stages of fractionation during cumulate solidification, as a consequence of either compaction or displacement by convecting interstitial liquids. Two samples, however, show the opposite: late phases are relatively more abundant than early ones, which is consistent with a convection-driven replacement of primitive interstitial liquid by more evolved liquid. These results have important implications for the interpretation of the Lower Zone and, by extension, for layered intrusions in general: (1) interstitial sulphide mineralisation may be introduced into a cumulate through infiltrating melts, i.e. the liquid components of a sulphur-saturated crystal mush are not withheld from further migration, upon interaction with a cumulate pile; (2) most importantly, late stage minerals, such as zircon, rarely crystallise from trapped liquid that was initially in equilibrium with the cumulate. Therefore, dating of interstitial zircon from cumulates is unlikely to record the actual timing of emplacement, but merely the crystallisation of a later episode of residual melt that migrated through the cumulate.     

Grain-size variations of cumulus plagioclase in the upper critical zone of the Bushveld Complex

Summary Due to the slow equilibration rate of feldspar, its zoning pattern is likely to be of primary origin. Initial studies of zoning patterns of cumulus feldspar within the interval between the UG2 chromitite and the Merensky Reef have shown postcumulus growth to affect only the outermost rims of grains. Therefore, present-day grain sizes of plagioclase are considered to resemble the original cumulus grain sizes. A correlation between grain size of plagioclase and its composition and zoning pattern has been established: larger, complexly zoned grains correlate with more calcic compositions. It is inferred that the residence time of neutrally buoyant plagioclases within a periodically replenished host liquid determined their size and zoning pattern. Older and more calcic grains are larger and more complexly zoned, whereas finer grain sizes, which are associated with relatively primitive (Mg-rich) orthopyroxenes, are the result of partial resorption of plagioclase. Grain sizes of plagioclase, furthermore, show regional variation: grains are larger in the vicinity of Union Section than in the southeastern parts of the Western Bushveld Complex, which is interpreted as a consequence of the increasing distance from a putative feeder zone located near Union Section.

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Variationen in der Korngröße von Kumulus-Plagioklas in der Upper Critical Zone des Bushveld Komplexes

Zusammenfassung Aufgrund der hohen Reaktionsträgheit von Feldspat ist dessen Zonierung höchstwahrscheinlich primärer Natur. Einführende Untersuchungen über Zonierungsmuster in Kumulus-Plagioklas im Interval zwischen der UG2 Chromitit-Lage und dem Merensky-Reef zeigten, daß Postkumulus-Wachstum nur den äußeren Rand der einzelnen Körner kennzeichnet. Demzufolge wird angenommen, daß die hier beobachteten Korngrößen den ursprünglichen Kumulus-Korngrößen entsprechen. Eine Korrelation zwischen der Korngröße von Plagioklasen und deren Chemismus und Zonierungs-muster konnte etabliert werden: größere, komplex zonierte Plagioklase haben einen höheren Anorthit-Gehalt. Diese Beziehung erklärt sich aus der relativ niedrigen Dichte von Plagioklas, die ein gravitatives Absinken verhindert. Demzufolge wurden Korngröße und Zonierungsmuster der in Schwebe befindlichen Plagioklase von der Verweildaner innerhalb einer sich periodisch ernenernden Schmelze bestimmt. Ältere, Ca-reiche Plagioklase sind relativ groß und komplex zoniert, während feinere Korngrößen, die zusammen mit relativ primitiven (Mg-reichen) Orthopyroxenen auftreten, das Ergebnis partieller Assimilation sind. Regionale Unterschiede existieren insofern, als daß Plagioklas in der Nähe einer postulierten Magmen-Zufuhrzone im Bereich von Union Section grobkörniger ist als in den südöstlichen Bereichen des westlichen Bushveld Komplexes.

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With 9 Figures     

Tectonic deformation and recrystallisation of oligoclase

Naturally deformed and partially recrystallised oligoclase porphyroclasts were studied in a high voltage electron microscope. The oligoclases had a highly deformed outer mantle zone, which contained high densities of dislocations and albite and pericline deformation twins, and less deformed cores containing fewer dislocations and albite twins but no pericline deformation twins. There was little evidence for recovery; apparently the internal strain energy due to deformation was relieved by recrystallisation. Strain free nuclei developed in areas with the highest defect densities. The resultant new grains had a lower anorthite content than their parents. It is suggested that the recrystallisation processes were aided by strain enhanced diffusion.     

Anorthosite formation by plagioclase flotation in ferrobasalt and implications for the lunar crust

The Sept Iles layered intrusion (Quebec, Canada) is dominated by a basal Layered Series made up of troctolites and gabbros, and by anorthosites occurring (1) at the roof of the magma chamber (100-500 m-thick) and (2) as cm- to m-size blocks in gabbros of the Layered Series. Anorthosite rocks are made up of plagioclase, with minor clinopyroxene, olivine and Fe-Ti oxide minerals. Plagioclase displays a very restricted range of compositions for major elements (An₆₈-An₆₀), trace elements (Sr: 1023-1071 ppm; Ba: 132-172 ppm) and Sr isotopic ratios (⁸⁷Sr/⁸⁶Sr_i: 0.70356-0.70379). This compositional range is identical to that observed in troctolites, the most primitive cumulates of the Layered Series, whereas plagioclase in layered gabbros is more evolved (An₆₀-An₃₈). The origin of Sept Iles anorthosites has been investigated by calculating the density of plagioclase and that of the evolving melts. The density of the FeO-rich tholeiitic basalt parent magma first increased from 2.70 to 2.75 g/cm³ during early fractionation of troctolites and then decreased continuously to 2.16 g/cm³ with fractionation of Fe-Ti oxide-bearing gabbros. Plagioclase (An₆₉-An₆₀) was initially positively buoyant and partly accumulated at the top of the magma chamber to form the roof anorthosite. With further differentiation, plagioclase (<An₆₀) became negatively buoyant and anorthosite stopped forming. Blocks of anorthosite (autoliths) even fell downward to the basal cumulate pile. The presence of positively buoyant plagioclase in basal troctolites is explained by the low efficiency of plagioclase flotation due to crystallization at the floor and/or minor plagioclase nucleation within the main magma body. Dense mafic minerals of the roof anorthosite are shown to have crystallized from the interstitial liquid.The processes related to floating and sinking of plagioclase in a large and shallow layered intrusion serve as a proxy to refine the crystallization model of the lunar magma ocean and explain the vertically stratified structure of the lunar crust, with (gabbro-)noritic rocks at the base and anorthositic rocks at the top. We propose that the lunar crust mainly crystallized bottom-up. This basal crystallization formed a mafic lower crust that might have a geochemical signature similar to the magnesian-suite without KREEP contamination, while flotation of some plagioclase grains produced ferroan anorthosites in the upper crust.