Morphology and microstructure of chromite crystals in chromitites from the Merensky Reef (Bushveld Complex, South Africa)

The Merensky Reef of the Bushveld Complex consists of two chromitite layers separated by coarse-grained melanorite. Microstructural analysis of the chromitite layers using electron backscatter diffraction analysis (EBSD), high-resolution X-ray microtomography and crystal size distribution analyses distinguished two populations of chromite crystals: fine-grained idiomorphic and large silicate inclusion-bearing crystals. The lower chromitite layer contains both populations, whereas the upper contains only fine idiomorphic grains. Most of the inclusion-bearing chromites have characteristic amoeboidal shapes that have been previously explained as products of sintering of pre-existing smaller idiomorphic crystals. Two possible mechanisms have been proposed for sintering of chromite crystals: (1) amalgamation of a cluster of grains with the same original crystallographic orientation; and (2) sintering of randomly orientated crystals followed by annealing into a single grain. The EBSD data show no evidence for clusters of similarly oriented grains among the idiomorphic population, nor for earlier presence of idiomorphic subgrains spatially related to inclusions, and therefore are evidence against both of the proposed sintering mechanisms. Electron backscatter diffraction analysis maps show deformation-related misorientations and curved subgrain boundaries within the large, amoeboidal crystals, and absence of such features in the fine-grained population. Microstructures observed in the lower chromitite layer are interpreted as the result of deformation during compaction of the orthocumulate layers, and constitute evidence for the formation of the amoeboid morphologies at an early stage of consolidation. An alternative model is proposed whereby silicate inclusions are incorporated during maturation and recrystallisation of initially dendritic chromite crystals, formed as a result of supercooling during emplacement of the lower chromite layer against cooler anorthosite during the magma influx that formed the Merensky Reef. The upper chromite layer formed from a subsequent magma influx, and hence lacked a mechanism to form dendritic chromite. This accounts for the difference between the two layers.     

A study of the soil geochemistry of the Platreef in the Bushveld Complex, South Africa

A study of the soil profiles in the central Transvaal showed that in most areas a thick top layer of transported soils is present. Since this transported horizon has moved for considerable distances and because mixing took place in the process, the use of soil geochemistry in the search for hidden ore bodies tends to yield erratic results. In the present investigation an attempt was made to use volatile elements such as Hg, As, S and C as CO₂ as geochemical indicator elements and to establish whether epigenetic haloes of these elements are imprinted on the transported soils. The geochemical behaviour of these elements are compared with elements such as Ni, Co, Cu and Fe, which would tend to migrate with the transported soil.The geochemical study was done on the Platreef in the Bushveld Complex, north of Potgietersrus. The Platreef consists of a stratified horizon which contains appreciable amounts of chalcopyrite, pyrrhotite, pentlandite and platinum-group elements. Four traverses across the ore body were investigated and the results indicate that in cases where the soil cover is relatively thin, approximately 1 to 2 metres, all the elements studied can be used to locate the ore body. Those traverses where the soil cover is thicker and where transported soils are definitely present, only mercury yields significant anomalies, while siderophile elements give erratic and poor results. The mercury anomalies are usually not displaced and even tend to delineate mineralized sub-zones of the Platreef.Arsenic, sulphur and carbon measured as total CO₂, also give erratic results in the cases where a thick soil cover is present. The fact that arsenic tends to be fixed relatively easily in soils, either in laterite particles or in surface limestone, apparently is the prime reason for its erratic behaviour. The results indicate that both S and C would yield unsatisfactory results if these elements are measured as total S and C in the soils. The use of gas species may be more successful.A surprising but consistent result, was the tendency for mercury to be concentrated in the soils above faults, which intersect the ore body at depth. The behaviour of mercury in soils taken along the strike of a fault indicated that a logarithmic relationship exists between the mercury concentration in the soil and the depth of the ore body.     

Argon diffusion in plagioclase and implications for thermochronometry: A case study from the Bushveld Complex, South Africa

Plagioclase is not only the most abundant mineral in the Earth’s crust, but is present in almost all terrestrial tectonic settings and is widespread in most extraterrestrial material. Applying the K-Ar system to this common mineral would provide a powerful tool for quantifying thermal histories in a wide variety of settings. Nonetheless, plagioclase has rarely been used for thermochronometry, largely due to difficulties in simultaneously acquiring precise geochronologic data and quantifying argon diffusion kinetics from a mineral with low-K concentration. Here we describe an analytical technique that generates high-precision ⁴⁰Ar/³⁹Ar data and quantifies Ar diffusion kinetics of low-K minerals. We present results of five diffusion experiments conducted on single crystals of plagioclase from the Bushveld Complex, South Africa. The observed diffusion kinetics yield internally consistent thermochronological constraints, indicating that plagioclase is a reliable thermochronometer. Individual grains have activation energies of 155-178 kJ/mol and ln(D₀/a²) varies between 3.5 and 6.5. These diffusion parameters correspond to closure temperatures of 225-300 °C, for a 10 °C/Ma cooling rate. Age spectra generally conform to single-domain diffusive loss profiles, suggesting that grain-scale diffusion dominates argon transport in this fairly simple plagioclase. Conjointly examining several single-grain analyses enables us to distinguish episodic reheating from slow cooling and indicates that the Bushveld Complex cooled rapidly and monotonically from magmatic temperature to <300 °C over 3 Ma, followed by protracted cooling to ambient crustal temperatures of 150-200 °C over ∼600 Ma.     

Partial Melting of Metapelitic Rocks Beneath the Bushveld Complex, South Africa

Metapelitic rocks in the aureole beneath the Bushveld Complexpreserve evidence for both high- and low-a_H2O anatexis. Theaureole is characterized by an inverted thermal structure inwhich suprasolidus rocks potentially interacted with an H₂O-richvolatile phase derived from underlying, dehydrating rocks. Atlower grade (T < 700°C) the rocks contain fibrolite matsand seams that record local redistribution of volatiles. Incongruentreactions consuming biotite produced small quantities (<1mol %) of liquid and peritectic cordierite that remained trappedwithin the mesosome. Larger volumes of melt (3–4%), preservedas coarse-grained discordant leucosomes, were produced by congruentmelting following a structurally focused influx of H₂O. Subhorizontalvolatile-phase flow was concentrated within thin (     

Plagioclase content of cyclic units in the Bushveld Complex,South Africa

Contributions to Mineralogy and Petrology - The Upper Critical Zone of the Bushveld Complex, South Africa, has been divided into so-called cyclic units. Ideally, they should consist of (from the...     

Modeling the Merensky Reef,Bushveld Complex,Republic of South Africa

The Merensky pegmatoid (normal reef) in the western Bushveld Complex is commonly characterized as a pyroxene-rich pegmatoidal unit with a base that is enriched in chromite and platinum-group element-bearing sulfides overlying a leuconorite footwall. Models for its formation have ranged from those that view it as entirely a magmatic cumulate succession to those that have suggested that it is a zone of volatile-induced remelting. The consequences of the latter interpretation are investigated using the numerical modeling program IRIDIUM, which links diffusive and advective mass and heat transport with a phase equilibration routine based on the MELTS program. The initial system consists of a simple stratigraphic succession of a partially molten leuconorite overlain by a partially molten pyroxenite, both initially at 1,190°C and 2 kbar. 2 wt% of a volatile fluid composed of 75 mol% H₂O, 20 mol% CO₂ and 5 mol% H₂S is then added to the lower 20 cm of the pyroxenite. The system is then allowed to evolve under conditions of chemical diffusion in the liquid. The addition of the volatile components results in a modest increase in the amount of melt in the pyroxenite. However, chemical diffusion across the leuconorite–pyroxenite boundary leads to more extensive melting at and below the boundary with preferential loss of opx from the underlying leuconorite, preferential re-precipitation of sulfide and chromite and concentration of the PGE at this boundary. These results mimic actual mineral and compositional profiles across the Merensky pegmatoid and illustrate that long-term diffusion process can effectively produce mineralogical and compositional layering not present in the original assemblage.     

Compressive deformation in the floor rocks to the Bushveld Complex (South Africa): evidence from the Rustenburg Fault Zone

The north-northwest-south-southeast striking Rustenburg Fault Zone in the western Transvaal Basin, South Africa, has been extensively mapped in order to unravel its tectonic history. In post-Pretoria Group times, but before the intrusion of the Bushveld Complex at 2050 Ma, the area surrounding the fault zone was subjected to two compressive deformational events. The shortening direction of the first event was directed northeast-southwest, producing southeast-northwest trending folds, and the shortening direction of the second was directed north-northwest - south-southeast, producing east-northeast - west-southwest trending folds. The second set of folds refolded the first set to form typical transitional Type 1-Type 2 interference folding. This compression ultimately caused reactivation of the Rustenburg Fault, with dextral strike-slip movement displacing the Pretoria Group sediments by up to 10.6 km. The subsequent intrusion of the Bushveld Complex intensely recrystallised, and often ponded against the strata along the fault zone. The fault rocks within the fault zone were also recrystallised, destroying any pre-existing tectonic fabric. Locally, the fault zone may have been assimilated by the Bushveld Complex. After the intrusion of the Bushveld Complex, little movement has occurred along the fault, especially where the fault passes under areas occupied by the Bushveld Complex. It is thought that the crystallisation of the Bushveld Complex has rheologically strengthened the neighbouring strata, preventing them from being refaulted. This model is at variance with previous assumptions, which suggest that continuous regional extension during Pretoria Group sedimentation culminated in the intrusion of the Bushveld Complex.     

Geochemical exploration for platinum-group elements in the Bushveld Complex, South Africa

Analyses of stream sediment and soil samples from the Bushveld Complex, South Africa have revealed enhanced precious metal concentrations, which can be related both to mining activities and the presence of hidden concentrations of platinum-group elements (PGEs) and gold. The economically important PGE deposits hosted by the Upper Critical Zone of the Rustenburg Layered Suite are revealed by a high PGE and Au content in the overlying soils. A second zone of elevated precious metal concentrations straddles the boundary between the Main and Upper Zones and has to date been traced for more than 100 km. This zone follows the igneous layering of the Rustenburg Layered Suite and is offset by the Brits Graben. It is therefore thought to be the reflection of a magmatic PGE-Au mineralisation. Received: 31 May 1996 / Accepted: 7 January 1997     

Chromite in the Platreef (Bushveld Complex, South Africa): occurrence and evolution of its chemical composition

The regional distribution and chemical composition of massive and disseminated chromitites through a Platreef sequence and along a strike distance of over ∼20 km were investigated to correlate them both within the framework of the northern limb and to the eastern and western limbs of the Bushveld Complex. The chromitite layers and seams of the Platreef form two main chromite-bearing zones: the Upper Chromitite that occurs as two to three discontinuous seams in feldspathic pyroxenite at approximately 20 m below the Platreef top contact and the Lower Chromitite that is composed of multiple seams in feldspathic harzburgite, pyroxenite and norite close to the bottom contact of the Platreef with footwall. Electron micro-probe analyses reveal that the chemical composition of chromite depends on the host rock type. Norite and pyroxenite host chromite with the highest Cr₂O₃ content while harzburgite-hosted chromites are Cr and Mg poor. The wide range in chromite compositions is explained by the influence of late-magmatic processes including post-cumulus growth and re-equilibration, interaction with fluid- and sulphide-saturated magmatic liquid and contact metamorphism. Each of these processes is characterised by its own distinct geochemical signature, but generally they lead to a decrease in Mg and Al and an increase in both di- and tri-valent Fe in the chromite. The occurrence of chromitite locally on the different distance from the contact between the upper Platreef sills and the overlying Main Zone magma suggests erosion of the upper Platreef by the Main Zone as it was emplaced. The localisation of chromitites supports an independent development of the northern limb during the Critical Zone emplacement although the chemical composition of chromite and co-existing silicates from ultramafic rocks suggest a Critical Zone affinity with the eastern and western limbs of the Bushveld Complex.     

Trace elements in the Merensky Reef and adjacent norites Bushveld Complex South Africa

Trace elements were analysed in rocks and minerals from three sections across the Merensky Reef in the Rustenburg Platinum Mine in the Bushveld Complex of South Africa. Whole rocks and separated minerals were analysed by inductively coupled plasma-mass-spectrometer (ICP-MS) and in situ analyses were carried out by ion microprobe and by laser-source ICP-MS. Merensky Reef pyroxenites contain extremely high concentrations of a wide range of trace elements. These include elements incompatible with normal silicate minerals as well as siderophile and chalcophile elements. For major elements and compatible trace elements, the measured concentrations in cumulus phases and the bulk rock compositions are similar. For highly incompatible elements, however, concentrations in bulk rocks are far higher than those measured in the cumulus phases. In situ analyses of plagioclase have far lower concentrations of Th, Zr and rare earth elements than ICP-MS analyses of bulk separates of plagioclase, a difference that is attributed to the presence of trace-element-rich accessory phases in the bulk mineral separates. We used these data to calculate the trace-element composition of the magmas parental to the Merensky Unit and adjacent norites. We argue that there is no reason to assume that the amount of trapped liquid in the Merensky orthopyroxenite was far greater than in the norites and we found that the pyroxenite formed from a liquid with higher concentrations of incompatible trace elements than the liquid that formed the norites. We propose that the Bushveld Complex was fed by magma from a deeper magma chamber that had been progressively assimilating its crustal wall rocks. The magma that gave rise to the Merensky Unit was the more contaminated and unusually rich in incompatible trace elements, and when it entered the main Bushveld chamber it precipitated the unusual phases that characterize the Merensky Reef. The hybrid magma segregated sulphides or platinum-group-element-rich phases during the course of the contamination in the lower chamber. These phases accumulated following irruption into the main Bushveld chamber to form the Merensky ore deposits.     

Textures of orthopyroxenites from the Burgersfort bulge of the eastern Bushveld Complex, Republic of South Africa

A suite of 30 samples was collected from the Burgersfort bulge area of the eastern Bushveld Complex for a detailed quantitative textural study. The studied section represents a stratigraphic column approximately 1,000 m thick, of mainly orthopyroxenites from the lower zone (LZ) and lower critical zone. Crystal size distribution (CSD) plots and quantitative fabric analyses reveal a history of crystal aging and compaction, as evident from the loss of smaller-sized crystals and the development of foliation with little evidence of mineral lineation. Compared with the thicker section from Jagdlust (~80 km north), the LZ orthopyroxenites at Burgersfort are consistently finer grained. Foliation is equally well developed at both locations and is consistent with a relatively thin compaction zone at the top of the mush column. Grain size is largely a function of local cooling history with a grain size growth exponent estimated to be between 2.0 and 3.4.     

Origin of phlogopite-orthopyroxene inclusions in chromites from the Merensky Reef of the Bushveld Complex,South Africa

About 30% of the chromite grains of variable sizes in a chromitite seam at the base of the Merensky Reef of the Bushveld Complex on the farm Vlakfontein contain abundant composite mineral inclusions. The inclusions are polygonal to circular with radial cracks that protrude into the enclosing chromite. They vary from a few microns to several millimeters in diameter and are concentrated in the cores and mantles of chromite crystals. Electron backscattered patterns indicate that the host chromites are single crystals and not amalgamations of multiple grains. Na-phlogopite and orthopyroxene are most abundant in the inclusions. Edenitic hornblende, K-phlogopite, oligoclase and quartz are less abundant. Cl-rich apatite, rutile, zircon and chalcopyrite are present at trace levels. Na-phlogopite is unique to the inclusions; it has not been found elsewhere in the Bushveld Complex. Other minerals in the inclusions are also present in the matrix of the chromitite seam, but their compositions are different. The Mg/(Mg+Fe²⁺) ratios of orthopyroxene in the inclusions are slightly higher than those of orthopyroxene in the matrix. K-phlogopite in the inclusions contains more Na than in the matrix. The average compositions of the inclusions are characterized by high MgO (26 wt%), Na₂O (2.4 wt%) and H₂O (2.6 wt%), and low CaO (1.1 wt%) and FeO (4.4 wt%). The δ¹⁸O value of the trapped melt, estimated by analysis of inclusion-rich and inclusion-poor chromites, is ∼7‰. This value is consistent with the previous estimates for the Bushveld magma and with the δ¹⁸O values of silicate minerals throughout the reef. The textural features and peculiar chemical compositions are consistent with entrapment of orthopyroxene with variable amounts of volatile-rich melts during chromite crystallization. The volatile-rich melts are thought to have resulted from variable degrees of mixing between the magma on the floor of the chamber and Na-K-rich fluids expelled from the underlying crystal pile. The addition of fluid to the magma is thought to have caused dissolution of orthpyroxene, leaving the system saturated only in chromite. Both oxygen and hydrogen isotopic values are consistent with the involvement of a magmatic fluid in the process of fluid addition and orthopyroxene dissolution. Most of the Cr and Al in the inclusions was contributed through wall dissolution of the host chromite. Dissolution of minor rutile trapped along with orthopyroxene provided most of the Ti in the inclusions. The Na- and K-rich hydrous silicate minerals in the inclusions were formed during cooling by reaction between pyroxene and the trapped volatile-rich melts.     

An overview of the geology of the Transvaal Sequence and Bushveld Complex,South Africa

The Late Archaean-Early Proterozoic Transvaal Sequence is preserved within the Transvaal, Kanye and Griqualand West basins, with the 2050 Ma Bushveld Complex intrusive into the upper portion of the succession within the Transvaal basin. Both Transvaal and Bushveld rocks are extensively mineralized, the former containing large deposits of iron, manganese, asbestos, andalusite, gold, fluorine, lead, zinc and tin ores, and the latter some of the World's major occurrences of PGE, chromium and vanadium ores. Transvaal sedimentation began with thin, predominantly clastic sedimentary rocks (Black Reef-Vryburg Formations) which grade up into a thick package of carbonate rocks and BIF (Chuniespoort-Ghaap-Taupone Groups). These lithologies reflect a carbonate-BIF platform sequence which covered much of the Kaapvaal craton, in reaction to thermal subsidence above Ventersdorp-aged rift-related fault systems. An erosional hiatus was followed by deposition of the clastic sedimentary rocks and volcanics of the Pretoria-Postmasburg-Segwagwa Groups within the three basins, under largely closed-basin conditions. An uppermost predominantly volcanic succession (Rooiberg Group-Loskop Formation) is restricted to the Transvaal basin. A common continental rift setting is thought to have controlled Pretoria Group sedimentation, Rooiberg volcanism and the intrusion of the mafic rocks of the Rustenburg Layered Suite of the Bushveld Complex. The dipping sheets of the Rustenburg magmas cut across the upper Pretoria Group stratigraphy and lifted up the Rooiberg lithologies to form the roof to the complex. Subsequent granitic rocks of the Lebowa and Rashoop Suites of the Bushveld Complex intruded both upper Rustenburg rocks and the Rooiberg felsites.     

Platinum-group elements in the Boulder Bed,western Bushveld Complex,South Africa

Concentrations of platinum-group elements in samples from the Boulder Bed at five localities in the western Bushveld Complex range between 50 ppb and 70 ppm. Boulders thus have much more variable, and sometimes highly enriched, PGE contents relative to the other lithologies in the immediate foot-wall sequence of the Merensky Reef. The PGE enrichment can largely be modelled as a result of primary magmatic processes including collection of PGE by segregating sulphide melt and fractionation of mss. Other features of the Boulder Bed, such as the selvages of pure anorthosite and the chromitite stringers surrounding some of the boulders, bear evidence of recrystallisation. A model is proposed by which the Boulder Bed formed as a result of a combination of early and late magmatic processes. The PGEs were collected by magmatic sulphide melt which accumulated in a pyroxenite layer. The host rock to the pyroxenite was a thick package of norites which recrystallised in response to upward-migrating magmatic fluids. The fluids caused partial hydration melting of the norites adjacent to the pyroxenite, producing anorthosite. The boulders represent the broken-up remnants of the pyroxenite layer. The selvages of chromite and pure anorthosite around some of the boulders remain poorly understood, but may represent the latest recrystallisation event, in response to localised late-magmatic fluid overpressure upon cooling.     

Continuity between eastern and western Bushveld Complex, South Africa, confirmed by xenoliths from kimberlite

The three-dimensional shapes of mafic layered intrusions have to be inferred from surface outcrops, in some cases aided by drilling and/or geophysical data. However, geophysical models are often equivocal. For the 2.06?Ga Bushveld Complex of South Africa, early geological models proposed a shape of a single, gently inward dipping lopolith. Subsequent resistivity and gravity data were interpreted to suggest that the eastern and western limbs were discrete, dipping wedge-shaped intrusions separated by ~150?km. A more recent gravity model that takes crustal flexure into account allows continuity and the reversal to the original model. Distinguishing between these possibilities is difficult from surface-based studies because the central regions of the Complex are obscured by large volumes of younger granites and sedimentary/volcanic cover rocks. Here, we describe xenoliths from the Cretaceous Palmietgat kimberlite pipe, located mid-way between the exposed western and eastern lobes of the Complex. They are chromite-bearing feldspathic pyroxenites considered equivalent to those of the typical outcropping Critical Zone of the Bushveld Complex. This result provides strong support for a regionally interconnected Bushveld Complex, implying its emplacement as a single sill-like body. Confirming the continuity of the Bushveld Complex greatly expands exploration opportunities and implies that other layered mafic intrusions could have similar geometry.     

Pyroxenes of the Bushveld Intrusion, South Africa

New analyses are presented, for major, minor, and trace elements,of eleven Ca-rich pyroxenes, four bronzites, and two invertedpigeonites from the Bushveld layered basic intrusion. The twenty-threeanalyses now available are believed to represent the entireBushveld fractionation sequence. The Ca-rich pyroxene trendis from Ca_45.4Mg_49.6Fe_5.0 to Ca_42.7Mg_0.6Fe_56.8, the ferrohedenbergitesshowing no evidence of inversion from ferriferous ß-wollastonites.The Ca-poor pyroxene trend is from bronzite (Ca_2.8Mg_85.0Fe_12.2)through pigeonites to ferropigeonites (approximately Ca₃Mg₂₇Fe₅₅).All the pigeonitic pyroxenes have inverted to orthopyroxene. The compositional trends are remarkably similar to those ofthe Skaergaard pyroxene series, but the Bushveld sequence isthe most complete known for a single fractionated intrusion.The compositional and other variations of the pyroxenes, consideredtogether with those of the coexisting olivines and feldspars,leave little doubt that the Bushveld rocks originated by crystalaccumulation from a slowly cooled and fractionated intrusionof tholeiitic basalt magma. The slight but significant differences between the Bushveldand Skaergaard pyroxene trend characteristics can be explainedin terms of a displacement, in one intrusion as compared withthe other, of the liquidus and solidus surfaces relative tothe solvus and inversion surfaces in the system Wo—En—Fs.This may be due to minor differences in the initial magma compositionsof the two intrusions. Differences in the Mg/Fe ratios of Bushveldand Skaergaard coexisting pyroxene pairs are believed to bedue, at least in part, to the greater depth of the Bushveldmagma chamber. The Bushveld trends are briefly discussed in the light of recentexperimental studies on compositions within the Di-Hed-En-Fspyroxene quadrilate     

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