Formation of transgressive anorthosite seams in the Bushveld Complex via tectonically induced mobilisation of plagioclase-rich crystal mushes

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.

Formation of anorthosite on the Moon through magma ocean fractional crystallization

Lunar anorthosite is a major rock of the lunar highlands,which formed as a result of plagioclasefloatation in the lunar magma ocean（LMO）.Constraints on the sufficient conditions that resulted in the formation of a thick pure anorthosite（mode of plagioclase 〉95 vol.%） is a key to reveal the early magmatic evolution of the terrestrial planets.To form the pure lunar anorthosite,plagioclase should have separated from the magma ocean with low crystal fraction.Crystal networks of plagioclase and mafic minerals develop when the crystal fraction in the magma（φ） is higher than ca.40-60 vol.%,which inhibit the formation of pure anorthosite.In contrast,when φ is small,the magma ocean is highly turbulent,and plagioclase is likely to become entrained in the turbulent magma rather than separated from the melt.To determine the necessary conditions in which anorthosite forms from the LMO,this study adopted the energy criterion formulated by Solomatov.The composition of melt,temperature,and pressure when plagioclase crystallizes are constrained by using MELTS/pMELTS to calculate the density and viscosity of the melt.When plagioclase starts to crystallize,the Mg~# of melt becomes 0.59 at 1291 C.The density of the melt is smaller than that of plagioclase for P 〉 2.1 kbar（ca.50 km deep）,and the critical diameter of plagioclase to separate from the melt becomes larger than the typical crystal diameter of plagioclase（1.8-3 cm）.This suggests that plagioclase is likely entrained in the LMO just after the plagioclase starts to crystallize.When the Mg~# of melt becomes 0.54 at 1263 C,the density of melt becomes larger than that of plagioclase even for 0 kbar.When the Mg~# of melt decreases down to 0.46 at 1218 C,the critical diameter of plagioclase to separate from the melt becomes 1.5-2.5 cm,which is nearly equal to the typical plagioclase of the lunar anorthosite.This suggests that plagioclase could separate from the melt.One of the differences between the Earth and the Moon is the presence of water.If the terrestrial magma ocean was saturated with H_2O,plagioclase could not crystallize,and anorthosite could not form.

High-pressure phase transitions of anorthosite crust in the Earth's deep mantle

We investigated phase relations, mineral chemistry, and density of lunar highland anorthosite at conditions up to 125 GPa and 2000 K. We used a multi-anvil apparatus and a laser-heated diamond-anvil cell for this purpose. In-situ X-ray diffraction measurements at high pressures and composition analysis of recovered samples using an analytical transmission electron microscope showed that anorthosite consists of garnet,CaAl_4Si_2O_(11)-rich phase(CAS phase), and SiO_2 phases in the upper mantle and the mantle transition zone.Under lower mantle conditions, these minerals transform to the assemblage of bridgmanite, Ca-perovskite,corundum, stishovite, and calcium ferrite-type aluminous phase through the decomposition of garnet and CAS phase at around 700 km depth. Anorthosite has a higher density than PREM and pyrolite in the upper mantle, while its density becomes comparable or lower under lower mantle conditions. Our results suggest that ancient anorthosite crust subducted down to the deep mantle was likely to have accumulated at660-720 km in depth without coming back to the Earth's surface. Some portions of the anorthosite crust might have circulated continuously in the Earth's deep interior by mantle convection and potentially subducted to the bottom of the lower mantle when carried within layers of dense basaltic rocks.

The origin of ferroan and magnesian anorthosites in the lunar crust

1 Introduction The widely accepted standard model for the lunar feldspathic crust is:the early Moon was wholly or mostly molten,forming Lunar Magma Ocean(LMO).Olivine and pyroxene crystallized first from that magma ocean and sank

Petrography of Bethampudi Anorthosites Layered Complex from the Khammam Schist Belt, Telangana, India

The Bethampudi layered anorthosite complex at the border zone of Archaean supracrustal rocks of Khammam district, Eastern Ghats shows normal stratification predominantly in the form of rhythmic layering and often exhibits of zebra layering. Graded bedding and cumulate structures are also noticed. The rocks of the study area are classified based on petrography into anorthositic rocks, gabbroic rocks and ultramafic rocks and amphibolites. The field relations and major element composition suggest that these anorthosite rocks are of calc-alkaline in nature and petrogenitically related to the gabbroic rocks by the fractional crystallization at ℃.

Landsat 8 and Alos DEM geological mapping reveals the architecture of the giant Mesoproterozoic Kunene Complex anorthosite suite(Angola/Namibia)

The quasi-monomineralic composition and huge spatial extent of massif-type anorthosites make detecting lithological regions and boundaries challenging.We use processed Landsat 8 OLI multispectral images and ALOS digital elevation models integrated with field and petrographic observations to characterize the architecture of the≥17,000 km2 Mesoproterozoic Kunene Complex(KC)anorthosite suite in Angola and Namibia.Images of false colour composite bands 6,4 and 1 and band ratios 4/2(ferric minerals),6/5(ferrous minerals)and 6/7(OH-bearing minerals),as well as assessment of the PCA and MNF matrices of eigenvectors and eigenvalues from Landsat 8 data using available spectral libraries,have substantially improved the interpretation of the Kunene Complex rock types and structures.The dataset shows that the reflectance signature of KC anorthositic rocks is primarily a function of the degree of metasomatism,which is most significant in olivine-poor rock types.The weathering intensity of the olivine-bearing anorthosite substrate is another control on the remote sensing signal.Our remote sensing and field-based approach has enabled us to divide the KC anorthosite suite into six distinct spectral and architecture domains and at least four distinct magmatic plutons when considering available high-precision geochronological data.The northernmost pluton(ca.1380 Ma)of massive,olivine-bearing anorthosite shows distinct remote sensing signatures in the band ratio and MNF images marked by the dominance of OH-bearing and subordinate ferric minerals.The central pluton(1412–1400 Ma)is composed of NNE-to N-striking steeply dipping interlayered olivine-bearing and olivinepoor anorthosite,which correspond to ridges of dark-coloured,low albedo rocks with subordinate slightly oxidised ferrous mineral spectral signatures,and valleys of low albedo and OH-bearing mineral spectral signatures.A NNE-striking tectonic zone along a linear belt of KC granite gneiss separates the northern and central plutons.To the south is a NNW-to NNE-striking layered pluton(ca.1390 Ma),marked by olivine-bearing anorthosite that forms ridges and metasomatized olivine-poor and pyroxene-bearing anorthosite that forms valleys.This domain is separated from the similarly layered and E–W-striking Zebra Lobe–Oryeheke anorthosite by a top-to-the north thrust zone.A small domain of ca.1438 Ma olivine-bearing anorthosite sensu lato is situated to the SE of the main Kunene Complex.This work highlights the variability in layered and massive textures,the extent of metasomatism,and the importance of internal tectonic boundaries in the architecture of the Kunene Complex anorthosite suite.Our approach can be applied in other anorthositic terrains and large igneous bodies elsewhere on Earth,especially those where outcrop is poor,or access is inhibited.

A possible anorthositic continent of early Mars and the role of planetary size for the inception of Earth-like life

The Moon has an anorthositic primordial continental crust. Recently anorthosite has also been discovered on the Martian surface. Although the occurrence of anorthosite is observed to be very limited in Earth＇s extant geological record,both lunar and Martian surface geology suggest that anorthosite may have comprised a primordial continent on the early Earth during the first 600 million years after its formation. We hypothesized that differences in the presence of an anorthositic continent on an Earthlike planet are due to planetary size. Earth likely lost its primordial anorthositic continent by tectonic erosion through subduction associated with a kind of proto-plate tectonics（PPT）. In contrast, Mars and the Moon, as much smaller planetary bodies, did not lose much of their anorthositic continental crust because mantle convection had weakened and/or largely stopped, and with time, they had appropriately cooled down. Applying this same reasoning to a super-Earth exoplanet suggests that, while a primordial anorthositic continent may briefly form on its surface, such a continent will be likely transported into the deep mantle due to intense mantle convection immediately following its formation. The presence of a primordial continent on an Earth-like planet seems to be essential to whether the planet will be habitable to Earth-like life. The key role of the primordial continent is to provide the necessary and sufficient nutrients for the emergence and evolution of life. With the appearance of a ＂trinity＂ consisting of（1） an atmosphere,（2） an ocean, and（3） the primordial continental landmass, material circulation can be maintained to enable a ＂Habitable Trinity＂ environment that will permit the emergence of Earth-like life. Thus, with little likelihood of a persistent primordial continent, a super-Earth affords very little chance for Earth-like life to emerge.

Petrogenesis and Metallogenesis of the Niumaoquan Gabbroic Intrusion Associated with Fe-Ti Oxide Ores in the Eastern Tianshan, NW China

The Niumaoquan layered gabbroic intrusion is in the southern margin of the Central Asian Orogenic Belt in North Xinjiang, China, and hosts a Fe-Ti oxide deposit in its evolved gabbroic phases. In this paper, we report zircon U-Pb age, Sr-Nd-Hf isotopes, plagioclase chemistry, and whole-rock geochemistry of the Niumaoquan layered gabbroic intrusion. Zircon grains separated from an anorthosite sample analyzed by laser ablation inductively coupled plasma mass spectrometry yielded a concordia age of 314.7±0.74 Ma, indicating that the Niumaoquan ore-bearing gabbroic intrusion was emplaced during the Late Carboniferous. The olivine gabbro texture and plagioclase chemistry suggest that plagioclase was an early crystallized silicate phase that crystallized prior to olivine. Fractional crystallization and accumulation of plagioclase significantly controlled the evolution of the Niumaoquan gabbroic intrusion and contributed to the formation of anorthosite layers, causing metallogenic elements to become enriched in the residual melt. The Niumaoquan gabbroic intrusion is characterized by the enrichment of large ion lithophile elements and depletion of high field strength elements, positive zircon εHf（t） values（＋2.1 to ＋12.2）, positive εNd（t） values（＋3.3 to ＋5.2）, and low initial ^（87）Sr/^（86）Sr ratios（0.7039 to 0.7047）, suggesting that the parental magma was produced by interactions between metasomatized lithospheric mantle and depleted asthenospheric melts at an early post-collision stage. The Fe-Ti oxide mineralization of the Niumaoquan intrusion benefited from interactions between depleted asthenospheric melts and lithospheric mantle, and fractional crystallization of abundant plagioclase and magnesian minerals.

Subduction of the primordial crust into the deep mantle

The primordial crust on the Earth formed from the crystallization of the surface magma ocean during the Hadean.However,geological surveys have found no evidence of rocks dating back to more than 4 Ga on the Earth＇s surface,suggesting the Hadean crust was lost due to some processes.We investigated the subduction of one of the possible candidates for the primordial crust,anorthosite and KREEP crust similar to the Moon,which is also considered to have formed from the crystallization of the magma ocean.Similar to the present Earth,the subduction of primordial crust by subduction erosion is expected to be an effective way of eliminating primordial crust from the surface.In this study,the subduction rate of the primordial crust via subduction channels is evaluated by numerical simulations.The subduction channels are located between the subducting slab and the mantle wedge and are comprised of primordial crust materials supplied mainly by subduction erosion.We have found that primordial anorthosite and KREEP crust of up to - 50 km thick at the Earth＇s surface was able to be conveyed to the deep mantle within 0.1-2 Gy by that mechanism.

Element mapping the Merensky Reef of the Bushveld Complex

The Merensky Reef hosts one of the largest PGE resources globally.It has been exploited for nearly 100 years,yet its origin remains unresolved.In the present study,we characterised eight samples of the reef at four localities in the western Bushveld Complex using micro-X-ray fluorescence and field emission scanning electron microscopy.Our results indicate that the Merensky Reef formed through a range of diverse processes.Textures exhibited by chromite grains at the base of the reef are consistent with supercooling and in situ growth.The local thickening of the Merensky chromitite layers within troughs in the floor rocks is most readily explained by granular flow.Annealing and deformation textures in pyroxenes of the Merensky pegmatoid bear testament to recrystallisation and deformation.The footwall rocks to the reef contain disseminations of PGE rich sulphides as well as olivine grains with peritectic reaction rims along their upper margins suggesting reactive downward flow of silicate and sulphide melts.Olivine-hosted melt inclusions containing Cl-rich apatite,sodic plagioclase,and phlogopite suggest the presence of highly evolved,volatile-rich melts.Pervasive reverse zonation of cumulus plagioclase in the footwall of the reef indicates dissolution or partial melting of plagioclase,possibly triggered by flux of heat,acidic fluids,or hydrous melt.Together,these data suggest that the reef formed through a combination of magmatic,hydrodynamic and hydromagmatic processes.

The experimental studies on electrical con-ductivities and P-wave velocities of anortho-site at high pressure and high temperature

Results of P-wave velocity (vP) and electrical conductivity measurements on anorthosite are presented from room temperature to 880 C at 1.0 GPa using ultrasonic transmission technique and impedance spectra technique respec-tively. The experiments show that the P-wave velocities in anorthosite decrease markedly above 680 C following the dehydration of hydrous minerals in the rock, and the complex impedances collected from 12 Hz to 105 Hz only indicate the grain interior conduction mechanism at 1.0 GPa, from 410 C to 750 C. Because the fluids in the rock have not formed an interconnected network, the dehydration will not pronouncedly enhance the electrical conduc-tivity and change the electrical conduction mechanism. It is concluded that the formation and evolution of the low-velocity zones and high-conductivity layers in the crust may have no correlations, and the dehydration can result in the formation of the low-velocity zones, but cannot simultaneously result in the high-conductivity layers.

Effect of crustal porosity on lunar magma ocean solidification

The lunar ferroan anorthosites,formed by plagioclase flotation from the crystallization of the lunar magma ocean,have an age span of over~200 Ma.However,previous thermal models predicted a much shorter time range.We propose that a much smaller thermal conductivity of anorthositic crust due to its high porosity may have delayed the solidification of the lunar magma ocean.Our thermal simulation results,using the thermal conductivity of porous lunar crust,show that crystallization of a 1000 km deep magma ocean could be prolonged to tens of millions of years,and up to 180 Ma under some extreme conditions.The porous crust alone can’t explain the large crustal age span,however.Other circumstances must be taken into consideration,such as a thick lunar soil.

Petrology and Geochemical Characteristic of the Younger Gabbros of Wadi Shianite Area, Southeastern Desert, Egypt

The present work is a petrological study of the gabbroic rocks of wadi Shianite Southeastern Desert of Egypt. Chemical analyses for major and trace elements showed that there are 3 main gabbro types. These are: 1)?pyroxene hornblende gabbronorite;?2) hornblende gabbro;?and 3) anorthosite.?The opaque minerals study of the gabbroic rocks showed that they composed mainly of ilmenite, magnetite and sulphides. The present gabbroic rocks work are related to calc-alkaline magma type, similar to the younger gabbros in other areas in the Eastern Desert.