Igneous Nature Research Articles

A Mars 2020 Perseverance SuperCam Perspective on the Igneous Nature of the Máaz Formation at Jezero Crater and Link With Séítah, Mars

AbstractThe Máaz formation consists of the first lithologies in Jezero crater analyzed by the Mars 2020 Perseverance rover. This formation, investigated from Sols (Martian days) 1 to 201 and from Sols 343 to 382, overlies the Séítah formation (previously described as an olivine‐rich cumulate) and was initially suggested to represent an igneous crater floor unit based on orbital analyses. Using SuperCam data, we conducted a detailed textural, chemical, and mineralogical analyses of the Máaz formation and the Content member of the Séítah formation. We conclude that the Máaz formation and the Content member are igneous and consist of different lava flows and/or possibly pyroclastic flows with complex textures, including vesicular and non‐vesicular rocks with different grain sizes. The Máaz formation rocks exhibit some of the lowest Mg# (=molar 100 × MgO/MgO + FeO) of all Martian igneous rocks analyzed so far (including meteorites and surface rocks) and show similar basaltic to basaltic‐andesitic compositions. Their mineralogy is dominated by Fe‐rich augite to possibly ferrosilite and plagioclase, and minor phases such as Fe‐Ti oxides and Si‐rich phases. They show a broad diversity of both compositions and textures when compared to Martian meteorites and other surface rocks. The different Máaz and Content lava or pyroclastic flows all originate from the same parental magma and/or the same magmatic system, but are not petrogenetically linked to the Séítah formation. The study of returned Máaz samples in Earth‐based laboratories will help constrain the formation of these rocks, calibrate Martian crater counting, and overall, improve our understanding of magmatism on Mars.

Time lag between metamorphism and crystallization of anatectic granites (Córdoba, Argentina)

SHRIMP and LA-ICP-MS analyses carried out on zircons from the Río de los Sauces granite revealed their metamorphic and igneous nature. The metamorphic zircons yielded an age of 537±4.8 (2σ)Ma that probably predates the onset of the anatexis during the Pampean orogeny. By contrast, the igneous zircons yielded a younger age of 529±6 (2σ)Ma and reflected its crystallization age. These data point to a short time lag of ca. 8Myr between the High Temperature (HT) metamorphic peak and the subsequent crystallization age of the granite. Concordia age of 534±3.8 (2σ)Ma, for both types of zircon populations, can be considered as the mean age of the Pampean HT metamorphism in the Sierras de Córdoba.

Reavaliação do Complexo Jequié: Um Olhar Geocronológico Sobre a Geologia da Área Jequié-Itagi (Bahia, Brasil)

New geological and litogeochemical data plus La-ICP-MS U-Pb isotopic study on the Jequie Complex, or Jequie “Block”, a granulite facies terrain of the Northeastern Sao Francisco Craton, allowed to propose that a new lithodemic unit, the Volta do Rio Plutonic Suite, should be created as a lower rank unit. The Jequie Complex is defined here as an intrusive complex metamorphosed in the granulite facies, containing an assemblage of plutonic calc-alkalic mafic to intermediate rocks, fractionated trondhjemites, tonalites and granodiorites, besides normal calc-alkaline tonalites/ granodiorites and rare metasedimentary remnants plus two different sets of leucogranites. The Volta do Rio Plutonic Suite is proposed as a lower range lithodemic unit belonging to the Jequie Complex. It is composed of granodiorites and monzogranites with porphyroclastic texture, even- -grained granitoids and fine-grained granitoids, besides an association of amphibole-bearing leucogranites and mafic to intermediate rocks. The metamorphosed mafic, intermediate and felsic rocks of the Jequie Complex compose a Cordilleran-type magnesian calc-alkalic association, which age is 2.7 Ga. In contrast, all the metagranitoids of the Volta do Rio Plutonic Suite show a distinctive ferroan (“A-type”) geochemical signature and the mafic and intermediate rocks associated to metaleucogranites show alkaline characteristics and host locally high-grade REE mineralizations contained in chevkinite group minerals. In the Volta do Rio Plutonic Suite, the porphyroclastic granites were dated at 2.6 Ga.The provisional age of alkaline mafic rocks with magmatic-hydrothermal REE mineralizations and of possibly coeval leucogranites is 2.5 Ga. These mineralizations are inedit in the world and the obtained time frame indicates the need to re-evaluate the geology and metallogenic potential of the Jequie Complex by considering its primordial igneous nature and by screening out its “granulitic” or “charnockitic” nature.

Ore Formation Conditions of the Aunik F-Be Deposit (Western Transbaikalia)

The paper presents the results of studying of the ore composition, ore-forming solutions, formation conditions, and sources of Be mineralization based on the example of the Aunik F-Be deposit, which is part of the Western Transbaikalia beryllium-bearing province; the main factors responsible for the beryllium mineralization are also assessed. The ores of the deposit are represented by feldspar-fluorite-phenakite-bertrandite metasomatites that formed after carboniferous limestones during its metasomatic replacement by hydrothermal solutions bearing F, Be, and accompanying admixture elements. It is established that the early phenakite-fluorite assemblage formed from high-fluoride CO2-bearing solutions with high alkalinity and a salinity of ~10.5–12 wt % NaCl eq. in the temperature range ~370–260°C at pressures from 1873 to 1248 bar. Later fluorite and bertrandite were formed by solutions with a salinity of 6.4–7.7 wt % NaCl eq. in the temperature range of ~156–110°C and pressure range of 639–247 bar. Isotopic composition studies of the ore assemblage minerals have confirmed the apocarbonate nature of the main ores at the deposit, making it possible to establish the igneous nature of the ore-forming paleohydrothermal solutions, the source of which were subalkaline leucogranites. The main factors controlling the formation of F-Be ores were the decrease in F activity in solutions owing to bonding of Ca and F in fluorite and the reduction in temperature during ore deposition. The increased alkalinity of the ore-forming solutions controlled the low solubility of Be complexes, which resulted in a low Be content in ores and a relatively small-scale mineralization at the deposit.

Carbonatitic versus hydrothermal origin for fluorapatite REE-Th deposits: Experimental study of REE transport and crustal “antiskarn” metasomatism

Nolans-type ore deposits contain REE and Th mineralization hosted in fluorapatite veins. These veins intrude granulite facies rocks and are surrounded by a diopside selvage. Nolans-type deposits are thought to form by REE, F and P-rich hydrothermal fluids derived from alkali or carbonatitic intrusions. However, REE are not effectively transported in F and P-rich systems. REE ore deposits are commonly hydrothermally overprinted, possibly obscuring the igneous nature of the primary mineralization. We conducted a series of piston cylinder “sandwich” experiments, testing the hydrothermal fluid hypothesis, and a newly suggested process of carbonatite metasomatism. Our results confirm theoretical predictions that REE are hydrothermally immobile in these systems and the experimental phase assemblage is not compatible with the natural rocks. Our results show that fluorapatite can only host several weight percent levels of REE at temperatures higher than ∼600 °C. Below that temperature, a miscibility gap exists between REE-poor fluorapatite and REE-rich silicates such as britholite or cerite. In contrast, experiments reacting P and REE-rich carbonatite with silicate rock above 700 °C closely resemble natural rocks from Nolans-type deposits. Selvage mineralogy is sensitive to the MgO content of the carbonatite. A diopside selvage formed at carbonatite MgO/(CaO+MgO) ≈ 0.2 while wollastonite and forsterite formed at lower and higher ratios, respectively. Phosphate solubility in carbonatites decreases with decreasing MgO contents. As diopside formed, REE-rich fluorapatite preferentially crystallized from the selvage inwards. Thus, carbonatites are effective at simultaneously mobilizing REE, F and P to the site of deposition. Nolans-type deposits are the cumulate residue of this reaction, with the carbonatite liquid migrating elsewhere. At temperatures below 700 °C the carbonatite–silicate reaction additionally formed monticellite, cuspidine and magnesioferrite, resembling a skarn assemblage. Whereas skarns form by infiltration of silicate magmas or related fluids to carbonate rocks, our experiments are the opposite: intrusion of carbonatite into silicate rock. These mid-crustal skarn-like rocks may host elevated ore elements of carbonatitic affinity, such as F, P, Y, REE, Th, Ba, Sr, and Nb. We propose the term “antiskarn” to describe such systems, and suggest they trace the migration of carbonatite liquids through the crust. Hydrothermal reworking, retrogression, or metamorphism of antiskarns may obscure the carbonatitic genesis of the rocks. These metasomatic zones are the crustal equivalent of wehrlites that form by peridotite–carbonatite reaction at mantle depths.

THE ALUMINOUS GNEISSES OF KOLA SERIES, BALTIC SHIELD (GEOCHEMISTRY, NATURE AND AGE OF PROTOLITH)

The geochemical and isotopic data were obtained for gneisses of the Kola Series, Baltic Shield. They are metamorphosed in granulite facies and interpreted according to the igneous nature of zircons and gneisses. The biotitic and garnet-biotitic gneisses are paralleled to the rocks of TTG associations in terms of the structure of correlation between the main elements. Sil ± Crd-Grt-Bt gneisses are formed on Grt-Bt gneisses as a result of blastomylonitization and partial melting. U-Pb zircon ages (SHRIMP-II) were determined for both gneiss species. All analytical points of magmatic zircons in the concordia diagram for Grt-Bt gneisses form a continuous trend extended in concordia in the range of 2909 ± 8 – 2732 ± 13 Ma. It is interpreted as the result of lead loss from magmatic zircons during the slow cooling of tonalite in the lower crust. Minimal age of magmatism and early granulitic metamorphism estimated by most ancient zircon is 2.9 Ga. The time of manifestation of younger granulite metamorphism determined by newly-formed zircon shells from Crd-Sil-Grt-Bt gneiss corresponds to the interval 2707 ± 9 – 2656 ± 8 Ma. Part of the zircon from Crd-Sil-Grt-Bt gneiss is profoundly altered crystals with a discordant age of 2740 ± 6 – 2639 ± 9 Ma, which is similar to that of metamorphic shells, suggesting an event happened at that time which disrupted the crystalline grid and caused the loss of radiogenic lead. Three ancient zircons with a maximum age of 3461 ± 5 MA are interpreted as captured zircons, which indicate the presence of continental crust at this point in the northern part of the Baltic Shield.

Volcanic passive margins: another way to break up continents.

Two major types of passive margins are recognized, i.e. volcanic and non-volcanic, without proposing distinctive mechanisms for their formation. Volcanic passive margins are associated with the extrusion and intrusion of large volumes of magma, predominantly mafic, and represent distinctive features of Larges Igneous Provinces, in which regional fissural volcanism predates localized syn-magmatic break-up of the lithosphere. In contrast with non-volcanic margins, continentward-dipping detachment faults accommodate crustal necking at both conjugate volcanic margins. These faults root on a two-layer deformed ductile crust that appears to be partly of igneous nature. This lower crust is exhumed up to the bottom of the syn-extension extrusives at the outer parts of the margin. Our numerical modelling suggests that strengthening of deep continental crust during early magmatic stages provokes a divergent flow of the ductile lithosphere away from a central continental block, which becomes thinner with time due to the flow-induced mechanical erosion acting at its base. Crustal-scale faults dipping continentward are rooted over this flowing material, thus isolating micro-continents within the future oceanic domain. Pure-shear type deformation affects the bulk lithosphere at VPMs until continental breakup, and the geometry of the margin is closely related to the dynamics of an active and melting mantle.

Transfer of natural radionuclides from soil to plants in tropical forest (Western Ghats - India).

Soil and 2 wild plant species (Evodia roxburghiana and Eleaocarpus oblangus)have been collected from the western Ghats region and analysed for 210Po and238U. The radioactivity concentrations in plants and soil reflect the impact of the existence of igneous nature of rock in the area of study. Concentration ratios (CR) of these radionuclides, between plants and underlying soil, have been studied and the results shows that the concentration ratios (CR) seem to be depend on radionuclides in soil. Key words: Primordial radionuclides, western Ghats, monazite, igneous rock, CR-concentration ratio.

English

Twelve specimens of intrusive rocks, ranging from granitic to ultramafic, were ground and subjected to the static-test neutralization-potential (NP) protocol so that the results could be compared with those computed by using the NP values previously obtained by Sobek tests of the constituent minerals. The quantitative mineralogy of the rocks was determined by Rietveld refinements of X-ray powder diffraction data, and was supplemented by optical microscopy, fizz tests, and analyses of total carbon to determine the presence of carbonate minerals. Despite the igneous nature of the suite, most samples were found to be carbonate-bearing; optical microscopy and fizz tests of the coarse (minus 6 mm) fractions were observed to be more sensitive to the presence of carbonates than was the minus 60-mesh fraction that is used in the Sobek protocol. For some minerals, notably olivine and serpentine, the acid-digestion period in the Sobek test has a pronounced effect on the resulting NP, and this part of the test protocol is in need of new standardization. Mineralogical prediction of the NP values is sensitive to the composition of the plagioclase because these feldspars are typically a major component of igneous rocks and the NP of the calcic end- member, anorthite, is about 12 times that of the sodic end-member, albite.

Hot-fluid Driven Metasomatism of Samalpatti Carbonatites, South India: Evidence from Petrology, Mineral Chemistry, Trace Elements and Stable Isotope Compositions

The magmatic heritage of carbonatites can be identified on the basis of a combination of geological criteria such as, their mode of occurrence, the nature of associated igneous rocks, the presence of minerals of igneous origin, fenitization, characteristic trace element contents and isotopic composition. Late Proterozoic Samalpatti carbonatites were studied in view of these criteria, and were found to contain metamorphic minerals that normally form under thermal metamorphic conditions and which have unusual chemical compositions. A combination of criteria points clearly to a magmatic origin for these carbonatites. Field relations indicate that the dominant modes of intrusion of carbonatite into the encompassing pyroxenites and syenites include small dykes, veins, or lenses. The igneous nature of these carbonatites has been described elsewhere and chemically they are classified as calico-carbonatites. Currently, very little is known about the metamorphic textures and mineralogy observed in the Samalpatti carbonatites. In this study, several metamorphic minerals are reported including diopside, grossularite, vesuvianite, K-feldspar and wollastonite, and a hornfelsic texture is described. These mineral phases and texture characterize thermal metamorphism under low pressure and high temperature (LP-HT) metamorphic conditions (650°_750°C) or metasomatism aided by hot-fluid advection. The metamorphic nature of minerals reported is also confirmed by electron microprobe study. The Samalpatti carbonatite samples show much lower values of characteristic trace elements (P, Sr, Ba, Zr, Nb, Th, Y and REEs) than average concentrations for magmatic carbonatite. Stable isotopic (d 13C and d 18O) compositions of Samalpatti carbonatites do not fall in the primary igneous carbonatite (PIC) domain. The petrological and chemical signatures of these carbonatites suggest metasomatism in conjunction with fluid advection. Such a metasomatic process may drastically change the chemistry of the rocks in addition to enrichment of heavier stable isotopes. During this metasomatic process, characteristic elements would be dissolved in the high d 18O fluid, and together with Rayleigh fractionation would contribute to enhanced concentrations of 13C and 18O in Samalpatti carbonatites.

Laser ablation ICPMS dating of zircons in Erzgebirge orthogneisses: evidence for Early Cambrian and Early Ordovician granitic plutonism in the western Bohemian Massif

U-Pb isotopic data obtained by laser ablation ICPMS analysis of nine zircons with centre to margin oscillatory growth zones from a K-feldspar-rich augen gneiss in the allochthonous Lower Crystalline Nappe of the Erzgebirge domain of the western part of the Bohemian Massif yield a concordia age of 524 ± 10 Ma (2 sigma). This Early Cambrian age represents the time of magmatic crystallization of the protolith of a representative, from near Měděnec, of the allochthonous “Red gneiss” whose igneous nature is shown by the presence of (deformed) xenoliths. Data from TIMS analysis of zircons with variable proportions of unzoned xenocrystic cores surrounded by oscillatory-zoned overgrowths point to magma derivation from upper Proterozoic, or older, rocks. Data obtained for five zircon grains from another “Red gneiss” in the Lower Crystalline Nappe (in the Klinovec anticline) plot below the concordia with the age of the one point that is near concordant being 519 ± 26 Ma (2 sigma). These data, together with internal features of the zircons, are consistent with Early Cambrian granitic plutonism also in this part of the Erzgebirge but with later Pb loss, possibly associated with considerable fluid movement during thrust nappe development. Another sample of a coarse-grained orthogneiss from the autochthonous St Catherine9s dome yielded a significantly younger Early Ordovician age of 480 ± 10 Ma (2 sigma) calculated from eight zircon analyses. However, three zircon grains from the same sample gave a significantly older near-concordant Late Proterozoic age of ca. 620 Ma. Provided that the age difference of ca . 40 Ma between orthogneisses from Měděnec - Klinovec and St Catherine9s dome holds also for other orthogneisses in the Erzgebirge, zircon U-Pb age data could be used to discriminate between allochthonous and autochthonous units in this region. The ca. 25 Ma difference between the Early Cambrian protolith age of the augen gneiss from near Měděnec determined by the laser ablation ICPMS technique and a previously reported older age of 550 ± 9 Ma for a nearby sample determined by the Pb-Pb evaporation technique is accounted for on the basis of the latter not being adequate for dating zircons with a small xenocrystic component. This study demonstrates the importance of high spatial resolution dating techniques, such as SHRIMP or laser ablation ICPMS, in dating zircons with complex growth history that are common in crustally-derived melts.

Sources of Svecofennian granitoids in the light of ion probe UPb measurements on their zircons

The presence of 1.91–1.93 Ga old granitoids at the Archean–Proterozoic boundary along the Raahe–Ladoga zone in Finland has been demonstrated on various occasions. These rocks have been considered to represent juvenile crustal material, as their ε Nd values are markedly positive. However, as Svecofennian metasediments contain detrital zircons derived from a ca. 2 Ga old source, the possibility has existed that the 1.92 Ga age may have been a mixture between 2 and 1.89 Ga old zircon populations, as such mixing would not markedly affect their neodymium isotopic properties. Also, some syntectonic 1.89 Ga old Svecofennian granitoids contain heterogeneous zircon populations, but it has been impossible to determine the age and origin of the older zircons by conventional methods. NORDSIM ion probe results on three samples from the 1.92 Ga age group confirm the earlier conclusions. Especially important is that no zircons older than 1.95 Ga were detected in the 1.92 Ga group samples. Thus, the 1.92 Ga event was the beginning of the formation of new continental crust in the primitive Svecofennian island arc and these granitoids formed by partial melting of basaltic magmas derived from a depleted mantle source. One sample also contains a younger zircon population formed during the orogenic culmination at 1.89 Ga. In contrast, one grain from a sample representing the 1.89 Ga age group contains an Archean core, which is considered to represent sedimentary detritus assimilated during either magma formation or intrusion. While the results prove the true igneous nature of the 1.92 Ga event, they also rule out these rocks as a possible provenance for the ca. 2 Ga old zircons encountered in the Svecofennian metaturbidites. Thus, there is still no direct evidence from granitoid rocks for an extensive Svecofennian protocrust, the existence of which has been postulated on the basis of geochemical and SmNd isotopic data.

A rare composite xenolith from Salt Lake Crater, Oahu: high-pressure fractionation and implications for kimberlitic melts in the Hawaiian mantle

A composite xenolith of olivine-bearing garnet clinopyroxenite wall rock intruded by two spinels + garnet veins is described. Vein minerals exhibit textural evidence of a reaction relationship with the mineral phases in the wall rock. Wall rock clinopyroxene contains exsolved blebby garnet and very fine lamellar exsolution of orthopyroxene, indicating that this xenolith had undergone considerable subsolidus cooling. Garnet–clinopyroxene thermometry suggests that the xenolith last equilibrated in the mantle at a temperature of about 1,060 (±25 °C). The spinels in the veins are of two kinds: pleonaste (that occurs with vein garnet) and a high-Mg, high-Al titanomagnetite (MAT spinels). Intriguingly, the MAT spinels are chemically very similar to the spinels found as groundmass in kimberlites, are moderately subhedral to euhedral, have a weakly developed cumulate texture, and, at places, show a reaction relation with the pleonaste + garnet (cumulate?) assemblage in the vein. Based on petrographic, chemical, and phase equilibrium considerations, we propose the following evolutionary history of this composite xenolith. (1) In the first stage the olivine-bearing garnet clinopyroxenite formed as crystal extracts (cumulates) as a result of high pressure fractionation of an alkaline melt in the deepest levels of Hawaiian lithosphere/uppermost asthenosphere (100–110 km). (2) In the second stage, igneous veining (the melt composition of this vein is not precisely known but could be kimberlitic) occurs in the already existing wall rock resulting in the precipitation of pleonaste + garnet. A reaction relation between the igneous veins and the wall rock also characterizes this stage. (3) The last igneous episode in this xenolith is recorded by MAT spinels in the wall rock and their precipitation close to the previous pleonaste + garnet veins. The last igneous stage could well be due again to high pressure fractionation of a kimberlitic melt (the residual melt after precipitation of pleonaste + garnet). The time relationship between exsolution and the later igneous veining stages is not known. The MAT spinels are not a result of sub-solidus solvus processes as partial reaction (melt present) between the pleonaste + garnet (from the second igneous stage) and MAT spinel exists, pointing to the igneous nature of the MAT spinel. Based on striking similarity between the MAT spinels in our xenolith and those found as groundmass in kimberlites, we propose that the veining stages could well have been kimberlitic. Thus, even though kimberlitic melts are not seen on the Koolau shield, this particular xenolith clearly shows the existence of such melts at great depths beneath Hawaii. We also propose that the initial wall rock, which represents crystal extracts (even though it does not exhibit definitive cumulate texture) as a result of high-pressure fractionation of an alkaline melt and subsequent veining episodes, are of pre-Koolau age. This implies that the Koolau shield volcano may have had a pre-shield alkalic stage.

An audiomagnetotelluric study of the Leona Batholith, Senegal

Scalar audiomagnetotelluric surveys over the frequency range 8–5000 Hz were conducted across the Leona batholith (northwest Senegal) along two traverses to map resistivity contrasts associated with the Senegal basin margins. The apparent resistivity data from 12 sites, measured in perpendicular directions, ranged from nearly isotropic to anisotropic with decreasing frequency. Modelling indicates a major change in structure at depth with a pronounced vertical contact. Resistivitie exceeding 20,000 ohm·m are obtained for a dome of igneous nature, contrasting with values of about 2000 ohm·m from the Senegal basin sediments. A conductive layer (50–200 m thick), overlying the more resistive layer, is associated with a fractured contractmetamorphic aureole to the intrusive body.

松ケ平‐母体変成岩類中の角閃岩類のＫ‐Ａｒ年代とその意義

K-Ar ages of five hornblendes were determined for the Matsugadaira-Motai Metamorphics. They are dated as 479-524 Ma for three hornblendes and 225-239 Ma for two. The dated hornblendes from the Matsugadaira and Yamagami epidotea-mphibolites are barroisitic indicating high pressure type, and one hornblende of the Ohachimori amphibolites from the Motai Metamorphics retains igneous nature in its core. The values of the 479-524 Ma of two Ohachimori amphibolites and one of Matsugadaira epidote-amphibolite suggest the age of metamorphism, and are in harmony with the stratigraphic evidence. The 225-239 Ma ages of the Yamagami epidoteamphibolites suggest the effect of the Cretaceous granites or some other events. The results of ca. 500 Ma age may constrain consideration of the tectonic regime of the Southern Kitakami Belt in pre-Silurian time.

Diversite des clinopyroxenites a grenat associees aux massifs ultramafiques orogeniques; eclogites, ariegites, griquaites et grospydites; une discussion

Abstract Chemical composition of rocks belonging to the garnet clinopyroxenite clan (eclogites, griquaites, grospydites) are plotted in the Al <sub>2</sub> O <sub>3</sub> -FeO-MgO diagram. Compositions of eclogites from metamorphic series define a trend corresponding to liquids resulting from the low-pressure cotectic precipitation of ol+cpx+pl. These data support the igneous nature of the material and suggest that the original rocks experienced crystallization of the crust. Garnet clinopyroxenite inclusions from kimberlites and alkali basalts as well as garnet clinopyroxenite lenses from high temperature peridotite bodies define a different trend that can be interpreted as resulting from cpx+gt precipitation from a basic melt. The unusual position of these latter rocks justify that the term "griquaite" be applied to them. Corundum and/or kyanite bearing eclogites from metamorphic series as well as "grospydites" included in kimberlites or alkali basalts, or found as lenses in ultramafic bodies), plot toward the Al-rich apex of the Al-F-M diagram, but show different SiO <sub>2</sub> /Al <sub>2</sub> O <sub>3</sub> : the term "grospydite" appears fully justified to describe the latter, and distinguish them from the former (metamorphic) ones. The origin of grospydites is still in debate: they may be the products of high pressure crystallization of basic liquids. or of metasomatic crystallization of gabbros recycled within the mantle. The coexistence within the Beni Bousera massif in northern Morocco of griquaites and grospydites indicates the large heterogeneity of this upper mantle fragment. This heterogeneity can only be explained by a complex geodynamic evolution characterized by either 1) the succession of two distinct episodes of basic melt production; or 2) a cyclical mechanism implying the subduction in the mantle of a lithospheric fragment followed by the adiabatic ascension of this fragment towards the surface.

Application of INAA and XRF to the analysis of a basic alkaline rock

A study of a basic alkaline rock has been carried out using instrumental neutron activation (INAA), X-ray fluorescence (XRF) and wet chemical analyses techniques. Elemental composition and major oxides have been reported. The rock is found to be rich in K2O, Na2O, MgO, CaO, Al2O3, and Ti−Fe minerals. The transition elements are also fairly concentrated in the rock. The analyses show the igneous nature of the rock from a high potassic origin.

Petrology of the alkaline rocks around Sivamalai, Coimbatore District, Tamilnadu, India

The alkaline rocks around Sivamalai are represented by syenites, nepheline syenites and basic alkaline rocks. The syenites are comprised of olivine-pyroxene, pyroxene, hornblende and biotite syenites and syenodiorites, pegmatite phases and quartz syenite. Nepheline syenites include pyroxene, hornblende and biotite bearing varieties. Syenites with pockets of garnet, magnetite or corundum occur in some places between the nepheline syenites and the country rocks represented dominantly by granitic gneisses. The basic alkaline rocks are represented by barkevikite melteigite and they occur as xenoliths amidst the nepheline syenites. The field mineralogical and petrochemical characteristics indicate the igneous nature of these rocks. The syenites and nepheline syenites are inferred to be evolved from pyroxene syenodiorite and barkevikite melteigite respectively. The irregular distribution of garnet and corundum is attributed to the metasomatism of the country rocks by the alkaline magma.

Parameters critical to the morphology of fluidization craters

The anomalously high number of craters with diameter less than 2.8 km, the igneous nature of rocks from the Apollo landing sites, and the possibility of outgassing magmas in the lunar crust, suggest that fluidization may be a viable mechanism for producing many of the smaller lunar craters, Fluidization craters were formed in the laboratory by blowing gas through various thicknesses of particulate material. Gas pressure, ‘regolith’ thickness, and the duration of gas streaming were controlled over practical experimental limits and compared with the resultant crater morphology. Low to moderate fluidization pressures on ‘coarsely’ crushed limestone (Mo = 0.40, So = =0.50) with low cohesion (o - 43°) produced bowl shaped, basin shaped, and flat bottomed craters. Bowl shaped craters change into basin shaped and/or flat bottomed craters with long durations of gas streaming. Cone, funnel, and flat-funnel shaped craters are indicative of high fluidization pressures. Craters formed in ‘finely’ crushed limestone (Mo - 1.55, So - 0.85) that is electrostatically charged by the streaming gas, are flat bottomed. Terraced craters develop from slumping during and after the discontinuation of gas flow. Central mounds inn terraced craters result from slumping into a confined space. In particulate material, fluidization craters have high circularity and axial symmetry, similar to those produced by impact. The use of an impact model and crater morphology (‘normal’, ‘flat bottomed’, and ‘concentric’) for estimating lunar regolith depth is questioned because similar craters can be produced by fluidizationn processes in a thicker regolith.

Geochemical characters of muscovite from the Pan African Mandi Granite, and its emplacement and evolution

DOI = 10.3126/hjs.v5i7.1291 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.98