Microtextural and mineral chemical analyses of andesite–dacite from Barren and Narcondam islands: Evidences for magma mixing and petrological implications

Andesite and dacite from Barren and Narcondam volcanic islands of Andaman subduction zone are composed of plagioclase, orthopyroxene, clinopyroxene, olivine, titanomagnetite, magnesio-hornblende and rare quartz grains. In this study, we use the results of mineral chemical analyses of the calc-alkaline rock suite of rocks as proxies for magma mixing and mingling processes. Plagioclase, the most dominant mineral, shows zoning which includes oscillatory, patchy, multiple and repetitive zonation and ‘fritted’ or ‘sieve’ textures. Zoning patterns in plagioclase phenocrysts and abrupt fluctuations in An content record different melt conditions in a dynamic magma chamber. ‘Fritted’ zones (An₅₅) are frequently overgrown by thin calcic (An₇₂) plagioclase rims over well-developed dissolution surfaces. These features have probably resulted from mixing of a more silicic magma with the host andesite. Olivine and orthopyroxene with reaction and overgrowth rims (corona) suggest magma mixing processes. We conclude that hybrid magma formed from the mixing of mafic and felsic magma by two-stage processes – initial intrusion of hotter mafic melt (andesitic) followed by cooler acidic melt at later stage.     

Petrography and geochemistry of uranium mineralised Precambrian granitic-pegmatitic rocks of Mawlait, West Khasi Hills district, Meghalaya

During radiometric investigation at Mawlait, significant uranium mineralisation (0.024–0.22%U₃O₈) was located mainly within the small pegmatite (garnet bearing quartzofeldspathic rock), which are locally segregated within migmatite at Umiang River section. Pink granite and granite gneisses are the dominant lithounits of the study area showing fertile character and spotty radioactivity at several places. Radioactivity in these rocks is mainly contributed by discrete uraninite grains along with some zircon and xenotime. Granites are peraluminous, low-Ca in nature and their geochemical signatures suggest derivation from a felsic source. Discriminant diagrams using Rb, Nb and Y indicate ‘within plate’ to ‘volcanic arc’ nature of the rock. The uraniferous pegmatitic veins within migmatite appear to have formed due to localised metamorphic segregation during late stage of anatexis. Petromineralogical and geochemical studies suggest that the uranium mineralisation in granitic-pegmatitic rocks of the area is mainly syn-magmatic type.     

Intergrowths of nepheline-potassium feldspar and kalsilite-potassium feldspar: A re-examination of the ‘pseudo-leucite problem’

The alkalic ultramafic Batbjerg intrusion of East Greenland contains rocks in which nepheline and leucite are important constituents. In addition, there are vermicular, finger print intergrowths of nepheline with potassium feldspar, and patchy to micrographic intergrowths of kalsilite with potassium feldspar. The history of the pseudoleucite problem is reviewed, and it is suggested that the term pseudoleucite be restricted to intergrowths of nepheline with alkali feldspar that appear to be pseudomorphs with the crystal morphology of leucite. It is further suggested that flame-like or feather-like finger print intergrowths of nepheline with alkali feldspar, that are either interstitial to the other minerals of the rock or have grown perpendicularly on relative large and often euhedral nepheline grains are an entirely different problem and are best explained by late-stage magmatic crystallization within the system NaAlSiO₄-KAlSiO₄-SiO₂-H₂O.In the Batbjerg intrusion the early crystallization of nepheline was followed by the co-crystallization of nepheline with leucite, or in some cases by nepheline and a silica-rich leucite. Although the magma was essentially dry, as indicated by the dominantly pyroxenitic character of the rocks, water pressure rose toward the late stages of crystallization as indicated by the presence of phlogopite and occasionally both amphibole and zeolite. Shrinkage of the leucite stability field attendant upon this rise in left the liquid that was crystallizing nepheline and leucite stranded on the nepheline-alkali feldspar cotectic. Shrinkage occurred too rapidly for the liquid to remain at the reaction point of the system, and leucite, therefore, was not resorbed. The remaining liquid crystallized rapidly as flames of vermicular intergrowth of nepheline with potassium feldspar (composition Ne 24.0, Ks 45.9, Qz 30.1), a texture that might be attributable to supercooling. Silica-rich leucite compositions (Ks 68.8, Qz 31.2) decomposed to intergrowths of kalsilite with potassium feldspar but reaction kinetics, or possibly variations in throughout the intrusion, prevented the breakdown of leucite.     

Deformation-enhanced reaction in experimentally deformed plagioclase-olivine aggregates

The effects of deformation on the kinetics of the net-transfer reaction anorthite + forsterite → cpx + opx + spinel ± gt were studied using static and shear deformation experiments. Experiments were performed on dry anorthite-olivine (An₉₂–Fo₉₃) samples at 900°C and pressures between 1,000 and 1,600 MPa in a Griggs apparatus. Deformed (‘non-hydrostatic’) and undeformed (‘static’) samples are compared in terms of phase petrology, reaction rate and reaction mechanisms. Anorthite + olivine reactions are diffusion-controlled as seen from reaction rim structures. In undeformed samples, delayed reaction onset and low reaction rates demonstrate sluggish nucleation of reaction products and slow rates of diffusion at dry conditions, even at 700–900 MPa confining pressure overstepping. The reaction rate is enhanced in deformed An–Fo samples. The higher rate is mainly attributed to a combination of high stresses and viscous deformation processes of the reactants and products, which cause an increase in the nucleation rate of products. The results imply that viscous deformation processes alone can be responsible for the initiation and localisation of metamorphic reactions in dry rocks in the absence of fluid infiltration. A. A. de Ronde was supported by the Swiss National Fond grants 2100-057092.99 and 2000-065041.01.     

Geochemistry of Santo Antônio da Barra Kamafugites, Goiás, Brazil

This work focuses on the kamafugites from Santo Antônio da Barra, Minas–Goiás Alkaline Province. These rocks contain olivine, clinopyroxene, titanomagnetite, perovskite, leucite (pseudomorphs), kalsilite, nepheline, and phlogopite.The rocks investigated are ultrabasic, with high contents of CaO, FeO, and TiO₂, high to moderate contents of Al₂O₃, alkalis, and P₂O₅, and low contents of MgO. The alkaline characteristic of the rocks is reflected in TiO₂, K₂O, and Na₂O contents and in the frequent presence of normative nepheline and leucite. K₂O contents are not primary since most of the leucite was replaced by analcime.The negative K anomaly verified in the extended incompatible element distribution diagram for kamafugites seems to be mainly related to alteration. Kamafugites are characterized by a marked enrichment in incompatible and large ion lithophile elements together with other typical compatible elements.The Santo Antônio da Barra kamafugites are less enriched in titanium, niobium, zirconium, and REE than the Mata da Corda and most of the Toro-Ankole ones. San Venanzo–Cupaello rocks have much lower titanium contents.     

Melt variability in percolated peridotite: an experimental study applied to reactive migration of tholeiitic basalt in the upper mantle

Melt-rock reaction in the upper mantle is recorded in a variety of ultramafic rocks and is an important process in modifying melt composition on its way from the source region towards the surface. This experimental study evaluates the compositional variability of tholeiitic basalts upon reaction with depleted peridotite at uppermost-mantle conditions. Infiltration-reaction processes are simulated by employing a three-layered set-up: primitive basaltic powder (‘melt layer’) is overlain by a ‘peridotite layer’ and a layer of vitreous carbon spheres (‘melt trap’). Melt from the melt layer is forced to move through the peridotite layer into the melt trap. Experiments were conducted at 0.65 and 0.8 GPa in the temperature range 1,170–1,290°C. In this P-T range, representing conditions encountered in the transition zone (thermal boundary layer) between the asthenosphere and the lithosphere underneath oceanic spreading centres, the melt is subjected to fractionation, and the peridotite is partially melting (T _s ~ 1,260°C). The effect of reaction between melt and peridotite on the melt composition was investigated across each experimental charge. Quenched melts in the peridotite layers display larger compositional variations than melt layer glasses. A difference between glasses in the melt and peridotite layer becomes more important at decreasing temperature through a combination of enrichment in incompatible elements in the melt layer and less efficient diffusive equilibration in the melt phase. At 1,290°C, preferential dissolution of pyroxenes enriches the melt in silica and dilutes it in incompatible elements. Moreover, liquids become increasingly enriched in Cr₂O₃ at higher temperatures due to the dissolution of spinel. Silica contents of liquids decrease at 1,260°C, whereas incompatible elements start to concentrate in the melt due to increasing levels of crystallization. At the lowest temperatures investigated, increasing alkali contents cause silica to increase as a consequence of reactive fractionation. Pervasive percolation of tholeiitic basalt through an upper-mantle thermal boundary layer can thus impose a high-Si ‘low-pressure’ signature on MORB. This could explain opx + plag enrichment in shallow plagioclase peridotites and prolonged formation of olivine gabbros.     

Experimental determination of the reaction dolomite + 2 coesite = diopside + 2 CO2 to 6 GPa

 The carbonation reaction CaMg(CO₃)₂ (dolomite)+2SiO₂ (coesite)=CaMgSi₂O₆ (diopside)+2 CO₂ (vapor) has been determined experimentally between 3.5 and 6 GPa in a multiple-anvil, solid-media apparatus. This reaction, a candidate for carbonation of eclogites (garnet+clinopyroxene) in the Earth’s mantle, lies at higher pressure for a given temperature than do the carbonation reactions for peridotites (olivine+orthopyroxene±clinopyroxene). A depth interval may exist within the Earth’s mantle under either ‘normal’ or ‘subduction’ thermal regimes where carbonated peridotite could coexist with carbonate-free, CO₂-bearing eclogite. Received: 25 May 1994/Accepted: 13 June 1995     

Intrafolial folds and associated structures in a progressive strain environment of Darjeeling-Sikkim Himalaya

The Dating rocks and Darjeeling gneisses, which constitute the Sikkim dome in eastern Himalaya, as well as the Gondwana and Buxa rocks of ‘Rangit Window’, disclose strikingly similar sequences of deformation and metamorphism. The structures in all the rocks belong to two generations. The structures of early generation are long-limbed, tight near-isoclinal folds which are often intrafolial and rootless. These intrafolial folds are associated with co-planar tight folds with variably oriented axes and sheath folds with arcuate hinges. Penetrative axial plane cleavage and mineral lineation are related structures; transposition of bedding is remarkable. This early phase of deformation (D ₁) is accompanied by constructive metamorphism. The structures of later generation are open, asymmetrical or polyclinal; a crenulation cleavage or discrete fracture may occur. The structures of early generation are distorted by folds of later generation and recrystallized minerals are cataclastically deformed. Recrystallization is meagre or absent during the later phase of deformation (D ₂). The present discussion is on structures of early generation and strain environment during theD ₁ phase of deformation. The concentration of intrafolial folds in the vicinity of ductile shear zones and decollement or detachment surface (often described as ‘thrust’) may be considered in this context. The rocks of Darjeeling-Sikkim Himalaya display minor structures other than intrafolial folds and variably oriented co-planar folds. The state of finite strain in the rocks, as observed from features like flattened grains and pebbles, ptygmatic folds and boudinaged folds indicate combination of flattening and constrictional type strain. The significance of the intrafolial folds in the same rocks is discussed to probe the environment of strain during progressive deformation (D ₁).     

Natural and synthetic leucites studied by solid state 29-Si and 27-Al NMR and 57-Fe Mossbauer spectroscopy

The 29-Si NMR spectra of natural and synthetic leucites (KAlSi₂O₆) are found to contain a number of resonances which are interpreted in terms of the known structure of low-temperature (tetragonal) leucite. Computer simulation of the spectra suggests that the most distorted tetrahedral lattice site T ₁ contains a higher proportion of Al than the other two tetrahedral sites. The occurrence of some ordering of the tetrahedral Si and Al in leucite is confirmed by Mossbauer studies of synthetic iron-containing leucites, including the fully ferric end-member KFeSi₂O₆, in which the three tetrahedral sites can be distinguished. On replacement of about half this Fe by Al, the most distorted of these sites is lost from the spectrum, reflecting the preference of Al for this site. A linear relationship is found between the unit cell dimensions of all these leucites and their iron content.     

Phase equilibria in the silica-undersaturated part of the KAlSiO4– Mg2SiO4– Ca2SiO4– SiO2– F system at 1 atm and the larnite-normative trend of melt evolution

The evolution of nephelinitic melts in equilibrium with mica-bearing liquidus assemblages and melting relations have been studied on two silica-undersaturated joins of the KAlSiO₄– Mg₂SiO₄– Ca₂SiO₄– SiO₂– F system at atmospheric pressure by quench runs in sealed platinum capsules. Fluorine has been added to the batch compositions by the direct exchange of fluorine for oxygen (2F⁻ = O²⁻). The first join is the pseudo-ternary Forsterite – Diopside – KAlSiO₃F₂ system. Forsterite, diopside, F-phlogopite and leucite crystallisation fields and a fluoride-silicate liquid immiscibility solvus are present on the liquidus surface of the join. Sub-liquidus and sub-solidus phases include akermanite, cuspidine, spinel, fluorite and some other minor fluorine phases. The second system is the pseudo-binary Akermanite – F-phlogopite join that intersects the Forsterite – Diopside – KAlSiO₃F₂ join. Akermanite, forsterite, diopside, F-phlogopite, leucite and cuspidine are found to crystallise on the join. Forsterite (fo) and leucite (lc) are related to F-phlogopite (phl) by a reaction with the fluorine-bearing liquid: fo + lc + l = phl, and the reaction proceeds until forsterite or leucite are completely consumed. The reaction temperature and resulting phase association depend on batch composition. Thus, leucite is not stable in the sub-solidus of the Akermanite – F-phlogopite join, but is preserved in a part of the Forsterite – Diopside – KAlSiO₃F₂ system where forsterite reacts out, or does not crystallise at all. The phlogopite-in reaction has an important effect on the composition of the coexisting liquid. The liquids initially saturated in forsterite evolve to extremely Ca rich, larnite-normative residuals. The experimental data show that larnite-normative melilitolites can crystallise from evolved melilititic melts generated from “normal” melanephelinitic parental magmas with no normative larnite. The evolution towards melilitites requires fractionation of phlogopite-bearing assemblages under volatile pressure. Received: 3 June 1997 / Accepted: 5 January 1998     

Geochemistry and petrogenesis of lamprophyric dykes and the associated rocks from Eslamy peninsula, NW Iran: Implications for deep-mantle metasomatism

Eslamy peninsula in NW of Iran is formed by a strato-volcano with collapsed calderon, which is intruded by lamprophyric dykes with minette composition. Also trachytic and microsyenitic dykes have intruded the volcanic rocks. The oldest volcanic activity includes eruption of leucite basanite, leucite tephrite, basanite and tephrite, which are associated with pyroclastic rocks. Lamprophyric dykes are distinguishable with large mica phenocrysts. Mica-clinopyroxenite xenoliths can be found in the rocks. The source magma of the rocks had a ultrapotassic to shoshonitic nature, rich in LREE and LILE. Eslamy peninsula lamprophyres are between alkaline and calc-alkaline lamprophyres in terms of REE patterns and spider diagrams for trace elements, but are closer to clac-alkaline lamprophyres. The behaviour of trace elements studied by the means of spider diagrams show that the magma, producing the lamprophyres, is generated from deep-mantle probably from a garnet-bearing source (garnet lherzolite) with high CO₂/H₂O content. The resulted magma had interacted with crustal materials and had formed Eslamy peninsula lamprophyres in a post-collisional tectonic setting. Geochemistry of rare elements indicate an extensive rutile-rich metasomatism in the source magma of the lamprophyres.     

Are Majhgawan-Hinota pipe rocks truly group-I Kimberlite?

The diamond bearing pipe rocks in Majhgawan-Hinota (more than four pipes) occur as intrusives in sandstones of Kaimur Group. These Proterozoic (974 ±30-1170 ±20 Ma) intrusive rocks, occupying the southeastern margin of Aravalli craton, were called as ‘micaceous kimberlite’ in tune with the reported kimberlite occurrences from other parts of the world. Judging from the definition of kimberlite, as approved by the IUGS Subcommission on Systematics of Igneous Rocks, it is not justified to call these rocks as ‘micaceous kimberlite’. Rather the mineralogical assemblages such as absence of typomorphic mineral monticellite (primary), abundance of phlogopite cognate, frequent presence of barite and primary carbonate mostly as calcite coupled with ultrapotassic and volatile-rich (dominantly H₂O) nature and high concentration of incompatible elements (such as Ba, Zr, Th, U), low Th/U ratios, low REE and no Eu-anomaly clearly indicate a close similarity with that of South African orangeites. Thus orangeites of Proterozoic age occur outside the Kaapvaal craton of South Africa which are much younger (200 Ma to 110 Ma) in age.     

Geodynamic setting,structure, and composition of continuous trachybasalt-trachyandesite-rhyolite series in the north of Altai-Sayan area: the role of crust-mantle interaction in continental magma formation

The geologic positions and geochemical and isotope parameters of the Ordovician-early Silurian and Early-Middle Devonian continuous volcanic series of the Minusa basin and its mountainous framing are compared. Both series are composed mostly of moderately alkaline rocks with variations in SiO₂ contents from 45 to 77 wt.%. The Ordovician-early Silurian series differs from the Early-Middle Devonian one in lower contents of TiO₂ (< 1.7 wt.%) and Fe₂O_3tot and higher contents of Al₂O₃ in all rock varieties and in the more fractionated REE patterns of trachybasalts. The compositions of both series reflect two simultaneous mechanisms of magma evolution. The main process was fractional crystallization leading to the formation of rocks from trachybasalts to trachyrhyodacites. The accessory mechanism was the contamination of fractionated melts by crustal material, anatectic melting of crust, and mixing of deep-seated magmas with crustal melts. These processes had specifics at each stage and were controlled by the composition of the sources of parental melts. Their geochemical and isotopic parameters (high alkalinity, high contents of lithophile elements, negative anomalies of Nb, Ta, and Ti, and enrichment in radiogenic Sr) point to the interaction of mantle plumes with the lithospheric mantle that was metasomatically transformed during the preceding Vendian-early Cambrian subduction processes.     

The Mesoproterozoic Midsommersø dolerites and associated high-silica intrusions, North Greenland: crustal melting, contamination and hydrothermal alteration

The Midsommersø dolerites and the flood basalts of the Zig-Zag Dal Basalt Formation, eastern North Greenland, represent a major Mesoproterozoic (∼1,380 Ma) igneous event. The intrusive rocks form large sheets within a succession of feldspathic sandstones which underlie the basalts. The geochemistry of the basalts has recently been re-investigated and reported elsewhere in this Journal; here we present new trace element and Nd-, Sr- and Pb-isotopic data for the intrusive rocks. Unlike the basalts, the intrusions yield evidence of considerable interaction and contamination with upper crustal rocks, especially the sandstones. High-silica rocks (80–90 wt% SiO₂) occur in sheets, up to 60 m thick. They were formed by mobilisation of sandstones, and indicate a very high rate of emplacement of hot basic magma into the sandstones at depth. These mobilised sandstones (‘rheopsammites’) are among the most SiO₂-rich intrusive rocks on earth. Sheets of remobilised granitoid rocks from the crystalline basement (∼70% SiO₂) are also present. Hydrothermal activity, associated with the igneous event, significantly changed the compositions of the silicic rocks as well as that of many dolerites. Sheets of hydrothermally altered (‘red’) dolerites and silicic rocks invariably have borders of dark, fresh dolerite; this is interpreted to be the result of intrusion from zoned magma chambers. Nd isotope data confirm the crustal origin of the silicic rocks as well as the contamination of some dolerites by components derived from crustal sources, while Sr- and Pb-isotopic systems are strongly affected by the hydrothermal alteration, and give little information on the petrogenesis of the rocks. Recent loss of Sr from hydrothermally altered rocks further affected the Sr isotope systems, and earlier age determinations by the Rb–Sr whole-rock isochron method (1,230 Ma) have proved to be in error. The dolerites and the basalts are geochemically very similar, but most dolerites have moderately negative Eu anomalies that are not observed in the basalts. Eu anomalies in the dolerites could be related to contamination by sandstone at depth, but it is not clear why the basalts escaped a similar contamination.     

Relative contributions of crust and mantle to the origin of the Bijli Rhyolite in a palaeoproterozoic bimodal volcanic sequence (<Emphasis Type="Italic">Dongargarh Group</Emphasis>), central India

New mineralogical, bulk chemical and oxygen isotope data on the Palaeoproterozoic Bijli Rhyolite, the basal unit of a bimodal volcanic sequence (Dongargarh Group) in central India, and one of the most voluminous silicic volcanic expressions in the Indian Shield, are presented. The Bijli Rhyolite can be recognized as a poorly sorted pyroclastic deposit, and comprises of phenocrystic K-feldspar + albite ± anorthoclase set in fine-grained micro-fragmental matrix of quartz-feldspar-sericite-chlorite-iron-oxide ± calcite. The rocks are largely metaluminous with high SiO₂, Na₂O + K₂O, Fe/Mg, Ga/Al, Zr, Ta, Sn, Y, REE and low CaO, Ba, Sr contents; the composition points to an ‘A-type granite’ melt. The rocks show negative Cs-, Sr-, Eu- and Ti- anomalies with incompatible element concentrations 2–3 times more than the upper continental crust (UCC). LREE is high (La/Yb ∼ 20) and HREE 20–30 times chondritic. δ¹⁸O_whole-rock varies between 4.4 and 7.8‰ (mean 5.87±1.26‰). The Bijli melt is neither formed by fractionation of a basaltic magma, nor does it represent a fractionated crustal melt. It is shown that the mantle-derived high temperature basaltic komatiitic melts/high Mg basalts triggered crustal melting, and interacted predominantly with deep crust compositionally similar to the Average Archaean Granulite (AAG), and a shallower crustal component with low CaO and Al₂O₃ to give rise to the hybrid Bijli melts. Geochemical mass balance suggests that ∼ 30% partial melting of AAG under anhydrous condition, instead of the upper continental crust (UCC) including the Amgaon granitoid gneiss reported from the area, better matches the trace element concentrations in the rocks. The similar Ta/Th of the rhyolites (0.060) and average granulite (0.065) vs. UCC (0.13) also support a deep crustal protolith. Variable contributions of crust and mantle, and action of hydrothermal fluid are attributed for the spread in δ¹⁸O^whole-rock values. The fast eruption of high temperature (∼ 900°C) rhyolitic melts suggests a rapid drop in pressure of melting related to decompression in an extensional setting.     

The geochemistry and petrogenesis of K-rich alkaline volcanics from the Batu Tara volcano,eastern Sunda arc

Mineralogical, major and trace element, and isotopic data are presented for leucite basanite and leucite tephrite eruptives and dykes from the Batu Tara volcano, eastern Sunda arc. In general, the eruptives are markedly porphyritic with phenocrysts of clinopyroxene, olivine, leucite ±plagioclase±biotite set in similar groundmass assemblages. These K-rich alkaline volcanics have high concentrations of large-ion-lithophile (LIL), light rare earth (LRE) and most incompatible trace elements, and are characterized by high ⁸⁷Sr/⁸⁶Sr (0.70571–0.70706) and low ¹⁴³Nd/ ¹⁴⁴Nd (0.512609–0.512450) compared with less alkaline volcanics from the Sunda arc. They also display the relative depletion of Ti and Nb in chondrite-normalized plots which is a feature of subalkaline volcanics from the eastern Sunda arc and arc volcanics in general. Chemical and mineralogical data for the Batu Tara K-rich rocks indicate that they were formed by the accumulation of variable amounts of phenocrysts in several melts with different major and trace element compositions. The compositions of one of these melts estimated from glass inclusions in phenocrysts is relatively Fe-rich (100 Mg/(Mg + Fe²⁺)=48–51) and is inferred to have been derived from a more primitive magma by low-pressure crystal fractionation involving olivine, clinopyroxene and spinel. Mg-rich (mg 90) and Cr-rich (up to 1.7 wt. % Cr₂O₃) zones in complex oscillatory-zoned clinopyroxene phenocrysts probably also crystallized from such a magma. The marked oscillatory zoning in the clinopyroxene phenocrysts is considered to be the result of limited mixing of relatively evolved with more primitive magmas, together with their phenocrysts, along interfaces between discrete convecting magma bodies.     

Mineral chemistry and petrogenetic aspects of the Vico lavas,Roman volcanic region,Italy

A detailed study of the phase chemistry of selected lavas from Vico shows that the “excess silica” in leucite is a significant and widespread crystal-chemical characteristic which may be interpreted in terms of an extended solubility of KAlSi₃O₈ and/or differential loss of potassium from the leucite structure. Plagioclase geothermometer data suggest increasing plagioclase control in the differentiation with decreasing Mg/Mg+Fe²⁺ ratios for the bulk rocks. The pyroxene compositions show moderate amounts of ferri-Tschermak's and Ca-Tschermak's molecules, probably controlled by temperature and compositional variations in the cooling liquids. Both microphenocrystal and late-crystallizing micas correspond to phlogopite compositions throughout the suite. The phase chemistry is discussed in relation to the differentiation of the suite and current petrogenetic models.     

Leaching of silica bands and concentration of magnetite in Archean BIF by hypogene fluids: Beebyn Fe ore deposit, Yilgarn Craton, Western Australia

The ~2,752-Ma Weld Range greenstone belt in the Yilgarn Craton of Western Australia hosts several Fe ore deposits that provide insights into the role of early hypogene fluids in the formation of high-grade (>55 wt% Fe) magnetite-rich ore in banded iron formation (BIF). The 1.5-km-long Beebyn orebody comprises a series of steeply dipping, discontinuous, <50-m-thick lenses of magnetite–(martite)-rich ore zones in BIF that extend from surface to vertical depths of at least 250 m. The ore zones are enveloped by a 3-km-long, 150-m-wide outer halo of hypogene siderite and ferroan dolomite in BIF and mafic igneous country rocks. Ferroan chlorite characterises 20-m-wide proximal alteration zones in mafic country rocks. The magnetite-rich Beebyn orebody is primarily the product of hypogene fluids that circulated through reverse shear zones during the formation of an Archean isoclinal fold-and-thrust belt. Two discrete stages of hypogene fluid flow caused the pseudomorphic replacement of silica-rich bands in BIF by Stage 1 siderite and magnetite and later by Stage 2 ferroan dolomite. The resulting carbonate-altered BIF is markedly depleted in SiO₂ and enriched in CaO, MgO, LOI, P₂O₅ and Fe₂O_3(total) compared with the least-altered BIF. Subsequent reactivation of these shear zones and circulation of hypogene fluids resulted in the leaching of existing hypogene carbonate minerals and the concentration of residual magnetite-rich bands. These Stage 3 magnetite-rich ore zones are depleted in SiO₂ and enriched in K₂O, CaO, MgO, P₂O₅ and Fe₂O_3(total) relative to the least-altered BIF. Proximal wall rock hypogene alteration zones in mafic igneous country rocks (up to 20 m from the BIF contact) are depleted in SiO₂, CaO, Na₂O, and K₂O and are enriched in Fe₂O_3(total), MgO and P₂O₅ compared with distal zones. Recent supergene alteration affects all rocks within about 100 m below the present surface, disturbing hypogene mineral and the geochemical zonation patterns associated with magnetite-rich ore zones. The key vectors for identifying hypogene magnetite-rich Fe ore in weathered outcrop include textural changes in BIF (from thickly to thinly banded), crenulated bands and collapse breccias that indicate volume reduction. Useful indicators of hypogene ore in less weathered rocks include an outer carbonate–magnetite alteration halo in BIF and ferroan chlorite in mafic country rocks.     

Trace element variations in olivine phenocrysts from Ugandan potassic rocks as clues to the chemical characteristics of parental magmas

Olivine phenocrysts in ugandite and leucite basanite from the western branch of the East African Rift have been analysed for up to 34 trace elements by Laser-ICP-MS with detection limits as low as 1 ppb. A combination of point analyses with varying ablation crater diameters and line scans allow the identification of subtle zonations from core to rim, as well as characterization of the chemical effects of contamination along cracks. Trace element concentrations are remarkably uniform between large and small phenocrysts; fractionated leucite basanites (Mg# 59) have higher D _Ca and D _Al, and less fractionated LREE/HREE than MgO-rich ugandites (Mg# 75–80). Minor zonation is seen in elements with cation charges from 5+ to 2+ (P, Ti, Zr, Cr, Al, Sc, V, Cu, Mn, Ni) and show correlation between Ti and Al, but not P. Early phenocryst cores with high Li or Ni, low Mn, or enrichments in many trace elements can be identified, whereas xenocrysts have exceptionally low Na, Cr, Ti, V and Co. Partition coefficients for Ni are 31–35, less than in lamproites, with which they demonstrate an approximately linear correlation with K₂O content, K₂O/Al₂O₃ and K₂O/Na₂O of the melt, but none with SiO₂ content or Mg#. D-values for Cr, Mn and Co overlap with those of basalts, whereas those for Sc (0.011–0.018), Zn (0.44–0.49) and Ga (0.006–0.007) are lower. D _V of various potassic rocks (0.015 in the Ugandan rocks) confirms the dependence on fO₂ calibrated by the Fe³⁺/(Fe³⁺+Fe²⁺) of spinels; the Ugandan potassic rocks crystallized at fO₂ = FMQ to FMQ + 1. The ugandite olivines have some trace element characteristics reminiscent of those in metasomatized Kaapvaal peridotites, but not ocean islands. Line scan analyses are contaminated in Al, Ca, Cu, Ga, Sr, Zr, Nb, La and Ce, elements that are also concentrated in microcracks between subgrains, indicating smearing out during polishing, and demonstrating that large spot analyses produce the best results.     

Geochemistry of granitization and magmatic replacement of basic volcanic rocks in the contact aureole of the Yurchik gabbronorite intrusion, Ganal Range, Kamchatka

Geological and geochemical data indicate that the formation of the granulite-like rocks in the contact aureole of the Yurchik gabbronorite intrusion of the Ganal Range, Kamchatka, was caused by the contact metamorphism, metasomatism, and local melting of the initial volcanosedimentary rocks of the Vakhtalka Sequence of the Ganal Group. The temperature in the inner part of the aureole reached 700–800°C and caused the transformation of the basic volcanic rocks of the sequence into two pyroxene-plagioclase, clinopyroxene-amphibole-plagioclase, and amphibole-plagioclase hornfelses, while sedimentary rocks were converted into garnet-biotite ± cordierite hornfelses. The hornfelsed basic volcanic rocks were locally subjected to metasomatic alteration and magmatic replacement with formation of biotite-orthopyroxene-plagioclase metasomatic bodies containing biotite-orthopyroxene-plagioclase ± garnet veinlets and aggregates. During these processes, sedimentary interlayers were converted into garnet enderbites at 700–800°C and 3.2–4.8 kbar. The comparison of the chemical composition of basic volcanic rocks of the Vakhtalka Sequence and their transformation products indicates that the metasomatic alteration and magmatic replacement correspond to siliceous-alkaline metasomatism (granitization) and cause subsequent and uneven influx of SiO₂, Al₂O₃, Na₂O, K₂O, Rb, Ba, Zr, Nb, and Cl and removal of Fe, Mg, Mn, Ca, Cr, Co, Ti, Y, and S. REE data on basic metavolcanic rocks, hornfelses, and metasomatites suggest that the processes of hornfelsation, metasomatism, and magmatic replacement of the initial volcanic rocks were accompanied by significant increase in LREE and slight decrease in HREE. The Sr and Nd isotope study of the rocks in the aureole showed that the initial basic volcanic rocks of the Vakhtalka Sequence are isotopically close to both mature island arc tholeiites and mid-ocean ridge basalts. The metasomatic alteration and magmatic replacement of volcanic rocks in the aureole lead to the decrease of ¹⁴³Nd/¹⁴⁴Nd and increase of ⁸⁷Sr/⁸⁶Sr approximately parallel to mantle array. Pb isotopic ratios in the studied rocks become more radiogenic from initial metavolcanic rocks to metasomatites. It is suggested that the processes of metamorphism, metasomatism, and magmatic replacement were caused by highly mineralized mantle fluids, which percolated along magmatic channels serving as pathways for gabbroid magma.