Peralkaline nephelinites. I. Comparative petrology of Shombole and Oldoinyo L'engai,East Africa

Shombole, a nephelinite-carbonatite volcano in south Kenya, erupted silicate lavas, carbonatite dikes and tuffs, and pyroclastic rocks similar to those at other East African alkaline centres. Shombole lavas containing cpx + nepheline + accessory minerals range from perovskite-bearing nephelinites (43% SiO₂, volatile-free) to sphene-bearing and phonolitic nephelinites (46–49% SiO₂) and phonolites (49–56% SiO₂) and have low peralkalinity ([Na+K]/Al 1.15) which does not correlate with SiO₂. Early fractionation of olivine and clinopyroxene strongly depleted Ni and Cr concentrations (10 ppm); fractionation of perovskite, melanite, sphene, and apatite produced negative correlations of all REE with SiO₂. Many lavas contain cognate intrusive xenoliths and xenocrysts and oscillatory zoning is a common feature of clinopyroxene, nepheline, and melanite crystals, indicating recycling of intrusive material. Irregular calcite-rich bodies in many samples are interpreted as quenched immiscible Ca-carbonatite liquid, and [Ca-carbonate]-silicate liquid immiscibility is observed in experiments with one nephelinite. Chemical variation in the Shombole suite can be modeled as a combination of crystal fractionation (clinopyroxene and heavy minor phases) and retention of neutral density nepheline derived from disaggregated xenoliths entrained during emplacement of dike swarms. Six newly analyzed lavas from Oldoinyo L'engai, northern Tanzania, are geochemically similar to Shombole nephelinites except that they have relatively high Na₂O+K₂O (average 18% vs 12%) and Zr (average 680 ppm vs 400 ppm). They are extremely peralkaline and are not typical of nephelinites from other centres. Three with [Na+K]/Al1.5 contain euhedral wollastonite phenocrysts; three with [Na+K]/Al2.0 contain combeite (Na₂Ca₂Si₃O₉) phenocrysts and pseudomorphs after wollastonite. Both types contain abundant sodalite phenocrysts (+nepheline+clinopyroxene+melanite+sphene). Seven other wollastonite nephelinite samples from L'engai have been described, but it is a lava type rarely seen in other centres. Combeite has been described from only two other locations (Mt. Shaheru, Zaire; Mayener Feld, Eifel). The hyperalkaline L'engai nephelinites have compositions similar to those of experimental silicate liquids immiscible with natrocarbonatite. Textural evidence for both carbonate-silicate (as carbonate globules) and silicate-silicate (as two optically discrete glasses with distinct compositions) liquid immiscibility is observed in the samples.     

Oversaturated Peralkaline Glassy Trachytes from Kenya

Peralkaline felsic lavas from the central part of the KenyaRift Valley were used by Bowcn as an illustration of the finalstages of fractional crystallization in nature. Despite theimportance of trachyte to this and other theories on the evolutionof peralkaline rhyolites, only one analysis of a pantelleritictrachyte obsidian is available from the literature. Comparisonof four newly analysed trachyte obsidians, from the Menengaivolcano, with the pantellerites from other centres in the Kenyanprovince shows that they form a chemically coherent group. Plottingthe analyses in the system Na₂O-K₂O-Al₂-SiO₂, reveals that thepantelleritic trachytes cannot be related to the pantelleritessimply by crystal liquid equilibria. At least in the case ofthe Kenyan specimens the conventional theory of fractional crystallizationfrom trachyte to rhyolite needs re-examination. Data on theKenyan rocks suggest that alkali feldspar liquids alkali-bearingvapour equilibria have controlled their evolution.     

Peralkaline acid liquids: A petrological study

Electron-microprobe analyses of the feldspars and associated ferromagnesian minerals in the peralkaline volcanics, comendites and pantellerites, are presented together with new data on the major and trace-elements of the rocks and residual glasses. The feldspar phenocrysts in the pantellerites span a narrower range (Or₃₃–Or₃₉) than those of the comendites (Or₃₀–Or₄₆); both sets show only limited increase in Or outwards, and the zoning is greatest in quartz-bearing assemblages. The feldspar microlites in the residual glasses are invariably more potassic (2–4% Or) than their associated phenocrysts. In pantellerites the feldspars become more potassic as the residual liquids become more sodic; thus the most potassic feldspar is found in the most sodic (and peralkaline) pantellerite.Of the ferromagnesian phenocrysts, aenigmatite is the most ubiquitous and is commonly associated with hedenbergite±fayalite, or ferrorichterite; in the later stages of crystallization (groundmass), it is associated with acmite, arfvedsonite and tuhualite. Aside from slight variation in Ti/Fe+Ti ratio, aenigmatite is virtually constant in composition. The pyroxenes from the different assemblages have zones which together almost span the range acmitehedenbergite. Both ferrorichterite and arfvedsonite incorporate F but not Cl, and are slightly potassic. Tuhualite exists as two varieties; one blue and potassic, the other violet and sodic; both varieties reject halogens. Using (estimated) free-energy data, a field in fo₂, T space is postulated in which Fe-Ti oxides are absent; their place is taken by pyroxene and aenigmatite. The no-oxide field will be intercepted by a cooling liquid in which peralkalinity is increasing and in which fo₂ is near but above the FMQ buffer.The characteristic pattern of trace-elements in peralkaline volcanics (e.g., high Nb, Ta, Zr, Mo, Zn, Cd, R.E, etc; low Sr, Ba, Mg) are considered to be as much evidence for the peralkaline (salic) condition as of the genetic process. Several lines of evidence suggest that at liquidus temperatures, peralkaline rhyolites are essentially anhydrous.     

Peralkaline silicate lavas at Oldoinyo Lengai, Tanzania

A detailed study of Oldoinyo Lengai has led to the recognition of two major cone-building stages. An early, predominantly phonolitic stage, Lengai I, forms the southern cone. The recent nephelinitic Lengai II developed following a major sector collapse event over Lengai I. Petrography of Lengai II lavas show that nephelinite is combeite- and wollastonite-bearing. All Oldoinyo Lengai lavas are peralkaline and highly evolved in terms of low Mg#, Ni and Cr values. Within the unique Lengai II combeite–wollastonite–nephelinite (CWN) peralkalinity increases with time to extreme values (Na + K)/Al = 2.36. Mineralogical expression of peralkalinity is the presence of combeite and Na-rich clinopyroxene. In addition, exceptionally high Fe₂O₃ (up to 10.28 wt.%) in nepheline is an indicator for alumina deficiency. Combeite also shows high Fe³⁺. Phonolite and CWN of Lengai I and Lengai II show similarly enriched LILE and LREE values and generally parallel patterns in PM normalized and REE plots.     

Phase Relations of Peralkaline Silicic Magmas and Petrogenetic Implications

The phase relationships of three peralkaline rhyolites fromthe Kenya Rift have been established at 150 and 50 MPa, at oxygenfugacities of NNO - 1·6 and NNO + 3·6 (log fO₂relative to the Ni–NiO solid buffer), between 800 and660°C and for melt H₂O contents ranging between saturationand nominally anhydrous. The stability fields of fayalite, sodicamphiboles, chevkinite and fluorite in natural hydrous silicicmagmas are established. Additional phases include quartz, alkalifeldspar, ferrohedenbergite, biotite, aegirine, titanite, montdoriteand oxides. Ferrohedenbergite crystallization is restrictedto the least peralkaline rock, together with fayalite; it isreplaced at low melt water contents by ferrorichterite. Riebeckite–arfvedsoniteappears only in the more peralkaline rocks, at temperaturesbelow 750°C (dry) and below 670°C at H₂O saturation.Under oxidizing conditions, it breaks down to aegirine. In themore peralkaline rocks, biotite is restricted to temperaturesbelow 700°C and conditions close to H₂O saturation. At 50MPa, the tectosilicate liquidus temperatures are raised by 50–60°C,and that of amphibole by 30°C. Riebeckite–arfvedsonitestability extends down nearly to atmospheric pressure, as aresult of its F-rich character. The solidi of all three rocksare depressed by 40–100°C compared with the solidusof the metaluminous granite system, as a result of the abundanceof F and Cl. Low fO₂ lowers solidus temperatures by at least30°C. Comparison with studies of metaluminous and peraluminousfelsic magmas shows that plagioclase crystallization is suppressedas soon as the melt becomes peralkaline, whatever its CaO orvolatile contents. In contrast, at 100 MPa and H₂O saturation,the liquidus temperatures of quartz and alkali feldspar arenot significantly affected by changes in rock peralkalinity,showing that the incorporation of water in peralkaline meltsdiminishes the depression of liquidus temperatures in dry peralkalinesilicic melts compared with dry metaluminous or peraluminousvarieties. At 150 MPa, pre-eruptive melt H₂O contents rangefrom 4 wt % in the least peralkaline rock to nearly 6 wt % inthe two more peralkaline compositions, in broad agreement withprevious melt inclusion data. The experimental results implymagmatic fO₂ at or below the fayalite–quartz–magnetitesolid buffer, temperatures between 740 and 660°C, and meltevolution under near H₂O saturation conditions. KEY WORDS: peralkaline; rhyolite; phase equilibria     

Peralkaline acid tendencies in Gran Canaria (Canary Islands)

The study of a volcanic series from the island of Gran Canaria (Canary Islands) in which alkaline and peralkaline, saturated and undersaturated rocks coexist, is reported here. Materials with high volatile content (ignimbritic trachytes) were first emitted and the series ended with the eruption of phonolitic lavas. The average peralkalinity index in these rocks is typically about 1.0 and, therefore, peralkaline rocks coexist with non-peralkaline ones. However, a maximum in peralkalinity is found in the ignimbritic rocks of the lower part of the series. In spite of the evident acid peralkaline tendencies of these rocks, it does not seem appropriate to classify them as pantellerites or comendites. Nor are they consistent with the genetic processes proposed for rocks of similar composition and oceanic environment.The crystallization of the feldspars controls the variation trends among the different magmas but the fractionation alone does not sufficiently explain the genesis of successive fluids. Various factors seem to point to the important role which a gas-transfer process causing a geochemical stratification inside the magmatic chamber may have played.The occurrence of peralkaline silicics at Gran Canaria, which is located for away from the active Mid-Atlantic ridge, is not related to transitional basalts. These rocks are a deviation from the main undersaturated alkalic trend which characterizes the volcanism of the Canary Islands, their genesis being related to the realization of favourable local volcanic conditions.     

Geochemistry of High-silica Peralkaline Rhyolites, Naivasha, Kenya Rift Valley

The Recent (<15000 y) volcanic complex of southwest Naivasha,Kenya, consists of mildly peralkaline (comenditic) rhyolitedomes, lava flows, air fall pumices, and lake sediments, withminor, peripheral, basalts and hawaiites. The comendites areeither aphyric or sparsely porphyritic, few samples containing>5 per cent phenocrysts. Phenocryst minerals are quartz-sanidine-ferrohedenbergite-fayalite-titanomagnetite-ilmenite-riebeckite-arfvedsonite-aenigmatite-biotite-zircon.Ferrohedenbergite and zircon are restricted to less peralkaline,and amphibole, aenigmatite, and biotite to more peralkaline,rocks. The comendites show unusually strong enrichment in Cs, F, Hf,Nb, Rb, REE, Ta, Th, U, Y, Zn, and Zr, and extreme depletionin Mg, Ca, Ba, Co, and Sr. REE patterns are moderately LREE-enriched,with large, negative Eu anomalies. Values of LIL/HFS elementratios, such as Th/Ta and Rb/Zr, are unusually high for peralkalinerhyolites, and are consistent with a substantial crustal componentin the comendites. Parameters such as LREE/HREE and Zr/Nb ratiosindicate that the Naivasha rhyolites represent several pulsesof closely related, but subtly different, magmas. Sanidine/glasspartition coefficients for Ba, Pb, Rb, Sr, U, and the REE arepresented for one specimen. Major and trace element modelling, and feldspar-rock relationships,show that closed system crystal fractionation cannot alone accountfor the overall compositional variations in the comendites.A model involving partial melting of variable crustal sourcerocks and migration of dissolved volatile-metal complexes maybe appropriate at Naivasha.     

Peralkaline magma evolution and the tephra record in the Ethiopian Rift

The 3.119 ± 0.010 Ma Chefe Donsa phreatomagmatic deposits on the shoulder of the Ethiopian Rift mark the northern termination of the Silti-Debre Zeyit Fault Zone, a linear zone of focused extension within the modern Ethiopian Rift. These peralkaline pumice fragments and glass shards span a wide range of glass compositions but have a restricted phenocryst assemblage dominated by unzoned sanidine. Glass shards found within the ash occupy a far more limited compositional range (75–76 wt% SiO₂) in comparison with the pumice (64–75 wt% SiO₂), which is rarely mingled. Thermodynamic modeling shows that liquids broadly similar to the least evolved glass composition can be achieved with 50–60 % fractionation of moderately crustally contaminated basalt. Inconsistencies between modeled solutions and the observed values of CaO and P₂O₅ highlight the important role of fluorine in stabilizing fluor-apatite and the limitations of current thermodynamic models largely resulting from the scarce experimental data available for the role of fluorine in igneous phase stability. On the basis of limited feldspar heterogeneity and crystal content of pumice at Chefe Donsa, and the difficulties of extracting small volumes of Si-rich melt in classical fractional crystallization models, we suggest a two-step polybaric process: (1) basaltic magma ponds at mid-upper-crustal depths and fractionates to form a crystal/magma mush. Once this mush has reached 50–60 % crystallinity, the interstitial liquid may be extracted from the rigid crystal framework. The trachytic magma extracted at this step is equivalent to the most primitive pumice analyzed at Chefe Donsa. (2) The extracted trachytic liquid will rise and continue to crystallize, generating a second mush zone from which rhyolite liquids may be extracted. Some of the compositional range observed in the Chefe Donsa deposits may result from the fresh intrusion of trachyte magma, which may also provide an eruption trigger. This model may have wider application in understanding the origin of the Daly Gap in Ethiopian magmas—intermediate liquids may not be extracted from crystal-liquid mushes due to insufficient crystallization to yield a rigid framework. The wide range of glass compositions characteristic of the proximal Chefe Donsa deposits is not recorded in temporally equivalent tephra deposits located in regional depocenters. Our results show that glass shards, which represent the material most likely transported to distal depocenters, occupy a limited compositional range at high SiO₂ values and overlap some distal tephra deposits. These results suggest that distal tephra deposits may not faithfully record the potentially wide range in magma compositions present in a magmatic system just prior to eruption and that robust distal–proximal tephra correlations must include a careful analysis of the full range of materials in the proximal deposit.     

塔里木大火成岩省瓦吉里塔格霞石岩稀有气体同位素特征及其地质意义

稀有气体被广泛用作地球化学示踪剂,本文对塔里木大火成岩省西北缘瓦吉里塔格霞石岩中的橄榄石和辉石单矿物进行了稀有气体同位素测定。结果表明,瓦吉里塔格霞石岩中的橄榄石和辉石单矿物具有较低的~3He/~4He值(分别为2.0~2.4 Ra和0.65~0.85 Ra)和略高于大气值的~(40)Ar/~(36)Ar值(342.3~651.7),反映了由古板块俯冲导致的较低的He、Ar同位素比值特征。研究表明,早中古生代南天山洋向南俯冲到塔里木板块之下,将富U或富~4He以及含有大气组分的流体带入到深部地幔,在塔里木地幔柱的作用下地幔源区发生低程度部分熔融产生霞石岩岩浆。     

Peralkaline rhyolites from the ordovician tweeddale lavas,peeblesshire, scotland

The riebeckite-bearing rhyolite of Winkston Hill, Peebles, is shown to be peralkaline and may be named a pantellerite. Trace element comparison with other more altered members of the Tweeddale lavas from Wrae Hill suggests that the whole suite is peralkaline. It is thought most unlikely that the suite was developed in association with an Upper Ordovician subduction zone, and it was perhaps erupted on an oceanic island.     

The Peralkaline Volcanic Suite of Aden and Little Aden, South Arabia

The volcanic rocks of Aden, Little Aden, and Ras Imran, heredesignated as belonging to the Aden Volcanic Series, were eruptedthrough central-vent, strato-volcanoes about 5 m.y. ago. Inits major element chemistry the Aden Volcanic Series is intermediatebetween the alkaline and tholeiitic associations, and this isdemonstrated by comparing it with the alkaline suite of Hawaiiand the tholeiitic series of Thingmuli, Iceland. It is proposedthat the most acceptable ‘parental’ magma is a mildlyalkaline olivine basalt which, on fractionation, produced aseries ranging from trachybasalts through trachyandesites andtrachytes to rhyolites. These rhyolites are peralkaline as themolecular proportion of alumina is less than that of the combinedalkalis, and are comenditic as the series is poor in normativefemic constituents. Trace element data suggest that the peralkalinesilicic eruptives are chemically comparable with those of MayorIsland, New Zealand, where a mildly alkaline olivine basaltparent has also been postulated. Although the age of eruption of c. 5 m.y., given by K-Ar measurements,is entirely consistent with an age deduced from geomorphologicalcriteria, an ⁸⁷Sr/⁸⁶Sr versus ⁸⁷Rb/⁸⁶Sr isochron plot suggeststhat the series is related to a thermal event some 20-30 m.y.older than the age of eruption. As this earlier age correspondsdirectly to the age of the previous magmatic episode, the eruptionof the Yemen Trap Series, the upper part of which is petrologicallysimilar to the Aden Volcanic Series, and as the initial ⁸⁷Sr/⁸⁶Srratios suggest that the magma originated in the mantle, it isproposed that the most acceptable petrogenetic scheme, whichwould also explain the anomalously old Rb-Sr age, is: (a) Partialfusion in the upper mantle giving rise to the alkaline YemenTrap Series, (b) After the cessation of surface activity, alarge body of magma existed in the upper mantle and this magma,on crystallizing, fractionated to produce a layered sequence,(c) About 5 m.y. ago some event, either pressure relief or furtherthermal activity, resulted in the partial remelting of thisfractionated plutonic sequence and the liquids so formed reachedthe surface without significant mixing or chemical fractionation.     

The Kuiqi Peralkaline Granitic Complex (SE China): Petrology and Geochemistry

The Mesozoic volcano-plutonic belt of SE China is characterized,in the Kuiqi area, by acid volcanics followed by the Yanshangranites. The Kuiqi granitic complex, which belongs to the latterunit, is made up of a calc-alkaline and a peralkaline group.The calc-alkaline group consists of two intrusions, the Danyangmonzogranite and the Fuzhou syenogranite, emplaced 10310 Maand 104 5 Ma ago, respectively (Rb-Sr whole-rock isochrons).Formation of the Danyang monzogranite can be explained by athree-stage model: (1) partial melting of a metasomatized mantlegenerated a dioritic magma known in the area as the Nanyu diorite;(2) the magma was contaminated (25%) by lower continental crust;(3) large amounts (70–80%) of fractional crystallizationof hornblende and plagioclase at depth gave rise to the magmaticsuite. The Fuzhou syenogranite is more fractionated and itsformation involved crystallization of plagioclase + biotite+ K-feldspar + apatite. Intrusion of the peralkaline group isdated at 93 1 Ma (Kuiqi peralkaline granite) and at 91.80.9(Bijiashan peralkaline granite). These units are homogeneousand their petrogenesis is less constrained than for the calc-alkalinesuite. Nevertheless, a multistage process can be proposed: (1)partial melting of a metasomatized mantle produced a dioriticmagma; (2) fractional crystallization began with segregationof hornblende + plagioclase ilmenite and/or magnetite; subsequently,hornblende no longer crystallized; (3) the last stage of fractionationcorresponded to the crystallization of K-feldspar + plagioclase+ REE-rich accessory phases. Mineralogical study indicates thatduring the last stage, fluids played a prominent role and controlledthe nature of the crystallizing minerals. The magma evolvedfrom F- and S-rich, and water-undersaturated to water-oversaturated,leading to the exsolution and dissociation of an H₂O vapourphase and to the loss of H₂. The change from calc-alkaline toperalkaline magmatism is related to inferred changes in thetectonic environment. The calc-alkaline granites were generatedin a subduction setting in which water was supplied by dehydrationof the downgoing slab. The peralkaline granites were producedin a crustal thinning environment where little water was available,thus necessitating high temperatures to initiate partial melting.On the other hand, petrogenetic modelling shows that both calc-alkalineand peralkaline granites could have been derived from the samesource which is metasomatized mantle. This possibly indicatesthat the volcanic are source, active during subduction, persistedbeneath the continent and was reactivated during the post-orogenicmagmatism. The reactivation was caused by a deep crustal fault.     

Experimental Constraints on the Relationships between Peralkaline Rhyolites of the Kenya Rift Valley

Crystallization experiments on three comendites provide evidencefor the genetic relationships between peralkaline rhyolitesin the central Kenya rift valley. The crystallization of calcicclinopyroxene in slightly peralkaline rhyolites inhibits increasein peralkalinity by counteracting the effects of feldspar. Fractionationunder high fO₂ conditions produces residual liquids that areless, or only slightly more, peralkaline than the bulk composition.In contrast, crystallization under reduced conditions (<FMQ,where FMQ is the fayalite–magnetite–quartz buffer)and at high fF₂ inhibits calcic clinopyroxene and yields residualliquids that are more peralkaline than coexisting alkali feldspar,whose subsequent crystallization increases the peralkalinityof the liquid. A marginally peralkaline rhyolite [molar (Na₂O+ K₂O)/Al₂O₃ (NK/A) = 1·05] can yield a more typicallycomenditic rhyolite (NK/A = 1·28) after 95 wt % of crystallization.This comendite yields pantelleritic derivatives (NK/A >1·4)after 25 wt % crystallization. Upon further crystallization,extreme peralkaline compositions (NK/A     

Ba- and Ti-rich oxymica from nephelinites in the Middle Atlas Volcanic Province,northern Morocco

Mineralogy and Petrology - A Ba- and Ti-rich mica (up to 14.0&nbsp;wt% BaO and 13.1&nbsp;wt% TiO2) occurs in nephelinites from the Middle Atlas Volcanic Province, Morocco. The rocks show a...     

Experimental and Thermodynamic Constraints on the Sulphur Yield of Peralkaline and Metaluminous Silicic Flood Eruptions

Many basaltic flood provinces are characterized by the existenceof voluminous amounts of silicic magmas, yet the role of thesilicic component in sulphur emissions associated with trapactivity remains poorly known. We have performed experimentsand theoretical calculations to address this issue. The meltsulphur content and fluid/melt partitioning at saturation witheither sulphide or sulphate or both have been experimentallydetermined in three peralkaline rhyolites, which are a majorcomponent of some flood provinces. Experiments were performedat 150 MPa, 800–900°C, fO₂ in the range NNO –2 to NNO + 3 and under water-rich conditions. The sulphur contentis strongly dependent on the peralkalinity of the melt, in additionto fO₂, and reaches 1000 ppm at NNO + 1 in the most stronglyperalkaline composition at 800°C. At all values of fO₂,peralkaline melts can carry 5–20 times more sulphur thantheir metaluminous equivalents. Mildly peralkaline compositionsshow little variation in fluid/melt sulphur partitioning withchanging fO₂ (D_S 270). In the most peralkaline melt, D_S risessharply at fO₂ > NNO + 1 to values of >500. The partitioncoefficient increases steadily for S_bulk between 1 and 6 wt% but remains about constant for S_bulk between 0·5 and1 wt %. At bulk sulphur contents lower than 4 wt %, a temperatureincrease from 800 to 900°C decreases D_S by 10%. These results,along with (1) thermodynamic calculations on the behaviour ofsulphur during the crystallization of basalt and partial meltingof the crust and (2) recent experimental constraints on sulphursolubility in metaluminous rhyolites, show that basalt fractionationcan produce rhyolitic magmas having much more sulphur than rhyolitesderived from crustal anatexis. In particular, hot and dry metaluminoussilicic magmas produced by melting of dehydrated lower crustare virtually devoid of sulphur. In contrast, peralkaline rhyolitesformed by crystal fractionation of alkali basalt can concentrateup to 90% of the original sulphur content of the parental magmas,especially when the basalt is CO₂-rich. On this basis, we estimatethe amounts of sulphur potentially released to the atmosphereby the silicic component of flood eruptive sequences. The peralkalineEthiopian and Deccan rhyolites could have produced 10¹⁷ and10¹⁸ g of S, respectively, which are comparable amounts to publishedestimates for the basaltic activity of each province. In contrast,despite similar erupted volumes, the metaluminous Paraná–Etendekasilicic eruptives could have injected only 4·6 x 10¹⁵g of S in the atmosphere. Peralkaline flood sequences may thushave greater environmental effects than those of metaluminousaffinity, in agreement with evidence available from mass extinctionsand oceanic anoxic events. KEY WORDS: silicic flood eruptions; sulphur; experiment; Ethiopia; Deccan     

U-Th Disequilibrium and Rb-Sr Age Constraints on the Magmatic Evolution of Peralkaline Rhyolites from Kenya

Mildly peralkaline rhyolites of the Olkaria Volcanic Complex,located in the Kenyan sector of the East African rift valley,have low Sr concentrations and elevated Rb/Sr ratios (Sr 1·3–2ppm; ⁸⁷Rb/⁸⁶Sr = 748–1769) that potentially allow theresolution of time differences on the order of 1 ka by conventionalSr isotope determination. Because of their young eruption ages(     

Geochemistry and Petrogenesis of Siwana Peralkaline Granites, West of Barmer, Rajasthan, India

The bimodal Malani suite, West of Barmer, Rajasthan is characterized by discontinuous, ring shaped outcrops of Siwana peralkaline granite with minor outcrops of basalt. The peralkaline, within- plate and A-type nature of granite are evident by its chemical characteristics. The granite is characterized by high Na₂O+K₂O, Fe/Mg, Zr, Nb, Y, Zn; low Al₂O₃, CaO and Sr and is significantly low in absolute abundance of trace and REE elements compared to type area Siwana granite. The granite is correlated to the “Pan-African” event and its petrogenesis and tectonic significance are discussed.     

浙江桃花岛碱性和普陀山铝质A型花岗岩副矿物对比研究

浙江舟山群岛的桃花岛和普陀山分别出露有大片中国东部沿海典型的燕山期碱性和铝质A型花岗岩,二者的岩石化学和主要造岩矿物存在明显差别。利用电子探针进行的测试分析表明,它们在副矿物组合及副矿物特征方面的差异也很显著：桃花岛碱性A型花岗岩中副矿物组合为富钍锆石、褐帘石、硅钛铈矿、钛磁铁矿和富锰钛铁矿;普陀山铝质A型花岗岩中的副矿物组合为贫钍锆石、富钍独居石、磁铁矿、金红石和红钛锰矿。二者更可从单颗粒矿物（特别是锆石）的成份和内部结构上加以区分。这些副矿物上的差异暗示了两类A型花岗岩的源区成分、结晶环境和物理化学特征存在明显不同：（1）碱性A型花岗岩的源岩相对较多地包含了来自较深部的地幔组分岩浆;（2）碱性A型花岗岩形成的温度相对较高,而铝质A型花岗岩结晶阶段的氧逸度较高;（3）碱度和磷活度的差异也是导致一些副矿物种类结晶差异的关键因素。     

Origin of Ross Island basanitoids and limitations upon the heterogeneity of mantle sources for alkali basalts and nephelinites

Primary basanitoids from Ross Island, Antarctica have REE patterns and Pb isotope ratios similar to those for primary alkali basalts and nephelinites on ocean islands. The lead data from all volcanics on Ross Island have a spread of 4% in the 206/204 ratio and give a two-stage model lead age of 1500 m.y. The age is interpreted to be the time since the development of the chemical heterogeneity of the mantle source, presumably during an earlier melting process. Comparison of REE, K, Rb, Sr, Ba and P₂O₅ concentrations for alkali basalts and nephelinites shows that the chondrite normalized mantle source is enriched in light REE with average La/Sm=3.4, Ce/Sm=2.6, Nd/Sm=1.6. Assuming a mantle source with heavy REE abundances of three times chondrites, nephelinites require 3 to 7% partial melting of the mantle source and alkali basalts require 7 to 15% partial melting. The patterns of K, Cu, V and Ti abundances suggest that phlogopite is a residual mineral for most nephelinite, but not alkali basalt mantle sources, and that a sulfide phase and a titanium-rich mineral are in the residual mantle source for both alkali basalts and nephelinites. Small positive Eu anomalies (2–5%) in near primary alkali basalts and nephelinites suggest that the xxx of the mantle sources is 10^?6 to 10^?9 atm. The progressive enrichment of light REE and incompatible elements in the mantle sources for nephelinites and alkali basalts is proposed to result by intrusion of veins of basaltic melt due to very low percentages of melting 1 000 to 3 000 m.y. ago when this part of the deeper mantle was previously involved in convection and partial melting.     

Peralkaline syenite autoliths from Kilombe volcano, Kenya Rift Valley: Evidence for subvolcanic interaction with carbonatitic fluids

Mineral chemistry, textures and geochemistry of syenite autoliths from Kilombe volcano indicate that they crystallized in the upper parts of a magma chamber from peralkaline trachytic magmas that compositionally straddle the alkali feldspar join in the “residuum system” (ne = 0–1.03; qz = 0–0.77). Mineral reaction and/or overgrowth processes were responsible for the replacement of (i) Mg–hedenbergite by aegirine–augite, Ca–aegirine and/or aegirine, (ii) fayalite by amphiboles, and (iii) magnetite by aenigmatite. Ti–magnetite in silica-saturated syenites generally shows ilmenite exsolution, partly promoted by circulating fluids.

By contrast, the Fe–Ti oxides in the silica-undersaturated (sodalite-bearing) syenites show no signs of deuteric alteration. These syenites were ejected shortly after completion of crystallization. Ilmenite–magnetite equilibria indicate fO₂ between − 19.5 and − 23.1 log units (T 679–578 °C), slightly below the FMQ buffer. The subsequent crystallization of aenigmatite and Na-rich pyroxenes suggests an increase in the oxidation state of the late-magmatic liquids and implies the influence of post-magmatic fluids.

Irrespective of silica saturation, the syenites can be divided into (1) “normal” syenites, characterized by Ce/Ce ratios between 0.83 and 0.99 and (2) Ce-anomalous syenites, showing distinct negative Ce-anomalies (Ce/Ce 0.77–0.24). “Normal” silica-saturated syenites evolved towards pantelleritic trachyte. The Ce-anomalous syenites are relatively depleted in Zr, Hf, Th, Nb and Ta but, with the exception of Ce, are significantly enriched in REE.

The silica-saturated syenites contain REE–fluorcarbonates (synchysite-bastnaesite series) with negative Ce-anomalies (Ce/Ce 0.4–0.8, mean 0.6), corroded monazite group minerals with LREE-rich patches, and hydrated, Fe- and P-rich phyllosilicates. Each of these is inferred to be of non-magmatic origin. Fractures in feldspars and pyroxenes contain Pb-, REE- and Mn-rich cryptocrystalline or amorphous material. The monazite minerals are characterized by the most prominent negative Ce-anomalies (Ce/Ce_mean = 0.5), and in the most altered and Ca-rich areas (depleted in REE), Ce/Ce is less than 0.2.

It is inferred that carbonatitic fluids rich in F, Na and lanthanides but depleted in Ce by fractional crystallization of cerian pyrochlore, percolated into the subvolcanic system and interacted with the syenites at the thermal boundary layers of the magma chamber, during and shortly after their crystallization.

Chevkinite–(Ce), pyrochlore, monazite and synchysite-bastnaesite, occurring as accessory minerals, have been found for the first time at Kilombe together with eudialyte, nacareniobsite–(Ce) and thorite. These latter represent new mineral occurrences in Kenya.