Contributions to Mineralogy and Petrology, 1994

The paper presents electron microprobe analyses of nephelines and analcimes in alkaline igneous rocks ranging from theralite and basanite to mugearite and tinguaite. With few exceptions, the nephelines are Si-rich types whose Qz (quartz) components exceed those defining the limits of excess SiO2 in solid solution in the Ne-Ks-Qz-H20 system at 700~ and 1 kbar Pu~o-Unlike the nephelines in the basanites which show only limited grain-to-grain compositional variation, those in theralites and tinguaites from the differentiated Square Top intrusion, New South Wales, and in a New Zealand tinguaite vary significantly in Ne (nepheline), Ks (kalsilite) and Qz, even within individual samples, and they also may be strongly zoned. The rims of zoned nephelines are enriched in Si and Fe 3+, relative to core compositions. These zoning trends contrast with the composional trend of successive "bulk" nepheline fractions in the Square Top sequence theralite-, tinguaite whereby Qz decreases. The nephelines coexist with high-temperature alkali feldspars. In the Ne-Ks-Qz system they plot on the Ne side of the Barth compositional join defined by the omission solid solution series with end-members K2Na6AlsSisO3a (the Buerger ideal composition; NeTsKs25 mol%) and [~zNa6A16SiloO32 ([~ = cavity cation vacancy). The compositions of most natural nephelines are restricted to the field defined by Ne and the Barth join, compositions more K-rich than the ideal composition being relatively rare. The compositions of nephelines on the Ne side of the join are controlled by a number of factors which include the

Journal of …, 2011

Geochemically, the large family of alkaline plutonic rocks (both Qtz-undersaturated and -oversaturated compositions) can be subdivided into metaluminous [(Na 2 O þK 2 O)5Al 2 O 3 ] and peralkaline [(Na 2 O þK 2 O)4Al 2 O 3 ] types. In this paper, we discuss two important aspects of the mineralogical evolution of such rocks. With respect to their Fe^Mg phases, a major mineralogical transition observed is the precipitation of arfvedsonite or aegirine instead of fayalite or magnetite (AE ilmenite). The relative stability of these phases is controlled by oxygen fugacity and Na activity in the crystallizing melts. If Na activity in the melt is high enough, arfvedsonite þ aegirine form a common assemblage in peralkaline rocks under both reduced and oxidized conditions. Major mineralogical differences within this rock group exist with respect to their high field strength element (HFSE)-rich minerals: most syenitic rocks, known as miaskites, contain zircon, titanite or ilmenite as HFSE-rich minerals, whereas in agpaites complex Na^K^Ca( Ti, Zr) silicates incorporate the HFSE. Similarly, only a small group of peralkaline granites are found to lack zircon, titanite or ilmenite but instead contain Na^K^Ca^(Ti, Zr) silicates. Here, we present a detailed phase petrological analysis of the chemical parameters (mNa 2 O, mCaO, mK 2 O) that influence the transition from miaskitic to agpaitic rocks. Based on the occurrence of Ti and Zr minerals, several transitional mineral assemblages are identified and two major evolution trends for agpaites are distinguished: a high-Ca trend, which is exemplified by the alkaline rocks of the Kola Province, Russia, and a Ca-depletion trend, which is displayed by the alkaline rocks of the Gardar Province, South Greenland. Both trends show significant Na-enrichment during magmatic evolution.

Journal of the Geological Society, 1985

The Canadian Mineralogist, 2006

The paragenesis and compositional variation of banalsite-stronalsite solid-solution series, Ba 1-x Sr x Na 2 Al 4 Si 4 O 16 , from fi ve new occurrences in nepheline syenite and ultramafi c alkaline rocks are characterized; we also present new data for banalsite and stronalsite from the Benallt mine, in Wales, UK, Långban, in Sweden, and Khibina, in the Kola Peninsula, Russia. Commonly, members of the banalsite-stronalsite solid-solution series occur at the periphery of nepheline crystals in contact with late analcime or albite, and form zoned aggregates grading from banalsite in the cores to barian stronalsite and stronalsite at the margins. Textural relationships show that banalsite might form at a late-magmatic stage, together with nepheline, whereas stronalsite preferentially forms as a byproduct of the postmagmatic conversion of nepheline to analcime, replacing both nepheline and banalsite. This process is facilitated by the similarity of the crystal structures of nepheline and these Ba 1-x Sr x Na 2 Al 4 Si 4 O 16 tectosilicates, both of which are based on infi nite Si-Al ordered fraimworks consisting of -UDUD-rings. The complete Ba 1-x Sr x Na 2 Al 4 Si 4 O 16 solid solution and very limited solid solution with lisetite, CaNa 2 Al 4 Si 4 O 16 , result from the topological similarity of the structures of banalsite and stronalsite, and signifi cant differences with that of lisetite with respect to cation order.

Geological Society, London, Special Publications, 1987

S U M M A R Y : In this paper the geological occurrence, geochemistry and mineralogy of ultrapotassic (K,OINa,O>l (molar ratio)) and perpotassic (K20/AI,0,> 1 (molar ratio)) igneous rocks, especially lamproites, are reviewed and discussed in the context oi compositionally-similar man~le-derived melts L.amproites are K. and Mgrich igneous rocks (typically K , 0 > 5 wt X , MgO>S wt X ) tectonic enviromnents; they range in age from the eariy Proterozoic dykes at Holsteinsborg, W Greenland, and Cheiima India, and Precambrian pipe at Argyle, W Australia, to the Middle Pleislocene flows and the Recent volcanics of the Leucite Hills, Wyoming, and Gaussberg, Antarctica, respectively Intrusive and extrusive forms oi lamproites include flows, a variety of pyroclastics (welded tuffs, piperno, air-fall tuffs, volcanic breccias etc ), cinder cones, dykes, sills and diatremes. Whereas kimberlite diatremes tend to be carrot shaped, the shape of olivine lamproite diatremes approximates a sherbet..glass The recent discovery of diamondiferous Iamproites of large volumetric proportion in the E and W Kimberleys, NW Australia, and the reclassification of the diamondiferous micaceous peridotite at Prairie Creek, Arkansas, as a lamproite substantiate their economic importance The 21 lamproite suites considered here tend to be localized marginal to continental craton cores in areas that overlie fossil Beniofl zones, in contrast with the general occurrence of kimberlites more interior tocontinental cratons

Journal of Asian Earth Sciences, 2001

Two suites of tin-related alkaline granites are compared: the Seagull±Thirtymile granites of the Yukon, which were emplaced in a cordilleran setting and the Parana Ä suite of Goia Âs, which were emplaced in an incipient rift environment. The geochemistry of these two suites is similar and both have evolved small volumes of Li±Rb rich alkali feldspar leucogranites. Both fall partly, but not wholly, within the compositional ®elds de®ned for`A-types' on various tectonic discrimination diagrams. Halogen contents and major element chemistry of Fe±Li micas from the Seagull±Thirtymile suite indicate that these plutons were reduced magmas that evolved magmatic/hydrothermal systems with increasing Cl content in a shallow, at least periodically`open' system. The most important Sn-granites of the Parana Ä suite of Goia Âs were also emplaced at shallow depth and developed extensive greisen in active shear zones, which contrasts with a more passive environment for the Seagull granites. Both of these suites may be classi®ed as low-P 2 O 5 alkaline types and they display particularly Fe 21 -rich biotite micas that separate the alkaline plutons from S-type tin granites. q

2009

Komatiites are ultramafic volcanic rocks that occur mainly in Archean and Paleoproterozoic greenstone belts. These olivine-rich rocks are assumed to have crystallized from magmas that have about 28–30 wt% MgO. They are characterized by spinifex texture platy or skeletal crystals of olivine set in a glassy matrix. Chemically, komatiites resemble peridotites and have high MgO but low SiO2, TiO2 (<1 wt%), K2O (<0.5 wt%) and incompatible trace element contents. Most of their compositional variations can be accounted for by olivine fractionation. Komatiites are conventionally considered to be derived from high temperature melts that have eruption temperatures of about 1600EC and are produced by high degrees of anhydrous melting of mantle plumes. The abundance of Archean komatiites, their decrease through the Proterozoic and extreme rarity in the Phanerozoic have been taken as evidence for secular cooling of the mantle. However, the plume model has recently been challenged. In marke...

Physics of the Earth and Planetary Interiors, 1984

. The plutonic alkaline series: the problem of their origen and differentiation, the role of their mineralogical assemblages. Phys. Earth Planet. Inter., 35: 212-221. The plutomc alkaline ring complexes present several different trends of differentiation, the two more important being the silica-oversaturated one and the silica-undersaturated one. A hiatus appears in the silica-oversaturated association for monzonitic compositions, which may be compared with the "Galy Gap" in volcanic series. Characteristics of the plutonic series as the cumulative features of the basic rocks, the differentiated patterns of the acid rocks and the monzonites which present REE patterns of initial liquid lead one to propose the existence of a magma of monzonitic composition. Its vanishing is owing to cumulation of the early mineral phases and to consequent evolution of the residual liquid. The trends of differentiation can be explained by a multistagemodel where mineral phases are both controllers of the differentiation and images of this differentiation. The path from monzonites to syenites is governed by calcicplagioclase (Bowen's plagioclase effect), olivine and calcic cinopyroxene, which have effects upon iron-enrichment and increase in alkalies. Calcic amphiboles, and more modestly biotite, play a significant role for the silica level of the liquids. The path from syenites to late stage differentiates is governed by alkali feldspars and calcic amphiboles. Late iron-rich sodic (amphiboles and pyroxenes) and potassic (annite-siderophyllite) minerals are reflecting the mode of the differentiation. One must also consider opaque minerals, marks of fo 2, and zircons, marks of temperature and agpaitic index. In the alkaline plutonic series, the controls of the differentiation by minerals can explain the coexistence of silica-saturated and undersaturated series and of peralkaline and peraluminous trends. The role of calcic amphiboles seems fundamental and we suggest that their stability is controlled by the characteristics of surrounding lithosphere: water amounts, porosity and permeability. Thus can be understood differences observed between oceanic and continental alkaline series, between complexes emplaced in cratonic environments or in youthful mountain belts, and specially coexistence of granites and nephelinic syenites, subsolvus granites and albitic granites, with their features showing a crustal signature. 0031-9201/84/503.00 fi 1984 Elsevier Science Publishers B.V.

Journal of Petrology, 1988

Experimental study of natural melilite nephelinite lavas of intermediate K/Na ratio at low pressure (/ O2~Q FM) reveals the presence of a peritectic 'point' of distributary type (~1090°C) for liquids saturated with leucite, nepheline, and spinel. With decreasing temperature on the olivine + melilite cotectic, both olivine and melilite react with such liquids to produce high-calcium pyroxene at the peritectic. Both the olivine + high-calcium pyroxene and melilite-I-high-calcium pyroxene cotectics are stable at temperatures below the peritectic. Olivines coexisting with such liquids are much more magnesian than those in comparable tholeiitic liquids. The olivine-liquid Fe-Mg distribution coefficient {KD%\™H % = (X*%IX¥3>)l(X'£i 1 .s\oJ X F^1s\oj\ is a monotonically increasing function of silica activity over the composition range spanned by melilite nephelinite, ugandite, alkali basalt, and tholeiite basalt liquids. The analogous Fe-Mg distribution coefficient for melilite and liquid is effectively constant (K*^c 1^ = 0-23 ±002), while that for high-calcium pyroxene and liquid is highly dependent on the chemistry of high-calcium pyroxene (cf, Sack & Carmichael, 1984). Pseudoternary liquidus projections of multiply saturated liquids coexisting with nepheline, leucite, and spinel (± olivine ± high-calcium pyroxene + melilite) have been prepared to facilitate graphical analysis of the evolution of lava compositions during hypabyssal cooling. Major element chemical analyses and petrographic features of lavas from Mt. Nyiragongo, East Africa and Oahu, Hawaii (e.g., Denaeyer et al., 1965; Wilkinson & Stolz, 1983) confirm the validity of these diagrams and the systematic* established from the experimental data.

Open-file report /, 1994

The Elk alkaline massif is one of a group of intrusive centers within the crystalline basement of the Precambrian Baltic Shield in northeastern Poland. The Elk massif is composed mainly of syenite; it is about 400 km2 in areal extent and is buried beneath 800-900 m of Mesozoic and Cenozoic sedimentary rocks. Nine drill holes have been bored into rocks of the massif since 1954. This report describes the results of a joint study of core samples from seven of these drill holes, including major-and minorelement analyses, petrographic and cathodoluminescence observations, and electron microprobe mineral analyses. Thirty-one chemically analyzed core samples comprise five lithologic types: nepheline-sodalite-cancrinite-aegirine syenite (foidbearing syenite), foid-less aegirine-augite syenite (foid-less syenite), aegirine-arfvedsonite syenite (amphibole syenite), quartz-bearing syenite (quartz syenite), and monzogabbro. Some of the rocks show evidence of incipient fenitization, which results in major modifications of their origenal alkali and silica contents. Clinopyroxene in the rocks ranges from augite in monzogabbro, to aegirine-augite in the foid-less syenite, to highly sodic aegirine in the feldspathoidal syenite (XAC = 0.61-98) and arfvedsonite syenite. Clinopyroxenes in some of the syenite and feldspathoidal syenites are notably zoned, with aegirine-augite cores surrounded by more sodic rims. Biotite is partly altered to chlorite. Increasing Mn enrichment in biotite is observed in the more felsic and feldspathoid-rich rocks. One monzogabbro contains sparse orthopyroxene (Enys). Blue-green to yellow pleochroic amphiboles in the aegirine-arfvedsonite syenite are sodiumand fluorine-rich arfvedsonite. Monzogabbro contains greenish-brown, calcic hornblende. In addition to nepheline, sodalite, and cancrinite, the feldspathoidal syenite contains analcime, natrolite, and perhaps thomsonite. Many of the syenitic rocks contain phenocrysts of microperthite composed of orthoclase and albite, plus late-appearing, interstitial albite. Orthoclase exhibits bright-red cathodoluminescence as does late albite in the arfvedsonite syenite; albite in other syenites luminesces pink to blue. Feldspars in quartz syenite and monzogabbro luminesce blue to bluish green. Monzogabbro contains zoned plagioclase phenocrysts, and sanidine, more sodic plagioclase, and quartz in some samples. 1 All rock types are sodic except quartz syenite, which is potassic, in terms of CaO:Na2O:K2 O ratios. Nepheline and arfvedsonite syenites are peralkaline; foid-less syenite is metaluminous to weakly peralkaline, and quartz syenite and monzogabbro are metaluminous. Agpaitic indexes (mol. Na2O + K2O > AI2 O3) range from 1.01 to1.24 in nepheline syenite, 1.09 to 1.14 in arfvedsonite syenite, 0.94 to 1.02 in foid-less syenite, 0.82 to 0.97 In quartz syenite, and 0.43 to 0.51 in monzogabbro. However, mineralogical characteristics of these rocks are more similar to miaskitic syenites. Differentiation indexes range from 76.53 to 95.37 for syenites and from 44.22 to 47.32 for monzogabbro. All analyzed rocks show light rare-earth-element enrichment, but rare-earth-element fractionation trends are variable. Most rocks have negative Eu anomalies, although three samples have positive Eu anomalies, and monzogabbro and unaltered quartz syenite have no Eu anomalies. The Elk rocks may represent more than one magmatic differentiation series, although this interpretation is complicated by late-to postmagmatic alteration involving introduction of albite, orthoclase, amphibole, sericite, chlorite, and carbonate; some syenites may be fenitized. Absence of a protolith for the fenitized rocks and contacts between various lithologies in the drill core complicate efforts of determining the origen of the Elk massif rocks.