Sodic Pyroxene Research Articles

Chromian sodic pyroxene, phengite and allanite from the Sanbagawa blueschists in the eastern Kii Peninsula, central Japan

Chromian aegirine – aegirine-augite, phengite and allanite occur in blueschists, which are interbedded with serpentinite conglomerates, in the Ise area of the Sanbagawa belt, central Japan. Aggregates of chromian phengite±sodic pyroxene are scattered in the blueschists, and often include detrital chromian spinel. Sodic pyroxene in the aggregate shows solid solution between aegirine and kosmochlor components with a maximum 12.8 wt.% Cr2O3 (kosmochlor component = 38 mol%). Sodic pyroxene also occurs as individual crystal in the matrix, and has lower Cr2O3 (up to 0.7 wt.%). Phengite has a maximum 17.8 wt.% Cr2O3 showing a substitution of Cr for Al in octahedral sites. Allanite occurs in an aggregate of chromian phengite, sodic pyroxene and detrital chromian spinel, and contains up to 7.5 wt.% Cr2O3. Chromium in sodic pyroxene, phengite and allanite is derived from detrital chromian spinel. Distinct difference in Cr2O3 content between sodic pyroxenes in the aggregate and matrix suggests limited diffusion of Cr during the recrystallization.

A‐types granites and rhyolites: Is A for ambiguous?

A symposium entitled “;A‐Type Granites and Rhyolites: Is A for Ambiguous?,” held last May at the AGU 1992 Spring Meeting in Montreal, Canada, provided a forum for continued discussion and scientific debate of the complexities inherent in A‐type granites. The concept of A‐type granites [Loiselle and Wones, 1979] originally distinguished granites that are relatively anhydrous, with alkaline affinities, and which are formed in anorogenic tectonic settings, commonly after a previous episode of crustal melting. These granites were noted for their distinctive mineralogy and chemistry. A‐type granites commonly contain sodic pyroxenes and amphiboles, late crystallizing iron‐rich amphiboles and biotites, fluorite, and exotic accessory minerals (for example, chevkinite). They have high SiO2, Na2O+K2O, Fe/Mg, F, and low CaO and Sr. They are enriched in high field strength elements (for example, zirconium, niobium, gallium, yttrium) and REEs except europian.

An experimental study of glaucophanic amphiboles in the system Na2O-MgO-Al2O3-SiO2-SiF4 (NMASF): some implications for glaucophane stability in natural and synthetic systems at high temperatures and pressures

The phase relations of glaucophanic amphiboles have been studied at 18–31 kbar/680–950°C in the synthetic system Na2O−MgO−Al2O3−SiO2−SiF4 (NMASF) using the bulk composition of fluor-glaucophane, Na2Mg3Al2Si8O22F2. Previous experimental studies of glaucophane in the water-bearing system (NMASH) have been hampered by problems of fine grain size (electron microprobe analyses with low oxide totals and contamination by other phases), and consequently good compositional data are lacking. Fluor-amphiboles, on the other hand, generally have much higher thermal stabilities than their hydrous counterparts. By using the fluorine-analogue system NMASF, amphibole crystals sufficiently coarse for electron microprobe analysis have been obtained. Furthermore, NMASH amphibole phase relations are directly analogous to those of the NMASF system because SiF4 fills the role of H2O as the fluid species. High-pressure NMASF amphibole parageneses are comparable to those obtained for NMASH amphiboles under similar pressure-temperature conditions, except that the NMASF solidus was not encountered. In the pressure-temperature range of the NMASF experiments, fluor-glaucophane is unstable relative to glaucophanenyboite-Mg-magnesio-katophorite amphiboles. Variations in synthetic fluor-amphibole composition with P and T are discussed in terms of changes in the thermodynamic activities of the principal amphibole end-members, such as glaucophane (aGp) and nyboite (aNy) using an ideal-mixing-on-sites model. The most glaucophanic amphiboles analysed have aGp=0.50–0.60 and coexist with jadeite and coesite at 30 kbar/800°C. Amphiboles become increasingly nyboitic with decreasing pressure through the NaAlSi-1 exchange, which is the principal variation observed. The most nyboitic amphiboles have aNy =0.65–0.70 and coexist with fluor-sodium-phlogopite and quartz at 21–24 kbar/800–850°C. At 800°C amphiboles are essentially glaucophane-nyboite solid solutions. At 850°C there is some minor displacement along MgMgSi-1, but Mg-magnesio-katophorite activities are very low (<0.06). Activities of the eight other NMASF amphibole end-members are <0.001, except for eckermannite activity which varies from 0.01–0.11. Our results indicate that: (a) synthetic amphiboles mimic the essential stoichiometries observed in blueschist amphiboles; (b) synthetic studies should be relevant to petrologically important high-pressure parageneses and reactions involving glaucophanicamphiboles, sodic pyroxenes, albite and talc; (c) the high-pressure stability limit of fluorglaucophane lies at pressures higher than those reached in this study (31 kbar); (d) in natural systems an approach to glaucophane stoichiometry should be favoured by high water activities as well as high pressures.

Petrology of very‐high‐pressure eclogitic rocks from the Brossasco‐Isasca Complex, Dora‐Maira Massif, Italian Western Alps

Petrographical and mineral chemical data are given for the eclogites which occur in the garnet‐kyanite micaschists of the Penninic Dora‐Maira Massif between Brossasco, Isasca and Martiniana (Italian Western Alps) and for a sodic whiteschist associated with the pyrope‐coesite whiteschists of Martiniana. The Brossasco‐Isasca (BI) eclogites are fine grained, foliated and often mica‐rich rocks with a strong preferred orientation of omphacite crystals and white micas. Porphyroblasts of hornblende are common in some varieties, whilst zoisite and kyanite occur occasionally in pale green varieties associated with leucocratic layers with quartz, jadeite and garnet. These features differentiate the BI eclogites from the eclogites that occur in other continental units of the Western Alps, which all belong to type C. Garnet, sodic pyroxene and glaucophane are the major minerals in the sodic whiteschist.Sodic pyroxene in the eclogites is an omphacite often close to Jd50Di50, with very little acmite and virtually no AlIV, and impure jadeite in the leucocratic layers and in the sodic whiteschist. Garnet is almandine with 20–30 mol. % for each of the pyrope and grossular components in the eclogites and a pyrope‐rich variety in the sodic whiteschist. White mica is a variably substituted phengite, and paragonite apparently only occurs as a replacement product of kyanite. Amphibole is hornblende in the eclogites, but the most magnesian glaucophane yet described in the sodic whiteschist. Quartz pseudomorphs of coesite were found occasionally in a few pyroxenes and garnets.The P‐T conditions during the VHP event are constrained in the eclogites by reactions which define a field ranging from 27–28 kbar to 35 kbar and from 680 to 750° C. These temperatures are consistent with the results of garnet‐pyroxene and garnet‐phengite geothermometry which suggest that the eclogites may have equilibrated at around 700° C. In the sodic whiteschist pressures ranging from 29 to 35 kbar can be deduced from the stability of the jadeite‐pyrope garnet‐glaucophane compatibility. As in the eclogites water activity must have been low. Such conditions are close to the P‐T values estimated for the early Alpine recrystallization of the pyrope‐coesite rock and, like petrographical and mineralogical features, set aside the BI eclogites from the other eclogites of the Western Alps, instead indicating a close similarity to some of the eclogite bodies occurring in the Adula nappe of the Central Alps. An important corollary is that glaucophane stability, at least in Na‐ and Mg‐rich compositions and under very high pressures, may extend up to 700° C, in agreement with the HT stability limit suggested by experimental studies.

Diverse Sources for Igneous Blocks in Franciscan Melanges, California Coast Ranges

Igneous blocks in Franciscan melanges are of three chemical-petrologic types: (1) tholeiitic basalts of both arc and spreading center origin, with depletions in light relative to heavy rare-earth elements, $$3\%> TiO_{2} >1\%$$, high Y/Zr and Y/Ti ratios, and relict augites that generally have low Al and Ti and well-defined iron-enrichment trends; (2) basalts of probable seamount origin with marked enrichments in light relative to heavy rare-earth elements, $$5\%> TiO_{2} >1\%$$, lower Y/Zr and Y/Ti than (1), and Ti-Al-rich augites showing little if any iron-enrichment trends; and (3) hypabyssal intrusives having $$SiO_{2} >52\%, TiO_{2} <1\%$$, flat or only slightly fractionated rare-earth-abundance patterns, and diopsidic augites that are very low in Ti and Al and show no iron-enrichment trends. All of the blocks are metamorphosed; most are undeformed pumpellyite-bearing greenstones, and a few contain sodic amphibole ± lawsonite ± sodic pyroxene. The melanges are probably olistostromal in origin, deriving their igneous block detritus both from the downgoing Pacific plate (ocean floor basalts and seamounts) and from the hanging wall of the Franciscan trench (basalts and arc-related silicic intrusive rocks). The silicic intrusive rocks and some of the basalts are eroded fragments of the fore-arc crust that ultimately became the Coast Range Ophiolite. These fragments were incorporated into the Franciscan trench fill and subducted. Our results suggest that the igneous blocks in ophiolitic melanges provide important information about melange formation and about the tectonics and paleogeography of the regions in which the melanges are found.

Mn-rich amphiboles from quartz schists of the Iimori district Sambagawa terrane, Wakayama.

Calcic to sodic amphiboles rich in Mn have been found in association with Mn-rich sodic pyroxenes and spessartine garnets in quartzose schists of the Sambagawa terrane of the limori district, Wakayama Prefecture, Southwest Japan. The amphiboles range in composition from subcalcic tremolite through winchite to crossite. One of the amphiboles is tirodite, as it has considerably high content of MnO (14.27 wt. %).

Mantle eclogites: evidence of igneous fractionation in the mantle

Despite recent geochemical studies advocating an origin of eclogite xenoliths in kimberlite by subduction of altered sea-floor basalts and crustal cumulates, there are numerous examples showing abundant exsolution of garnet and kyanite from clinopyroxene that could only have occurred on cooling from near-solidus temperatures at pressures in excess of 3 GPa. Samples showing extensive exsolution tend to have more calcic garnets and sodic pyroxenes than the average eclogite, but do not form a group that is geochemically distinct in major, minor, or trace elements. Also, composite samples containing both kyanite and bimineralic eclogites demonstrate the co-genetic relation of extremes of composition of the eclogite suite. Accordingly, we present an igneous fractionation model to account for the major and trace element variations observed in these rocks: grospydites and kyanite eclogites are early pyroxene, or pyroxeneplus-corundum, cumulates that have undergone extensive subsolidus exsolution of kyanite and garnet. These were then followed by more Fe-rich eclogites which had pyroxene plus garnet as solidus phases and fractionated towards more magnesian bimineralic eclogites.

Evolution of peralkaline calcite carbonatite magma in the Fen complex, southeast Norway

In the Fen complex, Telemark, SE Norway, peralkaline calcite carbonatite (pyroxene søvite) occurs associated with nepheline syenite, alkali pyroxenite and rocks of the melteigite-ijolite-urtite series. Nepheline line syenite and pyroxene søvite have identical liquidus mineral assemblages (sodic pyroxene + apatite, followed by nepheline + alkali feldspar and, as the last phase, calcite), and pyroxenes defining virtually identical compositional trends (Di 55Hd 25Ac 20 to Ac >90). The pyroxenite formed as a pyroxene cumulate from a magma related to ijolite, and contains a felsic interstitial mineral assemblage (alkali feldspar + nepheline + calcite + apatite) corresponding to a CO 2-rich nepheline syenite in composition. Ocelli of calcite within the nepheline syenite, and microstructural evidence for two concomitant liquids in the interstitial aggregates in the pyroxenite (nepheline syenite and peralkaline calcite carbonatite) suggest that two magma types corresponding to nepheline syenite and pyroxene søvite were present as immiscible liquids in the shallow crust during emplacement of peralkaline magmas in the Fen complex. The overlapping pyroxene trends of these rock types confirm their origin from immiscible liquids. The common parent magma was related to the ijolitic rocks of the Fen complex. The present findings question the relevance of the concept of a universal primitive carbonatite magma, regardless of its assumed composition.

The Peralkaline Nepheline Syenites of the Junguni Intrusion, Chilwa Province, Malawi

AbstractThe mineralogy of the highly peralkaline Junguni nepheline syenite intrusion of the Chilwa alkaline province has been investigated. The rocks comprise alkali feldspar, very abundant nepheline, locally exceptionally abundant sodalite, sodic pyroxenes, scarce biotite, rare amphibole, and an extensive range of accessory minerals. Electron microprobe analyses indicate that the pyroxenes define an evolutionary trend from salite through aegirine-augite to aegirine, which is unusual in its broadness and ill-definition. This is explained by a series of overlapping trends produced by fluctuating Fe3+/Fe2+ ratios caused by variations in alkali content of the magma, probably produced by periodic alkali loss by surface de-gassing. Much of the sodalite occurs as a primary liquidus phase, but the paragenesis of certain ramifying sodalite veins is more problematical. It is possible that such ramifying masses are the product of coupled migration of alkalis and volatiles by diffusion in a gravitation field under pressure gradients generated by eruptive events, or by the formation of immiscible Na- and Cl-rich liquids. Both early calcic and late mangan-fluor eckermannitic/arfvedsonitic amphiboles occur. The micas vary from biotites with Mg: Fe ratios &lt; 0·5 to almost pure annites; they are fluor-micas and characterized by high Mn contents. Analyses of niobian rutile, mangan ilmenite, ferroan pyrophanite, manganese-rich eucolite, låvenite, mangan-titan låtvenite, rosenbuschite, wöhlerite, pyrochlore, eudialyte and what is believed to be only the second occurrence of kupletskite are given. Many of these minerals are rich in Zr, Nb, Na and Mn and thus typical of the assemblages found in extreme agpaitic complexes such as Ilimaussaq and Lovozero.

神居古たん峡谷地域の広域変成作用

The Kamuikotan Gorge area, west of Asahikawa, is mainly underlain by the high-pressure Kamuikotan metamorphic rocks. In this area, the Kamuikotan metamorphic rocks form a coherent metamorphic sequence, and are classified into two formations, i. e., the Ino-gawa Green-stone Formation and the Harushinai Pelite Formation. In the latter formation, serpentinites are not uncommon, and sometimes enclose blocks of amphibolites, jadeite+quartz-bearing rocks, and metabasites. In the Kamuikotan metamorphic rocks, lawsonite, epidote, pumpellyite, glaucophane, actinolite, sodic pyroxene, and chlorite commonly occur. Various mineral assemblages are observed in metabasite, and their stability relations were analyzed by Schreinemakers' method. The petrogenetic grids derived from this method indicate that the Kamuikotan metamorphism of the Kamuikotan Gorge area mainly belongs to a high pressure subfacies of the lawsonite-albite facies, in which aragonite is stable, and that the Harushinai Pelite Formation was metamorphosed at the higher-pressure field than that of the Ino-gawa Greenstone Formation in narrow tempera-ture range. Its metamorphic grade is similar to that of the Horokanai area.

TEM study of the transformation of augite to sodic pyroxene in eclogitized ferrogabbro

Transmission and analytical electron microscopy has been used to study relicts of augite that occur in various stages of transformation to sodic pyroxene. The augite relicts are characterized by a hatching produced by two sets of former “001” exsolution lamellae that possess high dislocation densities and were altered completely to sodic pyroxene, even where the augite matrix is still fresh. With further alteration, the sodic pyroxene in these “001” lamellae recrystallized and grew into the augite matrix, resulting in irregular lamellae that consist of subgrains having low dislocation densities. Needles and thin (100) lamellae of sodic pyroxene developed on the “001” lamellae. Alteration of the augite matrix proceeded by growth of areas with defects (dislocations, stacking faults). All sodic pyroxenes in these microstructures have the same orientation as the precursor augite, indicating a topotactic reaction mechanism. The reactions occurred at roughly constant Si and mainly involved replacement of Ca and Mg by Na and Al. Dislocations may have played a prominent role in the transformation by acting as diffusion pathways and by migrating into untransformed augite, leaving sodic pyroxene in their wake. At the grain boundaries of the augite, discrete grains of sodic pyroxene formed without any fixed orientation relation with the augite, consistent with a non-topotactic reaction. The predominance of the topotactic reaction inside the augite over the non-topotactic grain-boundary reaction is attributed to the scarcity of fluids during eclogite metamorphism.

The mineralogy of nepheline syenite complexes from the northern part of the Chilwa Province, Malawi

AbstractThe mineralogy, including the accessory phases låvenite, rosenbuschite, and catapleiite, and consequent petrogenetic implications have been investigated for a group of four overlapping nepheline syenite complexes (Chikala, Chaone, Mongolowe, and Chinduzi) and for spatially associated silica-saturated and over-saturated perthosites, from the northern part of the Chilwa Alkaline Province, Malawi.The complexes are thought to have formed by injection into high-level chambers of magma pulses genetically related to a common source magma at depth. Evidence for the source magma is preserved in salitic cores observed in the pyroxenes and a trend to more hedenbergite-rich compositions is believed to have formed by evolution of this magma. Subsequent trends of acmite enrichment followed magma injection into the higher-level chambers; the actual pyroxene trend associated with each individual complex is a function of the evolution attained by the source magma, oxidation potential, and perhaps even alkali activity. On the basis of such a two-stage model, the pyroxene data suggest emplacement of the Chaone and Mongolowe magmas somewhat earlier than that of Chikala, with the Chinduzi magma migrating even later.Amphiboles and biotites are believed to have formed after high-level injection of the magmas. Their compositions broadly reflect the nature of the crystallizing pyroxenes in that magnesian hastingsitic hornblendes and more Mg-rich biotites are associated with more Mg-rich sodic pyroxenes, whereas katophorites and annite-rich micas are generally associated with sodic pyroxenes somewhat richer in hedenbergite. Sub-solidus crystallization in some of the complexes is represented by aegirine and magnesio-arfvedsonite. Nepheline compositions indicate broadly similar crystallization temperatures within the complexes, namely 950 to 750°C. Oxygen fugacities for these magmas obtained from biotite/annite compositions vary from 10−19 to 10−14 bars for this temperature range. Mineralogical data, particularly from pyroxenes and amphiboles, strongly suggest that the perthosites, spatially associated with the nepheline syenite complexes, are genetically unrelated.

A newly discovered high‐pressure terrane in eastern Hokkaido, Japan

Abstract The upper Jurassic Nikoro greenstone complex of eastern Hokkaido suffered high‐pressure intermediate type metamorphism. Characteristic minerals include lawsonite, aragonite, sodic pyroxene of the aegirinejadeite series, winchite. sodic amphibole of the glaucophane‐riebeckite series, pumpellyite, epidote and actinolite.High‐pressure metamorphism of the Nikoro greenstone complex is related to subduction of the Kula plate toward the Palaeo‐Okhotsk Land during Cretaceous time.

The catalysis of mineral reactions by water and restrictions on the presence of aqueous fluid during metamorphism

AbstractThe problems of characterizing inter-granular regions and of estimating rates of intergranular diffusion in metamorphic rocks are discussed. Inter-granular regions can be anhydrous, hydrated but under-saturated with H2O, or saturated with H2O, but only in the latter case can a free aqueous fluid phase be present. Estimates of intergranular diffusion coefficients (DIGR) at 550°C derived from a variety of published experimental work, vary from ∼ 10−8m2s−1for diffusion of species through an intergranular fluid film to ⩽ 4 × 10−24m2s−1for diffusion of SiO2or O in anhydrous grain boundaries in quartzite. Estimates ofDIGRfor hydrated grain boundaries vary from ∼ 10−13m2s−1to ∼ 10−21m2s−1; the concentration of H2O in the grain boundaries and the identity of the diffusing species (generally unknown) may be important controlling factors, and there exists the possibility of a spectrum of values between these two extremes.Using available kinetic data it is shown that a free aqueous fluid could never have been present in parts of the basement terrane of the Sesia Zone (Western Alps) during uplift from the eclogite facies, except possibly late in the cooling history. The breakdown of sodic pyroxene + quartz occurred in response to the localized infiltration of catalytic aqueous fluid, possibly over a time interval as short as 6–6000 a, and possibly under conditions remote from equilibrium. H2O-present conditions during a dehydration reaction in metapelites of the Adula nappe (central Alps) could also have been of short duration. These examples are consistent with a model in which basement rocks at deep crustal levels are dry for long periods of time and in which the development of equilibrium mineral assemblages and microstructures generally occurs over relatively short periods of time under transitory fluid-present conditions (caused by devolatilization and/or infiltration).

Jadeite-kosmochlor solid solution and chromian sodic amphiboles in jadeitites and associated rocks from Tawmaw (Burma)

Jadeitites from Tawmaw (Burma) and associated amphibolites display chromian pyroxene — and amphibole-bearing parageneses, formed during a high pressure — low temperature metamorphic event. Sodic pyroxene compositions confirm the occurrence of kosmochlor in terrestrial rocks ; they show a wide range of solid solution between pure jadeite and kosmochlor 85, but suggest the existence of a miscibility gap between Ko25 and Ko50 under these metamorphic conditions. The sodic amphiboles, katophorite, glaucophane and eckermannite, contain up to 10 % Cr2O3. These exceptional compositions were formed in a particular chemical environment resulting from metasomatic reaction between an albitite dyke containing chromite xenocrysts and the enclosing serpentinites during metamorphism.

Non-mafic rocks in the greenschist, blueschist and eclogite facies

Mineral relations in pelitic rocks having undergone regional metamorphism in greenschist and amphibolite facies are reasonably well understood in terms of reactions in the felsic model system, KNASH, and in the mafic system + muscovite, KFMASH, where biotite is the only mixed “felsic—mafic” phase. However, in blueschist and eclogite metamorphism albite and chlorite become unstable relative to the mixed felsic—mafic phase glaucophane, and a sodic pyroxene, plus quartz, may coexist with AFM phases. The appearance of sodic amphibole or pyroxene in high-pressure, low-temperature metamorphism indicates greater mineralogical similarities of pelitic rocks to mafic rocks than are found in lower-pressure facies series. Existing petrogenetic grids of pelitic rocks in the greenschist and amphibolite facies can be extended into high-pressure conditions through an examination of the discontinuous reactions relating chloritoid—staurolite—chlorite—talc—kyanite—garnet together with amphibole, pyroxene and mica. Using compositional data derived from coexisting phases in natural assemblages, FeMg equilibria among the AFM phases is further modified by the Tschermak (Al IV Al VIMg −1Si −1 reaction and theoretical isopleths for the Tschermak exchange in such reactions may be estimated. Whole-rock reactions in the mafic and non-mafic rocks are examined in H 2O-conserving and H 2O-excess compositional spaces. Although mafic rocks in the blueschist and eclogite facies are spectacular in the field, the assemblages in coexisting pelitic rocks are perhaps even more useful in evaluating mineral relations in the higher-pressure facies series, as is the case at lower pressures.

Coexisting sodic augite and omphacite in a Sanbagawa metamorphic rock, Japan

Coexisting sodic augite and omphacite were found in a zoisite amphibolite from the Iratsu epidote amphibolite mass in the Sanbagawa metamorphic terrain of central Shikoku, Japan. The occurrences of the sodic augite-omphacite pairs are classified into four types by texture: independent, composite, intergrowth and exsolution types. Sodic augite and omphacite of the independent and composite types (pair A) have XNa (=Na/(Na + Ca)) = 0.15 and 0.35, respectively, and were stable in the epidote amphibolite facies during the Sanbagawa progressive metamorphism. On the other hand, XNa values of sodic augite and omphacite of the intergrowth and exsolution types (pair B) are 0.10 and 0.44, respectively. The Na-poor augite and Na-rich omphacite of the pair B were formed by re-equilibration of the pair A at lower temperature. The pair A of the Iratsu sample suggests that a compositional gap lies between sodic augite and C2/c omphacite under epidote amphibolite facies conditions, and is in marked contrast to the coexistence of sodic augite and P2/n omphacite reported from some low-grade, high-pressure metamorphic terrains. A possible phase diagram to explain the chemistry and mode of occurrence of the coexisting sodic pyroxenes is proposed.

High pressure metamorphism in Ghiaros Island, Cyclades, Greece

Summary The various rock types encountered in Ghiaros island have suffered an episode of progressive regional metamorphism which produced high pressure low temperature assemblages with Na-pyroxene and glaucophane. Later in their metamorphic history the rocks were affected by retrograde metamorphism. Glaucophane crystals became continuously zoned and sodic pyroxene crystals developed a patchy composition. Zoning in amphiboles extends from glaucophanitic cores through intermediate Na-Ca winchite compositions to actinolite rims. The compositional differences observed between the various patches in individual pyroxene crystals are interpreted to reflect contrasting P-T conditions which drive the continuous reaction jadeitic pyroxene + quartz ⇋ albite + acmitic pyroxene + quartz, to progressively less jadeite-rich pyroxene.

High-pressure metamorphism between the pelagonian Massif and Vardar Ophiolite Belt, Yugoslavia

AbstractA reported occurrence of metamorphic rocks containing sodic pyroxenes, in the transition zone between the Pelagonian Crystalline Massif and the Vardar Ophiolite Belt, in Macedonia, Yugoslavia, has been re-investigated, and pyroxenes, micas, plagioclase, and clinozoisite analysed by microprobe. The presence of the high-pressure metamorphic assemblage quartz + omphacite+albite is confirmed, giving the first definite indication of blueschist-facies metamorphic conditions in Yugoslavia.

The role of speciation in alkaline igneous fluids during fenite metasomatism

Fenites associated with alkaline igneous rocks show a progression from a high temperature assemblage consisting of sodium-rich alkali feldspar + a sodium-ironrich mafic mineral, to an extreme end member assemblage consisting of pure potassium feldspar + iron oxide. The latter assemblage is only found in association with low temperature carbonatites. Segments of this distribution trend can be found in the contact aureole of single intrusive centers. In the east African Kisingiri nephelinite volcano, ijolite intruded a granodioritic basement, producing a fenitized contact aureole. During metasomatism of granodiorite, according to the mass balance model of Rubie (1982), feldspar only participated in an alkali exchange reaction, while quartz was replaced by sodic pyroxene. Outward from the intrusive contact, with decreasing temperature, feldspar became progressively K-enriched, while pyroxene was enriched in the acmite component. It is predicted that alkaliexchange reactions were controlled by NaCl0-KCl0 aqueous complexes close to the intrusive contact, while further out in the aureole Na+-K+ ions dominated at the lower temperatures and enhanced the level of potassium metasomatism of feldspar. With decreasing temperature in the aureole, the K/Na ratio of the fluid decreased and consequently the activity of acmite increased. Around carbonatites, where the level of CO2 in the escaping fluid can be expected to be high, Na2CO 3 0 -K2CO 3 0 complexes may dominate. Alkali exchange between feldspar and these aqueous species enhances, even further, the stabilization of pure potassium feldspar. Boiling may also play an important role in potassium metasomatism as carbonatites are frequently associated with pyroclastic rocks. Formation of hematite instead of sodic pyroxene may be attributed to low $$a_{SiO_2 } $$ , high $$a_{Fe^{3 + } } $$ and a CO2-rich fluid. Important variables which determine the products of alkali metasomatism are shown to be temperature, pressure and CO2 content of the fluid, as well as the K/Na ratio of the fluid.