Nepheline-bearing rocks from the Poohbah Lake complex,Ontario: Malignites and Malignites

Malignites from the Poohbah Lake complex of northwestern Ontario, Canada are melanocratic cumulates. Cumulus pyroxene and apatite are poikilitically enclosed in a groundmass of large plates of intercumulus orthoclase and nepheline. Nepheline-feldspar fingerprint-like intergrowths occur. Nephelines are commonly zeolitized and pyroxenes altered to aggregates of biotite and/or garnet by deuteric alteration. Pyroxenes are weakly zoned from Di₇₁ Hd¹⁸Ac₁₁ to Di₆₃Hd₂₂Ac₁₅, and are similar to the least evolved pyroxenes of other alkaline rocks. Nephelines all have compositions within the Morozewicz-Buerger convergence field and feldspars have a limited compositional range from Or⁸⁸ to Or⁹⁵. Perthites are absent.Inconsistancies in the usage of the terms malignite and juvite are discussed and it is considered that a non-genetic petrographic classification of nepheline syenites leads to the obscuration of a group of potassic nepheline syenites, characterized by the presence of nepheline plus orthoclase which are typically associated with saturated to over-saturated alkaline rocks, contain pseudo-leucite or nepheline-orthoclase intergrowths, are emplaced in mobile belts and are not associated with rocks of the ijolite-carbonatite suite.A genetic classification of nepheline syenites is suggested and it is proposed that; (1) mafic-rich nepheline syenites be referred to as mela-nepheline syenites (sensu lato) rather than as malignites; (2) the term malignite be used for magmatic potassic nepheline syenites characterised by the presence of nepheline plus a single potassium-rich feldspar (orthoclase or microcline) and devoid of exsolution perthite under subsolvus conditions; (3) the metasomatic malignites and juvites of ijolite-carbonatite complexes be referred to as varieties of fenites.     

The Klokken Gabbro--Syenite Complex, South Greenland: Cryptic Variation and Origin of Inversely Graded Layering

The Klokken stock has an outer sheath of vertically banded alkaligabbro which passes through syenogabbros into unlaminated syenitessurrounding a central layered syenite core. The layered seriescomprises sheets of granular syenite interleaved with drusylaminated syenite which shows repeated inversely graded minerallayering in which normal leucosyenite becomes progressivelymore hedenbergite-rich upwards. Tops to layers are nearly feldspar-free,fayalite—magnetite horizons. Model cumulate textures areexhibited, and cross-bedding and normally graded channel structuresare encountered. Granular layers often rest on ultramafic layersand load structures occur at the junctions. The series is cutby a sheet of biotite-syenodiorite and finally by quartzsyeniteaplites. Plagioclase (An₅₇ in gabbros) is progressively mantled by ternaryalkali feldspar and is absent only in the layered series inwhich alkali feldspars cluster around Ab₆₅ Or₃₅. Pyroxene showsunbroken evolution with respect to Fe/Fe + Mg, from augite toferrohedenbergite and sodie hedenbergite, with acmite in aplites,but Al and Ti decrease suddenly between syenogabbro and unlaminatedsyenite stages. Olivines show parallel evolution but there isa jump in Fe: Mg between gabbros and unlaminated syenites fromFo₅₂ to Fo₃₄, with subsequent steady evolution to Fo₃Te₅Fa₉₂in laminated syenites. Biotites show steady increase in Fe/Fe+ Mg throughout the syenites, approaching pure annite, but thegabbro-syenogabbro series show a reversal, interpreted as indicatingcrystallization under conditions of increasing f_o2. Ilmenomagnetitesshow systematically varying minor element ratios, with a suddendrop in MgAl₂O₄ content between gabbros and syenites. Amphiboles(ferroedenites and sodic hastingsites) appear only in the mostfractionated syenites in the layered series. The Klokken chamber was lined by gabbro with only limited insitu fractionation to syenogabbro. The termination of gabbrocrystallization was marked by ingress of water into the conduit,perhaps causing the liquid to embark on its fractionation trendtowards oversaturated residua. The mineralogical breaks areconsistent with a drop in temperature, and increase in a_SIO2at this point. The chamber was subsequently filled by a drytrachytic liquid which fractionated in situ, initially by congelationagainst the walls as unlaminated syenite. The layered seriesdeveloped within this chamber. The granular syenites show invertedcryptic and phase-layering (downward appearance of amphibole),and downward increase in grain size. They are sheets stopedoff the roof of the intrusion, representing chilled equivsalentsof the fractionating trachytic liquid of which the laminatedsyenites are the bottom accumulated products. In the inverselygraded layers the cumulus phases have slightly more advancedcompositions than the same phases in either normal rock or whenpresent as an intercumulus phase in adjacent parts of layers.The layering is attributed to crystallization at varying degreesof undercooling in a magma in which all phases exhibit a narrowcrystallization interval and which was subject to rhythmic P-build-upand sudden release. Crystal accumulation took place under nearstagnant to rhythmie P-build-up and sudden release. Crystalaccumulation took place under near stagnant conditions in athin chamber immediately beneath the roof of the intrusion.     

The orthoclase-microcline inversion: A high-resolution transmission electron microscope study and strain analysis

High-resolution electron microscopy of an intermediate microcline (Or₉₃) from a granodiorite in southeastern Australia reveals anen echelon arrangement of triclinic lens-shaped domains, twinned on the albite law. The domains are tabular on (010), are only a few unit cells wide, but extend 20 or 30 unit cells alongx, until they merge into a zone of monoclinic cells roughly aligned in the rhombic section. The domains are longer and less clearly terminated alongz. Strain calculations show that the energy released by Al/Si ordering, producing the orthoclase-microcline inversion, is equal to the strain energy developed when triclinic domains are forced to retain the original monoclinic crystal shape. This balance of strain energies thus explains the metastable persistence of intermediate microcline into the region of maximum microcline stability. Shearing along faults during deformation of the granodiorite released the strain in some of these feldspars, allowing maximum microcline to develop, and so giving rise to a bimodal distribution of triclinicities throughout the pluton. The value of measured for the intermediate microcline is the average of a range of values throughout each domain, and may be considerably closer to 90° than from an unstrained crystal with the same degree of Al/Si order.     

Enthalpies and Volumes Related to K--Na Mixing and Al-Si Order/Disorder in Alkali Feldspars

In order to investigate the thermodynamic properties of alkalifeldspars, three new feldspar ion-exchange series have beensynthesized, two based on monoclinic parent materials havingintermediate degrees of Al—Si order, the other on Amelialow albite. Acid solution calorimetric measurements have beencarried out in 20?1% HF at 50?C under isoperibolic conditionson 30 members of the three series, and compared with revisedvalues for a previously reported sanidine—analbite series.Molar volumes have been determined for all feldspars, and foran additional series based on Eifel sanidine. Enthalpies of K—Na mixing (A_ex) calculated from the 50?Cheats of solution are dependent on Al—Si distributionfor both topochemically monoclinic and triclinic alkali feldspars,and in general can be expressed as where N_Or and N_Ab are mole fractions of KAlSi₃O₈ and NaAlSi₃O₈,respectively, and Z is an ordering parameter defined as twicethe difference in the mole fraction of Al in the T1 vs the T2tetrahedral sites. A_ex values for all but the most disorderedseries are maximized toward sodic compositions, and increaseboth in magnitude and asymmetry as ordering increases. For topochemically monoclinic alkali feldspar series, volumesof K—Na mixing(V_ex) are asymmetric with N_Or, but withinthe precision of present data do not depend on Al—Si distribution: Microcline-low albite feldspars appear to have volumes of mixingwith the opposite asymmetry, but expressions of for these differ somewhat among various investigators. Since no single thermodynamic mixing property is markedly asymmetricwith respect to composition, the excess Gibbs energies impliedfrom solvus data for alkali feldspars, and maximized at sodiccompositions, are apparently the result of additive effectsof subtle asymmetries in the volumes, enthalpies, and entropiesof K—Na mixing in these minerals. The thermodynamic properties of an alkali feldspar at any compositionare significantly affected by the distribution of Al and Sibetween T1 and T2 tetrahedral sites. The enthalpy of formationat 50?C of a monoclinic potassium feldspar with perfect order(Z=1) differs by 2?19 kcal/mol from one with a completely randomAl—Si distribution (Z=0), while a value of 2?86 kcal/molapplies to analagous sodium end members. ConverselyY-ordering(between T1O andT1m sites) seems to have little or no effecton the enthalpy of formation of either end member, evidencedby the fact that most of the enthalpy differences for the lowmicrocline to sanidine and corresponding low albite to analbitetransitions (1?73 and 2?79 kcal/mol, respectively) can be attributedto Al—Si exchanges between T1 and T2 sites. Observed enthalpydifferences in alkali feldspars are probably related to strainat domain boundaries, whether the domains are extremely small,or somewhat larger as in modulated structures. Neither Z-nor Y-ordering has a substantial effect on the molarvolumes of alkali feldspars.     

Zonal Distribution of Variations in Structural State of Alkali Feldspar within the Rader Creek Pluton, Boulder Batholith, Montana

The granodioritic Rader Creek pluton of the composite Boulderbatholith contains microperthitic alkali feldspar of bulk compositionOr₆₅ to Or₈₆ with a structurally variable potassic phase. Completecell parameters, 2V measurements, and bulk composition are givenfor 11 feldspar samples. The 131 and 131 reflections for theseand 58 additional samples show the following structural typesin the potassic phase: orthoclase only; orthoclase with subordinatemaximum or near-maximum microcline (obliquity = 0.75–1.00);orthoclase with subordinate intermediate microcline (obliquity= 0.64–0.71); and intermediate microcline (obliquity =0.56–0.77) with subordinate orthoclase. Within the plutondifferent feldspar structural types occur in zones whose boundariesare approximately parallel to contacts with younger intrusiverocks cutting the Rader Creek pluton but are, in places, nearlyperpendicular to zonation within the pluton defined by rockcomposition. In general, the orthoclase zone is closest to thecontact with younger intrusives; the intermediate microclinezone is the most distant. Bulk compositions of alkali feldsparare more potassic in the orthoclase zone than elsewhere. Thedata suggest a complex history for the alkali feldspar, involvingat least two stages: 1. Exsolution and partial inversion oforthoclase to intermediate microcline during cooling of theRader Creek pluton; 2. Transformation of the intermediate-microclineassemblage to orthoclase during reheating of the pluton at thetime of intrusion of younger plutons of the batholith. The transitionalstage in this transformation is characterized by orthoclaseco-existing with subordinate microcline, whose obliquity usuallyapproaches that of maximum microcline.     

Partial homogenization of cryptoperthites in an ignimbrite cooling unit

A single-crystal x-ray study of alkali feldspars of bulk composition Or_39–43 and Or_62–64, from a single cooling unit of Battleship Rock Tuff, northern New Mexico, reveals a trend of decreasing degree of exsolution, from the non-welded zone toward the densely welded center of the cooling unit. Crystals of bulk composition Or_62–64 range from cryptoperthite with both phases monoclinic in the nonwelded zone to virtually unexsolved crystals in the welded center of the cooling unit. Crystals of bulk composition Or_39–43 include crytpoperthites with both phases monoclinic, and cryptoperthites with Pericline-twinned sodic lamellae, with ^* of the sodic phase increasing systematically from 87.3° in the nonwelded zone to 90° in the densely welded zone. Composition estimates based on unitcell parameters show decreasing compositional differences between coexisting lamellae toward the welded zone. The feldspar crystals studied are interpreted to be xenocrysts, which had undergone exsolution prior to incorporation in the erupting magma, and which were then partially homogenized during emplacement and post-emplacement cooling. The data indicate a maximum re-equilibration temperature of the feldspars of about 500° C, and a more rapid cooling of the tuff than calculated for simple conduction in a uniform slab.     

The Stability of Sodalite in a Synthetic Syenite plus Aqueous Chloride Fluid System

Natural feldspathoidal syenites may be approximated by assemblagescontaining some or all of the phases sodalite, nepheline, oneor two alkali feldspars, and aqueous chloride fluid in the systemNaAISi₃O₈-KAISi₃O₈-NaAISiO₄-KAISiO₄-NaCI-KCI-H₂O. The stabilityof sodalite in these assemblages was studied in the range 500–700°C and 600–2000 bars fluid pressure. Sodalite appears to be a stable phase on the vapor-saturatedliquidus in this system over a wide range of pressure. At or near the vapor-saturated liquidus minimum in this system,three distinct types of sodalite-bearing syenite can crystallize.Nepheline-sodalite-one alkali feldspar rocks, nepheline-sodalite-twoalkali feldspars rocks and sodalite-analcime-bearing rocks crystallizebelow 1600 bars, between 1600 and 2750 bars and above 2750 barsfluid pressure, respectively. The effects of closed-system cooling on the assemblage sodalite-nepheline-twoalkali feldspars-aqueous fluid are different and distinguishablefrom the effects of metasomatism. Closed-system cooling resultsin replacement of K-feldspar by albite, feldspathoids remainingnearly unchanged, while metasomatism generally results in sismultaneousenrichment or impoverishment in sodalite plus K-feldspar.     

X-ray diffraction study of the displacive phase transition in anorthoclase,grain-size effects and surface relaxations

The temperature evolution of the displacive order parameter of hypersolvus, Al-Si disordered alkali feldspars with composition Or₃₁ and Or₂₀ was measured using X-ray powder diffractometry. The monoclinic — triclinic transition shows second-order behaviour and bilinear order parameter-strain coupling. The transition temperatures are 443 K (Or₃₁) and 750 K (Or₂₀). Temperature evolution of the peak width, Γ, of the 132 reflection was found to depend on the grain size of the sample with an anomalous increase of Γ at T _c in fine-grained material. This effect has been rationalised in terms of surface relaxations occuring as T approaches T _c . No anomalous line broadening occurs in coarse-grained material.     

Alkali Feldspars with Water at 5 kb Pressure

Alkali feldspars with water at 5 kb begin to melt at an isobariceutectic: 703±2 °C, Or ₂₈.₅ (wt. per cent) recalculatedanhydrous. The liquidus of albite-H₂O at 5 kb is 758±3°C. The K feldspar-rich, water-saturated liquidus is essentiallythat of Yoder, Stewart, & Smith (1957), who furthermorefound orthoclase-H₂O to melt at 876 °C. The alkali feldspar solvus, determined by synthesis from glassand a few reversals using crystalline material, has a calculatedcritical temperature of 730 °C and a critical compositionof Or₃₁ (wt. per cent) using parametric equations (Thompson& Waldbaum, 1969). The Margules equation yields T_c = 738°C. The feldspars produced are structural equivalents oflow sanidine on the basis of their 060 and 04 powder X-ray diffractionpeaks. The critical line intersects the beginning of meltingcurve for the system Ab-Or-H₂O at 4.2 kb, 715 °C, the minimumconditions for the coexistence of two feldspars and liquid inthis system. The slope of the critical line, using Orville's (1963) 2 kbresults, is 18.3 °C/kb, leading to an intersection withthe kyanite-sillimanite curve of Richardson, Bell, & Gilbert(1968) at about 10.6 kb, 834 °C. The intersection with theandalusite-sillimanite curve of these authors (1969) falls atabout 3.5 kb, 703 °C. Such intersections may be used toestimate limits of P and T for a variety of crustal rocks. Because isobaric crystallization of feldspars in the water-deficientregion must enrich liquids in H₂O to the saturation point, haplosyeniticliquids with a finite initial water content must, at 5 kb totalpressure, eventually crystallize two feldspars.     

Mineral Chemistry and Crystallization Conditions of the Mboutou Layered Gabbro-Syenite-Granite Complex, North Cameroon

The Mboutou complex is one of a line of early Tertiary ringcomplexes which runs from Lake Chad to the Gulf of Guinea, noneof which has hitherto been described in detail. The main rocktypes are layered gabbros and gabbronorites, with minor bodiesof quartz-syenodiorite, quartz-syenite and hypersolvus granite.Feldspars form a continuum with exceptional compositional range,from An₈₅Ab₁₃Or₂ to around An₁Ab₄₆Or₅₅, and form an entirelyhypersolvus sequence with very strong zoning in the syenodiorites.Ca-rich clinopyroxenes (salite and calcic augite) and olivines(Fo_78–62) have restricted range. Orthopyroxene-bearingleucogabbros and syenodiorites contain minor orthopyroxene (En₆₂Fs₃₅Wo₃)and quartz; olivine and orthopyroxene never coexist. In moreevolved rocks amphibole (magnesio-hornblende to ferroedenite)and minor biotite, showing progressive Fe-enrichment, are theonly mafic silicates.Major-element rock chemistry, minor elementsin clinopyroxenes and biotite chemistry show that, notwithstandingits thoroughly anorogenic setting, Mboutou was, at the outset,only very mildly alkaline. Its more evolved members embarkedon a line of evolution with some calc-alkaline characteristics,probably because of ingress of water into residual batches ofmagma, a possibility supported by stable isotope data. Thischange in behaviour corresponded with the sudden appearanceof quartz and orthopyroxene, which was not in equilibrium withclinopyroxene on the two-pyroxene surface. Amphibole then becamethe main mafic silicate with further increase in . The more evolved rocks are relatively highly altered,but Fe-Ti oxide pairs suggest that was maintained near to and above the QFM buffer and the rangeof biotite compositions further suggests crystallization undera regime of decreasing . Biotites maintain alkaline characteristics throughout the sequence. Zoningpatterns in the ternary feldspars in the syenodiorites, andthe hypersolvus character of the final granite, limit maximumvalues of to < 1 kb, and suggest minimum temperatures for the end of crystallizationin the syenodiorites of{small tilde} 850 ?C.     

An elevated temperature X-ray study of synthetic disordered Na-K alkali feldspars

The temperature dependence of cell parameters for three disordered, synthetic alkali feldspars (Or₁₉, Or₃₈, and Or₁₀₀) has been determined up to 1,000 °C. The samples show no change in composition or degree of Si-Al disorder during the experiments. The triclinic-monoclinic inversion in the sample of composition Or₁₉ occurs at 560 °±10 °C and is accompanied by changes in the rates of expansion of a, b and c; the rate for a increases and those for b and c decrease above the inversion. The b and c parameters in Or₁₀₀ show small decreases with increasing temperature and this may be due to thermal motion effects causing a contraction of cell directions that are fully expanded at room temperature. Calculation of the thermal expansion ellipsoids for the monoclinic phases shows that the major expansion coefficients (₁) for all three samples are more than an order of magnitude greater than the intermediate (₂) and minor (₃) coefficients. Thus the thermal expansion of these phases is dominated by that of ₁ which makes an angle of 22 ± 4 ° with+a; this orientation is parallel to that of the short M-OA2 bonds. The thermal expansion mechanism for monoclinic, disordered alkali feldspars may involve tilting within the framework releasing compression along this direction and allowing the M-OA2 bonds to show high expansion rates. The stretching of the crankshaft units, which are parallel to a, may only play a subordinate role in controlling the expansion of the feldspar framework.     

http://www.sciencedirect.com/science/article/pii/S1674987113000893

Two petrologically distinct alkali feldspar syenite bodies （AFS-1 and AFS-2） from Chhotaudepur area, Deccan Large Igneous Province are reported in the present work. AFS-1 is characterized by hypidio-morphic texture and consists of feldspar （Or55Ab43 to Or25Ab71）, ferro-pargasite/ferro-pargasite horn-blende, hastingsite, pyroxene （Wo47, En5, Fs46）, magnetite and biotite. AFS-2 exhibits panidiomorphic texture with euhedral pyroxene （Wo47-50, En22-39, Fs12e31） set in a groundmass matrix of alkali feldspar （Or99Ab0.77 to Or1.33Ab98）, titanite and magnetite. In comparison to AFS-1, higher elemental concentra-tions of Ba, Sr and PREE are observed in AFS-2. The average peralkaline index of the alkali feldspar syenites is w1 indicating their alkaline nature. Variation discrimination diagrams involving major and trace elements and their ratios demonstrate that these alkali feldspar syenites have a shoshonite affinity but emplaced in a within-plate and rifting environment. No evidence of crustal contamination is perceptible in the multi-element primitive mantle normalized diagram as well as in terms of trace elemental ratios. The enrichment of incompatible elements in the alkali feldspar syenites suggests the involvement of mantle metasomatism in their genesis.     

The Klokken Gabbro-Syenite Complex, South Greenland: Quantitative Interpretation of Mineral Chemistry

The layered syenite series in the Klokken stock formed by continuousin situ fractionation of a trachytic magma in a chamber linedby gabbro with 3000 m of cover rocks. The following mineralsand reactions are assessed as geothermometers and barometers:two feldspars; hypersolvus ternary feldspars; ferrohedenbergite-ß-wollastonite;clinopyroxene-olivine Fe-Mg exchange; Fe-Ti oxides; sanidine-magnetite-annite;ferroedenite stability. Estimates of silica activity are obtainedfrom the silica-magnetite-fayalite assemblage. The gabbros ended magmatic crystallization at > 1000–1050°C.The less fractionated members of the syenite range probablycrystallized with P_H2O < P_total, at T > 870°C and,P_H2O 800 bars. In the more fractionated syenites P_H2O = P_totalduring intercumulus feldspar growth, and all magmatic phasescrystallized within the interval 940–830°C and P_H2O< 1100 bars. Magmatic f_O2 (bars) was 1 log unit below theQFM buffer. a_SIO2 in gabbros was slightly above the albite-nephelinebuffer, but rose suddenly to just <1 in the syenites, a jumpmirrored by minor elements in pyroxenes and opaque oxides. Biotitegrew subsolidus in most rocks, at f_O2 < QFM, except in intermediaterocks when f_O2 > QFM and was defined by the sanidine-magnetite-biotiteassemblage. In these rocks P_H2O of 450 bars at 760°C isobtained using existing experimental data, but application ofthis data to Fe-rich biotites in the layered series (where biotiteis an intercumulus phase) requires P > 10 kb at magmatictemperatures. High TiO₂ or F: OH probably accounts for increasedT stability of natural annites at low P. The syenitic liquid fractionated down a low temperature zonein a multicomponent system precipitating alk fsp + ol + cpx+ mt and the more fractionated members of the layered serieshad a negligible crystallization interval, a prerequisite forthe development of the unique Klokken-type inversely gradedmineral layering.     

Mafic Mineralogy of Ferroaugite Syenite from the Coldwell Alkaline Complex, Ontario, Canada

A 1500 m thick sheet-like body of ferroaugite syenite is divisiblemineralogically into an upper and lower series of syenites.The lower syenites are characterized by well developed igneouslayering defined by mafic cumulus minerals. The syenites aresaturated to oversaturated and contain as cumulus phases alkalifeldspar, olivine (Fa_83–93), ferroaugite (Di₅₀Hd₄₅Ac₅–Di₅Hd₉₀Ac₅)and ilmeno-magnetite. Amphiboles which crystallized from theintercumulus liquid range in composition from ferrohastingsitichornblende to ferroedenitic hornblende to ferroactinolitic edenite.The upper series are coarse grained cumulates with poorly definedlayering and abundant patch pegmatites. Cumulus phases are alkalifeldspar, olivine (Fa₉₃), and acmitic-hedenbergite (Di₅Hd₅₀Ac₅–Ac₅₀Hd₅₀).Intercumulus liquids are peralkaline and crystallized to aenigmatiteand amphiboles which range in composition from ferrorichteritickatophorite to ferrorichterite, Patch pegmatites are peralkalinerocks composed of ferrorichterite, ferroactinolite, alkali feldspar,aenigmatite, quartz and zircon. Extreme differentiation of ferroaugitesyenite magma generates residua which are ironrich, oversaturatedand peralkaline. Initial and final temperatures of crystallizationare estimated from mineral stability data to be 800–900°C to 500–550 °C respectively. Thermodynamic dataand mineral compositions indicate that during crystallizationthe oxygen fugacity of the magma decreased from approximately10^–15 to 10^–23–10^–24 bars. Ferroaugitesyenite pyroxene compositional trends are similar to those ofundersaturated peralkaline syenites (llimaussaq) and demonstratethat acmite enrichment trends are independent of silica activityand take place under decreasing oxygen fugacities.     

Geochemistry, petrogenesis and radioactivity of El Hudi I-type younger granites, South Eastern Desert, Egypt

The present study deals with geochemical characteristics and petrogenesis of three younger granite varieties (coarse-grained biotite-muscovite granites (CBG), garnetiferous muscovite granites (GMG) and Abu Aggag biotite granites (AAG)) in El-Hudi area, east of Aswan, southeastern desert of Egypt. Mineral chemistry and whole rock chemistry data revealed that all granites have high SiO₂ (70.8-74.7 wt.%), Al₂O₃ (12.8-14.3 wt.%), Na₂O and K₂O (>3.2 wt.%) contents with high Na₂O/K₂O ratios (～>1). Plagioclase feldspars range in composition from albite to oligoclase (An_9-27) in CBG, oligoclase (An_13-18) in GMG and albite (An_2-6) in AAG. Potash feldspars are mainly perthitic microcline and exhibit chemical formulae as (Or_93-96 Ab_7-4 An₀) in CBG, (Or_95-98 Ab_5-2 An₀) in GMG and (Or_82-98 Ab_18-2 An₀) in AAG. Biotites from CBG and GMG are enriched in (Mg and Ti) and depleted in (Al, Fe, Mn and K) compared with those of AAG. Biotites from CBG and GMG had been derived from calc-alkaline magma, whereas those from AAG had been derived from peraluminous magma. Chlorites from CBG and GMG are Mg-Fe bearing, while that from AAG is Fe-rich chlorite (chamosite). The CBG and GMG are Mg-rich monzogranites originated from high-K calc-alkaline magma with metaluminous to mildly peraluminous nature. The AAG are Fe-rich monzogranites to syenogranites generated from high-K calc-alkaline peraluminous magma. Both CBG and GMG are late- to post-orogenic granites, while the AAG are post-orogenic granites. All three granite varieties are considered as evolved I-type granites, formed under low to moderate water pressures (～ 0.5-7 kbars) and relatively high ranges of crystallization temperatures (～700-890°C). They were generated from partial melting of crustal materials at lower (CBG >30 km depth) and intermediate (GMG & AAG ～20-30 km depth) levels. The crystal fractionation was the predominant process during differentiation of parent magmas of these granites. Geochemical characteristics manifest that AAG represent the highly fractionated member of magma cycle differs from that produced CBG and GMG. The CBG are relatively enriched in both U and Th existing only within the accessory minerals such as zircon, sphene, and allanite.     

Cryptoperthites and cooling rate in a layered syenite pluton: a chemical and TEM study

Alkali feldspars in the Klokken layered syenite (South Greenland) are optically either coarse, turbid, deuteric patch perthites or clear, unaltered, fine braid microor cryptoperthites. Irregular, clear volumes can be found in most turbid grains. Electron microscopy shows that all clear crystals or volumes are cryptoperthites in which the periodicity of the exsolution lamellae increases systematically with distance from the roof of the intrusion, from 40 nm to 300nm. They are composed of low albite and diagonally associated maximum microcline. A secondary coarsening is responsible for optically visible braid microperthites.The layered series consists of interleaved, granular syenites and coarser grained, laminated syenites with mineral layering. Microprobe analyses show a downward decrease in Ca and Ba in the granular syenite feldspars consistent with chilling from the roof inwards. Most laminated syenite feldspars contain 1% An increasing to 5% in mafic layers.The periodicity of the exsolution lamellae in the laminated syenite feldspars is consistently up to twice that in adjacent granular syenites. Periodicity is not influenced by Ab:Or or An content, but high-An feldspars have continuous zig-zag intergrowths, while others are more lozengeshaped. Lamellar periodicities largely reflect cooling rate in the pluton. Cooling times in the exsolution interval were probably about 10³–10⁴ years at the top of the series, whereas existing experimental data suggest that the periodicities would be reached in a few days or years. The periodicities at the top of the granular series are only slightly greater than experimentally determined initial wavelengths for spinodal decomposition, suggesting that little or no coarsening occurred despite the long cooling times. Coarsening did occur lower in the pile. Framework ordering and twinning must slow or stop coarsening.The difference in periodicity between the granular and laminated syenite series may be caused by differences in amount of water incorporated in the feldspar structure from the magma. The exsolution of this water at low temperatures could be the cause of the deuteric unmixed patch perthites. Small volumes of cryptoperthite which have escaped deuteric unmixing may be common in igneous rocks in general and may be useful relative indicators of cooling rate.     

Crystallization Conditions of the Sybille Monzosyenite, Laramie Anorthosite Complex, Wyoming

The Sybille Monzosyenite, associated with the Laramie AnorthositeComplex, consists of rocks ranging in composition from monzogabbroto monzosyenite. There is a continuous range of mineral compositionswith plagioclase varying from An₄₅ to An₂₅ and olivine fromFa₇₅ to Fa₉₈ Strongly ternary (Or, Ab, An all > 10 mol%)feldspars–presently mesoperthites–are found in allrock types and define a continuous trend also in the feldsparternary system. The mineral compositions suggest that the rocktypes of the Sybille Monzosyenite could be part of a singledifferentiation sequence; contamination of the later units byassimilation of, or admixture of partial melts from, countryrock is also likely. Original magmatic temperatures of approximately950–1050?C are indicated by estimated original compositionsfor pyroxenes and feldspars; pressure was near 3 kb, as indicatedby the most magnesian olivine that coexists with quartz. Oxygenfugacity of crystallization is estimated as 1.5 to 2.0 log unitsbelow FMQ by using the displaced equilibrium: SiO₂ + 2Fe₂TiO₃= 2FeTiO₃ + Fe₂SiO₄. Such oxygen fugacities are consistent withthe occurrence of graphite and CO₂-rich fluid inclusions, whichsuggest that crystallization took place in the presence of aCO₂ vapor phase. Temperatures indicated by the present mineralassemblages show that all geothermometers used were reset duringcooling, first by intergrain and then by intragrain processes.     

Ternary feldspar thermometry in granulites from the Oaxacan Complex,Mexico

Coexisting feldspars from across 2,000 km² of the granulite facies Oaxacan Complex, southern Mexico exhibit variable amounts of solid solution from nearly binary (Ab-An and Ab-Or) to substantially ternary (Ab-An-Or). Reintegrated analyses of 21 coarsely exsolved perthite (AF)-plagioclase (PL) pairs yield AF=Or_30–63 Ab_30–56An_2–15 and PL=Or_1–2Ab_70–84An_11–28. These data have been used to test existing two feldspar geothermometers for this extended composition range.For all compositions, temperature estimates show relatively little spread in value (660° to 795° C, 7 kbar) using the Haselton et al. (1983) calibration (HHHR). These temperatures are in fair agreement with estimates of 750±40° C for feldspar pairs with nearly binary compositions using the Stormer (1975) thermometer (STO). However, STO temperatures increase significantly (to 990° C) with increasing ternary solid solution in AF, suggesting that thermometers derived for binary systems are inaccurate for ternary compositions. Isotherms drawn from HHHR which take into account variable anorthite solution in alkali feldspar show that estimated temperature decreased by 50–100° C for each 5 mole percent anorthite in alkali feldspar.Experimentally determined solvus relations (Seck 1971) require feldspars with significant ternary solid solution to have crystallized or to have equilibrated at higher temperature than feldspars with more binary compositions. However, petrographic and field relations of ternary and binary feldspars in the Oaxacan Complex suggest they were all equilibrated at similar metamorphic pressures and temperatures and do not support a model where ternary feldspars have preserved higher premetamorphic temperatures. The composition of coexisting feldspars from other Precambrian granulite-facies terranes are also inconsistent with Seck's (1971) results. Hence, thermometers which fit Seck's solvus relations may not yield accurate temperatures in high grade metamorphic terranes. Parallel tie-lines for ternary and binary feldspars in the Oaxacan Complex and the consistency of inferred temperatures (HHHR) for many granulite terranes suggest that estimation of temperature using tie-line slopes rather than solvus width may yield more accurate results for these samples.Peak metamorphic conditions in the Oaxacan Complex are inferred to have been 730±50° C, 7±1 kbar. Pressure estimates from four garnet-plagioclase barometers show good agreement. Results of feldspar thermometry are consistent with diopside-forsterite equilibria in marbles which restrict T=720–765° C at P=7 kbar.     

Partially Fused Granite Blocks from Mt. Elephant, Victoria, Australia

Petrological and chemical evidence is presented to show thatsmall patches of brown glass found in blocks of granite eruptedin basaltic scoria at Mt. Elephant are the result of the partialfusion of biotite and quartz. The glass has moderate SiO₂ (63–66per cent), high AI₂O₃ (17.5–19 per cent), high FeO (3.8–5.2per cent) and very high K₂O (7.0–7.8 per cent) relativeto Na₂O (3.3–3.7 per cent) and is similar in compositionto many high K alkali syenites and trachytes.     

Quantification of Magmatic and Hydrothermal Processes in a Peralkaline Syenite-Alkali Granite Complex Based on Textures, Phase Equilibria, and Stable and Radiogenic Isotopes

The Puklen complex of the Mid-Proterozoic Gardar Province, SouthGreenland, consists of various silica-saturated to quartz-bearingsyenites, which are intruded by a peralkaline granite. The primarymafic minerals in the syenites are augite ± olivine +Fe–Ti oxide + amphibole. Ternary feldspar thermometryand phase equilibria among mafic silicates yield T = 950–750°C,a_SiO2 = 0·7–1 and an f_O2 of 1–3 log unitsbelow the fayalite–magnetite–quartz (FMQ) bufferat 1 kbar. In the granites, the primary mafic minerals are ilmeniteand Li-bearing arfvedsonite, which crystallized at temperaturesbelow 750°C and at f_O2 values around the FMQ buffer. Inboth rock types, a secondary post-magmatic assemblage overprintsthe primary magmatic phases. In syenites, primary Ca-bearingminerals are replaced by Na-rich minerals such as aegirine–augiteand albite, resulting in the release of Ca. Accordingly, secondaryminerals include ferro-actinolite, (calcite–siderite)_ss,titanite and andradite in equilibrium with the Na-rich minerals.Phase equilibria indicate that formation of these minerals tookplace over a long temperature interval from near-magmatic temperaturesdown to