Mechanical twinning and slip in experimentally deformed plagioclases

Mechanical albite and pericline twins were induced in seven feldspars, (Na, Ca) (Al, Si)₄O₈, in triaxial compression tests at 800° C, 8 to 10 kb confining pressure. Two were initially in a high temperature structural state (disordered) (An₁, An₅₉). Three (An₃₂, An₃₉, and An₅₃) were slightly disordered, and two (An₇₆ and An₉₅) had the transitional and primitive anorthite structure, respectively. No microscopic twinning was detected in comparable tests on ordered albite and oligoclase (An₁, An₂₀), feldspars in the peristerite range (An₁₁ An₁₄) or in a low structural state labradorite (An₅₁). Other deformational features include lamellae 60 to 80° from the c axis and (010) slip in the opposite sense for (010) twinning.Work performed under the auspices of the U. S. Atomic Energy Commission.     

Cellular plagioclase intergrowths as a result of crystal-magma mixing in the Proterozoic Åland rapakivi batholith,SW Finland

Finely cellular plagioclase intergrowths have been studied in xenocrystic andesine (An₃₂) and andesine mantled K-feldspars within mafic magmatic enclaves in a quartz-feldspar porphyry from the Proterozoic subvolcanic Hammarudda complex, Åland rapakivi batholith, SW Finland. The cellular intergrowths usually occur as 0.2–2.0 mm mantles around xenocrysts but also as entirely cellular grains, and are built up of a network of two distinct phases: one relatively Na-rich (An₃₁) and one relatively Ca-rich (An₅₀). The grains are also covered by a thin (0.08–0.12 mm), continuous, normally zoned rim outside the cellular mantle. Small inclusions (0.01–0.05 mm) of Fe–Mg minerals are concentrated in the Ca-rich part of the network. Compositionally, the Na-rich phase of the network is close to the inner non-cellular andesine of the xenocrysts. However, it has a lower Or- and a slightly lower An-content. The Ca-rich phase has the same composition as the inner part of the normally zoned rim, which outwards grades into lower An-contents that overlap the An-content of the matrix plagioclases. The cellular network was developed after the andesine xenocrysts (or andesine mantled K-feldspars) were engulfed in mafic magmatic enclaves during a mixing event. The xenocrysts became heated to a temperature just below the liquidus of the mafic magma. Dissolution of the xenocrysts developed a spongy cellular texture which was penetrated by enclave magma. Ca-rich plagioclase crystallized in the cells in equilibrium with the enclave magma, trapping Fe–Mg-rich melt. As the enclaves cooled the outermost thin rim and matrix plagioclases crystallized from the mafic melt. These processes operated in fairly large enclaves, as the one studied here, which has a diameter of 70cm. Smaller enclaves, on the other hand, were cooled more rapidly to temperatures close to the solidus of the enclave magma, and consequently had no time to dissolve the xenoxrysts.     

Stability of scapolite in the system Ab-An-NaCl-CaCO3 at 4 kb and 750°C

Scapolite solid solution has been synthesized at 750°C and 4 kbar and is stable relative to plagioclase + calcite + halite over the range of plagioclase compositions from Ab₈₅An₁₅ to Ab₇₀An₃₀, although albite + halite is stable relative to marialite, Na₄Al₃Si₉O₂₄Cl, and anorthite + calcite is stable relative to meionite, Ca₄Al₆Si₆O₂₄CO₃. A chloride-free scapolite, mizzonite, has been synthesized at the approximate composition NaCa₃Al₅Si₇O₂₄CO₃ (Ab. 2An. CaCO₃). In the absence of chloride, a three-phase invariant assemblage, sodic plagioclase (~Ab₆₀An₄₀) + scapolite + calcite is stable relative to plagioclase + calcite over the approximate range of plagioclase composition Ab₆₀An₄₀-Ab₃₅An₆₅ and another three-phase invariant assemblage, calcic plagioclase (~Ab₁₅An₈₅) + scapolite + calcite is stable over the approximate range Ab₃₀An₇₀-An₁₅An₈₅.Unit-cell dimensions and refractive indices have been determined for the scapotite synthesized in these experiments and are compared with values for chemically analyzed natural scapolites.Scapolite must be regarded as a ternary solid solution in which, at a given equivalent An-content, the Cl/CO₃ ratio in the large anion site can vary as a function of NaCl and CaCO₃ activities.     

Mineralogy and geochemistry of the hydrothermally altered archaean granodioritic rocks of the ventersdorp dome,Western Transvaal,South Africa

Summary Archaean granodioritic rocks from the Ventersdorp Dome have been hydrothermally altered in two stages: epidotization and K-feldspatization. The alteration locally is both pervasive or occurs along veins. Mass balance calculations reveal CaO, Fe₂O₃, Sr and Y addition and loss of Na₂O,K₂O, MgO, Pb and Ba during epidotization. The combined epidotization and K-feldspatization involved a gain in SiO₂, Fe₂O₃, CaO, K₂O, Ba, Sr and Rb. A1₂O₃ and Nb were immobile during both alteration stages. The alteration is accompanied by a change in the An-content of the plagioclase (An₁₁ to An₁).

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Mineralogie und Geochemie der hydrothermal veränderten, archaischen Granodiorite des Ventersdorp Domes (westliches Transvaal, Südafrika)

Zusammenfassung Archaische Granodiorite des Ventersdorp Domes sind durch zwei Typen hydrothermaler Verdnderung, Epidotisierung und Kalifeldspatisierung überprägt worden. Die Veránderung tritt sowohl durchdringend als auch auf Adern auf. Massenvergleichsberechnungen ergaben, daß während der Epidotisierung CaO, Fe₂O₃, Sr und Y angereichert und Na₂O, K₂O, MgO, Pb und Ba abgeführt worden sind. Während der kombinierten Epidotisierung und Kalifeldspatisierung treten eine Zufuhr von SiO₂, Fe₂O₃, CaO, K₂O, Ba, Sr und Rb auf. A1₂O₃ und Nb verhielten sich während beider Verdnderungen immobil. Die hydrothermale Veränderung wurde von einer Verminderung des Ca-Gehaltes der Plagioklase begleitet (An₁₁ zu An₁).

     

Structural characterization of labradorite-bytownite plagioclase from volcanic,plutonic and metamorphic environments

The transmission electron microscope and the electron microprobe are used to characterize calcic plagioclase (An₆₅ to An₈₅) from a variety of geological environments. The cooling histories of samples from volcanic, plutonic and metamorphic environments are estimated and the transformation and exsolution sequence is inferred from observations in the transmission electron microscope. Several distinctive textural modifications occur depending both on bulk composition and cooling history. (1) Exsolution occurs in increasingly calcic bulk compositions upon slower cooling, and the coexisting phases are An₆₆ intermediate plagioclase and An_85–90 P¯1, c=14 Å plagioclase in the sample from the metamorphic environment, (2) the morphology of b antiphase boundaries (APBs) in An₇₅ to An₈₅ plagioclase changes from smoothly curving (rapid cooling and calcic compositions) to zig-zag (slower cooling or sodic compositions). (3) The concentration of defects in the intermediate plagioclase superstructure changes from a high density in rapidly cooled plagioclase to a lower density in slowly cooled plagioclase. In all plagioclases except for the rapidly cooled, volcanic specimens there is evidence in images and diffraction patterns for short-range ordered domains with P¯1 symmetry. The observations allow the microstructure of a single zoned plagioclase to be used as an indication of the geologic environment under which it cooled.     

Composition,physical properties and origin of sodic anorthites from the ejected plutonic blocks of the Soufrière Volcano,St. Vincent,West Indies

Three complete analyses and one partial analysis are recorded of unzoned sodic anorthites having compositions An_94,9, An_94,0, An_92,7 and An₉₂ (mol per cent). Although crystallising at a high temperature the K and Sr contents are low. X-ray parameters show the material to be in a relatively high structural state comparable with material from Japanese volcanics. Available data indicate small but real differences in optical parameters between sodic anorthites of volcanic and plutonic origin but this needs further clarification. Refractive indices and specific gravities of the analysed samples are also recorded. Anorthite (An₉₀-An₁₀₀) occurs widely as phenocrysts in basic lavas and as crystal ejecta in the calc-alkaline rock suites of the circumoceanic islands. Anorthite can be precipitated from sub-alkaline magmas under certain conditions and does not require the postulation of special magmas or of contamination. It is apparent that factors other than the normative feldspar composition of the magma determine the composition of the plagioclase precipitated. Evidence from experimental work on synthetic systems indicates that an increase in both hydrostatic pressure and water vapour pressure will influence the plagioclase composition but it is not possible to evaluate these effects fully at the present time. The occurrence of anorthite is interpreted as evidence of extensive crystal accumulation at depth from fractionating basic magmas in circumoceanic calc-alkaline suites.     

The thermodynamics of the I \overline{1} –P \overline{1} phase transition in Ca-rich plagioclase from an assessment of the spontaneous strain

In-situ powder diffraction measurements between 90 and 935?K on four anorthite-rich plagioclase samples (An₁₀₀, An₉₆Ab₄, An₈₉Ab₁₁ and An₇₈Ab₂₂) were used to determine the detailed evolution of these samples through the $I \overline{1} $ – $P \overline{1} $ phase transition. The c-type reflections indicative of $P \overline{1} $ symmetry were detected only in An₁₀₀, An₉₆Ab₄, whereas deviations in the evolution of the unit-cell parameters with temperature were observed in all samples, most prominently in the β unit-cell angle. The c-type reflections disappear at ~510 and ~425?K in An₁₀₀ and An₉₆Ab₄ respectively, and their intensity decreases according to a tricritical trend $ I^{2} \propto \left( {T - T_{\text{c}} } \right) $ . The cell parameter changes were used to determine the spontaneous strains arising from the transition which were modelled with Landau theory, allowing for low-temperature quantum saturation, in order to determine the thermodynamic behaviour. In An₁₀₀ tricritical behaviour was observed [T _c?=?512.7(4)?K; θ_s?=?394(4)] in good agreement with previous studies, and the c-type superlattice reflections indicative of $P \overline{1} $ symmetry persist up to the T _c determined from the spontaneous strain, and then disappear. The evolution of the spontaneous strain in An₉₆Ab₄ is tricritical at low temperatures [T _c?=?459(1) K, θ_s?=?396(5)] up to the temperature of disappearance of c-type reflections, but becomes second order beyond ~440?K. In An₈₉Ab₁₁ the strain displays second-order behaviour throughout [T _c?=?500(1) and θ_s?=?212(5)], and the c-type reflections are not detected in the powder diffraction patterns at any temperature. The apparent discrepancy between the absence of c-type reflections in temperature ranges where the cell parameters display significant spontaneous strain is resolved through consideration of the sizes of the anti-phase domains within the crystals. It is deduced that the tricritical phase transition occurs in well-ordered crystals with large domains in which the behavior of individual domains is dominant (i.e. in pure anorthite) or where the $P \overline{1} $ distortions within the domains are large enough to dominate the structural coherency strains between the domains. When both the magnitude of the $P \overline{1} $ pattern of displacements of the tetrahedral framework become smaller and the influence of the structural coherency between anti-phase domains becomes significant, the thermodynamic behavior becomes 2nd-order in character, the c-type reflections disappear, and the orientation of the spontaneous strain changes.     

Schiller effects and exsolution in sodium-rich plagioclases

Plagioclase feldspars with mean compositions Ab_91,3Or_4,7An_4,0 and Ab_88,7An_10,1Or_1,2 have been studied by transmission electron microscopy and electron diffraction. The substructure consists of thin lamellae of albite and oligoclase. Two types of orientations of the lamellar planes were observed. The orientation of the more common type was found to change from (08 \(\bar 1\) ) to about ( \(\bar 1\) , 21, \(\bar 2\) ) as a function of the mean potassium content. The plane of the other type was found to be near ( \(\bar 7\) 12). Only the first type of lamellae produces visible Schiller colours.     

Thermodynamics of plagioclase III: Spontaneous strain at the I\overline 1 - P\overline 1 phase transition in Ca-rich plagioclase

The lattice parameters of anorthites An₉₈Ab₂ and An₁₀₀ have been measured from 22 to 1100 K. The spontaneous strain arising from the \(I\overline 1 - P\overline 1\) displacive transition in An₉₈ follows second order Landau behaviour. The spontaneous strain (? _s) couples quadratically to the order parameter (Q ⁰) with ? _s∝Q ⁰²∝(T _c ^* ?T) and T _c ^* =530 K in An₉₈. This is in contrast to the tricritical behaviour observed in pure anorthite. These observations are consistent with a Landau model for the free energy of Ca-rich plagioclases in which Al/Si order and Na content renormalize the fourth order coefficient.     

Dissolution kinetics of plagioclase in the melt of the system diopside-albite-anorthite,and origin of dusty plagioclase in andesites

The textures and kinetics of reaction between plagioclase and melts have been investigated experimentally, and origin of dusty plagioclase in andesites has been discussed. In the experiments plagioclase of different compositions (An₉₆, An₆₁, An₅₄, An₂₃, and An₂₂) surrounded by glasses of six different compositions in the system diopside-albite-anorthite was heated at temperatures ranging from 1,200 to 1,410° C for 30 min to 88 h. Textures were closely related to temperature and chemical compositions. A crystal became smaller and rounded above the plagioclase liquidus temperature of the starting melt (glass) and remained its original euhedral shape below the liquidus. Whatever the temperature, the crystal-melt interface became rough and often more complicated (sieve-like texture composed of plagioclase-melt mixture in the scale of a few m was developed from the surface of the crystal inward; formation of mantled plagioclase) if the crystal is less calcic than the plagioclase in equilibrium with the surrounding melt, and the interface remained smooth if the crystal is more calcic than the equilibrium plagioclase. From these results the following two types of dissolution have been recognized; (1) a crystal simply dissolves in the melt which is undersaturated with respect to the phase (simple dissolution), and a crystal is partially dissolved to form mantled plagioclase by reaction between sodic plagioclase and calcic melt (partial dissolution). The amount of a crystal dissolved and reacted increased proportional to the square root of time. This suggests that these processes are controlled by diffusion, probably in the crystal.Mantled plagioclase produced in the experiments were very similar both texturally and chemically to some of the so-called resorbed plagioclase in igneous rocks. Chemical compositions and textures of plagioclase phenocrysts in island-arc andesites of magma mixing origin have been examined. Cores of clear and dusty plagioclase were clacic (about An₉₀) and sodic (about An₅₀), respectively. This result indicates that dusty plagioclases were formed by the partial melting due to reaction between sodic plagioclase already precipitated in a dacitic magma and a melt of intermediate composition in a mixed magma during the magma mixing.     

Thermal expansion of plagioclase feldspars

The volume thermal expansion coefficient and the anisotropy of thermal expansion were determined for nine natural feldspars with compositions, in terms of albite (NaAlSi₃O₈, Ab) and anorthite (CaAl₂Si₂O₈, An), of Ab₁₀₀, An₂₇Ab₇₃, An₃₅Ab₆₅, An₄₆Ab₅₄, An₆₀Ab₄₀, An₇₈Ab₂₂, An₈₉Ab₁₁, An₉₆Ab₄ and An₁₀₀ by high resolution powder diffraction with a synchrotron radiation source. Unit-cell parameters were determined from 124 powder patterns of each sample, collected over the temperature range 298–935 K. The volume thermal expansion coefficient of the samples determined by a linear fit of V/V ₀ = α(T − T ₀) varies with composition (X _An in mol %) as:

Dynamic character of the primitive to body-centered phase transition in anorthite

We show from elastic neutron diffraction data that anorthite CaAl₂Si₂O₈ (An₁₀₀) undergoes a primitive to body-centered phase transition at T _c =237±1°C. The transition is reversible, and T _c is well defined. Our measurements demonstrate that the I-lattice at the high temperature phase applies to all structural elements; in other words, the time-averaged lattice is exactly body-centered and not just in the space average of An₁₀₀, as concluded earlier by other authors.     

Plagioclase in the Skaergaard intrusion. Part 1: Core and rim compositions in the layered series

The anorthite content of plagioclase grains (X_An) in 12 rocks from the layered series of the Skaergaard intrusion has been studied by electron microprobe (typically ∼30 core and ∼70 rim analyses per thin section). Mean core compositions vary continuously from An₆₆ at the base of the layered series (LZa) to An_32–30 at the top. On the other hand, crystal rims are of approximately constant composition (An_{50 ± 1}) from the LZa to the lower Middle Zone (MZ). Above the MZ, core and rim compositions generally overlap. Profiles across individual plagioclase grains from the lower zone show that most crystals have an external zone buffered at X_An ∼50 ± 1. The simplest explanation for these features is that during postcumulus crystallization in the lower zone, interstitial liquids passed through a density maximum. This interpretation is consistent with proposed liquid lines of descent that predict silica enrichment of the liquid associated with the appearance of cumulus magnetite.     

Trapped intercumulus liquid in the Main Zone of the eastern Bushveld Complex, South Africa

Lateral variations in the amount of trapped intercumulus liquid in the Main Zone of the eastern Bushveld Complex are constrained by new Sr-isotopic, whole-rock and mineralogical data from three profiles that are separated laterally by ca.100 km and represent thicknesses of 551–1,127 m of Main Zone gabbronorites below the Pyroxenite Marker. An analysis of the An-contents (100×Ca/(Ca+Na)) of plagioclase cores within the Thornhill (north), Roossenekal and Stoffberg (south) profiles show similar systematic, up-section variations from An₆₆ to An₅₉. In contrast, both the An-contents of bulk plagioclase separates and the Mg-numbers (100×Mg/(Mg+Fe_T)) of orthopyroxenes show pronounced lateral variations from Thornhill (An_67-61; Mg#_67-61), through Roossenekal (An_64-58; Mg#_64-55) to Stoffberg (An_59-55; Mg#_59-50). These mineralogical variations are interpreted to be the result of reaction between cumulus minerals and an increasing amount of trapped liquid from north to south. Modelling of the trapped liquid shift of orthopyroxene compositions suggests that the amount of trapped liquid in the cumulates increased from near 0% at Thornhill, through 10–30% at Roossenekal to 30–45% at Stoffberg. A two- to eightfold southward increase in whole-rock concentrations of P, Ti, Y, Zr and Ba is consistent with the trapped liquid model. However, the 14-fold increase in Rb from Thornhill to Stoffberg is too great to be accounted for by trapped liquid alone, but can be explained by local assimilation of partial melts of the country rock. Constant initial ⁸⁷Sr/⁸⁶Sr isotopic ratios of Main Zone plagioclase separates (0.7081–0.7085) in all the three profiles do not preclude assimilation of adjacent basaltic to rhyolitic country rock with initial ratios between 0.6924–0.7096. The southward increase in the amount of trapped liquid is ascribed to an increased cooling rate by enhanced heat loss and partial assimilation of country rock xenoliths in the distal cumulate sequence at Stoffberg. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorised users.     

Dynamics of magma mixing and degassing recorded in plagioclase at Stromboli (Aeolian Archipelago,Italy)

Crystal-rich materials (scoriae and lava flows) emitted during the 1985–2000 activity of Stromboli were taken into consideration for systematic study of bulk rock/matrix glass chemistry and in particular for the study of chemical and textural zoning of plagioclase, the most abundant mineral phase. Over the considered time period, bulk rock composition remained fairly constant in both major (SiO₂ 49.2–50.9 wt% and K₂O 1.96–2.18 wt%) and trace elements. The quite constant chemistry of matrix glasses also indicates that the degree of crystallization of magma was maintained at around 50 vol%. Plagioclase ranges in composition between An₆₂ and An₈₈ and is characterized by alternating, <10–100 m thick, bytownitic and labradoritic concentric layers, although the dominant and representative plagioclase of scoriae is An_68. The labradoritic layers (An_62–70) show small-scale (1–5 µm), oscillatory zoning, are free of inclusions, and appear to record episodes of slow crystal growth in equilibrium with a degassed liquid having the composition of the matrix glass. In contrast the bytownitic layers (An₇₀-An₈₈) are patchy zoned, show sieve structure with abundant micrometric glass inclusions and voids, and are attributed to rapid crystal growth.A key to understanding the origin of bytownitic layers can be retrieved from the texture and composition of the coronas of plagioclase xenocrysts, inherited from crystal-rich magma, in nearly aphyric pumice which are erupted during more energetic events and represent a deep, volatile-rich, HK-basaltic magma. They show a continuum from fine-sieve to evident skeletal texture from the inner to the outer part of the corona associated with normal compositional zoning from An₉₀ to An₇₅. In the light of these observations, we propose that input of H₂O-rich melt blobs, and their mixing with the residing magma, causes partial dissolution of the labradoritic layers followed by the growth of bytownitic composition whose sieve texture attests of rapid crystallization occurring under undercooling conditions mainly induced by degassing. As a whole, the zoning of plagioclase in the scoriae records successive and discrete intrusions of volatile-rich magma blobs, its degassing and mixing with the resident degassed magma at shallow level.Editorial responsibility: T.L. GroveAn erratum to this article can be found at     

Comparative petrology of Archaean anorthosites in amphibolite and granulite facies terranes, SW Greenland

We report new field and petrographic observations, and mineral-chemical data, on the amphibolite-facies Buksefjorden and granulite-facies Nordland anorthosites, which occur in different tectonostratigraphic terranes within the Archaean gneiss complex of SW Greenland. The Buksefjorden body [from the Akulleq (middle) terrane] is dominated by plagioclase and Ca-amphibole, but shows widespread effects of retrograde hydration (epidote, chlorite). Most plagioclase compositions are in the An_60–82 range, with the majority of samples showing average core compositions ∼An₇₆, whereas rims or recrystallized margins are ∼An₆₅. Most grains in the An_70–82 range display optically visible Huttenlocher intergrowths. Amphiboles at Buksefjorden are mainly magnesio-hornblende with X _Mg ranging from 0.70 to 0.45. The Nordland anorthosite [from the Akia (northern) terrane] is also dominated by plagioclase and Ca-amphibole, but contains additional clinopyroxene (∼Ca₄₇Mg₃₈Fe₁₅) as well as minor orthopyroxene (∼En₆₈), spinel and corundum. Plagioclase at Nordland shows an equilibrated, equigranular texture, consistent with prolonged slow cooling from high temperatures. Despite this textural equilibration, plagioclase at Nordland shows a striking range of compositions from An₂₈ to An₉₇, most of which is found in single thin sections. A distinctive feature is the presence of discrete anorthite (+ spinel ± corundum) domains in some samples. Although a number of explanations may apply, we consider these domains to result from prograde mass transfer reactions involving Ca-amphibole and plagioclase. Amphibole compositions at Nordland show similar X _Mg to those at Buksefjorden, but are more aluminous, alkalic, and titanian. This shift to more pargasitic compositions is consistent with the contrasts in metamorphic grade between the two anorthosite bodies. At Buksefjorden, there is no correlation between the amount of modal Ca-amphibole and plagioclase composition, which would be expected if amphibole was produced solely through metamorphism. Our results suggest, alternatively, that the primary igneous mineralogy of these rocks may have been plagioclase (∼An₇₆) + hornblende + pyroxene + magnetite. The primary mineralogy at Nordland is less certain, but it is noteworthy that no rocks contain anorthite of unambiguous igneous origin, in contrast to some other occurrences of Archaean anorthosites. Received: 17 January 1996 / Accepted: 12 March 1997     

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.     

Study of peristerites using transmission electron microscopy and energy dispersive X-ray analysis

Peristerite feldspars from Seiland in Norway have been studied by a combination of transmission electron microscopy and energy dispersive X-ray analysis. The different types of microtextures have been correlated with the local mean composition, and it is found that the miscibility gap between An₄ and An₁₇ extends into the ternary Or—Ab—An region. The orientation of the more common type of exsolution lamellae varies from (08¯1) to about (¯1, 21, ¯2) with increasing potassium content.     

The superstructure of plagioclase feldspars

A modulation function representing the position and density of (Na, Ca) atoms in the superstructure of the e-plagioclase has been derived from the average structures of different plagioclase and a general modulation theory. Based on this function the superstructure of bytownite (An₇₃) has been studied with the single crystal X-ray method. The cell dimensions by Megaw's axes are a=7.946(3)A, b=67.09(2)A, c=12.236(4)A, α=39.03(1)°, β=45.63(1)° and γ=59.63(1)°. Z=18(Na, Ca) Al(Al, Si)Si₂O₈. The initial phase factor of the modulation function for bytownite has been obtained from the intensity data of the satellite reflections. This modulation function indicates a coherent small-scale alternation of the Na-rich and Ca-rich bands in the superstructure. This superstructure has been refined by applying the albite and anorthite structures to the Na-rich and Ca-rich bands, respectively. The change of the superstructure of the e-plagioclase due to the compositional change has been described based on the movements of the satellites in reciprocal space. The direction of the coherent small-scale intergrowth of the anorthite-like and albite-like bands is perpendicular to the t vector. The thickness of the intergrowth is 1/|t|. Both direction and thickness change regularly from An₇₅ to An₂₅.     

Late Quaternary uplift and subsidence of the west coast of Tanna,south Vanuatu,southwest Pacific: U–Th ages of raised coral reefs in the Median Sedimentary Basin

An X‐ray determinative curve, based on the parameter (Smith &; Gay, 1958) has been derived from twelve analysd plagioclases in the range An₆₂‐₈₇. The line differs from previous calibrations of against anorthite content, and it is suggested that this is due to the differing thermal histories of the materials used.

A break in slope in the determinative line at about An₇₅ may correspond to the change in lattice type from low intermediate plagioclase, to “body centred” anorthite. Heating of the plagioclases results in an increase in and the disappearance of the discontinuity.