Mantled feldspars in alkali rhyolite from Oki-Dogo island,Japan

Summary Feldspar phenocrysts in alkali rhyolite from Oki-Dogo island in the Sea of Japan show mantled textures with cores of anorthoclase and rims of sanidine. These feldspars were examined by electron microscopy, X-ray diffraction and X-ray microanalysis. Anorthoclase first crystallized, then was partially resorbed, and finally sanidine overgrew on the anorthoclase. Saw-tooth or comb-like interfaces between the cores and rims were likely formed at the magmatic stage of resorption and overgrowth. Optically perthitic intergrowths appear in thin sections cutting saw-tooth or comb-like interfaces of the mantled feldspars. The sanidine preserves primary cryptoperthitic textures of a periodicity smaller than 10 nm, which are considered to have been produced by subsolidus exsolution after the resorption event ended. The anorthoclase has no exsolution texture under an electron microscope.

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Feldspatkristalle in Alkali-Rhyolith von der Insel Oki-Dogo, Japan

Zusammenfassung Feldspatkristalle in Alkali-Rhyolith von der Oki-Dogo Insel im Japanischen Meer zeigen ummantelte Texturen mit Kernen von Anorthoklas und Rändern von Sanidin. Diese Feldspate wurden mit Elektronenmikroskopie, Röntgendiffraktion und Mikrosondenanalyse untersucht. Anorthoklas kristallisierte zuerst, wurde dann teilweise resorbiert und schließlich wuchs Sanidin über den Anorthoklas. Sägezahn- und Kamm-ähnliche Grenzen zwischen Kernen und Rändern wurden wahrscheinlich wahrend des magmatischen Stadiums von Resorption und Überwachsung gebildet. Unter dem Mikroskop erkennt man, daß perthitische Verwachsungen durch Sägezahn- oder Kamm-artige Grenzen der ummantelten Feldspäte hinwegsetzen. Der Sanidin erhellt primäre kryptoperthitische Texturen mit einer Periodizität von > 10 nm, die als Produkte einer Subsolidus-Entmischung nach der Resorption interpretiert werden. Anorthoklas läßt unter dem Elektronenmikroskop keine Entmischungstexturen erkennen.

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With 6 Figures     

Post-emplacement fluids and pluton thermobarometry: Mount Stuart batholith,Washington Cascades

The effects of post-emplacement infiltration of externally derived, high-temperature fluids into arc-related batholiths are often not well characterized. Such infiltration can have far-reaching effects on the elemental and light isotopic chemistry of a batholith and on its mineral phases. At high temperature, fluid infiltration can be less easily detected, especially if widespread. The Mount Stuart batholith of the Washington Cascades is offered as an example of high-temperature infiltration of high δ¹⁸O fluids derived from its contact aureole. Some of the fluid infiltration coincided with and may have been partly derived from a kyanite-grade, post-emplacement metamorphic event that affected northern portions of the batholith. However, the effects of the fluid infiltration were far reaching and affected the entire margin of the batholith, including southerly portions that did not experience post-emplacement metamorphism. The result led to an oxygen isotopic zonation of the batholith, which is viewed as secondary in origin, with expected effects on mineral chemistry, including derived estimates of thermobarometry, a portion of which is also substantially subsolidus in origin. Our revised emplacement barometry of the Mount Stuart batholith, excluding areas affected by fluid infiltration, demonstrates that it was emplaced at ～350–400 MPa. Soon after emplacement, the batholith was tilted to the north by loading processes and subsequently was righted and unroofed during erosion in the Eocene. Its current near palaeohorizontal orientation has implications for palaeomagnetic studies supporting northward transport (the Baja–British Columbia hypothesis), but these results need further study, given the batholith's complex metamorphic and deformation history and the nature of its dominant magnetic mineralogy (pyrrhotite).     

A garnet-cordierite granite porphyry containing rapakivi feldspars in the Cabeza de Araya batholith (extremadura,spanish Hercynian belt)

Summary The garnet-cordierite granite porphyry is a large dyke associated with the Cabeza Araya batholith. The batholith shows S typology but with intermediate characteristics between the aluminous granite series, spatially and genetically related to the Hercynian regional metamorphism, and the calc-alkaline series of the Iberian massif. The granite porphyry is rich in megacrystic alkali feldspar often with rapakivi texture. Its mineralogy consists mainly of quartz + plagioclase + alkali feldspar + biotite I + cordierite I + garnet + biotite 11 + cordierite II ± muscovite + chlorite.Cordierite-garnet thermobarometry and stability relationships of ternary feldspars are used to estimate the T-P conditions of crystallization and the evolution of these rocks. The equilibrium temperature obtained from the cordierite-garnet pair is about 800°C (4 ± 0.5 Kb). This shows the xenocrystic origin of cordierite and garnet, in accordance with other geological and textural evidence. Garnet transformation, the genesis of Cordierite II and the formation of mantled textures are interpreted as the result of an isothermal decompression accompanying the emplacement of the porphyry.

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Eine Rapakivi-Feldspat führender Granat-Codierit-Granitporphyr aus dem Cabeza de Araya Batholith (Extremadura, Hereynischer Gürtel, Spanien)

Zusammenfassung Der Granat-Cordierit-Granit stellt einen mit dem Cabeza Araya Batholith assoziierten, mächtigen Gang dar. Der Batholith zeigt S-Typ Charakter, jedoch mit bergangsmerkmalen zu aluminösen Granitserien, und steht räumlich und genetisch im Zusammenhang mit der hercynischen Regionalmetamorphose und den Kalkalkaliserien des iberischen Massivs. Der Granitporphyr führt häufig Megakristalle von Alkalifeldspat, die oft Rapakivitextur zeigen. Die Mineralogie des Granitporphyr besteht aus Quarz + Plagioklas + Alkalifeldspat + Biotit 1 + Cordierit 1 + Granat + Biotit II + Cordierit II ± Muscovit ± Chlorit. Die P-T Bedingungen der Kristallisation und die magmatische Entwicklung dieserDie wurde, P-T Bedingungen der Kristallisation und die magmatische Entwicklung dieser Gesteine wurde, unter Anwendung der Cordierit-Granat Thermobarometrie und den Stabilitätsbeziehungen der Feldspäte, ermittelt. Die Gleichgewichtstemperatur aus Cordierit-Granat-Paaren beträgt um 800°C (4 ± 0.5 Kb). Dies zeigt die Bildung von Cordierit und Granat als Xenokristalle, in Übereinstimmung mit anderen geologischen und texturellen Merkmalen. Die Transformation des Granat. die Genese von Cordierit II und das Entstehen von mantled Texturen, kann als Resultat einer isothermalen Dekompression interpretiert werden, die die Platznahme des Porphyrs begleitet hat.

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With 5 Figures     

Magmatic andalusite from the South Mountain batholith,Nova Scotia

Accessory andalusite has been found in some late-stage granitic differentiates of the South Mountain batholith. The andalusite is petrographically distinct from the andalusite of the thermal aureole, and it occurs in regions of the batholith which show little evidence of contamination by the country rocks. Analyses of biotites from andalusite-bearing and andalusite-free phases suggest that those coexisting with andalusite actually grew in equilibrium with the andalusite. It is concluded that magmatic andalusite must be confined mainly to water-saturated, peraluminous, epizonal granites, and some possible P-T paths for the crystallization of magmatic andalusite are considered.     

Turbid alkali feldspars from the Isle of Skye,northwest Scotland

Partially turbid alkali feldspars from hydrothermally altered Tertiary granites on the Isle of Skye (the Red Hills granites) were studied using light microscopy, scanning and transmission electron microscopies, and energy-dispersive X-ray spectroscopy. Limpid cores and turbid rims of individual crystals were compared to determine the causes of the turbidity. The limpid cores were cryptoperthitic, with lamellar widths of 0.1–0.3 μm. In contrast, the turbid rims contained K-rich and Narich areas coarsened to >0.5 μm. Turbid regions contained abundant inclusions, whereas limpid regions did not. Two generations of turbidity were recognized. Feldspars from the Beinn an Dubhaich granite, a granite with near-normal values for ¹⁸O/¹⁶O possessed limpid cores surrounded by turbid rims that cast a reddish-brown hue in transmitted light. When viewed in darkfield light microscopy, the regions with the reddish-brown turbidity were blue. This is consistent with the hypothesis that the cloudy appearance of these turbid regions arises from the scattering of light by micrometerto submicrometer-sized inhomogeneities in refractive index caused by fluid-filled cavities. Feldspars from the Loch Ainort granite, a granite with low values for ¹⁸O/¹⁶O possessed limpid and reddish-brown-turbid cores surrounded by turbid rims that cast a blackish hue in transmitted light. Ion thinning of the turbid areas produced an abundance of small holes (≤1–2 μm) apparently the remains of fluid inclusions. Transmission electron microscopy revealed that some holes from regions of reddish-brown turbidity contained non-feldspar material, including halite and metal-rich phases of various compositions. In contrast, blackish turbid regions contained cavities filled with alteration products, such as kaolinite. Hence, the feldspars from granites on the Isle of Skye apparently record interactions with at least two fluids: a saline fluid (possibly a late-stage magmatic fluid) and a meteoric fluid.     

Phytoplankton as an Indicator of Improving Water Quality in the Golden Horn Estuary

The variation in phytoplankton community as a response to improving water quality has been studied in the severely polluted Golden Horn Estuary (GHE). Phytoplankton data collected monthly for 4 years and a detailed set of environmental parameters clearly showed the remarkable change in the GHE. The GHE ecosystem did not respond to numerous rehabilitation efforts to improve water quality, unless strict measures were taken to enhance water circulation. The opening of a bridge operating on large buoys and release of high amounts of fresh water from a dam acted as the turning point for the whole system. The weakening of light limitation and anoxia at upper regions is followed by consecutive blooms of different species/groups and resulted in super-saturated dissolved oxygen concentrations and a significant decrease in nutrient concentrations. When compared to the pre-remediation period, average total eukaryotic phytoplankton abundance increased 11-fold in the lower estuary, while the increase in the upper estuary was over 3 × 10⁴ in the post-remediation period. In addition, species richness continuously increased through the study period and community structure showed very strong variability. Since the estuarine ecosystem is controlled by precipitation in the last decade, the increase in anthropogenic/terrestrial inputs following heavy rainfall had adversely affected water quality and phytoplankton.     

Extreme ductility of feldspars from a mylonite,Parry Sound,Canada

Single feldspar crystals in mylonites, from the Grenville structural province of Canada, have accumulated extreme strains by ductile mechanisms. The samples studied are from a deep-crustal shear zone and were collected at Parry Sound, Ontario. The mylonites were derived from granite and leucogabbro, and the feldspar crystals originated in late syntectonic pegmatite dykes.Optical and transmission electron microstructures of microperthitic alkali feldspars show evidence of syntectonic and synchronous dislocation climb (leading to recovery), Si/Al ordering, and Na/K interdiffusion. Evidence for the operation of these processes is, respectively, ubiquitous subgrains, development and coarsening of tweed texture and transition of monoclinic to triclinic K-feldspar, and exsolution domain reorganization. These processes occurred concurrently with extreme straining of the host crystals by dislocation and diffusion mechanisms.The effects of ductile deformation preserved in these crystals are quite different from those usually observed in feldspars. The unusual microstructures are thought to be due to the conditions of high temperature and confining pressure which existed during deformation, in concert with a deformation-enhancing point defect chemistry, possibly associated with incorporation into the crystals of a species of ‘water’.     

福建南平花岗伟晶岩中钾长石的矿物学研究

钾长石是南平伟晶岩中分布广泛的一种造岩矿物，它的结晶作用几乎贯穿于伟晶岩形成－演化过程的所有阶段。通过对不同类型伟晶岩和同一类型伟晶岩中不同世代钾长石的产出条件、物理性质、矿物粉晶X光衍射、化学成分等方面研究，发现钾长石在伟晶岩中的分布有明显的规律性。这不仅深化了对南平伟晶岩形成机制的了解，而且对钾长石的综合利用也有重要意义。     

Nature of the crust in Maine,USA: evidence from the Sebago batholith

 Neodymium and lead isotope and elemental data are presented for the Sebago batholith (293±2 Ma), the largest exposed granite in New England. The batholith is lithologically homogeneous, yet internally heterogeneous with respect to rare earth elements (REE) and Nd isotopic composition. Two-mica granites in the southern/central portion of the batholith (group 1) are characterized by REE patterns with uniform shapes [Ce_N/Yb_N (chondrite normalized) = 9.4–19 and Eu/Eu* (Eu anomaly) = 0.27–0.42] and ɛ _Nd(t) = −3.1 to −2.1. Peripheral two-mica granites (group 2), spatially associated with stromatic and schlieric migmatites, have a wider range of total REE contents and patterns with variable shapes (Ce_N/Yb_N = 6.1–67, Eu/Eu* = 0.20–0.46) and ɛ _Nd(t) = −5.6 to −2.8. The heterogeneous REE character of the group 2 granites records the effects of magmatic differentiation that involved monazite. Coarse-grained leucogranites and aplites have kinked REE patterns and low total REE, but have Nd isotope systematics similar to group 2 granites with ɛ _Nd(t) = −5.5 to −4.7. Rare biotite granites have steep REE patterns (Ce_N/Yb_N = 51–61, Eu/Eu* = 0.32–0.84) and ɛ _Nd(t) = −4.6 to −3.8. The two-mica granites have a restricted range in initial Pb isotopic composition (²⁰⁶Pb/²⁰⁴Pb = 18.41–18.75; ²⁰⁷Pb/²⁰⁴Pb = 15.60–15.68; ²⁰⁸Pb/²⁰⁴Pb = 38.21–38.55), requiring and old, high U/Pb (but not Th/U) source component. The Nd isotope data are consistent with magma derivation from two sources: Avalon-like crust (ɛ _Nd>−3), and Central Maine Belt metasedimentary rocks (ɛ _Nd<−4), without material input from the mantle. The variations in isotope systematics and REE patterns are inconsistent with models of disequilibrium melting which involved monazite. Received: 8 December 1995 / Accepted: 29 April 1996     

Petrology of the Chilliwack batholith,North Cascades,Washington: generation of calc-alkaline granitoids by melting of mafic lower crust with variable water fugacity

Calc-alkaline granitoid rocks of the Oligocene-Pliocene Chilliwack batholith, North Cascades, range from quartz diorites to granites (57–78% SiO₂), and are coeval with small gabbroic stocks. Modeling of major element, trace element, and isotopic data for granitoid and mafic rocks suggests that: (1) the granitoids were derived from amphibolitic lower crust having REE (rare-earth-element) and Sr-Nd isotopic characteristics of the exposed gabbros; (2) lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting. The main effect of f_{H 2 O} variation is to change the relative proportions of plagioclase and amphibole in the residuum. The REE data for intermediate granitoids (quartz diorite-granodiorite; Eu/Eu^*=0.84–0.50) are modeled by melting with f_{H 2 O}<1 kbar, leaving a plagioclase + pyroxene residuum. In contrast, data for leucocratic granitoids (leuco-granodiorites and granites; Eu/Eu^* =1.0–0.54) require residual amphibole in the source and are modeled by melting with f_{H 2 O}=2–3 kbar. Consistent with this model, isotopic data for the granitoids show no systematic variation with rock type (⁸⁷Sr/⁸⁶Sr_i =0.7033–0.7043; Nd(0)=+3.3 to +5.5) and overlap significantly with data for the gabbroic rocks (⁸⁷Sr/⁸⁶Sr_i =0.7034–0.7040; Nd(0)=+3.3 to +6.9). The f_{H 2 O} variations during melting may reflect additions of H₂O to the lower crust from crystallizing basaltic magmas having a range of H₂O contents; Chillwack gabbros document the existence of such basalts. One-dimensional conductive heat transfer calculations indicate that underplating of basaltic magmas can provide the heat required for large-scale melting of amphibolitic lower crust, provided that ambient wallrock temperatures exceed 800°C. Based on lithologic and geochemical similarities, this model may be applicable to other Cordilleran batholiths.     

江西雅山花岗岩长石中磷的分布及意义

雅山富氟高磷花岗岩具较高的全岩磷含量（０．１５％～０．５５％）,长石是磷的主要赋存矿物,磷以ＰＡｌＳｉ－２替换方式进入长石结构中。各种结晶习性的长石的磷含量各不相同。早世代钠长石和雪球结构中钠长石包裹体的结晶较早,平均磷含量分别为０．１８％和０．１９％,而晚世代钠长石和环带钾长石中钠长石嵌晶的结晶稍晚,具较高的平均磷含量（分别为０．２５％和０．２３％）。钾长石的磷含量相对于钠长石总体上较低,早期结晶的雪球结构主晶钾长石和环带钾长石的磷含量最低,分别为０．０４％和０．０２％,晚期细粒钾长石的平均磷含量略高（０．１９％）。长石对全岩磷的贡献率还与磷锂铝石等磷酸盐副矿物是否达到饱和有关,当无磷锂铝石结晶时,长石磷的贡献率约为７６％,反之则约为３１％。     

Tourmalinites from the Golden Dyke Dome,Northern Australia

Tourmalinites occur at five stratigraphic levels within the low metamorphic grade multiply deformed Lower Proterozoic metasediments and metavolcanics of the Golden Dyke Dome. The geological setting, exhalite associations, sedimentary structures and tourmaline chemistry all preclude a granitic origin. Tourmalinite derived from the isochemical metamorphism of an exhalite comprising silica and tourmaline. Tourmalinite and other exhalites are hosted by carbonaceous pelitic metasediments and were deposited in deeper basinal areas. The ore fluid was probably dolerite-heated seawater which leached a thick pile of argillaceous sediments and acquired metals and boron by leaching. The first and second increase in geothermal gradient produced tourmalinite, the third produced auriferous tourmalinite and iron formations with tourmalinite and exhalative Pb-Zn deposits forming during the period of maximum geothermal gradient. More tourmalinite formed during the subsequent decline in geothermal gradient. Tourmalinites are regarded as B-rich iron formations which can be the host for or associated with exhalative deposits of Au and base metals.     

Evidence for the magmatic origin of quartz-topaz rocks from the New England batholith,Australia

Quartz-topaz rocks from the New England district, New South Wales, have mineralogical, textural and field relationships suggesting a magmatic origin. These rocks (called topazites) occur as dykes and sills intruding a biotite granite and sediments in a roof pendant. Where they have intruded into sediments, the topazites have a narrow aureole of induration or hornfels. One type of primary solid inclusion, thought to be silicate glass, has a composition ranging from that of the topazite towards that of nearby granite. Primary fluid inclusions contain an aqueous solution of alkali chlorides with concentrations of total salts to 57 wt%. These fluid inclusions indicate crystallization temperatures in the range 570–620° C, close to the experimentally determined solidus of a vapour-saturated, topaz-normative melt. The presence of primary fluid inclusions indicates crystallization of topazite following saturation of a granitic magma with water and the formation of immiscible silicate and aqueous phases. Partitioning of alkali metals into the aqueous phase left a silicate melt that could only crystallize quartz and topaz.     

Mesolite from Horní Jílové, Bohemia

Summary Hey(1936) called attention to discrepancies in the identification of the metascolecite (Michel, 1920) from the Horní Jílové (Ober Eulau) locality in Bohemia. In a check study this mineral has been determined as mesolite, a zeolite hitherto not described from this locality. The mineral was identified using optical, chemical, thermal, IR-spectroscopic and X-ray methods. Radial aggregates of mesolite occur in fissures of contact metamorphosed marls in the proximity of a phonolite intrusion. In terminal parts mesolite often changes into scolecite.

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Mesolit aus Horní Jílové, Böhmen

Zusammenfassung Hey(1936) hat auf die Widersprüche in der Bestimmung des Metaskolezites (Michel, 1920) aus der Horní Jílové (Ober Eulau) Lokalität in Nordböhmen aufmerksam gemacht. Auf Grund einer Revisionsstudie wurde dieses Mineral als ein aus dieser Lokalität bisher nicht beschriebener Zeolit, nämlich Mesolit, bestimmt. Das Mineral wurde mit optischen, chemischen, thermischen, IR-spektroskopischen und röntgenometrischen Methoden identifiziert. Die radial-strahligen Aggregate des Mesolits kommen in den Phonolitintrusionen vor. In terminalen Teilen gehen sie oft in Skolezit über.

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With 4 Figures     

On the constitution of the alkali feldspars

Summary The crystalline variants of the alkali feldspars differ from each other in the pattern of Al/Si disorder. The interrelations of the several variants are graphically shown in Figs. 1, 2, and 3. In high albite thekind of disorder is different from that in any other alkali feldspar; therefore the transition high albitemonalbite is diffusive; but the transition monalbiteanalbite is displacive. See Fig. 10.The phase diagram of the system NaAlSi₃O₈–KAlSi₃O₈ is presented in Fig. 4. The volume relations of the mixed crystals (Figs. 5 and 8) indicate that albite exhibiting monoclinic symmetry at room temperature has a defect lattice (defect monalbite) with vacant Na sites. A vacant cation site behaves as if a large cation (for example K) were present; therefore the defect monalbite behaves like a mixed crystal with some K replacing Na.

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Zusammenfassung Die verschiedenen kristallinen Modifikationen der Alkalifeldspate unterscheiden sich bezüglich der Art und Weise ihrer Al/Si-Verteilung. Die Interrelationen sind in den Abb. 1, 2 und 3 dargestellt. Im Hochalbit ist die Al/Si-Verteilung andersartig als in irgendeinem anderen Alkalifeldspat. Die meisten Alkalifeldspate haben eine zwifache Unendlichkeit von Variationsmöglichkeiten der Al/Si-Verteilung (entsprechend der in Abb. 2 eingezeichneten Ebene), Hochalbit aber steht einzig da und weist eine dreifache Unendlichkeit von Möglichkeiten auf. Infolgedessen entspricht dem Übergang HochalbitMonalbit ein diffuser Umwandlungsmechanismus, dem Übergang MonalbitAnalbit aber ein displaciver (Abb. 10).Das Phasendiagramm des Systems NaAlSi₃O₈–KAlSi₃O₈ ist in Abb. 4 dargestellt. Die Volumverhältnisse der Mischkristalle (Abb. 5 und 8) deuten darauf hin, daß der bei gewöhnlicher Temperatur monokline Albit ein defektes Gitter hat (= defekter Monalbit mit teilweise unbesetzten Na-Punktlagen). Die Wirkung einer unbesetzten Kation-Punktlage im Gitter ist der Wirkung eines großen Kations (z. B. Kalium) gleich; hierdurch erklärt sich die Tatsache, daß der defekte Monalbit die Eigenschaften eines etwas K-haltigen Mischkristalls nachahmt.

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With 10 Figures

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Dedicated to ProfessorF. Machatschki on the occasion of his 70th birthday.     

Rock-Forming feldspars of the Khibiny alkaline pluton,Kola Peninsula,Russia

This paper describes the structural-compositional zoning of the well-known Khibiny pluton in regard to rock-forming feldspars. The content of K-Na-feldspars increases inward and outward from the Main foidolite ring. The degree of coorientation of tabular K-Na-feldspar crystals sharply increases in the Main ring zone, and microcline-dominant foyaite turns into orthoclase-dominant foyaite. The composition of K-Na-feldspars in the center of the pluton and the Main ring zone is characterized by an enrichment in Al. This shift is compensated by a substitution of some K and Na with Ba (the Main ring zone) or by an addition of K and Na cations to the initially cation-deficient microcline (the central part of the pluton). Feldspars of volcanosedimentary rocks occurring as xenoliths in foyaite primarily corresponded to plagioclase An_15–40, but high-temperature fenitization and formation of hornfels in the Main ring zone gave rise to the crystallization of anorthoclase subsequently transformed into orthoclase and albite due to cooling and further fenitization. Such a zoning is the result of filling the Main ring fault zone within the homogeneous foyaite pluton with a foidolite melt, which provided the heating and potassium metasomatism of foyaite and xenoliths of volcanosedimentary rocks therein. The process eventually led to the transformation of foyaite into rischorrite-lyavochorrite, while xenoliths were transformed into aluminum hornfels with anorthoclase, annite, andalusite, topaz, and sekaninaite.     

Internal textures and chemical compositions of anti-rapakivi mantled feldspars from Oki-Dogo Island,Japan

Summary The cooling history of the feldspars of a hypersthene-augite trachyte lava of Oki-Dogo island, Japan was investigated by optical microscope, electron microscope and X-ray microanalyzer. Anti-Rpakivi mantled feldspars in the alkaline volcanic rocks consist of anhedral plagioclase cores and subhedral to euhedral sanidine mantles. The interfaces between the cores and mantles are wavy, saw-tooth-like, or comb-like under the optical microscope, suggesting sanidine overgrowth after plagioclase was partially resorbed. Perthitic lenses or lamellae of plagioclase are also observed in the sanidine near the interfaces. After the formation of such mantled feldspars at the magmatic stage, perthitic lamellae were produced in sanidine due to subsolidus exsolution. Periodicity of the perthitic lamellae is below 10 nm.[/ab]

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Zusammenfassung Die Abkühlungs-Geschichte von Feldspäten einer Hypersthen-Augit-Trachyt-Lava von der Insel Oki-Dogo, Japan, wurde mikroskopisch, elektronenmikroskopisch und mit der Mikrosonde untersucht. Anti-Rapakivi-Feldspate in den Alkali-Vulkaniten bestehen aus anidiomorphen Plagioklas-Kernen and idiomorphen Sanidin-Rändern. Die Grenzen zwischen Kernen and Randern erscheinen unter dem Mikroskop gewellt, sägezahnartig oder kammartig. Dies deutet darauf hin, daß Überwachsung mit Sanidin nach teilweiser Resorption von Plagioklas stattgefunden hat. Perthitische Linsen oder Lamellen von Plagioklas im Sanidin wurden im Grenzbereich der beiden Feldspäte beobachtet. Perthit-Lamellen im Sanidin entstanden durch Subsolidus-Entmischung nachdem die Feldspäte wahrend der magmatischen Phase gebildet worden waren. Die Periodizität der Perthit-Lamellen liegt unter 10 nm.

     

Origin of biotite-apatite-rich enclaves, Achala batholith, Argentina

The Achala batholith of Argentina contains very unusual layered enclaves containing up to 30% apatite and 50% biotite in some layers. This modal mineralogy produces bulk-rock compositions that cannot represent liquids, having as little as 29% SiO₂ and up to 8% P₂O₅. Nor can the enclaves represent metasedimentary xenoliths because: (1) none of the Precambrian wall rocks has these compositions; (2) none of the metasedimentary xenoliths present within the batholith shows any degree of transition to the mica-apatite-rich enclaves; (3) the compositions and textures in the enclaves are inconsistent with metasediments; (4) a geochronological study of zircon from an enclave gives an age of 368 ± 2 Ma, the exact age of zircons in the granitic host rocks. For these reasons, we conclude that the enclaves are neither xenoliths of Precambrian wall rocks nor restite of a Precambrian source. The identical age of the enclave and the host granites, coupled with textural, mineralogical, and bulk-rock characteristics of the enclaves, indicates that the enclaves are magmatic segregations, i.e., cumulates. The F-rich nature of the stubby-shaped apatites and biotites indicates a high F content of the magma parental to the enclaves. We infer that the viscosity of the melt was lowered sufficiently to allow cumulates to form in spite of the granitic composition of the melt. Received: 12 December 1996 / Accepted: 11 August 1997