Nucleation on perthite-perthite boundaries and exsolution mechanisms in alkali feldspars

A transmission electron microscope study of intracrystalline boundaries between two perthites of markedly different composition in composite crystals, one a tenary mesoperthite (Or₂₆Ab₅₂An₂₂, initially a homogeneous potassian monalbite) the other a more potassic cryptoperthite (Or₆₁Ab₃₃An₆, initially a homogeneous sodian sanidine), shows that the two perthites are in nearly parallel intergrowth. Most boundaries examined were of (hkO) type; (010) boundaries are straight, whereas other (hkO) boundaries are curved or stepped. Exsolution occurred first in the potassian monalbite (mesoperthite) and was unaffected by the boundary. Subsequent exsolution in the sodian sanidine (cryptoperthite) was affected by the boundary, but for up to only a few micrometers. Exsolution occurred by heterogeneous nucleation and growth of oligoclase on and from the intracrystalline boundary. At almost the same time the rest of the volume of sanidine exsolved by spinodal decomposition. 1–2 μm from the boundary in the intervening K-rich matrix of the sodian sanidine, further exsolution occurred by homogeneous nucleation. Time — temperature — transition curves for continuous cooling have been devised to account for the unusual complexity of the exsolution texture. Except in such exceptional circumstances as the example studied, the initial exsolution in high-temperature alkali feldspars of intermediate composition, unlike other minerals, probably does not occur by nucleation, but only by spinodal decomposition.     

Mixing geochemistry of cold water around non-volcanic thermal springs in high-grade metamorphic terrain,Sri Lanka

A geochemical survey was conducted on thermal water and cold water around non-volcanic geothermal fields at Mahaoya and Marangala in Sri Lanka. One hundred forty-two samples were analyzed for fifteen selected irons to investigate geochemical relationships resulting from water-rock interactions and mixing. Based on measurements using a Na-K-Mg geothermometer, the maximum temperatures of thermal reservoirs were estimated to be 148 °C in Mahaoya and 191 °C in Marangala, which were higher values than those obtained using Na-Li and Li-Mg geothermometers. This suggests that the reservoirs extend from intermediate to deep levels. Hydrogeochemistry of thermal waters is distinct from that of cold water; higher contents of Na, K, Cl, F, SO_4, and TDS in thermal water are most likely due to the dissolution of feldspar, mica, and sulfide minerals in the granitic rocks. Conversely, lower values of Fe, Mg, Cu, Zn, and Pb imply less ferromagnesian minerals in the basement. Classification based on major ions reveals a Na-K-SO₄ type of thermal water for Mahaoya and Marangala. Cold water is dominated by the Na-K-HCO₃ type, which indicates deep groundwater influence by iron exchange. Non-mixing cold water indicates a Ca-HCO₃ type. In general, chemistry of cold water wells (<400 m) close to the thermal water changes significantly due to direct mixing of thermal water and cold water. In comparison, a contrasting action occurs with increasing distance from the geothermal field. Gradual decline of SO₄ with increasing distance from thermal water may indicate a trend of clear oxidation. However, the chemistry of more distant wells demarcates deep circulations through fractures and faults in the basement.     

A new chemical index of weathering for metamorphic silicate rocks in tropical regions: A study from Sri Lanka

Weathered and fresh samples of metamorphic rocks from Sri Lanka were collected from various localities and analysed for major elements by XRF method (RIGAKU, KG-X System, Japan). The content of water was determined by the ignition method.

The XRF results, obtained from these samples form the basis of a new index of chemical weathering, particularly for Sri Lanka, which is named the Silica-Titania Index, and is calculated as follows: Silica-Titania Index = {(SiO₂/TiO₂)/[(SiO₂/TiO₂) × (Al₂O₃/TiO ₂) × (SiO₂/Al₂O₃)]} × 100(molecular proportions).

The index can be used to determine the degree of weathering in chemically weathered silicate rocks of other countries in tropical regions. A triangular diagram plots the position of this index. The point load strengths of fresh rocksand weathered rocks with different degrees of weathering were correlated with the values of this new chemical index. The relative variation in strengths of fresh rocks and weathered rocks clearly indicates its suitability and usefulness for engineering geologists.     

Geochemistry and genesis of granitoid rocks from Southern Sri Lanka

Summary Pyroxene granulites of intermediate to felsic composition comprise a major part of the Precambrian terrain exposed in southern Sri Lanka. Chemical characteristics and field relations reveal that there are two closely associated groups of granitoids both occurring as discrete layers or deformed bodies. Granitic-adamellitic rocks are more common and can be distinguished from tonalitic-trondhjemitic rocks. Granitoids of these two groups and minor metabasalt and metadiorite/meta-andesite have been subjected to high-grade metamorphism and were mapped as charnockite or charnockitic rocks. Rocks of granitic composition are mostly meta-aluminous and, in the AFM diagram, fall in the tholeiitic field. These rocks are mainly A-type granites characterized by high K₂O/Na₂O, FeO/MgO and high Zr, Nb and LREE contents, most belonging to the relatively Ce- and Y-enriched A₂-type. Tonalitic rocks are meta-aluminous to per-aluminous and display a calc-alkaline trend. These rocks are classified as I-type, although some of them have very low K₂O and REE contents.Tonalites and granites are not products of a consanguineous fractionation process. In spite of their close field association, they appear to be polygenetic. The parent magmas of the A-type granites seem to have been differentiated melts derived from felsic continental crust. They were emplaced into normal or attenuated continental crust in a non-Compressive (extensional or trans-tensional) tectonic regime. Tonalites most probably represent volcanic arc granitoids emplaced in a compressive tectonic setting. Present intercalation of these polygenetic granitoids, generated in different tectonic environments, is a result of the complex tectonic activity which prevailed before or during Pali-African (550–610 Ma) granulite facies metamorphism.

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Geochemie und Genese von Granitoiden aus dem süchen Sri Lanka

Zusammenfassung Pyroxengranulite intermediärer bis saurer Zusammensetzung bilden einen Hauptbe standteil des präkambrischen Terrains im südlichen Sri Lanka. Chemische Untersuchungen und Feldbeziehungen ermöglichen die Unterscheidung von zwei räumlich eng assoziierten Gruppen von Granitoiden, die beide lagenförmig oder in Form deformierter Körper auftreten. Granitisch-adamellitische Gesteine sind häufiger und sind von tonalitisch-tronhjemitischen Gesteinen zu unterscheiden. Diese Granitoide und untergeordnet vorkommende Metabasalte und Metadiorite/Metaandesite wurden von hochgradiger Metamorphose erfaßt und als Charnockite bzw. charnokitische Gesteine kartiert. Gesteine granitischer Zusammensetzung sind meist metaluminös und fallen im AFM Diagramm ins Feld der Tholeiite. Es sind A-Typ Granite des an Ce und Y angereicherten A2-Subtyps mit hohem K₂O/Na₂O, FeO/MgO und hohen Gehalten an Zr, Nb und LREE. Tonalitische Gesteine sind met-bis peraluminös und folgen einem kalk-alkalischen Trend. Sie werden als I-Typ Granite klassifiziert, obwohl einige Proben sehr niedrige K₂O und REE Gehalte aufweisen. Die Tonalite und Granite sind nicht die Produkte eines cogenetischen Fraktionie rungsprozesses und sind, trotz ihrer räumlichen Assoziation, polygenetischen Ursprungs. Die Ausgangsmagmen der A-Typ Granite scheinen differentierte Schmelzen saurer krustaler Herkunft zu sein, die in normal dicker oder verdünnter Kruste in einem nicht kompressionellen tektonischen Regime (ex- oder transtensional) intrudierten. Die Tonalite sind wahrscheinlich in einem Kompressionsregime enstandene Granitoide eines Vulkanbogens. Das heutige Nebeneinander dieser polygnetischen, in unterschiedlichen geotektonischen Bereichen gebildeten, Granitoide ist das Ergebnis komplexer tektonischer Aktivität vor oder während der panafrikanischen (550–610 Ma) granulitfaziellen Metamorphose.

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With 11 Figures     

Rare earth elements in high-grade metamorphic rocks from the western Alps

A sequence of amphibolite to granulite facies metasedimentary and mafic metaigneous rocks from the western Italian Alps has been analysed for rare earth elements (REE). In this sequence, the metasedimentary granulites have probably been affected by a melting event while the metaigneous granulites remained unaffected. Metasedimentary granulites have a less fractionated chondrite-normalized REE pattern than equivalent amphibolite facies rocks. The granulites tend to have a higher content of heavy REE and lower abundances of light REE (LREE). The leucosomes of migmatitic granulites have lower REE content than the melanocratic bands and both these rock types have variable relative abundances of Eu. The mafic granulites have LREE enriched patterns while the amphibolites are slightly depleted in LREE. The differences between the mafic granulites and amphibolites are probably of pre-metamorphic origin.     

Co-existing cordierite-almandine-a key to the metamorphic history of Sri Lanka

A co-existing cordierite—almandine assemblage is described from the pelitic metasedimentary rocks of the Precambrian granulite-facies terrain of Sri Lanka. The chemical control on this assemblage is demonstrated, as has been observed in granulite-facies terrains elsewhere. The previous view of a low-pressure origin of cordierite in pelitic rocks, and the subdivision of the central granulite-facies belt of Sri Lanka based thereon, is hence doubtful.     

Sapphires from Sri Lanka

A rural industry of small‐scale mining, for sapphires and a host of other gemstones, is alive and thriving at Ratnapura in Sri Lanka. The minerals are extracted from alluvial gravels and colluvium, and most are then cut and polished locally for use in jewellery.     

The occurrence of högbomite in high-grade metamorphic rocks

Högbomite has generally been considered to be a rare accessory phase in metamorphic rocks. While investigating high-grade peraluminous metamorphites in the Benson Mine District, Adirondack Mountains, New York and the Manitouwadge Massive Sulfide District, Ontario, Canada, we have found several högbomite occurrences and believe that högbomite is more widespread in high-grade aluminous rocks than previously recognized. At Benson Mine, an iron-rich högbomite (Hög) occurs with K-feldspar-magnetite (Mt)-ilmenite (Ilm)-biotite-almandine (Alm)-sillimanite (Sil)-quartz (Qz)-hercynite (Hc)-corundum (Cor)-rutile (Ru). At Manitouwadge, Fe -Zn högbomite is found with gedrite-cordierite-staurolite-hercynite-magnetite±quartz ±ilmenite±rutile±biotite±cassiterite. Because composition varies with structure type, it is essential to determine the structure of högbomite utilized in specific reactions. Högbomite from Benson Mine has an 8H structure type, while that at Manitouwadge has a complex mixed structure. Both are more iron-rich than previously reported högbomites, and their composition can be approximated by the ideal formula Fe₅Al₁₆TiO₃₀(OH)₂. Proposed reactions for 8H-högbomite are Hög=Ilm+Hc+Cor+V, Hög=Ru+Hc+Cor+V, Hög+Ru=Ilm+Cor+V, and Hög+Ilm=Ru+Hc+V. These reactions can be combined with the experimentally determined reactions Alm+Sil=Hc+Qz and Ru+Alm=Ilm+Sil+Qz to derive reactions in the system FeO-Al₂O₃-TiO₂-SiO₂-H₂O that limit the stability of the assemblages Hög+Alm and Hög+Sil. Oxidation-sulfidation reactions define a wedge-shaped stability field for högbomite that is closed on the high f S₂ side.Contribution No. 456 from the Mineralogical Laboratory, University of Michigan, Ann Arbor, USA     

Halite and sylvite as solid inclusions in high-grade metamorphic rocks

Solid inclusions of halite and sylvite, formed during regional and contact metamorphism have been identified by microscopy and by electron microprobe analysis in rocks from Campolungo, Switzerland and Cornone di Blumone, Italy. The solid inclusions occur in several of the major minerals crystallized during metamorphism and have been observed as idiomorphic crystals and dendrites. The compositions measured in 100 analyses from Campolungo, Switzerland and 40 analyses from Cornone di Blumone, Italy extend across the two-phase region in the system, KCl-NaCl, indicating that the salt inclusions are high temperature precipitates. In both localities compositionally zoned and unzoned crystals have been found. Measured compositions on the temperature maximum of the two-phase region indicate at least 500° C which can be compared with 500°±20° C determined by Mercolli (1982) and Walther (1983) from the Mg content of calcites from Campolungo. The solid inclusions have been trapped apart from CO₂-rich and saline, H₂O-rich fluid inclusions which have been described by Mercolli (1982) as the earliest preserved fluid inclusions in the rocks. The early precipitation of salt minerals at Campolungo indicates that fluids were saturated with NaCl and KCl at 500° C and pressures of 2,000 bars or higher. Similar relationships exist between solid and fluid inclusions in the rocks of Cornone di Blumone which formed at temperatures as high as 800° C and pressures between 0.5 and 1 kilobar (Ulmer 1983). The entrapment of halite and sylvite as solid inclusions preserves the composition of the minerals which may therefore be useful as geothermometers.     

大别-苏鲁造山带高级变质岩中锆石微区U，Th和Pb化学组成特征统计

对大别-苏鲁造山带27个高级变质岩(以榴辉岩和片麻岩为主)中锆石进行了337次微区离子探针U.Th和Pb测定和U-Pb年龄研究。根据测定点年龄的不同，可以把变质岩中锆石分成三种不同成因区域，一是变质岩原岩(继承)锆石区域，它遭受了不同程度的变质重结晶作用，其^206Pb/^238U年龄值大于277Ma；二是三叠纪高压.超高压变质阶段形成的变质锆石区域，其年龄限定为273-184Ma；三是碰撞造山变质作用后形成的锆石区域，其年龄小于148Ma。对三种类型锆石中U，Th和Pb特征分析发现：片麻岩锆石中U的含量是榴辉岩类锆石中U的丰度的两倍，片麻岩类和榴辉岩类锆石中Th的丰度大致相近。各类岩石变质阶段形成的变质锆石和原岩锆石区域相比，U含量有升有降，而Th含量则大幅下降，下降幅度一般在2-10倍，高者可达20-30倍以上。除石榴石橄榄岩外，各类岩石变质锆石中区域的Th/U比都小于0.1，这是判断高压-超高压变质成因锆石的极重要化学指标。大别山超高压变质阶段后形成的锆石的Th/U比多数类似于其原岩锆石，少数接近变质锆石，说明白垩纪后期热扰动对变质岩中已存在的锆石的影响如同岩浆结晶过程。各类岩石中锆石Pb的含量变化比较复杂，Pb-U正相关性表明锆石中普通铅含量很低，绝大部分为放射成因铅。     

Ordering and unmixing of alkali feldspars in a porphyritic granite and its country rocks

Summary 50 alkali feldspars from a Variscan porphyritic granite and 27 from two country rocks of Caledonian and Variscan age respectively have been investigated for their bulk chemistries and their structural states. Ordering is illustrated in thebc-plot afterStewart andWright (1974); unmixing is illustrated in a newly designed diagram with the Or-contents of the bulk feldspar and that of the unmixed K-rich phase on the coordinates. The feldspars from the granite have a greater variability with regand to chemistry, degree of ordering and degree of unmixing than those of the country rocks. Ordering and unmixing depend mainly on the action of a hydrous fluid phase. During the in situ formation of the granite, water must have accumulated in the granitic magma and must have remained in the granite after consolidation. Temperatures derived by means of the two feldspar geothermometer indicate selective loss of sodium during the postmagmatic stage and before unmixing of the alkali feldspars. Loss of sodium makes unmixing increasingly sluggish.

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Ordnung und Entmischung von Alkalifeldspäten in einem porphyrischen Granit und seinen Nebengesteinen

Zusammenfassung 60 Alkalifeldspäte aus einem varistischen porphyrischen Granit und 27 Alkalifeldspäte aus zwei verschiedenen Nebengesteinen (Metamorphosealter kaledonisch bzw. varistisch) wurden chemisch analysiert und auf ihren Strukturzustand hin untersucht. Der Ordnungszustand wird imbc-Diagramm nachStewart undWright (1974) dargestellt, die Entmischung in einem Variationsdiagramm mit den Or-Gehalten des Gesamtfeldspats bzw. der entmischten K-reichen Phase als Achsen. Die Feldspäte des Granits sind hinsichtlich Chemismus, Ordnungsgrad und Entmischungsgrad variabler als die der Nebengesteine. Ordnung und Entmischung hängen im wesentlichen vom Einfluß der wäßrigen fluiden Phase ab. Während der in situ-Bildung des Granits muß sich H₂O im Magma angereichert haben und nach der Erstarrung noch im Granit gewesen sein. Anhand des Zwei-Feldspat-Geothermometers wurden Modelltemperaturen abgeleitet. Die erhaltenen Werte weisen auf einen selektiven Velust von Natrium aus dem Alkalifeldspat während des postmagmatischen Stadiums vor der Entmischung hin. Der Na-Verlust macht den Entmischungsvorgang träger.

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With 6 Figures     

Zoning and exsolution in alkali feldspars from Laacher See volcano (Western Germany): constraints on temperature history prior to eruption

Compositional zoning and exsolution patterns of alkali feldspars in carbonatite-bearing cognate syenites from the 6.3 km³ (D.R.E) phonolitic Laacher See Tephra (LST) deposit in western Germany (12.9 ka) are reported. These rocks represent the cooler outer portion and crystal-rich products of a cooling magma reservoir at upper crustal levels. Major and trace-element difference between cores and rims in sanidine crystals represent two generations of crystal growth separated by unmixing of a carbonate melt. Trace-element differences measured by LA–ICP–MS are in accordance with silicate–carbonate unmixing. Across the core–rim boundary, we extracted gray-scale profiles from multiple accumulations of back-scattered electron images. Gray scales directly represent K/Na ratios owing to low concentrations of Ba and Sr (<?30 ppm). Diffusion gradients are modeled to solve for temperature using known pre-eruptive U–Th zircon ages (0–20 ky) of each sample (Schmitt et al., J Petrol 51:1053–1085, 2010). Estimated temperatures range from 630 °C to 670 °C. For the exsolution boundaries, a diffusive homogenization model is constrained by the solvus temperature of ~ 712^_725 °C and gives short time scales of only 15–50 days. Based on our results, we present a model for the temperature–time history of these rocks. The model also constrains the thermal variation across the cooling crystal-rich carapace of the magma reservoir over 20 ka and suggests a thermal reactivation of cumulates, the cooling carapace, and probably the entire system only a few years prior to the explosive eruption of the remaining molten core of the phonolitic magma reservoir.     

The nature of potassium feldspar,exsolution microtextures and development of dislocations as a function of composition in perthitic alkali feldspars

Transmission electron microscope data on the morphology of exsolution lamellae, the nature of the potassium feldspar and the development of dislocations at lamellar interfaces in coherent cryptoperthites and fine microperthites are reviewed. Dislocations have been reported previously in only two crystals, and periodic dislocations noted in only one, an Or-rich microperthite. Periodic dislocations (spacing 100–150 nm) are here described from a ternary mesoperthite (Or₂₆ Ab₅₂ An₂₂). Small crystallites (<30 nm) of other phases have sometimes nucleated on the dislocations. The 020 lattice fringes of the feldspar phases have been imaged; the difference in 020 spacings can be almost entirely accommodated by the regular dislocations, so that the boundaries may be termed nearlyperfectly semicoherent.Dislocations have been found so far only in cryptoperthites with lens-shaped or straight lamellae, either in Or-rich feldspars or in Ab-rich ternary ones. In intermediate compositions with wavy or zig-zag albite lamellae, or lozengeshaped albite areas (braid microperthites) dislocations have not been observed. Strain reduction in intermediate compositions occurs by migration of lamellar interfaces from (¯601) to near (¯6¯61) as microcline forms in the diagonal association. In Ab-rich ternary feldspars the relatively high Ancontent blocks interface migration, and strain reduction occurs by nucleation of dislocations; the Or-rich feldspar phase is tweed orthoclase. In Or-rich bulk compositions the low volume of albite exerts insufficient stress to promote microcline formation, and tweed orthoclase develops. Interfaces do not migrate, and dislocations again develop. Fields in which different potassium feldspar polymorphs occur and in which the different exsolution textures are developed are summarized on a ternary diagram.     

General geology and petrology of some Precambrian crystalline rocks from the Vijayan Complex of Sri Lanka

Detailed field studies of the Precambrian Vijayan Complex terrain, Sri Lanka, reveal the occurrence of granites, gneisses and migmatites in association with calc-silicate gneisses, quartzites and dolerites. Microcline-rich granites and gneisses show both sharp and gradational contacts with the adjacent migmatites. Petrological observations favor a magmatic origin for the granites and gneisses. These bodies seem to have intruded into pre-existing metasedimentary rocks which were subsequently subjected to retrograde metamorphism under amphibolite facies conditions.     

The spatial extent of chemical equilibrium in some high-grade metamorphic rocks from the grenville of southeastern Ontario

Many of the recent studies of chemical equilibrium in metamorphic rocks may have been hampered by sampling which did not reflect single regions or domains of local equilibration. This paper records results of tests performed to determine the specific volume of rock in chemical equilibrium with respect to a certain phase, under a particular set of conditions. Two samples of high-grade metamorphic gneisses from the Grenville Series of southeastern Ontario were selected for intensive study. The garnets of one specimen, possessing a biotite-garnet-cordierite assemblage, underwent in situ laser microprobe spectrochemical determinations for Fe, Mg, Mn and Ca. Conclusions are thus possible as to the volumes over which free diffusion and chemical equilibrium took place. These volumes were found to be on the order of only a few cubic centimeters and their shapes are strongly controlled by rock structures such as foliation and lineation. A higher-grade pyroxene granulite was also investigated with respect to garnet and biotite equilibrium. Somewhat larger volumes of equilibration were noted in this rock.     

Non-equilibrium mixing in natural alkali feldspars

Chemical-optical and X-ray analysis of some perthite groups, from cristalline rock massifs, came into evidence that in perthites with the same order degree of Si/Al distribution, the observed variations of 2V_X are strictly related to the Na content in the potassic phase; i.e. 2V_X decreases when Ab mixed in the K-phase increases.This fact suggests that for any Si/Al, distribution, not in equilibrium, exists a certain range of non equilibrium mixing between Or and Ab.     

中天山北缘华力西期造山作用——变质岩锆石U-Pb年代学限定

笔者对中天山微陆块北缘托克逊干沟地区角闪岩相变质岩中的锆石进行了U-Pb年代学研究,结果证明变质沉积岩中的碎屑锆石记录了从太古宙至元古宙(3320～530 Ma)的源区岩浆热事件,变质火成岩中的岩浆锆石记录了新元古代晚期(550 Ma)的岩浆作用,而变质锆石记录了晚泥盆纪(385～360 Ma)的变质作用。这一定年结果表明,中天山微陆块北缘的造山作用很可能发生在华力西期,中天山微陆块形成于新元古代以前,但并没有经历前寒武纪变质作用,具有与塔里木克拉通明显不同的前寒武纪构造演化历史。因此,中天山微陆块很可能是一个独立的块体,并不支持其是从塔里木板块分离出来的观点。     

Cross-twinning in a natural spinel from Sri Lanka

A modified cross-twinning growth mechanism is put forward to explain the anomalous morphology of a spinel multiple-twin from Sri Lanka, flattened crosswise the twin planes. Cross-twinning in spinel was found also in other specimens from Pegu (Myanmar), and the results were published in a previous paper. This particular type of twinning is derived from the combination of cyclic twinning with lamellar twinning, so that these samples may be thought of as partial fivelings (cubic cyclic {111} twins with five components sharing a common <110> pseudo-fivefold axis). In the present paper, the sample from Sri Lanka has been suitably cut with the aim of focusing the study on the cross-twinning region. The transformation matrices that link the orientation states of each couple of twin components have been determined by means of White Beam Synchrotron Radiation Topography. They showed that the specimen is made up of four twin components (A, B, C and D), with three twin planes: and They also showed that the cross-twinned individuals (B and D) actually are not twinned to each other, and that a simple crystallographic relationship holds between them. X-ray diffraction topography by conventional source allowed to image the crossing-region and to determine that the cross-twinned individuals are in contact through a semi-coherent boundary, with twinning dislocations contributing to relieve the coherency strains. Electron probe microanalyses with wave dispersive spectroscopy showed that the chemical composition is almost homogeneous, at least within the spatial resolution limit of this technique. The similar growth features observed in the spinel sample from Sri Lanka and in those from Myanmar are interpreted as growth marks, indicators of a similar origin: in both cases they are found in impure dolomitic marbles. In particular, the specimen from Sri Lanka results from the interaction of thermal and metasomatic effects due to contact metamorphism. An unusual stepped morphology of the face close to the twin boundary, possibly due to corrosion and re-growth processes acted preferentially at a re-entrant corner by metasomatic fluids, is interpreted as indicator of a metasomatic event that succeeded to the crystal growth, the latter occurred by thermal effect.

Inclusion/host relations involving accessory minerals in high-grade metamorphic and anatectic rocks

In general, accessory minerals are expected to participate in partial fusion of their host rocks to a degree determined simply by their solubilities in the melt. The possibility must be recognized, however, that a given accessory grain may be physically isolated from the melt by inclusion within a residual major mineral. Because of the importance of accessory minerals to crustal-rock trace element and isotope geochemistry, and because of their common existence as inclusions in major phases, we undertook an evaluation of the factors that affect inclusion formation during ultrametamorphism. Three approaches are taken: 1) a review of interfacial energy considerations is used to show that the free energy of the system is lowered by location of accessory minerals at major-phase grain perimeters, and that the magnitude of this effect is proportional to the square of the accessory grain radius; 2) annealing and partial melting experiments (1000° C, 10 and 15 kbar) on rock analogs are described, and the results are shown to confirm the predicted tendency of accessory minerals to occupy grain boundaries; and 3) the results of a study of accessory phase (rutile and zircon) distribution in a migmatite from the Tibetan slab are reported, again in confirmation of the prediction that accessories tend to be situated at majorphase grain perimeters. The latter two aspects of the study reveal that, although included accessory grains are common, their generally small size results in only a minor contribution to the bulk-rock budget of accessory mineral components: Most of the mass of these components is contained within populations of generally larger accessory grains located at major-phase grain boundaries. Accordingly, the assumption that accessory minerals are involved in crustal melting is generally valid.     

Zoned ternary feldspars in the Klokken intrusion: exsolution microtextures and mechanisms

The microtextures developed during relatively slow cooling as a function of bulk composition in zoned ternary feldspars from syenodiorites and syenites in the Klokken intrusion, described in the preceding paper, were determined by TEM and their origin and evolution deduced. The feldspars normally have a plagioclase core and an alkali feldspar rim; cores become smaller and rims larger and the An content of both decrease with distance from the contact of the intrusion. The following microtextural sequence was observed. The inner plagioclase cores are homogeneous oligoclase-andesine with Albite growth twins only, but are crypto-antiperthitic towards the outer core. At first small platelets of low sanidine a few nanometres thick and up to 10 nm long occur sporadically only on Albite-twin composition planes. With further increase in bulk Or they are homogeneously distributed in the plagioclase. Thicker, through-going plates in platelet-free areas are found, which induce Albite twins in the surrounding plagioclase. The microtextures in the rims are regular cryptomesoperthitic, with (¯601) lenses or lamellae, depending on the bulk Or-content, of low sanidine in Albite-twinned low oligoclase-andesine. Albite and Pericline twins in plagioclase in an M-twin relationship, together with lenticular low sanidine, were found in only one small area. The overall diffraction symmetry of the mesoperthites is monoclinic, showing that exsolution started in a monoclinic feldspar, whereas that of the antiperthites is triclinic. The intermediate zone between the core and rim is more complex and microtextures vary over distances of a few micrometres.The cryptomesoperthites are very regular where Or-rich and probably arose by spinodal decomposition. The platelets in the outer cores arose by heterogeneous nucleation on twin composition planes and by homogeneous nucleation elsewhere. Near the intermediate zone they coarsened to give larger plates which induced Albite-twins in the plagioclase. Because of the zoning, microtextures that were initiated in areas of given composition, can propagate laterally into zones of different composition. A diagram is given showing the relationship between ternary bulk composition and the microtexture developed in coherent perthitic alkali feldspars and plagioclases from slowly-cooled rocks.CRPG contribution 730