Plagioclase lamellae in hypersthene,Tikkoatokhakh Bay,Labrador

Abundant lamellae of plagioclase are present in the (100) planes of hypersthene megacrysts in andesine anorthosite along Tikkoatokhakh Bay, northwest of Nain, Labrador. Spongy intergrowths of plagioclase in hypersthene also occur. Plagioclase lamellae have mean compositions ranging from An₄₃ to An₉₂, with extreme compositions from An₃₉ to An₉₇; the calcic compositions are the more abundant. Such lamellae are always accompanied in the hypersthene by grains or lamellar segments of magnetite, and rarely by lamellae of olivine, augite, magnetite, or ilmenite. Some calcic plagioclase lamellae contain antiperthitic spindles of orthoclase. The host rocks of the hypersthene megacrysts are layered leuconorites and anorthosites with mean plagioclase compositions ranging from An₄₁ to An₅₅. The plagioclase lamellae in hypersthene are characteristically much more calcic than the host-rock plagioclase. There is little doubt that the lamellae exsolved from a pyroxene host, dominantly by a coupled redox reaction which generated magnetite, thereby releasing silica to combine with the Ca-Tschermak and jadeite components of the precursor pyroxene. Rapid growth of megacrysts may account for their aluminous nature.     

Repeated recharge,assimilation, and hybridization in magmas erupted from El Chichón as recorded by plagioclase and amphibole phenocrysts

Compositional and isotopic zoning patterns in plagioclase and amphibole phenocrysts from El Chichón record multiple cycles of country rock assimilation, magma injection, hybridization, and mixing. Laser ablation ICP-MS and electron microprobe analyses of plagioclase crystals from 7 eruptions spanning 3100 years reveal four types of zoning. These compositional and isotopic zones are often associated with textural changes observed in the crystals in thin section (e.g. sieved or patchy regions). Amphiboles are frequently zoned in Al and Si, and, in two magmas, may have clinopyroxene rims. Interestingly, most plagioclase show multiple and repeated zoning patterns. Moreover, all magmas contain all zoning patterns and textures, and crystals with substantially different sequences of zones occur within mm of one another. The most reasonable explanation for the origin of these textures is a frequently recharged chamber. Plagioclase zones with increasing anorthite contents (An) and decreasing ⁸⁷Sr/⁸⁶Sr ratios record injection by a hotter, possibly wetter, and more primitive magma (lower ⁸⁷Sr/⁸⁶Sr ratio). Zones with decreasing An and increasing ⁸⁷Sr/⁸⁶Sr ratios record assimilation of country rock and/or hybridization of the host and injected magmas; injection of hot magma may provide the heat for country rock assimilation. Changes in An without corresponding changes in ⁸⁷Sr/⁸⁶Sr ratio likely record slight variations in pressure or temperature during crystallization, or the far-field thermal effects of magma injection. Variations in ⁸⁷Sr/⁸⁶Sr ratio unaccompanied by Anzoning record assimilation of country rock. Amphibole zoning patterns also record periodic heating events; amphibole with clinopyroxene rims record episodes where the magma was heated beyond the amphibole stability field. Bulk compositional homogeneity and the juxtaposition of many crystals with disparate zoning patterns in single pumice require the magmatic system to be well mixed. Strontium diffusion rates indicate that the plagioclase zoning patterns cannot have persisted at magmatic pressures and temperatures for more than ~ 500 years, thus cycles of injection and assimilation occur on timescales equal to or shorter than the eruption recurrence interval. Long-term compositional and isotopic homogeneity indicate that there is a balance between recharge, assimilation, and crystallization.     

Occurrence of chromian,hercynitic spinel (“pleonaste”) in Apollo-14 samples and its petrologic implications

Many isolated grains of a reddish pleonaste-type spinel occur in fines and metabreccia samples, particularly 14 319. Electron microprobe analyses (104) of spinels and their associated phases include 58 of pleonaste which show Mg/(Mg + Fe) 0.44–0.62 and Cr/(Cr + Al) 0.017–0.134 (atomic), plus minor amounts of other ions, and differ greatly from almost all previously recorded lunar spinels; almost no spinels of intermediate composition were found. Two types of compositional zoning exist: a diffuse primary one with cores lower in Ti, and a narrow secondary one from reaction with matrix yielding rims higher in Cr, Ti, and Mn. At contacts with breccia matrix there is a narrow corona of almost pure plagioclase (An₈₀-An₉₄), free of opaque minerals and pyroxene. Two types of solid inclusions found in the pleonaste are calcic plagioclase, and tiny spherical masses of nickel-rich sulfide.     

Metavolcanic rocks and their origin,Southern Adirondack mountains,New York

One of the problems associated with massif-type anorthosite in high grade metamorphic terranes is the absence of anorthositic extrusives. However, if anorthosite formed by crystal segregation, volcanic equivalents of the final fluid differentiate might occur. Massive, stratiform hornblende-garnet granitic gneisses occur between two metasedimentary sequences in asynclinorium south of the main Adirondack anorthosite massif and north of the Thirteenth Lake anorthosite dome. The granitic gneisses are divided into three stratigraphic units on the basis of mineralogy. The entire stratigraphic configuration has been traced 70 kilometers laterally, and the interpretation of the literature indicates that it may extend for over 100 kilometers. Earlier workers considered the granitic gneisses to be sill-like intrusives. The following evidence suggests a volcanic origin for these gneisses:

1. A thin (3 to 5 meters) but continuous zone of cale-silicates and quartzite occurs within the sequence of granitic gneisses at the same horizon for some 25 kilometers laterally and does not appear to be truncated nor intruded by the gneisses. This observation virtually precludes an intrusive origin.
2. 2V, of plagioclase (An_24,25) in granulated, augen-shaped zones in the granitic gneisses variesfrom 109° to 120°, whereas 2V, for groundmass plagioclase (also An_{20, 25}) is 84° to 88°. According to the data of Slemmons, the 2V of the granulated augen is characteristic of volcanic (high temperature) plagioclase and that of the groundmass is characteristic of plutonic (low temperature) plagioclase.
3. The great lateral extent of the granitic gneisses suggests that the materials were deposited as volcanic ash or pyroclastics.
4. The stratigraphic coherency of metasedimentary sequences and granitic gneisses indicates that the gneisses are an integral part of the stratigraphy.

A jotunite-mangerite-charnockite suite of intrusives in this area is related to the main anorthosite massif. Since plagioclase augen or their remnants exist in both the granitic gneisses and in the intrusive suite, it is possible that the granitic gneisses and the intrusives were derived from the same magma. It has been suggested that differentiation of the anorthosite suite occurs at considerable depth whereas differentiation of the same magma at shallow depth produces the rapakivi-laboradorite porphyry association of the Fennoscandian region. If the stratiform granitic gneisses and the anorthosite suite in the Adirondacks are comagmatic, differentiation of the magma must have occurred at moderate depth and the parental magma may have been granodioritic and thus more silicic than presumed by most workers.     

Petrographic investigation and high-voltage electron microscopy of an anorthosite fragment from Luna 20 landing site

A 2-mm fragment of an anorthosite was investigated in transmitted light and by high-voltage electron microscopy. A modal composition of 76% plagioclase (anorthite), 23% pyroxene (pigeonite) and about 1% spinel (pleonaste) was deduced. The results indicate a slight olivine-normative chemical composition. The rock is hypidiomorphic to idiomorphic fine-grained with a well-defined parallel-oriented fabric.Under the petrographic microscope, and even in the submicroscopic range, the plagioclase crystals show twinning only along (010) composition planes (predominantly Carlsbad twins). The dislocation density is low and the dominating defect structures are small-angle grain boundaries. The subtexture of the plagioclases is characterized by aI1-P1 phase transformation and by the existence of large antiphase domains.Features indicative of shock were not observed. From the present results it is concluded that the rock fragment is in its original state. Most likely, crystallisation occurred under plutonic conditions in a deep crustal region with the dominant assemblage: plagioclase + pyroxene + spinel.     

Amphibolitization within the lower crust in the termination area of the Godzilla Megamullion,an oceanic core complex in the Parece Vela Basin

Gabbroic rocks and amphibolites were collected from the KR03‐01‐D10 dredge site located on the West Arm Rise of the Godzilla Megamullion, close to the Parece Vela Rift which appears to correspond to the termination area of a detachment fault, the Philippine Sea. The gabbroic rocks and amphibolites reveal the occurrence of a high hydrothermal activity in the lower crust close to a paleo‐ridge. In the gabbroic rocks, plagioclase compositions of both porphyroclasts and matrix were transformed into sodium‐rich compositions close to albite. Amphiboles are of secondary rather than igneous origin based on their microstructural occurrences. In the amphibolites, anorthite contents of porphyroclasts and matrix plagioclase are relatively lower than those of the gabbroic rocks, whereas the chemical compositions of amphibole within the amphibolites are similar to those of amphibole within the gabbroic rocks. Amphibolites represent the product of retrograde metamorphism associated with hydrothermal alteration of the gabbroic body by the reaction: clinopyroxene + calcic plagioclase + fluid → amphibole + sodic plagioclase. The estimated temperatures of the amphibolites derived from the amphibole thermobarometer and the gabbroic rocks derived from the hornblende–plagioclase geothermometer show ～700–950°C and 650–840°C, respectively. The hydrothermal alteration recorded in the gabbroic rocks possibly occurred under high‐T conditions; the rocks were then metamorphosed to the amphibolites during a retrogressive stage. Our study indicates that amphibolitization took place with various degrees of deformation. It may imply that the hydrothermal activity increased as the Godzilla Megamullion developed as an oceanic core complex in the paleo‐ridge.     

江西盛源盆地橄榄玄粗岩系列火山岩长石矿物特征及成因信息

江西盛源盆地橄榄玄粗岩系列火山岩中的长石矿物特征为：斜长石斑晶具有反环带结构和交代净边结构,基质中存在大量的斜长石微晶,且在电子探针下研究发现基质中斜长石微晶具有钾长石环边的矿物学特点,为此类火山岩归属于橄榄玄粗岩系列火山岩提供了矿物学证据。通过对长石矿物组合特征进行研究,探讨该地区橄榄玄粗岩系列火山岩的成因以及成岩时的物理化学条件等方面的信息。     

Flotation and remelting of plagioclase in the Kalka Intrusion,central Australia: Petrological implications for anorthosite genesis

Flotation of early-formed plagioclase in the layered gabbroic Kalka Intrusion of central Australia was accompanied by gravitational settling of co-crystallizing pyroxenes. Remelting of the rising plagioclase upset the normal gravitational crystal-extractive processes which control classic basaltic fractionation trends. Together with high pressures of crystallization which cause orthopyroxene rather than olivine to be the first phase to precipitate, this may help to explain the eventual production of at least 800 m of plagioclase cumulates (anorthosites) as late-stage products of fractionation in Kalka. Addition or removal of melts at various stages of fractionation are further processes affecting the differentiation trends of layered intrusions. Variations in the occurrence of such processes may explain many of the differences between anorthosite suites throughout the world.     

Magnetic properties of the Cunene Anorthosite Complex,Angola

The Cunene Complex is the largest known anorthosite body and outcrops across the border between Angola and South West Africa. Palaeomagnetic results are reported from a traverse across the dark troctolitic facies of the anorthosite in Angola which yielded fifteen sites with two additional sites in gabbro bodies. Fourteen sites are stable to a.f. demagnetisation and a single site in the cumulative border zone of the anorthosite is reversed with respect to the remainder. Twelve sites combine to give a mean direction of D = 259°, I = ?46° (k = 7) with a virtual geomagnetic pole at 255°E and 3°S. The low overall precision is probably due to apparent polar movement during cooling of the Complex. Radiometric data are currently conflicting and imply that the anorthosite has an age between 1100 and 2600 m.y.; the only clear feature to emerge from age studies is a thermal overprinting at ca. 1100 m.y. The directions of magnetisation are shown to be most consistent with an age of ca. 2100 m.y. with cooling through the Curie point continuing to ca. 2000 m.y.A variety of magnetic tests demonstrate that magnetite is the principal remanence carrier in the dark troctolitic anorthosite where it occurs both as discrete grains and as fine rods in plagioclase. Lowrie-Fuller tests suggest that both these components include single domains but results from separated mineral fractions demonstrate that the bulk of the high coercivity remanence resides in magnetite rods within the feldspar.     

The geology, petrology, and petrogenesis of Saba Island, Lesser Antilles

Saba is the northernmost volcano along the Lesser Antilles island-arc chain. The Lesser Antilles arc results from the west-northwest subduction of the Atlantic lithosphere beneath the Caribbean Plate. Sediment thickness along the trench decreases northward away from sediment sources on the continent of South America. We focused our attention on Saba precisely because it is the furthest away from documented geochemical effects in the southern arc volcanics of the large sediment thicknesses — normally attributed to both source or upper level contamination (i.e. assimilation).Field mapping, petrology, mineralogy, K–Ar dating, and geochemical analyses (major and trace element) indicate a complex history of magma petrogenesis including crystal fractionation, magma mixing, and, surprisingly, crustal assimilation. This is the first time assimilation has been documented in the northern section of the Lesser Antilles arc. Magma mixing shows up in the field as banded pumice and petrographically and mineralogically as complex zoning in phenocrysts (such as reverse zoning in plagioclase), disequilibrium mineral assemblages (e.g. quartz and olivine), and disequilibrium between minerals and whole-rock compositions (e.g. forsterite content of olivine). Mass-balance modeling of major and trace elements support our contention that crystal fractionation (including amphibole) played an important role in magma evolution. However, various geochemical trends can only be explained by assimilation-fractional crystallization based on the fact that the trends of various trace elements and trace-element ratios vary with increasing silica. Finally, we could find no evidence of sediment source contamination in the most mafic rocks. It may exist but is overprinted by the later assimilation effects.     

Thermodynamic conditions of framework grain dissolution of clastic rocks and its application in Kela 2 gas field

Feldspar and clastic debris are the most important constituent framework grains of sedimentary clastic rocks and their chemical dissolution plays an essential role in the formation and evolution of the secondary pore in the reservoir rocks. On the basis of thermodynamic phase equilibrium, this study investigates the chemical equilibrium relationships between fluid and various plagioclase and K-feldspar in diagenesis of the sediments, particularly, the impact of temperature and fluid compositions (pH, activity of K+, Na+, Ca2+ and so on) on precipitation and dissolution equilibria of feldspars. Feldspar is extremely easily dissolved in the acid pore water with a low salinity when temperature decreases. The dissolution of anorthite end-member of plagioclase is related to the Ca content of the mineral and the fluid, higher Ca either in the mineral or in the fluid, easier dissolution of the feldspar. Moreover, the dissolution of albite end-member of plagioclase is related to Na of both the mineral and fluid,     

Compositions of spinels from cumulative anorthites of the Mutnovskii,Ksudach, Golovnina,and Malyi Semyachik Volcanoes,in Kamchatka

Microprobe studies of unzoned plagioclases (An_92–96) from crystal tuff of Mutnovskii Volcano and allivalite nodules of rocks from the Ksudach, Malyi Semyachik, and Golovnina Volcanoes revealed small inclusions of a dark-colored mineral that was later identified as spinel. Microprobe analyses showed that the grains are unzoned and spinel inclusions of different chemical compositions may occur in one plagioclase crystal. The spinel compositions form a clear extended single trend corresponding to the solvus zone of a solid solution that has not been described in the literature. The existence of this spinel trend in the solvus zone might have been due to early capturing of spinel grains by growing plagioclase crystals and their rapid cooling soon after eruption, resulting in hardening of the metastable solution. These spinels are supposed to form synchronously with plagioclase crystallization. The diversity of spinel compositions is explained by thermo diffusive equalizing of originally zonal spinel crystals after they were captured by plagioclase crystals or by their growth in crystallization haloes of anorthite.     

Structure and development of the lower crust and upper mantle of Southwestern Japan: Evidence from petrology of deep-seated xenoliths

Abstract Basic and ultrabasic xenoliths included in Cenozoic alkali basalts from the Kibi and Sera plateaus, Southwest Japan, can be classified into five groups on the basis of mineral association and texture. Their equilibration P-T conditions estimated from paragenesis and mineral chemistry indicate that the dominant rock type from the lower crust to upper mantle changes with increasing depth as follows: (i) pyroxene granulite (Group V) and meta-sediments; (ii) garnet gabbro (Group 111) and corundum anorthosite (Group IV); (iii) spinel pyroxenite (Group 11); and (iv) spinel peridotite and pyroxenite (Group I). Groups I1 and I11 show a lower degree of recrystallization than Groups I and V, and have similarities in composition and mineral chemistry to host basalts. Based on these facts along with the P-T conditions of equilibration, Groups I1 and I11 are interpreted as formed from basaltic magma that intruded beneath the crust-mantle boundary at an early stage of the magmatism of the alkali basalts, where the lower crust and uppermost mantle had consisted of Group V and metasediments, and Group I, respectively. It follows that the crust has grown downward due to underplating of basaltic magma beneath the bottom of pre-existing crust. Group IV has commonly the same mineral assemblage, corundum + calcic plagioclase + aluminous spinel, and shows locally, nearby kyanite crystals, almost the same texture as fine-grained aggregates in a quartzite xenolith. The aggregates appear to have been formed by reaction between kyanite and host basalt, and accordingly Group IV is interpreted as formed by reaction between metasediments and basaltic magma at the time of the underplating. The Kibi, Sera and Tsuyama areas are distinguished from the areas nearby the Sea of Japan by the occurrence of the garnet gabbro and corundum anorthosite xenoliths, by the absence of the association of olivine + plagioclase in basic and ultrabasic xenoliths, and by the lower temperature of equilibration of basic xenoliths. From these facts it is stressed that in general the crust becomes thinner and geothermal gradient becomes higher towards the back-arc side. Such a regional variation in crustal structure must reflect the tectonic situation of Southwest Japan at the time of the magmatism of the alkali basalts, namely rifting and shallow-level magmatism at the back-arc side.     

Thermodynamics of the oxygen isotope fractionation involving plagioclase

The thermodynamic relationship between the oxygen isotope fractionation properties of plagioclase and its composition has been derived by treating plagioclase as a “reciprocal solution” consisting of independent cationic and anionic substitutions, namely (NaAl)⁵⁺?(CaSi)⁵⁺and¹⁸O?¹⁶O. The results show that the logarithm of the oxygen isotope fractionation factor, α, between plagioclase and a coexisting phase varies linearly with the anorthite content of plagioclase. The proportionality constant is given by the oxygen isotope fractionation factor between albite and anorthite, and has been derived from the experimental data of two groups of workers, O'Neil and Taylor [2] and Matsuhisa et al. [3], on the isotopic fractionation between each plagioclase end-member and aqueous solutions. It is found that O'Neil and Taylor's data on isotopic exchange of plagioclase end-members with only 2–3 M chloride solution, rather than with both pure water and the chloride solution, lead essentially to zero intercept of the ln α(Ab-An) vs. 1/T² relation, in accord with Bottinga and Javoy's [10] conclusion about the oxygen isotope fractionation between two anhydrous silicates at T<500°C.     

Olivine basalts at the Karymskii Volcanic Center: Mineralogy, petrogenesis, and magma sources

The olivine basalts of the Karymskii Volcanic Center (KVC) can be traced during the history of the area from the Lower Pleistocene until recently (the 1996 events); they are typical low-and moderate-potassium tholeiite basalts of the geochemical island-arc type. We have investigated the compositions of phenocryst minerals represented by plagioclase, olivine, clinopyroxene, as well as solid-phase inclusions of spinel in olivine, and more rarely in anorthite. The evolutionary trends of the rock-forming minerals provide evidence of the comagmaticity of these basalts, and thus of a long-lived intermediate magma chamber in the interior of the structure. The activity of this chamber is related to periodic transport of high temperature basalt melts to the surface. The geochemistry of the basalts is controlled by their origin at the same depleted magma source close to N-MORB, by successive crystallization of the primary melt, and by restricted mixing with magma components that are crystallizing at different depths. It is hypothesized that the solid-phase inclusions of high alumina spinel (hercynite?) found in olivine (and anorthite) of the basalts in the KVC north sector are of relict origin.     

Mineral composition variation in Alpine Schist, Southern Alps, New Zealand: Implications for recrystallization and exhumation

Abstract Compositional variation of silicates (plagioclase, K-feldspar, epidote, titanite, garnet, white mica, biotite, chlorite), ilmenite, carbonates (calcite, ankerite) and apatite, in quartzofeldspathic lithologies of the Alpine Schist, New Zealand, is discussed in terms of increasing metamorphic grade and possible isograd-producing reactions. The mineral data, in conjunction with geological considerations, are used to determine polychronous P-T arrays of an early high P/T event (c. 16°C/kb; 5°C/km) overprinted by a lower P/T event (c. 50°C/kb; 15°C/km) that provides an estimation of Mesozoic and Cenozoic exhumation of schist of 11 to 13 km and 19 to 22 km respectively. The effects of possible shear heating and recrystallization to form K-feldspar zone schist near the Alpine Fault is consistent with movement along a mid to lower crustal detachment surface during Cenozoic shortening, and near isothermal exhumation of the schists to form the Southern Alps.     

The petrochemistry and significance of jogimardi traps,chitaldrug schist belt,Mysore

Forty-two samples of the Jogimardi trap from Chitaldrug schist belt, Mysore, were analysed for their major elements. It is inferred that the Jogimardi trap is a submarine lava flow of eugeosynclinal environment. The composition of the suite varies from tholeiitic basalt to andesite and spilite. The original magma, which was probably tholeiitic in nature, might have given rise to this association due to varying and incomplete degree of spilitisation, which in turn seems to be related to the short period of contact between metasomatic liquid and the tholeiitic flows. The suite bears the imprint of low grade metamorphism of green schist facies. This resulted in the development of mineral assemblage comprising chlorite, epidote and plagioclase but without bringing about any change in the original basaltic texture. At and near the contact with granite, the tholeiitic lava was converted into orthoamphibolite, most probably due to granitie intrusion.     

Petrogenesis of Tauhara Dacite (Taupo Volcanic Zone,New Zealand) - Evidence for magma mixing between high-alumina andesite and rhyolite

Tauhara dacites have petrographic, geochemical and isotopic characteristics which indicate an origin by magma mixing between andesite and rhyolite. Phenocrysts typically exhibit strong zoning near their rims, are resorbed or display fusion textures. Assemblages are not in equilibrium with host lavas and compositions are bimodal: plagioclase An_23–43 and An_66–91; orthopyroxene En_44–51 and En_69–79. Chemical and isotopic trends pass through the bulk compositions of high-alumina andresite and rhyolite which crop out in the vicinity of the dacite domes. Least squares mixing models indicate 40–75% of a rhyolite endmember mixed with andesite can generate the full range of dacite compositions. Subtle geochemical differences between domes suggest that magma mixing may have proceeded as three or more general episodes, each punctuated by several events. These episodes may have catalyzed some of the larger pyroclastic flow eruptions of Taupo Volcanic Zone in the past 50,000 years.     

Genetic relations of shoshonitic and absarokitic magmas, Absaroka Mountains, Wyoming

The nature, origin, and tectonic significance of shoshonitic volcanism is currently the subject of widely differing views. In the type locality in the Absaroka Range, the rocks consist of a diverse group of lavas, all of Mid-Eocene age. High in the volcanic pile are subordinate volumes of absarokite and shoshonite flows, both of which contain calcic plagioclase and sanidine coexisting in the groundmass. Shoshonites contain plagioclase, olivine, and pyroxene phenocrysts; absarokites contain only olivine and pyroxene phenocrysts. A few absarokites contain modal leucite. A chemical study was made of six shoshonites and three absarokites from this area.Although resorption and zoning indicate disequilibrium in both types of rocks, there is no geochemical evidence of magmatic contamination. Calculated crystal/liquid distribution coefficients are in close agreement with previously determined values for basalts. Decrease in pressure during ascent is a likely cause for the observed disequilibrium.Mass balance calculations show that it would be possible to form a shoshonite by fractionation of olivine and pyroxene from an absarokitic magma. Trace element abundances are consistent with this relationship. Petrographic evidence suggests, however, that fractionation of plagioclase played a role in the formation of shoshonites.A high-pressure origin is consistent with rare earth compositions, and a high magmatic temperature is indicated by the composition of the plagioclase. A calculation of ln a_K^liquid2O for a reaction involving eclogite and a reaction involving a tested rock (Marsh and Carmichael, 1974) suggests a high-pressure origin for a latitude underlying the shoshonites and absarokites. This calculation is subject to large variances because of its sensitivity to estimated equilibrium temperatures.Comparison with lavas of similar composition indicates that shoshonites and absarokites of the Absaroka Range are influenced by their continental setting. There is little evidence for the generation of these magmas in a subduction environment.     

Origin of strongly reversed rims on plagioclase in cumulates

Narrow reversed rims on plagioclase are ubiquitous in troctolites and olivine gabbros of the Kiglapait intrusion and may be a common feature of all such cumulates. The rims occur at plag/plag, plag/ol, and less strongly at plag/aug grain boundaries. They are optically obvious atΔAn < 10 mol.% and can reachΔAn = 32 mol.% or more. In parallel,K/Na drops sharply. Although ubiquitous from sample to sample, the reversed rims are only locally present at grain boundaries even for the same pair of crystals in contact; they are prominent in linear networks suggesting the last trace of intercumulus liquid. A subsolidus origin is ruled out by the absence of reactants at plag/plag and plag/ol boundaries and by the local rather than pervasive development of rims. The rims are required to grow from intercumulus liquid, in which the partition of An component between crystals and liquid increases with the trapped augite component of the liquid. Calculations from published experimental data show thatΔAn > 30 can easily be achieved by such a process. It is also probable that the trapped liquid is part of an An-rich boundary layer generated by solute rejection during adcumulus growth. The ability of the rims to sustain steepK/Na gradients despite a long subsolidus cooling history proves that the KNa exchange rate is vanishingly small over a geologic time scale in An-rich feldspar, suggesting that at low K content the potassium is site-bound to the tetrahedralAl/Si distribution. Reversed rims therefore provide important information on diffusion limits as well as on the late-stage solidification history of plagioclase-rich cumulates. Moreover, they demonstrate that plagioclase geothermometry cannot be divorced from effects of liquid composition and structure as monitored, for example, by augite content.