The activity of silica in kimberlites,revisited

The activity of silica in a silicate liquid in equilibrium with olivine and orthopyroxene decreases with increasing pressure. In contrast, the activity of silica in an unbuffered silicate liquid changes little with pressure. Although the implications of these pressure dependencies have been considered by previous authors in terms of inferring pressures of origin of magmas, less consideration has been given to the implications of these dependencies on the evolution of the magma en route to the surface, or to the mantle through which the magma passes. In this paper, a combination of Schreinemakers’ analysis in isothermal section and calculated reactions in space is used to (a) rationalize the absence of orthopyroxene xenocrysts in kimberlites and the relative abundance of olivine “megacrysts” therein, (b) propose another reason for the paucity of xenocrystic mantle-derived carbonates in kimberlites, (c) explain why clinopyroxene is much less reactive in the kimberlite melt than is orthopyroxene, and (d) explore the implications of the relative stabilities of olivine, orthopyroxene, and clinopyroxene in kimberlitic magma for the mantle through which the magma transits.

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Ab initio study of the high-pressure behavior of CaSiO3 perovskite

Using density functional simulations, within the generalized gradient approximation and projector-augmented wave method, we study structures and energetics of CaSiO₃ perovskite in the pressure range of the Earths lower mantle (0–150 GPa). At zero Kelvin temperature the cubic CaSiO₃ perovskite structure is unstable in the whole pressure range, at low pressures the orthorhombic (Pnam) structure is preferred. At 14.2 GPa there is a phase transition to the tetragonal (I4/mcm) phase. The CaIrO₃-type structure is not stable for CaSiO₃. Our results also rule out the possibility of decomposition into oxides.

A redox profile of the Slave mantle and oxygen fugacity control in the cratonic mantle

The authors report a redox profile based on Mössbauer data of spinel and garnet to a depth of 210 km from mantle xenoliths of the northern (N) and southeastern (SE) Slave craton (northern Canada). The profile transects three depth facies of peridotites that form segments of different bulk composition, represented by spinel peridotite, spinel–garnet peridotite, low-temperature garnet peridotite, high-temperature garnet peridotite, and pyroxenite. The shallow, more depleted N Slave spinel peridotite records lower oxygen fugacities compared to the deeper, less depleted N Slave spinel–garnet peridotite, consistent with their different spinel Fe³⁺ concentrations. Garnet peridotites show a general reduction in log fO₂ (FMQ)s with depth, where values for garnet peridotites are lower than those for spinel–garnet peridotites. There is a strong correlation between depletion and oxygen fugacity in the spinel peridotite facies, but little correlation in the garnet peridotite facies. The strong decrease in log fO₂ (FMQ) with depth that arises from the smaller partial molar volume of Fe³⁺ in garnet, and the observation of distinct slopes of log fO₂ (FMQ) with depth for spinel peridotite compared to spinel–garnet peridotite strongly suggest that oxygen fugacity in the cratonic peridotitic mantle is intrinsically controlled by iron equilibria involving garnet and spinel.

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Two diamond-bearing peridotite xenoliths from the finsch kimberlite,South Africa

Two diamond bearing xenoliths found at Finsch Mine are coarse garnet lherzolites, texturally and chemically similar to the dominant mantle xenoliths in that kimberlite. A total of 46 diamonds weighing 0.053 carats have been recovered from one and 53 diamonds weighing 0.332 carats from the other. The diamonds are less corroded than diamonds recovered from the kimberlite. Geothermobarometric calculations indicate that the xenoliths equilibrated at 1,130° C and pressures 50 kb which is within the diamond stability field; this corresponds to depths of 160 km and would place the rocks on a shield geotherm at slightly greater depths than most coarse garnet lherzolites from kimberlite. The primary minerals in the two rocks are very similar to each other but distinctly different to the majority of mineral inclusions in Finsch diamonds. This suggests a different origin for the diamonds in the kimberlite and the diamonds in the xenoliths although the equilibration conditions for both suites are approximately coincident and close to the wet peridotite solidus.     

Magmatism-related localized deformation in the mantle: a case study

A deformed composite peridotite-pyroxenite xenolith in Pliocene alkali basalts from the Pannonian Basin (Szentbékkálla, Bakony—Balaton Highland Volcanic Field) has been studied in detail. A narrow shear zone of intense deformation marked by porphyroclast elongation and recrystallization runs along the peridotite-pyroxenite contact. The xenolith contains a large volume of euhedral olivine neoblasts and tablet grains of olivine away from the shear zone interpreted as products of annealing and recrystallization in the presence of grain boundary fluid. Estimates of the time required for growth of recrystallized olivine grains suggest that the annealing took place in situ in the mantle and not during transport of the xenolith in the basalt magma. The grain boundary fluid present during recrystallization was a vapor rich silicate-melt different from the host basaltic melt that entrained the xenolith. This study demonstrates that high-stress deformation zones and associated fluid-assisted recrystallization, which are common features in kimberlite mantle xenoliths, also occur in some mantle xenoliths from alkali basalts. The suggested high-stress deformation zones in the mantle beneath the Pannonian Basin may be produced by paleoseismic events in the lithosphere associated with faulting related to the ascent of basalt magma.Editorial responsibility: J. Hoefs

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Weathering of sulfidic shale and copper mine waste: secondary minerals and metal cycling in Great Smoky Mountains National Park,Tennessee, and North Carolina,USA

Metal cycling via physical and chemical weathering of discrete sources (copper mines) and regional (non-point) sources (sulfide-rich shale) is evaluated by examining the mineralogy and chemistry of weathering products in Great Smoky Mountains National Park, Tennessee, and North Carolina, USA. The elements in copper mine waste, secondary minerals, stream sediments, and waters that are most likely to have negative impacts on aquatic ecosystems are aluminum, copper, zinc, and arsenic because these elements locally exceed toxicity guidelines for surface waters or for stream sediments. Acid-mine drainage has not developed in streams draining inactive copper mines. Acid-rock drainage and chemical weathering processes that accompany debris flows or human disturbances of sulfidic rocks are comparable to processes that develop acid-mine drainage elsewhere. Despite the high rainfall in the mountain range, sheltered areas and intermittent dry spells provide local venues for development of secondary weathering products that can impact aquatic ecosystems.Electronic Supplementary Material  Supplementary material is available for this article if you access the article at . A link in the frame on the left on that page takes you directly to the supplementary material.

X-Ray investigations on the deformation of experimentally shock-loaded quartzes

Quartz single crystals submitted to dynamic pressures higher than 200 kbar show intensive anisotropic postshock cell expansions and lattice disordering which gradually increase along with the degree of shock compression. Maximum expansion and lattice distortion occur parallel to [10.0] followed by [21.0] and [20.1], whereas the lowest expansion rates and comparatively little lattice damage can be observed parallel to [10.2] and [00.1]. [10.0], [21.0], and [20.1] represent short Burgers vectors within the quartz lattice. They are probably preferred directions of a structure-controlled deformation.Annealing experiments carried out at 300, 605 and 900 ° C make the expanded cell parameters approach the values of unshocked quartz. Two different types of recrystallization can be observed:

Diamondiferous garnet harzburgites from the Finsch kimberlite,Northern Cape,South Africa

Two xenoliths of garnet harzburgite from the Finsch kimberlite, South Africa, have been found to contain diamond. One of the xenoliths has mineral compositions typical of low-T coarse textured garned peridotites, whereas minerals in the other are similar but not identical to most peridotite-suite minerals included in diamonds, especially in the low-CaO content of garnet. Geothermobarometric calculations show both xenoliths equilibrated at temperatures above 1,100°C and pressures>55 kbar, which is near the low-pressure end of the range of equilibration conditions for diamond-free garnet lherzolites and garnet harzburgites from Finsch. The chemistries of the minerals in the two rocks are distinctly different to most of the mineral inclusions in Finsch diamonds. This, as well as the different ¹³C compositions between xenolith diamonds (-2.8 to-4.6) and diamonds in the kimberlite (generally<-4.3) suggest different origins or sources for the diamonds.     

Sulfur isotopes and the origin of stibnite mineralisation in New England,Australia

Stibnite mineralisation in the antimony province of New England can be divided into Central type ores (veins of stibnite + quartz ± berthierite) and Peripheral type ores of stibnite + quartz + native antimony ± berthierite. The Central stibnites have ³⁴S_CDT values of –5±2 (1) which may represent equilibrium precipitation from mantle sulfur at about 200°C. Peripheral stibnites have ³⁴S values between 0 and –25, with a large group at 0±2. They represent precipitation from a limited supply of mantle sulfur and the acquisition of sedimentary sulfur. We consider that the different ore types were produced from distinct ore solutions derived from two immiscible melts. These originated in the deep mantle, were mobilised by tectonic activity and supplied the antimony and most of the sulfur to the ores.     

Approaches to optimal aquifer management and intelligent control in a multiresolutional decision support system

Despite remarkable new developments in stochastic hydrology and adaptations of advanced methods from operations research, stochastic control, and artificial intelligence, solutions of complex real-world problems in hydrogeology have been quite limited. The main reason is the ultimate reliance on first-principle models that lead to complex, distributed-parameter partial differential equations (PDE) on a given scale. While the addition of uncertainty, and hence, stochasticity or randomness has increased insight and highlighted important relationships between uncertainty, reliability, risk, and their effect on the cost function, it has also (a) introduced additional complexity that results in prohibitive computer power even for just a single uncertain/random parameter; and (b) led to the recognition in our inability to assess the full uncertainty even when including all uncertain parameters. A paradigm shift is introduced: an adaptation of new methods of intelligent control that will relax the dependency on rigid, computer-intensive, stochastic PDE, and will shift the emphasis to a goal-oriented, flexible, adaptive, multiresolutional decision support system (MRDS) with strong unsupervised learning (oriented towards anticipation rather than prediction) and highly efficient optimization capability, which could provide the needed solutions of real-world aquifer management problems. The article highlights the links between past developments and future optimization/planning/control of hydrogeologic systems.

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Resumen  A pesar de nuevos avances notables en hidrología estocástica y las adaptaciones de métodos avanzados de investigación de operaciones, control estocástico, e inteligencia artificial, las soluciones de problemas complejos del mundo real en hidrogeología han sido bastante limitadas. La principal razón es la dependencia definitiva en modelos de primer-principio que conducen a ecuaciones parciales diferencias de parámetro distribuido complejas (PDE) a una escala dada. Mientras que la adición de incertidumbre, y por lo tanto, estocasticidad o aleatoriedad ha incrementado la profundidad y resaltado relaciones importantes entre la incertidumbre, confiabilidad, riesgo, y su efecto en la función de costo, la adición también ha permitido (a) introducir complejidad adicional que resulta en potencia computacional excesiva aún para un solo parámetro incierto/aleatorio; y (b) llevar a reconocer nuestra discapacidad para evaluar la incertidumbre completa aún cuando se incluyen todos los parámetros inciertos. Se introduce un cambio paradigmático: una adaptación de nuevos métodos de control de inteligencia que relajará la dependencia en PDE estocásticas, rígidas y de uso computacional intensivo, cambiando el énfasis hacia un sistema de apoyo de decisiones de propósitos múltiples (MRDS) adaptivo, flexible, y orientado a objetivos con fuerte aprendizaje sin supervisión (orientado a la anticipación más que a la predicción) con fuerte capacidad de optimización eficiente, lo cual podría aportar las soluciones necesarias a los problemas de manejo reales con los acuíferos. El artículo resalta los vínculos entre desarrollos pasados y control/planificación/optimización futura de sistemas hidrogeológicos.

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Résumé  Malgré de remarquables nouveaux développements en hydrologie stochastique ainsi que de remarquables adaptations de méthodes avancées pour les opérations de recherche, le contrôle stochastique, et lintelligence artificielle, solutions pour les problèmes complexes en hydrogéologie sont restées assez limitées. La principale raison est lultime confiance en les modèles qui conduisent à des équations partielles complexes aux paramètres distribués (PDE) à une échelle donnée. Alors que laccumulation dincertitudes et, par conséquent, la stockasticité ou laléat a augmenté la perspicacité et a mis en lumière dimportantes relations entre lincertitude, la fiabilité, le risque, et leur effet sur les coûts de fonctionnement, il a également (a) introduit une complexité additionnelle qui résulte dans un pouvoir prohibitif des moyens de calcul informatique même pour une simple estimation de lincertitude; et (b) a conduit a une reconnaissance de notre manque daptitude à maîtriser lincertitude totale même en introduisant tous les paramètres connus de lincertitude. La représentation du changement est introduit: une adaptation de nouvelles méthodes de contrôle intelligent qui va relâcher la dépendance à la rigidité des algorithmes, aux calculs informatiques intensifs, à la PDE stockastique, et qui modifiera lemphase entre les MRDS—systèmes interactifs daide à la décision de multiresolutionelle (flexibles, adaptables et orientables selon les objectifs)—avec un fort apprentissage non (orienté vers lanticipation plutôt que la prédiction), et une capacité doptimisation efficiente très élevée, qui pourrait apporter le besoin de solutions pour la modélisation des problèmes de management des aquifères réalistes. Cet article met en lumière les liens entre les développements passés et les futurs moyens doptimisation, de gestion et de contrôle des systèmes hydrogéologiques.

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Non-equilibrium,metasomatic18O/16O effects in upper mantle mineral assemblages

Kyser, O'Neil, and Carmichael (1981, 1982) measured the ¹⁸O values of coexisting minerals from peridotite nodules in alkali basalts and kimberlites, interpreting the nodules as equilibrium assemblages. Based mainly on the systematics revealed in ¹⁸O-olivinevs. ¹⁸O-pyroxene diagrams, we have re-interpreted the Kyser et al. data as non-equilibrium phenomena. On such- diagrams, the mantle nodules exhibit data arrays that cut across the ¹⁸O=zero line; these arrays strongly resemble the non-equilibrium quartz-feldspar and feldspar-pyroxene ¹⁸O arrays that we now know arediagnostic of hydrothermally altered plutonic igneous rocks. Thus, the peridotites appear to have been open systems that underwent metasomatic exchange with an external, oxygen-bearing fluid (CO₂ magma, H₂O, etc.); during this event, the relatively inert pyroxenes exchanged at a much slower rate than did the coexisting olivines and spinels. This accounts for the correlation between ¹⁸O pyroxene-olivine and the whole-rock ¹⁸O of the peridotites, which is a major difficulty with the equilibrium interpretation. The metasomatic¹⁸O-enrichments of the peridotites can be related to metasomatic enrichments in LIL elements and the development of amphibole and phlogopite. This type of precursor metasomatic activity can explain the development of alkali basalt magmas, as well as leucitites and nephelinites (all of which tend to be slightly¹⁸O-rich relative to MORB, with ¹⁸O=+6 to +7.5). Fluids with appropriate ¹⁸O values to explain the open-system metasomatic effects can be produced by exchange with ancient subducted oceanic crust (eclogite). However, fluid/rock ratios of about 0.4 to 2.5 are required, indicating that this cannot be a mantle-wide phenomenon. Also, these non-equilibrium effects are apparently transient phenomena, probably associated with the eruptive events that brought the nodules to the surface; at characteristic mantle temperatures, the effects would likely disappear in a few tens of millions of years, or less, implying that the ultramafic nodules are not typical samples of the upper mantle.Contribution No. 4156, Publications of the Division of Geological and Planetary Sciences, California Institute of Technology     

Sr,Nd and Pb isotopic and REE geochemistry of St. Paul's Rocks: the metamorphic and metasomatic development of an alkali basalt mantle source

Surface samples of peridotites and hornblendite mylonites from St. Paul's Rocks, and dredge samples from the flanks of the massif, have been analyzed for Sr, Nd and Pb isotopic ratios and Rb, Sr, and REE concentrations. This data, coupled with previous K and REE data, are used to develop a self-consistent model for the genesis of these ultramafic rocks. This model involves metasomatism of an ocean island-type mantle about 155 m.y. ago by a strongly light-REE-enriched metasomatic fluid, probably derived from the same mantle. This metasomatism produced light-REE-enriched materials which were isotopically homogeneous on a small scale (100 m), and isotopically heterogeneous on a large (km) scale. The geochemical relationships between the peridotites and the hornblendites were established by metamorphic equilibration on a relatively small scale (<10 m). The average mantle produced by these events is characterized by⁸⁷Sr/⁸⁶Sr=0.7034,¹⁴³Nd/¹⁴⁴Nd=0.51291,²⁰⁶Pb/²⁰⁴Pb=19.33 and 207/204=15.63. An alkali basalt which postdates the mylon-itization of the ultramafic massif has an isotopic character which is identical to the average ultramafic massif; it also lies on the five-dimensional isotopic mantle plane of Zindler et al. (1982). With respect to major elements, trace elements, and Sr, Nd and Pb isotopes, the average ultramafic rock of the St. Paul's massif is an ideal candidate for a mantle source from which alkali basalts can be derived by partial melting; the St. Paul's massif is in fact the first such example of an ultramafic rock which meets all the requirements to be an alkali basalt source.     

Beating words to life: subtitles,assemblage(s)capes,expression

Ostensibly, subtitles in films serve as linguistic approximations of meaning. Expectedly then, much of the debate surrounding subtitling has been concerned with representational accuracy, fidelity and authenticity. In this article I argue that by encountering subtitles as affective bodily expressions, as opposed to approximate representations of pre-existent meanings or intentions, filmic experiences may be(come) transformed and differently transformational. As a result, meaning and accuracy in subtitles as superimposed signifiers or static representations become secondary to subtitles as spatially affective- and expressive-movements intimately part of filmic scapes. The creative use of subtitling in Bekmambetov’s Russian language film Night Watch (Nochnoy dozor; 2006 [2004]) is discussed.

Sediment dispersal from Vogelsberg Basalt,Hessen, West Germany

Volcanic detritus in sand size stream sediment spreading outward from Vogelsberg extrusive and hypabyssal rocks decreases in abundance from percentages exceeding 90 % to less than 5 % within 20 km of the source area. The rapid decrease results largely from dilution of the stream sediment resulting from disintegration of friable Bunter Sandstone on the plains surrounding the crystalline rocks. The effect is more pronounced in the fine sand fraction of the stream sediment because the Bunter sand is fine grained.If a geologist of the post-Holocene were to sample this stream sand in outcrop, he would have to be located within about 10 km of the present Vogelsberg mass to infer its existence.

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Zusammenfassung Vulkano-klastische Flußsedimente im Korngrößenbereich von Sanden, die sich aus den Ergußgesteinen und hypabyssischen Gesteinen des Vogelsbergs herleiten, nehmen die Häufigkeit von mehr als 90 % bis auf weniger als 5 % innerhalb von 20 km vom Ursprungsgebiet ab.Die starke Abnahme ist als Folge einer Verdünnung der Flußsedimente entstanden. Die Verdünnung ist durch die Zersetzung des leicht verwitterbaren Buntsandsteins auf den Ebenen, die diese vulkanischen Gesteinen geben, verursacht.Da der Buntsandstein von feinkörniger Natur ist, wird der Verdünnungseffekt in der Feinsandfraktion der Flußsedimente besonders deutlich beobachtet.Würde ein Geologe einst Gesteinsproben entnehmen, die aus diesen Sedimenten entstanden sind, so müßte er sich in einem Umkreis von 10 km des heutigen Vogelsberggebietes befinden, um dessen Existenz nachweisen zu können.

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Résumé Les débris volcaniques dans les sédiments fluviaux de granularité sableuse provenant des roches extrusives et hypabyssales du Vogelsberg voient leur proportion décroÎtre de 90 % à moins de 5 % sur une distance de 20 km. à partir de leur origine. Cette décroissance rapide provient principalement de la dilution du sédiment fluvial par suite de la désintégration des grès bigarrés sur les plaines avoisinant les roches cristallines. L'effet est encore plus prononcé dans la fraction fine du sédiment fluvial à cause de la finesse du sable bigarré.Un géologue des temps futurs qui aurait à échantillonner ce sable fluvial dans ses affleurements devrait rester dans les 10 km. de l'actuel Vogelsberg pour en présumer l'existence.

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Mixing of magmas from enriched and depleted mantle sources in the northeast Pacific: West Valley segment,Juan de Fuca Ridge

The 50 km-long West Valley segment of the northern Juan de Fuca Ridge is a young, extension-dominated spreading centre, with volcanic activity concentrated in its southern half. A suite of basalts dredged from the West Valley floor, the adjacent Heck Seamount chain, and a small near-axis cone here named Southwest Seamount, includes a spectrum of geochemical compositions ranging from highly depleted normal (N-) MORB to enriched (E-) MORB. Heck Seamount lavas have chondrite-normalized La/Sm_cn0.3, ⁸⁷Sr/⁸⁶Sr=0.70235–0.70242, and ²⁰⁶Pb/²⁰⁴Pb=18.22–18.44, requiring a source which is highly depleted in trace elements both at the time of melt generation and over geologic time. The E-MORB from Southwest Seamount have La/Sm_cn1.8, ⁸⁷Sr/⁸⁶Sr=0.70245–0.70260, and ²⁰⁶Pb/²⁰⁴Pb=18.73–19.15, indicating a more enriched source. Basalts from the West Valley floor have chemical compositions intermediate between these two end-members. As a group, West Valley basalts from a two-component mixing array in element-element and element-isotope plots which is best explained by magma mixing. Evidence for crustal-level magma mixing in some basalts includes mineral-melt chemical and isotopic disequilibrium, but mixing of melts at depth (within the mantle) may also occur. The mantle beneath the northern Juan de Fuca Ridge is modelled as a plum-pudding, with plums of enriched, amphibole-bearing peridotite floating in a depleted matrix (DM). Low degrees of melting preferentially melt the plums, initially removing only the amphibole component and producing alkaline to transitional E-MORB. Higher degrees of melting tap both the plums and the depleted matrix to yield N-MORB. The subtly different isotopic compositions of the E-MORBs compared to the N-MORBs require that any enriched component in the upper mantle was derived from a depleted source. If the enriched component crystallized from fluids with a DM source, the plums could evolve to their more evolved isotopic composition after a period of 1.5–2.0 Ga. Alternatively, the enriched component could have formed recently from fluids with a less-depleted source than DM, such as subducted oceanic crust. A third possibility is that enriched material might be dispersed as plums throughout the upper mantle, transported from depth by mantle plumes.     

Characterization of Quaternary faults by electric resistivity tomography in the Andean Precordillera of Western Argentina

El río San Juan, situado en la Provincia de San Juan (Argentina) cruza la Precordillera y otras unidades geológicas incluyendo la Depresión de Ullum y la Zona de La Laja, entre las latitudes 31°S y 32°S. El curso del río tiene un cierto caracter antecedente como puede deducirse por sus dos trazas perpendiculares unidas por otra casi paralela a las alineaciones estructurales principales. En la zona de la Precordillera, el valle del río San Juan muestra numerosos abanicos aluviales, situados en las zonas de confluencia entre el río principal y sus tributarios. Las superficies de los abanicos aluviales cuaternarios estan cortadas por una serie de escalones que consideramos como terrazas aluviales generadas por episodios repetitivos de agradación y degradación. El sector estudiado incluye una zona con una importante actividad sísmica reciente(La Laja), otra sin una importante actividad sísmica reciente (Precordillera), y una zona subsidente (Ullum) donde se formó un gran lago natural hace unos 6500 años. El antiguo río San Juan fue capturado por el valle de la Quebrada de Ullum mediante una incisión del orden de 25 m, que implicó una nueva adecuación del gradiente del río mediante los efectos de la erosión remontante. El gradiente del río San Juan muestra algunas irregularidades que, aunque no se presenten relacionadas directamente con las estructuras principales, estan relacionadas con la propia dinámica fluvial que acentúa la diferenciación litológica. La anchura del valle del río principal, la geometria y el gradiente de cada tributario, junto a las litologias del basamento y a las dimensiones de cada area fuente local, son los factores principales que controlan los procesos de la generación de las terrazas aluviales. En la zona de La Laja, donde la terraza mas alta soporta un nivel de travertino, la datación de los depósitos travertínicos proporciona datos como para suponer una tasa de incisión del orden de 0,9–1 mm/año, asociada a la actuación periódica de la falla de La Laja.

Petrology of the alkaline rocks around Sivamalai,Coimbatore District,Tamilnadu, India

The alkaline rocks around Sivamalai are represented by syenites, nepheline syenites and basic alkaline rocks. The syenites are comprised of olivine-pyroxene, pyroxene, hornblende and biotite syenites and syenodiorites, pegmatite phases and quartz syenite. Nepheline syenites include pyroxene, hornblende and biotite bearing varieties. Syenites with pockets of garnet, magnetite or corundum occur in some places between the nepheline syenites and the country rocks represented dominantly by granitic gneisses. The basic alkaline rocks are represented by barkevikite melteigite and they occur as xenoliths amidst the nepheline syenites. The field mineralogical and petrochemical characteristics indicate the igneous nature of these rocks. The syenites and nepheline syenites are inferred to be evolved from pyroxene syenodiorite and barkevikite melteigite respectively. The irregular distribution of garnet and corundum is attributed to the metasomatism of the country rocks by the alkaline magma.

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Zusammenfassung Die alkalischen Gesteine um Sivamalai umfassen Syenite, Nephelinsyenite und basische alkalische Gesteine. Die Syenite sind aus Olivin-Pyroxen, Pyroxen, Hornblende und der Biotitsyenit, Syenodiorit, Pegmatitphasen und Quarzsyenit zusammengesetzt. Die Nephelinsyenite schließen Varietäten mit Pyroxen, Hornblende und die Biotite ein. Syenite mit Granatlinsen, Magnetit oder Korund schalten sich an einigen, Stellen zwischen Nephelinsyenite und die weiter verbreiteten granitischen Gneise. Die basischen Alkaligesteine werden durch barkeviktischen Melteigit vertreten und finden sich als Xenolithe im Nephelinsyenit. Der mineralogische und petrochemische Feldbefund läßt auf eruptiven Charakter dieser Gesteine schließen. Es wird angenommen, daß die Syenite und Nephelinsyenite sich aus Pyroxensyenodiorit und Barkevikitmelteigit entwickelt haben. Die ungleichförmige Verteilung von Granat und Korund wird auf die Metasomatose der umgebenden Gesteine durch alkalisches Magma zurückgeführt.

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Résumé Les roches alcalins autor de Sivamalai sont représentées par des syénites, des syénites éléolitiques et de roches alcalines basiques. Les syénites comprennent des syénites à péridot, pyroxene, hornblende et biotite, et des syénodiorites, des phases pegmatitiques et des syénites quartziféres. Les syénites éléolitiques comprennent des variétés à pyroxene, hornblende et biotite. Des syénites avec poches à grenat, magnétite ou corindon se rencontrent en quelques endroits entre les syénites éléolitiques et les roches encaissantes représentées principalement par gneiss granititiques. Les roches alcalines basiques sont représentées par des melteigites barkévictiques et se rencontrent en enclaves dans les syénites à néphéline. Les caractéristiques sur le terrain, minérologiques et pétrochimiques, indiquent le caractère igné de ces roches. Les syénites et les syénites éléolitiques sont considérées comme dérivant respectivement de syénodiorites pyroxéniques et de melteigites barkévictiques. La répartition irregulière du grenat et du corindon est attribuée à la métasomatose des roches encaissantes sous l'influence du magma alcalin.

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