Contribution of enclave studies to the understanding of origin and evolution of granitic magmas

Enclaves give important information on the origin and evolution of granitic magmas.The presence in a granite of surmicaceous enclaves (restites) is good evidence of continental crust contribution to magma genesis. The presence of dark microgranular enclaves is an indication of mantle contribution. The joint presence in the same granite of the two types of enclaves indicates that its magma has probably more than one source.Dark microgranular enclaves, and their host granites as well, are hybridized rocks resulting from incomplete mixing (or mingling) of more acidic and basic components. Enclave studies allow one to specify mixing conditions.When mixing occurs relatively early with respect to granite crystallization, only earlier-formed granitic minerals (zircon, apatite ...) may be incorporated in the basic magma. When mixing occurs later all granitic minerals may be involved.When all the dark microgranular enclaves of a granite are similar, the intervention of a single mixing event can be assumed. On the contrary, when several chemically and mineralogically contrasted varieties occur, several mixing events can be assumed. The order of these events may be deduced from the nature of the granitic minerals present in the enclaves (early of late minerals) or sometimes by observation of double enclaves.Enclave shapes bring information on the distance between the place of mixing and the place of observation. Enclaves with irregular or crenulated margins, or with chilled margins as well, always occur near mixing places. The enclave transport in the magma makes these characteristics disappear.In spite of hybridization processes, the primitive composition of the basic component of the mixture may sometimes be recognized. A systematic identification of this component in the different granites from the same orogen should allow to understand the space and time evolution of basic magmatism in an orogen.Magma mixing is a cause of heterogeneity in granites, but it is also an important factor of homogenization as the thermal contribution of the basic magma produces new mingling of the components and sometimes new intrusive processes.

---

Zusammenfassung Einschlüsse in Graniten geben wichtige Auskünfte über die Herkunft und Entwicklung der granitischen Magmen.Das Vorkommen restitischer glimmerreicher Einschlüsse (»enclaves surmicacées«) beweist die Teilnahme kontinentaler Kruste an der Magmenbildung. Die kleinkörnigen dunklen Einschlüsse dagegen sind Hinweise auf die Beteiligung von Mantelmaterial. Das Auftreten beider Arten von Einschlüssen ist ein starkes Argument für das Zusammenwirken beider Prozesse.Die kleinkörnigen dunklen Einschlüsse und der sie einschlie\ende Granit sind hybride Gesteine, sie entstehen durch unvollständige Mischung eines basischen und eines sauren Magmas. Durch genauere Untersuchung der Einschlüsse lassen sich die Vorgänge der Vermischung noch weiter erschlie\en.Wenn die Vermischung in einem Zustand eintritt, in welchem die Magmen noch wenig kristallisiert sind, dann werden nur die frühesten Mineralbildungen des Granits (Zirkon, Apatit) in die Einschlüsse aufgenommen. In späteren Stadien können alle jeweils schon kristallisierten Mineralphasen ausgetauscht werden. Tritt nur ein Typ basischer Einschlüsse in einem Granit auf, so ist dies ein Argument für nur einen Akt der Magmenmischung. Kommen dagegen mehrere Arten basischer Einschlüsse in einem Granit vor, die chemisch und mineralogisch deutlich verschieden sind, dann kann mit mehreren Vermischungsphasen gerechnet werden. Ihre Abfolge kann aus der Natur der ausgetauschten Minerale (Frühbis Spätausscheidungen) oder gegebenenfalls aus »doppelten« Einschlüssen abgeleitet werden.Die Gestalt der basischen Einschlüsse hängt von dem Abstand zwischen dem Ort der ersten Magmenvermischung und dem jetzt erreichten Ort im Granitkörper ab. Einschlüsse mit unregelmä\igen oder gekräuselten (crenulated) Rändern befinden sich stets noch in der Nähe des Ortes der Vermischung. Einschlüsse mit glatten Umrissen und ovoidaler Gestalt haben einen mehr oder weniger weiten Transport hinter sich.Gelegentlich kann noch die ursprüngliche Zusammensetzung des basischen Magmas ermittelt werden. Die Kenntnis dieser Magmenanteile in allen Graniten eines Orogens könnte die Entwicklung der basischen Magmen während der Orogenese erkennen lassen.Die Vermischung mit basischem Magma ist zwar eine Ursache der Heterogenität von Granitkörpern, auf der anderen Seite bewirkt aber die Wärmezufuhr durch den basischen Anteil und der damit verbundene »Rühreffekt« eine Homogenisierung und Belebung des Intrusionsvorganges.

---

Résumé Les enclaves donnent d'importantes informations sur l'origine et l'évolution des magmas granitiques.La présence dans un granite d'enclaves surmicacées (restites) est une preuve de participation de la croûte continentale à la genèse de son magma. Celle d'enclaves microgrenues sombres est une preuve de contribution mantellique. La présence des deux types d'enclaves dans le mÊme granite est un fort argument pour une origine double.Les enclaves microgrenues sombres et le granite qui les entoure sont des roches hybrides résultant du mélange incomplet de magmas plus acides et plus basiques. L'étude des enclaves permet de préciser les conditions de mélange.Quand le mélange intervient à un stade où les magmas sont encore peu cristallisés, seuls les minéraux précoces (zircon, apatite) du granite sont incorporés dans les enclaves. Quand il se produit plus tardivement, toutes les phases minérales peuvent Être échangées.La présence d'un seul type d'enclaves basiques dans un granite est un argument pour l'intervention d'un seul stade de mélange. Au contraire, l'existence de plusieurs familles d'enclaves basiques, chimiquement et minéralogiquement bien différentes, témoigne de l'existence de plusieurs stades. L'ordre de ces événements peut Être déduit de la nature des minéraux échangés (précoces ou tardifs) ou parfois grâce à l'existence d'enclaves doubles.La forme des enclaves basiques renseigne sur la distance entre lieu de mélange et lieu d'observation. Les enclaves à bords irréguliers ou crénelés, ou avec bordures figées sont toujours proches du lieu de mélange. Les enclaves ovoÏdes, aux formes régulières, ont subi un transport plus ou moins long.La composition originale des magmas basiques mélangés peut parfois Être identifiée. Cette identification dans tous les granites d'un mÊme orogène permettrait de connaÎtre la distribution et l'évolution du magmatisme basique au cours de l'orogenèse.Les mélanges de magmas sont assurément une cause d'hétérogénéité des granites, mais ils sont aussi un important facteur d'homogénéisation car l'apport thermique des magmas basiques favorise le brassage magmatique et la reprise du mécanisme intrusif.

---

. ( enclaves surmicacées) , . . , . . , , , ( , ). . , . , , , , . (, ), . pc- . , . , . . . ; , .

     

La végétation macrophytique d'un lac hautement eutrophe du Plateau suisse: Le Baldegger See

The study of the qualitative and quantitative distribution of the Baldeggersee macrophytic vegetation is undertaken in 1980 with the help of colour aerial photographies and verification on the field. The comparison with old data allows then a short survey of the macrophytic evolution in relation to the eutrophication.      

Microcrystalline textures resulting from rapid crystallization in a pseudotachylite melt in a meta-anorthosite

The very fine-grained (1 m) polygonal microcrystalline texture occurring in the groundmass of pseudotachylite veins in the Harris meta-anorthosite and its variation with position are described and an origin by crystallization from a melt at very large undercooling suggested. The intrusive nature of the veins is shown by their geometry and internal structures. Clasts, which are almost always only plagioclase, are generally concentrated towards the centres of veins. Flow of tens of millimetres can account for this concentration in millimetre-thick veins as a result of the Bagnold effect. The veins are generally thin (5 m to 5 mm or more), are frequently zoned and always contain transparent granules of high relief (probably Al-rich pyroxene) and opaque granules of magnetite up to a few micrometres in size. The granules are either uniformly distributed in microcrystalline textures or concentrated locally giving cellular textures. In some veins, spherulitic or bow-tie textures occur. The coarsest textures are found in the centres of the thickest veins. The groundmass of the pseudotachylite is never completely isotropic but consists of a mosaic of transparent plagioclase crystals decreasing in size from the centres of thick veins to less than 1 m in thin veins or in the margins of thicker veins. This fine microcrystalline texture was studied by both scanning and transmission electron microscopy and consists of polyhedral crystals of regular size in the range 0.2–1.5 m, which show little sign of deformation. The local composition of the pseudotachylites varies little from the average compositions of the rocks in optically homogeneous veins, the variation being within the compositional space defined by the minerals of the host rock. This shows that homogenization of the pseudotachylite has occurred. In cellular veinsdifferentiation has occurred as the compositions of the cell centres lie outside those of the minerals of the host rock. This was produced by segregation of the granules, pyroxene being absent from the host rock. The plagioclase in the pseudotachylite is more disordered than that in the host rock. All the microtextures described are absent from the associated cataclasites and cannot be due to recrystallization of a fine-grained and intensely strained rock powder. The physical state on and after intrusion was that of a melt and injection was followed by crystallization. The melt was produced by more or less total fusion of the host rock minerals at shallow depth by heat produced during local faulting and perhaps during crack propagation. The fine microcrystalline texture very closely resembles that produced during hypercooling of molten metals and alloys. It thus possibly formed not by devitrification but from a melt at much greater degrees of undercooling than the spherulitic and bow-tie textures.     

An Inductively Coupled Plasma Spectrometric Method for the Determination of Six Indicator Elements for Mineralization Processes in Granites

A simultaneous multi-element method is described for the determination of six elements of importance in geochemical investigations of mineralization processes in granites. After a mixed acid sample digestion, the elements Be, Li, Nb, W, Mo and Sn are determined by inductively coupled plasma spectrometry. Possible sources of bias due to incomplete sample dissolution and from spectral interferences were investigated. Analysis of 16 geological reference standard samples and a number of enriched samples demonstrates the accuracy and precision of the method for the study of mineralized granites and for several other geochemical sample types as well.     

Tectonics of the East Pacific Rise near 12°50′N: a submersible study

Using the diving saucer “Cyana”, a portion of the East Pacific Rise between 12°38′ and 12°54′N has been studied and mapped. Four main zones are defined: (1) an active volcanic zone located in the central graben; the graben displays a variable relief along strike from less than 10 m up to 55 m and has an en-échelon pattern in plan view; (2) an active tectonic zone comprising the central graben and extending up to about 2 km from either side of the rise axis; (3) an inactive tectonic zone starting 2 km away from the axis and formed by discontinuous horst and graben type of structures; and (4) off-axis volcanoes (seamounts) located less than 20 km on either side of the rise axis. At 12°50′N, the active tectonic zone is extremely narrow and the mode of deformation is through fissuring in strong contrast with that observed in slow or intermediate rate ridges. The presence or absence of a magma reservoir at shallow depth explains the difference in the width of the active tectonic zone. The state of isostatic equilibrium or lack of equilibrium explains the different modes of deformation.     

Multistage alkaline permanganate degradation of a type II kerogen

The products of a 27-step alkaline permanganate degradation of a type II kerogen from a sample of Toarcian shale, Paris Basin, have been studied. The high yield of oxidation products consisted of 1.86% neutrals and bases, 24.48% ether-soluble acids, and 45.95% precipitated, ether-insoluble acids, based on weight of original kerogen. The ether-soluble acids and the soluble products of further permanganate degradation of precipitated acids were found to consist mostly of saturated unbranched C₆–C₂₂ α,ω-dicarboxylic and C₉–C₂₅ monocarboxylic acids. Significant amounts of aromatic monocarboxylic, dicarboxylic and tricarboxylic acids were also found. Alkane tri- and tetracarboxylic acids were obtained in small concentration.     

The mechanisms of overlapping of the Bresse graben by the Jura formations in the Vignoble area (France)

This paper aims to underline how powerful and important the balanced cross-section constructions are for understanding structural mechanisms, especially when several interpretations are possible. In the Vignoble area, between Lons le Saulnier and Arbois — French foothills of Jura, the frontal overlapping is proved by several drillholes. The classical interpretation is a thrusting through listric reverse faults and continuation of the décollement level underlying the Jura at the cover—basement interface. The balanced cross-section construction demonstrates that this assumption is not valid due to the necessary thinning and stretching of the overlapping formations in this region. This paper proposes a new solution where the overlapping, through listric normal faults, is like a huge gravity landslide, without any relation to the shortening and décollement of the internal Jura. The new interpretation is supported by the very consistent balanced crosssection of the displaced cover and by the extension structures observed in the field.     

The geologic history of the Caribbean-Cocos plate boundary with special reference to the Nicoya ophiolite complex (Costa Rica) and D.S.D.P. results (Legs 67 and 84 off Guatemala): A synthesis

The Pre-Upper Senonian basement of Costa Rica crops out in the Santa Elena and Nicoya peninsulas. From south to north and from base to top the basement includes: the Esperanza, Matapalo and Santa Elena units. The Esperanza unit is Albian-Santonian in age and consists mainly of pillow basalt and massive basalt flows. The Matapalo unit includes Callovian to Cenomanian radiolarite and includes massive basalt flows, basalt, and dolerite basement. The Santa Elena unit contains ultramafic and mafic rocks in which harzburgite is the major component. The most important tectonic features of the Nicoya Complex are the large Santa Elena and Matapalo nappes. Nappe emplacement was from north to south during upper Santonian time. The sedimentary cover of the Nicoya Complex comprises:

1. (1) the Campanian El Viejo Formation that consists of shallow-water sediments in the north (Santa Elena Peninsula) and the Campanian-Maastrichtian Sabana Grande Formation of deep-water origin in the South (Nicoya Peninsula);

2. (2) Paleocene strata indicating deposition in a deep-water environment comprises the Rivas, Las Palmas and Samara Formations;

3. (3) a post-upper Eocene (?) sequence that consists of the shallow-water Barra Honda and Montezuma Formations.

Two unconformities are significant geological features of the upper-Senonian to Tertiary history of Costa Rica. The lower one is at the base of the Sabana Grande Formation and marks a major change in the geologic conditions (basalt is scarce in the Campanian-Tertiary series); the upper unconformity at the base of the Barra Honda and Montezuma Formations is not as major as the lower one. During post-Campanian time, normal faulting occurred in two stages separated by a strong erosional phase.The geology of the landward slope of the adjacent Middle America Trench is outlined by interpreting multifold seismic reflection records off the west coast of Costa Rica and the DSDP Legs 67 and 84 transects off Guatemala. The western Caribbean plate boundary may have been under extensional stress for the last 75 m.y. The strong landward-dipping reflectors of the Middle America Trench landward slope off Guatemala could be equivalent to the on-land pre-Campanian overthrusts of Costa Rica. The available data are consistent with the Convergent Extensional margin concept.