Thèses sur le sujet « Contact aureole »

16 samples of increasing metamorphic grade from the Ballachulish Igneous Complex and Aureole, located in the west of Scotland, were studied in order to analyze the sulfide mineralogy and to what extent they were affected by contact metamorphism. The samples were collected from two lithologies, the Creran Succession and the Ballachulish Slate lithology, as well as from the igneous complex. The sulfides of main interest in the samples are pyrite and pyrrhotite. At the onset of contact metamorphism, pyrite disappears while pyrrhotite gets more abundant as metamorphic grade increases. Pyrrhotite also undergoes multiple changes such as 1) elongation and thinning of the grains, 2) development of 120° grain-boundaries, 3) development of pyrite-zones within the pyrrhotite and 4) the decomposition of pyrrhotite and alignment of pyrite along its grain-boundaries at high temperature. The elongation of the grains occurs in both the Creran Succession and the Ballachulish Slate. The rest of the textures, however, can only be found in the Creran Succession. The two lithologies differ by the high graphite content in the Ballachulish Slate. The elongated grains as well as the pyrite inclusions in the pyrrhotite both are strong evidence of recrystallization. The absence of pyrite in the Ballachulish Slate was most probably caused by the buffering properties of the graphite-rich fluid in these rocks, causing more reducing conditions. There is evidence against a heavy, pervasive fluid flow through the aureole. However, the inner contact zone seems to have been affected by a more pronounced fluid flow. This could have been caused by the metamorphic fluid working in conjunction with fluids released from the intrusion. Regarding the mobility of S in the aureole, no strong evidence could be found, other than the decomposition of pyrrhotite grain-boundaries in the high-grade metamorphic samples.

The 2.06 to 2.054 Ga Bushveld Igneous Complex intruded into the sedimentary rocks of the Transvaal Supergroup and generated an extensive contact metamorphic aureole mainly developed in the upper Pretoria group. The studied samples represent the Silverton Daspoort and Timeball Hill formations and are divisible into garnet bearing hornfels (DY918, DY954 and DY956) and garnet-free staurolite-bearing metapelites (DY916, DY982 and DY987). The garnet-bearing hornfelses marks the garnet zone within the aureole and the garnet formation is controlled by different reactions forming from 490 to 630 0C. On the other hand, the garnet free staurolite-bearing Fe-Al rich metapelites define the staurolite zone restricted to the Timeball Hill formation. The recorded P-T conditions in G0 and G1 garnets of the DY954 hornfels imply that the two garnets formed under different conditions indicating two stages of metamorphism. However, the Lu-Hf isotope systematics of these garnets records a 2061 Ma age for all garnet porphyroblasts in both the DY918 and DY954 hornfelses, which support co-genetic garnet growth regardless of their stratigraphic positions. Therefore, the 2061 Ma garnet age denote the emplacement age of the Lower Zone and Critical Zone magmas which was synchronous with the extrusion of the Rooiberg Group volcanics. The fact that all analysed garnets do not record the 2059 – 2054 intrusion of the Main Zone and Upper Zone magmas probably means that the crystallisation temperatures of the later magma pulse was not significant enough to shift the Lu-Hf isotopic signatures. Euhedral staurolites are widespread within the Fe-Al rich metapelites with grain sizes of up 4mm; texturally the majority of them have been altered or overgrown by biotite and chloritoid. The alteration or of these staurolite porphyroblasts is due to isobaric cooling during uplift, and the St-Bt assemblage represent the peak equilibrium conditions and marks the upper stability limit of the Chl-Ctd assemblage.
Dissertation (MSc)--University of Pretoria, 2013.
gm2014
Geology
unrestricted

The entire thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file; a non-technical public abstract appears in the public.pdf file. Title from PDF of title page (University of Missouri--Columbia, viewed on January 15, 2010). Thesis advisor: Dr. Peter Nabelek. Includes bibliographical references.

The effects of fluid evolution and infiltration in the Alta stock contact aureole, Utah, were studied by evaluating chemical additions and depletions in igneous, replacement skarn, and carbonate whole-rock samples. Of the major and trace elements studied, boron proved to be the most interesting tracer of fluid, due in part to extensive borate mineralization (ludwigite and kotoite) in carbonates near the stock contact, and in part to the physical-chemical properties of boron which make it a useful element for isotopic studies and for spatial distribution analysis via alpha track mapping. Boron is enriched throughout the Alta aureole and is hosted by a variety of metamorphic and metasomatic silicate minerals (forsterite and humite group are the most significant). Aureole whole-rock B concentrations increase toward the contact, indicating that B originated in the stock and migrated down-temperature in exsolved magmatic fluids. Boron stable isotope measurements of carbonates, skarns and mineral separates range from $-$7.2 to +1.6$\perthous\ \delta\sp{11}$B. A value of ${-}6.0\perthous$ for a B-enriched igneous sill falls within this range and also indicates that the stock is the source of aureole B-enrichments.

L’Isola d’Elba (Mar Tirreno Settentrionale) è collocata in posizione intermedia tra la Toscana (Appennino Settentrionale) e la Corsica (Corsica Alpina). Il complesso edificio tettonico dell’Isola d’Elba, è ben noto per gli evidenti rapporti tra la messa in posto di corpi magmatici acidi mio-pliocenici e le ultime fasi tettoniche. Tali plutoni granitoidi, oltre a provocare il termo metamorfismo nelle unità incassanti (oceaniche liguri e piemontesi e continentali toscane), hanno alterato con strutture sia a basso che ad alto angolo l’originale impilamento delle falde tettoniche avvenuto a partire dal Cretaceo Superiore fino al Neogene e provocato deformazioni sia in regime duttile che in quello fragile. In particolare plutone del Monte Capanne, che domina la parte occidentale dell’isola, mostra alla propria base gli effetti della propria risalita forzata in quello che rimane dell’originale copertura incassante. Il plutone ha le caratteristiche di un laccolite complesso, formato del diverse iniezioni di magma che hanno fatto rigonfiare il corpo intrusivo fino a far scollare e scaricare lateralmente le unità di copertura. Le unità dell’aureola sono costituite da successioni ofiolitiche (serpentiniti, gabbri e la copertura sedimentaria e vulcanica attribuita al periodo compreso tra il Giurassico superiore ed il Cretaceo inferiore) in gran parte ricristallizzate, all’interno delle quali si sono intrusi prima i dicchi e i laccoliti (come le Apliti di Capo Bianco, il Porfido di Portoferraio e il Porfido di San Martino) e di seguito il corpo magmatico del Monte Capanne. Le rocce di tale aureola mostrano, nelle diverse zone dell’anello, gradi metamorfici differenti che vanno dall’alto al basso grado, in funzione della distanza dal plutone e della circolazione di fluidi “magmatici”. Successivamente tali rocce sono state dislocate a causa di movimenti tettonici che hanno portato in contatto direttamente sul plutone rocce di basso grado. Inoltre i rapporti primari tra i corpi intrusivi e le coperture incassanti sono spesso evidenti, come anche i segni della tettonica polifasica sia duttile che fragile legata alle diverse fasi evolutive del sistema in risalita. Nell’anello termometamorfico del Capanne infatti sono ben note le fasce di taglio fragili, con la sviluppo di set di fratture ben documentati. Tuttavia talvolta sono evidenti localizzati aumenti del grado metamorfico legati ad incrementi relativi di permeabilità delle rocce incassanti (dovute ad esempio a fenomeni di idrofratturazione) che hanno favorito la risalita di fluidi metasomatizzanti e la formazione di fasce di taglio duttili milonitiche soprattutto a carico delle litologie calcaree. Invece le litologie pelitiche hanno reagito in maniera tendenzialmente fragile, come avviene generalmente in un contesto di metamorfismo di contatto. Il lavoro presenta i dati relativi alla zona di taglio duttile presente nelle zone di Cavoli-Colle Palombaia e Spartaia, con lo sviluppo di miloniti calcaree di spessore da decimetrico a plurimetrico (in quelle che prima delle deformazione e del metasomatismo erano i Calcari a Calpionelle) con elementi, da centimetrici a metrici, sia metapelitici che meta-basaltici inglobati all’interno della foliazione milonitica. Lo scopo della tesi era: Ricostruzione di dettaglio della litostratigrafia e della zoneografia metamorfica Correlazione delle formazioni dell’aureola Caratterizzazione dell’assetto geologico-strutturale per risalire ad un modello evolutivo dell’Isola d’Elba occidentale Carta geologica di dettaglio (1:5000) The Elba Island is located in the Northern Tyrrhenian Sea at midway between Tuscany (Northern Apennines Chain) and Corsica (Alpine Corsica structural pile). The complex Elba Island stack of nappes, which is considered the innermost outcrop of the Northern Apennines Chain, is also well known for its Fe-ore bodies and the relationships between the emplacement of the Mio-Pliocene magmatic bodies and tectonics. These granitoid plutons, in addition to causing the thermal metamorphism in the host units (oceanic Ligurian and Piedmontese and continental Tuscan), have modified the original tectonic stacking of the nappes. Overlapping occurred from the Upper Cretaceous to Neogene, and in combination with the emplacement of granitoids causing deformation both fragile and ductile regime. (e.g. detachements). The plutonic complex of Monte Capanne (6.9 Ma), which dominates the western part of the island, produce the effects of his forced ascent in the host units, in large part termo-metamorphosized. In particular, the aureole units consist of ophiolitic sequences of the Upper Jurassic-Lower Cretaceous, inside which are intruders before acidic dikes and laccoliths (“Christmas-tree” laccolithic complex, Capo Bianco Aplite (8-8.5 Ma), Portoferraio Porphyry (8 Ma) and San Martino Porphyry (7.4-7.2 Ma)) and then the magmatic body of Monte Capanne. The rocks of this aureole show, in different parts of the ring, different metamorphic grades ranging from high to low grade, as a function of distance from the pluton and circulation of fluids. Sometimes they are evident localized increases of metamorphic grade likely related to amplifications in relative permeability of the host rocks (e.g. due to phenomena of hydro-fracturation) that have favored the ascent of metasomatic fluids. Furthermore in the area the Promontory of Fetovaia, in present the not metamorphic Eocene turbidite unit of Punta le Tombe which lies tectonically above the ophiolitic units. Subsequently, these rocks have been displaced as a result of tectonic movements that led in direct contact the pluton with metamorphic rocks of medium and low-grade (e.g. S.Piero). In addition, the primary relationships between intrusive bodies and their covers are often evident, as well as the signs of polyphase tectonic activity, both ductile and brittle, due to the different evolutive stages of emplacement of the pluton. Ductile deformation occurring especially on carbonate lithologies, while the pelitic lithologies have reacted basically so as fragile, more coherently with contact metamorphism. Important set of fractures with fragile shear zone are well known in the termometamorphic ring. The paper presents for the first time data on the ductile shear zone present in the outcrops of ophiolitic units of Cavoli-Colle Palombaia and Spartaia, with the development of mylonites in thickness from decimeter to plurimetric (probably derived from the Calpionelle Limestones), consisting mainly of crystalline carbonate foliated body in which are present elements, from centimetric to metric, made of meta-pelitic and meta-basaltic embedded within the mylonitic isorientation. The analysis was performed on the kinematic indicators and fold structures; the main direction of the milonitic foliation and the senses of shear observed by kinematic indicators (sigma and delta type porphyroclasts, asymmetric folds and domino structures) show a trend of transport generally toward the outside of the intrusive massif of Monte Capanne. The direction of movement indicated by kinematic indicators in the analyzed area, since the location of the outcrops with respect to the main body of the pluton of Monte Capanne is consistent with a radial pattern of the stress field. This radial trend is typical feature of the discharge phenomena related to a sub-circular up-rising central body. In several outcrops is evident the presence of different deformation regimes, both simple shear and pure shear. Locally the occurrence of the combination of the two end-member (sub-simple shear) testify the extreme variability and complexity of the processes at the time of the development of deformation. Furthermore, the data resulting from the microscopic analysis indicate that the foliation and the microstructures are associated with metamorphic events of medium to high grade (HT-LP), consistent with the dynamo-thermal effect due to the emplacement of the Monte Capanne pluton.

Thesis (M.S.)--University of Wisconsin--Madison, 1998.
Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 29-33).

Krkonose-Jizera plutonic complex is one of the largest composite plutonic bodies in the Czech massif. The emplacement of this pluton relates to the origin of the structural and thermal aureole. The processes of contact metamorphism are most evident in the northern and southern part. The topic of this work is to find influence of the contact metamorphism in the host rocks. Partial information about contact metamorphism in this area could be found in several papers, but there aren't work which discuss this topic in detail. Goal of this work is to make complex interpretation of contact metamorphic processes connected with intrusion of the Krkonose-Jizera plutonic complex and use this interpretation for discussion about model of magma intrusion. In rocks of contact aureole of Krkonose-Jizera plutonic complex (KJPC) were identified relicts of Variscan regional metamorphism, connected to creation of regional metamorphic fabric S1 and S2. Original mineral associations and deformation fabrics were in near-contact zone of KJPC 1 km in average in width heterogeneously overprinted by effects of the contact metamorphism, which intensity increases towards the intrusive contact. In the mineral associations of contact-metamorphic rocks appears characteristic minerals such as cordierite and andalusite, rarely...