On the Aegean island of Naxos, four major fluid flow events (lp, 2r, 3r and 4r) occurred during Miocene greenschist- to amphibolite-facies metamorphism. I have made a study of grain-scale fluid pathways and the extent of isotopic equilibration at a number of localities across the island. I have combined ion microprobe analyses of ¹⁸O/¹⁶O,¹³C/¹²C,<i> </i>Mn and Fe with detailed cathodoluminescence imaging to obtain direct evidence of grain-scale infiltration mechanisms, the timing of different fluid flow events and subsequent isotopic modification, fluid compositions, and the time scales of fluid infiltration. On Naxos, fluid flow was channelled and heterogeneous at all scales of observation. At the km-scale, the isotopic and petrological record of fluid flow is very variable. The metamorphic fluids (lp and 2r) were chemically and isotopically heterogeneous. Fluid pathways were probably controlled by deformation and lithological contrasts. A detailed study of a marble band was made at one locality. At the m-scale, the pattern of infiltration (resulting from buoyancy-driven flow) was controlled by the structure and permeability contrast of the lithological layering. An isotopic front formed at the bottom contact of the band as a result of advection of fluid along a network of cracks and grain boundaries, combined with volume diffusion into grains. The front represents the final product of at least two fluid flow events. Cm- to m-­scale δ¹⁸O data provide only limited information about fluid flow events: multiple events cannot be distinguished. Modelling of bulk data may give misleading results as some of the assumptions made in applying chromatographic theory to metamorphic fluid flow are not valid. In summary, metamorphic fluid flow on Naxos was spatially and chemically heterogeneous at all scales of observation. Permeability was dynamically created and maintained: fluid pathways were controlled by deformation and lithological contrasts. Grain-scale fluid networks are transient: time scales of fluid flow are extremely short compared with time scales of metamorphism.

The island of Réunion in the Indian Ocean consists of two volcanic massifs: the active volcano, Piton de la Fournaise; and Piton des Neiges, which is now extinct. During the shield-building stage (Phases I and II) of Piton des Neiges, olivine-phyric, transitional alkali basalts were erupted. The products of Phase I and II magmatism constitute 90% of the edifice by volume. The focus of magmatism migrated to Piton de la Fournaise c. 350,000 years ago. Simultaneously, Piton des Neiges entered Phase III when it started to erupt evolved magmas, producing the first Differentiated Series lavas. Some 220,000 years ago Piton des Neiges magmas became sufficiently evolved to erupt explosively; these eruptions mark the base of Phase IV, the subject of this study. The products of Phase IV consists of mugearite to quartz trachyte lavas , intrusives, and pyroclastic deposits; the most recent explosive eruption may have occurred as little as 12,000 years ago. There are two categories of pyroclastic formation: (i) heterogeneous deposits (Salazie Formation, St Louis Formation, Dalle Soudée Formation), which show evidence for mixing between mugearite or benmoreite and quartz trachyte magmas; and (ii) homogeneous deposits (St Gilles Formation, St Pierre Formation), containing aphyric, geochemically homogeneous, benmoreite to quartz-trachyte pumices, with little or no evidence for magma mixing. Compositions of juvenile material from the heterogeneous deposits lie on a coherent whole-rock fractionation line. The Salazie Formation is zoned, with a mixed benmmoreite-trachyte lower flow-unit and a mugearite upper flow-unit. Evidence for magma mixing in the heterogeneous deposits includes: co-existence of fiamme/pumices with different compositions and phenocryst populations; reverse zoning in phenocrysts; and banded pumice. The most evolved material in any deposits is enriched by a factor of 2 to 5 in incompatible elements (REE, LILE, REE, K and Na), and is similarly depleted in compatible elements (e.g. Mg, Ca, transition metals). Post-magmatic alteration has affected pumices from the homogeneous deposits, and accounts for much of the observed within-deposit variation. The homogeneous deposits have lower Ca, P, Mn, Fe, Na, Ba, Sc, and Zn; and higher Al, Sr, Rb, Pb, Cu, and Zr for the same SiO<SUB>2</SUB> than those in the heterogeneous deposits.

No description available.

No description available.

The Complex is emplaced partly among Cambrian limestones, and partly among thrust Lewisian and Cambrian rocks of the Benmore Nappe. It has the form of a laccolith but consists of two distinct igneous phases of which the later occupies the higher part of the mass. The earlier rocks have been emplaced mainly before, am the later rocks after the principal movements on the Benmore Thrust. The earlier rocks are of four main types : spotted rocks and rocks derived from them, borolanites, potash-feldspar-mica-ore rocks and potash-feldspar-muscovite rocks; rocks of Ledmore type, shonkinites, ma.lignites and ledmorites; ultramafio rocks, cromaltite, hornblendite ani pyroxenite; and finally miscellaneous rocks including nepheline syenites, fenitised quartzites and a. phlogopite-rich rock. The spotted rocks of the 'Eastern Tract' are derived from a feldspar porphyry by metamorphic and metasomatio processes involving mineralogioa.l, textural and chemical changes. The spotted borolanites are not pseudo-leucite rocks. The rocks of Ledmore type are derived from a shonkinite-pulaskite magma, but some members have been changed, by nephelinisation and melani tisation, to ledmorites am borolanites of Ledmore type. It is suggested that the ultramific rooks were formed by metamorphic segregation representing chemical culminations of iron, lime, magnesia and titania. In contrast to the earlier rocks, the later rocks, quartz syenites perthosites am nordmarkites, differentiated from a magua by processes of progressive crystallisation.The earlier and later rocks are shown to belong to ohemically distinct groups, and a similar grouping can be recognised among the rocks of the nearby Loch Ailsh Complex, as well as among the hypabyssal rocks that are distributed more widely in the Assynt district. Finally, it is postulated that the earlier rocks were formed by metasanatism, and by anatexis and mobilisation, these processes taking place in advance of am through the agency of a syenitic or granitic magma which was eventually' emplaced within the metasomatised assemblage.

No description available.

No description available.

The highest grade pelitic and semi-pelitic rocks of the Ballachulish contact aureole show structural and textural features indicative of partial melting. Three locations were selected for detailed petrographic study which show the development of boudinage and breccia structures, discrete leucosomes, and small-scale network features. One more extensively melted area (the Chaotic Zone) showed boudinage and breccia structures developed on a much larger scale. Rocks from the Chaotic Zone were divided into several groups: hornfels fragments, laminated metasediment, granulose metasediment, granitic material, and penetrative vein systems. The development of the above features is related to melting processes. Discrete leucosomes and small-scale network features in the less extensively melted rocks represent accumulations of melt which could not be contained along the grain boundaries in the host rock. In the Chaotic Zone large bodies of rock have melted to such a degree that the critical melt fraction was exceeded, and the rock behaved as a ductile body. The hydraulic fracturing responsible for formation of boudinage and breccia structures operates on a larger scale in this area due to the larger degrees of melting involved. An important control on the distribution of melt, and melt-related processes, is the original compositional layering of the rocks concerned. Throughout the melting history, movements of the melt phase are thought to have been minor and the orientation of original layering is often preserved, even after large degrees of melting. Quantitative study of the angular relationships between mineral grains showed that the approach to textural equilibrium in these rocks is poor. In rocks which contain abundant evidence of melting, non-equilibrium dihedral angles (in the range 50-60<SUP>o</SUP>) are commonly preserved which coincide with experimentally-derived wetting angles for granitic-type silicate liquids. In rocks where melting is thought to have been at a minimum, the approach to textural equilibrium is better. Dihedral angles measured are in the range 90<SUP>o</SUP> to 110<SUP>o</SUP>, which corresponds to expected values for equilibrium solid/solid angles between the phases present. A δ18O study of leucocratic segregations shows that in general, they have the same δ18O composition as the immediately adjacent matrix material, consistent with a local origin. Extensively melted rocks are coincident with large-scale oxygen isotope homogenisation. This indicates enhanced communication between rock layers and thus supports petrographic evidence suggesting the presence of an interconnected melt phase over large distance. Consideration of isotopic equilibration between the mixed grains in these rocks shows that late-stage fluid activity was at a minimum. A comparison of traditional methods of testing for equilibrium, and application of Giletti's model for slow cooling in a closed system showed that Giletti's model provided a closer approach to the observed δ18O compositions.

No description available.

No description available.