No description available.

Deformation, metamorphism and fluid infiltration are important processes in crustal evolution. This thesis is concerned with how these processes interact to remodel the crust after orogenesis. Localisation of strain, fluid<i> </i>infiltration and metamorphic reactions all play a part in controlling the rheological properties of rocks deep in the crust and these factors will therefore influence the process of exhumation, and control the structural and metamorphic development of an exhuming terrain<i>. </i>It has been shown that these processes do not act independently. Complex patterns of inter-linked deformation and hydration of metamorphic assemblages are often superimposed, such that orogenic fabrics and mineral assemblages are partially overprinted by exhumation fabrics and assemblages. Thus, a chronology of deformation, metamorphism and infiltration can be determined and by identifying the depths and temperatures at which events occurred, the exhumation process can be unravelled. The island of Syros in the Attic Cycladic blueschist belt of Greece was chosen for study as it is characterised by a high pressure blueschist facies assemblage (T = 450 °C and P >14 Kbars) which has been variably overprinted within the greenschist facies stability field. The partial overprinting has resulted in the preservation of high pressure fabrics. This preservation combined with local down-pressure superimposition of deformation fabrics and recrystallisation allows a chronology of events to be determined making Syros an ideal locality for the study of structural and metamorphic evolution during exhumation. Syros consists of interleaved schist and marble units, with local metabasite horizons. The units are layered and generally dip shallowly to the NE. The partial overprinting of metamorphic assemblages has been used to link down-pressure fabric development, fluid infiltration and metamorphism, whilst the interleaving of lithologies has allowed an assessment to be made of the lithological controls on fluid channelling.

No description available.

No description available.

No description available.

The Belingwe greenstone belt in southern Zimbabwe contains one of the most well preserved Archean volcanic successions in the world. The komatiites in this succession have been studied in great detail, but the associated basalts have received much less attention. A detailed study of these basalts in the Zeederbergs Formation has revealed the existence of a previously unrecognised lava type which has important implications for the petrogenesis of the suite. The Zeederbergs Formation and the underlying Reliance Formation form the 2.7 Ga Ngezi Group volcanics, which are underlain by thin, generally shallow water sediments of the Manjeri Formation. These in turn rest unconformably on 3.6 Ga and 2.9 Ga granitoid gneisses in the east and on older (2.9 Ga) greenstones in the west. The nature of the basal contact of the Ngezi Group volcanics on the sediments of the Manjeri Formation has been the subject of recent controversy, and is interpreted here as comformable. The type section of the Zeederbergs Formation in the Ngezi River is logged and described in detail for the first time. Combined with correlation of geochemical marker horizons in other sections this has led to a re-evaluation of the vertical thicknesses of the Zeederbergs Formation - estimated here to be approximately 3km. Study of the geochemical stratigraphy has revealed a horizon of basalts with low Zr/Nb and high CaO/Al<SUB>2</SUB>O<SUB>3</SUB> compared to the rest of the formation. The basalts in this horizon are called Type II (as opposed to the Type I basalts which make up the majority of the formation). Examination of the petrography of the Zeederbergs Formation basalts has revealed that no subdivision into different rock types on petrographic grounds is practical. The lavas are generally fine grained, sparsely phyric and altered to hydrated low greenschist assemblages. The 'spheroids' in the lavas are considered in some detail, and are thought to represent products of spherulitic devitrification.

No description available.

No description available.

High grade blueschist metamorphism on the Aegean island of Syros involves the widespread growth of the calc-silicate minerals lowsonite, zoisite and epidote/clinozoisite in a wide range lithologies. In calcareous metasediments the only realistic lawsonite-generating reaction appeared to be one involving calcite-breakdown and thus for significant reaction CO<SUB>2</SUB> would have to have been flushed from the rock by the infiltration of a water-rich fluid. The distribution of lawsonite was mapped in the metasediments. Structural mapping was used to elucidate the strain history of the syn-blueschist deformation. Detailed petrographic and microfabric analysis of relevant samples was undertaken to document their petrologic and microstructural development and to identify lawsonite forming reactions. Changes in bulk rock composition accompanying the growth of lawsonite were assessed. The results show that in the greyschist units of northern Syros lawsonite growth is controlled by a syn-deformational, fracture-controlled fluid infiltration event traces of which are visible as quartz veins. Ca-bearing, water-rich fluid pervasively infiltrated the schists adjacent to the fractures resulting in the growth of large amounts of lawsonite by metasomatic reactions involved the breakdown of mica. The scale of fluid transport and therefore the extent of metasomatic alteration is controlled by the deformation and compositional variation. These observations confirm the suggestion in the previous studies that lawsonite growth is at least partially controlled by the infiltration of an externally-derived fluid. The preservation of small scale variation in fluid composition within significant parts of the metasediments is explained by the channelized nature of the event.

There are a number of geochemically, geographically and, in most cases, stratigraphically distinct magma types around the igneous centres of Tayvallich and Loch Awe which represent separate phases of magmatism. These have been characterised by trace element geochemistry. Relationships have been established between some of the magmas, however the full range of rocks in the area (from alkaline to tholeiitic) cannot be accounted for by melting or fractional crystallisation processes, implying more than one melt source for the igneous activity. Extension across the area produced a number of parallel basins, but movement along the Cruachan Lineament allowed rotational opening of the Loch Awe basin which became a major volcanic centre. During this period, the centre of igneous activity moved in successive stages from the Tayvallich Peninsula to the Loch Awe area, producing a series of magmas, ranging from alkaline to transitional and tholeiitic. The data confirm a general increase in melt fraction and a source which was being gradually modified. A similar series can be seen on Islay and Jura. There is a strong correlation between different magma types and individual basins and sub-basins, from which we conclude that the magmatism was controlled by basin formation and evolution. The igneous activity was therefore passive and was driven by the extensional tectonics prevailing at that time. The relationship between the Green Beds and the Loch Avich Volcanics has been re-assessed. The igneous component in the Green Beds is derived from erosion of Loch Awe lavas, and the younger Loch Avich Lavas represent a final, minor episode of igneous activity. Their anomalous geochemistry is due to contamination of the small volume melt prior to eruption.