This article explores a CD-based courseware package for the teaching and consolidating of geological field skills used for the interpretation of folding sequences in deformed rocks, focussing on examples from Anglesey in North Wales. The article briefly considers the advantages and disadvantages of virtual fieldwork and then discusses the rationale, structure, development and production of the CD courseware package.
Phase equilibria calculated with thermocalc for simplified mantle ultrabasic and basic bulk compositions in Na2O–CaO–FeO–MgO–Al2O3–SiO2 (NCFMAS) at depths down to the Transition Zone and uppermost lower mantle are presented here for the first time. This capitalizes on a new equation of state for solid phases and ties in deep mantle phases (wadsleyite, ringwoodite, akimotoite, garnet, MgSi-perovskite, CaSi-perovskite, cf phase, ferropericlase, nal, corundum and stishovite) to the thermodynamic dataset used at lower pressures in thermocalc. Activity–composition relationships for these complex phases are formulated that combine the end-member properties, allowing such phase equilibria to be calculated. The results are presented in the form of P–T and P–X pseudosections showing the dependence of mineral assemblage on conditions of formation and composition in basic and ultrabasic compositions, and also in terms of modal proportion and mineral composition diagrams. Such thermodynamic calculations will allow quantitative assessment of the mineral assemblages in the mantle, as well as in subducting lithospheric slabs. The results will be a key input into geophysical modelling, allowing questions such as the controls on the penetration of subducted slabs into the lower mantle to be addressed. The calculated mineral assemblages can also help in the interpretation of deep mantle phase inclusions trapped within diamonds. Calculations are made more realistic than in pre-existing models by inclusion in this study of a chemically complex nal phase as well as inclusion of a wide range of substitutions in the cf phase and in both the Mg- and Ca-perovskite phases.
Abstract Metasediments of the Tayvallich Subgroup of the Dalradian at Kinnairds Head, Fraserburgh are metamorphosed to sillimanite + K-feldspar grade and form part of the classic high- T low- P Buchan metamorphic terrain. Pelitic samples constrain peak-metamorphic conditions to 615±13°C and 2.2±0.2 kbar. At or close to the metamorphic peak, irregular garnetiferous aplites and autopegmatite bodies intruded the metasediments. Thin marble bands within the sequence are dominated by calcite with diopside, and equilibrated with relatively CO 2 -rich, internally buffered fluids. Where these are in close proximity to granitoid pegmatites, wollastonite dominates the matrix, and fractures and veins running through the rock contain concentrations of grossular and vesuvianite. With increasing distance from the pegmatite, vesuvianite and then grossular disappear, and wollastonite is only patchily developed. Such occurrences require a flushing of the marble by metasomatic (siliceous and aluminous) aqueous fluids which were derived from the de-watering of the adjacent pegmatite as it crystallized. The large quantities of dissolved silica led to pervasive wollastonite formation for several metres. The smaller quantities of Al reacted to form Ca-Al-silicates which were confined to the fractures.
This article explores a CD-based courseware package for the teaching and consolidating of geological field skills used for the interpretation of folding sequences in deformed rocks, focusing on examples from Anglesey in North Wales. The article briefly considers the advantages and disadvantages of virtual field work and then discusses the rationale, structure, development and production of the CD-ROM courseware package.
Synopsis Calc-silicate rocks from the Southern Highland Group of the Dalradian at Banff are metamorphosed carbonate concretions. Many original sedimentary features are preserved even though the calc-silicates now have amphibolite facies mineral assemblages. Those occurring in the andalusite and staurolite zones contain layered mineral assemblages that are characterized by the presence of calcite, clinopyroxene, hornblende, cummingtonite, and biotite from centre to margin. It is unlikely that the layering formed principally by synmetamorphic diffusional processes since diffusion modelling demonstrates that the relative values of phenomenological diffusion coefficients required to reproduce this set of layers are inconsistent with known values. The layer order, with pyroxene adjacent to carbonate and amphibole adjacent to pelite, is also inconsistent with their formation by infiltration of an H 2 O-rich fluid into a pure carbonate body. The layers can be best explained as resulting from metamorphism of compositionally zoned, diagenetic carbonate concretions. If the metamorphism was essentially isochemical, then composition profiles present in the calc-silicates may be compared with those in unmetamorphosed concretions and interpreted in terms of known diagenetic processes. A decrease in CaO from centre to margin results from the progressive elimination of the primary porosity during compaction of the sediment, a conclusion supported by the presence of relict sedimentary structures. Such calc-silicates contain important information about the early burial history of the sedimentary basin where their host rocks were deposited prior to the onset of metamorphism. There is some evidence that the alkali profiles have been modified during metamorphism. However it is not appropriate to use such calc-silicates for studies of synmetamorphic diffusion or other synmetamorphic processes without taking pre-metamorphic composition gradients into account.
Abstract A steeply dipping set of quartz-chlorite-muscovite-biotite veins was emplaced, at depths of at least 14 km and temperatures >400°C, into Môna Complex metasedimentary rocks of Holy Island, after the D4 event. Vein trends range from northeast-southwest to north-northwest-south-southeast. Vein offshoots, consistently oriented c. 10–35° anticlockwise relative to the main vein, are common. Quartz fibres within different veins show a range of orientations, from northwest-southeast to east-west, and are interpreted as tracking vein opening directions. Vein and fibre orientations are integrated into a four-stage model for vein emplacement. At each stage, new main veins either open extensionally, with fibres subnormal to their boundaries, or by hybrid extension-shear. Hybrid main veins commonly have extensional offsoots. Extensional main veins are orientated progressively further anticlockwise with time from a dominantly northeast-southwest trend to a dominantly north-south trend, but hybrid main veins have out-of-sequence orientations and probably utilized pre-existing fractures. The dominance of extensional and hybrid extension-shear vein opening indicates that the differential stress was small and pore fluid pressure was high. A study of length-thickness relationships has revealed that this vein set is self-affine with vein widening progressing more quickly than elongation during growth. Linear transect analyses show that the vein spacings have fractal characteristics.
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