No description available.

No description available.

Petrological, structural and stable isotope studies of variably calcareous metapelites, metacarbonates and interbedded metabasite sills from the Argyll and Southern Highland Groups in the Dalradian Supergroup from the SW and Central Scottish Highlands have constrained the mechanisms and timing of metamorphic fluid flow. In the SW Highlands, infiltration of the H<SUB>2</SUB>O-CO<SUB>2</SUB> fluids accompanied greenschist facies metamorphism. The syn-metamorphic fluids were channelled through the Ardrishaig Phyllites, infiltrating metabasite sills and driving carbonation reactions in the sill margins. High fluid pressures resulted in hydrofracture in metabasite sills, and infiltration via microfractures was an important mechanism for fluid transport. Carbonation reactions in the metabasites were kinetically limited by diffusion away from microfractures, but in areas of high fluid flux, metabasite sills were completely carbonated. Calcite-quartz microveins developed as a direct result of fluid infiltration and occur in carbonated metabasite units, in the phyllites and more rarely, in psammitic units. The highest density of microveins occurs in axial regions of the regional Ardrishaig Anticline in the SW Highlands, coinciding with zones of high fluid flux, measured by Skelton et al. (1995). High fluid fluxes and microveining in axial regions also coincide with isotopic homogenisation of calcite and quartz in microveins and their host rocks. Therefore, syn-metamorphic fluid flow was focused in axial regions of the Ardrishaig Anticline Calcite-quartz microveins and all homogenised rock units in axial regions have low negative δ<SUP>13</SUP>C calcite values, indicating that the fluid derived its CO<SUB>2</SUB> from oxidation of graphite-bearing metasediments at depth. In low fluid flux zones, i.e. on the limbs of the Ardrishaig Anticline, incomplete fluid:rock interaction has resulted in heterogeneous δ<SUP>18</SUP>O and δ<SUP>13</SUP>C calcite values. Petrological evidence indicates that metamorphic fluids did not transport heat to the Ardrishaig Phyllites, even in zones of large fluid flux.

The Proterozoic Younger Giant Dyke Complex (YGDC) was emplaced during a period of widespread extensional continental magmatism in southwest Greenland and North America. The complex consists of a series of interconnected branching dyke segments up to 800m wide, which can be traced intermittently over 145km. The dykes were intruded into granitic basement and appear to have been halted at the unconformity between the basement and overlying supracrustal rocks. This project focusses on the Tugtut^oq-Narsaq area of the complex. The magmas producing the dykes were critically-undersaturated alkali olivine basalts to hawaiites with whole-rock MgO = c. 5wt%. They were carrying phenocrysts of olivine, stellate glomerocrysts of plagioclase + olivine, and anorthosite xenoliths up to 100m across. These xenoliths are found in the Narsaq area which represents a roof zone to the dykes. The magmas crystallised to form predominantly troctolitic cumulates. In western Tugtut^oq these are olivine-plagioclase cumulates but in eastern Tugtut^oq and Narsaq, magnetite and ilmenite (± apatite) joined the fractionating assemblage. Olivine and plagioclase core compositions range from Fo_68 to Fo_49 and An_65 to An_30. Salitic to ferrosalitic pyroxene is entirely intercumulus except in two evolved pods of syenogabbro and syenite enclosed by troctolite at the eastern end of Tugtot<SUP>o. These pods are ovoid in shape, elongated along the dyke, and up to 3km long. They are thought to have differentiated <i>in situ</i>. The distinction between syenogabbro and syenite is made on the presence or absence of plagioclase cores to alkali feldspar crystals. The order of appearance of fractionating minerals was ol</SUP>+ pl at c. 5 wt% MgO, Fe-Ti oxides + apatite at c. 4 wt% MgO, cpx at c. 3.5 wt% MgO and alkali feldspar at 3 wt% MgO or less. In ovoid pods at irregular intervals along the dyke segments, modal variation and/or plagioclase lamination define synformal layering. The modal variation takes the form of troctolite alternating with generally narrower layers of mafic cumulates. In the western YGDC these are olivine cumulates (gabbro picrites) but in the east they are olivine + magnetite + ilmenite ± apatite cumulates. Modal layering is best developed in the western YGDC where plagioclase lamination is absent. It may take the form of parallel mafic layers (1-30cm thick) or gabbro picrites filling channels (up to 8m wide and 4m deep) which plunge towards the synform axis. At two localities breccias of gabbro picrite blocks within a troctolite matrix occupy the axial zone. They are inferred to have resulted from the breakup of thick picrite layers. The breccias and channels provide evidence that the synformal structure was primary.

Olivine and orthopyroxene with smaller amounts of garnet dominate the mineralogy of the ‘clastic’ fragments. The matrix assemblage largely consists of phlogopite and ilmenite with a lesser proportion of orthopyroxene and, rarely, garnet. The cores of individual fragmented mineral grains exhibit an extensive range of major-minor element compositions compatible with derivation from various mantle peridotites, eclogites and megacrysts. In contrast, the rims of specific mineral phases often converge on common compositions, which may also be found in matrix phases. Thus the rims appear to be overgrowths around the original fragmental grains and represent crystallisation products from the melt transporting the fragments. The polymict matrix shows similarities of mineral assemblage and composition to the ilmenite-rutile-phlogopite-sulphide (IRPS) metasomatic suite documented in mantle peridotites from Matsoku and elsewhere. The polymict xenoliths therefore provide evidence that the melt type responsible for this metasomatism, is widely distributed though of presumably very small volume. Trace element compositions show more complex patterns of variation than the major-minor elements. Some convergence of rim matrix compositions is seen in some phlogopites, but the orthopyroxenes show a scatter of compositions suggestive of highly localised crystallisation and perhaps partial re-equilibration. In the one case showing matrix garnet, the garnet rim and matrix trace element compositions are similar, and the rims of orthopyroxene and phlogopites also show convergence. The trace element compositions from this particular polymict are therefore considered to be unequivocally indicative of the matrix melt composition. A new set of mineral-melt partition co-efficients derived from the Matsoku suite of xenoliths are used to calculate the trace element composition of the melt which interacted with this polymict peridotite. This melt is similar to that of a kimberlitic melt. Calculated models show that fractionation of small amounts of garnet and larger amounts of orthopyroxene and phlogopite from a melt of megacrystic composition produces a melt similar to that which interacted with the polymicts.

No description available.

Archaean orthogneisses in the Rauer Group and Vestfold Hills have been examined through a combined field, petrological, geochemical and isotopic study in order to document and evaluate regional terrane correlations in the East Prydz Bay region of East Antarctica, and assess models for early Precambrian crustal growth, both regionally and in general terms. Within the Archaean domain in the Rauer Group, two layered ultramafic-mafic associations, the Torkler-Tango and the Scherbinina Layered Complexes (TTLC & SLC), are hosted in <I>ca</I>. 2800 to 3300-3470 Ma Composite Layered Orthogneiss (CLO). The SLC and TTLC are spatially, compositionally and probably temporally separate metaigneous suites. The TTLC is older than <I>ca</I>. 2800 Ma CLO intrusive sheets, and geochemical similarities between the TTLC and <I>ca</I>. 3300 Ma tonalite-trondhjemite-granite components of the CLO imply that there <I>may</I> be a genetic link between these gneiss units, as both have distinctive arc-like geochemical signatures. The SLC is also cross-cut by <I>ca</I>. 2800 Ma CLO components, but SHRIMP U-Pb zircon dating of the complex reveals that it is only marginally older with a crystallization age of 2844 ± 6 Ma. Geochemically, the SLC has very weak arc-like signatures which are attributed to small amounts of crustal contamination (by CLO), and primary geochemical signatures are inferred to have been derived from an E-MORB-like source. A tectonomagmatic setting similar to that deduced for the early Tertiary East Greenland Margin is envisaged as a possible analogue for events at 2844 Ma in the Rauer Group. However, petrogenetic models for the Rauer Group Archaean domain are <I>tentative</I>, because Sr and Nd isotopes indicate that significant open-system behaviour occurred during fluid infiltration associated with a regionally recognised Pan-African (<I>ca</I>. 500 Ma) event. Detailed mapping of intrusive relations that are preserved in Archaean othogneisses in the Vestfold Hills has allowed a reconstruction of the early high-grade geological history of the terrane. A number of spatially, temporally and compositionally distinct intrusions were emplaced throughout the Crooked Lake magmatic episode between D<SUB>1</SUB>-M<SUB>1</SUB> and D<SUB>2</SUB>-M<SUB>2</SUB>. The duration of this episode has been precisely defined by SHRIMP U-Pb dating axial planar leucosomes to 2496.3 ± 0.7 Ma (D<SUB>1</SUB>-M<SUB>1</SUB>) and 2475.3 ± 0.7 Ma (D<SUB>2</SUB>-M<SUB>2</SUB>). The distribution of Crooked Lake Gneiss (CLG) indicates that regional-scale tectonic repetition of major units was local rather than regional, and the amount of strain associated with the main folding and fabric producing event (D<SUB>2</SUB>) is heterogeneous.

No description available.

Upper Dalradian sediments are exposed on the north coast of Banffshire and Aberdeenshire between Boyne Bay and Fraserburgh. The section is almost at right angles to the trend of the main structural features, but there are strike sections of more than a quarter of a mile along the sides of inlets, and vertical sections on cliffs up to five hundred feet high. Information on these rocks has been published by several geologists (see sections 2:1 and 4:2). Their descriptions of the structural geology and sedimentary petrology are extended here and new interpretations proposed (sections 2 and 4) in the light of modern work on the origin of folds and sediments. Techniques involving the use of a computer, which might be useful in studies of this kind, are proposed in section 3.

No description available.