Deformation-Induced and Reaction-Enhanced Permeability in Metabasic Gneisses, Iona, Scotland: Controls and Scales of Retrograde Fluid Movement

The spatial distribution of greenschist-facies retrograde reaction products in metabasic gneisses from Iona, western Scotland, has been investigated. The retrograde products may be broadly accounted for by a single reaction, but their different spatial and temporal development indicates that a series of reactions occur with significantly different scales of metasomatic transfer. After initial fluid influx linked to deformation-induced high permeability, reaction-enhanced permeability, coupled to cycling of fluid pressure during faulting, strongly controls the pervasive retrogression. Ca-plagioclase and pyroxene in the gneisses are replaced by albite and chlorite in pseudomorphic textures, and this is followed by localized epidotization of the albite. Two main generations of epidote are formed in the gneisses. Epidosite formation is associated with prominent zones of cataclasite indicating a strong link between faulting and fluid influx. In contrast, complete alteration of albite to epidote in the host metabasic gneisses is spatially complex, and areas of pervasive alteration may be constrained by both epidote-rich veins and cataclasites. In other instances, reaction fronts are unrelated to structural features. Volume changes associated with individual stages of the reaction history strongly control the localized distribution of epidote and the earlier more widespread development of chlorite and albite. Such behaviour contrasts with adjacent granitic gneisses where epidotization is restricted to local structural conduits. Many small-scale mineralized fractures with evidence of having previously contained fluids do not enhance the pervasive retrogression of the metabasic gneisses and represent conduits of fluid removal. Retrogression of these basement gneisses is dominated by a complex combination of reaction-enhanced and reaction-restricted permeability, kinetic controls on the nucleation of reaction products, changes in fluid composition buffered by the reactions, and periodic local migration of fluids associated with fault movements. This combination generates spatially complex patterns of epidotization that are limited by cation supply rather than fluid availability and alternations between focused and pervasive types of retrogression.

Low temperature grain-scale retrograde alteration of detrital minerals in Ajali formation from Benin Flank Of Anambra Basin, Nigeria

Petrographical study of detrital and authigenic minerals from the Ajali Formation in the Benin flank of Anambra Basin was carried out with the aim of interpreting the textural characteristics and alteration patterns. Samples were collected from Ayowgiri sand quarry site and along Fugar-Agenebode road where good exposures of the Ajali Sandstone occurred. Heavy minerals were concentrated using bromoform according to standard procedures. The heavy mineral concentrates were mounted on glass slides with araldite glue. In addition, thin sections of sandstone were prepared. The samples were studied with the aid of polarizing microscopes. The textural features and relationships among the heavy minerals were used in the interpretations. The results indicate that the detrital heavy minerals (Fe-Ti oxides and garnet) have undergone a very low-temperatureretrograde changes rather than ordinary in-situ dissolution by weathering and burial diagenesis. The alteration of Fe-Ti oxides resulted into the formation of leucoxene and chlorite while that of garnet resulted only into the formation of chlorite. Detrital magnetite was partially dissolved with skeletal remains while the authigenic grains showed well preserved structures. The re-equilibration of these minerals and partial dissolution were probably due to rise in temperature and change in the chemistry of the formation water. Reducing and acidic conditions at elevated temperature probably favoured dissolution and alteration of the detrital Fe-oxides while the formation of authigenic hematite was aided by oxidizing condition. Temperature increase was probably induced by either underneath thin lithospheric plate or igneous activities at the center of the basin. The conversion of the detrital minerals to authigenic chlorite and partial replacement structures represent a typical case of retrograde reaction which could be described as evidence of a very low-temperature anchi-metamorphism at mineral grain level. Key words: Authigenic, chlorite, Fe-oxides, garnet, retrograde, anchi-metamorphism.

A Pan-African zincian staurolite imprint on Namaqua quartz–gahnite–sillimanite assemblages

Quartz-gahnite-sillimanite asssemblages are described from a supracrustal enclave at Kraaifontein, 45 km south-west of Springbok in the Namaqualand Metamorphic Complex, South Africa. The assemblage formed by prograde reaction in the granulite facies zone during the 1100 Ma Namaqua event, possibly as the result of desulphidation of sphalerite. Subsequent lower-amphibolite-facies retrogression occurred in close proximity to shear zones during an early Pan-African metamorphic event at 700 Ma. Zincian staurolite formed as overgrowths on gahnite in a hydration reaction involving the consumption of gahnite, quartz, and sillimanite. Compositional zoning to more zinc-rich rims in gahnite at Kraaifontein is unrelated to the retrograde reaction, but is interpreted as a result of changing conditions during the prograde Namaqua event.