Petrophysical and geochemical constraints on alteration processes in granites

The hydrothermal alteration of granites has large influence on their petrophysical properties. To reveal the impact of alteration on magnetic and porosity properties of granites we have conducted a complex study of effects of two largely independent alteration processes, related to chemically different fluids, in granites of the Vysoký Kámen stock (the Krudum granite body, Czech Republic). It includes the whole-rock geochemical, magnetic and pore-space characterization. The alkali feldspathization resulted in decomposition of Li-mica, quartz removal, depletion in mafic cations and growth of new alkali feldspars (albite, K-feldspar), decreasing the overall magnetic susceptibility and disrupting the pore space by its discontinuation. The preservation of the orientation of the principal susceptibility axes is likely related to insignificant influence of the feldspathization process on the paramagnetic and diamagnetic phases orientation acquired during the magma emplacement. The greisenization, on the other hand had considerably more significant effects on microstructure and physical properties of the granite. The microstructure was modified by the growth of large amounts of new phases (Li-mica, quartz and topaz). This changed the mineral density of the rock, the porosity, size and character of pores to larger, flatter and probably more connected. This led also to the complete reworking of the original anisotropy of magnetic susceptibility during the greisenization.     

Fabric transpositions in granite plutons — An insight from non-scaled analogue modelling

Investigations on a set of experimental models of highly viscous intrusions were carried out in order to study the internal strain pattern during vertical ascent and emplacement of granite intrusions. The strain pattern was determined by means of anisotropy of magnetic susceptibility (AMS) resulting from the orientation of magnetite particles in a liquid plaster medium. The modelled intrusions show distinct fabrics reflecting the flow of a rheologically complex, non-Newtonian material. During the vertical growth of the intrusion, constrictional vertical fabrics are transposed into flattening fabrics, and along with the development of low-intensity fabric domains are passively transported upwards. Vertical growth takes place along subvertical thrust shear zones that satisfactorily explain the discordant magmatic fabrics in granites along intrusion sides. The resulting complex fabric patterns suggest that the vertical movement of material in ascending intrusions is accommodated by various flow mechanisms operating simultaneously.     

Anwendung derF-Verteilung bei der Suche nach optimaler Form einer approximativen Funktion

Summary In processing observed data one frequently encounters the problem of substituting a set of observed values of two variables by a continuous functions. This function is selected in the form of a polynomial with powers of the variable x. A test characteristic is then constructed, F, which has a Fisher probability distribution. The test characteristic has a simple form when orthogonal polynomials are used. By succesive application of the test of the hypothesis that the function is expressed by a polynomial of a given degree the degree and the form of the polynomial is determined.     

Origin and orientation of microporosity in eclogites of different microstructure studied by ultrasound and microfabric analysis

This work contributes to the experimental investigations of the origin and 3-D orientation of micropores in low porosity crystalline rocks. The origin and spatial orientation of microporosity in two eclogites with different microstructures were studied by 1) quantitative and qualitative microstructural analysis of grains and grain boundaries, 2) measurement of lattice preferred orientation using the SEM-EBSD method and 3) experimental measurement of velocity of elastic P-waves in spherical samples in 132 directions under confining pressures up to 400 MPa. Results show good correlation between the elastic properties and the orientation of grain boundaries and cleavage planes in clinopyroxene. The magnitude and anisotropy of velocity change with pressure shows that microporosity in the fine-grained sample is relatively large and strongly preferentially oriented, whereas it is significantly lower and less preferentially oriented in the coarse-grained sample. Seeing that the lattice preferred orientation of clinopyroxene is similar in both samples we can deduce from velocity changes that the grain size of the rock forming minerals controls the amount of microporosity. Also, the orientation of microporosity depends mostly on preferred orientation of grain boundaries and somewhat less on the orientation of cleavage planes. Grain boundaries are therefore the most important contributors to the bulk microporosity in the studied rocks.     

Long-lasting Cadomian magmatic activity along an active northern Gondwana margin: U–Pb zircon and Sr–Nd isotopic evidence from the Brunovistulian Domain,eastern Bohemian Massif

International Journal of Earth Sciences - Cadomian magmatic complexes of the Brunovistulian Domain crop out at the eastern termination of the Bohemian Massif. However, the age, nature and...