| Abstrakt: |
A detailed investigation of changes in granites of the Krkonoe-Jizera Composite Massif (KJCM) resulting from intrusion of the basic and ultrabasic dykes was carried out using a scanning electron microscopy (SEM), electron-backscattered diffraction (EBSD), electron probe microanalysis (EPMA), and magnetometric survey. The first group of samples representing a contact of olivine melilitite with xenoliths of the Liberec Granite comes from one of the ultrabasic dykes that are exposed in an abandoned quarry on the Výina Hill in Liberec (Figs 1b, 2a). This olivine melilitite dyke is a member of the Čertovy Zdi dyke swarm inside the KJCM. It is 61.9 ± 3.0 Ma old (K/Ar whole rock dating), and encloses xenoliths of the Liberec Granite of Variscan age (320 Ma, U/Pb zircon age). The melilitite emplacement into the granite is accompanied and marked by fragmentation, microfracturing, partial melting of the granite feldspars, and by conversion of biotite into magnetite. This mineral assemblage gives evidence of heating and rapid cooling of the melilitite dyke enclosed in the Liberec Granite. The microstructure of granite xenoliths in melilitite, which in hand specimens resembles pseudotachylite (Fig. 2a), is resulting from a combination of fracturing, thermal alteration, and partial melting processes. The second group of samples represents a contact of the Tertiary olivine bazanite (about 30 Ma. K/Ar whole rock dating) with the Jizera Granite (Figs 1b, 2b) collected in an abandoned quarry on the southern slope of the Buková Hill near Jiřetín pod Bukovou. Both rocks are welded along the sharp vertical contact boundary. The bazanite magma influence on the Jizera Granite is indicated by conversion of biotite into fine coatings of magnetite, and by K-feldspars partial melting, and formation of mullite. As concerns the melilitite, its microstructure shows less damage and less pronounced metasomatic alteration along the contact. The ground magnetometric survey at both localities was carried out (Fig. 4) in order to reveal the thermal effect of the basic magma (the Buková abandoned quarry) and to determine the dyke orientation (the Výina abandoned quarry). The magnetic susceptibility values of the Jizera Granite along the contact observed in a quarry at the Buková Hill was found similar to bazanite, whereas the magnetic susceptibility of the thermally unaffected granite is almost negligible. The thermal effect of the bazanite magma is traceable by magnetic survey to a distance of about 15-20 meters from the bazanite - granite contact. Moreover, the thermal effect recorded by magnetometric measurements appears to be a suitable field method for detecting the spatial orientation of contact metamorphism and the thickness of thermal interaction zone. [ABSTRACT FROM AUTHOR] |
