| Abstrakt: |
The Liberec and Jizera granites are most dominant rocks in the western part of the Krkonoe-Jizera Composite Massif (KJCM). These two types of biotite granite show mutual transitions. On mesoscopic scale, these rock types differ in grain size and in quantity and size of K-feldspar phenocrysts. There are also differences in their magnetic susceptibility. Typical Liberec Granite (LG) shows the most frequent MS values of 1.7 × 10-3 SI units, whereas the authentic Jizera Granite (JG) is characterized by values about 10 times lower, i.e. about 0.13 × 10-3 SI units. Medium-grained strongly porphyritic biotite granite (Jizera type) is the main rock-type in the Jizerské hory Mts. The strongly porphyritic biotite granite shows in places linear to planar orientation of K-feldspar phenocrysts and mafic microgranular enclaves and schlieren. The fresh rock is light grey, often with slightly pink shade. Groundmass is composed of plagioclase, K-feldspar, biotite, chlorite, rare hornblende and accessory minerals (zircon, apatite, titanite, allanite, and ilmenite). Subhedral K-feldspar of pink to light red colour forms phenocrysts 3-5 cm, and exceptionally 7 cm long. Plagioclase forms smaller phenocrysts. K-feldspar phenocrysts are often rimmed by white plagioclase resembling the rapakivi texture. Coarse- to mediumgrained porphyritic biotite granite (Liberec type) constitutes deeper parts of the Krkonoe-Jizera Composite Massif. It crops out on the surface as the rock underlying the Jizera granite, mainly in deeply eroded parts of the massif and along its southern boundary. Fresh rock has a pinkish colour caused by phenocrysts of K-feldspar, which occupies nearly half of its volume. The phenocrysts are usually short prismatic, up to 3 by 2 cm in size. The coarse-grained groundmass (5-10 mm) is composed of plagioclase, quartz, K-feldspar, some biotite, minor hornblende, muscovite, and accessory zircon, apatite, titanite, allanite, magnetite, ilmenite, and pyrite. Visual discrimination of these two very similar granites has until now been rather subjective, bringing complications and uncertainty during the field mapping. The main problem is definite recognition of the individual rock types in the vicinity of their demarcation line because of their gradational nature. Field parameters are not always sufficient for classification of both rocks within their domains. Specific geochemical characteristics of the Liberec and Jizera granites have been applied to demarcate their boundaries. Based on more precise geochemical discrimination between these two granites it is possible to state that the Liberec Granite is overlain by the Jizera type and that the Liberec Granite represents a more advanced differentiation stage in the magmatic evolution of the Krkonoe-Jizera Composite Massif. Based on magnetic susceptibility (MS) survey of these granitoids also two major granite types have been recognized: (i) Coarse-grained porphyritic Liberec Granite (LG), (ii) Medium-grained porfyritic Jizera Granite (JG). The basic statistical parameters MS values (10-3 SI) measured at various outcrops and average MS of each rock type are summarized in Table 1, and frequency (%) distributions of MS values in several classes for various granite types are also shown in the histogram (Fig. 3). The observed variations in MS values have been classified into magnetite series (> 3.0 × 10-3 SI) and ilmenite series ( ≤ 3.0 × 10-3 SI) granites, which corresponds to oxidized type and reduced type granites respectively, and are indeed a reflection of oxygen fugacity (fO2) intrinsic to source regions. The highest MS values were recorded in lowland region between the Liberec and Tanvald towns mostly on LG outcrops. Liberec Granite belongs to magnetite series (75 % strongly oxidized) granites. Most widely distributed Jizera Granite in the central part of western part of the Krkonoe-Jizera Composite Massif belongs to ilmenite (95 % strongly reduced) granites. [ABSTRACT FROM AUTHOR] |
