Wpływ granitu karkonoskiego na gnejsy izerskie

Teresa Oberc-Dziedzic


Stwierdzono, że najdalej od kontaktu z granitem zaznacza się zmiana pleochroizmu biotytu, nieco bliżej rekrystalizacja kwarcu. Zasięg rekrystalizacji skaleni nie przekracza strefy występowania nowo powstałych minerałów, z których największy zasięg ma andaluzyt i kordieryt, nieco mniejszy siUimanit, a najmniejszy korund i hercynit. Ustalono, że w strefie bezpośredniego kontaktu osiągnięta została facja piroksenowo-hornfelsowa, a nieco dalej hornblendowo·hornfelsowa.


Intrusive Variscan Karkonosze granite contacts Proterozoic micaceous schists and Izera gneisses in the north lts influence on the latter rocks is reflected by changes in structure of gneisses, properties of some minerals. and origin of new ones. The changes in structure are connected with three- to five-fold increase in size of quartz and feldspar grains, not affecting ordered orientation of optical axes of quartz. Recrystallized grains are characterized by straight boundaries, quadrilateral to hexagonal outline Table 1, Figs. 2 -4; Table II, Fig. 5), banded structure and, in the case of K-feldspars, disappearance of twinning chequerboard microcline. Pleochroic colours of biotite change from brown to red and green.

The newly formed minerals, mainly representing products of decay of micas, include sillimanite, cordierite, andalusite, hercynite, and corundum (Table II, Fi.gs. 6, 7; Tables III and IV). The cordierite + sillimanite + hercynite + corundum paragenesis is found in zone of direct contact of granite, showing that cover rocks locally reached hornfels-pyroxene facies. In zone stretching somewhat further of the contact there is recorded. the sillimanite + cordierite paragenesis, and still further -the cordierite + andalusite paragenesis, indicative of conditions of the hornblende-hornfels facies (as interpreted by F.J. Turner, 1981).

Figure 1 shows extent of the above changes. The change of pleochroic colours of biotite from brown to red is found the farthest from the contact, even in places where any other evidence for thermal effect of granite is missing. The extent of quartz recrystallization is somewhat smaller, and that of feldspar recrystallization - still smaller. Andalusite appears to be the most widely distributed of the newly formed minerals. The extent of cordierite is smaller than that of the above mineral, and that of sillimanite - the smallest. Distribution of corundum, especially hercynite, appears limited to the direct neighbourhood of granite, where melting of gneisses has locally taken place also. Extent of changes evoked by granite appears relatively wide which may be due to fact that it rests flat beneath gneisses. Top surface of granite dips northwards at the angle of about 120 in the Piechowice-Bobrowe Skały hills section. Changes in pleochroism of biotite and traces of recrystallization of quartz are also found at large distances from outcrops of the granite. which may indicate existence of local culminations of its top.

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