Próba zastosowania analizy zbliźniaczeń plagioklazów do ustalenia genezy gnejsów i granitoidów NW części bloku izerskiego (Sudety Zachodnie)

Abstract

AN ATTEMPT TO USE ANALYSIS OF PLAGIOCLASE TWINNINGS IN STUDIES ON ORIGIN OF GNEISSES AND GRANITOIDS FROM NW PART OF THE IZERA BLOCK (WESTERN SUDETY MTS) Summary The origin of gneisses and granitoid rocks of the Izera block , Western Sudetes, has been the subject of large controversy in the last decades. The rocks were thought to have been of either metasedimentary or magmatic origin. The Gorai's method of a statistical analysis of plagioclase twins was used by the present author in order to test the latter inferences. The results of this study evidence a compound origin of the rock series in question. The analysis of plagioclase twinning implies that some rocks, such as laminated gneisses and leucogneisses, are of metasedimentary origin, while flaser (streaky) gneisses and veins of the fine-grained Izera granites are magmatic in their nature. The Izera granite-gneisses and granitoid rocks appear to be of a twofold origin. In various parts of the region these rocks may be of either infracrustal or supracrustal origin. Their parent rock assemblage is thought to have originally been of considerable lithological variability, but later became gradually homogenized due successive stages of metamorphosis. As a final effect of these metamorphic alterations there originated the gneisses, and them the homophanous granites of the Izera region.