A petrogenetic comparative study of zircons rom the mylonites of the Niemcza Shear Zone and the gneisses of the Góry Sowie Block (SW Poland)

Abstract

The morphological features and typological distributions of zircon in the mylonites of the Niemcza Shear Zone (NZ) and in the gneisses and migmatites of the Góry Sowie Block (GSB), in the NE part of the Bohemian Massif, SW Poland, provide important petrogenetic indicators in the strongly deformed metamorphic rocks. The observed similarities between the zircon populations (combined with other field and petrographic evidence) strongly suggest that at least a part of the mylonites developed at the expense of rocks similar to the GSB gneisses and migmatites. The protoliths of the gneisses and migmatites (both in the GSB and within the NZ) were predominantly of sedimentary character, but the zircons suggest that crustal-type granites (in the case of the NZ gneiss and mylonite protoliths) and hybrid mantle/crustal-type granites (in the case of the GSB migmatite protoliths) could have been important sources for the original, mostly detrital (?) material. The large proportion of zircon grains in the NZ mylonites, showing effects of disintegration, can result from sedimentary abrasion of detrital material, and this apparently corroborates the hypotheses that a part of the NZ mylonites derived from protoliths other (more strongly reworked by sedimentary processes?) than those typical of the gneisses and migmatites of the GSB. However, there is also evidence that mylonitization could have influenced the morphometric features of the zircon crystals, generally increasing the proportion of fractured and broken crystals and, most spectacularly, reducing the mean size of the zircon grains in the mylonites. The controversy remains open and to find better constraints would require further detailed petrological studies..