Deformazioni di feldspati alcalini in laccolite, un esempio da rioliti dei Colli Euganei.

According to the experiments conducted by Willaime & Gandais, (1977) and Kovacs & Gandais (1980) concerning the deformation of single crystals of sanidine under high confining pressures (1500 MPa) and subjected to differential stress variables as a function of temperature (700 °, 800 °, 900 °) it is possible to obtain the activation of the slip system <100> (010) and / or <101> (010). In particular, the results obtained show the achievement of a constant strain rate at higher temperatures and at a differential stress of 340 MPa. Based on these experimental data we have tried to explain the causes activation of natural systems above slip in some deformed sanidine crystals found in the rhyolitic laccolith of Monte Brusà. Thanks to the increasingly sophisticated analyzes (analysis by light microscopy of thin sections, electron microprobe analysis, X-ray fluorescence analysis, analysis by SEM-EBSD), applied to natural deformed sanidine crystals, we arrived to propose a likely hypothesis about the cause of their natural deformation. According to this there has been a transfer of stress from the magma just outpouring from the conduit to that stagnant at the edges (at the time in conditions of Critical Reologcal Melt Fraction) creating along the margins of the sub-volcanic body a system of simple shear that has deformed crystals for dislocation glide. In addition, during the execution of this work discrepancies are highlighted between the experimental results obtained from Willaime & Gandais, (1977) and Kovacs & Gandais (1980) and data collected by K-feldspar found in the laccolith of Monte Brusà. In fact, in this site, the magma intrudes, of course, under conditions of temperature and differential stress lower than the experimental ones producing slip systems <100> (010) and / or <101> (010). The here obtained results show that the deformation achieved by sanidines under natural conditions far exceeds the strain reached in experiments, already a few MPa of differential stress (about 10 MPa).