Polymineralic orientation analysis of magmatic rocks using ElectronudBack-Scatter Diffraction: Implications for igneousudfabric origin and evolution

摘要

Electron Back-Scatter Diffraction (EBSD), which provides an easy way of acquiring large numbers of individual crystallographicudorientation data from different phases, has been applied to the study of magmatic fabrics. Using this technique, the crystallographicudpreferred orientation (CPO) of plagioclase, biotite, orthopyroxene, hornblende and quartz in natural tonalites and quartzdiorites (fromudthe Santa Olalla Igneous Complex, SW Iberia) deformed during the magmatic stage have been determined. Plagioclase is the coarserudphase defining the main fabric in each sample, whereas biotite can display either the same fabric as plagioclase or a completelyuddifferent one. The differences between these two phases occur because: (1) smaller phases interact with the larger ones to produceudmore random orientations, (2) under simple shear, finer phases can completely rotate giving a girdle included in the XZ plane, (3) finerudphases can more easily preserve relict fabrics, while the coarser phases are completely reoriented by the last stress tensor. The lastudphases to crystallize show weak to completely random CPOs (hornblende) or completely random distributions (quartz). The study wasudcompleted with a shape preferred orientation analysis using the Intercept Method in order to detect weak magmatic lineations, andudnumerical modelling simulations of theoretically equivalent simple shear situations for each sample.