Polymineralic orientation analysis of magmatic rocks using Electron Back-Scatter Diffraction: Implications for igneous fabric origin and evolution

Abstract

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