Complete texture analysis of a deformed amphibolite: comparison between neutron diffraction and U-stage data

Abstract

This paper describes a new technique of analysing complex neutron diffraction patterns, allowing the quantitative texture analysis of each rock-forming mineral up to trielinic lattice symmetry in polymineralic rocks. The method is demonstrated using hornblende and plagioclase fabrics in Alzenau amphibolite from the Spessart Mountains (Mid-German Crystalline Rise). Hornblende and plagioclase exhibit a pronounced shape preferred orientation with respect to the macroscopic fabric elements. For control purposes, crystallographic preferred orientations were also determined independently by individual grain measurements using a U-stage. From the neutron texture analyses we obtain only superposed pole figures (hornblende and hornblende, hornblende and plagioclase, and plagioclase with plagioclase) of low-indexed lattice planes. Therefore, the critical crystallographic directions were recalculated by the orientation distribution function (ODF) which is given by texture components. The optical and neutron-derived pole figures are in a good agreement. All pole figures display strong preferred orientation, although the plagioclase texture is less pronounced. The hornblende texture consists of a [001] maximum parallel to the stretching lineation, with [010] showing a tendency to develop a girdle around the lineation. [100]-axes are distributed with a clearly split maximum representing the monoclinic angle between [100] and [001]. The plagioclase texture is a rare one with (001) subparallel within the foliation and [100] oriented subparallel to the lineation. [010] of plagioclase is concentrated normal to lineation within the foliation.The technique outlined here is potentially the most useful one for texture analysis of polyphase rocks, independent of crystal symmetry.