Interpreting magmatic fabric patterns in plutons

Most plutons have widespread magmatic fabrics, the interpretation of which remains controversial. We propose a method to constrain likely causes of fabric patterns, the application of which indicates the following: (1) preserved fabric patterns often form after chamber construction and only rarely provide information about ascent or emplacement; (2) fabrics are poor recorders of total strain and are easily reset, preserving only the last increment of strain during crystallization; (3) in magmatic systems mechanically decoupled from host rocks, patterns may result from strain during internally driven flow, filter pressing or porous flow in relatively static chambers, or by final increments of strain during emplacement; (4) with greater emplacement depths, fabric patterns increasingly reflect strain caused by regional deformation; and (5) given that magmatic fabrics are easily reset and reflect only the last increment of strain of comparatively weak materials, they may provide a relatively direct record of paleostress in orogenic belts. (C) 1998 Elsevier Science B.V. All rights reserved.