INTERPRETING MAGMATIC FABRIC PATTERNS IN PLUTONS

Most plutons have widespread magmatic fabrics, the interpretation of w hich remains controversial. We propose a method to constrain likely ca uses of fabric patterns, the application of which indicates the follow ing: (1) preserved fabric patterns often form after chamber constructi on and only rarely provide information about ascent or emplacement; (2 ) fabrics are poor recorders of total strain and are easily reset, pre serving only the last increment of strain during crystallization; (3) in magmatic systems mechanically decoupled from host rocks, patterns m ay result from strain during internally driven flow, filter pressing o r porous flow in relatively static chambers, or by final increments of strain during emplacement; (4) with greater emplacement depths, fabri c 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 ma y provide a relatively direct record of paleostress in orogenic belts. (C) 1998 Elsevier Science B.V. All rights reserved.