shape fabric of paramagnetic minerals, such as biotite and amphibole. If these
minerals were present when the rock was deformed, then the AMS and mineral
fabrics will be identical.
The Mono Creek pluton is a good example of a magnetite dominated system
(Bouchez, 2000). It was emplaced into earlier components of the Sierra
Nevada Batholith, California, during a period of regional dextral transgres-
sion. The AMS does not follow the border of the intrusion, as would be
expected if fabric were due to convection or other internal processes (Saint-
Blanquat & Tikoff, 1997). Instead, the AMS defines a broad sigmoidal shape
that accords with emplacement during tectonism. Towards the end of solidi-
fication the deformation was localised and became more intense, producing a
stronger AMS fabric. Deformation continued after solidification with the
production of a shear zone.
In many granitoids rock fabric, as expressed by AMS, clearly indicates a
sense of deformation, but this deformation may be a late event, unrelated to
the main phase of the emplacement of the intrusion. Cruden and Launeau
(1994) have investigated the Lebel syenite intrusion, Ontario. AMS fabrics,
defined by magnetite, indicate that the intrusion has a sheet-like form. They
propose a model in which the magma rose up an E–W fault and was then
injected horizontally as a tongue of magma into the host rocks. Launeau and
Cruden (1998) extended this study by the analysis of mineral fabrics using the
intercept method. They found that the mineral fabrics were generally parallel
to the AMS ellipsoids: however, in some samples bimodal mineral fabrics were
attributed to overprinting or complex movements of grains with different
shapes. The experiments of Iezzi and Ventura (2002) confirm that minerals
with different shapes can behave differently during magmatic deformation.
Crystal size distributions of pyroxene and magnetite were straight and this was
interpreted to mean that these minerals crystallised over a protracted interval.
Hence, the AMS was mostly produced during the main phase of crystallisation
of the body.
The fabric of some granitoids has been investigated using optical methods at
the same time as other aspects of the texture were quantified. The Halle
Volcanic Complex, Germany, is a porphyritic rhyolite intrusion with abun-
dant quartz, orthoclase and quartz phenocrysts (Mock et al., 2003). Optical
measurement of sections, some of them orthogonal, consistently showed sig-
nificant preferred orientations at 458 and 1358. This was interpreted as con-
jugate zones, developed during crystal settling. However, the consistent
direction for both vertical and horizontal sections, and the similar, weak
orientations, suggests that it may just be an artefact of the measurement
process.
5.8 Typical applications 195