Analogue and numerical modelling of shape fabrics: application to strain and flow determination in magmas

Research areas:
Year:
2000
Authors:
Journal:
TRANSACTIONS OF THE ROYAL SOCIETY OF EDINBURGH-EARTH SCIENCES
Volume:
91
Number:
1-2
Pages:
97-109
ISSN:
0263-5933
BibTex:
Note:
4th Hutton Symoposium on the Origin of Granites and Related Rocks, CLERMONT FERRA, FRANCE, SEP 20-25, 1999
Abstract:
We summarise numerical and analogue models of shape fabrics, and discuss
their applicability to the shape preferred orientation of crystals in
magmas. Analyses of flow direction and finite strain recorded during the
emplacement of partially crystallised magmas often employ the analytical
and numerical solutions of the Jeffery's model, which describe the
movement of noninteracting ellipsoidal particles immersed in a Newtonian
fluid. Crystallising magmas, however, are considered as dynamic fluid
systems in which particles nucleate and grow. Crystallisation during
magma deformation leads to mechanical interactions between crystals
whose shape distribution is not necessarily homogeneous and constant
during emplacement deformation. Experiments carried out in both
monoparticle and multiparticle systems show that shape fabrics begin to
develop early in the deformation history and evolve according to the
theoretical models for low-strain regimes. At large strains and
increasing crystal content, the heterogeneous size distribution of
natural crystals and contact interactions tend to generate steady-state
fabrics with a lineation closely parallel to the direction of the
magmatic flow. This effect has been observed in all three-dimensional
experiments with particles of similar size and for strain regimes of
high vorticity. On the other hand, studies of feldspar megacryst
sub-fabrics in porphyritic granites suggest that these record a
significant part of the strain history. Thus, the fabric ellipsoid for
megacrysts evolves closer to the strain ellipsoid than for smaller
markers. This behaviour results from the fact that the matrix forms of
the melt and smaller crystals behave like a continuous medium relative
to the megacrysts. Consequently, in the absence of these markers, and
because the fabric intensities of smaller particles such as biotite are
stable and lower than predicted by the theory, finite strain remains
indeterminate. In that case, strain quantification and geometry of the
flow requires the addition of external constraints based on other
structural approaches.