Texture and fabric
The maps of identified grains and grain boundaries for DS03, BC11,
BC302, and HC01 (shallowest to deepest in terms of paleodepth, least to
most mafic, and highest to lowest Rb/Sr ratios) are shown in Fig. 7. The
thin sections were cut along planes that were originally perpendicular
to the surface (assuming a 45ยบ westward postemplacement tilt) and were
striking NS but that had since been tilted (Fig. 7a). Only grains away
from the edges were analyzed leading to some vacant space on certain
portions of the maps. Rose diagrams (last two column of Fig. 7) show the
number of particles oriented at a given angle (azimuth) with respect to
the bottom edge of the image. The tan rose diagrams include all
identified grains while the pink rose diagrams only include phenocryst
grains (euhedral plagioclase, alkali feldspar, and biotite) for each
sample. The intensity of fabric development can be described by the
alignment factor (Barraud, 2006). The alignment factor (AF) can be
calculated by first introducing an orientation tensor,
Tn, for each grain:
\(T^{n}=\ L_{n}\par
\begin{bmatrix}\operatorname{}\alpha_{n}&\cos\alpha_{n}\sin\alpha_{n}\\
\cos\alpha_{n}\sin\alpha_{n}&\operatorname{}\alpha_{n}\\
\end{bmatrix}\) (1)
Here, n , refers to a grain of interest and L is the major axis
length. The bulk orientation tensor, Mij, can be related
to Tn as:
\(2.\ \ M_{\text{ij}}=\frac{1}{N}\sum_{1}^{N}T_{\text{ij}}^{n}\) (2)
Finally, AF is defined as:
\(alignment\ factor\ (AF)=100\ \times\ \frac{e_{1}-e_{2}}{e_{1}}\)(3)
where e1 and e2 are eigenvalues of
Mij. Conceptually speaking, AF represents the extent of
agreement between orientations of individual grains.