| This study is being done with the objective of trying to understand whether
the anisotropy of magnetic susceptibility (AMS) data can provide information
about strain in quartzites with negative magnetic susceptibility. For this,
nine quartzite samples have been collected from Rengali Province (located in
the eastern part of India) with bulk magnetic susceptibility between -13.6 x
10$^{-6}$ SI units and -3.06 x 10$^{-6}$ SI units. Since these rocks did not
show any visible foliation or lineation, AMS analysis was performed using
KLY-4S Kappabridge and the orientation of three principal axes of the AMS
ellipsoid (K$_{1}>$K$_{2}>$K$_{3})$ were determined. Thin sections were
prepared parallel to the K$_{1}$K$_{3}$ plane of the AMS ellipsoid (plane
parallel to lineation and perpendicular to foliation), which is equivalent
to the XZ plane of the strain ellipsoid. SEM based electron
backscatter diffraction (EBSD) analysis, shape preferred orientation (SPO)
analysis and strain analysis were carried out in these sections.
Recently, Renjith et al. (2016) used the same samples to establish that the
AMS in quartzites gives information about the SPO and not the CPO. To
further evaluate the robustness of AMS in strain analysis, the authors have
integrated the degree of magnetic anisotropy (P$_\mathrm{j}$ - a measure of the
eccentricity of AMS ellipsoid; Tarling and Hrouda, 1993) with the intensity
of SPO ($\kappa $; Piazolo and Passchier, 2002), and the strain ($E$ -
calculated using AMOCADO; Gerik and Kruhl, 2009) from the same samples from
Rengali. EBSD data were used as the basis for the above calculations. Whilst
the orientation of long axis of quartz grains from EBSD statistical data was
used to calculate $\kappa $, the grain boundary map generated from EBSD
analysis was used as the basis to determine strain ($E)$. It is found that the
sample with minimum P$_\mathrm{j}$ also has a minimum $\kappa $ and $E$, and
vice-versa. Hence it is concluded that one-to-one correlation exists between
the degree of magnetic anisotropy, strain and intensity of SPO in deformed
quartzites that have a negative magnetic susceptibility. Since the
application of AMS as a strain-intensity gauge in quartzites with mean
susceptibility below 50 x 10$^{-6}$ SI units has been questioned in the past
(Hrouda, 1986), the present findings open up a further avenue of research
that can be addressed using AMS.
\textbf{References:}
Gerik, A., Kruhl, J.H., 2009. Towards automated pattern quantification:
time-efficient assessment of anisotropy of 2D patterns with AMOCADO.
Computers and Geoscience 35, 1087-1097.
Hrouda, F., 1986. The effect of quartz on the magnetic anisotropy of
quartzite. Studia Geophysica et Geodaetica 30, 39-45.
Piazolo, S., Passchier, C.W., 2002. Controls on lineation development in low
to medium grade shear zones: a study from the Cap de Creus peninsula, NE
Spain. Journal of Structural Geology 24, 25-44.
Renjith, A.R., Mamtani, M.A., Urai, J.L., 2016. Fabric analysis of
quartzites with negative magnetic susceptibility -- does AMS provide
information of SPO or CPO of quartz? Journal of Structural Geology 82,
48-59.
Tarling, D.H., Hrouda, F., 1993. The Magnetic Anisotropy of Rocks. Chapman
and Hall, London, p. 217. |