Magnetic susceptibility anisotropy: A new petrofabric tool in migmatites

Abstract

[1] The rheological behavior of the lower to middle continental crust during orogenic deformation is poorly understood. Migmatite, common at this crustal level, remains difficult to analyze from a structural viewpoint because of pervasive recrystallization. We present new anisotropy of magnetic susceptibility (AMS) data from 217 specimens of migmatites of the Superior Province in Minnesota with the aim of investigating its potential as a petrofabric tool. Quarry exposures provide the ideal sampling playground for such a study. The migmatites are coarse grained, are granitic to tonalitic in composition, and display a centimeter-scale compositional layering. The layering and the regional foliation are subhorizontal to shallowly dipping to the NE. Mineral lineations are difficult to observe due to recrystallization but trend around 70°N with gentle plunges. Magnetic properties are carried mainly by multidomain (MD) primary magnetite grains ranging in size from 20 to 200 mm. The magnetic susceptibilities are generally high, around (5-10) Â 10 À3 SI. The degree of magnetic anisotropy is large, between 1.2 and 1.3. The principal axes of the AMS ellipsoids coincide with those of the finite strain, and the degree of anisotropy broadly correlates with the mineral fabric intensity. This study is the first detailed investigation of magnetic fabrics in migmatites. These preliminary results suggest that the AMS technique is an easy and powerful tool for the petrofabric analysis of migmatites.

Viscoplastic flow in migmatites deduced from fabric anisotropy: An example from the Naxos dome, Greece

“…The value of P′ is generally low (<1.2) below the K para* but increases greatly for bulk magnetic susceptibilities greater than the K para* . P′ is positively correlated with bulk susceptibility in other studies [e.g., Rochette et al, 1992;Bouchez, 1997;Ferré et al, 1997Ferré et al, , 2003 and is interpreted as resulting from mixing of paramagnetic and ferromagnetic contributions.…”

“…These cubes were cut along the horizontal, N-S vertical, and E-W vertical planes according to the procedure described by Ferré et al [2003Ferré et al [ , 2004.…”

“…[26] In migmatites where the magnetic fabric is controlled by magnetite, the low-field AMS is a good proxy for the shape fabric of magnetite grains during high-temperature plastic flow [Ferré et al, 2003[Ferré et al, , 2004. To elucidate the relationship between the subfabric of biotite and the origin of low-field AMS in the dominantly paramagnetic Naxos migmatites, we have investigated the lattice-preferred orientation (LPO) of biotite using electron backscatter diffraction (EBSD).…”

“…Our proposed methodology builds upon relatively few other investigations that evaluate the relationship between structural and magnetic fabric in migmatites. Ferré et al [2003Ferré et al [ , 2004 provided the first detailed analysis of magnetic fabric in dominantly ferromagnetic migmatites, documenting a strong correlation between the principal axes of the AMS and the shape fabric of magnetite grains developed during high-temperature plastic flow. More recently, studies of the AMS technique in migmatites have focused on tectonic applications [Teyssier et al, 2005;Charles et al, 2009] or on evaluating relationships between AMS fabric and evolving mechanical anisotropy during progressive melting [Hasalová et al, 2008;Schulmann et al, 2009].…”

Evolução estrutural e térmica de um batólito sin-cinemático no orógenos Neoproterozóico Araçuaí (leste do Brasil)

“…A large body of data supports that AMS measurements, if used carefully, may be a reliable and therefore useful tool to map the internal fabric in magmatic, migmatitic and metamorphic rocks (e.g. Borradaile, 1988;Henry, 1989;Tarling and Hrouda, 1993;Borradaile and Henry, 1997;Bouchez, 2000;Borradaile, 2001;Ferré et al, 2003;Borradaile and Jackson, 2004).…”