FLOW OF PARTIALLY MOLTEN CRUST RECORDED BY THE ANISOTROPY OF MAGNETIC SUSCEPTIBILITY IN LEUCOGRANITES

The Shuswap metamorphic core complex, British Columbia exposes a ~15 km thick structural section composed of an upper unit that preserves Mesozoic rocks and structures, separated from underlying high-grade rocks by a low-angle detachment zone. The high-grade rocks in the Thor-Odin region of the Shuswap metamorphic core complex are intruded by a network of leucogranitic sills and dikes, which feed into large laccoliths at the level of the detachments. Geochronologic, petrologic, and structural data suggest that leucogranitic melt was produced during partial melting of the crust coeval with high-temperature decompression. Both solid state and magmatic state fabrics are recorded in the leucogranites with mylonitic fabrics developed in the detachment zone. In order to determine the kinematics of flow of the partially molten crust we have used the anisotropy of magnetic susceptibility (AMS) to characterize the regional fabric recorded by the leucogranites.

All the leucogranites display a low magnetic susceptibility (K) and three main types are distinguished: (1) diamagnetic garnet-bearing leucogranites with K < 10 ^-6 SI; (2) paramagnetic biotite-muscovite-bearing granites ± magmatic andalusite with K = 10 to 30.10 ^-6 SI; (3) ferromagnetic pyrrhotite-biotite-bearing granites with K = 30 to 100.10 ^-6 SI. The degree of anisotropy (P) ranges from 1.02 to 1.10 and increases slightly with K. Most stations comprise one only of the above magnetic types.

The diamagnetic types (10% of stations) have a small within-station AMS fabric consistency due to the low intrinsic anisotropy of quartz and feldspars. The ferromagnetic types (40%) have a large variability due to presence of a ferrimagnetic carrier in trace amounts only. The paramagnetic types (50%) display consistent AMS fabrics, and are concordant with foliations and lineations measured in the field. Therefore, leucogranites in which the magnetic carriers are paramagnetic can be used to characterize regional fabrics in metamorphic core complexes. These data can provide important kinematic information for the deformation of partially molten crust.