RECOVERING FLOW FABRICS IN DIATEXITE: LOW-FIELD ANISOTROPY OF MAGNETIC SUSCEPTIBILITY (AMS) IN THE NAXOS MIGMATITE DOME, GREECE

Gneiss domes are common in orogens and are typically cored by high melt-fraction migmatite (diatexite with > 30% melt). However, structural studies of diatexite are commonly hindered by weak to absent mappable foliation and especially lineation. We apply the anisotropy of magnetic susceptibility (AMS), widely used in fabric studies of granitoids, to recover magnetic foliation and lineation in diatexite, and to test the AMS technique as a petrofabric tool to evaluate flow within partially molten crust. The Naxos dome (Greece) developed during Alpine extension and is mantled by a sequence of schist and marble. Migmatite foliation, defined by thin biotite layers, outlines a broad dome (12x4 km) containing at least one km-scale subdome partially wrapped by an entrained marble layer.

Bulk magnetic susceptibilities, measured in low field on 32 diatexite samples (~12 cubes per sample) in the subdome, are generally low (2-100 x 10-6 SI), suggesting the magnetic properties are likely carried by biotite. The magnetic anisotropy factor is up to 30% and is positively correlated with increasing oblate shape of the AMS ellipsoid. AMS data show a strong correlation between the macroscopic foliation orientation and the magnetic foliation. The shape of the AMS ellipsoid, the bulk susceptibility, and the directions of the AMS principal axes vary with position in the subdome. Magnetic lineation shows the most systematic variation; it is steeply plunging in the core of the subdome and against the marble bordering the subdome on the west, whereas more intermediate to shallowly plunging magnetic lineations girdle the subdome on the other sides. The planar and linear magnetic fabric obtained from AMS, and the somewhat systematic spatial variation of these data in the sampled subdome, indicates that AMS is a sensitive indicator of rock fabric in diatexite. Therefore, the AMS technique significantly contributes to the study of migmatites by tracking flow planes and directions in order to better understand the dynamics of partially molten crust and the development of gneiss domes.