Segregation of leucogranite microplutons during syn-anatectic deformation: an example from the Taylor Valley, Antarctica

A suite of migmatites in uppermost amphibolite facies schists of the Koettlitz Group exposed in the Taylor Valley, Antarctica, provides direct evidence of the behaviour of partially molten rock during syn-anatectic deformation. The geometry of the migmatites is directly related to their position relative to the hinge of a kilometre-scale antiform. Migmatitic rocks on the fold limbs are characterized by extensional shears and fractures, filled with leucosome material, that intersect the pervasive foliation and millimetre-thick stromatic leucosomes. Vein- and dyke-like leucosomes become more common and thicker from the limb to the hinge region of the antiform. Rocks characterized by high leucosome-to-rock ratios near the antiform hinge are xenolithic in appearance. Major parasitic folds within the hinge contain leucogranite 'microplutons' up to 50 m across beneath refractory 'cap-rock' layers.
Angular boudinage structures in schists surrounded by leucosomes indicate a relatively low yield strength in the leucosome, which is compatible with a molten rather than solid leucosome. Leucogranite-bearing extensional shears and fractures indicate that repeated extensional fracturing and shearing promoted by high fluid (melt) pressure is an important mechanism of melt segregation. Dilation in the hinges of developing folds aids the migration of melt into fold hinges and the development of 10–50-m-wide 'microplutons' of xenolith-rich leucogranite.
Lack of vapour-absent melting and consequent low melt-to-rock ratios allowed the Koettlitz Group to maintain its structural coherency on a kilometre scale. Consequently, leucosome 'microplutons' did not exceed 50 m in width, and therefore observed leucosomes have not contributed to the development of adjacent plutonic-scale granitoids.