Koettlitz group meta-sediments and intercalated orthogneisses from the mid Taylor Valley and Ferrar Glacier regions

Abstract

Koettlitz Group rocks and intercalated orthogneisses in the mid Taylor Valley and Ferrar Glacier regions form a 1-4km wide, north-northwest trending belt, intruded by a variety of undeformed granitoid pIutons along either side. Two phases of deformation affect the Koettlitz Group and intercalated orthogneisses. The earlier phase resulted in isoclinal folding, transposition of Koettlitz Group-orthogneiss contacts and development of a pervasive foliation in all lithologies, while the second phase produced open to tight, upright, shallow plunging, north-northwest trending folds on all scales and minor, sporadic crenulation cleavage in psammitic schists.

The Koettlitz Group rocks are dominated by marbles and migmatitic quartzofeldspathic to psammitic schists with lesser amounts of amphibolite, calc-silicate rock, non-migmatitic quartzofeldspathic to psammitic schists and very minor amounts of pelitic and semi-pelitic schist. Field relations and assemblages indicate that calc-silicate rocks are products of metasomatism, but most amphibolites are metamorphosed, basic, igneous rocks.

Sporadic development of orthopyroxene-bearing granulite fades assemblages is controlled by metre scale variations in aH₂O and possibly bulk rock composition. Assemblages developed in pelitic and semi-pelitic rocks are characteristic of the uppermost amphibolite facies. Geothermobarometry based on the assemblage; sillimanite - garnet - quartz - cordierite - plagioclase biotite - K-feldspar, found at Nussbaum Riegel , indicate peak metamorphic temperatures between 675°C and 705°C and pressures between 3.8kb and 5.7kb. Garnet-biotite geothermometry using semi-pelitic rocks from other localities and an estimated pressure of 4.8kb yields temperatures between 679°C and 714°C.

Granitic to gabbroic orthogneisses intercalated with the Koettlitz Group have chemistries suggestive of active plate margin settings, unlike ultramafic, alkaline amphibolites within the Koettlitz Group whose chemistries are suggestive of intraplate settings. Characteristics of mineral inclusions in augen from augen gneiss intercalated with the Koettlitz Group indicate that augen gneiss is a deformed, pre-F₁ intrusive and not the product of metasomatism.

Segregation of leucosome material in some meta-sediments and orthogneisses was promoted by extensional fracturing and tensional shearing on the limbs of some F₂ folds, caused by high grain boundary fluid pressure and appropriate strain rates. Initial segregation of leucosome material by compaction and filter pressing mechanisms may have aided leucosome infiltration of shears and fractures. Boudinage structures in amphibolite and psammitic schist layers surrounded by leucosome indicate leucosomes possessed a much lower yield stress and were therefore molten. Repeated failure along shears and fractures produced dikes, sills and veins of anatectic melt up to 50cm thick. Dilation in fold hinge zones during deformation promoted propogation of these sills and dikes towards the hinges of F₂ folds, resulting in the development of "microplutons" of xenolith-rich anatectic leucogranite up to 50 metres across beneath impermeable, refractory layers of amphibolite and marble. Continuing deformation of fold hinges containing incompressible, molten, "microplutons" of anatectic melt caused refractory "cap rock" layers to fracture and allowed intrusion of anatectic leucogranite dikes through the fold hinge. Low melt fractions prevailing during anatexis prevented the small amounts of anatectic leucogranite from forming pluton scale bodies.