Abstract
D.Sc
Neumayerskarvet forms a continuous outcrop of high-grade gneiss within the northern
Kirwanveggen in western Dronning Maud Land, Antarctica. A detailed geological study was carried out to obtain an evolutionary history for Neumayerskarvet. The work involved field mapping to provide a structural framework for further metamorphic and isotopic investigations. U-Pb zircon SHRIMP analysis, Rb-Sr, Sm-Nd and Ar-Ar mineral analysis were used to provide absolute time constraints on different tectono-metamorphic periods and cooling histories. Petrographic investigations, coupled with mineral chemistry on kyanite-bearing leucogneisses, provided information on the P-T conditions. An understanding of the crustal evolution of the high-grade gneisses was obtained through whole-rock geochemistry and isotope analysis. The dominant lithotectonic unit preserved at Neumayerskarvet is biotite-garnet migmatite gneiss, which is inter-fingered with quartzofeldspathic gneisses and banded quartz-feldspar gneisses. Several magmatic phases have intruded these sequences. Three tectonometamorphic
cycles have been established for the region. The first two cycles are assigned to a
period between 1390 Ma and 970 Ma while the third cycle is constrained between 650 Ma and 450 Ma. An age of ca. 1390 Ma for the biotite-gamet migmatite gneiss provides a maximum age for the first tectono-metamorphic cycle. Zircon growth and magmatism during this tectonometamorphic cycle constrains deformation (D1a) between ca. 1160 Ma and ca. 1110 Ma. Deformation is marked by the development of a penetrative planar foliation and isoclinal recumbent folding. High-pressure metamorphic conditions during this cycle have been suggested from previous investigations but are not confirmed in this investigation as the kyanite-bearing leucogneisses intruded during the second tectono-metamorphic cycle. It is possible that the first and second tectono-metamorphic cycles are part of a progressive deformational cycle. The second tectono-thermal cycle represents a major period of magmatism and tectonism constrained between ca. 1110 Ma and ca. 970 Ma. Major folding occurred during this tectonic episode, represented by isoclinal recumbent folds, sheath folds and re-folded fold interference patterns (D1b). The structural fabric elements produced a complicated relationship of transposed coplanar and colinear composite fabrics. Fabric geometries suggest NNW-SSE
tectonic transport directions. Garnet-kyanite-muscovite-biotite-quartz assemblages (Mn+1 (nkv)) provide P-T estimates of 710-760 °C and 7.8-8.5 kb. Later metamorphic assemblages of sillimanite-muscovite-high Ca-garnet-biotite-quartz (Mn+2 (nkv)) provide P-T estimates of 630- 690 °C and 6.0-7.4 kb. The whole-rock isotope data indicate that material accreted during the second tectono-metamorphic cycle experienced a short crustal residence time. The third tectono-metamorphic cycle is constrained by isotopic ages between 650 Ma and 450 Ma. Deformation (D2) that re-works earlier tectonic fabrics may represent signatures of this cycle, but the exact nature of the deformation remains enigmatic. Tectonic fabric styles and geometries are similar to the more dominant D1 tectonic episode, making recognition of temporal relationships difficult. Diffusional P-T data from garnet-biotite rims (Mn+3 (nkv)) provide P-T cooling estimates of 560-570 °C and 4.4-4.6 kb. Re-working of the high-grade gneisses during the third tectono-metamorphic cycle, with no addition or accretion of new crustal material is indicated by the isotopic data. A final tectonic episode (D3) comprising late brittle deformation and uplift is equated to Gondwana break-up.