$$^{40}Ar/^{39}Ar$$ Mineral Age Constraints on the Paleozoic Tectonothermal Evolution of High-Grade Basement Rocks within the Ross Orogen, Central Transantarctic Mountains

Abstract

The Precambrian to early Paleozoic orogenic history of the central Transantarctic Mountains is obscured by uncertainties in. the regional extent, relative intensities, and tectonic significance of three or more events: the Nimrod, Beardmore, and Ross orogenies. Significant problems exist in correlation of structural styles, structural geometry, and lithotectonic units across major tectonic boundaries, as well as in conflicting geochronological results. In an effort to resolve these disparities, ages were determined for hornblende and muscovite from several lithotectonic subunits of the Nimrod Group exposed in the Miller Range, including amphibolitic gneiss, ca. 1.7 Ga orthogneiss, and metasedimentary mica schist. These rocks display ductile deformation fabrics that formed prior to intrusion of ca. 500 Ma, post-kinematic granitic plutons. Hornblende concentrates from eight gneiss samples in the Miller Range display internally discordant apparent age spectra due to variable intracrystalline contamination with extraneous argon components (total-gas ages range between 510 and 738 Ma). vs. isotope correlations are generally well defined (MSWD < 2.0) and yield ages between ca. 525 and 487 Ma. Two muscovite concentrates from mica schist samples in the Miller Range record plateau ages of 508 and 486 Ma, which are interpreted to date cooling through temperatures required for intracrystalline retention of argon. The data indicate that conventional K-Ar dates (up to 1100 Ma) previously reported for hornblende from the Miller Range probably have no geologic significance because of the widespread presence of extraneous argon. They cannot, therefore, be regarded as dating any "pre-Ross" event(s). Intracrystalline argon systems in samples from this study appear to have been completely rejuvenated during granite emplacement associated with the Ross orogeny. Structural fabric elements were partially annealed in many dated Nimrod Group samples, likely reflecting contact effects imposed by emplacement of post-tectonic granite plutons at ca. 500 Ma.