Mid crustal processes : a study of fluid flow and deformation in the Renegar Glacier region, Southern Victoria Land, Abtarctica

Simpson, Guy D. H. (Guy Donald Hugh), 1970-

Basement lithologies which form the basis of this report include Koettlitz Group metasediments along with diorites and gabbros of the Renegar Pluton. Metasediments are typically highly calcareous and include pelitic schist, mafic schist, felsic schist, calc-silicate, and marble. Also common are metamorphic veins which consist of quartz (dominant), feldspar, and calcite cores surrounded by a reaction zone comprising amphibole and/or diopside, plagioclase, quartz, and titanite. The Renegar Pluton forms a small body (approx. 4 x 10 km) which is elongate in a NE/SW orientation. lt is intruded into metasediments, the foliations of which are closely parallel to the margins of the pluton. Throughout the area mapped there is a close correlation between the extent of transposition, and the abundance of both metasomatic-derived calc-silicates, and metamorphic veins. Structural transposition aids in metasomatism by not only increasing the surface area of marble available for reaction but also by increasing the rate of reaction. Metasomatic decarbonation reactions then release large quantities of C02 (and Ca) rich fluid into the adjacent rocks to form veins. Reaction zones developed either side of the veins are the result of dehydration of the host rock, induced by an increase of xco2. In the less deformed regions chemical exchange between the vein and host rock can be described in terms of a simple closed system reaction approximated by diffusion metasomatic processes. However in intensely deformed regions higher fluid velocities were maintained and chemical components were lost from the rock as the result of veining. Such veins are best described in terms of infiltration metasomatism. Fluids were preferentially focussed within incompetent lithologies. However, within intensely veined units the addition of veins has had a strengthening effect on the rock type. The result is that further strain (and veining) is focussed elsewhere. The Renegar Pluton is an elongate reversely zoned body which comprises a core of gabbro cumulates surrounded by gabbroic differentiates and an outer ring of diorites. The contact between the gabbroic rocks and the diorites is highly intermixed forming a steeply dipping foliated zone which is parallel to the margins of the gabbro core. The rocks within the contact zone have undergone chemical mixing to form a suite of rocks which are chemically transitional to both the diorites and the non-cumulate gabbros. The diorites may have been derived by extraction of a combination of plagioclase, clinopyroxene, olivine (and minor orthopyroxene) from a gabbroic liquid. The gabbro cumulates are consistent with this. All rocks of the Renegar Pluton have undergone varying degrees of mineral reconstitution. Recrystallisation can be subdivided into two stages. The first of these probably occured late in the crystallisation history of the pluton, and involved uralitization. The second stage of recrystallisation was associated with deformation around the margin of the pluton. This involved the conversion of amphibole to biotite to form a foliated diorite-like rock termed the foliated marginal zone (FMZ). Despite being extensively recrystallised, these rocks have identical geochemistry to the undeformed diorites. Rocks of the Renegar Pluton are chemically and mineralogically indistinguishable from both the Dromedary Gabbro and the Dromedary Diorite. These rocks do not, however, appear to be genetically related to the Glee intrusives. This suite was probably formed by mixing the most mafic member of the Glee intrusives with a granite.

1 v. (various pagings) : ill. (some col.), maps ; 30 cm.

1994Simpson