Petrographic and geochemical characterisation of a layered garnetite-pyroxenite xenolith and of garnet in the Breaksea Orthogneiss, Fiordland New Zealand

The Breaksea Orthogneiss (BOG) represents the deepest pluton of the Western Fiordland Orthogneiss (WFO) suite. It has been emplaced and recrystallized at P-T conditions around1.8GPa and 850°C and shows structural and compositional properties characteristic for the transition zone of granulite and eclogite facies. Extensive parts of the BOG are composed of plagioclase, garnet and omphacite. This study presents an investigation of one xenolith in BOG which is a layered garnetite-pyroxenite and the examination of garnet in different textural settings in the surrounding host rock, which is an omphacite-granulite. Polished thin sections produced from samples collected on a fieldtrip in February 2010 have been examined by the petrographic microscope and by electron microprobe (EMP). The results have been compared to measurements from previous workers, which also include trace element data. For reasons of comparison a data base has been produced. This contains data of comparable rocks found in the literature and includes a selection of related literature. It is added on a CD to this work. Three texturally different kinds of garnet have been recognized. Geochemical and petrographic properties indicate that two of them may be metamorphic. One cannot definitely be assigned to either an igneous or metamorphic process but from the textural arrangement an igneous origin is conceivable. From the tectonic, petrographic and geochemical context it seems also likely that the layered garnetite-pyroxenite is igneous and cognate. But evidence for that remains ambiguous. Based on these analyses an interpretation of the origin of different garnets in the omphacite-granulite and a hypothetical model for the origin of the layered garnetite-pyroxenite xenolith is presented. In addition to that, density measurements of diverse rock types of the BOG which were collected on a fieldtrip in February 2009 have been carried out. Furthermore, zircon mineral grains have been separated from one sample of omphacite-granulite and U-Pb as well as Hf isotope-analyses have been made by inductively coupled plasma mass spectrometry (ICP-MS) in order to date the host rock. The achieved Pb206/U238 age of 116 Ma might be too young and probably results from Pb-loss and metamorphic neo-crystallisation of zircon.