Evolution of the Median Batholith, Fiordland, New Zealand

The Median Batholith is the remains of a Mesozoic magmatic arc now exposed in Fiordland, New Zealand. There are three groups of plutonic rocks found in the area; the Triassic-Cretaceous Darran Suite; the Cretaceous Separation Point and Western Fiordland Orthogneiss suites. The Darran Suite rocks are LoSY, whilst the younger rocks are HiSY. During fieldwork samples were collected which were subsequently identified largely on the basis of their High Sr and low Y concentrations. The suites have distinctive rare earth element patterns, with the Separation Point Suite being depleted in HREE relative to the other suites. This is interpreted as being due to partial melting at a depth where garnet was stable, but plagioclase was not. Geochemically the Western Fiordland Orthogneiss occupies an intermediary position between the Darran and Separation Point Suites. Some of the Western Fiordland Orthogneiss rocks were depleted in Rb, Th, U and K and this has been interpreted as partial melting in a region that has already experienced melt depletion. Initial isotope ratios indicate that the suites have not all come from a single source from which they have all fractionated. Sr-Nd modelling reveals that the isotopic signatures of the rocks could be produced by the mixing of depleted mantle with a very small amount of metasediment from the area. Whilst this is possible, it is not the only or most likely mechanism in the genesis of these rocks.