6559 20 archive 266 disk0/00/00/65/59 2008-06-02 10:52:05 2008-07-18 11:00:10 2008-07-16 17:26:51 article show jahalpin@utas.edu.au White RW rwhite@uni-mainz.de Powell R powell@unimelb.edu.au Halpin JA jahalpin@utas.edu.au pub 040304 260102 restricted diffusion; granulite facies; metapelite; NCKFMASHTO; partial melting The definitive version is available at www.blackwell-synergy.com Large garnet poikiloblasts hosted by leucosome in metapelitic gneiss from Broken Hill reflect complex mineral–melt relationships. The spatial relationship between the leucosomes and the garnet poikiloblasts implies that the growth of garnet was strongly linked to the production of melt. The apparent difficulty of garnet to nucleate a large number of grains during the prograde breakdown of coexisting biotite and sillimanite led to the spatial focussing of melting reactions around the few garnet nuclei that formed. Continued reaction of biotite and sillimanite required diffusion of elements from where minerals were reacting to sites of garnet growth. This diffusion was driven by chemical potential gradients between garnet-bearing and garnet-absent parts of the rock. As a consequence, melt and peritectic K-feldspar also preferentially formed around the garnet. The diffusion of elements led to the chemical partitioning of the rock within an overall context in which equilibrium may have been approached. Thus, the garnet-bearing leucosomes record in situ melt formation around garnet porphyroblasts rather than centimetre-scale physical melt migration and segregation. The near complete preservation of the high-grade assemblages in the mesosome and leucosome is consistent with substantial melt loss. Interconnected networks between garnet-rich leucosomes provide the most likely pathway for melt migration. 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