Spatially-focussed melt formation in aluminous metapelites from Broken Hill, Australia

R. W. White, R. Powell, J. A. Halpin*

*Corresponding author for this work

    Research output: Contribution to journalArticle

    186 Citations (Scopus)

    Abstract

    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 centimetrescale 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. Decimetre-scale, coarse-grained,garnet-poor leucosomes may represent areas of melt flux through a large-scale melt transfer network.

    Original languageEnglish
    Pages (from-to)825-845
    Number of pages21
    JournalJournal of Metamorphic Geology
    Volume22
    Issue number9
    DOIs
    Publication statusPublished - Dec 2004

    Keywords

    • Diffusion
    • Granulite facies
    • Metapelite
    • NCKFMASHTO
    • Partial melting

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