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Spatially-focussed melt formation in aluminous metapelites from Broken Hill, Australia
journal contribution
posted on 2023-05-17, 10:28 authored by White, RW, Powell, R, Jacqueline HalpinJacqueline HalpinLarge 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. © 2004 Blackwell Publishing Ltd.
History
Publication title
Journal of Metamorphic GeologyVolume
22Issue
9Pagination
825-845ISSN
0263-4929Department/School
School of Natural SciencesPublisher
Blackwell Publishing LtdPlace of publication
9600 Garsington Rd, Oxford, England, Oxon, Ox4 2DgRights statement
The definitive published version is available online at: http://www3.interscience.wiley.com/Repository Status
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