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

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 garnetbearing 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.

Melt Reintegration Modelling: Testing against a Subsolidus Reference Assemblage

“…This stepwise melt reintegration method is now well-established and has been used to remove melt or to rebuild protolith compositions in numerous studies e.g., [11,12,[17][18][19]. This method adds 6 mol% melt to the system when the modal proportion of melt in the initial composition reaches 1 mol%, equivalent to the 7% melt connectivity transition (MCT) of [46].…”

“…It is one of the few methods that allows investigation of the petrological and chemical evolution of deep-crustal rocks, as it provides an opportunity to define equilibrium relationships between melt and solid mineral species for particular bulk compositions at a given set of P-T conditions e.g., [13,14]. Several previous studies have utilised this technique to explore the effects of reintegrating melt back into lower-crustal rocks [16,18,19,41,78,79]. However, rarely do studies attempt to assess the veracity of the melt reintegration process to create valid subsolidus rock compositions.…”

“…However, significant underestimations of melt productivity may still occur, as exact protolith compositions are unknown in most cases. While useful, this process relies on assumptions regarding the prograde metamorphic path and volume of melt that has been lost and is commonly done up to a point at which a water-saturated solidus is achieved [18,19]. The determination of a final composition is unconstrained, as the prograde compositions often no longer exist for comparison, except in some rare cases e.g., [20,21].…”

Conflicting P–T paths within the Central Zone of the Limpopo Belt: A consequence of different thermobarometric methods?

“…High-grade rocks that have undergone partial melting inevitably loose a certain proportion of melt . The loss of melt from the system effectively alters the bulk-composition of the rock, which in turn will alter the P-T-X range of phase stability fields within the pseudosection (White et al, 2004). A melt re-integrated composition would be required for an accurate assessment of the prograde conditions during metamorphism.…”

“…However, the bulk-composition selected for pseudosection modelling is only appropriate for investigating peak-to-retrograde conditions, as potential melt loss at, or near to the thermal peak of metamorphism has not been taken into account (c.f. White et al, 2004). Therefore the prograde path, in terms of an exact P-T trajectory should be regarded with caution.…”