Disequilibrium melting of plagioclase during biotite fluid-absent anatexis of metapelites in the South Marginal Zone of the Limpopo belt, South Africa

[摘要] ENGLISH ABSTRACT: This study investigated the behaviour of plagioclase during fluid-absent biotite melting in a metapelitic source from which peraluminous granite magma had segregated. The study focused on the Bandelierkop Quarry (South Marginal Zone of the Limpopo belt), where stromatic and discordant nebulous leucosomes contain peritectic garnet crystals produced by the following melting reactions; Bt + Sil + Qtz + Pl = Grt + Ilm + Melt; and, Bt + Qtz + Pl = Grt + Ilm + Opx + Crd + Melt. In rare cases, large (10 to 20mm) garnet crystals within both leucosome types are growth zoned. These garnet crystals host plagioclase inclusions, which are commonly euhedral and occur as single crystals, clusters of crystals and as polymineralic inclusions which typically consist of biotite, rutile and orthopyroxene. The inclusions display considerable chemical heterogeneity. Matrix plagioclase in both leucosome types ranges in composition from An27 to 35, whilst that in the residuum adjacent to the leucosomes ranges from An32 to 39. Euhedral plagioclase inclusions are considerably more calcic with compositions that range from An50 to 83. Composition of the inclusions shows no relationship with position within the garnet crystal, or with size of the plagioclase crystal. These crystals are also significantly zoned, yet their zoning is not systematic, with inclusions displaying both Na- and Ca-enriched rims. Garnet zonation around the inclusions was investigated in detail and demonstrates that the Ca-rich nature of the inclusions is not a consequence of Ca-uptake from garnet. In contrast, anhedral plagioclase within large amoeboid polymineralic inclusions displays compositions similar to the matrix plagioclase. Garnet also hosts very small (≤ 10μm) inclusions of quartz + plagioclase ± biotite. These micro-granite inclusions are interpreted as melt inclusions.The complexity of plagioclase behaviour recorded within the peritectic garnet crystals is interpreted to reflect disequilibrium during anatexis due to slow diffusion in plagioclase; only the outer portions of the plagioclase crystals participate in the melting reaction and they do so by combined dissolution and precipitation of new, Ca-rich crystals. Modelling demonstrates that the Ca-contents of these new crystals is inversely proportional to the amounts of plagioclase that participate in the reaction. Consequently, the euhedral plagioclase inclusions are interpreted to be peritectic, where these occur as polymineralic inclusions they are small and plagioclase is volumetrically dominant, so the melt volume in the inclusion was low. The more Ca-rich rims in euhedral plagioclase inclusions may reflect zonation produced by the melting reaction, whilst Na-rich rims may reflect crystals that are too calcic to equilibrate with melt within the time-scale available for melting , the volume of equilibrium is extremely locally controlled, which in turn induces Na-rich rims. The preservation of these garnet crystals, possibly due to rapid and efficient melt loss from the leucosome structures, provides a rare insight into the details of the anatectic process. In the vast majority of migmatitic granulites these are lost due to comprehensive extensive recrystallization and homogenization of mineral compositions.

[发布机构] Stellenbosch University