Selective entrainment of peritectic garnet into S-type granitic magmas: Evidence from Archaean mid-crustal anatectites

摘要

Entrainment of restite is commonly invoked to explain both the origin of relatively mafic granites and granodiorites, as well as the chemical connection between granite magmas and their sources. This concept has become linked to models for magma migration out of the source, as restite entrainment is considered to take place when diatexitic sources mobilise en masse. This is at odds with the common occurrence of relatively mafic granites as high level intrusions in the crust or their eruptive equivalents that must have formed from markedly water-undersaturated magmas that ascended through narrow conduits. We investigate pelitic migmatites from the Mkhondo Valley Metamorphic Suite (MVMS) in Swaziland, where a mid-crustal heating event produced metatexitic migmatites with minimal post-anatectic recrystallisa-tion. In these rocks all the garnet is peritectic, having arisen through biotite fluid-absent melting, which produced garnet poikiloblasts characterised by inclusions of melt, quartz and biotite. Leucosomes that represent sites of melt transfer carry similar, smaller (typically <1 mm), entrained garnet poikiloblasts, which were capable of amalgamating to form larger composite grains. In anatectic structures where melt was present for longer, entrained garnet was extensively recrystallised, via a dissolution-precipitation process, to adopt a more magmatic character. The peritectic garnet in the pelitic source appears to have grown out of equilibrium with feldspar and HREE-rich accessory phases, while the recrystallised garnet in the larger melt-filled structures became progressively better equilibrated with these minerals. Thus, peritectic garnet in the source grew sufficiently rapidly to prevent trace element equilibrium with the bulk-rock composition, and, concurrent rapid magma segregation prevented the development of diatexitic source conditions. The segregated magma consisted of melt, the peritectic assemblage (principally garnet) and the accessory minerals monazite and zircon. These rocks illustrate that mafic granites may arise purely as mixtures of melt and the peritectic assemblage produced by the incongruent melting reaction. Importantly, under the circumstances which produced the MVMS anatectites, peritectic garnet is entrained as <1 mm poikiloblasts, demonstrating how mafic granitic magmas can migrate out of the source without the source becoming diatexitic.