Mafic schlieren, crystal accumulation and differentiation in granitic magmas: an integrated case study

Abstract

Observations in the S-type granites (s.l.) of the Wilson’s Promontory batholith demonstrate that one type of schlieren in granitic rocks represent accumulations of mainly mafic magmatic minerals, with internal layering formed through pulsed magma flow. Loss of interstitial magmatic liquid played, at most, a minor role in shaping the preserved compositions of the schlieren; filter pressing was not involved, and simple gravity settling of crystals was also insignificant. Through dissolution-reprecipitation and reactions with residual magmatic liquids in the Wilsons Promontory schlieren, the original accumulated crystals of mafic minerals were largely supplanted by later generations or completely new phases. In the present case, the original accumulating minerals were garnet and orthopyroxene, with minor biotite and accessory minerals. The schlieren retain some of the early, euhedral, compositionally distinct, accumulated biotite, but most biotite formed through reaction of accumulated orthopyroxene and garnet with residual liquid. Some early, accumulated garnet remains, but this is not the peritectic garnet that was originally entrained into the magmas, at source depths. Rather, these are magmatic crystals formed, at mid-crustal depths, through dissolution–reprecipitation of the original peritectic garnet. At emplacement level, another episode of garnet dissolution–reprecipitation occurred, close to the solidus, extensively reorganising the grain-scale igneous textures. Although schlieren preserve structural, chemical and some textural features that can be used to infer their origins, their present microtextures do not fully reflect their initial formation mechanisms. Also, the physical mechanisms of schlieren formation most commonly have little similarity with the processes that were responsible for the main chemical variations in the batholith. Nevertheless, the presence of schlieren indicates that the granitic magmas were flowing in sheet-like laminae, and they provide information on the high-temperature igneous minerals that were suspended in their parent magmas, prior to emplacement.