Polymineralic synneusis in silicic magmas revealed by megacryst-inclusion orientation relationships

Charlotte Gordon , David Wallis

Crystal aggregation via synneusis is well-documented in many common igneous minerals, such as quartz and olivine. Synneusis is typically described as a monomineralic process whereby two crystals of the same mineral adhere in specific low-energy orientation relationships. In contrast, unlike minerals are generally assumed to be antipathetic. Nonetheless, the occurrence of polymineralic synneusis (systematically oriented attachment of unlike mineral pairs) has been postulated but never quantitatively demonstrated. Therefore, the occurrence and characteristics of polymineralic synneusis, and its impacts on the behaviour of magmatic systems, remain poorly understood.
We apply spatially correlated microstructural analyses, including electron backscatter diffraction and cathodoluminescence mapping, to test whether polymineralic crystal aggregates can form via synneusis. The abundant mineral inclusions within K-feldspar megacrysts in granites have previously been postulated to arise from polymineralic synneusis, therefore we chose to study examples of this texture from two unrelated plutons. Textural observations confirm that inclusions of plagioclase, amphibole and various accessory minerals grew independently in the magma before attaching to the megacrysts.
Analysis of the crystallographic orientations of plagioclase, amphibole, titanite and magnetite relative to their host K-feldspar reveal variably systematic orientation relationships. Plagioclase, amphibole and titanite show distinct suites of orientations within each growth sector of the K-feldspar, while magnetite does not. Titanite orientations appear to be controlled mainly by shape, whereas amphibole orientations cannot easily be explained by shape alone, so may also have a crystallographic control. Plagioclase orientations are tightly crystallographically controlled due to the lattice similarities between K-feldspar and plagioclase. Some of the observed K-feldspar-plagioclase orientation relationships follow established feldspar twin laws, which represent low-energy grain interfaces. However, we also observe orientation relationships that have not been documented as growth twins and are diagnostic synneusis relationships. In particular, the predominance of combined Baveno-type relationships resulting from the attachment of (010) and (001) feldspar faces appears to be unique to synneusis. We infer that crystals encountering each other in the magma attached, and then rotated into more favourable orientations, due to the minimisation of interfacial energy.
Synneusis has been considered diagnostic of low-viscosity, low-crystallinity magmas, but our results demonstrate that this is not always the case, as the megacrysts grew in relatively viscous, silicic, crystal-rich magma. The occurrence of polymineralic synneusis even in these granitic systems suggests that it may be an important and widespread magmatic process, creating crystal clusters that potentially impact both the rheological behaviour and geochemical evolution of the host magma.