Variations in orientation relationships between rutile inclusions and garnet host relate to magmatic growth zoning

TL;DR

This study shows how the orientation of rutile inclusions in garnet reflects changes in magmatic growth conditions.

Contribution

The paper introduces a new method to interpret magmatic growth zoning using crystallographic orientation relationships in rutile-garnet inclusions.

Findings

Rutile inclusions in garnet cores and rims show distinct crystallographic orientation relationships.

The shape and orientation of rutile needles correlate with specific garnet growth directions.

Variations in rutile microstructures reflect changes in melt composition during magmatic crystallization.

Abstract

Rutile inclusions in almandine-spessartine garnet from a peraluminous pegmatoid from the Moldanubian zone (Bohemian Massif, AT) show distinct changes in aspect ratio, shape preferred orientations (SPO) and crystallographic orientation relationships (COR) along the transition between microstructurally different growth zones in the garnet core and rim. For identification of the COR characteristics we pool specific CORs based on their common axial relationship into three COR groups: Group 103R/111G, Group 001R/111G and Group 001R/100G. The rutile inclusions in the garnet core domains are elongated along the four Grt\documentclass[12pt]{minimal}
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\begin{document}$$\rangle$$\end{document}⟩ directions and are dominated by COR Group 103R/111G. The garnet rim zone additionally contains rutile needles elongated along Grt\documentclass[12pt]{minimal}
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\begin{document}$$\rangle$$\end{document}⟩. Here, Group 001R/111G and 001R/100G are more abundant than in the garnet core. Needle-shaped rutile in the rim shows a systematic correlation between SPOs and CORs as needles elongated parallel to Grt\documentclass[12pt]{minimal}
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\begin{document}$$\rangle$$\end{document}⟩ are dominated by Group 103R/111G and 001R/111G, whereas those needles elongated parallel to Grt\documentclass[12pt]{minimal}
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\begin{document}$$\rangle$$\end{document}⟩ exclusively pertain to CORs of 001R/100G. Furthermore, the frequency of each particular SPO in the garnet rim clearly depends on the local growth direction of the particular Grt{112} sector. Facet-specific variations in rutile SPO frequencies in different sectors and growth zones of garnet were observed even between equivalent directions, indicating that the microstructures and textures of rutile inclusions reflect varying parameters of garnet growth. The characteristic differences in COR groups of different garnet growth zones are referred to compositional changes in the bulk melt or compositional boundary layer, associated with magmatic fractional crystallisation.

The online version contains supplementary material available at 10.1007/s00410-024-02146-9.

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