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Olivine-rich rocks containing olivine + orthopyroxene + spinel + Ca-amphibole ± clinopyroxene ± garnet are present in the central Ötztal–Stubai crystalline basement associated with eclogites of tholeiitic affinity. These rocks contain centimetre-sized garnet layers and lenses with garnet + clinopyroxene ± corundum. Protoliths of the olivine-rich rocks are thought to be olivine + orthopyroxene + spinel dominated cumulates generated from an already differentiated Fe-rich (  \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(X_{\mathrm{Mg}}^{\mathrm{melt}}\ =\ 0{\cdot}57{\mbox{--}}0{\cdot}65\) \end{document}  ) tholeiitic magma that was emplaced into shallow continental crust. Protoliths of the garnet-rich rocks are interpreted as layers enriched in plagioclase and spinel intercalated in a cumulate rock sequence that is devoid of, or poor in, plagioclase. U–Pb sensitive high-resolution ion microprobe dating of zircons from a garnet layer indicates that emplacement of the cumulates took place no later than 517 ± 7 Myr ago. After their emplacement, the cumulates were subjected to progressive metamorphism, reaching eclogite-facies conditions around 800°C and &gt;2 GPa during a Variscan metamorphic event between 350 and 360 Ma. Progressive high- P  metamorphism induced breakdown of spinel to form garnet in the olivine-rich rocks and of plagioclase + spinel to form garnet + clinopyroxene ± corundum in the garnet layers. Retrogressive metamorphism at  T  ≤ 650–680°C led to the formation of Ca-amphibole, chlorite and talc in the olivine-rich rocks. In the garnet layers, högbomite formed from corundum + spinel along with Al-rich spinel, Ca-amphibole, chlorite, aspidolite–preiswerkite, magnetite, ilmenite and apatite at the interface between olivine-rich rocks and garnet layers at  P  &lt; 0·8 GPa. Progressive desiccation of retrogade fluids through crystallization of hydrous phases led to a local formation of saline brines in the garnet layers. The presence of these brines resulted in a late-stage formation of Fe- and K-rich Ca-amphibole and Sr-rich apatite, both characterized by extremely high Cl contents of up to 3·5 and 6·5 wt % Cl, respectively.

Metamorphic Evolution of Iron-rich Mafic Cumulates from the Ötztal–Stubai Crystalline Complex, Eastern Alps, Austria