Metamorphic evolution of iron-rich mafic cumulates from the Ötztal-Stubai crystalline complex, Eastern Alps, Austria

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 (X_Mg^melt = 0· 57-0·65) 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 > 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 <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.