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

Olivine-rich rocks containing olivine + orthopyroxene + spinel + Ca-amphibole ‘ clinopyroxene ‘ garnet are present in the central Oštztal-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 (XMgmelt = 0ž 57-0ž65) 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 e0-680°C led to the formation of Ca-amphibole, chlorite and talc in the olivine-rich rocks. In the garnet layers, hošgbomite 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