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

Abstract

Olivine-rich rocks containing olivine þ orthopyroxene þ spinel þ Ca-amphibole clinopyroxene garnet are present in the central Otztal–Stubai crystalline basement associated with eclogites of tholeiitic affinity. These rocks contain centimetre-sized garnet layers and lenses with garnet þ clinopyroxene corundum. Protoliths of the olivine-rich rocks are thought to be olivine þ orthopyroxene þ spinel dominated cumulates generated from an already differentiated Fe-rich (X Mg 1⁄4 0 57– 0 65) tholeiitic magma that was emplaced into shallow continental crust. Protoliths of the garnetrich 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 7Myr 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 360Ma. Progressive high-P metamorphism induced breakdown of spinel to form garnet in the olivine-rich rocks and of plagioclase þ spinel to form garnet þ 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, hogbomite formed from corundum þ 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 8GPa. 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 Feand 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.