Evidence for Late Carboniferous subduction-type magmatism in mafic-ultramafic cumulates of the SW Tauern window (Eastern Alps)

Abstract.

Hectometric bodies of fresh mafic-ultramafic cumulates have been discovered within the Central Gneiss of the Zillertal massif, SW Tauern window (eastern Alps, Italy). The cumulates, intruded by the Central Gneiss granitoids, are amphibole-bearing harzburgites and norites made of cumulitic olivine (Fo_73–80), spinels, sulphides and plagioclase (An_79–87), included in orthopyroxene (En_76–83) and Ti-pargasite (Mg#=0.73–0.81). Major and trace element geochemistry indicates that these rocks represent olivine + spinel ± plagioclase cumulates, in which interstitial melt crystallized as orthopyroxene + Ti-pargasite. The parental melt has trace element patterns typical of subduction zone magmas. The crystallization sequence, mineral compositions, and modes indicate that cumulates formed from a H₂O-rich basaltic andesite, which intruded at low-pressure (~2 kbar) and temperatures of 1,050–1,100 °C. SHRIMP U-Pb dating of zircons from ultramafic cumulates and adjacent metagranodiorite yielded ages of 309±5 and 295±3 Ma, respectively. In agreement with field relationships, these results show that the mafic-ultramafic cumulates represent a co-genetic, early product of the Late Carboniferous plutonic activity in the western Tauern window, which started in the Westphalian, earlier than previously thought. Our data on the most primitive rocks in the Zillertal massif permit, for the first time, insight into the parental magma and thus into the origin of this Late Carboniferous calc-alkaline magmatism, which was most likely related to slab break off during the Late Variscan convergence.