Magmatic evolution of the Mesozoic ophiolites in Austria

Abstract

The Mesozoic ophiolites in the Eastern Alps are restricted to the Penninic windows: the Lower Engadin Window, Tauern Window and Rechnitz Window Group. The ophiolite sequence starts with serpentinites (harzburgitic chemistry) and some metamorphosed ultramafic cumulates which include rodingites and chloritites. The plutonic section comprises leucogabbros, ferrogabbros and rare plagiogranites. Pillow lavas, where recognizable, massive lava flows, and sometimes tuffs build up the extrusive series. Small bands of radiolarites, cherts and thick calc schists form the sedimentary cover. Ophicarbonates on top of the serpentinites are rare (Rechnitz Window Group). All ophiolites, apart from some oceanic alterations, have undergone a complex history of Alpine regional metamorphism of variable grade (pumpellyite-actinolite facies in the Lower Engadin Window, to greenschist to amphibolite facies in the Tauern Window). Geochemically, the plutonic section can be explained in terms of their Ti, V, Zr and Y concentration reflecting a fractional crystallization sequence with olivine, plagioclase, clinopyroxene, and later ilmenite and magnetite as cumulus phases. The volcanics show mid-ocean ridge basalt affinities. Five groups based on major-element variations can be observed: plag, ol and ilm-mag accumulations, and ol-plag-(cpx) and cpx-ol-plag fractionations. This grouping is also recognized in the trace elements. The variability in Zr/Y ratios within and between the groups can be explained by different degrees of partial melting and slight variation in the mantle source. This complex magmatic history indicates an oceanic ridge environment for the origin of the ophiolites with a structure similar to that of the North Atlantic and low spreading rates. Fracture zones may also play a certain role.