FISHNET AND ATOLL GARNETS FROM THE TAUERN WINDOW, EASTERN ALPS: CONDITIONS AND MECHANISMS OF FORMATION

Rocks from the Eclogite Zone (EZ) in the Tauern Window, Eastern Alps, contain garnets with unusual habits. Metasedimentary rocks contain fishnet garnets, where fine webs of garnet line grain boundaries between strain-free but inclusion-filled quartz grains. Quartz in the surrounding matrix has textures indicative of strain accommodation via dislocation creep. EBSD analysis shows that the garnet webs are crystallographically continuous, and that the quartz inclusions are randomly oriented. Garnet zoning in Mn is patchy and discontinuous, but there is no systematic difference between garnet webs and adjacent massive garnet crystals. In metabasites, some garnets show atoll morphologies, with core regions replaced by coarse omphacite + white mica or a carbonate phase. Atolls developed preferentially in horizons in which early garnets were dismembered into multiple, puzzle-like pieces during deformation. Euhedral Mn enriched cores are randomly crosscut by the atoll inclusions. Previous studies indicate that equilibration of all units in the Eclogite Zone occurred at 18-25 kbar and 600-650°C. The EZ is tectonically sandwiched between lower-P units (P<12 kbar) that lack the fishnet and atoll garnet morphologies. This observation implies that development of the unusual garnets in the EZ is related to high-P processes, which may provide insight into rheologies at high P. Atoll garnets likely developed in response to high-strain-rate events and/or over-pressuring of an immobile, non-wetting fluid. In contrast, fishnet garnets record growth either via diffusive mass-transfer and precipitation of garnet-forming elements from a wetting fluid, or through co-nucleation of gar + qtz from a supercritical silicate/aqueous gel. High-P miscibility of silicate melt and aqueous fluid to form a gel would have allowed strain to be partitioned into the metasedimentary horizons and may have facilitated transport of the EZ from depth.