Scambelluri, M., Rampone, E., Braga, R. and Malaspina, N. 2010. The Variscan garnet peridotites from the Eastern Alps (Ulten Zone): records of subduction metasomatism in the mantle wedge. In: (Eds.) Marco Beltrando, Angelo Peccerillo, Massimo Mattei, Sandro Conticelli, and Carlo Doglioni, The Geology of Italy: tectonics and life along plate margins, Journal of the Virtual Explorer, Electronic Edition, ISSN 1441-8142, volume 36, paper 4, doi:10.3809/jvirtex.2010.00259
The Variscan garnet peridotites from the Eastern Alps (Ulten Zone): records of subduction metasomatism in the mantle wedge
At subduction zones, influx in the mantle wedge of fluids released by subducting plates affects the properties and composition of large mantle domains. These effects are investigated through field-based studies of mantle peridotites hosted in high- and ultrahigh-pressure (HP-UHP) terrains. We review the case-study of the Ulten Zone peridotite lenses inside the eclogite-facies Variscan continental crust (Eastern Alps). These rocks record transition from spinel- (1200 °C; 1.5 GPa) to garnet-facies (850 °C; ~2.5 GPa) and represent mantle wedge tectonically emplaced in the subducted Variscan continental slab. Stability of amphibole and locally dolomite in the garnet-peridotites indicates modal metasomatism by C-O-H fluids. Radiometric constraints indicate that HP metamorphism of crust and garnet-bearing mantle rocks took place simultaneously at ~ 330 Ma.
Significant enrichment in Cs, Ba, Pb, U of the Ulten Zone garnet (+ amphibole ± dolomite) peridotites indicates that fluids were sourced by the associated crust and infiltrated the peridotites at HP, in a tectonic environment comparable to a slab-mantle wedge interface. Measurement of Fe3+ contents of garnet from Ulten Zone peridotites enabled to estimate oxygen fugacities (FMQ to FMQ +2) higher than those of garnet-peridotite xenoliths from sub-cratonic mantle, thereby indicating an oxidizing environment during fluid influx in the Ulten Zone mantle. Comparable features pertain to UHP mantle wedge rocks from Western Norway, suggesting that similar metasomatic processes potentially act on a large range of subduction depths. The Ulten Zone peridotites are thus relevant to assess the fluid-mediated crust-to-mantle element transfer during subduction, a major process for the chemical differentiation of the Earth.
Table of Contents
- Geological background of the Ulten Zone and evolution of crustal rocks
- Petrology of the Ulten Zone peridotites
- The geochemical fingerprint of the Ulten Zone garnet-bearing peridotites: record of crustal metasomatism by subduction fluid
- The redox state of Ulten Zone peridotites and inference on fluid phase speciation
- Major implications and comparison with metasomatized mantle rocks from greater depths