28 Petrology, Igneous (Posters)

Sunday, 9 October 2011
9:00 AM - 6:00 PM
Hall C

UNRAVELING COMPLEX MICROSTRUCTURES IN PERIDOTITES: NEW EVIDENCE FOR THE TIMING OF MELT PERCOLATION IN THE RONDA PERIDOTITE MASSIF, SOUTHERN SPAIN

Paper number: 28-7
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

The Ronda peridotite massif in the Betic orogen of southern Spain is a ~6 km thick slice of sub-continental lithospheric mantle, which has recorded several P-T assemblages and composite microstructures reflecting a range of solid state and melt-present processes. As a result, it displays mineralogical and textural zonation, containing in structural sequence from bottom to top: plagioclase tectonites, granular spinel peridotites, spinel tectonites, and garnet-spinel bearing mylonites, which represent equilibration within a thermal gradient during exhumation of the body from >20 kb to <8 kb. For this study, we focus on the development of the two spinel bearing zones, the lower of which has dunite and Cr diopside-rich pyroxenite layers and a granular texture suggestive of annealing recrystallization and the percolation of basaltic melt. The overlying spinel tectonites show a deformational fabric defined by coarse-grained porphyroclastic olivine and pyroxene. This fabric strengthens towards the top of the unit, where the upper margin of the massif is mylonitic and could represent the ductile fault along which the peridotite massif was exhumed.

We have identified interstitial pyroxene grains with curved and cuspate boundaries between coarser olivine within the granular zone. Similar interstitial pyroxenes are found in the deformed units above, and some show signs of deformation including subgrains and sweeping extinction. These textures are evidence of melt interaction with the peridotite prior to deformation as part of the same melt percolation event that created the granular zone. The microstructural evidence suggests that the melt percolation event occurred prior to the development of the shear zone, which localized to form the marginal mylonites. This reinterpretation solves a number of problems concerning the evolution of the massif and suggests the following sequence of events leading to its emplacement into the crust of the Betic orogen: 1) Delamination of part of the lithospheric mantle at ~90 km depth, leading to heating and melt percolation; 2) extensional exhumation along a shear zone several km wide; 3) progressive cooling and localization of deformation into a shear zone ~500 m wide; 4) thrust emplacement of the largely exhumed peridotite and its thinned crustal envelope onto the Iberian margin.