Fluid‐Enhanced Annealing in the Subcontinental Lithospheric Mantle Beneath the Westernmost Margin of the Carpathian‐Pannonian Extensional Basin System

L E Aradi,C J Garrido,C Szabó,A Tommasi,I J Kovács,R Klébesz,K Hidas

doi:10.1002/2017tc004702

Abstract

AbstractMantle xenoliths from the Styrian Basin Volcanic Field (Western Pannonian Basin, Austria) are mostly coarse granular amphibole‐bearing spinel lherzolites with microstructures attesting for extensive annealing. Olivine and pyroxene CPO (crystal‐preferred orientation) preserve nevertheless the record of coeval deformation during a preannealing tectonic event. Olivine shows transitional CPO symmetry from [010]‐fiber to orthogonal type. In most samples with [010]‐fiber olivine CPO symmetry, the [001] axes of the pyroxenes are also dispersed in the foliation plane. This CPO patterns are consistent with lithospheric deformation accommodated by dislocation creep in a transpressional tectonic regime. The lithospheric mantle deformed most probably during the transpressional phase after the Penninic slab breakoff in the Eastern Alps. The calculated seismic properties of the xenoliths indicate that a significant portion of shear wave splitting delay times in the Styrian Basin (0.5 s out of approximately 1.3 s) may originate in a highly annealed subcontinental lithospheric mantle. Hydroxyl content in olivine is correlated to the degree of annealing, with higher concentrations in the more annealed textures. Based on the correlation between microstructures and hydroxyl content in olivine, we propose that annealing was triggered by percolation of hydrous fluids/melts in the shallow subcontinental lithospheric mantle. A possible source of these fluids/melts is the dehydration of the subducted Penninic slab beneath the Styrian Basin. The studied xenoliths did not record the latest large‐scale geodynamic events in the region—the Miocene extension then tectonic inversion of the Pannonian Basin.

Highlights

The Carpathian-Pannonian region (CPR) is a Miocene Mediterranean-style back-arc extensional basin (e.g., Horváth et al, 2015) situated in central Europe (Figure 1a)
Mantle xenoliths from the Styrian Basin Volcanic Field (Western Pannonian Basin, Austria) are mostly coarse granular amphibole-bearing spinel lherzolites with microstructures attesting for extensive annealing
Based on the correlation between microstructures and hydroxyl content in olivine, we propose that annealing was triggered by percolation of hydrous fluids/melts in the shallow subcontinental lithospheric mantle

Summary

Introduction

The Carpathian-Pannonian region (CPR) is a Miocene Mediterranean-style back-arc extensional basin (e.g., Horváth et al, 2015) situated in central Europe (Figure 1a). In the middle of the basin, xenoliths from the Bakony-Balaton Highland Volcanic Field (e.g., Bali et al, 2008; Berkesi et al, 2012; Créon et al, 2017; Downes et al, 1992; Embey-Isztin, 1976; Embey-Isztin et al, 2001, 1989; Hidas et al, 2010) and the Little Hungarian Plain Volcanic Field were analyzed (e.g., Downes et al, 1992; Embey-Isztin et al, 2001, 1989; Falus et al, 2007) Most of these studies focused on the mineral and whole rock major and trace element geochemistry, Nd-Sr-Pb isotopes, and silicate melt and fluid inclusions in peridotites and pyroxenites (see Szabó et al, 2004, for a review). A few report microstructural and crystal-preferred orientation (CPO) data

Results

Discussion

Conclusion