Crystal chemistry of clinopyroxenes from upper-mantle xenolith series in the Balaton-Bakony volcanic area (Carpathian-Pannonian region, Hungary)

A detailed crystal chemical study of clinopyroxenes (cpx) from a suite of mantle xenoliths of the Balaton-Bakony volcanic highland (Carpathian-Pannonian region (CPR), Hungary) has been undertaken by means of X-ray single-crystal diffraction and electron microprobe. The study aims to evaluate the response of cpx crystal chemistry to textural changes and increasing deformation, and to estimate equilibrium pressure conditions of texturally heterogeneous spinel-peridotite xenolith series. Moreover, our interest also focussed on cpx crystal chemistry of mosaic and poikilitic textured xenoliths. Results demonstrate that cpx from protogranular, porphyroclastic and some equigranular xenoliths define the "primary" compositional trend that reflects structural and textural changes. This cpx-trend also suggests that deformation increase correlates with pressure (and temperature) decrease. Protogranular xenoliths record pressure near to the garnet stability field whereas equigranular xenoliths record lower pressure approaching the plagioclase stability field. Some cpx from equigranular samples show a crystal chemistry similar to that of cpx from poikilitic and mosaic xenoliths which depart from the "primary" trend. This suggests that the latter cpx are likely formed at low pressure near to or within the plagioclase stability field. The inverse relationship between pressure and deformation in the main series can be explained geodynamically by the presence of a mantle diapir beneath the region, which could have caused significant deformation and lithosphere thinning in the centre of the CPR.