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Spinel lherzolite xenoliths from Upper Pliocene potassic trachybasalts at Grad, NE Slovenia

Peridotite xenoliths are scarcely encountered in the debris of a collapsed Upper Pliocene monogenetic volcano situated in the SW marginal region of Pannonian Basin, south of Styrian Basin. The volcano exclusively erupted potassic trachybasalts accompanied with abundant pyroclastics and was assigned as the southwesternmost edifice of Plio-Pleistocene extension-related mafic alkaline volcanism post-dating the since-Eocene subduction-related calc-alkaline volcanism in the Carpathian-Pannonian region. We collected only two xenoliths and their petrologic and geochemical signatures are briefly discussed herein. The samples are fresh spinel lherzolites (LOI < 0.25 wt.% ; Al2O3 = 2.94 wt.% ; CaO = 2.96 wt.% ; Na2O = 0.46 wt.%) showing coarse-grained protogranular to poikilitic textures. Fine-grained clusters of orthopyroxene, clinopyroxene and spinel, which are interpreted to represent the decomposition of former garnet, were encountered in both samples. Beside primary olivine (Mg# = 89.4-90.6), orthopyroxene (Mg# = 89.2-91.1), Na-rich clinopyroxene (Mg# = 88.7-91.5 ; 1.6-1.8 wt.% Na2O) and accessory spinel (Mg# = 79.3-81.7 ; Cr# = 8.7-11.8) one sample contains also texturally equilibrated pargasitic amphibole (Na/K = 7.3). All mineral phases are chemically homogeneous suggesting ambient equilibrium P-T conditions. The equilibrium temperatures calculated for coarse-grained pyroxene compositions range from 1073º ; C to 1038º ; C (assumed pressure of 1.2 GPa) is consistent with temperatures estimated for at least deformed peridotite xenoliths hosted in Pliocene-Pleistocene alkali basalts from the Pannonian Basin. Glass veins up to 1 mm thick are found in both samples. Glass shows variable compositions: it is silica saturated (58.7-68.7 wt.% SiO2), enriched in Al2O3 (16.2-23.1 wt.%) and total alkalis (4.6-9.5 wt.% ; Na2O/K2O = 0.8-2.2) with Mg# ranges from 37.8 to 73.5. The vein liquid fractionates microlites of olivine (Mg# = 89.0-90.7), clinopyroxene (Mg# = 87.7-91.1 ; 0.7-1.3 wt.% Na2O), spinel (Mg# = 68.5-72.1 ; Cr# = 21.2-44.8) and rounded patches of carbonates, and contain empty blobs. The carbonate in pargasite-free xenolith is Mg-calcite with around 3.5 wt.% of MgCO3. The carbonates in pargasite-bearing xenolith show more complex compositions in terms of MgO, FeO and MnO (up to 1.83 wt.%, 2.57 wt.% and 2.93 wt.%, respectively.). Orthopyroxene was not observed in the analysed samples. Silicate microlites may contain glassy inclusions with matrix glass compositions confirming their igneous origin. Based on a limited amount of data our results indicate that untectonized and lithologically monotonous upper mantle characterising the Styrian Basin near Kapfenstein spreads to the Mura depression at south. Partially crystallized veins in the xenoliths suggest interaction of melting, crystallization and mixing of melts and fluids in the underneath mantle. On the basis of textural and geochemical constraints in the analysed xenolith the pargasitic amphibole may be excluded as the source of these melts and fluids. We believe that silicate glass containing carbonate-bearing assemblage formed from an alkali elements rich hydrous assemblage located deep in the mantle. The released carbonatite liquid may have triggered partial melting of overlain mantle. After the reaction with the mantle peridotite minerals the carbonatite liquid left behind carbonates, silica-rich-glass and microlites. This model does not required subduction-related melts or fluids and links the litospheric mantle metasomatism to the same tectonomagmatic event caused mafic alkaline host volcanism in the Mura depression.

mantle xenoliths; mineral chemistry; geotermobarometry; NE Slovenia

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