Mantle source characteristics of Late Miocene-Pleistocene alkaline basalts, western Pannonian Basin, Austria

Late Miocene-Pleistocene alkaline mafic magmas were erupted in the Carpathian-Pannonian Region following the Eocene to Miocene subduction-related calc-alkaline volcanism. The lavas range from hy-normative basalts through alkali olivine basalts andbasanites to nephelinites.
   The majority of basaltic lavas are fresh, olivine-phyric and holocrystalline, whereas olivine is coupled by clinopyroxene phenocrysts in some samples. The olivine phenocrysts contain occasionally spinel inclusions. The matrix consists of plagioclase, clinopyroxene and olivine along with titanomagnetite, ilmenite, and apatite. Phlogopite, interstitial alkali feldspar, nepheline and leucite also occur in some of the lavas. In contrast, nephelinites are rich in clinopyroxene and nepheline and contain titanomagnetite in the groundmass along with subordinate amounts of olivine, apatite and leucite.
   Olivines mg-numbers vary between 88-66. They are generally zoned and began to crystallize with Fo around 87 and 79. In the course of crystallization Fo decreases to 84-66 at the rims of phenocrysts being similar to Fo in the groundmass. The NiO of the olivines decreases with decreasing MgO content, while CaO and MnO increase.
   Clinopyroxene compositions range from augite to diopside. They exhibit both oscillatory and sector zoning as a result of disequilibrium crystallization. The compositional difference between cores and rims follow the normal pyroxene fractionation trend; the cores are richer in Mg, Si and Cr and poorer in Fe, Mn and Ti than the rims. The majority of the clinopyroxenes have Al^VI/Al^IV (0.0-0.65) typical for low pressure clinopyroxene and support shallow level crystallization.
   Most alkali basalt corresponds to the criteria for primitive rocks having high mgnumber (>0.62), high MgO (>9 wt. %) and high Ni (>192 ppm) and Cr (>286 ppm). These magmas underwent only minor olivine±clinopyroxene±spinel fractionation and apparently approach a primary melt composition.
   The silica saturation index (vary from -59 to +2) and trace element ratios (La_N/Yb_N=11-31) generally suggest that these lavas have experienced different degrees of partial melting. The hynormative basalts of Oberpullendorf have the highest degree of melting while Stradnerkogel nephelinites have the lowest. Those rocks that formed via a low degree of melting possess high Zr/Hf ratios (60-66), and negative K and Ti anomalies similar to those of carbonatites. All studied rock varieties have high Nb/La ratios (>1) suggesting OIB-like asthenospheric mantle source. The absence of LILE enrichments (K, Rb and Ba) indicating no interaction with fluids possibly derived from subducted slab. The steep REE patterns and the high Dy_N/Yb_N ratio (average 1.8) strongly suggest that garnet was a residual phase during the partial melting in the source region.
   The ⁸⁷Sr/⁸⁶Sr isotopic compositions of the studied lavas are low (0.703505-0.704279), and the ¹⁴³Nd/¹⁴⁴Nd ratios are high (0.512736-0.512858). Thus, they are isotopic depleted relative to the bulk Earth and similar to HIMU-OIB. Moreover, they are similar to those of Romanian basalts and Neogene alkali basalts throughout Europe.