SPINEL-PERIDOTITE XENOLITHS FROM KAPFENSTEIN (GRAZ BASIN, EASTERN AUSTRIA) - A GEOCHEMICAL AND PETROLOGICAL STUDY

Ultramafic xenoliths from the basaltic tuff of Kapfenstein (Graz Basin ) are mainly protogranular and protogranular- to- porphyroclastic spin el Iherzolites. In addition to these common rocks, a coarse-grained sp inel harzburgite and a heterogeneous spinel peridotite consisting of a clinopyroxene-poor harzburgite and a clinopyroxene-rich Iherzolite ha ve been found. Interstitial amphiboles (pargasite and kaersutite) are present in most xenoliths. The magmatic activity which brought these x enoliths to the surface is part of the Plio-Pleistocene extension-rela ted alkali basalt volcanism which took place in the Carpatho-Pannonian Region, post-dating the subduction-related calc-alkaline volcanism wh ich formed the Inner Carpathian arc. Mineralogical and bulk geochemica l data for Kapfenstein xenoliths are similar to those for other sub-co ntinental shallow mantle xenolith suites. Correlation diagrams for maj or and trace elements indicate that their chemical variations are rela ted to different degrees of partial melting due to the extraction of m afic melts. Rare earth element (REE) patterns of separated clinopyroxe nes from the xenoliths are generally light rare earth element (LREE)-d epleted, although LREE-enriched and more complex patterns have also be en found. Sr-87/Sr-86 and Nd-143/Nd-144 isotopic ratios of separated c linopyroxenes vary between 0.70193 and 0.70330 (epsilon(Sr) = -36.4 to -17.0) and 0.512824 and 0.513589 (epsilon(Nd) = 4.2 to 18.1), respect ively. The highest Sr-87/Sr-86 and lowest Nd-143/Nd-144 ratios occur i n those samples which have the most refractory character and also have the most unusual REE patterns. REE determined by ion microprobe on se parated interstitial amphibole grains indicate equilibrium with the ho st peridotite in most of the xenoliths but disequilibirum conditions h ave been observed, including some extreme LREE enrichments. Striking t extural and some geochemical and isotope geochemical similarities have been recognised between Kapfenstein xenoliths and those from the East ern Transylvanian Basin, both of which are situated on the edges of th e diapiric upwelling located in the central part of the Pannonian Basi n. The deformation, depletion and enrichment processes which occurred in these two areas are thought to be similar. The LREE-enriched inters titial amphiboles and the presence of cross-cutting amphibole veins su ggest that the shallow depleted lithospheric mantle beneath the Graz B asin has been affected by metasomatising events related to the mafic a lkaline host magmas rather than by subduction-related fluids or melts.