Multiple Metasomatism beneath the Nógrád-Gömör volcanic field (Northern Pannonian Basin) revealed by upper mantle peridotite xenoliths

Peridotite xenoliths from the Nógrád-Gömör Volcanic Field (NGVF) record the geochemical evolution of the subcontinental lithospheric mantle beneath the northern margin of the Pannonian Basin. This study is focused on spinel lherzolites and presents petrography, and major and trace element geochemistry for 51 xenoliths selected from all xenolith-bearing localities of the NGVF. The xenoliths consist of olivine, orthopyroxene, clinopyroxene and spinel ± amphibole. No correlations between modal composition and textures were recognized; however, major and trace element geochemistry reveals several processes, which allow the distinction of xenolith groups with different geochemical evolution. The xenoliths have undergone varying degrees (~7-25%) of partial melting with overprinting by different metasomatic processes. Based on their Mg#, the xenoliths can be subdivided into two major groups. Group I has olivine Mg# between 89 and 91, whereas Group II has Mg# < 89, significant enrichment of Fe and Mn in olivine and pyroxenes, and high Ti in spinel. Trace element contents of the xenoliths vary widely, allowing a further division based on light rare earth element (LREE) enrichment or depletion in pyroxenes. REE patterns of amphiboles match those of clinopyroxenes in each xenolith where they appear, and are inferred to have different origins based on their Nb (and other high field strength element) contents. It is proposed that Nb-poor amphiboles record the oldest metasomatic event, caused by subduction-related volatilebearing silicate melts or fluids, followed by at least two further metasomatic processes: one that resulted in U-Th-(Nb-Ta)-LREE enrichment and crystallization of Nb-rich amphibole, affecting selective domains under the entire NGVF, and another evidenced by Fe-Mn-Ti-LREE enrichment, which overprinted early geochemical signatures. We suggest that the metasomatic agents in both cases were basaltic silicate melts, compositionally similar to the host basalts. These melts were generated during the Miocene extension of the Pannonian Basin. The effects of heating and subsequent cooling are evident in significantly different equilibration temperatures.