Symplectite in spinel lherzolite xenoliths from the Little Hungarian Plain, Western Hungary: A key for understanding the complex history of the upper mantle of the Pannonian Basin

Two spinel lherzolite xenoliths from Hungary that contain pyroxene–spinel symplectites have been studied using EPMA, Laser ablation ICP-MS and universal stage. Based on their geochemical and structural characteristics, the xenoliths represent two different domains of the shallow subcontinental lithospheric mantle beneath the Pannonian Basin. The occurrence of symplectites is attributed to the former presence and subsequent breakdown of garnets due to significant pressure decrease related to lithospheric thinning. This implies that both mantle domains were once part of the garnet lherzolitic upper mantle and had a similar history during the major extension that formed the Pannonian Basin.

Garnet breakdown resulted in distinct geochemical characteristics in the adjacent clinopyroxene crystals in both xenoliths. This is manifested by enrichment in HREE, Y, Zr and Hf towards the clinopyroxene porphyroclast rims and also in the neoblasts with respect to porphyroclast core compositions. This geochemical feature, together with the development and preservation of the texturally very sensitive symplectites, enables us to determine the relative timing of mantle processes. Our results indicate that garnets had been metastable in the spinel lherzolite environment and their breakdown to pyroxene and spinel is one of the latest processes that took place within the upper mantle before the xenoliths were brought to the surface.     

Composition and evolution of lithosphere beneath the Carpathian–Pannonian Region: a review

Our knowledge of the lithosphere beneath the Carpathian–Pannonian Region (CPR) has been greatly improved through petrologic, geochemical and isotopic studies of upper mantle xenoliths hosted by Neogene–Quaternary alkali basalts. These basalts occur at the edge of the Intra-Carpathian Basin System (Styrian Basin, Nógrád-Gömör and Eastern Transylvanian Basin) and its central portion (Little Hungarian Plain, Bakony-Balaton Highland).The xenoliths are mostly spinel lherzolites, accompanied by subordinate pyroxenites, websterites, wehrlites, harzburgites and dunites. The peridotites represent residual mantle material showing textural and geochemical evidence for a complex history of melting and recrystallization, irrespective of location within the region. The lithospheric mantle is more deformed in the center of the studied area than towards the edges. The deformation may be attributed to a combination of extension and asthenospheric upwelling in the late Tertiary, which strongly affected the central part of CPR subcontinental lithosphere.The peridotite xenoliths studied show bulk compositions in the following range: 35–48 wt.% MgO, 0.5–4.0 wt.% CaO and 0.2–4.5 wt.% Al₂O₃ with no significant differences in regard to their geographical location. On the other hand, mineral compositions, particularly of clinopyroxene, vary according to xenolith texture. Clinopyroxenes from less deformed xenoliths show higher contents of ‘basaltic’ major elements compared to the more deformed xenoliths. However, clinopyroxenes in more deformed xenoliths are relatively enriched in strongly incompatible trace elements such as light rare earth elements (LREE).Modal metasomatic products occur as both hydrous phases, including pargasitic and kearsutitic amphiboles and minor phlogopitic micas, and anhydrous phases — mostly clinopyroxene and orthopyroxene. Vein material is dominated by the two latter phases but may also include amphibole. Amphibole mostly occurs as interstitial phases, however, and is more common than phlogopite. Most metasomatized peridotites show chemical and (sometimes) textural evidence for re-equilibration between metasomatic and non-metasomatic phases. However, amphiboles in pyroxenites are sometimes enriched in K, Fe and LREE. The presence of partially crystallized melt pockets (related to amphiboles and clinopyroxenes) in both peridotites and pyroxenites is an indication of decompression melting and, rarely, incipient partial melting triggered by migrating hydrous melts or fluids. Metasomatic contaminants may be ascribed to contemporaneous subduction beneath the Carpathian–Pannonian Region between the Eocene and Miocene.Sulfide inclusions are more abundant in protogranular and porphyroclastic xenoliths relative to equigranular types. In mantle lithologies, sulfide bleb compositions vary between pentlandite and pyrrhotite correlating with the chemistry and texture of the host xenoliths. While sulfides in peridotites are relatively rich in Ni, those in clinopyroxene-rich xenoliths are notably Fe-rich.     

Evolution of Mafic Alkaline Melts Crystallized in the Uppermost Lithospheric Mantle: a Melt Inclusion Study of Olivine-Clinopyroxenite Xenoliths, Northern Hungary

Olivine-clinopyroxenite xenoliths exhumed in alkali basalts(sensu lato) in the Nógrád–GömörVolcanic Field (NGVF), northern Hungary, contain abundant silicatemelt inclusions. Geothermobarometric calculations indicate thatthese xenoliths crystallized as cumulates in the upper mantlenear the Moho. These cumulate xenoliths are considered to representa period of Moho underplating by mafic alkaline magmas priorto the onset of Late Tertiary alkaline volcanism in the Carpathian–Pannonianregion. The major and trace element compositions of silicatemelt inclusions in olivine display an evolutionary trend characterizedby a strong decrease in CaO/Al₂O₃. The parental melt of thecumulates was a basanite formed by low-degree ( 2%) partialmelting of a garnet peridotite source. The compositional trendof the silicate melt inclusions, textural features, and modellingwith pMELTS show that the parental melt evolved by major clinopyroxeneand minor olivine crystallization followed by the appearanceof amphibole simultaneously with significant resorption of theearlier clinopyroxene and olivine. The resulting residual meltwas highly enriched in Al₂O₃, alkalis and most incompatibletrace elements. This type of melt is likely to infiltrate andreact with surrounding mantle peridotite as a metasomatic agent.It might also form high-pressure pegmatite-like bodies in themantle that might be the source of the amphibole and sanidinemegacrysts also found in the alkali basalts of the NGVF. Preferentialremelting of the later-formed (i.e. lower temperature) mineralassemblage (amphibole, sanidine, residual glass) might havesignificantly contaminated the host alkaline mafic lavas, increasingtheir Al₂O₃ and total alkali contents and, therefore, reducingtheir MgO, FeO and CaO content. KEY WORDS: silicate melt inclusions; geochemistry; petrogenesis; Nógrád–Gömör Volcanic Field; Pannonian Basin