Poikilites szövetű felsőköpeny peridotit xenolitok eredete = The origin of poikilitic upper mantle peridotite xenoliths

A pályázati munka során részletes összehasonlító kőzettani, geokémiai és izotóp-geokémiai vizsgálatokra került sor a Pannon medence felsőköpenyéből származó rendhagyó poikilites szövetű peridotit xenolit sorozaton, valamint a közönséges felsőköpeny peridotit xenolitokon. A részletes petrográfiai, elektronmikroszonda, oldatos ICP-MS, lézer ablációs (LA) ICP-MS, valamint Sr, Nd és Pb izotóp analízisek alapvető és szisztematikus eltérésekre derítettek fényt a két szembenálló xenolit-csoport között. Világossá vált, hogy a poikilites xenolitok eltérnek a parciális olvadási trendtől, mely a közönséges peridotit xenolitok kémiai összetételi változásainak fő oka. Az új adatok ellentétben állnak az eddig javasolt keletkezési modellek egyik vagy másik vonásával, ezzel szemben egy komplex keletkezési modellt valószínűsítenek. E különleges kőzetek képződésében egyaránt fontos szerepet játszott az olvadék/szilárd köpenykőzet kölcsönhatása, a mellékőzet magas Mg- és Ni-tartalmú olivinjének pufferhatása, a kromatografikus, pórusokban történő olvadékszivárgás, a metaszomatózis, a mellékkőzet alkotórészeinek visszaoldása, és kumulusz fázisok kiválása (telér kumulátok). A poikilites peridotit xenolitok képződése szorosan kapcsolódhat a litoszféra kivékonyodását okozó eróziós folyamatokhoz. | A detailed petrological, geochemical and isotope study on a suite of anomalous poikilitic peridotite xenoliths as opposed to normal upper mantle peridotite xenoliths from the Pannonian Basin has been carried out. Microprobe, solution ICP-MS, (LA) ICP-MS, as well as Sr, Nd and Pb isotope work has revealed a fundamental and systematic difference between these contrasting groups of xenoliths. Poikilitic xenoliths deviate from the partial fusion trend that shapes the chemistry of normal mantle peridotites. The new data are in conflict with several aspects of previously proposed models, but they are consistent with a complex scenario for the formation of these peculiar rocks, involving liquid/solid mantle rock interaction, buffering effect of the high Mg-, and Ni-olivine of the host, chromatographic porous flow, metasomatism, dissolution of wall rock constituents, and precipitation of cumulate phases (dyke cumulates). It has been argued that the formation of poikilitic peridotites may be intimately linked to a process of erosion of the lithosphere

The role of melt depletion versus refertilization in the major element chemistry of four-phase spinel peridotite xenoliths

Mantle peridotites are interpreted as either residues after partial melting and melt extraction or products of igneous refertilization of refractory peridotites. The simple distinction between these models is difficult to assess because in chemical variation diagrams, both processes lead essentially to the same results. The only exception is the Ti-in-Cpx versus Ti-in-whole-rock plots, which can successfully discriminate between these models. In this study, a modified version of Ti-in-Cpx versus Mg#-in-olivine plots was applied to ∼1,500 spinel peridotite xenoliths from worldwide localities. The results showed that the vast majority of shallow mantle samples are consistent with the partial melting model; however, a minority of samples may indicate refertilization of formerly refractory mantle domains

A szubkontinentális litoszféraköpeny nagy mélységeiben lejátszódó fluidmigrációs folyamatok összehasonlító vizsgálata = Comparative study of the fluid migration processes in the deep lithospheric mantle

A kutatás során a köpenyben nagy mélységben migráló C-O-H fluidumok szerepét vizsgáltuk a köpenymetaszomatózis és gyémántkristályosodás folyamataiban. Negyvennél több, dél Afrikából származó diamondit xenolitot elemeztünk; meghatároztuk a gyémánttal összenőtt gránátot és klinopiroxént fő- és nyomelem (LAM-ICPMS) tartalmát, valamint a gyémánt C és N izotóparányait. A diamonditban található gránátok 5 csoportra oszthatók: peridotitos, átmeneti és 3 eklogitos (E1, E2, E3) csoportokra. A P gránátot tartalmazó diamondit karbonatit-kimberlit összetételű fluidumból kristályosodott, míg az E gránátot tartalmazó diamonditot kristályosító fluidum nyomelemei az alkáli bazaltokhoz hasonlítanak. A gyémántot kristályosító fluidumok redukált szenet tartalmazó mély köpeny eredetű fluidumokból oxidáció révén képződtek. Ez a folyamat C izotóp frakcionációval járt. Az így képződött karbonatit fejlődött tovább, és kristályosította a P és E típusú diamonditokat. Természetes és szintetikus karbonát-eklogit rendszeren végeztünk kísérlekete 8.5 GPa nyomáson és 1700-1800 oC hőmérsékleten. A kis viszkozitású karbonát olvadék gyorsan eljutott a gyémántkristályosodás helyére, nagyfokú gyémántkristályosodás volt megfigyelhető, és a gyémántokkal együtt gránát és cpx is kristályosodott. A természetes diamondit xenolitok vizsgálata és a kísérleti eredmények megerősítik, hogy a kontitnentális kratonok mélyén a gyémánt kristályosodás karbonatitos közegben megy végbe a köpenymetaszomatózis eredményeképp. | Migration of C-O-H fluids in the subcontinental lithospheric mantle and their role in mantle metasomatism and crystallization diamondites have been studied. Garnet and clinopyroxene intergrowths from over 40 diamondite xenoliths from southern Africa have been analysed form major and trace elements (LAM-ICPMS), and diamonds have been analysed for C and N isotope ratios. Five garnet groups could be distinguished: peridotitic, transitional and 3 eclogitic (E1, E2,E3). Diamondites with P garnets crystallised from fluids in the carbonatite-kimberlite spectrum, while the parental fluids for diamondites with E garnets had trace element composition similar to alkali basalts. Parental fluids for diamondites evolved from reduced carbon species of deep mantle origin through oxidation. The resulted carbonatitic melt would further evolve and crystallize the P and E diamondites. Experiments on natural and synthetic carbonate-eclogite systems have been made at 8.5 GPa and 1700-1800 oC in order to simulate processes for the formation of diamondites. The processes observed are as follows: quick migration of mobile low-viscosity carbonate-silicate melts into zones of diamondite formation, extremely high rate of diamondite crystallization from a carbonate- silicate melt and formation of syngenetic inclusions of garnet and cpx inside the pores and cavities of diamondites. Diamondites crystallised in carbonatitic environment as a result of mantle metasomatism in the roots of the cratons

Lower crustal zircons reveal Neogene metamorphism beneath the Pannonian Basin (Hungary)

Neogene alkaline intraplate volcanic deposits in the Pannonian Basin (Hungary) contain many lower crustal granulite-facies xenoliths. U-Pb ages have been determined for zircons separated from a metasedimentary xenolith, using LA-ICPMS and SHRIMP techniques. The zircons show typical metamorphic characteristics and are not related to the hostmagmatism. The oldest age recorded is late Devonian, probably related to Variscan basement lithologies. Several grains yield Mesozoic dates for their cores, which may correspond to periods of orogenic activity. Most of the zircons show young ages, with some being Palaeocene-Eocene, but the majority being younger than 30Ma. The youngest zircons are Pliocene (5.1-4.2 Ma) and coincide with the age of eruptions of the host alkali basalts. Such young zircons, so close to the eruption age, are unusual in lower crustal xenoliths, and imply that the heat flow in the base of the Pannonian Basin was sufficiently high to keep many of them close to their blocking temperature. This suggests that metamorphism is continuing in the lower crust of the region at the present day

Post-collisional Tertiary–Quaternary mafic alkalic magmatism in the Carpathian–Pannonian region: a review

Mafic alkalic volcanism was widespread in the Carpathian–Pannonian region (CPR) between 11 and 0.2 Ma. It followed the Miocene continental collision of the Alcapa and Tisia blocks with the European plate, as subduction-related calc-alkaline magmatism was waning. Several groups of mafic alkalic rocks from different regions within the CPR have been distinguished on the basis of ages and/or trace-element compositions. Their trace element and Sr–Nd–Pb isotope systematics are consistent with derivation from complex mantle-source regions, which included both depleted asthenosphere and metasomatized lithosphere. The mixing of DMM-HIMU-EMII mantle components within asthenosphere-derived magmas indicates variable contamination of the shallow asthenosphere and/or thermal boundary layer of the lithosphere by a HIMU-like component prior to and following the introduction of subduction components. Various mantle sources have been identified: Lower lithospheric mantle modified by several ancient asthenospheric enrichments (source A); Young asthenospheric plumes with OIB-like trace element signatures that are either isotopically enriched (source B) or variably depleted (source C); Old upper asthenosphere heterogeneously contaminated by DM-HIMU-EMII-EMI components and slightly influenced by Miocene subduction-related enrichment (source D); Old upper asthenosphere heterogeneously contaminated by DM-HIMU-EMII components and significantly influenced by Miocene subduction-related enrichment (source E). Melt generation was initiated either by: (i) finger-like young asthenospheric plumes rising to and heating up the base of the lithosphere (below the Alcapa block), or (ii) decompressional melting of old asthenosphere upwelling to replace any lower lithosphere or heating and melting former subducted slabs (the Tisia block)

Origin and ascent history of unusually crystal-rich alkaline basaltic magmas from the western Pannonian Basin

The last eruptions of the monogenetic Bakony-Balaton Highland Volcanic Field (western Pannonian Basin, Hungary) produced unusually crystal- and xenolith-rich alkaline basalts which are unique among the alkaline basalts of the Carpathian- Pannonian Region. Similar alkaline basalts are only rarely known in other volcanic fields of the world. These special basaltic magmas fed the eruptions of two closely located volcanic centres: the Bondoró-hegy and the Füzes-tó scoria cone. Their uncommon enrichment in diverse crystals produced unique rock textures and modified original magma compositions (13.1-14.2 wt.% MgO, 459-657 ppm Cr, 455-564 ppm Ni contents). Detailed mineral-scale textural and chemical analyses revealed that the Bondoró-hegy and Füzes-tó alkaline basaltic magmas have a complex ascent history, and that most of their minerals (~30 vol.% of the rocks) represent foreign crystals derived from different levels of the underlying lithosphere. The most abundant xenocrysts, olivine, orthopyroxene, clinopyroxene and spinel, were incorporated from different regions and rock types of the subcontinental lithospheric mantle. Megacrysts of clinopyroxene and spinel could have originated from pegmatitic veins / sills which probably represent magmas crystallized near the crust-mantle boundary. Green clinopyroxene xenocrysts could have been derived from lower crustal mafic granulites. Minerals that crystallized in situ from the alkaline basaltic melts (olivine with Cr-spinel inclusions, clinopyroxene, plagioclase, Fe-Ti oxides) are only represented by microphenocrysts and overgrowths on the foreign crystals. The vast amount of peridotitic (most common) and mafic granulitic materials indicates a highly effective interaction between the ascending magmas and wall rocks at lithospheric mantle and lower crustal levels. However, fragments from the middle and upper crust are absent from the studied basalts, suggesting a change in the style (and possibly rate) of magma ascent in the crust. These xenocryst- and xenolith-rich basalts yield divers tools for estimating magma ascent rate that is important for hazard forecasting in monogenetic volcanic fields. According to the estimated ascent rates, the Bondoró-hegy and Füzes-tó alkaline basaltic magmas could have reached the surface within hours to few days, similarly to the estimates for other eruptive centres in the Pannonian Basin which were fed by "normal" (crystal- and xenolith-poor) alkaline basalts

Tectonic significance of changes in post-subduction Pliocene-Quaternary magmatism in the south east part of the Carpathian-Pannonian Region

The south-eastern part of the Carpathian–Pannonian region records the cessation of convergence between the European platform/Moesia and the Tisza–Dacia microplate. Plio-Quaternary magmatic activity in this area, in close proximity to the ‘Vrancea zone’, shows a shift from normal calc-alkaline to much more diverse compositions (adakite-like calc-alkaline, K-alkalic, mafic Na-alkalic and ultrapotassic), suggesting a significant change in geodynamic processes at approximately 3 Ma. We review the tectonic setting, timing, petrology and geochemistry of the post-collisional volcanism to constrain the role of orogenic building processes such as subduction or collision on melt production and migration. The calc-alkaline volcanism (5.3–3.9 Ma) marks the end of normal subduction-related magmatism along the post-collisional Călimani–Gurghiu–Harghita volcanic chain in front of the European convergent plate margin. At ca. 3 Ma in South Harghita magma compositions changed to adakite-like calc-alkaline and continued until recent times (< 0.03 Ma) interrupted at 1.6–1.2 Ma by generation of Na and K-alkalic magmas, signifying changes in the source and melting mechanism. We attribute the changes in magma composition in front of the Moesian platform to two main geodynamic events: (1) slab-pull and steepening with opening of a tear window (adakite-like calc-alkaline magmas) and (2) renewed contraction associated with deep mantle processes such as slab steepening during post-collisional times (Na and K-alkalic magmas). Contemporaneous post-collisional volcanism at the eastern edge of the Pannonian Basin at 2.6–1.3 Ma was dominated by Na-alkalic and ultrapotassic magmas, suggesting a close relationship with thermal asthenospheric doming and strain partitioning related to the Adriatic indentation. Similar timing, magma chamber processes and volume for K-alkalic (shoshonitic) magmas in the South Apuseni Mountains (1.6 Ma) and South Harghita area at a distance of ca. 200 km imply a regional connection with the inversion tectonics