33 research outputs found
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
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
Cryptic metasomatism in clino- and orthopyroxene in the upper mantle beneath the Pannonian region
Clino- and orthopyroxenes in anhydrous spinel peridotite xenoliths from Pliocene alkali basalts of the western Pannonian Basin have been analysed for trace elements by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Clinopyroxenes show highly variable mantle normalized REE (rare earth elements) patterns but basically can be classified into three major groups: LREE-depleted, LREE-enriched and U-shaped patterns. As the REE patterns of clinopyroxenes usually reflect the REE patterns of the host peridotite, the three major REE patterns define three geochemically different groups of xenoliths. LREE-depleted xenoliths generally have undeformed protogranular textures, while the more deformed xenoliths with porphyroclastic and equigranular textures have LREE-enriched trace element patterns. The U-shaped pattern is very distinctive and is generally associated with poikilitic textures. The HREE content of the clinopyroxenes suggest that most of the xenoliths experienced less than15% partial melting, with the lowest degree occurring in the LREE-depleted xenoliths, and the highest degree in LREE-enriched xenoliths. Cryptic metasomatism frequently accompanies deformation. Metasomatic enrichment of incompatible trace elements can be observed not only in clinopyroxenes but also in coexisting orthopyroxenes. The metasomatic agents were probably alkaline mafic melts of asthenospheric origin and some may relate to upper Cretaceous alkali lamprophyre magmatism. Geochemical signatures of subduction-related melts or fluids have not been found in the anhydrous LREE-enriched xenoliths, although poikilitic xenoliths with U-shaped normalized REE patterns may indicate the influence of subduction-related melts
Crystal chemistry of clinopyroxenes from upper-mantle xenolith series in the Balaton\u2013Bakony volcanic area (Carpathian\u2013Pannonian region, Hungary)
A detailed crystal chemical study of clinopyroxenes (cpx) from a suite of mantle xenoliths of the Balaton\u2013Bakony volcanic highland (Carpathian\u2013Pannonian region (CPR), Hungary) has been undertaken by means of X-ray single-crystal diffraction and electron microprobe. The study aims to evaluate the response of cpx crystal chemistry to textural changes and increasing deformation, and to estimate equilibrium pressure conditions of texturally heterogeneous spinel-peridotite xenolith series. Moreover, our interest also focussed on cpx crystal chemistry of mosaic and poikilitic textured xenoliths. Results demonstrate that cpx from protogranular, porphyroclastic and some equigranular xenoliths define the \u2018\u2018primary\u2019\u2019 compositional trend that reflects structural and textural changes. This cpx-trend also suggests that deformation increase correlates with pressure (and temperature) decrease. Protogranular xenoliths record pressure near to the garnet stability field whereas equigranular xenoliths record lower pressure
approaching the plagioclase stability field. Some cpx from equigranular samples show a crystal chemistry similar to that of cpx from poikilitic and mosaic xenoliths which depart from the \u2018\u2018primary\u2019\u2019 trend. This suggests that the latter cpx are likely formed at low
pressure near to or within the plagioclase stability field.
The inverse relationship between pressure and deformation in the main series can be explained geodynamically by the presence of a
mantle diapir beneath the region, which could have caused significant deformation and lithosphere thinning in the centre of the CPR
Origin of amphibole megacrysts in the Pliocene-Pleistocene basalts of the Carpathian-Pannonian region
Major and trace element compositions, stable H and O isotope
compositions and Fe3+ contents of amphibole megacrysts of
Pliocene-Pleistocene alkaline basalts have been investigated to obtain
information on the origin of mantle fluids beneath the
Carpathian-Pannonian region. The megacrysts have been regarded as
igneous cumulates formed in the mantle and brought to the surface by the
basaltic magma. The studied amphiboles have oxygen isotope compositions
(5.4±0.2 ‰, 1 σ), supporting their primary mantle origin. Even within
the small δ18O variation observed, correlations with major and trace
elements are detected. The negative δ18O-MgO and the positive δ18O-La
/Sm(N) correlations are interpreted to have resulted from varying
degrees of partial melting. The halogen (F, Cl) contents are very low
(<0.1 wt. %), however, a firm negative (F+Cl)-MgO correlation (R2 =
0.84) can be related to the Mg-Cl avoidance in the amphibole structure.
The relationships between water contents, H isotope compositions and
Fe3+ contents of the amphibole megacrysts revealed degassing. Selected
undegassed amphibole megacrysts show a wide δD range from -80 to -20 ‰.
The low δD value is characteristic of the normal mantle, whereas the
high δD values may indicate the influence of fluids released from
subducted oceanic crust. The chemical and isotopic evidence collectively
suggest that formation of the amphibole megacrysts is related to fluid
metasomatism, whereas direct melt addition is insignificant
Origin of CO2 and carbonate veins in mantle-derived xenoliths in the Pannonian Basin
The origin and evolution of CO2 inclusions and calcite veins in
peridotite xenoliths of the Pannonian Basin, Hungary, were investigated
by means of petrographic investigation and stable isotope analyses. The
fluid inclusions recovered in paragenetic olivine and clinopyroxene
belong to distinct populations: type A (texturally early) inclusions
with regular shapes (often with negative crystal forms) forming
intragranular trails, type B (texturally late) inclusions defining
randomly oriented trails that reach grain boundaries Type B inclusions
are often associated with silicate melt (type C) inclusions Stable
carbon isotope compositions in inclusion-hosted CO2 were obtained by
vacuum crushing followed by conventional dual inlet as well as
continuous flow mass spectrometry in order to eliminate possible lab
artifacts. Olivines, clino- and orthopyroxenes of the host peridotite
have oxygen isotope compositions from 5.3 to 6.0 parts per thousand
(relative to V-SMOW). without any relationship with xenolith texture.
Some of the xenoliths contained calcite in various forms veins and
infillings in silicate globules in veins, secondary carbonate veins
filling cracks and metasomatic veins with diffuse margins The former two
carbonate types have delta C-13 values around -13 parts per thousand
(relative to V-PDB) and low Sr contents (<05 wt %), whereas the third
type,veins with high-temperature metasomatic features have a delta C-13
value of -5 0 parts per thousand and high Sr contents up to 34 wt.% In
spite of the mantle-like delta C-13 value and the unusually high Sr
content typical for mantle-derived carbonate, trace element compositions
have proven a crustal origin. This observation supports the conclusions
of earlier studies that the carbonate melt droplets found on peridotite
xenoliths in the alkaline basalts represent mobilized sedimentary
carbonate. The large delta C-13 range and the C-12-enrichment in the
carbonates can be attributed to devolanlization of the migrating
carbonate or infiltration of surficial fluids containing C-12-rich
dissolved carbon
Carbon isotope compositions of inclusion-hosted CO2 range from -17 8 to
-4.8 parts per thousand (relative to V-PDB) with no relation to the
amount of CO2 released by vacuum crushing. Low-delta C-13 values
measured by stepwise heating under vacuum suggest that the carbon
component is pristine and not related to surficial contamination, and
that primary mantle fluids with delta C-13 values around -5 parts per
thousand were at least partly preserved in the xenoliths Tectonic
reworking and heating by the basaltic magma resulted in partial CO2
release and local C-13-depletion. (C) 2010 Elsevier B V All rights
reserve
Origin and significance of poikilitic and mosaic peridotite xenoliths in the western Pannonian Basin: geochemical and petrological evidences
International audiencePeridotite xenoliths erupted by alkali basaltic volcanoes in the western Pannonian Basin can be divided into two fundamentally contrasting groups. Geochemical characteristics of the abundant protogranular, porphyroclastic and equigranular nodules suggest that these samples originate from an old consolidated and moderately depleted lithospheric mantle domain. In contrast, the geochemical features of the worldwide rare, but in the Pannonian Basin relatively abundant, poikilitic xenoliths attest to a more complex evolution. It has been argued that the origin of the peculiar texture and chemistry may be intimately linked to melt/rock reactions at successively decreasing liquid volumes in a porous melt flow system. The most likely site where such reactions can take place is the asthenosphere-lithosphere boundary. In this context, poikilitic xenoliths may provide petrological and geochemical evidence for reactions between magmatic liquids issued from the uprising asthenosphere and the solid mantle rocks of the lithosphere. These reactions are important agents of the thermal erosion of the lithosphere; thus, they could have considerably contributed to the thinning of the lithosphere in the Pannonian region. We suggest that in the Pannonian Basin, there could be a strong relation between the unusual abundance of poikilitic mantle xenoliths and the strongly eroded lithosphere
Fluid‐Enhanced Annealing in the Subcontinental Lithospheric Mantle Beneath the Westernmost Margin of the Carpathian‐Pannonian Extensional Basin System
Mantle xenoliths from the Styrian Basin Volcanic Field (Western Pannonian Basin, Austria) are
mostly coarse granular amphibole-bearing spinel lherzolites with microstructures attesting for extensive
annealing. Olivine and pyroxene CPO (crystal-preferred orientation) preserve nevertheless the record of coeval
deformation during a preannealing tectonic event. Olivine shows transitional CPO symmetry from [010]-fiber
to orthogonal type. In most samples with [010]-fiber olivine CPO symmetry, the [001] axes of the pyroxenes
are also dispersed in the foliation plane. This CPO patterns are consistent with lithospheric deformation
accommodated by dislocation creep in a transpressional tectonic regime. The lithospheric mantle deformed
most probably during the transpressional phase after the Penninic slab breakoff in the Eastern Alps. The
calculated seismic properties of the xenoliths indicate that a significant portion of shear wave splitting delay
times in the Styrian Basin (0.5 s out of approximately 1.3 s) may originate in a highly annealed subcontinental
lithospheric mantle. Hydroxyl content in olivine is correlated to the degree of annealing, with higher
concentrations in themore annealed textures. Based on the correlation between microstructures and hydroxyl
content in olivine, we propose that annealing was triggered by percolation of hydrous fluids/melts in the
shallow subcontinental lithospheric mantle. A possible source of these fluids/melts is the dehydration of the
subducted Penninic slab beneath the Styrian Basin. The studied xenoliths did not record the latest large-scale
geodynamic events in the region—the Miocene extension then tectonic inversion of the Pannonian Basin