Gábor Dobosi

Petrology and geochemistry of late Tertiary/Quaternary mafic alkaline volcanism in Romania

Abstract Alkaline volcanic activity occurred in the Persani Mountains (eastern Transylvanian Basin) and Banat (eastern Pannonian Basin) regions of Romania between 2.5 Ma and 0.7 Ma. This volcanism followed an extended period of subduction-related mostly andesitic and dacitic magmatism in the Eastern Carpathian arc. The Persani Mts. alkaline activity coincided with the last phase of subduction-related activity. Several lava flows and pyroclastic deposits in the Persani Mts. carry peridotitic mantle xenoliths and amphibole megacrysts. Major- and trace-element geochemistry indicates that the alkaline magmas are primitive, silica-undersaturated alkali basalts and trachybasalts (7.8–12.3 wt.% MgO; 119–207 ppm Ni; 210–488 ppm Cr) which are LREE-enriched. Mantle-normalised trace-element diagrams reveal an overall similarity to continental intraplate alkali basalts, but when compared with a global average of ocean island basalts (OIB), the Banat lavas are similar to average OIB, whereas the Persani Mts. basalts have higher Rb, Ba, K and Pb and lower Nb, Zr and Ti. These features slightly resemble those of subduction-related magmas, particularly those of a basaltic andesite related to the nearby older arc magmas. With 87 Sr 86 Sr varying from 0.7035-0.7045 and 143 Nd 144 Nd from 0.51273-0.51289, the Romania basalts are indistinguishable from those of the western Pannonian basin (Hungary and Austria) and Neogene alkali basalts throughout Europe. Amphibole megacrysts have similar isotopic signatures, and their REE patterns indicate derivation by crystallisation from a mafic alkaline magma. The age-corrected Sr and Nd isotopic compositions of a calc-alkaline basaltic andesite related to the preceeding period of subduction also lies within the field of the younger alkaline magmas. Pb isotopic ratios of the Romanian alkali basalts do not lie on the NHRL, but overlap the field of Tertiary alkali basalts from the western Pannonian basin, and have unusually high 207 Pb 204 Pb at a given 206 Pb 204 Pb . Thus it is probable that, although the Romanian alkali basalts have a strong asthenospheric (i.e. OIB-type mantle source) component, their Pb isotopic characteristics were derived from mantle which was affected by the earlier subduction.

Thermal evolution of the lithosphere beneath the western Pannonian Basin: Evidence from deep-seated xenoliths

Abstract A series of recently and previously collected peridotite xenolith samples from the western Pannonian Basin has intensively been studied using electron microprobe techniques and a thermo-barometric approach in order to obtain relevant information about the thermal evolution of the lithosphere. Most of the xenoliths have chemically homogeneous mineral phases reflecting ambient equilibrium P–T conditions. The results indicate hot and thinned lithosphere in agreement with heat flow, seismic and magnetotelluric properties of the area. A strong relationship has been observed between deformation and temperatures of peridotites, in as much as coarse-grained protogranular and poikilitic xenoliths have high temperatures (up to 1175°C), whereas fine-grained equigranular and mosaic xenoliths have low temperatures (800–900°C). This picture suggests that diapiric uplift of hot mantle material into a cooler uppermost mantle has probably taken place.

Magma fractionation replenishment, and mixing as inferred from green-core clinopyroxenes in Pliocene basanite, southern Slovakia.

Abstract Electron microprobe analyses were made of 43 green-core clinopyroxene phenocrysts and 5 clinopyroxene megacrysts (233 analyses in total) in two Pliocene basanite samples from the Hungarian-Slovakian border area. There are two varieties of clinopyroxene cores. The most common defines a fractionation trend of Mg#s [Mg = Mg/(Mg + Fe + Mn)] from 0.84 to 0.42, is Al-rich (9–13 wt.%) and has Al (VI) / Al (IV ) ratios of 0.34-0.71 and Ti/Al ratios of 0.12-0.17. They can be anhedral, euhedral and partly resorbed, either homogeneous or oscillatory zoned, and there are sometimes inner and outer cores with compositional gaps between them. Less common are highly resorbed Fe-rich diopside cores with lower Al and Ti but Al (VI) /Al (IV) and Ti/Al ratios similar to those of the first type. Mafic (Mg# 0.85-0.77) clinopyroxene mantles surround all cores and have Al (VI) / Al (IV) and Ti/Al ratios similar to the cores. Finally, all grains have outermost rims with Fe/Mg ratios and Ti compositionally continuous from clinopyroxene mantle compositions. Overall compositions of the rims are identical to those of the basanite groundmass clinopyroxenes. Megacryst compositions resemble those of the first core type and have the same kinds of mantles and rims. We interpret the green-core phenocrysts as originating in a mantle alkalic basalt magma reservoir undergoing fractionation, replenishment and convective mixing. While cores record liquids in various stages of compositional evolution, the clinopyroxene mantles indicate that all cores (and megacrysts) shared a common mafic liquid prior to eruption of the host basalt. Compositional continuity between clinopyroxene mantles and rims suggests that the mantles represent liquid parental to the host basanite. Al and Ti abundances in the Fe-rich diopside cores suggest that they are unrelated to the main core type and megacrysts, and are perhaps wall-rock xenocrysts that crystallized from different highly evolved magma. This study of green-core phenocrysts is basaltic rocks of southern Slovakia identifies processes of magma replenishment and mixing associated with alkalic basalt origins. But because green-core phenocrysts are not common in basalts, special conditions, such as low rates of magma replenishment, may be essential to allow magma evolution to produce these types of pyroxene compositions and zonation.

Clinopyroxene zoning patterns in the young alkali basalts of Hungary and their petrogenetic significance

In the Upper Pliocene, during the final phase of igneous activity within the Pannonian Basin, alkali basalts were erupted. Their occurrences are restricted to two main regions in Hungary: 1) Transdanubia; 2) Nógrád County. The clinopyroxene phenocrysts of these rocks are zoned titanaugites which often have distinctive colourless or green cores. Two kinds of green cores can be distinguished: 1. olive-green cores (fassaitic augite and fassaite), and 2. grassgreen cores (salite and ferrosalite). Both types of green cores are comparatively iron-rich and they occur only in the basalts of Nógrád County. The olive-green fassaites probably precipitated from relatively evolved melts which have been mixed into their present host magmas, whereas the grassgreen salites and ferrosalites are xenocrysts derived probably from upper mantle rocks. The clinopyroxene zoning patterns suggest, that after being generated by small-degree partial melting in the mantle the Transdanubian basalts ascended to the surface with little or no modification en route, whereas those of Nógrád County had a more complex evolution, in which fractionation at depth and magma mixing played an important role.

Origin of megacrysts and pyroxenite xenoliths from the Pliocene alkali basalts of the Pannonian Basin (Hungary)

Clinopyroxene and amphibole megacrysts and type II pyroxenite xenoliths occur in young (mainly Pliocene) alkali basalts of the Pannonian Basin. Major and trace element contents (the latter were determined by LAM-ICP-MS) of the clinopyroxenes and amphiboles suggest that megacrysts and type II xenoliths share a common origin. Both suites are fragments of igneous cumulates and pegmatitic veins that crystallised from alkali basaltic melts in the upper mantle at 11-14 kbar pressures. Trace element contents of the calculated hypothetical melts in equilibrium with the megacrysts and type II xenoliths are very similar to those of the host alkali basalts, but 8 7 Sr/ 8 6 Sr ratios are significantly lower and 1 4 3 Nd/ 1 4 4 Nd ratios are higher in the megacrysts and xenoliths than in the Pliocene alkali basalts that host them. The source rocks of the megacrysts and type II xenoliths had probably crystallised from earlier intrusions of alkali basaltic melts in the lithospheric mantle and were disrupted later by more voluminous intrusions of compositionally similar but isotopically more enriched alkali basaltic magmas.

Oxygen isotope ratios of phenocrysts from alkali basalts of the Pannonian basin: Evidence for an O-isotopically homogeneous upper mantle beneath a subduction-influenced area

Oxygen isotope compositions of olivine and pyroxene phenocrysts and pyroxene and amphibole megacrysts from Neogene alkali basalts of the Pannonian basin (0.5–11 Ma) have been determined by laser fluorination. Measured δ18O values in olivine and clinopyroxene phenocrysts show rather restricted variations from 5.00 to 5.20‰ and from 5.07 to 5.34%., respectively, with cpx-ol fractionations Δ18O(cpx-ol) ranging from + 0.04 to + 0.29‰. These δ18O values are significantly lower than those of the corresponding whole rocks, suggesting that low temperature alteration has increased the 18O16O ratios of the groundmass of host rocks, even in fresh looking samples, whereas their phenocrysts have retained original oxygen isotope compositions. The uniform oxygen isotope ratio in the phenocrysts suggests that the mantle source of the alkali basalts was also homogeneous with respect to its oxygen isotope composition, which is in contrast to the relatively wide variation of Sr, Nd and Pb isotope ratios in the source. Variations in radiogenic isotope compositions in the basalts have been explained by the interaction of subduction-related fluids with the mantle source of the basalts. If this is the case, then the fluids which caused significant changes in the Sr and Pb isotope ratios of the mantle source clearly did not noticeably modify its oxygen isotope composition. These data support the opinion that the upper mantle is more homogeneous with respect to its oxygen isotope composition than it was previously considered.

Pyroxene chemistry and evolution of alkali basaltic rocks from Burgenland and Styria, Austria

SummaryThe mineral chemistry of several Pliocene alkali basaltic rocks from Burgenland and Styria (Eastern Austria) have been investigated in order to determine the evolution path of the basalt magmas prior to eruption. With their wide range of substitutions, clinopyroxenes provide the best records of the evolution history of rocks. Pyroxene phenocrysts of the investigated basalts show both concentric and sector zoning. The investigation of sector zoned crystals shows, that not only Ti, Al and Fe contents are different in different sectors but there can be significant differences also in their Cr content. This fact apparently suggests that the distribution of Cr between clinopyroxene and melt could be influenced by crystallization kinetics.The depth of crystallization and differentiation of the basalts can be estimated from Ti and Al contents of clinopyroxene phenocrysts. From a combination of data on clinopyroxene composition, compatible trace element contents and mg-values of the rocks, it is concluded, that the alkali basalts of Pauliberg and Steinberg underwent slight olivine and clinopyroxene fractionation in shallow magma chambers prior to eruption, while the nephelinite of Stradnerkogel evolved mainly through clinopyroxene fractionation under high pressure conditions, probably in the upper mantle.ZusammenfassungEinige pliozäne alkalibasaltische Gesteine aus dem Burgenland und der Steiermark wurden mineral-chemisch untersucht, um Aufschluß über ihre Evolution vor der Eruption zu erhalten. Klinopyroxene mit ihren vielfältigen Subtitutionsmöglichkeiten erlauben am besten eine Abschätzung der Evolution der Basalte. Pyroxen-Einsprenglinge der Basalte zeigen sowohl konzentrischen als auch sektoralen Zonarbau. Die von uns untersuchten Einsprenglinge zeigen in den verschiedenen Sektoren nicht nur unterschiedliche Gehalte an Ti, Al und Fe, sondern vielfach auch unterschiedliche Cr-Gehalte. Dies macht es wahrscheinlich, daß die Verteilung von Cr zwischen Klinopyroxen und Schmelze von der Kristallisations-Kinetik beeinflußt wird.Die Tiefe in der die Basalte kristallisierten und differenzierten kann von den Ti- und Al-Gehalten der Klinopyroxen- Einsprenglinge abgeschätzt werden. Die Zusammensetzung der Klinopyroxene im Verein mit den Gehalten an kompatiblen Spurenelementen und denmg-Werten der Gesteine erlauben den Schluß, daß die Alkalibasalte von Pauliberg und Steinberg vor ihrer Eruption eine geringfügige Olivinund Klinopyroxen-Fraktionierung in einer seichten Magmakammer erlebten. Der Nephelinit vorn Stradnerkogel hingegen erfuhr hauptsächlich eine Klinopyroxen Fraktionierung unter Hochdruck-Bedingungen, möglicherweise im oberen Erdmantel.

Cryptic metasomatism in clino- and orthopyroxene in the upper mantle beneath the Pannonian region

Abstract 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.

Origin and significance of poikilitic and mosaic peridotite xenoliths in the western Pannonian Basin: geochemical and petrological evidences

Peridotite 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.

Petrology of a new basalt occurrence in Hungary

SummaryA new occurrence of basalt (minimum K/Ar age 57.9 ± 2.2 m.y.) is reported from Budaliget, near Budapest. Major and minor element concentrations show that the basalts are alkaline and potassic in nature ranging from olivine tholeiite to moderately undersaturated basanite. High mg-values and concentrations of Ni and Cr indicate that some of the samples may represent primary compositions. High pressure accidental xenoliths, xenocrysts and cognate megacrysts are frequent and the chemical zoning patterns of olivine and pyroxene phenocrysts indicate a complex evolution and polibaric conditions for the crystallization. The discovery of the new basalt occurrence is important from a geodynamical point of view: prior to the last two major geodynamical events (Plio-Pleistocene rifting with associated alkali basaltic volcanism and collision of microplates with associated Miocene calk-alkaline volcanism) the continental lithosphere below NE Transdanubia may have experienced another rifting period in the Paleocene or Upper Cretaceous.ZusammenfassungAus der Nähe von Budapest (Budaliget) wird ein neues Basaltvorkommen (minimum K/Ar Alter 57.9 ± 2.2 m. J.) beschrieben. Die Konzentrationen der Haupt- und Spurenelemente zeigen dass die Basalte Kali-betont sind und dass sie zwischen Olivintholeiiten und mässig untersättigten Basaniten variieren. Hohe mg-Zahlen sowie Ni- und Cr-Konzentrationen sprechen dafür, dass die Proben zum Teil primäre Zusammensetzungen aufweisen. Xenolithe mit Hochdruckparagenesen, Xenokristalle und cognate Megakristalle kommen haufig vor. Der chemische Zonarbau von Olivinund Pyroxenphenokristallen deutet auf eine komplexe Kristallisationsgeschichte bei wechselnden Druck hin. Die Entdeckung dieses neuen Basaltvorkommens ist von geodynamischer Bedeutung: Vor den letzten zwei geodynamischen Ereignissen (a) Entstehung von Rifttekonik im Plio-Pleistozän mit alkalibasaltischem Vulkanismus und (b) Kollision von verschiedenen Mikroplatten begleitet von miozänem kalkalkalischem Vulkanismus war die Lithossphäre unterhalb von NE Transdanubien einer weiteren Riftperiode im Paleozän oder in der oberen Kreide ausgesetzt.