Dejan Prelević

The analcime problem and its impact on the geochemistry of ultrapotassic rocks from Serbia

Abstract Tertiary ultrapotassic volcanic rocks from Serbia occasionally display low levels of K2O and K2O/ Na2O. In these rocks, analcime regularly appears as pseudomorphs after pre-existing leucite microphenocrysts. The process of leucite transformation in Serbian ultrapotassic rocks is very thorough: fresh leucite survives only in ugandites from the Koritnik lava flows as well as in rare inclusions in Cpx. This paper focuses on the impact of ‘analcimization’ on the mineralogy and geochemistry of the Serbian ultrapotassic rocks, using the samples where leucite survived as a monitor for the process. Analcimization has had a great impact on the geochemistry of the rocks, but affects only a restricted number of chemical parameters. These are the falsification of the original K2O/Na2O ratio, the decoupling of large-ion lithophile elements resulting in considerable depletion of Rb and K2O, but not of Ba, and sporadic, but extreme enrichment of Cs in some analcime-bearing samples (up to 900 ppm). Analcimization is also recognized by an increase in whole-rock S18O values of ~3‰ compared to fresh rocks, which correlates with the level of whole-rock hydration. Finally, the 87Sr/86Sr enrichment at nearly constant 143Nd/144Nd demonstrated by some rocks can also be explained by the analcimization of leucite. For samples with variable 87Sr/86Sr from the same lava flow, 87Sr/86Sr values correlate with modal analcime abundance (ex-leucite), loss on ignition of whole-rock and whole-rock δ18O values. The extreme depletion in K and enrichment in Na, together with modification of other geochemical parameters, may have led to the misinterpretation of the origin and geodynamic affiliations of the Serbian ultrapotassic rocks, had the effects of analcimization not been taken into account.

Orogenic vs anorogenic lamproites in a single volcanic province: Mediterranean-type lamproites from Turkey

Lamproites are mantle-derived ultrapotassic volcanic rocks, derived from phlogopite-bearing harzburgitic source. The origin of the metasomatism which enriched their mantle source is either an old event with a metasomatic component derived from the convecting mantle, or is a more recent introduction of an already aged metasomatic component. Together with different trace element signatures, this view serves for a general distinction between anorogenic and orogenic lamproites worldwide. In Turkey, lamproitic volcanism resulted from interplay of subduction/collisional and postcollisional/extensional regime since Miocene until Pliocene, in the Western Anatolia-Aegean and Kirka-Afyon-Isparta region. We present new set of Ar-Ar age data together with geochemical data, including Sr, Nd and Pb isotopes, of all Turkish lamproites. Our study revealed the most intriguing geochemical data: although the majority of lamproites have an orogenic affinity, with radiogenic 87Sr/86Sr, 207Pb/204Pb, and unradiogenic 143Nd/144Nd and 206Pb/204Pb, and high LILE/HFSE ratios, the lamproites from the most southern localities (Bucak area) exhibit geochemical features characteristic for anorogenic lamproites, with unradiogenic 87Sr/86Sr, 207Pb/204Pb, and radiogenic 143Nd/144Nd and 206Pb/204Pb, coupled with rather smooth incompatible trace element pattern with low LILE/HFSE ratios and high concentrations of Nb and Ti. The common and coeval occurrence of anorogenic and orogenic lamproites in a single volcanic province suggests that geodynamic distinction of lamproites based only on geochemistry may be questioned.

Mafic alkaline metasomatism in the lithosphere underneath East Serbia: Evidence from the study of xenoliths and the host alkali basalts

Abstract Effects of mafic alkaline metasomatism have been investigated by a combined study of the East Serbian mantle xenoliths and their host alkaline rocks. Fertile xenoliths and tiny mineral assemblages found in depleted xenoliths have been investigated. Fertile lithologies are represented by clinopyroxene (cpx)-rich lherzolite and spinel (sp)-rich olivine websterite containing Ti–Al-rich Cr-augite, Fe-rich olivine, Fe–Al-rich orthopyroxene and Al-rich spinel. Depleted xenoliths, which are the predominant lithology in the suite of East Serbian xenoliths, are harzburgite, cpx-poor lherzolite and rare Mg-rich dunite. They contain small-scale assemblages occurring as pocket-like, symplectitic or irregular, deformation-assisted accumulations of metasomatic phases, generally composed of Ti–Al- and incompatible element-rich Cr-diopside, Cr–Fe–Ti-rich spinel, altered glass, olivine, apatite, ilmenite, carbonate, feldspar, and a high-TiO2 (c. 11 wt%) phlogopite. The fertile xenoliths are too rich in Al, Ca and Fe to simply represent undepleted mantle. By contrast, their composition can be reproduced by the addition of 5–20 wt% of a basanitic melt to refractory mantle. However, textural relationships found in tiny mineral assemblages inside depleted xenoliths imply the following reaction: opx+sp1 (primary mantle Cr-spinel) ±phlogopite+Si-poor alkaline melt=Ti–Al-cpx+sp2 (metasomatic Ti-rich spinel)±ol±other minor phases. Inversion modelling, performed on the least contaminated and most isotopically uniform host basanites (87Sr/86Sr=c. 0.7031; 143Nd/144Nd=c. 0.5129), implies a source that was enriched in highly and moderately incompatible elements (c. 35–40× chondrite for U–Th–Nb–Ta, 2× chondrite for heavy rare earth elements (HREE), made up of clinopyroxene, carbonate (c. 5%), and traces of ilmenite (c. 1%) and apatite (c. 0.05%). A schematic model involves: first, percolation of CO2- and H2O-rich fluids and precipitation of metasomatic hydrous minerals; and, second, the subsequent breakdown of these hydrous minerals due to the further uplift of hot asthenospheric mantle. This model links intraplate alkaline magmatism to lithospheric mantle sources enriched by sublithospheric melts at some time in the past.

Potassium-rich magmatism from a phlogopite-free source

The generation of strongly potassic melts in the mantle is generally thought to require the presence of phlogopite in the melting assemblage. In the Mediterranean region, trace element and isotope compositions indicate that continental crustal material is involved in the generation of many potassium-rich lavas. This is clearest in ultrapotassic rocks like lamproites and shoshonites, for which the relevant chemical signals are less diluted by extensive melting of peridotite. Furthermore, melting occurs here in young lithosphere, so the continental crust was not stored for a long period of time in the mantle before reactivation. We have undertaken two types of experiments to investigate the reaction between crust and mantle at 1000–1100 °C and 2–3 GPa. In the first, continental crustal metasediment (phyllite) and depleted peridotite (dunite) were juxtaposed as separate blocks, whereas in the second, the same rock powders were intimately mixed. In the first series, a clear reaction zone dominated by orthopyroxene was formed between dunite and phyllite but no hybridized melt could be found, whereas analyzable pools of hybridized melt occurred throughout the charges in the second series. Melt compositions show high abundances of Rb (100–220 ppm) and Ba (400–870 ppm), and consistent ratios of Nb/Ta (10–12), Zr/Hf (34–42), and Rb/Cs (28–34), similar to bulk continental crust. These experiments demonstrate that melts with as much as 5 wt% K 2 O may result from reaction between melts of continent-derived sediment and depleted peridotite at shallow mantle depths without the need for phlogopite or any other potassic phase in the residue.

Geology of South-Eastern Europe

The region of South-Eastern Europe (SEE) occupies an important segment of the Alpine–Himalayan collisional orogenic belt and consists of several Phanerozoic mobile belts. The SEE region inherits its geology from the evolution of the Vardar Tethys ocean, which existed in-between the Eurasian (Europe) and Gondwana (Africa) continental plates and which relicts presently occur along the Vardar–Tethyan mega-suture. This synthesis, therefore, consists of (1) pre-, (2) syn- and (3) post-Vardar–Tethyan geology of SEE. Pre-Vardar–Tethyan geology on the European side is reflected by geological units formed from Precambrian to Mesozoic times and include the Moesian platform, the Dacia mega-unit and the Rhodopes. On the Gondwana side, it is represented by the External Dinarides, the Dalmatian-Ionian Zone and Stable Adria (Apulia), all principally formed from Paleozoic to Mesozoic times. The Syn-Vardar–Tethyan units encompass the bulk of the geological framework of SEE. They are a physical record of the former existence of the Mesozoic oceanic lithosphere, being represented dominantly by ophiolites and trench/accretionary wedge (melange) assemblages, which originated and were reworked during the life-span of the Vardar Tethys. The Post-Vardar–Tethyan geological evolution refers to the time period from the final closure of the Vardar Tethys until present. It comprises all rocks that stratigraphically overlie the Vardar–Tethyan mega-suture and seal the contacts between the mega-suture and the surrounding geological units. This is the time characterized by rapid extension coupled with exhumation of the lower crustal material, high heat flow, both intrusive and extrusive magmatism and considerable lithosphere thinning.

No need for involvement of a hidden mantle reservoir in the origin of lamproites from Mediterranean

ICP and XRF Spectroscopy methods were used to evaluate the metals (ppm) from soils for the mining dumps of Crucea- Botusana uranium deposit (Bistrita Mountains, Romania). The sequential extraction has emphasized the fact that U is associated with all the mineral fractions present in the soil samples. A great percentage of U can be found in the carbonate, organic and oxides fractions. The percentage of U detected in the exchangeable fraction is rather small. The fact that 21.77% of the total U can be found in the specifically absorbed and carbonate bound fraction, indicated the important role played by the carbonates in the retention of U; one the other hand this fraction is liable to release U if the pH should happen to change.Th appear in high-enough concentration in the soil is scarcely available because 70.29% is present in residual fraction, and about 21.78% in the organic and oxides fractions. This is certainly due to the fact that this naturally occurring radionuclide can be associated with relatively insoluble mineral phases like alumino-silicates and refractory oxides. Its association with the organic matter suggests that it can form soluble organic complexes that can facilitate its removal by the stream waters. In the case of Sr, the sequential extraction shows that it is very strongly fixed because the residual fraction concentrates the great amount of this element. What is interesting is the percentage of 2.65 % of Sr from the exchangeable fraction because it can be easily released and transported to the surrounding environment. Pb it is present in various relatively soluble pools (17.81% in carbonate boud and 34.85% in organically bound), which appears to be an efficient sink for this element. This fact may indicate a possible link between the biological activity and the Pb cycling into the soil. In addition, only 17.78% is present in the insoluble residual fraction. Although from our research it resulted that the radioactive metals does not concentrate in the exchangeable fraction (Th) or it concentrates very little in it (U and Sr), the isolation of the mineral fraction of soil rich in U, Th and Sr helps us in the future identification of the connections which control the cycle of the radioactive metals. These results have important implications for remediation strategies. The thorium and uranium from Crucea mining area are in labile, not strongly retained, fractions, thus making them amendable for remediation by phytoremediation.