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摘要
At the end of the syn-extensional phase in the Carpathian-Pannonian Region (CPR), alkaline mafic magmas were erupted during the Late Miocene in Burgenland.
The majority of these lavas (Pauliberg and Oberpullendorf) are basalts and tephrite basanites. They have been slightly affected by high-pressure (>1.3 GPa) clinopyroxene fractionation. In the Burgenland basalts, olivine and clinopyroxene phenocrysts, as well as groundmass olivine, clinopyroxene and plagioclase grew at lower pressures. The geochemistry of the Pauliberg lavas (alkali basalts and basanites) indicates that they originated by low, but variable, degrees of melting from the same source mantle. The Pauliberg basaltic rocks have significantly higher TiO2 and lower Al2O3 contents, compared to those of Late Miocene to Recent alkaline basalts from the Pannonian Basin. The Ti enrichment could be attributed to low degrees of melting (beneath thick lithosphere) of a peridotite source that had been affected by Ti-rich recycled ancient oceanic crust, whereas the low Al and high (La/Yb)(N) suggest that there was residual garnet after partial melting. The depletion in K, Rb and Ba relative to Nb and the high Nb/La and Ce/Pb ratios (OIB-like) in the Pauliberg basalts, rule out any interaction with subduction-related melts/fluids and/or contamination with crustal material.
Sr and Nd radiogenic isotopes range from 0.703687 to 0.704279 and from 0.512736 to 0.512774 respectively, with the basanites being the most depleted. The Oberpullendorf sample has Sr and Nd isotopic ratios (Sr-87/Sr-86 = 0.704279; Nd-143/Nd-144 = 0.512736) similar to those of other Pannonian basalts, e.g. Saghegy. The Burgenland basalts have relatively high Pb-206/Pb-204 isotopic ratios (19.6-19.7) suggesting a HIMU/OIB-like character (Embey-Isztin et al., 1993).
The calculated mantle potential temperature for the Pauliberg basalts is 1386 degrees C and a melt fraction of similar to 2%. Similar calculations for the Oberpullendorf basalt indicate clinopyroxene fractionation. This leads to an overestimate of the mantle potential temperature (1530 degrees C), making it impossible to calculate the degree of partial melting involved in the genesis of the primary magma. These calculations indicate that the Burgenland basalt melted from mantle at ambient temperature. i.e. no thermal anomaly is indicated, providing an additional argument against plume activity beneath the Pannonian Basin. Consequently we propose that late Miocene lithospheric extension of the Pannonian Basin gave rise to alkaline melt generation beneath the studied area through passive upwelling and adiabatic decompressional melting of an asthenospheric mantle source.
- 出版日期2011-1
