Origin of basaltic magmas of Per艧ani volcanic field, Romania: A combined whole rock and mineral scale investigation

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• 作者：Szabolcs Harangi ; Tam谩s S谩gi ; Ioan Seghedi ; Theodoros Ntaflos
• 关键词：Alkaline basalt ; Olivine ; Chromian spinel ; Monogenetic volcanic field ; Magmagenesis ; Carpathian&ndash ; Pannonian region
• 刊名：Lithos
• 出版年：November, 2013
• 年：2013
• 卷：180-181
• 期：Complete
• 页码：43-57
• 全文大小：2368 K

文摘

The Per艧ani volcanic field is a low-volume flux monogenetic volcanic field in the Carpathian-Pannonian region, eastern-central Europe. Volcanic activity occurred intermittently from 1200 ka to 600 ka, forming lava flow fields, scoria cones and maars. Selected basalts from the initial and younger active phases were investigated for major and trace element contents and mineral compositions. Bulk compositions are close to those of the primitive magmas; only 5-12% olivine and minor spinel fractionation occurred at 1300-1350 掳C, followed by clinopyroxenes at about 1250 掳C and 0.8-1.2 GPa. Melt generation occurred in the depth range from 85-90 km to 60 km. The estimated mantle potential temperature, 1350-1420 掳C, is the lowest in the Pannonian Basin. It suggests that no thermal anomaly exists in the upper mantle beneath the Per艧ani area and that the mafic magmas were formed by decompression melting under relatively thin continental lithosphere. The mantle source of the magmas could be slightly heterogeneous, but is dominantly variously depleted MORB-source peridotite, as suggested by the olivine and spinel composition. Based on the Cr-numbers of the spinels, two coherent compositional groups (0.38-0.45 and 0.23-0.32, respectively) can be distinguished that correspond to the older and younger volcanic products. This indicates a change in the mantle source region during the volcanic activity as also inferred from the bulk rock major and trace element data. The younger basaltic magmas were generated by lower degree of melting, from a deeper and compositionally slightly different mantle source compared to the older ones. The mantle source character of the Per艧ani magmas is akin to that of many other alkaline basalt volcanic fields in the Mediterranean close to orogenic areas. The magma ascent rate is estimated based on compositional traverses across olivine xenocrysts using variations of Ca content. Two heating events are recognized; the first one lasted about 1.3 years implying heating of the lower lithosphere by the uprising magma, whereas the second one lasted only 4-5 days, which corresponds to the time of magma ascent through the continental crust. The alkaline mafic volcanism in the Per艧ani volcanic field could have occurred as a response to the formation of a narrow rupture in the lower lithosphere, possibly as a far-field effect of the dripping of dense continental lithospheric material beneath the Vrancea zone. Upper crustal extensional stress-field with reactivation of normal faults at the eastern margin of the Transylvanian basin could enhance the rapid ascent of the mafic magmas.