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

Abstract

The Perşani volcanic field is a low-volume flux monogenetic volcanic field in the Carpathian–Pannonian region, eastern-central Europe. Volcanic activity occurred intermittently from 1200ka to 600ka, 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°C, followed by clinopyroxenes at about 1250°C and 0.8–1.2GPa. Melt generation occurred in the depth range from 85–90km to 60km. The estimated mantle potential temperature, 1350–1420°C, is the lowest in the Pannonian Basin. It suggests that no thermal anomaly exists in the upper mantle beneath the Perşani 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 Perşani 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 1.3years implying heating of the lower lithosphere by the uprising magma, whereas the second one lasted only 4–5days, which corresponds to the time of magma ascent through the continental crust. The alkaline mafic volcanism in the Perşani 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.