PETROLOGY AND GEOCHEMISTRY OF LATE TERTIARY QUATERNARY MAFIC ALKALINEVOLCANISM IN ROMANIA

Alkaline volcanic activity occurred in the Persani Mountains (eastern Transylvanian Basin) and Banat (eastern Pannonian Basin) regions of Ro mania between 2.5 Ma and 0.7 Ma. This volcanism followed an extended p eriod of subduction-related mostly andesitic and dacitic magmatism in the Eastern Carpathian are. The Persani Mts. alkaline activity coincid ed with the last phase of subduction-related activity. Several lava fl ows and pyroclastic deposits in the Persani Mts. carry peridotitic man tle xenoliths and amphibole megacrysts. Major- and trace-element geoch emistry indicates that the alkaline magmas are primitive, silica-under saturated alkali basalts and trachybasalts (7.8-12.3 wt.% MgO; 119-207 ppm Ni; 210-488 ppm Cr) which are LREE-enriched. Mantle-normalised tr ace-element diagrams reveal an overall similarity to continental intra plate alkali basalts, but when compared with a global average of ocean island basalts (GIB), the Banat lavas are similar to average GIB, whe reas the Persani Mts. basalts have higher Rb, Ba, K and Pb and lower N b, Zr and Ti. These features slightly resemble those of subduction-rel ated magmas, particularly those of a basaltic andesite related to the nearby older are magmas. With Sr-87/Sr-86 varying from 0.7035-0.7045 a nd Nd-143/Nd-144 from 0.51273-0.51289, the Romanian alkali basalts are indistinguishable from those of the western Pannonian basin (Hungary and Austria) and Neogene alkali basalts throughout Europe. Amphibole m egacrysts have similar isotopic signatures, and their REE patterns ind icate derivation by crystallisation from a mafic alkaline magma. The a ge-corrected Sr and Nd isotopic compositions of a calc-alkaline basalt ic andesite related to the preceeding period of subduction also lies w ithin the field of the younger alkaline magmas. Pb isotopic ratios of the Romanian alkali basalts do not lie on the NHRL, but overlap the fi eld of Tertiary alkali basalts from the western Pannonian basin, and h ave unusually high Pb-207/Pb-204 at a given Pb-206/Pb-204. Thus it is probable that, although the Romanian alkali basalts have a strong asth enospheric (i.e. OIB-type mantle source) component, their Pb isotopic characteristics were derived from mantle which was affected by the ear lier subduction.