SIGNIFICANCE OF PYROXENITES MANTLE XENOLITHS FROM THE PERSANI MOUNTAINS (EASTERN TRANSYLVANIAN BASIN) BASED ON PETROLOGIC AND GEOCHEMICAL STUDY

Abstract

The Persani Mts. in the Eastern Transylvanian Basin is the easternmost Plio-Pleistocene alkali-basaltic volcanic field in the Carpathian–Pannonian Region. The products of the volcanic activity are lava flows and pyroclastic rocks, in which peridotites as xenoliths from the upper mantle can often be found. Besides peridotite xenoliths, pyroxenites are also quite common. The mineralogical composition of these pyroxenite xenoliths is mostly clinopyroxene, olivine, garnet, spinel ± orthopyroxene and amphibole. According to their modal composition, they are olivine clinopyroxenites, clinopyroxenites and websterites. We classified the pyroxenites into spinel-bearing pyroxenites (representing type I/I and type II/I series) and garnet-bearing pyroxenites. In the type I/I series clinopyroxene is diopsidic whereas in type II/I series its composition is augitic. In spinel pyroxenites, besides clinopyroxene, olivine is the second common mineral in some cases accompanied by orthopyroxene and amphibole. In garnet-bearing pyroxenites olivine and amphibole are absent, whereas orthopyroxene can often be observed. Textural observations suggest deformation events and mineral reactions, the results of changes in stress and p-T conditions during the evolution of the mantle rocks. Both clinopyroxene and orthopyroxene occur in each other as exsolution lamellae, which in some cases are curved. Garnet often contains exsolved needles of rutile and is always surrounded by symplectitic intergrowth of secondary orthopyroxene, clinopyroxene, spinel and plagioclase. Thermobarometry was based upon EMP analyses of the “primary” rock forming minerals. Equilibrium pressure values estimated from single clinopyroxene compositions are between 1,1 and 1,4 GPa, whereas garnet orthopyroxene barometry yielded 1,3-1,6 GPa. Orthopyroxene-clinopyroxene equilibrium temperatures are in the range 1070-1140°C, temperatures obtained from garnet-clinopyroxene pairs are between 1030-1170°C (at 1,1-1,3 GPa). Thermobarometric calculations were also made on the symplectite-forming minerals. The clinopyroxene barometer yielded 1,0-1,3 GPa for the symplectite-formation. Temperature values calculated with the compositional data of symplectite-forming clinopyroxene-orthopyroxene are 1070-1120°C. The variation in lithology points to the heterogeneity of the upper mantle. Based on the textural relationships and the thermobarometric results, the evolution of the xenoliths can be outlined: the deformation and pressure decrease (upwelling) in the lithospheric mantle before alkaline basaltic volcanism. The inferred p-T-path of the Persani Mts. pyroxenites fits well previously studied garnet peridotites. We used geochemical data to determine the equilibrium conditions of the xenolith formation, and the p-T path along with the degree of the mantle upwelling in the Eastern- Carpathian region is estimated by thermobarometric calculations, too.