SIGNIFICANCE OF MANTLE SHEAR ZONES IN THE EASTERN TRANSYLVANIAN BASIN

Abstract

Peridotitic upper mantle xenoliths provide an opportunity to study physico-chemical processes in the upper mantle. They also offer an exclusive chance to correlate, control and construct geophysical models dealing with the upper mantle. The subduction of the European lithosphere occurred along the Carpathian arc. Numerous studies demonstrated the presence of the subducting slab in the Vrancea-zone beneath the south-easternmost segment of the arc. The earlier position of the subducted European lithosphere is suggested to be more to the northwest, beneath the southeastern portion of the Transylvanian Basin. Alkali basalts of the Eastern Transylvanian Basin Volcanic Field, erupted during the late-Miocene-Pleistocene, are extremely rich in mantle peridotites and pyroxenites, which represent a wide variety of textures, modal major and trace element and isotopic compositions, suggested to represent the full subcontinental lithospheric mantle column beneath the Eastern Transylvanian Basin. There is growing evidence that deformation, in response to subduction rollback and basin formation was significantly different in the lithospheric mantle beneath the Transylvanian Basin than beneath other portions of the Intra Carpathian Basin System. Special xenoliths with mylonitic textures (Fig.1), representing mantle shear zones, were collected from the Eastern Transylvanian Basin Volcanic Field. Such textural types have never been described in other xenolith locations of the whole basin system. These mylonites indicate extreme localization of deformation. Shear zones are believed to largely reduce the strength of the lithosphere. In the mantle their formation could be related to melt infiltration and melt weakening. Whereas, the extreme localization is attributed to the switch of deformation mechanism from grain size independent to grain size dependent deformation. The main aim of this study is to analyze the peridotite mylonites in a complex way regarding their petrologic and geochemical characteristics, as well as their structure and fabric. Special attention is paid to the structural development of the mylonitic rocks, whereas the role of melt, occurring now as interstitial glass (Fig. 2), in the mylonite formation is also thoroughly analyzed.