Lithosphere thermal structure at the eastern margin of the Bohemian Massif: a case petrological and geophysical study of the Niedzwiedz amphibolite massif (SW Poland)

摘要

The crustal section beneath amphibolite Niedzwiedz Massif (Fore-Sudetic Block in NE Bohemian Massif), modelled on the basis of geological and seismic data, is dominated by gneisses with subordinate granites (upper and middle crust) and melagabbros (lower crust). The geotherm was calculated based on the chemical anal-yses of the heat-producing elements in the rocks forming the crust and the measurements of their density and heat conductivity. The results were verified by heat flow calcu-lations based on temperature measurements from 1,600 m deep well in the Niedzwiedz Massif and by temperature-depth estimates in mantle xenoliths coming from the nearby ca. 4.5 My basanite plug in Lutynia. The paleocli-mate-corrected heat flow in the Niedzwiedz Massif is 69.5 mW m~(-2), and the mantle heat flow is 28 mW m~(-2). The mantle beneath the Massif was located marginally relative to the areas of intense Cenozoic thermal rejuvena-tion connected with alkaline volcanism. This results in geotherm which is representative for lithosphere parts located at the margins of zones of continental alkaline volcanism and at its waning stages. The lithosphere-asthenosphere boundary (LAB) beneath Niedzwiedz is located between 90 and 100 km depth and supposedly the rheological change at LAB is not related to the appearance of melt.