Geochemical and petrological constraints on mantle composition of the Ohře(Eger) rift, Bohemian Massif: peridotite xenoliths from the České Středohoří Volcanic complex and northern Bohemia

Abstract

The Ohře(Eger) rift in the Bohemian Massif, which represents part of the Cenozoic Central European Volcanic Province, contains abundant mantle xenoliths of spinel peridotite. Petrography, geothermometry, mineral compositions and whole-rock and clinopyroxene trace element compositions of xenolith suites from four localities in the central part Ceske Středohoři Volcanic complex (CSVC) and one locality in the eastern continuation (northern Bohemia) of the rift system display important differences that reveal small-scale compositional heterogeneity of the mantle. The xenoliths from the CSVC are mostly harzburgite that experienced high degrees of partial melting from ~17 to 21 %. However, xenoliths from one locality, Medvědický vrch, are predominantly fertile lherzolite. Subsequent pervasive metasomatism produced enrichments in light rare earth elements, large-ion lithophile elements, and U and Th, except for xenoliths from Medvědický vrch, which show marked depletions in all these elements. Such composition most likely reflects refertilization of the source rock by depleted melts. The trace element characteristics of the CSVC xenoliths indicate metasomatism of a depleted protolith by silicate melt at high melt/rock ratios. In contrast, harzburgite xenoliths from northern Bohemia experienced metasomatism at low melt/rock ratios by more evolved, alkaline and carbonate-rich melts.