Trace element and isotopic composition of mafic and ultramafic cumulate xenoliths in alkaline basalts from the Eastern Rhodopes, Bulgaria: Inferences on deep processes under the metamorphic core complexes

Abstract

Oligocene within-plate alkaline basalts, intruded in the Eastern Rhodope metamorphic core complexes Kesebir and Biala Reka, carry ultramafic and mafic xenoliths, consisting of clinopyroxenites, websterites, orthopyroxenites, 2-pyroxene- and clinopyroxene gabbros. These xenoliths are interpreted as samples of layered intrusions formed at upper mantle-lower crustal depths. Calculated parental liquids from LA ICP-MS analyses of clinopyroxene in the ultramafic clinopyroxenites, orthopyroxenite and websterite indicate that they may have formed from melts similar in composition to their host basanitic magma. The clinopyroxene REE patterns of least magnesian websterites suggest amphibole and clinopyroxene fractionation. Trace element compositions of gabbros and high-Fe clinopyroxenites indicate that they are the most differentiated product from the same parent. Calculated parental liquids for some of them, however, show geochemical features typical for subduction-related rocks. In addition, Sr and Nd isotopic compositions of evolved websterites and gabbros are more radiogenic than those of the host basanites. This transition from within- plate to subduction-related signature can be related to fractionation of amphibole, accompanied by crustal contamination of the basanitic magma with more radiogenic lower crustal material.