The jotunite of the Korosten AMCG complex (Ukrainian shield): Crust- or mantle-derived?

Abstract

The Korosten Plutonic Complex (KPC) of Palaeoproterozoic age is made up of an AMCG suite of rocks. Jotunitic/ferromonzodioritic magmas have been emplaced at the margins or within massive anorthosites and in hypabyssal dykes coeval with the AMCG plutonism. Major and trace element geochemistry shows a calc-alkalic to alkali calcic trend with large variations of composition with SiO2 ranging from 44% to 62% and Mg# from 19 to 36, high concentrations of Fe, Ti and P, up to 20% Fe2O3t, 4% TiO2 and 2% P2O5. The Korosten jotunitic sequence of rocks matches the jotunite liquid line of descent that has been defined in the Rogaland anorthosite province. Both trends are compared to experimental data in dry conditions and medium to high pressure on the evolution of olivine tholeiite melt. It is shown that the initial step of the jotunite evolution characterized by a decrease in SiO2 correlated with increases in Fe and P is not reproduced in the experiments at medium pressure. At high pressure such evolution is possible but cannot yield SiO2 rich melts with further fractionation because of the existence of a thermal barrier. Derivation of jotunite melts from mantle-derived olivine tholeiite would be possible in a polybaric evolution of the melt. However, the polybaric evolution of the anorthosite mush with extraction of the interstitial jotunite melt to form the mafic marginal intrusion is not supported by field evidence of filter press mechanism. Derivation of the jotunite from melting of gabbroic rocks in the crust and their evolution at medium pressure appears as a more convincing mechanism. The ages of formation of the Korosten and Rogaland anorthosites are separated by 800Myr. It is speculated that the secular cooling of the earth has modified the generation process of the anorthosite. In the KPC a hot zone developed in the crust due to an asthenosphere uprise concomitant with the development of Ni-rich tholeiitic dykes, and in Rogaland, the cooler and more rigid crust was able to subduct and form crustal tongues that were potential source of melts.