The lanthanide tetrad effect and its correlation with K/Rb, Eu/Eu∗, Sr/Eu, Y/Ho, and Zr/Hf of evolving peraluminous granite suites

Abstract

Lanthanide tetrad effects are often observed in REE patterns of more highly evolved Variscan peraluminous granites of mid-eastern Germany (Central Erzgebirge, Western Erzgebirge, Fichtelgebirge, and Northern Oberpfalz). The degree of the tetrad effect (TE 1,3) is estimated and plotted vs. K/Rb, Sr/Eu, Eu/Eu∗, Y/Ho, and Zr/Hf. The diagrams reveal that the tetrad effect develops parallel to granite evolution, and significant tetrad effects are strictly confined to highly differentiated samples. Mineral fractionation as a cause for the tetrad effect is not supported by a calculated Rayleigh fractionation, which also could not explain the fractionation trends of Sr/Eu and Eu/Eu∗. The strong decrease of Eu concentrations in highly evolved rocks suggests that Eu fractionates between the residual melt and a coexisting aqueous high-temperature fluid. Mineral fractionation as a reason for the tetrad effect is even more unlikely as REE patterns of accessory minerals display similar tetrad effects as the respective host rocks. The accessory minerals inherit the REE signature of the melt and do not contribute to the bulk-rock tetrad effect via mineral fractionation. These results point in summary to significant changes of element fractionation behavior in highly evolved granitic melts: ionic radius and charge, which commonly control the element distribution between mineral and melt, are no longer the exclusive control. The tetrad effect and the highly fractionated trace element ratios of Y/Ho and Zr/Hf indicate a trace element behavior that is similar to that in aqueous systems in which chemical complexation is of significant influence. This distinct trace element behavior and the common features of magmatic-hydrothermal alteration suggest the increasing importance of an aqueous-like fluid system during the final stages of granite crystallization. The positive correlation of TE 1,3 with bulk-rock fluorine contents hints at the importance of REE fluorine complexation in generating the tetrad effect. As the evolution of a REE pattern with tetrad effect (M-type) implies the removal of a respective mirroring REE pattern (W-type), the tetrad effect identifies open system conditions during granite crystallization.