Variable onset of magma segregation controls contrasting geochemical patterns of mineral assemblages in fractionating alkaline volcanic systems

Abstract

Three occurrences of primitive, alkaline, SiO₂-undersaturated, volcanic rocks from SW Germany show coarse-grained, mm- to cm-thin veinlets and pockets of differentiated lithologies that are a missing link in an evolutionary trend towards phonolites. (1) In the Hegau region (Hohenstoffeln), melilite-bearing olivine nephelinite contains ijolite pockets with skeletal perovskite, titanomagnetite, and euhedral fluorapatite. (2) Compositionally similar olivine melilitite at Urach (Sternberg) is crosscut by ijolite veinlets with late-magmatic lileyite and wadeite, in which subhedral–euhedral titanomagnetite and perovskite lack a skeletal shape. (3) Nepheline syenitic domains in a phlogopite-nepheline basanite from the Kraichgau region (Steinsberg) contain less clinopyroxene, titanomagnetite, and apatite, but accessory titanite. Differences between the fractionation products partly result from variable host magma compositions. However, the exact starting point of residual melt separation was the key factor explaining the trace element evolution of late mineral phases. Under conditions of advanced clinopyroxene crystallization and perovskite saturation (Urach), massive trace element fractionation led to high Nb/Ta and Zr/Nb, but low LREE/HREE and Zr/Hf ratios in clinopyroxene and perovskite. High F contents contributed to the stabilization of LILE, Zr, and Hf in the enriched residual melt which finally caused crystallization of an agpaitic assemblage. In contrast, melt separation before perovskite saturation (Hegau) resulted in strong, uniform enrichment of LILE, HFSE, and REE in clinopyroxene and perovskite. The nepheline syenite (Kraichgau) underwent a similar evolution with incompatible element enrichment in clinopyroxene and titanite, although its mineral assemblage differs from the ijolites due to higher SiO₂ and H₂O but lower F and P concentrations in the melt.