Timing of magma segregation governs distinct geochemical traits of late-stage mineral and melt phases in alkaline volcanic rocks

Three primitive alkaline SiO₂-undersaturated volcanic edifices from the southern Central European Volcanic Province (SW Germany) comprise coarse-grained, mm‑ to cm-thin veinlets and pockets of low-pressure differentiates, representing a missing link in an evolutionary trend towards phonolites. (1) In the Hegau region (Hohenstoffeln), a melilite-bearing olivine nephelinite contains ijolite pockets with skeletal perovskite, titanomagnetite, and euhedral fluorapatite. (2) Compositionally similar olivine melilitite from Urach (Sternberg) is crosscut by lileyite- and wadeite-bearing ijolite veinlets, in which titanomagnetite and perovskite lack 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. Despite variable host magma compositions, most differences between the occurrences are related to the exact timing of residual melt separation, a key factor in explaining the trace element evolution of late mineral and melt phases: at advanced clinopyroxene crystallization and perovskite saturation (Sternberg), massive trace element fractionation caused high Nb/Ta and Zr/Nb, but low LREE/HREE and Zr/Hf ratios in clinopyroxene and perovskite. Stabilization of LILE, Zr and Hf in the enriched residue, favoured by high F contents, finally lead to crystallization of an agpaitic assemblage. In contrast, melt separation before perovskite saturation (Hohenstoffeln) resulted in strong, uniform enrichment of LILE, HFSE, and REE in clinopyroxene and perovskite. The nepheline syenitic residues (Steinsberg) experienced a similar evolution with incompatible element enrichment in clinopyroxene and titanite, but their mineral assemblage differs from the ijolites due to higher aSiO₂ and aH₂O and lower F and P concentrations in the host melt.