Barium-rich sanidine megacrysts from the West Eifel (Germany)

Abstract

Sanidine megacrysts are erupted together with syenitic and phonolitic debris in a Quaternary carbonatite diatreme located within the West Eifel volcanic field (Germany). The sanidines have a limited compositional range (Or 7 0 - 8 0 ), which overlaps with that of sanidine phenocrysts from syenite and phonolite xenoliths from the same diatreme. The sanidines have BaO contents of c. 1 wt.%, low total REE, but large positive Eu anomalies (Eu/Eu* = 7.5-9.2). A contrast in xenocryst population between the carbonatite-phonolite diatreme and a neighbouring nephelinite volcano provides an insight into mineral stability regions. The key difference is sanidine from the carbonatite-phonolite diatreme and phlogopite from the nephelinite volcano. Both minerals are major potassic phases and have a significant Ba content. A subsolidus transition from phlogopite to feldspar stability is known to occur at around 25 kbar, indicating a maximum depth for the felsic rocks. Isotopically the felsic rocks are distinct from the nephelinite and carbonatite rocks and preclude a cogenetic relationship. Therefore the felsic rocks are thought to be derived either by interaction between lower crustal rocks and nephelinite, or they represent the crystallisation products of exotic silicic melts in the upper mantle.