Mixing and chemical interdiffusion of trachytic and latitic magma in a subvolcanic complex of the Tertiary Westerwald (Germany)

A special kind of magma mixing is extraordinarily well exposed in the Bitte rsberg subvolcanic complex in the Tertiary volcanic field of the German Wes tervald: A trachytic melt has been penetrated by a latitic dyke which has b een dispersed within the host magma as small spherical enclaves (globules). Whole rock analyses of the globules show a change in composition that cann ot be explained by a simple mechanical mixing between the endmembers. The m ost evolved globules have a phonolitic composition. Microprobe measurements in the microlithic matrix of the host rock and the guest indicate a diffus ive motion of the alkalis from the host into the globules. On the other han d, an opposite trend can be observed for Ca, Mg, Fe and Ti, which are impov erished in the globules. The trace elements and the middle rare earth eleme nts (MREE) has also been involved in the diffusive exchange. The REE-patter n of the most evolved (phonolitic) globules shows a characteristic trough i n the area of the MREE which is almost identical to the REE-pattern of many phonolites. The phonolites and the alkali-rich trachytes of the Westervald show similar globular textures as the Bittersberg volcanics. Therefore, ge neration of these rocks involving diffusive element exchange during mixing processes in a magma reservoir situated on a deeper crustal level may be po ssible. (C) 1999 Elsevier Science B.V. All rights reserved.