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3.2 i. Studies of mantle-derived xenoliths (C.S.J. Shaw, in collaboration with J. Eyzaguirre and C. Stanley/London, Canada)

The Quaternary and Tertiary volcanic events in Western Germany sampled a large region of the upper mantle in the form of mantle xenoliths. Ongoing studies in the Quaternary West Eifel and Tertiary Vogelsberg regions are documenting the petrologic characteristics of the mantle with a view to developing a spatial and temporal map of the mantle in this region.

Xenoliths from the West Eifel lavas display distinctive mineralogical and petrographic characteristics due to mantle metasomatism. At present two distinct metasomatic events have been identified: an early high temperature event that formed disseminated phlogopite - amphibole and a later event that was restricted to regions around conduits for the early stages of Eifel magmatism. This late event resulted in the formation of phlogopite, clinopyroxene - amphibole veins and megacrysts. Detailed study of the veined xenoliths and megacrysts indicate that the Eifel magmas sampled a region of the lithosphere cut by veins ranging in thickness from a few mm to more than 10 cm. The wide range of compositions of vein and megacryst minerals suggests that they record a polybaric fractionation trend of a similar magma type.

In contrast to the West Eifel xenoliths, those from the Tertiary volcanic rocks of the Vogelsberg region record evidence of an early depletion event related to partial melting but show little evidence for later metasomatism, at least involving growth of secondary phlogopite and amphibole. However, the Vogelsberg xenoliths do record abundant evidence for interaction with silicate melts. At present it is not possible to constrain accurately the conditions under which these reactions occurred, but it seems likely that they occurred during xenolith transport.

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