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3.6 c. Petrogenetic links between veined peridotite and megacrysts from the West Eifel Volcanic Field, Germany (C.S.J. Shaw, in collaboration with J. Eyzaguirre/London, Canada)

Composite (veined) peridotite xenoliths are generally interpreted as remnants of magma conduits in the mantle. The West Eifel region of Germany is a classic locality for the study of such veins as well as other manifestations of mantle alteration (i.e. modal and cryptic metasomatism). In a previous contribution (Annual Report 1998) we described the petrogenesis of megacrysts from the West Eifel and related them to fragmentation of veins formed by the passage of magma in the upper mantle. In this project we have used both veins and megacrysts to examine spatial variations in the composition of the magmas that formed veins in the sub-Eifel mantle.

Three types of vein have been identified: Olivine clinopyroxenite veins occur at Dreiser Weiher, amphibole - phlogopite - clinopyroxene (APC) veins occur at Dreiser Weiher and Meerfelder Maar whereas phlogopite - clinopyroxene (PC) veins are found at Kyllerkopf, Baarley and Gees. There are several types of megacryst that are similar in both major and trace element composition to vein minerals. Cr-diopside megacrysts derived from olivine-clinopyroxenite veins were found at Dreiser Weiher and Meerfelder Maar. Augite and phlogopite megacrysts similar in composition to the minerals in PC veins were found at at Gees, Baarley and Kyllerkopf. Amphibole and phlogopite megacrysts found at Meerfelder Maar are similar in composition to the minerals in APC veins.

The close similarity between vein mineral and megacryst compositions suggests that the megacrysts were derived from fragmentation of pegmatitic veins that cut the lithospheric mantle. Modelling of the composition of the magmas responsible for the various vein (and megacryst) types indicates that olivine - clinopyroxenite, PC and APC veins were dervied from different magma batches. The absence of amphibole from PC veins is due to differences in the composition of the vein forming magma in the Gees - Kyllerkopf - Baarley region. The magma parental to these veins was more potassic than the magma that invaded the mantle beneath Meerfelder Maar and Dreiser Weiher to form APC veins. Analysis of the lavas in these regions indicates that these differences in magma composition are reflected in the volcanic products erupted at the surface. These variations likely reflect regional differences in the composition of the source region, depth and amount of melting.

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