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Clinopyroxene, amphibole and phlogopite megacrysts are common in alkaline basaltic lavas. They may occur with a variety of peridotite and clinopyroxenite upper mantle xenoliths. The megacrysts may be derived from early high-pressure crystallisation of their host or they may represent fragments of pegmatitic veins that crystallised from differentiated alkaline mafic magmas that passed through the upper mantle.

In the alkaline lavas of the West Eifel region, Germany, we have identified several generations of megacrysts. There are three types of clinopyroxene megacryst: Type I megacrysts are Cr₂O₃-rich, TiO₂-poor diopside similar in major and REE composition to clinopyroxene found in olivine-clinopyroxenite veins that crosscut peridotite xenoliths. Type II clinopyroxene megacrysts are the most common and are augites that range in mg# = [(molecular MgO/(MgO + FeO)_x/oo)] from 72 to 87 and show a pattern of increasing TiO₂, Al₂O₃, CaO and Na₂O content with decreasing mg#. These megacrysts are similar in major and REE composition to clinopyroxene found in amphibole, phlogopite ± clinopyroxene veins that cut peridotite and discrete amphibole-phlogopite clinopyroxenites. The single type III clinopyroxene megacryst is distinctive in its low mg# and low TiO₂, Al₂O₃ and CaO and high Na₂O content. This sample has a similar REE pattern to the type I megacrysts and is similar in its major element composition to Ti-augite found as phenocrysts in the Eifel lavas. Pargasite and phlogopite megacrysts are similar in composition to pargasite and phlogopite found in amphibole, phlogopite ± clinopyroxene veins that cut peridotite and discrete amphibole-phlogopite clinopyroxenites.

The texture and composition of the megacrysts preclude a cognate origin via high-pressure crystallisation of their host magmas. Modeling of parent magma compositions using Fe-Mg partitioning between clinopyroxene and liquid and REE-melt partitioning for clinopyroxene and melt suggests that the megacrysts were derived from three different magma batches that crystallized over a range of pressure and temperature in the lithospheric mantle and crust underlying the Eifel region.

The close compositional correspondence between the megacrysts and various xenolith types suggests that: 1) Type I clinopyroxene megacrysts represent fragments of olivine clinopyroxenite veins that formed by crystallisation of magnesian magma in the lithospheric mantle. 2) Type II clinopyroxene, pargasite and phlogopite megacrysts represent fragments of clinopyroxene ± amphibole ± phlogopite veins that cut peridotite and discrete amphibole-phlogopite clinopyroxenite veins that crystallized from moderately magnesian magmas in the lithospheric mantle and possibly in the lower crust. The range of megacryst and xenolith compositions suggest that the magma was undergoing fractionation during this event. 3) The single type III clinopyroxene megacryst crystallised from an iron-rich magma at relatively high pressure within the middle to lower crust.