Origin of megacrysts in the mafic alkaline lavas of the West Eifel volcanic field, Germany

Megacrysts of clinopyroxene, pargasite and phlogopite are common in the lav as of the West Eifel volcanic field of Germany. Clinopyroxene megacrysts ha ve been found at all seven localities studied; phlogopite and pargasite meg acrysts are less common. Three types of clinopyroxene megacryst have been i dentified. All show undulatory extinction and local recrystallisation. Type I clinopyroxene megacrysts are Cr2O3-rich TiO2-poor diopsides that are sim ilar in major and rare earth element (REE) composition to clinopyroxene fou nd in olivine-clinopyroxenite veins that cross-cut peridotite xenoliths. Ty pe II clinopyroxene megacrysts are the most common. They are augites that r ange in mg# from 72 to 87 and show a pattern of increasing TiO2, Al2O3, CaO and Na2O with decreasing mg#. These megacrysts are similar in major and RE E composition to clinopyroxene found in amphibole, phlogopite +/- clinopyro xene veins that cut peridotite and discrete amphibole-phlogopite clinopyrox enites. The single type III clinopyroxene megacryst is distinctive in its l ow mg# and low TiO2, Al2O3 and CaO and high Na2O. This sample has a similar REE pattern to the type I megacryst and is similar in its major element co mposition to acmite found as phenocrysts in the Eifel lavas. Pargasite and phlogopite megacrysts are similar in composition to pargasite and phlogopit e found in amphibole, phlogopite +/- clinopyroxene veins that cut peridotit e and discrete amphibole-phlogopite clinopyroxenites. The texture and compo sition of the megacrysts preclude a cognate origin via high-pressure crysta llisation of their host magmas, Modelling of parent magma compositions usin g known Fe-Mg partition coefficients for clinopyroxene and liquid and REE-m elt partition coefficients for clinopyroxene-melt and amphibole-melt sugges ts that the megacrysts were derived from three different magma batches that crystallised over a range of pressure and temperature in the Lithospheric mantle and crust underlying the Eifel region. The close compositional corre spondence between the megacrysts and various xenolith types suggests that: (1) Type I clinopyroxene megacrysts represent fragments of olivine clinopyr oxenite veins that formed by crystallisation of relatively magnesian magma in the lithospheric mantle. (2) Type II clinopyroxene, pargasite and phlogo pite megacrysts represent fragments of amphibole, phlogopite +/- clinopyrox ene veins that cut peridotite and discrete amphibole-phlogopite clinopyroxe nite veins that crystallised from moderately magnesian magmas in the lithos pheric mantle and possibly in the lower crust. The range of megacryst and x enolith compositions suggest that there may have been fractionation of magm a during this event. (3) The single type III clinopyroxene megacryst crysta llised from an iron-rich ma,oma at relatively high pressure within the midd le to lower crust. (C) 2000 Elsevier Science B.V. All rights reserved.