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

Abstract

Megacrysts of clinopyroxene, pargasite and phlogopite are common in the lavas of the West Eifel volcanic field of Germany. Clinopyroxene megacrysts have been found at all seven localities studied; phlogopite and pargasite megacrysts are less common. Three types of clinopyroxene megacryst have been identified. All show undulatory extinction and local recrystallisation. Type I clinopyroxene megacrysts are Cr 2O 3-rich TiO 2-poor diopsides that are similar in major and rare earth element (REE) composition to clinopyroxene found in olivine–clinopyroxenite veins that cross-cut peridotite xenoliths. Type II clinopyroxene megacrysts are the most common. They are augites that range in mg# from 72 to 87 and show a pattern of increasing TiO 2, Al 2O 3, CaO and Na 2O 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 TiO 2, Al 2O 3 and CaO and high Na 2O. This sample has a similar REE pattern to the type I megacryst and is similar in its major element composition to acmite 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. Modelling of parent magma compositions using known Fe–Mg partition coefficients for clinopyroxene and liquid and REE–melt partition coefficients for clinopyroxene–melt and amphibole–melt suggests 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 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 relatively magnesian magma in the lithospheric mantle. (2) Type II clinopyroxene, pargasite and phlogopite megacrysts represent fragments of amphibole, phlogopite±clinopyroxene veins that cut peridotite and discrete amphibole–phlogopite clinopyroxenite veins that crystallised from moderately magnesian magmas in the lithospheric mantle and possibly in the lower crust. The range of megacryst and xenolith compositions suggest that there may have been fractionation of magma 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.