Abstract
Abstract. Granulite xenoliths from the Quaternary West Eifel Volcanic Field in Germany record evidence of magmatism in the lower crust at the end of the Permian. The xenoliths sampled two distinct bodies: an older intrusion (ca. 264 Myr old) that contains clinopyroxene with flat, chondrite-normalised rare earth element (REE) profiles and a younger (ca. 253 Myr old) intrusion that crystallised middle-REE-rich clinopyroxene. The younger body is also distinguished based on the negative Sr, Zr and Ti anomalies in primitive mantle-normalised multi-element plots. REE-in-plagioclase–clinopyroxene thermometry records the magmatic temperature of the xenoliths (1100–1300 ∘C), whereas Mg-in-plagioclase and Zr-in-titanite thermometry preserve an equilibration temperature of ca. 800 ∘C. These temperatures, together with a model of the mineral assemblages predicted from the composition of one of the xenoliths, define the pressure of crystallisation as ∼1 GPa. The xenoliths also preserve a long history of reheating events whose age ranges from 220 to 6 Myr. The last of these events presumably led to breakdown of garnet; formation of symplectites of orthopyroxene, plagioclase and hercynite; and redistribution of heavy rare earth elements into clinopyroxene. The data from the West Eifel granulite xenoliths, when combined with the existing data from granulites sampled in the East Eifel, indicate that the lower crust has a long a complex history stretching from at least 1.6 Ga with intrusive events at ca. 410 and 260 Ma and reheating from the Triassic to late Miocene.
Highlights
Crustal xenoliths in volcanic rocks carry valuable information about deep subsurface geology and petrology that cannot be accessed in any other way
Mantle xenoliths from the West Eifel field have attracted attention because they record multiple metasomatic events that have advanced our understanding of the evolution of the lithospheric mantle and the asthenospheric magmas that have interacted with it (Lloyd and Bailey, 1975; WittEickschen et al, 1998, 2003; Shaw et al, 2005; Mertz et al, 2015; Shaw et al, 2018)
This study examines the petrology and age of a new suite of quartz- and garnet-bearing mafic granulites that occur within the well-studied Rockeskyllerkopf Volcanic Complex in the West Eifel Volcanic Field (Shaw and Eyzaguirre, 2000; Shaw, 2009a; Shaw et al, 2010; Shaw and Woodland, 2012; Shaw et al, 2018)
Summary
Crustal xenoliths in volcanic rocks carry valuable information about deep subsurface geology and petrology that cannot be accessed in any other way. The textures, mineral assemblages and compositions of the xenoliths provide information on the age of the sampled crust, the pressure and temperature conditions of formation/equilibration. Knowledge of these permits correlation with regional- and continent-scale tectonic events. Xenoliths of granulite facies metamorphic rocks carry information on the petrology and textural evolution of the lower crust (Rudnick and Goldstein, 1990) and may be important in understanding interactions between mantle-derived magmas and the deep lithosphere (Ernst and Liou, 2008). The occurrence of accessory titanite in these xenoliths affords the opportunity to obtain robust geochronological information that will help to refine our understanding of the evolution of the crust below the Eifel volcanic fields
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