Pleistocene Underplating and Metasomatism of the Lower Continental Crust: a Xenolith Study

Abstract

Xenoliths from Engeln–Kempenich in the East Eifel volcanic field (Germany) comprise gabbroic to ultramafic cumulates, and meta-igneous and meta-sedimentary granulite- to amphibolite-facies lithologies. They provide evidence for Pleistocene heating and metasomatism of the lower continental crust by mafic magmas. The metamorphic xenoliths were divided into three types: (1) primitive type P, which are little affected by metasomatic replacement structures; (2) enriched type E1 defined by metasomatic replacement of primary pyroxene and garnet by pargasitic amphibole and biotite; (3) enriched type E2 defined by breakdown of hydrous phases. Type E rocks are geochemically related to type P and cumulate xenoliths by compositional trends. During modal metasomatism, type E rocks were oxidized. Type E1 rocks were typically enriched in Rb, Th, U, Nb, K, light rare earth elements (LREE) and Zr, and E2 enriched in Rb, Th, U, Nb, K, REE, Zr, Ti and Y, relative to type P rocks. Formation of the hydrous, chlorine-bearing phases amphibole and scapolite containing glass and fluid inclusions in the E1 rocks provides evidence for a water and Cl-bearing fluid phase coexisting with silicate melt. Accordingly, we calculated 10 mol % H2O back into the CO2-dominated fluid inclusions, in agreement with experimental data on the composition of a fluid phase coexisting with mafic alkaline melts at elevated pressure. Primary CO2-dominated fluid inclusions coexisting with glass inclusions in metamorphic corona phases and neoblasts, and in cumulate xenoliths, have overlapping densities. Fluid inclusion barometry using the corrected densities indicates that both cumulates and metamorphic xenoliths originated from the same depth at 22–25 km (650 ± 50 MPa). This is interpreted as being a main magma reservoir level within the upper part of the lower crust close to the Conrad discontinuity, where the xenoliths represent wall-rocks. The Conrad discontinuity separates an upper-crustal layer, consisting of preferentially ductile granodioritic and tonalitic gneisses, and more brittle lower-crustal mafic granulites. The brittle–ductile transition appears to be a preferred level of magma stagnation.